bulk polymerization of tpu for reactive processing using rheo- ftir

BULK POLYMERIZATION OF TPU FOR REACTIVE PROCESSING USING RHEO-

FTIR

By

JESSE L. GADLEY

Submitted in partial fulfilment of the requirements for the degree of

Doctor of Philosophy

Dissertation Adviser: Prof. João M. Maia

Department of Macromolecular Science and Engineering

Case Western Reserve University

August 2016

2

CASE WESTERN RESERVE UNIVERSITY SCHOOL OF GRADUATE STUDIES

We hereby approve the thesis/dissertation of

Jesse L. Gadley

candidate for the degree of Ph.D *.

Committee Chair

Dr. João M. Maia

Committee Member

Dr. Gary Wnek

Committee Member

Dr. Michael J. A. Hore

Committee Member

Dr. Jesse S. Wainright

Date of Defense

June 21st, 2016

*We also certify that written approval has been obtained for any proprietary material contained

therein.

3

Dedication

This dissertation is dedicated to my grandfather, Guy Ellis, for inspiring me since a very

young age to follow a career in science and engineering.

4

Table of Contents

Dedication ....................................................................................................................... 3

List of Figures ................................................................................................................. 8

List of Tables ................................................................................................................ 13

Acknowledgments......................................................................................................... 14

Abstract ......................................................................................................................... 15

Chapter 1 : Introduction .................................................................................................... 17

1.1 TPU Background .................................................................................................... 18

1.2 Reactive Extrusion .................................................................................................. 20

1.3 Rheo-Kinetics of TPUs ........................................................................................... 21

1.5 Dissertation Scope .................................................................................................. 23

1.6 References ............................................................................................................... 24

Chapter 2 : Rheo-Kinetic Study of Thermoplastic Polyurethanes using In Situ FTIR

Analysis............................................................................................................................. 27

2.1 Abstract ................................................................................................................... 28

2.2 Introduction ............................................................................................................. 29

2.3 Experimental ........................................................................................................... 31

2.3.1 Materials .......................................................................................................... 31

2.3.2 On-Plate Polymer Synthesis ............................................................................ 32

2.3.3 Characterization ............................................................................................... 33

5

2.3.3.1 Rheology ................................................................................................... 33

2.3.3.2 In Situ FTIR .............................................................................................. 33

2.4 Results and Discussion ........................................................................................... 34

2.5 Conclusions ............................................................................................................. 39

2.6 References ............................................................................................................... 41

2.7 Figures..................................................................................................................... 43

2.8 Tables ...................................................................................................................... 51

Chapter 3 : Investigation of Rheological Behavior During Onset of Thermoplastic

Polyurethane Reactions ..................................................................................................... 54

3.1 Abstract ................................................................................................................... 55

3.2 Introduction ............................................................................................................. 56

3.3. Experimental .......................................................................................................... 59

3.3.1 Materials .......................................................................................................... 59

3.3.2 Characterization ............................................................................................... 60

3.3.2.1 Rheology ................................................................................................... 60

3.3.2.2 FTIR .......................................................................................................... 61

3.3.2.3 X-ray ......................................................................................................... 62


3.5 Conclusions ............................................................................................................. 68

3.6 Figures..................................................................................................................... 73

6

3.7 Tables ...................................................................................................................... 80

Chapter 4 : Rheo-kinetic Modeling of TPU Using In Situ FTIR Analysis ....................... 81

4.1 Abstract ................................................................................................................... 82

4.2 Introduction ............................................................................................................. 83

4.3 Experimental ........................................................................................................... 85


4.4.1 Constant Composition Under Steady Shear ..................................................... 86

4.4.2 Varied Hard Segment Composition Under Oscillatory Shear ......................... 91

4.4.3 Comparison of Model Between Steady Shear and Oscillatory Shear.............. 94

4.5 Conclusions ............................................................................................................. 95

4.6 References ............................................................................................................... 97

4.7 Figures..................................................................................................................... 99

Chapter 5 : Effect of Soft-to-Hard Segment Ratio on Viscoelastic Behavior of Model

Thermoplastic Polyurethanes During Phase Transitions ................................................ 113

5.1 Abstract ................................................................................................................. 114

5.2 Introduction ........................................................................................................... 115

5.8 Figures................................................................................................................... 136

5.9 Tables .................................................................................................................... 146

Chapter 6 : Contribution of Inflow Conditions on Residence Time Output in Extrusion

Adapter Analysis ............................................................................................................. 147

7

6.1 Abstract ................................................................................................................. 148

6.2 Introduction ........................................................................................................... 149

6.3 Experimental ......................................................................................................... 152

6.3.1 Simulation Setup and Parameters .................................................................. 152

6.4 Results and Discussion ......................................................................................... 154

6.5 Conclusions ........................................................................................................... 160

6.6 References ............................................................................................................. 161

6.7 Figures................................................................................................................... 163

Bibliography ................................................................................................................... 172

8

List of Figures

Figure 1.1 - The reaction of an alcohol with isocyanate to form a urethane linkage.

Shown with reactants used for this work. ......................................................................... 25

Figure 1.2 - Typical phase separated structured developed in multi-block TPU systems. 26

Figure 2.1 - Experimental procedure developed for performing polymerization

experiments on the rheometer. .......................................................................................... 43

Figure 2.2 – Example of FTIR spectra collected throughout rheology experiments (26.4%

HS shown). ........................................................................................................................ 44

Figure 2.3 – A) Comparison of G’ and G” during isothermal reaction of the three

compositions at 100 °C. Closed symbols represent G’ while open symbols represent G”.

B-D) Absorbance peak at 2260 cm-1 representing free isocyante consumption in 26.4 wt.

% HS, 36.8 wt. % HS, and 55.6 wt. % HS. ...................................................................... 45

Figure 2.4- Fitting for reaction constants on second order plot of the three compositions

at 100C. ............................................................................................................................. 46

Figure 2.5 - Comparison of G’ and G” during isothermal reaction of the three


........................................................................................................................................... 47

Figure 2.6– Second order reaction plots of free isocyanate consumption at 200 °C. ....... 48

Figure 2.7 – Shear viscosity versus time under isothermal conditions of 36.8 wt. % HS by

varying shear rate from 0.1 sec-1 to 20 sec-1. .................................................................... 49

Figure 2.8 – Second order reaction plots of free isocyanate consumption in the 36.8 wt. %

HS system at various shear rates. ...................................................................................... 50

file:///C:/Users/jlg117/Documents/Maia%20Group/Thesis/Thesis%20Document/Thesis-1%20Formatted-Final-REV%20Post%20Def-Final.docx%23_Toc455067873

file:///C:/Users/jlg117/Documents/Maia%20Group/Thesis/Thesis%20Document/Thesis-1%20Formatted-Final-REV%20Post%20Def-Final.docx%23_Toc455067873

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Figure 3.1 - G' and G" plots during isothermal time study at 100 C of varying HS

content. .............................................................................................................................. 73

Figure 3.2 - FTIR waterfall plot demonstrating the data collected concurrently with

rheological measurements. ................................................................................................ 74

Figure 3.3 - Comparison of absorption bands throughout isothermal time study of 36.8

wt. % HS TPU................................................................................................................... 75

Figure 3.4 - Shift in wavenumber of carbonyl stretching peak throughout isothermal time

study. ................................................................................................................................. 76

Figure 3.5 - 1D WAXS plot at progressive time steps throughout the TPU reaction....... 77

Figure 3.6 Plot of SAXS data for the 36.8 wt.% HS sample with progressing reaction

time. .................................................................................................................................. 78

Figure 3.7 - Plot of increasing d-spacing with reaction time as a comparison between

multiple HS compositions. ................................................................................................ 79

Figure 4.1 - Viscosity versus time of 36.8 wt.% HS across a range of shear rates varying

from 0.1 sec-1 to 20 sec-1. .................................................................................................. 99

Figure 4.2 – Conversion versus time of 36.8 wt.% HS across a range of shear rates

varying from 0.1 sec-1 to 20 sec-1. ................................................................................... 100

Figure 4.3 Viscosity versus strain of 36.8 wt.% HS across a range of shear rates varying

from 0.1 sec-1 to 20 sec-1. ................................................................................................ 101

Figure 4.4 - Conversion versus strain of 36.8 wt.% HS across a range of shear rates

varying from 0.1 sec-1 to 20 sec-1. ................................................................................... 102

Figure 4.5 – Normalized viscosity versus conversion with fit using Equation 4.7. ....... 103

10

Figure 4.6 - Complex viscosity measured under oscillatory shear at 100 C for three

different hard segment compositions. ............................................................................. 105

Figure 4.7 – Normalized complex viscosity versus conversion for oscillatory shear

experiment at 100 °C. ..................................................................................................... 106

Figure 4.8 – Influence of total isocyanate consumed for compositions at 100C by

normalizing conversion by the initial concentration of NCO. ........................................ 107

Figure 4.9 - Viscosity versus time plot of polymerization under isothermal conditions at

200 C. .............................................................................................................................. 108

Figure 4.10 - Complex viscosity versus conversion for varied compositions under

oscillatory shear at 200 C. ............................................................................................... 109

Figure 4.11 - Master curve of complex viscosity versus conversion normalized by the

initial concentration of NCO in the system..................................................................... 110

Figure 4.12 - Comparison of viscosity versus conversion behavior between oscillatory

and steady shear experiments of 36.8 wt. % HS TPU. ................................................... 112

Figure 5.1 -DSC curves of TPUs with the endothermic peak become more pronounced

and shifted to higher temperatures with increase of HS. ................................................ 136

Figure 5.2 – Dynamic moduli versus time of TPUs at different temperatures: A) 26.4 wt.

% HS, B) 36.8 wt. % HS, and C) 55.6 wt. % HS where closed symbols represent G’, and

open symbols represent G” at each respective temperature. ........................................... 137

Figure 5.3 - Transient elongational viscosity pre-annealing data for A) TPU with 26.4%

HB at 100 °C, B) TPU with 36.8% HB at 150 °C, and C) TPU with 55.6% HB at 195 °C.

The dashed lines represent the linear viscoelastic envelope. .......................................... 139

Figure 5.4 - DSC curves for TPU samples before and after annealing. .......................... 140

11

Figure 5.5- SAXS patterns a) Comparison of differernt HS contentment and b) 36.8% HS

before and after annealing. .............................................................................................. 141

Figure 5.6 - WAXS before and after recrystallization of the 36.8 wt. % HS TPU. ........ 142

Figure 5.7 - Frequency sweep data after isothermal annealing for A) TPU with 26.4% HS

at 70 °C B) TPU with 36.8% HS at 150 °C, and C) TPU with 55.6% HS at 195 °C. .... 143

Figure 5.8- Transient elongational viscosity data after isothermal annealing for A) TPU

with 36.8% HB at 150 °C, and B) TPU with 55.6% HB at 195 °C. The dashed lines

represent the linear viscoelastic envelope. ...................................................................... 144

Figure 5.9- Top: Images of viscoelastic rupture under extensional flow (left) and a brittle-

like (right) for TPU with 36.8% HS. Bottom: Images of TPU with 36.8% after cessation

of flow before recrystallization (right) and after (left).................................................... 145

Figure 6.1 – 3D Model rendering of extrusion adapter used in simulation studies. ....... 163

Figure 6.2 – Extrusion adapter geometry showing meshed flow channel (A), inflow

boundary condition (B), zero wall velocity boundary surface (C), and outflow boundary

surface (D). ..................................................................................................................... 164

Figure 6.3 – Volume rendering of residence time (A), simplified inflow condition (B),

and velocity streamline plot (C). ..................................................................................... 165

Figure 6.4 – Helical inflow condition with Newtonian material characteristics (A) used to

generate a volume rendering of residence time (B), and velocity streamline plot (C). .. 166

Figure 6.5 - Helical inflow condition with PTT model material (A) used to generate a

volume rendering of residence time (B), and velocity streamline plot (C). ................... 167

Figure 6.6 – Depiction of purposely imbalanced inflow conditions based on material

exiting a counter-rotating twin screw extruder. .............................................................. 168

12

Figure 6.7 – Residence time volume rendering plot (A) shown with velocity streamline

plot (B) of an adapter with imbalanced inflow boundary conditions. ............................ 169

Figure 6.8– Comparison plot of material volume remaining versus residence time

between different adapter inflow boundary conditions. ................................................. 170

Figure 6.9 - Residence time distribution plots of various inflow conditions and material

models. ............................................................................................................................ 171

13

List of Tables

Table 2.1 – Rate constant comparison of three different composition TPUs under

isothermal conditions at 100 °C and 200 °C. .................................................................... 51

Table 2.2 - Rate viscosity buildup comparison between various shear rates of TPU under

isothermal conditions at 100 °C. ....................................................................................... 52

Table 2.3 – Rate constant comparison determined by FTIR of varied shear rates during

the formation of TPUs under isothermal conditions at 10 ................................................ 53

Table 3.1 - Comparison of transition time detected using FTIR and rheological

measurements. ................................................................................................................... 80

Table 4.1 - Fitting parameter comparison for model of TPU polymerization under steady

shear. ............................................................................................................................... 104

Table 4.2 - Fitting parameter comparison for model of TPU polymerization of varied HS

content under oscillatory shear. ...................................................................................... 111

Table 5.1– Molecular weight information of TPU before and after annealing. ............. 146

Table 6.1 - PTT Input Parameters for Simulations ......................................................... 154

14

Acknowledgments

I would like to start off by thanking my adviser, Dr. Joao Maia for providing me

with the opportunity to be a part of his research group. He has challenged me consistently,

introduced me to countless industrial projects, and supported all of my individual interest.

I can honestly say that he has had a significant impact on my development as a professional.

I would also like to thank my thesis committee: Dr. Gary Wnek, Dr. Michael Hore, and Dr.

Jesse Wainright for assisting me in the most crucial stages of this endeavor. From a

personal standpoint I would like to thank Dr. Lashanda Korley and Dr. David Schiraldi for

their guidance and support, specifically during some very challenging times.

Most importantly I would like to thank my family and friends for their support

throughout graduate school. I would not have made it this far if not for my friends,

especially Sid, Pat, Arman, Alex, Amanda, Stephen, and Nancy. I cannot stress enough

how important the support of my parents, Carrie and Terry, and my brother, Wayne, was

to my success. Without their help I certainly would not be where I am today.

Finally, I want to say thank you to my loving wife, Mahala, and daughter Paige.

Thank you for your support through all of the tough times and fun times alike. Finding

words to express my gratitude for putting up with me working late and all the stress of

these past five years is difficult. I love you both very much and look forward to all the

adventures we have ahead of us!

15

Abstract

Bulk Polymerization of TPU for Reactive Processing Using Rheo-FTIR

Jesse L. Gadley

Thermoplastic polyurethanes (TPUs) were studied to understand the complex

connection between changes in rheological characteristics and reaction behavior of bulk

polymerizations under flow. These materials are commonly produced using reactive

extrusion (REX) where the processing equipment is used as a reactor in which the

polymerizations take place. While under flow, the relationship between reaction progress,

melt viscosity, and processing conditions becomes very complex leading to difficulty in

targeting specific material properties and reproducibility. Generally, REX equipment is

viewed as a “black box” where carefully monitored reactants are added to the extruder, are

mixed in the presence of heat and shear, and the final product is then evaluated for the

desired results. While these systems have been the focus of significant research effort,

extending off-line experiments to processing conditions proves challenging by requiring

the inference of chemical behavior from rheological measurements or vice versa.

The focus of this work was to utilize simultaneous measurement of rheological

behavior and Fourier transform infrared spectroscopy (FTIR) to study the connection

between viscosity of the system and reaction kinetic behavior. First, an experimental

technique for monitoring bulk polymerization of TPUs in situ was developed. Using this

technique, the influence of hard to soft segment ratio within the TPU and the influence of

16

shear rate on these systems was investigated. This technique was also employed to study

the initial stages of polymerization and the potential influence of phase separation on the

development of mechanical properties throughout the reaction. The ability to

simultaneously measure changes in reactant concentration changes in time and viscosity

was then used to develop a model to connect conversion and viscosity under shear flow

conditions. Finally, the effect of thermal treatment on post processed materials were

studied under extensional flow. After processing, exposure to temperature near the phase

transition of the material resulted in changes in architecture and a subsequent change in

extensional flow properties. Overall, it was determined that varying hard to soft segment

composition resulted in a correlation between reaction rate, total isocyanate consumed

within the system, and final properties of the system. These TPU reactions also proved

sensitive to the shear rate applied due to the degree of mixing occurring during the reaction.

These observations provide important benchmarks moving forward when using TPUs in

reactive processing equipment. These results pave the way for future on-line studies of bulk

TPU polymerization processing methods.

17

Chapter 1 : Introduction

18

1.1 TPU Background

Polyurethanes (PU) are a class of polymers which contain urethane bonds as

linkages along the polymer backbone. Generally, the polyurethane bond results from the

reaction between isocyanate and a hydroxyl moiety. These materials are considered very

versatile due to a plethora a different diisocyanate and diol reactants to choose from,

providing access to a large array of potential polymer architectures. PUs are known for

their abrasion resistance, flexibility, and toughness which is conveniently tunable by

changing the chemical architecture of the system. While there are generally advantages

from a performance standpoint in application, many PUs are cross-linked thermosetting

resins. However, thermoset PUs lack processability and this restricts these materials to

casting techniques and reaction injection molding (RIM). While these materials are

undoubtedly useful, they are not recyclable and are application limited due to processing

technique compatibility.

In this work the PU subclass of materials, thermoplastic polyurethanes (TPUs), are

of primary interest. TPUs exhibit many of the advantages of polyurethanes while remaining

melt processable making them compatible with most traditional forming operations. These

materials typically exhibit properties somewhere between traditional rubber and plastic

materials currently on the market. In industry, TPUs are commonly produced through

reactive extrusion (REX) via bulk polymerization in the presence of shear and elevated

temperatures. These materials achieve these properties and process capability because they

are linear block copolymers rather than promoting crosslinking between polymer chains

with branching as occurs in traditional polyurethane materials. Instead of forming covalent

crosslinks, the TPUs form physical crosslinks through hydrogen bonding which occurs

19

between hard segments of adjacent chains. The high level of versatility in TPU systems

necessitate high precision process control to achieve the targeted product properties.

Typically, TPUs are produced through a step growth polymerization using a

diisocyante and two different diols as shown in Figure 1.1. The most commonly used

isocyanates consist of hexamethylene diisocyanate (HMDI), methylene diphenyl

diisocyante (MDI), and toluene diisocyanate (TDI). The isocyanate is reacted with a

mixture of short chain diols referred to as chain extender and a polyol which typically

ranges from 500 to 5,000 g/mol in molecular weight. The resultant material, when reacting

in a one-pot process, leads to a random distribution of isocyanates linked by either a very

short or very long chain yielding a segmented linear product. Consequently, the long

flexible soft segments (SS) and short rigid hard segments (HS) tend to phase separate into

soft and hard domains respectively. This phase separation usually results from an enthalpic

driving force due to chemical incompatibility between the HS and SS as shown in Figure

1.2. The hydrogen bonding between HS of adjacent polymer chains then act as physical

crosslinks which may be broken down by processing at temperatures above the order-

disorder temperature (ODT). TPUs owe their versatility and excellent mechanical

properties to this phase separated architecture. Through varying HS to SS content and the

relative level of flexibility within each phase, the phase separation behavior may be altered

drastically, thereby adjusting the ultimate performance of the system. Because of the near

limitless combinations of polyol molecular weights and chemical compositions that are

readily available, mechanical properties are readily tunable in TPU systems. While

versatility comprises the most significant advantage of TPUs, accessing the desired

20

properties through production of these materials in an efficient manner proves very

challenging.

1.2 Reactive Extrusion

The most common processing technique used to produce TPUs is reactive extrusion

(REX). Production through REX is a continuous process and has the capability of

producing larger volumes of material when compared to batch processes. This is

accomplished by feeding each of the reactants into the extruder separately under very

carefully metered conditions to ensure the correct stoichiometry is maintained throughout

the reaction. The reaction then proceeds due to the elevated temperature and mixing which

occurs within the extruder. The process is tuned to ensure the reaction completes prior to

exiting the die where the material is typically pelletized so it is usable in traditional forming

techniques for shaping into a final product. As a rule of thumb REX is most commonly

used for moderate to high reaction rate TPUs where slowly reacting systems are usually

limited to batch reaction type processes. In order to enhance the reaction rate for more

favorable extrusion conditions catalysts are commonly added to the extrusion process as

well. However, all of the materials used within this work were uncatalyzed systems which

react under heat and shear at a sufficient rate for reactive extrusion.

While the REX process is commonly implemented for production of TPUs, it is a

very complicated process. Reaching a target TPU composition using REX proves to be a

difficult task which is usually accomplished through experience of the operators. Because

of the complexity of these systems, producing them typically results in a very narrow

process window. Even though REX has economic and technical advantages over most

other polymerization methods, properly targeting the desired ultimate properties is

21

challenging. [1-4] The execution of tuning these processes typically result in a time

consuming and labor intensive endeavor driven by trial and error. [5] Since polymerization

drives the rheological behavior of these systems and these rheological properties in turn

drive the extrusion process, understanding the connection between the reaction progression

and subsequent changes of rheology is of the utmost importance to make the most of the

REX process.

1.3 Rheo-Kinetics of TPUs

While these materials have been heavily characterized for a multitude of reasons,

making connections to practice and between different measurement techniques proves

extremely difficult. Significant effort has been focused on understanding the chemical

reaction kinetics of TPUS and as a result their behavior has been well characterized.

Generally, TPUs are understood to follow second-order rate kinetics as shown in Equation

1.1. [6-8]

−𝑑[𝑁𝐶𝑂]

𝑑𝑡= 𝑘[𝑁𝐶𝑂][𝑂𝐻]

Equation 1.1

This equation is expressed in terms of the concentration of reactants (isocyanate

and hydroxyl groups) and a kinetic constant k. In order to connect this kinetic information

to a process, relationships between these rates, molecular weight, and subsequently

viscosity have been developed. It is known that the viscosity of a polymer in the melt state

is a function of shear rate, molecular weight, and temperature as shown by the relationship

depicted in Equation 1.2.

22

𝜂 = 𝑓(�̇�, 𝑀𝑤, 𝑇)

Equation 1.2

When considering a reactive system, particularly bulk polymerizations, all of these

variables change throughout production making the relationship between the viscosity and

reaction progress complicated. However, there are known relationships between viscosity

and the molecular weight that are also important to benchmarking reaction progress.

During polymerization of linear systems, the viscosity is proportional to the molecular

weight where the viscosity increases with molecular weight to the power 1 then increases

to the power 3.4 once entanglement molecular weight is reached.

𝜂 ∝ 𝑀𝑤𝑛

Equation 1.3

Further study of viscosity increasing over time has been shown to follow the

relationship in Equation 1.4. [1, 9, 10] Understanding how the viscosity is building directly

in time provides important insight to changes in the melt viscosity but is limited to specific

conditions and lacks a connection to the kinetics occurring during processing.

𝜂(𝑡) = 𝜂𝑜𝑒𝑘𝑛𝑡

Equation 1.4

The main scope of this work was to utilize a unique opportunity to simultaneously

measure chemical changes using Fourier transform infrared spectroscopy (FTIR) while

measuring viscosity under flow conditions. Using previously developed understanding of

these systems, the primary thrust of the work in this dissertation was to exploit this in situ

approach to help determine the connections linking viscosity changes over time and

23

consumption of reactants. These techniques hold promise for bridging the gap between

limited experimental techniques and bulk polymerization of TPUs in industrial processes.

1.5 Dissertation Scope

Chapter 2 focuses on developing a reliable experimental technique for the study of

bulk polymerization of TPUs. This chapter demonstrates the ability to produce reliable

results in these systems and determine potential connections between shear rate and

composition on rheo-kinetic behavior.

Chapter 3 focuses on a transition detected during the initial stages of bulk

polymerization of TPU. The relationship between early stages of the reaction and potential

influence of phase separation during the process was investigated.

Chapter 4 focuses on developing a model to connect viscosity of a TPU melt during

bulk polymerization and conversion as measured using FTIR. The influence of shear rate

on a single composition and the influence of varying composition were investigated. The

connection between these relationships and inconsistent viscosity buildup reported at high

conversions was investigated.

Chapter 5 focuses on the final properties of TPUs of various compositions. The

influence of post-processing and thermal treatment on extensional properties was

investigated.

Chapter 6 focuses on simulation techniques for extrusion adapters for use with

thermally sensitive materials. Various inflow conditions were coupled with Newtonian,

shear-thinning inelastic, and viscoelastic material models to investigate their influence on

residence time simulations.

24

1.6 References

[1] V. W. A. Verhoeven, A. D. Padsalgikar, K. J. Ganzeveld, and L. P. B. M. Janssen,

"A Kinetic Investigation of Polyurethane Polymerization for Reactive Extrusion

Purposes," Journal of Applied Polymer Science, vol. 101, pp. 370-382, 2006.

[2] J.-P. Puaux, P. Cassagnau, G. Bozga, and I. Nagy, "Modeling of Polyurethane

Synthesis by Reactive Extrusion," Chemical Engineering and Processing, vol. 45,

pp. 481-487, 2006.

[3] H. Madra, S. B. Tantekin-Ersolmaz, and F. S. Guner, "Monitoring of oil-based

polyurethane synthesis by FTIR-ATR," Polymer Testing, vol. 28, pp. 773-779,

2009.

[4] M. Szycher, Szycher's Handbook of Polyurethanes. Boca Raton: CRC Press,

1999.

[5] W. Michaeli, A. Greefenstein, and U. Berghaus, "Twin-Screw Extruders for

Reactive Extrusion," Polymer Engineering and Science, vol. 35, pp. 1485-1504,

1995.

[6] C. Hepburn, Polyurethane elastomers, 2nd ed. London ; New York

New York, NY, USA: Elsevier Applied Science ;

Sole distributed in the USA and Canada, Elsevier Science Pub. Co., 1992.

[7] G. G. Odian, Principles of polymerization, 4th ed ed. Hoboken, N.J.: Wiley, 2004.

[8] Z. Wirpsza and T. J. Kemp, Polyurethanes : chemistry, technology, and

applications. Chichester ; New York: E. Horwood, 1993.

[9] A. H. Navarchian, F. Picchioni, and L. P. B. M. Janssen, "Rheokinetics and effect

of shear rate on the kinetics of linear polyurethane formation," Polymer

Engineering and Science, vol. 45, pp. 279-287, Mar 2005.

[10] V. Sekkar, K. A. Devi, and K. N. Ninan, "Rheo-kinetic evaluation on the

formation of urethane networks based on hydroxyl-terminated polybutadiene,"

Journal of Applied Polymer Science, vol. 79, pp. 1869-1876, Mar 7 2001.

25

Figure 1.1 - The reaction of an alcohol with isocyanate to form a urethane linkage.

Shown with reactants used for this work.

26

Figure 1.2 - Typical phase separated structured developed in multi-block TPU systems.

Polyol

Chain Extender

Diisocyanate

Soft Segment Hard Segment

Hard Domain

Soft Domain

27

Chapter 2 : Rheo-Kinetic Study of Thermoplastic

Polyurethanes using In Situ FTIR Analysis

This chapter is partially based on:

Journal article:

J. L. Gadley and J. M. Maia Submitted (Polymer Testing)

28

2.1 Abstract

Thermoplastic polyurethanes (TPUs) are complicated yet highly versatile materials which

have greatly benefitted from the application of many characterization techniques for their

successful production. The focal point of this study was to utilize in situ FTIR (direct)

measurements concurrently with rheological (indirect) measurements to understand the

behavior of TPU reactions. Through the integration of these two methods chemical and

mechanical data were collected simultaneously and used to help eliminate potential short

comings of either technique alone. This work showed the combination of these

techniques indicates fundamental differences during the polymerization of different hard

segment (HS) ratio TPUs when using the same reactants. The complexity of these

systems were also showcased through studying a range of flow conditions within a single

composition which determined that these reactions are very sensitive to the shear rate

during the reaction. The results of this study indicated that the connection of information

between these direct and indirect measurement techniques provides a unique view of

these complex systems by permitting real time chemical changes to be compared to

viscosity development. This method provides a powerful tool for analyzing TPU systems

for industrial applications.

29

2.2 Introduction

Thermoplastic polyurethanes (TPUs) are commonly used materials which have

excellent versatility making them attractive for use across a broad range of applications.

These materials typically exhibit properties somewhere between traditional rubber and

plastic materials currently on the market. TPUs are known for their abrasion resistance,

flexibility, and toughness which is conveniently tunable by changing the chemical

architecture of the system. In industry, TPUs are commonly produced through reactive

extrusion (REX) via bulk polymerization in the presence of shear and elevated

temperatures.

The high level of versatility in TPU systems necessitate high precision process

control to achieve the targeted product properties. Because of the complexity of these

systems, producing them typically results in a very narrow process window. Even though

REX has economic and technical advantages over most other polymerization methods,

properly targeting the desired ultimate properties is challenging. [1-4] The execution of

tuning these processes typically result in a time consuming and labor intensive endeavor

driven by trial and error. [5]Since polymerization drives the rheological behavior of these

systems and these rheological properties in turn drive the extrusion process, understanding

the connection between the reaction progression and subsequent buildup of rheological

characteristics is of the utmost importance to make the most of REX processes.

Polyurethane reaction kinetics have been studied extensively through both direct

and indirect measurement techniques independently. Commonly, the direct methods used

to measure kinetics of these systems are Fourier transform infrared spectroscopy (FTIR)

and back titrations. [11, 12]Raman spectroscopy and NMR of these systems were studied

30

as well but much less commonly than the other direct methods. [13, 14]The sample

preparation process of these measurement methods is time intensive and sample

preparation quality radically affects the kinetic data collected. Furthermore, these

techniques are unable to directly measure these changes as they occur resulting in limited

practical relevance to industrial processes or bulk reactions. In order to circumvent some

of the aforementioned sample preparation issues it has been shown that FTIR in attenuated

total reflection (ATR) mode is capable of accurately capturing the consumption of free

isocyanate as confirmed by back titration [3, 15]

Indirect measurement techniques such as rheometry and differential scanning

calorimetry (DSC) are commonly pursued to study the reaction kinetics of these systems

due the simplicity of sample preparation and relevance to processing. While measuring

viscosity buildup is convenient and the kinetics of this process are crucial to understanding

the rheological transitions occurring to control processing, the high viscosity during

reaction in the bulk results in the inability to accurately estimate changes in functional

groups. [16]While direct measurement techniques, particularly in solvents, capture kinetics

much more accurately, it is known that bulk kinetics may be much different resulting in a

disconnect from predicted buildup of rheological properties. [14, 16, 17]In order to better

understand the reaction behavior under flow conditions in the melt state, the coupling of

both direct and indirect measurement techniques could provide feedback on changes in

rheological data with a direct connection to the chemical changes taking place.

The intent of this study was to use Rheo-FTIR to monitor the development of TPUs

under bulk reaction conditions. The combination of these techniques provide a unique

opportunity to monitor the consumption of free isocyanate within the system while

31

monitoring the changes in dynamic moduli in real time. The use of this method simplifies

the sample preparation procedure compared to that of traditional FTIR techniques.

Furthermore, performing the polymerization directly on the rheometer circumvents the

exposure of samples to additional thermal cycles which would alter the architecture or

reaction progress before measurements are attained.

This work consisted of a rheo-kinetic study using parallel plate rheometry in

tandem with in situ FTIR measurements collected under ATR mode. TPU reaction kinetics

were studied under small amplitude oscillatory shear (SAOS) and steady shear while

collecting FTIR measurements in real time. Bulk polymerization of TPUs containing 26.4

wt. %, 36.8 wt. %, and 55.6 wt. % HS was performed under isothermal conditions without

a catalyst. These compositions were achieved by varying the ratio of chain extender to

polyol in the system while maintaining a stoichiometric ratio of unity between hydroxyl

moieties and isocyanate groups. Varied HS content TPUs with the same reactants were

compared. The resulting changes in kinetic behavior were evaluated based on the

composition of a specific sample and the influence of different flow conditions were

investigated.

2.3 Experimental

2.3.1 Materials

The TPUs used in this study were polymerized through a bulk one-pot reaction

directly on the rheometer plate without the addition of a catalyst. The reaction was a three-

part system driven only by elevated temperatures to represent an uncatalyzed REX process

of a TPU system. The isocyanate component used was 4,4’-methylenedipenyl diisocyanate

(MDI) (Sigma Aldrich). The MDI was stored at -20 °C under a nitrogen blanket in a

32

desiccated vessel until immediately prior to use. The short chain diol used as a chain

extender was 1-4 butanediol (BDO) (Sigma Aldrich) which was stored in a sealed

container. The polyol component used was a poly(butylene adipate) with a molecular

weight of approximately 1000 g/mol. The polyol was stored as received in a sealed vessel.

2.3.2 On-Plate Polymer Synthesis

The on-plate reaction was performed by maintaining a 1:1 ratio of hydroxyl to free

isocyanate within the reaction mixture. Maintaining this ratio, the BDO to polyol ratios of

0.25:0.75, 0.50:0.50, and 0.75 :0.25 were used, resulting in 26.4 wt. %, 36.8 wt. %, and

55.6 wt. % HS respectively according to equation 1. The percentage of HS was calculated

by dividing the product of equivalent weights and number of equivalents of MDI and BDO

by the product of the total equivalent weight and number of equivalents of all three

components in the system.

𝐻𝑆% = 𝐸𝑞𝑊𝑡𝐻𝑆 ×𝐸𝑞𝐻𝑆

𝐸𝑞𝑊𝑡𝑇𝑜𝑡𝑎𝑙 ×𝐸𝑞𝑇𝑜𝑡𝑎𝑙 (2.1)

A metal ring was fixed to the bottom plate of the rheometer to create a reservoir and prevent

the initial low viscosity reactants from flowing out of the desired sampling area. The

bottom plate of the rheometer was set to the testing temperature and allowed to equilibrate

for 30 minutes. Once equilibrated the appropriate mass of polyol for the desired

composition was added to the testing fixture on the bottom plate and allowed to equilibrate

at temperature for 5 minutes. The appropriate volume of BDO was then added to the polyol

and stirred to provide a homogenous mixture of the diols. The MDI was added to the polyol

mixture and stirred briefly (less than 10 seconds). The reaction remained under isothermal

conditions for 1800 sec while rheological and FTIR measurements were taken.

33

2.3.3 Characterization

2.3.3.1 Rheology

Rheological experiments were conducted on a MARS III rheometer (Thermo

Scientific) using a round 20 mm stainless steel parallel plate as the testing geometry. These

experiments were carried out under isothermal conditions at temperatures of 100 °C and

200 °C. These two temperatures were chosen in order to demonstrate differences in the

bulk reaction behavior based on different levels of viscosity increase. First, an amplitude

sweep was performed on only the two diols (lowest viscosity material during the time

sweep) and on a fully reacted sample (highest viscosity experienced throughout

experiment) and a stress value of 100 Pa was chosen to ensure the experiment remained

within the linear viscoelastic regime (LVR) for its entirety. Using the stress within the

LVR, isothermal time sweeps were performed at a constant frequency of 6.28 rad/sec for

a total time interval of 1800 sec. In addition, these systems were studied under isothermal

steady shear at shear rates of 0.1, 1.0, 5.0, 10.0, 15.0, and 20.0 s-1 to probe the influence of

shear rate on the reaction kinetics of the system. All rheometry experiments were

performed at a gap height of 1.0 mm. The experimental procedure developed for this work

is shown in Figure 2.1.

2.3.3.2 In Situ FTIR

The in situ FTIR experiments were performed using the Rheonaut module (Thermo

Scientific). The Rheonaut is a bottom plate assembly for the rheometer that allows for

electrically controlled heating and collection of FTIR data in ATR mode. The ATR window

used for all experiments was a single reflection diamond crystal. The detector used in the

Rheonaut module was a mid-infrared region deutereated triglycine sulfate (DTGS) detector

34

with as spectral detection range of 400 – 4,000 cm-1. Prior to each experiment a background

scan was collected for use as a reference. During the rheometry experiments, at time zero

an FTIR scan was collected prior to any exposure of the system to oscillation or steady

shear. Immediately after the first scan the rheological measurements were started.

Throughout the duration of the experiment, FTIR spectra were collected continuously

using an instrument resolution setting of 32 on 16 scans per collection period. Under these

conditions an FTIR spectrum was collected approximately every 4 secs.

2.4 Results and Discussion

Typically, the peak positions of interest in IR spectra for TPU systems have

absorption bands located at approximately 1520 cm-1, 1720 cm-1, and 3340 cm-1. These

peaks correspond to amide II stretching associated with a urethane bond, carbonyl bending,

and N-H stretching respectively. In this study, the absorption band at 2260 cm-1 associated

with unbonded isocyanate was of primary interest. It has previously been demonstrated

that FTIR-ATR is a capable alternative to using back titration when tracking the

consumption of free isocyanate.[3] The spectra shown in Figure 2.2 is representative of the

data collected in situ during each isothermal rheological study. The resulting data is a

cascade plot with an IR spectrum collected every 4 sec. A cascade plot similar to Figure

2.2 was generated for each rheological experiment performed and analyzed to track the

consumption of free isocyanate within the system.

The storage modulus (G’) and loss modulus (G”) of the system was tracked over

time under isothermal conditions while maintaining a constant frequency of 6.28 rad/s. The

data shown in Figure 2.3 depicts the buildup of the dynamic moduli as the reaction was

progressing on the rheometer at 100 °C. The plot clearly shows an increase in both G’ and

35

G” approaching a plateau indicating the reaction was either completed or had become

inhibited by a diffusion limited state due to the high viscosity values reached. Further

investigation showed a free isocyanate remained in the system indicating that these

reactions did not reach 100% completion and the samples solidified throughout the

experiment indicating that the reaction had halted due to high viscosity. However, these

experiments still captured the kinetic behavior of the systems while the reaction persisted.

The rheological data showed a distinct difference in the progression of the reaction based

on changes in composition of the system. The moduli in Figure 2.3 indicates that as the HS

content in the system was increased, the reaction rate increased as was indicated by

plateauing earlier and by the difference in slope during the reaction period.

The trend of increasing viscosity at a higher rate was attributed to a localized

concentration effect. Although the isocyanate and hydroxyl groups were reacted at a 1:1

ratio, while trending toward a higher targeted HS content, the volume of BDO present in

the system compared to polyol is much greater. The higher concentration of BDO resulted

in a lower average distance between reactants promoting a higher rate of reaction as well

as a higher required amount of isocyanate within the system to balance the stoichiometric

ratio of the reactants while maintaining a constant sample volume for the experiment.

Considering the lowest HS content sample, it is then intuitive that increasing the polyol to

BDO ratio necessitates a reduction in isocyanate reactive groups to balance the

stoichiometry thereby reducing the effective concentration of the system and the rate of the

reaction. The increase in reaction rate with increasing HS of the same system has

previously been observed and attributed to the average distance between reactive groups

within the system.[16]

36

The FTIR data was analyzed in order to determine the kinetic parameters of these

samples. The change in the absorbance band of free isocyanate is plotted in Figure 2.4. The

reaction between aromatic isocyanates and polyester polyols are typically reported as

following second order reaction kinetics. [18-20] Therefore, the absorbance peak heights

of the 2260 cm-1 band were plotted after normalizing the absorbance as a function of time

(At) with the initial absorbance (Ao). This assumed that the initial measurement prior to

any rheological testing was representative of zero reaction progress or at least a small

enough reaction progression to remain negligible. Using this method (Ao/At) – 1 was

plotted as a function of time which is representative of a second order rate plot. The second

order rate plots are shown in figure 3B-D. These plots show a clear trend that increasing

the HS content of the system increases the rate of the reaction. The reaction rate constants

from the linear regression shown on the plots are listed in Table 2.1. Prior to the linear

behavior of the reaction a region exists which appears to have a very low reaction rate or

even no reaction progression until a certain time. Discontinuity such as this has been

reported and was usually attributed to complex changes in the reaction conditions or

differential reactivity between system components. [3] This appears to act as an induction

period related to the amount of HS component added as evident by the longer time with

the increase in polyol content. Additionally, the samples at 100 °C reacted and transitioned

to a solid material with a melting temperature above the temperature of the reaction. This

inevitably lead to an incomplete reaction and increased complexity due to the liquid to solid

transition occurring.

In order to further understand the applicability of in situ FTIR analysis coupled with

rheometry, the experiments were repeated at 200 °C at a temperature above the order-

37

disorder temperature (ODT) of the final material. The SAOS experimental results are

shown in Figure 2.5. The results indicate a similar trend that demonstrates an increase in

HS content within the system causes the material to react more rapidly, achieve a higher

modulus value, and reach a plateau modulus earlier. However, under these conditions the

26.4 wt. % and 36.8 wt. % HS reached a completion percentage greater that 99% and the

55.6 wt. % reached a value of approximately 92%. The lower completion of the highest

HS content system was likely due to the higher viscosity and closer relative vicinity to the

Tm of the material inhibiting the reaction in the final stages.

The FTIR results treated in the same fashion as at lower temperature are shown in

Figure 2.6. The first observation noted with the 200 °C samples was the lack of lower rate

stage induction period of the reaction. Under these conditions the reaction follows a second

order rate relationship from time zero. This is either a result of a more homogenous system

at the higher temperatures and less preference between reactive components or simply that

the reaction progresses too rapidly to detect any onset behavior under the experimental

conditions. A clear deviation from second order kinetics was observed once the viscosity

increased to a high enough value which is consistent with previous kinetic studies on bulk

systems. [3] Because of the discontinuity, the calculated rate constants were based solely

on the initial buildup of the reaction while a linear increase in storage modulus was still

occurring. The rate constants obtained are shown in Table 2.1.

The influence of steady shear on the reaction behavior was investigated by

performing simultaneous collection of FTIR spectra and rheological data while

maintaining a shear rate ranging from 0.1 sec-1 to 20 sec-1 using the same setup as was

studied under oscillation. To understand the effect of shear, the composition of the system

38

remained constant. The 36.8 wt. % HS system specifically was utilized due to the moderate

melting temperature and rate of reaction observed under oscillatory conditions. The

samples were exposed to isothermal steady shear conditions and buildup of viscosity was

tracked as shown in Figure 2.7. The initial slope was regarded as representative of the

reaction behavior to avoid an increasingly complex system as the viscosity reaches

relatively high values. Generally, the viscosity values follow a first order relationship with

time which is supported by previous studies associated with polyurethane formation.[21]

The rate of viscosity change is summarized in Table 2.1 which indicates little to no

variation in viscosity increase with increasing shear rate within the error of measurement.

These results suggest that the influence of flow conditions on the initial stages of the

reaction over a relatively small range in shear rate is minimal and the viscosity buildup

behavior does not change appreciably during these stages. This was attributed to the low

molecular weight likely generated throughout this period. FTIR data was collected

concurrently with these measurements and the consumption of free isocyanate was tracked.

The second order rate plots associated with isocyanate consumption are shown in Figure

2.8. Interestingly, this data indicates a strong dependence on the shear rate within the

system. The rate of this process is displayed in Table 2.3. This enhancement in reaction

rate was attributed to better mixing of the reactants through mechanically promoting their

diffusion to the reaction interface. The reactions in the initial stages still appears to follow

second order kinetics but indicate no induction period as was noted in the oscillatory

experiments suggesting that flow is more efficiently starting the reaction when compared

to the quasi-quiescent conditions of the oscillatory experiments. Furthermore, the FTIR

results suggest a significant change in reactant consumption as a function of shear value

39

which was undetectable through solely studying the rheological data. A shift to increased

reaction rate at higher shear rates ultimately related to different viscosity buildup

characteristics. By increasing the shear rate the viscosity increased more rapidly and

reached different final viscosity values. Considering these results, the FTIR has a higher

level of sensitivity at low viscosity values during the initial stages of the reaction.

Conversely, due to complications concerning penetration depth of the IR beam and

complexities in the bulk as viscosity reaches high values, the rheological measurements

likely provide more useful information after a threshold in reaction progress has been

reached. This behavior suggests that changes in flow conditions, although indistinguishable

in viscosity data, could affect the reaction’s progress ultimately leading to an altered final

architecture as evident by reaching different viscosity plateau values. Therefore, through

connecting direct and indirect measurement techniques simultaneously, new perspective is

gained when considering complex materials such as TPUs.

2.5 Conclusions

The work conducted within this study demonstrated the capability of a rheo-FTIR

system to track bulk reaction kinetics in a complicated TPU system. This technique showed

promise for understanding the effect of compositional changes within these systems and

sensitivity to adjustments in the flow environment during polymerization. Concerning

compositional changes, this work demonstrated that increasing HS content from 26.4 wt.

% to 55.6 wt. % HS resulted in an increased rate of polymerization, a decreased

polymerization time, and an increase in dynamic moduli. These results demonstrate the

complexity of TPUs and their reliance on precise control of reactants to achieve the desired

final product. In addition, the results indicate the complex relationship between reaction

40

rate and rheological response which must be understood to increase efficiency of their

production through reactive extrusion.

While maintaining a constant composition this study also indicated a change in flow

conditions has the potential to alter bulk reaction behavior even across a very small range

of shear rate. Changes in shear rate effect the rate at which reactants can interact due to

changes in mixing, ultimately affecting the reaction behavior of the system. Under these

conditions the FTIR proved more sensitive to changes, particularly in the early stages of

the reaction. Therefore, the advantage of simultaneously collecting FTIR and rheological

data on reactive systems is realized to enhance the overall picture associated with complex

changes taking place in the bulk. While these techniques are independently capable of

granting information on reactive TPU systems, the combination of direct and indirect

techniques provides the unique advantage of collecting all of the information under

precisely the same conditions in real time.

41

2.6 References






pp. 481-487, 2006.



2009.


1999.



1995.

[11] S. Ajithkumar, S. S. Kansara, and N. K. Patel, "Kinetics of Castor Oil Based

Polyol-Toluene Diisocyanate Reactions," European Polymer Journal, vol. 34, pp.

1273-1276, 1998.

[12] I. Yilgor, B. D. Mather, S. Unal, E. Yilgor, and T. E. Long, "Preparation of

segemented, high molecular weight, aliphatic poly(ether-urea) copolymers in

isopropanol. In-situ FTIR studies and polymer synthesis," Polymer, vol. 45, pp.

5829-5836, 2004.

[13] S. Parnell, K. Min, and M. Cakmak, "Kinetic Studies of Polyurethane

Polymerization with Raman Spectroscopy," Polymer, vol. 44, pp. 5137-5144,

2003.

[14] C. Dubois, S. Desilets, A. Ait-Kadi, and P. Tanguy, "Bulk Polymerization of

Hydroxyl Terminated Polyubutadiene (HTPB) with Toluene Diisocyanate (TD):

A Kinetics Study using 13C-NMR," Journal of Applied Polymer Science, vol. 58,

pp. 827-834, 1995.

[15] D. Kincal and S. Ozkar, "Kinetic Study of the Reaction Between Hydroxyl-

Terminated Polybutadiene and Isophorone Diisocyanate in Bulk by Quantitative

FTIR Spectroscopy," Journal of Applied Polymer Science, vol. 66, pp. 1979-

1983, 1997.

[16] V. Sekkar, K. Ambika, and K. Ninan, "Rheo-kinetic Evaluation on the Formation

of Urethane Networks Based on Hydroxyl-Terminated Polybutadiene," Journal of

Applied Polymer Science, vol. 79, pp. 1869-1876, 2001.

[17] O. Gunter, Polyurethane Handbook. Munich: Hanser Publishers, 1985.

[18] L. L. Ferstandig and R. A. Scherrer, "Mechanism of Isocyanate Reactions with

Ethanol," Journal of American Chemical Society, vol. 81, pp. 4838-4842, 1959.

[19] H. G. Wissman, L. Rand, and K. C. Frisch, "Kinetics of Polyether Polyols-

Diisocyanate Reactions," Journal of Applied Polymer Science, vol. 8, pp. 2971-

2978, 1964.

[20] M. Gambiroza-jukic, Z. Gomzi, and H. J. Mencer, "Kinetic Analysis of Bulk

Polymerizatio of Diisocyanate and Polyol," Journal of Applied Polymer Science,

vol. 47, pp. 513-519, 1993.

42

[21] A. H. Navarchian, F. Picchioni, and L. P. B. M. Janssen, "Rheokinetics and Effect

of Shear Rate on the Kinetics of Linear Polyurethane Formation," Polym. Eng.

Sci., vol. 45, pp. 279-287, 2005.

43

2.7 Figures

Addition of polyol onto

rheometer plate BDO added Polyol + BDO mixed

MDI added Experiment begins immediately

Figure 2.1 - Experimental procedure developed for performing polymerization

experiments on the rheometer.

44

Figure 2.2 – Example of FTIR spectra collected throughout rheology experiments (26.4%

HS shown).

45

Figure 2.3 – A) Comparison of G’ and G” during isothermal reaction of the three


B-D) Absorbance peak at 2260 cm-1 representing free isocyante consumption in 26.4 wt.

% HS, 36.8 wt. % HS, and 55.6 wt. % HS.

46

Figure 2.4- Fitting for reaction constants on second order plot of the three compositions at

100C.

47

Figure 2.5 - Comparison of G’ and G” during isothermal reaction of the three compositions

at 200 °C. Closed symbols represent G’ while open symbols represent G”.

48

Figure 2.6– Second order reaction plots of free isocyanate consumption at 200 °C.

49

Figure 2.7 – Shear viscosity versus time under isothermal conditions of 36.8 wt. % HS by

varying shear rate from 0.1 sec-1 to 20 sec-1.

50

Figure 2.8 – Second order reaction plots of free isocyanate consumption in the 36.8 wt. %

HS system at various shear rates.

51

2.8 Tables

Temperature Composition Rate Constant

(L/mole∙sec)

R2 of Fit

100 °C

26.4 wt. % HS 0.0022 0.994

36.8 wt. % HS 0.0186 0.975

55.6 wt. % HS 0.0507 0.997

200 °C

26.4 wt. % HS 0.0051 0.950

36.8 wt. % HS 0.0196 0.978

55.6 wt. % HS 0.0610 0.994

Table 2.1 – Rate constant comparison of three different composition TPUs under

isothermal conditions at 100 °C and 200 °C.

52

Shear Rate (sec-1) Rate Constant R2 of Fit

0.1 0.0361 0.988

1.0 0.0229 0.973

5.0 0.0295 0.977

10.0 0.0273 0.979

15.0 0.0283 0.979

20.0 0.0324 0.984

Table 2.2 - Rate viscosity buildup comparison between various shear rates of TPU under

isothermal conditions at 100 °C.

53

Shear Rate (sec-1) Rate Constant

(L/mole∙sec)

R2 of Fit

0.1 0.0093 0.984

1.0 0.0058 0.983

5.0 0.0170 0.998

10.0 0.0303 0.997

15.0 0.0537 0.992

20.0 0.0680 0.997

Table 2.3 – Rate constant comparison determined by FTIR of varied shear rates during

the formation of TPUs under isothermal conditions at 10

54

Chapter 3 : Investigation of Rheological Behavior

During Onset of Thermoplastic Polyurethane Reactions


Journal article:

J. L. Gadley and J. M. Maia Submitted (Rheologica Acta)

55

3.1 Abstract

The reaction conditions during thermoplastic polyurethane (TPU) polymerizations

have a complex relationship with the structural buildup within the system. Particularly,

during the initial stages of the reaction the rheological response is difficult to connect with

the reaction behavior. During these stages a sharp decrease in storage modulus (G’) was

detected. In order to understand the implications of this behavior on the developing TPU

system rheo-FTIR studies were performed. The aim of this work was to understand the

implication of the detected transition in G’ through connecting rheological and

spectroscopic measurements under precisely the same flow conditions. In doing so it was

determined that this transition is related to the onset of phase separation within the system

during competition in diffusion between reactants. The combination of these techniques

determined the transition in G’ could prove useful as a marker in understanding the

complex interactions occurring in the initial stages of TPU reactions which ultimately

dictates the architecture formed in these systems.

56

3.2 Introduction

One of the major challenges when manufacturing TPUs is achieving the desired

material composition during polymerization. Typically, TPUs are produced via reactive

extrusion (REX) which provides continuous output of material that is then capable of being

molded into a final product through most forming processes. However, the REX process is

very sensitive to any variation in stoichiometry which is only controlled by inflow

conditions of multiple reactants into the extruder. Since polymerization drives the

rheological behavior of these systems, and these flow properties dictate the proper

production conditions, the relationship between the extrusion process and material

properties quickly become extremely complex. Understanding the rheo-kinetic behavior of

these systems during the complex conditions present with increasing viscosity with

increased conversion would be highly beneficial for industrial applications.

Generally, synthesis of TPUs consist of a three-part system containing a

diisocyanate, low Mn diol, and a telechelic diol of a much higher Mn that is commonly

referred to as a polyol. Introducing a mixture of vastly different sized molecules results in

a system where under certain conditions the short chain diol has a relatively high diffusivity

when compared to the polyol which is expected to diffuse more slowly due to its size. This

is a direct result of a competition between reaction rate and mass transfer of reactants to

the reaction interface as well as products away from the reaction interface. [22, 23]

Consequently, these short chain diols can under certain conditions react much more readily

with the diisocyanate. In doing so they create the rigid portion of the polymer backbone,

termed a hard segment (HS), while the reaction product of the polyol and the diisocyanate

represents a flexible soft segment (SS) within the multiblock copolymer chain that is

57

generated. A unique architecture is then attainable through microphase separation which

results in a system comprised of urethane rich hard domains and polyol rich soft domains

that form via enthalpically unfavorable processes due to chemical incompatibility between

the HS and SS in the system. [24, 25] TPUs thus attain their high level of versatility from

the formation of a phase separated system comprised of hard and soft domains. The final

structure and mechanical behavior of TPU systems depend on a multitude of factors

including the HS to SS ratio, the distribution of these segments, molecular weight,

molecular weight distribution, hydrogen bonding and the chemical compatibility between

hard and soft phase. [26-30] These complex structures and subsequent relationship to

mechanical properties are reliant upon the reaction environment and processing to which

these materials are exposed during their formation. More specifically the different

conditions and configurations under which the reactants of these systems are permitted to

interact have shown drastically different reaction behavior and even final properties. Most

notably the microaggregation structure which occurs during a TPU reaction has been

studied and shown to be highly dependent on the reaction environment and interactions

between reactants of differing diffusivity. [31]

Various studies have been performed to enhance understanding of these TPUs

systems using different measurement techniques, including Fourier transform infrared

spectroscopy (FTIR), back titrations, Raman spectroscopy, nuclear magnetic resonance

(NMR), rheology, and differential scanning calorimetry (DSC). [9, 10, 32-36] While in

their own respect, each technique provides insightful information into the progress of TPU

reactions, rheological measurements are of particular interest in this work due to the

relevance to processing of these materials. Specifically, for studying bulk systems,

58

rheology is capable of tracking buildup of mechanical properties throughout the reaction.

Additionally, FTIR operated under attenuated total reflectance (ATR) mode has been

verified to accurately detect reaction progress through monitoring consumption of free

isocyanate in bulk polyurethane (PU) reactions. [33, 37] Under bulk polymerization

conditions, TPUs are typically viewed as second order reactions but is not always the case

due to complex shear scenarios and variations in reaction temperature. [38, 39] While many

studies have shown a relationship between rheological properties and polymerization

conditions, much less attention has been directed at understanding rheology during

polymerization processes. [40] In this work, we take advantage of a unique opportunity to

combine FTIR-ATR measurements with rheology through performing in situ experiments.

Using this method on a three component system, the chemical changes occurring were

detected in real time while tracking the buildup of mechanical response.

During rheo-kinetic studies of polyurethane systems, an abrupt drop in elastic

modulus (G’) causing an early peak in loss tangent (tan δ) has been observed and attributed

to onset of gelation in a thermoset system. [41-44] Through in situ study of a linear TPU

system the combination of ATR and mechanical response provides the opportunity to better

understand the transitions noted in the early stages of the reaction. Gaining a more complete

picture of reaction development during bulk polymerization enables a more accurate

targeting of compositional goals for end use. Due to the complexity of TPU systems,

accurately controlling morphology and HS composition poses a constant challenge in

application. Understanding the reaction progression even in the very early stages ultimately

provides an enhanced toolkit for use in developing these systems.

59

In order to understand the origin of the early peak in tan δ multiple wt. % HS

containing TPUs were studied. Small amplitude oscillatory shear (SAOS) was used to

measure the progress of the reaction while simultaneously collecting FTIR spectra in ATR

mode. The combination of these two techniques enabled for direct comparison of two

measurement techniques instead of relying upon one signal to speculate the cause of the

rheological transition detected in G’ in the early stages of the reaction. Complimentary to

this work, wide angle x-ray scattering (WAXS) and small angle x-ray scattering (SAXS)

synchrotron radiation experiments were performed at intervals throughout the reaction

connect structural development occurring in the system with the mechanical and chemical

data collected. Through the combination of these techniques a unique view of changes

occurring rapidly during the onset of the TPU reaction was attainable.

3.3. Experimental

3.3.1 Materials

The system studied in this work was a three-part system representative of a TPU

system commonly used in industry for reactive extrusion applications. The reactions were

performed directly on the bottom plate of the rheometer under isothermal conditions at

elevated temperature. These TPUs were polymerized as an uncatalyzed system in bulk as

a one-pot reaction. This particular system consisted of a diisocyanate, a chain extender

(short chain diol) and a polyol (long chain diol). The diisocyanate used for this study was

4-4’ methylenediphenyl diisocyanate (MDI) (Sigma Aldrich). The polyol utilized in this

system was poly(butylene adipate) with an Mn of approximately 1000 g/mol. The chain

extender used was 1-4 butane diol (BDO) (Sigma Aldrich). The MDI was stored under

nitrogen at -20 °C until prior to use for the experiment. The polyol was stored in a sealed

60

container then placed in a vacuum oven at 80 °C for 8 hours prior to use. The BDO was

stored in a sealed container in a desiccated vessel until immediately prior to use.

The on-plate synthesis of TPUs was executed by targeting different wt.% HS

compositions through varying the ratio of polyol to BDO in the system while maintaining

a stoichiometric ratio of unity between the available hydroxyl groups and free isocyanates

in the system. TPUs containing HS contents of 100 wt. %, 55.6 wt. %, 36.8 wt. %, 26.8 wt.

%, and 19.6 wt. % were synthesized through individual isothermal reactions on the

rheometer. These targeted HS contents were calculated by determining the product of the

equivalent weight and number of equivalents of MDI and BDO then dividing this value by

the product of the summation of equivalents in the system and total equivalent weight of

the system. In order to achieve reaction on the plate, a well was added onto the bottom

plate of the rheometer to prevent flow of the reactants from the desired geometric area

between the plates of the rheometer. The rheometer plates were heated to the desired

experiment temperature of either 100 °C or 200 °C and permitted to equilibrate for 30

minutes prior to use. The polyol was added to the testing fixture in the appropriate mass

for the targeted composition and allowed to equilibrate for 5 minutes. A composition

specific volume of BDO was added to the molten polyol and briefly stirred. The MDI was

then added to this mixture and stirred manually for approximately 5 seconds. The reactants

were then subjected to rheo-IR measurements for 30 minutes.

3.3.2 Characterization

3.3.2.1 Rheology

SAOS experiments were performed using a HAAKE MARS III parallel plate

rheometer (Thermo Scientific). The in situ reactions were performed using a standard

61

stainless steel parallel plate with a diameter of 20 mm. Initially, the linear viscoelastic

regime (LVR), where G’ and G” may be measured independent of the applied stress was

determined by performing a stress sweep at a constant frequency of 6.28 rad/sec. Two

stress sweeps were performed, one on the polyol only to represent the lowest viscosity

during the initial stages of the reaction and one on the final product after a benchtop

reaction of the highest wt. % HS material. A common stress of 100 Pa was chosen from

both amplitude sweeps was to guarantee the material was within the LVR for the entirety

of the reaction. The reactions were performed under isothermal conditions for a time

interval of 30 min. An initial study was performed to determine the proper experimental

gap by using different gap heights to ensure slip was not occurring at the plate during the

experiment. As a result, all of the reactions occurred with a geometry gap setting of 1.0

mm on the rheometer.

3.3.2.2 FTIR

In situ FTIR spectra was collected continuously throughout collection of SAOS

data. The enabling technology that provided this capability was the Rheonaut module

(Thermo Scientific) which interfaced directly with the HAAKE MARS III rheometer used.

The Rheonaut module contained a single reflection diamond crystal ATR window in an

electrically heated plate. The spectra were collected using a mid-infrared region deuterated

triglycine sulfate (DTGS) detector with a detection range of 400 – 4,000 cm-1. Before each

individual experiment a background reference spectrum was collected and the HAAKE

RheoWin software was used to perform a background subtraction on all spectra collected.

After the reactants were all added to the rheometer plate, the initial “time zero” scan was

taken then the SAOS experiment began immediately. During the oscillatory shear

62

experiment the Rheonoaut was set to collect a spectrum at a resolution setting of 32 with

16 scans per collection which generated one FTIR spectrum approximately every four

seconds. All FTIR spectra was normalized using the peak located at 2950 cm-1.

3.3.2.3 X-ray

Synchrotron radiation experiments were performed at Argonne National

Laboratory (ANL) Advanced Photon Source (APS) beamline 12-ID-B with a focused beam

spot size of 300 μm X 20 μm and a wavelength of 0.8856 nm. Simultaneous, two

dimensional (2D) WAXS and SAXS experiments were also performed on the TPU samples

in order to probe the chain spacing development during the reaction and growth of hard

domains respectively between specific time steps during the reaction. Samples for these

experiments were obtained by performing reactions under identical conditions used on the

rheometer then quenching the reaction in a cooling bath using solid CO2 and acetone.

Immediately after quenching the reaction the samples were placed into an auto sampler

array between two pieces of solid film Kapton tape and the scattering experiments were

performed.

The WAXS detector used was a Pilatus 300K set at a sample to collector distance

of 435.16 mm. Concurrently, a Pilatus 2M SAXS detector was utilized with a sample to

collector distance of 1,916.70 mm. The WAXS setup was calibrated using an aluminum

oxide (Al2O3) standard and the SAXS was calibrated using a silver behenate (AgBe)

standard. All experiments consisted of a 0.1 sec exposure time for five pulses. Prior to

exposure of the samples a background was collected using a “blank” sample under the

same exposure conditions. An established program using Matlab at the 12-ID-B

workstation used each samples collected transmission to overlay and average the five

63

pulses as well as subtract the background scan. Integration was performed on the corrected

WAXS and SAXS data to obtain 1D profiles used to form cascade plots as a function of

reaction time.


In order to understand the early stages of TPU reactions, isothermal time sweeps

were performed to track the development of viscoelastic properties while concurrently

collecting FTIR data. The initial stages of the reaction were of particular interest to gain

insight into the earliest stages of the reaction and the potential implications on the final

products achieved from polymerization. The HS content was varied in the system in

compositions of 100, 55.6, 36.8, 26.4, and 19.6 wt. % to probe the effect of composition

on the initial kinetic behavior of the reaction. Throughout the range of compositions, FTIR

data was collected concurrently with rheological data for the three central compositions

(26.4 wt. % HS, 36.8 wt. % HS, and 55.6 wt. % HS) while the two extremes were

characterized via rheology to capture the overall trend in rheological data. The isothermal

time sweeps during the polymerization of these TPUs are shown in Figure 3.1. When

comparing the storage modulus (G’) development across the different compositions, a

distinct transition occurs marked by a rapid drop and recovery in G’. The transition time

was defined as the time which G’ drops below a stable value until the signal recovered to

measure the same value. The drop in G’ varies based on composition which ranges from

438-695 sec (19.6 wt. % HS), 161-258 sec (26.4 wt. % HS), 40-90 sec (36.8 wt. % HS),

18-77 sec (55.6 wt. %HS), and did not appear in the 100 wt.% sample prepared. While this

behavior is regularly detected in rheological curing studies the implications of this type of

behavior on linear systems such as TPUs require further investigation. [41-44] Initial

64

investigation of the plot in Figure 3.1 shows the existence of a trend in the time at which

the transition was detected depending on the ratio of HS to SS in the system. Furthermore,

the disappearance of the effect when synthesizing the 100 wt. % HS (only reacting MDI

and BDO) indicated that the transition in G’ is related to interactions between HS and SS

within the system. The primary focus of this study is centered on understanding the

implications of these interactions in the initial stages of the reaction to the overall

development of rheological properties and what specifically is leading to the drop in G’

measurement signal.

In order to elucidate the fundamental cause of the mechanical response detected,

FTIR data was collected during the rheological experiments to track the reaction progress.

During the measurements of the 26.4 wt. %, 36.8 wt. %, and 55.6 wt. % HS systems an

array of FTIR data was collected as shown in Figure 3.2. These plots enabled the tracking

of the consumption of reactants and subsequent bond formation to understand the chemical

changes taking place during the earliest stages of the TPU reaction. The primary peaks of

interest that were tracked for these materials are located at 1525 cm-1 (H-N-C=O stretch),

1720 cm-1 (C=O stretch), 2260 cm-1 (free isocyanate), and 3350 cm-1 (N-H stretch).

Further investigation of the aforementioned absorbance peaks indicated the

presence of a transition in reaction progress in the consumption of free isocyanate as well

as formation of the urethane linkage detectable using ATR. The plots shown in Figure 3.3

show the evolution of these peaks throughout the experimental window in which the

rheological transition was observed. In all three of the compositions shown a transition was

detected in the spectra within the same time during the reaction as was shown in rheology.

The transition in FTIR signal was determined by the length of time between different rates

65

of free isocyanate consumption shown by a brief plateau in the data. The correlation

detected between FTIR and rheological results are shown in Figure 3.3. This confirms that

the time dependence of the change was due to the composition of the sample, and that the

signal in G’ should not be dismissed as an experimental artifact. While the trend was

evident in all compositions, to facilitate the presentation of the data a focus on the

discussion the composition containing 36.8 wt. % HS was chosen, believing it to be

representative of the behavior observed. More specifically, this particular composition was

selected because the occurrence of the transition was sufficiently removed from equipment

startup to ensure sufficient resolution was available to capture behavior while still

occurring early enough in the course of the polymerization to mitigate potential negative

environmental effects. Considering the trends in Figure 3.3 showing the formation of

urethane bonds in conjunction with a difference in consumption behavior of the reactive

isocyanate groups, this transition was thus an induction period attributed to either a phase

separation effect or a difference in reactivity within the system.

In order to provide additional insight into the origin of the hesitation in isocyanate

consumption and in urethane linkage formation, it was noted that a significant shift in the

carbonyl stretching peak near 1720 cm-1 occurred during the transition time period from

27-90 sec. The plot in Figure 3.4 indicates a shift from a wavenumber measurement of

1720 cm-1 to 1708 cm-1. The observation of this peak shift was attributed to the presence

of hydrogen bonding in a disordered configuration synonymous hydrogen bonding due to

phase separations as noted in similar systems near 100°C and reported in previous studies.

[45] Thus, this shift in wavenumber most likely correlates to a period of rapidly increasing

molecular mass which would correspond with the relative increase in urethane linkages

66

measured in the FTIR. Connecting the peak shift with the changes in absorbance all

happening within the same time period support the likelihood of a difference in reactivity

between reactants in the system. This connection supports the hypothesis that the smaller,

higher diffusivity diol may be interacting more rapidly in the early stages of the reaction

than the larger polyol. During the progression of the reaction the existence of phase

separation and physical configuration within the system was investigated using X-ray

scattering to compliment the rheological and FTIR experiments.

Offline reactions were performed under isothermal conditions at the same

temperature as the rheometry experiments and quenched in a dry ice cooling bath at

different reaction times immediately before collecting scattering data. The WAXS plots

collected for the 36.8 wt. % HS composition are shown in Figure 3.5. The WAXS data

indicates the development of a peak at a q value of 1.37 Å between 30 sec and 60 sec into

the reaction. The formation of this peak correlates to a spacing of approximately 4.6 Å

which is a known packing distance present in MDI-BDO hard segmented polyurethanes.

[46] The reaction time of the appearance of the peak correlates well to both the rheological

and FTIR results shown previously. The formation and growth of this peak indicates that

during this time frame the HS of adjacent chains have begun stacking beside one another,

supporting the molecular mass increase suggested in Figure 3.5. This stacking between

hard segments also supports the onset of phase separation where these individual segments

are beginning to cluster together and form domains. The SAXS patterns in Figure 3.6 were

then considered in order to compliment these WAXS results. Analysis of the SAXS results

show a shift toward lower q values starting at a time between 30 sec and 60 sec which

agrees with all previous experimental techniques. In this case, the d-spacing shifts from a

67

constant initial value of approximately 12 nm to a value of 16 nm at the end of the

experimental window considered. This indicates that initially the system is phase separated

likely with the reaction occurring at the interface between components. Within the

transition timeframe the domains begin to grow into larger hard and soft domains as the

polymerization progresses and builds larger molecular weight. The onset of growth is

shown in Figure 3.7. Through comparison of the d-spacing it becomes clear that each

composition indicates growth of these domains at the same time the change in G’ was

originally detected. All three systems begin from a similar state and the overall d-spacing

value reached throughout the reaction increases with the amount of HS present within the

system as expected.

Overall, through comparison of all techniques a consensus can be reached, one that

shows that the original rheological signal immediately after the transition in G’ corresponds

to a rapid increase in molecular weight and a subsequent growth in domain size of these

systems through phase separation. The drop in storage modulus is attributed to the fact that

the polymerization is exothermic in nature coupled with the presence of two diols which

are vastly different in size, diffusivity and hence reactivity under the reaction conditions

studied. During early reaction times, the BDO and MDI react preferentially due to the

higher diffusivity of the BDO to the interface at which the reaction occurs. The reaction

between these components result in heat generation within the system at a greater rate than

the buildup of molecular weight can offset, thus resulting in a reduction in G’. The localized

heat generation promotes diffusion of the larger polyol in conjunction with a lower

availability of BDO due to consumption. Upon reaching this critical point, the

polymerization accelerated due to the temperature increase and the molecular weight

68

begins to build more rapidly. The storage modulus then increases for the remainder of the

experiment as the molecular weight continues to build and increase the viscosity of the

system as would be expected.

Understanding the development of TPU systems and the complicated relationship

between rheological conditions, reaction behavior, and the effect of these factors on

property development are very important issues for industrial applications within this

material class. It is well understood that the initial reaction behavior within a specified

composition can affect the ultimate properties of the material. For example, it is known

that by maintaining a constant composition and creating a prepolymer as opposed to a

random configuration, or simply reacting at different temperatures, the properties of the

final material may behave significantly different. [31, 47-51] With this in mind, using

techniques such as rheo-FTIR studies to find markers for these behaviors such as in this

work, can assist in unraveling these complex behaviors. It can be envisaged that simply

changing temperature profiles in the initial zones of an extruder barrel while maintaining a

constant composition provides yet another tool to access the versatility TPUs offer for a

multitude of uses and aid in the efficiency of accomplishing these goals.

3.5 Conclusions

The study completed within this work was focused on determining the validity and

subsequent contributing factors to the transition detected in storage modulus during the

early stages of thermoplastic polyurethane reactions. The transition noted was marked by

sharp drop and recovery of the storage modulus signal. Further investigation showed that

the signal is tied to a transition in reactive behavior within the melt due to differences in

reactivity between the low molecular weight diols and the telechelic polyols present during

69

the reaction. This was concluded as the transition was strongly dependent upon the amount

of hard segment in the system. It is believed the transition in storage modulus results from

the exothermic nature of the reaction, i.e. the addition of heat to the system, and the

competition between heat generated within the system and the viscosity build up

characteristics of the system. This behavior was attributed to a differences in diffusivity of

reactants and the resultant molecular weight increase as the reaction progressed. Initially,

the more mobile species (BDO and MDI) react, increasing the heat present within the

system until the reaction with the polyol begins rapidly, thus increasing the molecular

weight of the system in a manner that outpaces the effect of heat on viscosity of the system.

FTIR data indicated a shift in the chemical composition that occurred within the same

timeframe as the rheological signal was detected, and was consistent with the rheological

findings. WAXS and SAXS were used to supplement this evidence by demonstrating that

chain packing as well as hard domain growth both rapidly increase during the time of this

transition. Understanding the importance of the early stages of reactions in TPU systems

is paramount when targeting specific behavior due to their high level of complexity.

Benchmarking a signal in rheology or through combined techniques such as rheo-FTIR

provides a quick and simple indication of factors which will influence the entire

polymerization in TPUs. Using these sorts of tools provides an additional approach to

industrial applications for efficiently producing TPUs for a multitude of uses.

References




70




[22] R. G. Pearson and E. L. Williams, "Interfacial Polymerization of an Isocyanate

and a Diol," Journal of Polymer Science Part a-Polymer Chemistry, vol. 23, pp.

9-18, 1985.

[23] R. G. Pearson and E. L. Williams, "A Comparison of Heterogeneous and

Homogeneous Kinetics in the Formation of a Polyurethane," Journal of Polymer

Science Part a-Polymer Chemistry, vol. 25, pp. 565-573, Feb 1987.

[24] R. W. Seymour, G. M. Estes, and S. L. Cooper, "Infrared Studies of Segmented

Polyurethan Elastomers .1. Hydrogen Bonding," Macromolecules, vol. 3, pp. 579-

&, 1970.

[25] G. M. Estes, S. L. Cooper, and A. V. Tobolsky, "Block Polymers and Related

Heterophase Elastomers," Journal of Macromolecular Science-Reviews in

Macromolecular Chemistry, vol. C 4, pp. 313-&, 1970.

[26] H. D. Kim, J. H. Huh, E. Y. Kim, and C. C. Park, "Comparison of properties of

thermoplastic polyurethane elastomers with two different soft segments," Journal

of Applied Polymer Science, vol. 69, pp. 1349-1355, Aug 15 1998.

[27] S. Velankar and S. L. Cooper, "Microphase separation and rheological properties

of polyurethane melts. 2. Effect of block incompatibility on the microstructure,"

Macromolecules, vol. 33, pp. 382-394, Jan 25 2000.


of polyurethane melts. 3. Effect of block incompatibility on the viscoelastic

properties," Macromolecules, vol. 33, pp. 395-403, Jan 25 2000.

[29] A. Kintanar, L. W. Jelinski, I. Gancarz, and J. T. Koberstein, "Complex

Molecular Motions in Bulk Hard-Segment Polyurethanes - a Deuterium Nmr-

Study," Macromolecules, vol. 19, pp. 1876-1884, Jul 1986.

[30] L. M. Leung and J. T. Koberstein, "Dsc Annealing Study of Microphase

Separation and Multiple Endothermic Behavior in Polyether-Based Polyurethane

Block Copolymers," Macromolecules, vol. 19, pp. 706-713, Mar 1986.

[31] S. Yamasaki, D. Nishiguchi, K. Kojio, and M. Furukawa, "Effects of

polymerization method on structure and properties of thermoplastic

polyurethanes," Journal of Polymer Science Part B-Polymer Physics, vol. 45, pp.

800-814, Apr 1 2007.

[32] C. Dubois, S. Desilets, A. Aitkadi, and P. Tanguy, "Bulk-Polymerization of

Hydroxyl Terminated Polybutadiene (Htpb) with Tolylene Diisocyanate (Tdi) - a

Kinetics Study Using C-13-Nmr Spectroscopy," Journal of Applied Polymer

Science, vol. 58, pp. 827-834, Oct 24 1995.

[33] D. Kincal and S. Ozkar, "Kinetic study of the reaction between hydroxyl-

terminated polybutadiene and isophorone diisocyanate in bulk by quantitative

FTIR spectroscopy," Journal of Applied Polymer Science, vol. 66, pp. 1979-1983,

Dec 5 1997.

[34] H. Kothandaraman and A. S. Nasar, "The Kinetics of the Polymerization Reaction

of Toluene Diisocyanate with Polyether Polyols," Journal of Macromolecular

Science-Pure and Applied Chemistry, vol. A31, pp. 339-350, 1994.

71

[35] S. Parnell, K. Min, and M. Cakmak, "Kinetic studies of polyurethane

polymerization with Raman spectroscopy," Polymer, vol. 44, pp. 5137-5144, Aug

2003.


segmented, high molecular weight, aliphatic poly(ether-urea) copolymers in


5829-5836, Aug 5 2004.



Oct 2009.

[38] E. B. Richter and C. W. Macosko, "Kinetics of Fast (Rim) Urethane

Polymerization," Polymer Engineering and Science, vol. 18, pp. 1012-1018,

1978.

[39] G. Anzuino, A. Pirro, O. Rossi, and L. P. Friz, "Reaction of Diisocyanates with

Alcohols .1. Uncatalyzed Reactions," Journal of Polymer Science Part a-Polymer

Chemistry, vol. 13, pp. 1657-1666, 1975.

[40] M. Cioffi, K. J. Ganzeveld, A. C. Hoffmann, and L. P. B. M. Janssen,

"Rheokinetics of linear polymerization. A literature review," Polymer

Engineering and Science, vol. 42, pp. 2383-2392, Dec 2002.

[41] E. Papadopoulos, M. Ginic-Markovic, and S. Clarke, "Reaction Kinetics of

Polyurethane Formation Using a Commercial Oligomeric Diisocyanate Resin

Studied by Calorimetric and Rheological Methods," Macromolecular Chemistry

and Physics, vol. 209, pp. 2302-2311, Nov 24 2008.

[42] J. S. Martin, J. M. Laza, M. L. Morras, M. Rodriguez, and L. M. Leon, "Study of

the curing process of a vinyl ester resin by means of TSR and DMTA," Polymer,

vol. 41, pp. 4203-4211, May 2000.

[43] A. Y. Malkin and S. G. Kulichikhin, "Rheokinetics of Curing," Advances in

Polymer Science, vol. 101, pp. 217-257, 1991.

[44] J. M. Laza, J. L. Vilas, F. Mijangos, M. Rodriguez, and L. M. Leon, "Analysis of

the crosslinking process of epoxy-phenolic mixtures by thermal scanning

rheometry," Journal of Applied Polymer Science, vol. 98, pp. 818-824, Oct 15

2005.

[45] M. M. Coleman, K. H. Lee, D. J. Skrovanek, and P. C. Painter, "Hydrogen-

Bonding in Polymers .4. Infrared Temperature Studies of a Simple Polyurethane,"

Macromolecules, vol. 19, pp. 2149-2157, Aug 1986.

[46] J. Blackwell and K. H. Gardner, "Structure of the Hard Segments in Polyurethane

Elastomers," Polymer, vol. 20, pp. 13-17, 1979.

[47] M. Furukawa, Y. Mitsui, T. Fukumaru, and K. Kojio, "Microphase-separated

structure and mechanical properties of novel polyurethane elastomers prepared

with ether based diisocyanate," Polymer, vol. 46, pp. 10817-10822, Nov 21 2005.

[48] K. Kojio, M. Furukawa, Y. Nonaka, and S. Nakamura, "Control of Mechanical

Properties of Thermoplastic Polyurethane Elastomers by Restriction of

Crystallization of Soft Segment," Materials, vol. 3, pp. 5097-5110, Dec 2010.

[49] M. S. Sanchez-Adsuar, E. Papon, and J. J. Villenave, "Influence of the synthesis

conditions on the properties of thermoplastic polyurethane elastomers," Journal of

Applied Polymer Science, vol. 76, pp. 1590-1595, Jun 6 2000.

72

[50] M. S. Sanchez-Adsuar, E. Papon, and J. J. Villenave, "Influence of the

prepolymerization on the properties of thermoplastic polyurethane elastomers.

Part II. Relationship between the prepolymer and polyurethane properties,"

Journal of Applied Polymer Science, vol. 76, pp. 1602-1607, Jun 6 2000.



Part 1. Prepolymer characterization," Journal of Applied Polymer Science, vol.

76, pp. 1596-1601, Jun 6 2000.

73

3.6 Figures

Figure 3.1 - G' and G" plots during isothermal time study at 100 C of varying HS

content.

74

Figure 3.2 - FTIR waterfall plot demonstrating the data collected concurrently with

rheological measurements.

75

Figure 3.3 - Comparison of absorption bands throughout isothermal time study of 36.8

wt. % HS TPU.

76

Figure 3.4 - Shift in wavenumber of carbonyl stretching peak throughout isothermal time

study.

77

Figure 3.5 - 1D WAXS plot at progressive time steps throughout the TPU reaction.

78

Figure 3.6 Plot of SAXS data for the 36.8 wt.% HS sample with progressing reaction

time.

79

Figure 3.7 - Plot of increasing d-spacing with reaction time as a comparison between

multiple HS compositions.

80

3.7 Tables

Composition (wt. % HS) G’ Transition Time (sec) FTIR Transition Time

(sec)

55.6 18-77 10-54

36.8 40-90 27-81

26.4 161-258 144-252

Table 3.1 - Comparison of transition time detected using FTIR and rheological

measurements.

81

Chapter 4 : Rheo-kinetic Modeling of TPU Using In

Situ FTIR Analysis


Journal article:

J. L. Gadley, A. Boromand and J. M. Maia To Be Submitted (ACS Macro Letter)

82

4.1 Abstract

Bulk polymerization of thermoplastic polyurethanes (TPUs) are widely known to exhibit a

complex relationship between reaction behavior, viscosity of the system, and flow conditions

during polymerization. The intent of this work was to determine the influence of both shear rate

and composition during polymerization on a simple TPU system.

Using rheometry coupled with in situ FTIR measurements the relationship between

viscosity and conversion was determined. Under steady shear conditions, the total strain applied

during the reaction was identified as the most influential parameter affecting viscosity growth.

This observation also determined that a critical strain existed where the reaction transitioned from

a diffusion controlled process to a mixing dominated process. Using a simple model, the viscosity

and corresponding conversion can be predicted. When considering three different weight percent

hard segment (HS) compositions (26.4 wt. %, 36.8 wt. %, and 55.6 wt. %), it was observed that

the reaction is dependent upon the total isocyanate consumed during polymerization regardless of

the composition.

Through tracking viscosity of TPUs systems such as these, the conversion under known

conditions could be determined through a single simple experiment. Utilizing these sorts of

relationships are very promising to efficiently controlling polymerizations in processes such as

reactive extrusion.

83

4.2 Introduction

Thermoplastic polyurethanes (TPUs) are a subset of polyurethanes (PUs) that are heavily

used in performance applications. These materials are of particular interest because they are able

to take advantage of many traditional PU properties while remaining melt processable. TPUs are

most commonly produced through bulk reactions in reactive extrusion (REX) or via batch reactors

in the case of slowly reacting systems. For obvious reasons, the prospect of continuously reacting

and producing TPUs via extrusion is very attractive for industrial production of these materials for

use in other traditional forming methods. However, controlling the REX process has proven

difficult due to many factors at play during the reaction under the complex flow conditions within

the equipment. Because of the high level of unpredictability present in this process, the reaction

kinetics of these systems has been an area of significant research throughout the years.

Due to the inherent complexities of polymerizing TPUs in industrial processes, many

different approaches have been taken to elucidate the specific behavior contributing to

inconsistencies in reactive processing.[33, 37, 52-54] These types of polymerizations that take

place concurrently while being processed ultimately lead to connections between processing

conditions and polymerization behavior which cannot be separated. For example, most TPU

monomers are immiscible and the rate of reaction observed is strongly dependent upon the flow

conditions present. In this case, increasing total interfacial area due to mixing would be expected

to affect the reaction behavior. The ability of monomer to diffuse to these interfaces would also

play an important role in the kinetics of the system. However, as the reaction progresses the

viscosity of the mixture inevitably increases affecting flow, mixing and diffusion of the reactants.

[9] Experimental efforts to understand this kinetic behavior have been taken using nuclear

magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), adiabatic

84

temperature rise (ATR), and ramen spectroscopy. [11, 12, 22, 32, 33, 37] While each of these

studies provide a unique view of TPU behavior and how system stoichiometry, temperatures, and

catalysts affect the behavior, they generally do not focus on how shear flows during polymerization

affect the rate of the reaction. More pertinent studies have been performed using rheometry and

batch mixers to track the effect of shear on reactive systems. [1, 9, 38, 54-57] These sorts of studies

encompass both linear TPU formation as well as thermoset reactions in PUs but have shown that

flow conditions during bulk polymerization drastically change the kinetics observed. Throughout

these works it is generally accepted that shear rate had little effect on the reactions especially at

low viscosity and shear rate ranges. However, other studies have shown that increasing shear rate

increases the time at which viscosity buildup behavior changes during the reaction. [9, 58] These

studies show promise in unraveling the connection between the polymerization progress and the

viscosity buildup occurring in these complex systems. Further study of this behavior has noted that

while shear is affecting the onset point where rapid viscosity buildup occurs, after reaching

approximately 50% conversion, the reaction kinetics appear to deviate from the accepted second

order behavior. [1] At this point, reaction rate has been shown to increase in some cases while

decreasing in others.[54] The increase has been theorized to result from an autocatalysis of the

urethane bond while deceleration was attributed to diffusion limitations.[1, 59] Specifically, when

studying whether the diffusion limitation caused the reaction change, the deceleration was only

observed for very short diols.[59] However, this did suggest that a difference in reactivity during

reaction progression may be playing a significant role in viscosity buildup behavior. These studies

all tend to have difficulty definitively detecting the causes of the observed behavior because of the

complexity of these systems. Therefore, finding a potential way to ‘bridge the gap’ between

85

processing and easily performed experiments would provide tremendous benefit to identifying

inconsistencies in industrial bulk processes.

The intent of this work is to combine spectroscopy and rheometry in a simultaneous test to

help connect changes in viscosity with real time conversion measurements. The methodology for

such an effort were developed in Chapter 2 and indicate that tracking the buildup of mechanical

behavior and consumption of the reactants simultaneously can be reliably achieved. The scope of

this investigation focuses on whether or not this methodology can be used to create a viscosity

model that allows for prediction of conversion or viscosity behavior from a relatively simple

experiment. This approach provides the unique advantage of measuring reaction behavior under

known shear conditions allowing a direct comparison to be made. This work focused on using

three different TPU compositions ranging from 26.4 wt. % HS to 55.6 wt.% HS. These

compositions were exposed to different temperatures and different shear rates to understand the

connection between viscosity buildup behavior and monomer ratio as well as understand how shear

rates may affect a specific composition of reaction.

4.3 Experimental

In order to generate the data for this study to capture the relationship between bulk reaction

kinetics and the resultant flow behavior, TPUs were synthesized on the rheometer plate according

to the technique developed in Chapter 2. Using this methodology, TPUs containing 26.4 wt.%,

36.8 wt. %, and 55.6 wt.% were synthesized under oscillatory and steady shear flow conditions.

The polymerizations were performed at two different temperatures, 100 °C and 200 °C. The

samples were subjected to oscillation and measured at both temperatures to determine the influence

of hard segment composition on the reaction’s behavior and subsequent buildup of viscosity. To

understand the effect of shear on the samples, a range of shear rates were run (0.1, 1.0, 5.0, 10.0,

86

15.0, and 20.0 sec-1) on the 36.8 wt. % composition at 100 °C. This particular temperature and

composition were chosen to ensure the reaction rate occurred fast enough to mitigate

environmental influence while occurring slowly enough to capture data within the experimental

resolution of the rheometer and FTIR equipment. When comparing compositions, the oscillatory

experiments were performed at 100 °C to create a zero shear baseline for the varied shear rate

experiments. The oscillatory experiments at 200 °C were performed to ensure all compositions

were reacted to high conversions. This was of particular interest for this study because we wanted

to investigate whether or not a change in viscosity buildup behavior was observed after a certain

conversion as previously reported. It was hypothesized that observing this behavior with both

rheological and spectroscopic techniques simultaneously could provide a unique understanding as

to what the root cause of the change in viscosity buildup.


4.4.1 Constant Composition Under Steady Shear

In an effort to move toward modeling these complicated systems in such a way where

conversion data alone could predict viscosity or vice versa, polymerization of a single composition

under steady shear was initially considered. The steady shear viscosity data collected as a function

of time across different shear rates are depicted in Figure 4.1. The data in Figure 4.1 indicates that

the different shear rates measure similar initial values as would be expected. The sample viscosity

across all shear rates then appears to increase at similar reaction times. However, the final viscosity

varies depending upon the shear rate applied to the system. In fact, increasing shear rate generally

caused an increase in the maximum viscosity value reached during polymerization. Although it

would be expected that these materials exhibit shear-thinning behavior, the increasing viscosity

with increasing shear rate indicates the flow condition was altering the reaction interface in the

87

melt enough to alter the reaction rate. The FTIR data collected during these experiments were used

to further investigate the relationship between shear rate and the viscosity. The conversion of free

isocyanate was determined by tracking the absorbance peak at 2260 cm-1 and plotted against the

reaction time as shown in Figure 4.2. This plot shows independently that as the shear rate of the

system was varied, the conversion rate and extent of conversion was shifted to higher values as

the applied shear rate increased. This indicates a change related to the flow behavior of the system.

For example, increasing shear rate can be causing increased interfacial area for the reaction. The

higher total conversion reached at higher shear rates was responsible for the increased in final

viscosity reached during the rheological measurement. The relationship shown in Figure 4.2 was

interpreted as a shift in reaction rate due to increased interfacial area available to reactants due to

the flow conditions applied to the system. In order to better understand the influence of flow on

the reactive system, the conversion and viscosity information were plotted as a function of total

strain in the system.

Considering the effect of shear rate on the FTIR measurements, the same values were

plotted as a function of total strain applied as shown in Figure 4.3. Considering the viscosity versus

total strain applied, a shift in strain at which the viscosity increases was observed as well as a

change in slope of viscosity buildup. After reaching a shear rate of 10 sec-1, further increases in

shear rate appeared to follow very similar behavior between shear rates. This effect was attributed

to a shift from a mostly diffusion controlled reaction environment to a flow dominated

environment. Also shown in the Figure 4.3 inlay, was the change in viscosity as a function of the

conversion during the reaction. The data indicated that at a critical conversion, the viscosity values

increase rapidly. The conversion reached was dependent on the shear rate applied to the system.

88

This behavior further supports the FTIR data indicating that applying shear to these reactions even

across a small range of shear rates has a significant effect on the polymerization behavior.

Further investigation of this behavior is depicted in Figure 4.4. However, it should be noted

that the viscosity measured for 15 sec-1 and 20 sec-1 increased and fluctuated erratically. This was

attributed to exceeding the sampling rate of FTIR in this case or due to the flow rate occurring

under the shear flow conditions. Because of this behavior, the maximum shear rate under these

experimental conditions was determined to be 10 sec-1. This curve indicates that consumption of

isocyanate in the system was related to the shear rate applied to the melt. This effect is likely due

to an increase in interfacial area between the reactants. Considering the dependence of the reaction

behavior on strain across the relatively small shear rate range measured suggests that a shift in

reaction behavior from diffusion dominated to flow dominated was observable. At low shear rates,

little strain is applied to the system throughout the reaction resulting in a mostly diffusion

controlled process because minimal distortion of the reaction interface is occurring. After reaching

a sufficiently high shear rate, in this case 10 sec-1, enough strain has been applied to the reaction

to transition to a regime where the reaction is drastically affected by the flow conditions present.

At a strain value of approximately 60, the isocyanate was consumed much more rapidly as the area

of the reaction interface increased due to flow which reduced the dependence on diffusion for the

reaction to occur. To better develop the connection between viscosity and conversion a model was

developed to help describe the relationship between shear rate and the behavior of these reactions.

The system was conceptualized to act as spherical particles for simplicity and treated as a growth

of these particles. This analogy can be thought of as behavior similar to that observed during

growth of crystals for example. Under this assumption, the relationship defined by Einstein was

89

used as shown in Equation 4.1 where it is known that in a particle system composed of hard

spheres, C1=2.5.

𝜂 = 𝜂0(1 + 𝐶1𝜑)

Equation 4.1

Considering a reacting system as is present in this case, more complex particle interactions

must be considered. Particularly, as these particles move very close together, as would happen

when approximating a reaction through this method. To capture this behavior the series is

expanded as shown in Equation 4.2.

𝜂 = 𝜂0(1 + 𝐶1𝜑 + 𝐶2𝜑2 + 𝐶3𝜑3 + ⋯ )

Equation 4.2

Given this equation it can be stated that:

𝜂 ∝ (∆𝜑)−1

Equation 4.3

Which can be written as:

𝜂 ∝1

𝜑𝑚𝑎𝑥 − 𝜑

Equation 4.4

The relationship can then be rewritten as:

𝜂 ∝ 1

𝜑𝑚𝑎𝑥(1 −𝜑

𝜑𝑚𝑎𝑥)

Equation 4.5

90

Using this relationship, the equation can be adapted with parameters for fitting to allow

for modeling the behavior of this system as shown in Equation 4.6.

𝜂 =1

𝜑𝑚𝑎𝑥⁄

(1 −𝜑

𝜑𝑚𝑎𝑥)𝐶

Equation 4.6

Finally, the form shown in Equation 4.7 can be written to allow for fitting across the

different viscosity changes even when approaching a very high conversion of a completed reaction.

𝜂 =1

(1 −𝛼𝐵)

𝐶

Equation 4.7

Using the combination of rheological measurements and FTIR data collected the

experimental data was fit using Equation 4.7 where η is viscosity, α represents conversion, B is a

fitting parameter containing αm, where αm is maximum conversion (ideally 1), and parameter C is

a fitting parameter. Ultimately, the ability to predict conversion or viscosity behavior based on one

measurement technique would provide a useful tool to predict viscosity buildup behavior for

processing applications. The resultant experimental data fit and corresponding fits are shown in

Figure 4.5. Due to the inconsistencies previously noted at higher shear rates with the FTIR data,

the maximum shear rate examined here was 10 sec-1. The fitting method used shows good

agreement with the experimental data indicating that this method provides a direct connection

between the viscosity and conversion values obtained experimentally. For these fits, B results in a

value of 4.239 (0.1 sec-1), 1.063 (1.0 sec-1), 1.017 (5.0 sec-1), and 1.001 (10 sec-1). Since these

91

values all approach a value of 1 within error, the model is accurately trending toward an ultimate

conversion value of unity as expected. The deviation from this behavior for the 0.1 sec-1 sample

was attributed to a difference in interaction between reaction sites under mostly diffusion

controlled conditions. The remaining fitting parameters are shown in

Shear Rate 0.1 sec-1 1.0 sec

-1 5.0 sec-1 10.0 sec

-1 B 4.239 1.063 1.017 1.001

C 24.99 3.155 2.354 1.519

R-Square 0.986 0.983 0.998 0.985

Table 4.1. Considering the trend shown in the parameters, particularly the exponent, c, the

equation shows a sharpening of the transition as shear rate was increased. Further investigation of

the viscosity versus conversion plot shows the influence of changing shear rate on a specific

composition of TPU. Increasing the shear rate causes a shift from a more gradual to a rapid increase

in viscosity at higher shear rates. This information could be very useful for tuning bulk reactions

under flow. Changes in viscosity and conversion behavior were apparent even when changing

shear rates across a small range. These results indicate that an increased shear rate provides a more

efficient route to reaching higher conversion. Using the relationship demonstrated here to predict

conversion knowing only the viscosity under specific flow conditions for example, provides a

powerful technique to control the relationship between a shear flow based process and the viscosity

of the system.

4.4.2 Varied Hard Segment Composition Under Oscillatory Shear

After determining that shear rate affects viscosity buildup and conversion rate within a

specific composition of bulk polymerized of TPU, oscillatory shear experiments were performed

to understand the influence of composition on polymerization behavior. In order to accomplish

92

this, TPUs were synthesized on the rheometer plate consisting of three different HS compositions,

26.4 wt. %, 36.8 wt. %, and 55.6 wt. %. These polymerizations progressed under SAOS, enabling

the tracking of viscosity growth while simultaneously monitoring chemical changes via FTIR.

First, the polymerization was tracked under isothermal conditions at 100 °C. The typical results of

such an experiment are shown in Figure 4.6 as a plot of complex viscosity versus time. While these

TPUs would typically be exposed to processing temperatures higher than 100 °C, these conditions

were chosen initially to delay reaction onset prior to measurement.

Considering Figure 4.6, increasing HS content affects the rate of viscosity buildup and

overall magnitude of viscosity as was noted in Chapter 1. Maintaining the goal of connecting the

polymerization viscosity with the conversion of the system, these measured variables were plotted

against each other as shown in Figure 4.7. These results indicated that at low conversion all three

compositions were behaving similarly. After reaching a conversion of approximately 50%, the

viscosity buildup behavior changes between the different HS content materials. At conversions

above this value, the relationship between viscosity and conversion develops differently based on

composition. The highest HS content system (55.6 wt. %) also indicated transition to a slower

increase in viscosity with conversion which was initially attributed to diffusion limitation of the

reaction at high viscosity values. The conversion values where then normalized by the initial

concentration of isocyanate in the system in order to directly compare the amount of isocyanate

consumed as shown in Figure 4.8. These results show that all three compositions develop viscosity

in the same manner under a conversion of approximately 50%. Above this value, all three

compositions rapidly increase in viscosity and form a master curve. This suggests that the reactions

under identical flow conditions and varied composition progress based on the total isocyanate

93

consumption up to any point in the reaction. The transition noted in the highest HS content to an

inhibited condition was an interesting result that required further study to identify potential causes.

In order to provide a more complete understanding of the influence of composition on

viscosity buildup in TPUs, the same experiments were performed under isothermal conditions at

200 °C to ensure the reaction remained in the melt state throughout the experiment. The viscosity

versus time data is depicted in Figure 4.9. Polymerization under the elevated temperature, more

representative of extrusion conditions and a significant difference in the data is immediately

apparent. In this case the complex viscosity rapidly increased then reached a stable value while

remaining in the melt state for the entirety of the experiment.

Next, the normalized complex viscosity was plotted against the conversion of isocyanate

as was detected simultaneously using FTIR. The data plotted in Figure 4.10 represents the

relationship between normalized viscosity and conversion as a function of different HS content

systems. As the HS content increases, the required conversion of isocyanate to reach a specific

viscosity is reduced. The data also indicated a transition was present during the polymerization

where viscosity was rapidly increasing with conversion then suddenly transitioned to a much lower

rate. Changes in viscosity behavior such as this has been previously reported and has been

attributed to difference in relativities between components, diffusion limitations, or the onset of

phase separation in the system. [9, 52, 53, 60, 61] Further investigation suggests the transition is

related to the magnitude of viscosity reached during the reaction. After the viscosity reaches a

critical value, the rate of viscosity increase is severely inhibited. To identify the cause of this

behavior, the complex viscosity was plotted versus the conversion normalized by the initial

concentration of isocyanate added to the system and is plotted in Figure 4.11. This normalization

allows for comparison of the viscosity of different HS compositions as a function of total

94

isocyanate consumption. Through treating the data in this manner it becomes apparent that the

viscosity differences measured were a result of a difference in reactant concentration. Since the

viscosities collapse on a master curve when accounting for the starting concentration of the

reaction, the buildup of viscosity in the system is governed by the total isocyanate consumed

throughout the reaction. This plot also suggests that at a critical normalized viscosity when of

approximately 1,000 Pa∙s marks the transition in viscosity buildup behavior However, the

transition also occurs after consuming the same total amount of isocyanate during the reaction.

This behavior was not attributed to degradation because of the viscosity measured over this time

remained stable, supporting significant degradation had not occurred. While the results clearly

indicate a transition in viscosity behavior after a specific amount of isocyanate has been consumed,

further study will be required to determine the root cause of this transition.

4.4.3 Comparison of Model Between Steady Shear and Oscillatory Shear

The normalized viscosity of the 26.4 wt. %, 36.8 wt. %, and 55.6 wt. % HS systems were

fit using the same model developed for variation in shear rate of a constant composition material

to determine if this model could be useful when varying HS composition as well. The resultant

fitting parameters are shown in Table 4.2. These parameters indicate that again, the B value

remains near one as was the case under steady shear. The value of parameter C increased as the

wt. % HS in the system was increased.

In order to further evaluate the model, the model curve of 36.8 wt.% HS under oscillation

at 100 °C was overlaid on the on the viscosity versus conversion plot of the steady shear data as

shown in Figure 4.12. The solid line represents the model of the same composition at the same

temperature under oscillation at 6.28 rad/s (1 Hz). It is interesting to note that the oscillatory curve

falls near the lower shear rate as was expected because very low strain was expected during the

95

oscillation experiment. More specifically, this curve appears to match very closely to the steady

shear experiment at 1 sec-1. While it was not expected that Cox-Merz apply to this system, the 1

Hz and 1 sec-1 data match very similarly. To further probe this relationship, further experiments

across multiple frequencies should be performed to determine the extent to frequency and shear

behavior match. If oscillatory data shows similar trends as a function of frequency as steady shear

does to shear rate, further exploring this methodology could widen the predictive scope of this

technique.

4.5 Conclusions

TPUs are complicated materials which exhibit a complex relationship between viscosity

and reaction behavior, particularly during bulk polymerizations. In this work the influence of both

shear rate and composition on bulk polymerization were studied to determine the connection

between changes in melt viscosity and the progress of the reaction itself. The results indicated that

while maintaining a constant composition and reaction temperature, the conversion behavior was

sensitive to the shear rate applied. As the shear rate was increased, the system increased viscosity

more rapidly. While the viscosity was sensitive to shear rate throughout the reaction due to shear-

thinning, it was determined that the FTIR data collected provided a master curve when plotted

against the total strain applied to the system. This dependence demonstrated that when under flow,

the conversion of isocyanate to urethane bonds was directly related to the strain in the system.

Through this relationship the viscosity versus conversion data collected could be fit with a simple

model allowing the prediction of viscosity from conversion or vice versa. This relationship also

indicated a critical strain below which the reaction progresses under diffusion controlled behavior.

After sufficient strain was applied the viscosity then increased rapidly as it moved toward a

scenario where mixing of the reaction interfaces dominated the reaction behavior.

96

The concept applied to steady shear experiments was then expanded upon and applied to

oscillatory experiments using 26.4 wt. % HS, 36.8 wt. % HS, and 55.6 wt.% HS compositions

under isothermal conditions. First the polymerization was compared at 100 °C and the viscosity

behavior was documented. In order to provide a more comprehensive understanding of the

relationship between reaction progress and viscosity of the system, the different compositions were

compared under isothermal conditions at 200 °C. Using the increased temperature allowed

complete or nearly complete conversion to be reached. These results determined that varying

composition indicated the presence of a critical conversion where viscosity begins increasing

rapidly. Increasing the HS content of the system shifted this increase to earlier overall conversion

during the reaction. The results also detected a transition in reaction behavior after reaching a

critical viscosity. Once normalized by the initial concentration of isocyanate present in the system,

a master curve was developed based on composition. The ability to create a master curve through

this normalization determined that the most important factor in buildup of viscosity, regardless of

composition, was the total amount of isocyanate consumed up to any specific point in the reaction.

Finally, the model developed in this work shows good agreement between steady shear

experiments and SAOS experiments. This provides a promising base as a predictive technique

across different compositions and shear rates that is relevant to these bulk polymerizations in

continuous processes.

While these polymerizations on a rheometer are oversimplified from a production

standpoint due to the relatively low shear rates applied, this information could be quite useful for

understanding critical factors which should be carefully controlled in the process. The relationship

developed suggests adequate mixing in the system and control of total reactants consumed are of

the utmost importance. This information was only achievable due to the unique combination of

97

rheometry coupled with in situ FTIR. Through tracking the initial concentration of a process and

approximate shear conditions the relationship determined here provides an avenue to tune a

process to ensure the reaction is completed efficiently and to the desired specification. These

relationships also provide potential for future use in in-line or on-line measurement techniques for

process control applications.

4.6 References

[1] V. W. A. Verhoeven, A. D. Padsalgikar, K. J. Ganzeveld, and L. P. B. M. Janssen, "A

Kinetic Investigation of Polyurethane Polymerization for Reactive Extrusion Purposes,"

Journal of Applied Polymer Science, vol. 101, pp. 370-382, 2006.

[9] A. H. Navarchian, F. Picchioni, and L. P. B. M. Janssen, "Rheokinetics and effect of

shear rate on the kinetics of linear polyurethane formation," Polymer Engineering and

Science, vol. 45, pp. 279-287, Mar 2005.

[11] S. Ajithkumar, S. S. Kansara, and N. K. Patel, "Kinetics of Castor Oil Based Polyol-

Toluene Diisocyanate Reactions," European Polymer Journal, vol. 34, pp. 1273-1276,

1998.

[12] I. Yilgor, B. D. Mather, S. Unal, E. Yilgor, and T. E. Long, "Preparation of segemented,

high molecular weight, aliphatic poly(ether-urea) copolymers in isopropanol. In-situ

FTIR studies and polymer synthesis," Polymer, vol. 45, pp. 5829-5836, 2004.

[22] R. G. Pearson and E. L. Williams, "Interfacial Polymerization of an Isocyanate and a

Diol," Journal of Polymer Science Part a-Polymer Chemistry, vol. 23, pp. 9-18, 1985.

[32] C. Dubois, S. Desilets, A. Aitkadi, and P. Tanguy, "Bulk-Polymerization of Hydroxyl

Terminated Polybutadiene (Htpb) with Tolylene Diisocyanate (Tdi) - a Kinetics Study

Using C-13-Nmr Spectroscopy," Journal of Applied Polymer Science, vol. 58, pp. 827-

834, Oct 24 1995.

[33] D. Kincal and S. Ozkar, "Kinetic study of the reaction between hydroxyl-terminated

polybutadiene and isophorone diisocyanate in bulk by quantitative FTIR spectroscopy,"

Journal of Applied Polymer Science, vol. 66, pp. 1979-1983, Dec 5 1997.


polyurethane synthesis by FTIR-ATR," Polymer Testing, vol. 28, pp. 773-779, Oct 2009.

[38] E. B. Richter and C. W. Macosko, "Kinetics of Fast (Rim) Urethane Polymerization,"

Polymer Engineering and Science, vol. 18, pp. 1012-1018, 1978.

[52] A. Y. Malkin, "Rheology in polymerization processes," Polymer Engineering & Science,

vol. 20, pp. 1035-1044, 1980.

[53] A. Y. Malkin, S. G. Kulichikhin, D. N. Emel'yanov, and I. E. Smetanina, "Rheokinetics

of free-radical polymerization," Polymer, vol. 25, pp. 778-784, 1984.

[54] V. W. A. Verhoeven, M. P. Y. Van Vondel, K. J. Ganzeveld, and L. P. B. M. Janssen,

"Rheo-kinetic measurement of thermoplastic polyurethane polymerization in a

98

measurement kneader," Polymer Engineering and Science, vol. 44, pp. 1648-1655, Sep

2004.

[55] S. D. Lipshitz and C. W. Macosko, "Kinetics and energetics of a fast polyurethane cure,"

Journal of Applied Polymer Science, vol. 21, pp. 2029-2039, 1977.

[56] S. D. Lipshitz, F. G. Mussati, and C. W. Macosko, "Kinetic and viscosity relations for

urethane network polymerizations," Soc. Plastics Eng. Antec. Tech. Papers21, pp. 239-

241, 1975.

[57] C. S. Schollenberger, K. Dinbergs, and F. D. Stewart, "Thermoplastic Polyurethane

Elastomer Melt Polymerization Study," Rubber Chemistry and Technology, vol. 55, pp.

137-150, 1982.

[58] B. J. Briscoe, M. B. Khan, and S. M. Richardson, "Rotary injection takes RIM to new

frontiers," Plastics and rubber international, vol. 13, pp. 20-24, 1988.

[59] P. Krol and Z. Wietrzynskalalak, "Study on Structure and Properties of Carbamates as

Model Compounds for Urethane Polymers," European Polymer Journal, vol. 31, pp.

689-699, Jul 1995.

[60] I. Y. Gorbunova, M. L. Kerber, I. N. Balashov, S. I. Kazakov, and A. Y. Malkin, "Cure

rheokinetics and change in properties of a phenol-urethane composition: Comparison of

results obtained by different methods," Polymer science. Series A, Chemistry, physics,

vol. 43, pp. 826-833, 2001.

[61] A. Y. Malkin, S. G. Kulichikhin, and G. K. Shambilova, "Strain Effect on the Phase State

of Polyvinyl Acetate Solutions," Vysokomolekulyarnye Soedineniya Seriya B, vol. 33, pp.

228-231, Mar 1991.

99

4.7 Figures

0 200 400 600 80010

-3

10-2

10-1

100

101

102

103

104

105

106

0.1 sec-1

1.0 sec-1

5.0 sec-1

10.0 sec-1

15.0 sec-1

20.0 sec-1

Vis

co

sity

Time (sec)

Figure 4.1 - Viscosity versus time of 36.8 wt.% HS across a range of shear rates varying

from 0.1 sec-1 to 20 sec-1.

100

0 100 200 300 400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

0.1s-1

1s-1

5s-1

10s-1

time(sec)

Figure 4.2 – Conversion versus time of 36.8 wt.% HS across a range of shear rates

varying from 0.1 sec-1 to 20 sec-1.

101

0.1 1 10 100 1000 1000010

-3

10-2

10-1

100

101

102

103

104

105

106

Vis

co

sity

Strain

0.1 sec-1

1.0 sec-1

5.0 sec-1

10.0 sec-1

15.0 sec-1

20.0 sec-1

Figure 4.3 Viscosity versus strain of 36.8 wt.% HS across a range of shear rates varying

from 0.1 sec-1 to 20 sec-1.

102

1 10 100 1000 10000

0.0

0.2

0.4

0.6

0.8

1.0

0.1 sec-1

1.0 sec -1

5.0 sec -1

10.0 sec-1

Co

nve

rsio

n

Strain

Figure 4.4 - Conversion versus strain of 36.8 wt.% HS across a range of shear rates

varying from 0.1 sec-1 to 20 sec-1.

103

0.0 0.2 0.4 0.6 0.8 1.0

200

400

600

800

1000

/

0

Conversion

Viscosity (0.1)

Viscosity (1)

Viscosity (5)

Viscosity (10)

Viscosity-Fit (0.1)

Viscosity-Fit (1)

Viscosity-Fit (5.0)

Viscosity-Fit (10.0)

Figure 4.5 – Normalized viscosity versus conversion with fit using Equation 4.7.

104

Shear Rate 0.1 sec-1 1.0 sec

-1 5.0 sec-1 10.0 sec

-1 B 4.239 1.063 1.017 1.001

C 24.99 3.155 2.354 1.519

R-Square 0.986 0.983 0.998 0.985

Table 4.1 - Fitting parameter comparison for model of TPU polymerization under steady

shear.

105

0 200 400 600 800 1000 120010

-1

100

101

102

103

104

105

106

107

(

Pa s)

Time (s)

55.6% HS

36.8% HS

24.6% HS

Figure 4.6 - Complex viscosity measured under oscillatory shear at 100 C for three

different hard segment compositions.

106

0.0 0.2 0.4 0.6 0.8 1.010

-1

100

101

102

103

104

105

106

107

26.4 wt. %HS

36.8 wt.% HS

55.6 wt. % HS

Conversion

Figure 4.7 – Normalized complex viscosity versus conversion for oscillatory shear

experiment at 100 °C.

107

0.0 0.5 1.0 1.5 2.010

-1

100

101

102

103

104

105

106

107

26.4 wt. %HS

36.8 wt.% HS

55.6 wt. % HS

*/

* 0

*[NCO0]

Figure 4.8 – Influence of total isocyanate consumed for compositions at 100C by

normalizing conversion by the initial concentration of NCO.

108

0 500 1000 1500 2000 2500 300010

-1

100

101

102

103

104

105

55.6 wt. % HS

36.8 wt. % HS

26.4 wt. % HS

(

Pas

)

Time (s)

Figure 4.9 - Viscosity versus time plot of polymerization under isothermal conditions at

200 C.

109

0.0 0.2 0.4 0.6 0.8 1.0

100

101

102

103

104

105 26.4 wt. %HS

36.8 wt.% HS

55.6 wt. % HS

*/

* 0

Conversion

Figure 4.10 - Complex viscosity versus conversion for varied compositions under

oscillatory shear at 200 C.

110

0.0 0.5 1.0 1.5 2.0

100

101

102

103

104

55.6 wt. %HS

36.8 wt. %HS

26.4 wt. % HS

*/

* 0

*[NCO0]

Figure 4.11 - Master curve of complex viscosity versus conversion normalized by the

initial concentration of NCO in the system.

111

Composition 26.4 wt. % HS 36.8 wt. % HS 55.6 wt. % HS

B 1.000 1.037 1.000

C 1.372 3.096 6.740

R-Square 0.975 0.984 0.958

Table 4.2 - Fitting parameter comparison for model of TPU polymerization of varied HS

content under oscillatory shear.

112

0.0 0.2 0.4 0.6 0.8 1.010

-1

100

101

102

103

104

105

106

0.1 sec-1

1.0 sec-1

5.0 sec-1

10.0 sec-1

OSC

Vis

cosity

Conversion

Figure 4.12 - Comparison of viscosity versus conversion behavior between oscillatory

and steady shear experiments of 36.8 wt. % HS TPU.

113

Chapter 5 : Effect of Soft-to-Hard Segment Ratio on

Viscoelastic Behavior of Model Thermoplastic

Polyurethanes During Phase Transitions


Journal article:

J. L. Gadley, R. J. Andrade, and J. M. Maia, Macromolecular Materials and Engineering,

2016, DOI: 10.1002/mame.201500450

114

5.1 Abstract

Model thermoplastic polyurethanes were prepared with the aim of investigating the

effect of soft-to-hard segment ratio on the phase transition and the resulting structure that

forms upon isothermal exposure to temperatures near their phase transition temperature.

The dynamic rheological properties of TPUs before exposure to these isothermal

conditions show a phase transition at high temperatures that is directly related to the content

of hard segments. The extensional viscosity data indicates a strain-hardening behavior that

becomes less pronounced with the increase of hard segments.

After isothermal treatment, the DSC results show that the high temperature endotherm peak

narrowed and shifted to higher temperatures, suggesting a transition in structure. SAXS,

WAXS, and AFM results indicate a phase-separated system in which the hard domain sizes

and crystallinity change during this process. The rheological data collected after

recrystallization shows a significant increase in both moduli, transitioning from

viscoelastic fluid-like to glassy behavior. Concurrently, the uniaxial elongation viscosity

presents a significant increase in absolute values, but with a shift from strain-hardening to

strain-softening behavior for all strain rates. A transition from traditional phase separated

viscoelastic melt behavior to more brittle rupture is also observed, marking a significant

fundamental difference in properties before and after recrystallization.

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5.2 Introduction

Thermoplastic polyurethanes (TPU) are multi-block copolymers typically constructed

of alternating hard and soft segments and have been the subject of a relatively strong effort

from both scientific and industrial communities due to their wide range of applications,

including coatings, fibers, films, footwear, wire and flexible tubing.[6, 62, 63] Hard

segments (HS) are composed of diisocyanate (e.g. 4,4’-methylenediphenyl diisocyanate

(MDI)) and short-chain diols (e.g. 1-4 butanediol (BDO)) as chain extenders, while soft

segments (SS) are composed of long-chain polyols, commonly polyesters or polyethers.

Through varying soft and hard segment chemistry and relative composition, a broad range

of physical properties are achievable, ranging from soft elastomers to hard plastics. The

wide property range of TPUs, specifically abrasion resistance, toughness, and flexibility,

is due to their high degree of chemical tunability. This allows TPUs to bridge the gap

between traditional rubber materials and plastics.

The structure and subsequent physical properties of TPUs are strongly affected by the

synthesis and processing conditions. These systems gain their mechanical property

versatility from the phase separation between hard and soft segments. Phase separation

occurs due to thermodynamically unfavorable phase-mixing conditions. A phase-separated

configuration reduces the enthalpic penalty of the system resulting in a more stable

configuration consisting of hard and soft domains. [64-66] While entropy is reduced when

phase separation occurs, this effect is primarily driven by the molecular weight of the SS

within the system. However, chemical compatibility of the HS and SS within the TPU leads

to an enthalpic driving force to generate phase separation where in most cases mixing the

SS and HS is energetically unfavorable. The ultimate properties of TPUs are strongly

116

influenced by the different morphologies which result from the phase separation character

of the system. These morphologies, while complex, commonly adopt a disordered globule

or fibrillar structure. At room temperature, TPUs offer a relatively high modulus and

extensibility, due to hydrogen bonding between HS acting as physical cross-links, while

preserving mobility of the flexible chains within the soft domains. When these systems are

heated above the melting temperature of the HS, phase mixing becomes more entropically

favorable over the phase separated morphology present at lower temperatures. The original

structure is recovered after cooling below the melting point of the HS. However, the rate

at which these materials traverse thermal transitions has a significant impact on the final

structure of the system.[48, 67, 68] This becomes particularly important when processing

TPUs or performing post-processing operations when specific final properties are desired.

In the past, extensive experimental studies have proven that a variety of thermal

transitions exist in TPUs that greatly influence the morphology of the system.[69-78] It is

well known that differential scanning calorimetry (DSC) experiments detect the existence

of multiple endothermic peaks during the melting process, particularly in multi-block

copolymers. Depending on the initial thermal and mechanical processing conditions, and

annealing conditions, up to three distinct peaks can be observed in DSC experiments: i)

Endotherm I at 60-80 °C, which is the glass transition of either the hard or soft segments;

ii) Endotherm II at 120-180 °C, influenced by the annealing on the hard segments; iii)

Endotherm III at temperatures around 200 ºC, associated to the microcrystalline peak. The

actual origins of the multiple endotherms have been attributed to different phase transitions,

such as phase separation, crystallization of the hard segments, and local restructuring of

the hard micro-domains.[79] In particular, studies show that in some systems, hard domains

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may remain amorphous and exhibit only phase separation, while in other systems hard

domains can achieve high levels of crystallinity resulting in a wide range of potential

configurations.[80, 81]

The final structure, and consequently the rheological behavior, are affected by the

chemical structure of hard and soft segments, hydrogen bonding, molecular weight,

molecular weight distribution, thermal history, and ratio of hard and soft blocks.[26, 28,

30] Most of the studies that investigate the phase transition behavior of TPUs were

conducted by DSC, small angle X-ray scattering (SAXS) wide angle X-ray scattering

(WAXS), infrared spectroscopy (IR), and atomic force microcopy (AFM).[82-89] While

the understanding of phase transition behaviors are important to understanding thermal

post-processing conditions, these techniques leave a gap in the relationship between

exposure conditions and the physical performance of the system.

In order to further understand the physical implications of phase transitions, rheological

measurements can also be used as an experimental tool to monitor the behavior.[90-92]

Recently, it was shown that investigating the evolution of the viscoelastic moduli allows

significant insights to be gained into the kinetics of phase transition in a commercial

TPU.[88] The micro-phase separation between hard and soft segments and the

crystallization of hard phase domains originates at a solution to gel (sol-to-gel) transition.

The thermal history that the TPU was exposed to directly relates to the microstructure at

the critical gel point; an increase in HS content induces the shift of phase transition kinetics

to higher temperatures, due to an enhancement in the tendency for phase mixing to occur

during the phase transition of the TPU.[88, 92]

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The aforementioned characteristics of TPUs demonstrate that both the phase separated

structure and rheological properties of the materials are affected by thermal history during

both synthesis and processing. However, very few investigations report on the influence of

thermal history on the shear and extensional rheology measurements at elevated

temperatures, where intermixing of the hard and soft domains occur.[87, 91-93] The effect

of thermal history on the evolution of the viscoelasticity of commercial TPUs were

investigated and proved to be very influential.[93] These changes in the rheological

character of the materials were attributed to either an order-to-disorder transition or

reconfiguration of the micro-phase separated structure present. However, minimal results

have been published on the effect of the phase transition structure on the elongational

viscosity. In one study, the effect of the molecular aggregation structure on the rheological

properties of TPUs was investigated and an increase in the micro-phase separation with the

increase of annealing temperature was observed.[87] These changes strongly influenced

the sol-to-gel transition temperature and the strain hardening of uniaxial elongational

viscosity. Recently, we reported the effect of uniaxial flow on the structural development

of commercial TPUs by showing a transition from strain-softening to strain-hardening at

strain rates above 1s-1.[94]

The purpose of this work is to further study the rheological behavior and related

structure development during the phase transition of TPUs upon their exposure to

temperatures near where phase transitions occur. In particular, we aim to understand the

effect of soft-to-hard segment ratio on the phase transition and structural and

morphological changes of well-characterized model thermoplastic polyurethanes upon

recrystallization at high temperatures. Downstream, from the applications point of view,

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this knowledge can be very relevant when trying to understand how post-processing these

complex systems can alter performance.

Experimental

Materials

The TPUs used in this work were synthesized through bulk polymerization of a three

part system. The reactants used were 4,4’-methylenediphenyl diisocyanate (MDI), 1,4-

butanediol, and a 1,014 g/mol molecular weight polyester polyol (poly(butylene adipate))

was obtained from an industrial production source. The hydroxyl and isocyante ratio

remained 1:1, while adjustments in the chain extender and polyol were used to give three

different compositions, 26.4 wt. %, 36.8 wt. %, and 55.6 wt. % HS. The compositions were

achieved by adding mole ratios of 1.0 MDI:0.25 BDO :0.75 Polyol (26.4 wt. % HS), 1.0

MDI: 0.50 BDO: 0.50 Polyol (36.8 wt. % HS), and 1.0 MDI: 0.75 BDO: 0.25 Polyol (55.6

wt. % HS). Preparation of the samples were performed under bulk polymerization using

only heat to drive the polymerization. These components were added to an insulated vessel,

which was allowed to equilibrate to 200°C. The appropriate mass for the desired

composition of each component was carefully weighed to achieve a 50 g sample of the

desired HS content while maintaining proper stoichiometry. The polyol, which was stored

in a sealed container, was added to the vessel and allowed to melt (60 sec). Once the polyol

was fully melted the liquid BDO was added to the vessel. These components were mixed

briefly using a mechanical mixer at 20 rotations per minute (RPM). The MDI remained

frozen at -20 °C until immediately before use and was then added to the reaction vessel.

Immediately after adding the MDI the mechanical mixer was set to 100 RPM and allowed

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to stir for 600 sec. The vessel was then covered and allowed to cool under ambient

conditions.

Thermal Analysis

The thermal properties of the TPU samples were investigated through the use of

differential scanning calorimetry (DSC). A TA Instruments Q100 DSC was used to

determine the enthalpy of melting of the various compositions. The samples used were 3-

5 mg of the TPU as reacted after remaining at ambient conditions for 24 h. The post small

angle oscillatory shear (SAOS) samples were also allowed to remain at ambient conditions

for 24 h before testing. The enthalpy of melting on the first heating was determined by

performing a heat/cool/heat experiment through integration of the endotherm bound by

deviation and return to the experimental baseline. DSC experiments used a lower

temperature of -80 °C, an upper temperature of 230 °C at a heating (and cooling) rate of

10 °C/min. This analysis was performed under continuous nitrogen purge at a flow rate of

50 ml/min.

X-ray diffraction

Two-dimensional (2D) small-angle X-ray scattering (SAXS) was used to investigate

morphological features of the TPU and 2D wide-angle X-ray scattering (WAXS) was used

to probe the crystallinity of the samples. The experiments were performed on the X27C

Advanced Polymers beamline (wavelength λ = 0.1371 nm) at Brookhaven National

Laboratory (BNL) National Synchrotron National Light Source – I (NSLS-I). A 2D Brüker

Smart 1500 X-ray CCD detector (1024 x 1024 pixels) The SAXS setup was calibrated

using a silver behenate (AgBe) standard with a 001 scattering vector (q) at 1.076 nm-1 to

calculate the sample-to-detector distance and calibrate the scattering vector. The sample-

121

to-detector distance was calculated to be 1783 mm. The WAXS setup was calibrated using

an aluminum oxide (Al2O3) standard with a 104 reflection at q = 18.6 nm-1 (2θ = 31.1°). A

sample-to-collector distance of 113.5 mm was calculated. For all experiments, plaque

samples were placed with their face perpendicular to the beam for an exposure time of 120

sec. The dark-field baseline and background scattering were collected for an exposure time

of 120 sec as well for consistency. The collected scattering images were then processed

using the software POLAR (Precision Works NY, Inc.) through correction with

background scattering and dark-field images. The corrected SAXS and WAXS data was

integrated to attain a one dimensional (1-D) intensity profile. The resulting 1-D WAXS

profiles were processed using peak fitting software assuming the amorphous region fit a

Gaussian distribution while the crystalline region fit a Lorentzian distribution. The

integrated difference of the amorphous and crystalline regions were used to determine the

percent crystallinity of the samples.

Atomic Force Microscopy (AFM)

The domain size and spacing in the TPUs was analyzed using AFM. Specifically, a

Veeco Multinode AFM was used with a NanoScope controller operating in tapping mode

under ambient conditions. The tips used for this work were Veeco antimony doped Si with

a spring constant of 20-80 N/m, an oscillating frequency of 327-383 kHz and a 3.5-4.5 μm

tip. The samples were prepared by cutting a small sample from a melt pressed disc prior

to performing rheological experiments. After the rheological experiments the disc was then

removed from the rheometer and a sample was prepared in the same manner. The samples

were placed on the stage face up by applying force to the edges to ensure adhesion to the

122

stage without damaging the test surface. The images were then analyzed using NanoScope

and ImageJ software.

Gel Permeation Chromatography (GPC)

The molecular weight and molecular weight distribution of the TPU before and after

annealing was measured using a Varian ProStar 210/215 GPC. Dimethylformamide (DMF)

was used as the eluent at a flow rate of 0.7 mL/min. Polymer laboratories PLGel columns

were used in conjunction with a Viscotek 270 Dual Dector multi-angle light scattering

(MALS) detector and a Varian ProStar Model 350 refractive index detector. The GPC

samples were prepared by creating a solution with a concentration of approximately 3

mg/mL in DMF for the analysis.

Rheology

A MARS III rheometer (Thermo Fisher Scientific), operated with a 20 mm parallel

plate setup was used to measure the dynamic rheological properties of the TPUs. Disk-

shaped TPU samples with 26.4 wt. %, 36.8 wt. %, and 55.6 wt. % hard segment of 20 mm

diameter and 1 mm thickness were molded in a compression molder at 140, 175, 225º C,

respectively. The melt pressing temperature was chosen at temperature above the

endothermic transition in the system in order to mitigate changes near the transition which

would be studied. The specimens were subject the respective temperature for 5 min during

molding. The system in the rheometer was first heated and kept at the desired temperature

for 2 min to reach equilibrium. All rheological experiments were performed under a

nitrogen atmosphere. Prior to the experiments, all samples were vacuum dried for 12 h at

70 C.

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Time sweep experiments were conducted at different temperatures for each TPU. These

experiments were performed at constant frequency at 1 Hz. Frequency sweeps at constant

temperature with oscillatory shear frequency between 1 and 100 rad/s were also carried

out. Storage modulus (G’), loss modulus (G’’), and damping factor (tan δ) were measured

in all rheological experiments described above. All experiments were performed in the

linear viscoelastic regime, confirmed independently by stress sweeps.

Uniaxial elongation viscosity was measured on a SER fixture mounted on the same

rheometer, under different constant strain rates. Samples with length, width, and height

dimensions of 12.7 x 10.0 x 0.80 mm (L x W x H) were compression molded following the

same conditions for the disk-shaped samples. Sample preparation and loading followed the

procedures recommended by Barroso et al.[95] The extensional samples were allowed to

equilibrate for the same two-minute time period as was used in the oscillatory experiments

to ensure identical thermal history was applied to all rheology samples. In addition, this

established a uniform start time (t = 0) and mitigated any structural changes which may

occur during equilibration time and testing.

Results and Discussion

Thermal analyses of the TPUs with different ratios of hard and soft segments are shown

in Figure 5.1. The DSC thermograms of the TPUs exhibit endothermic peaks, which

become more pronounced and shift to higher temperatures with increasing HS content.

These results indicate that increasing HS content leads to a system with a higher affinity

toward developing a crystallized structure or at least a more robust structure that requires

greater energy input requirements for dissociation. This behavior exemplifies the

124

significance of HS content on both the character of the transition as well as the temperature

required to influence the transition.

Oscillatory shear time sweeps were performed at different constant temperatures in

order to analyze the time stability of the TPUs at different temperatures and determine the

maximum allowable duration of oscillatory frequency sweep experiments. Understanding

the stability of the materials under these isothermal conditions is very important for

extensional testing to verify the accuracy of measurement for the duration of the

experiment. The rheological results are shown in Figure 5.2 and clearly show that in the

vicinity of the endothermic peak the material properties are not stable. Significant changes

in storage modulus were noted in the 26.4 wt. %, 36.8 wt. % and 55.6 wt. % HS samples

when the isothermal experiments were performed at or near the endothermic melting peak

of 105, 170, and 206 °C respectively. This response of the storage modulus to isothermal

conditions is typical of systems undergoing a transition from liquid-like to solid-like

behavior as would be evident during a period of crystallization. This behavior is further

supported by the crossover of G’ and G” while the sample remained at the endothermic

transition temperature (detected in DSC) of 120 °C, 150 °C, and 195 °C with increasing

HS content throughout the experiment. The crossover occurred at 348 sec, 664 sec, and

708 sec in the 55.6 wt.% HS, 36.8 wt. % HS, and 26.4 wt.% HS materials respectively.

Above the transition, the system remains disordered and liquid-like while below this

temperature the material remains solid. Similar results were recorded for TPUs with a

transition characterized by the passage through a critical gel state, albeit at much lower

temperatures.[88] A sharp increase in storage modulus was observed, which is consistent

with behavior expected from a material experiencing an increase in crystallinity or

125

restructuring to a more mechanically robust state. These time study results indicate that the

sharp increase in storage modulus is observed in all compositions but with a more

pronounced change as HS content is increased. This suggests that under the isothermal

conditions, crystallinity is increasing between hard segments within the hard domains. This

observation will be further investigated and discussed below.

The number average molecular weight (Mn), weight average molecular weight (Mw),

and dispersity (Ð) information was obtained via GPC before and after the time sweeps at

each material’s respective transition temperature in order to analyze the thermal stability

for all three TPU compositions as well as to confirm no crosslinking or branching was

occurring in the hard segments. As shown in Table 1, an increase in Mw and Mn was present

in all samples (42.5%, 40.1%, and 41.3% Mw increase for 26.4 wt. %, 36.8 wt. %, and 55.6

wt. % HS respectively). However, this increase does not account for the magnitude of

increase shown in the rheological data. For example, even for the 55.6 wt. % HS sample,

which is the one where the increase is more pronounced, the increase in molecular weight

is only enough to justify an increase of approximately one order of magnitude in the

viscosity, assuming an entangled polymer network is formed, whereas the rheological data

shows an increase of almost three orders of magnitude.

Comparing the clear increase in G’, and the mechanical change in behavior from liquid-

like to solid-like behavior and the GPC results support that a significant physical

rearrangement is occurring. Since a clear shift in rheological behavior is evident and there

is an absence of evidence indicating the existence of degradation (see Table 5.1), the results

at the endothermic peak captured in DSC are most likely the reflection of a phase transition

during the experiment. This is in agreement with the quick increase of the storage modulus

126

at short times during the time sweeps at 100, 150 and 195ºC (Figure 5.2), for the 26.4 wt.

%, 36.8 wt. % and 55.6 wt. % samples, respectively. The extensional measurements, shown

in Error! Reference source not found., are not affected by this phenomenon since all the e

xperiments were subjected to the same equilibration time as the oscillatory samples and

had a maximum experiment time of less than 20 seconds. These measurements are depicted

as representative samples based on a 5 sample set with acceptable repeatability. They show

a typical strain-hardening behavior for these materials, with the degree of strain-hardening

decreasing with the increase of HS percentage. A decrease in strain-hardening with an

increase in HS content is consistent with a decrease in the elastic (soft domains) within

overall system.42 Effectively this results in dispersing more hard domains throughout the

soft matrix, ultimately reducing the strain-hardening effect. These results suggest that

strain-hardening is dominated by the SS content and the respective fraction of soft domains

as was expected.

Results thus far in the present work indicate a clear influence of HS to SS ratio on the

melting and crystallization behavior of TPUs. This is particularly evident when considering

temperatures at or near the endothermic peaks in DSC (see Figure 5.1). Thus, DSC and

rheometry where used to probe the melting and crystallization behavior occurring at the

endothermic peak, through isothermal recrystallization of the TPU at these temperatures to

investigate implications on the structural property changes.

DSC experiments were performed on the TPU samples after isothermal annealing at

each sample’s transition temperature (post-annealing) on the rheometer for 120 minutes.

Immediately upon removal from the rheometer the samples were quenched in a solid CO2

bath then placed in the DSC to match conditions detected on the rheometer as accurately

127

as possible. The results were compared with the samples before annealing (pre-annealing)

and are presented in Figure 5.4. In both cases, the endothermic peaks shift to higher

temperatures and narrow after annealing, indicating the development of a well-defined

crystalline structure. The 26.4 wt.% sample is not shown, due to the absence of discernable

change in that composition via DSC. The enthalpy of melting was investigated before and

after annealing for the TPUs and the values for the TPU with 36.8 wt. % HS before and

after annealing are 15.90 J/g and 16.91 J/g, respectively, which is consistent with an

increase in crystallinity. Similarly, the TPU with 55.6 wt. % HS presented values of 6.77

J/g and 18.55 J/g, before and after annealing respectively, suggesting increased

crystallinity, the largest increase of the three samples.

The TPU samples were also characterized by SAXS and AFM pre and post-high-

temperature annealing, with the scattering patterns and the AFM phase images being shown

in Figure 5.5 (a and b). The different compositions show a scattering peak, implying the

presence of a two-phase structure before annealing. While the pre-annealing peak could be

related to the correlation hole effect, this was likely not the case due to the fact that this

system is known to form a phase separated morphology. Further investigation shows an

agreement between the domain size measured in SAXS and AFM therefore the measured

peak was attributed to phase separation within the system. Before the annealing process,

all three compositions show a very similar d-spacing and domain size distribution.

Therefore, changes in structure during time at the transition temperature was investigated

using the 36.8 wt. % HS sample. After annealing the peak shifts to lower q in the TPU

sample, suggesting the occurrence of a thickening effect within the hard domains.

Combining these results with AFM images (Figure 5.5 b) two distinct phases are observed

128

before annealing, with HS and SS appearing as light and dark regions respectively. After

annealing, the AFM image suggests that the HS have grown and become less uniform in

size supporting the observations in SAXS. The 1D SAXS curve appears to show no long

range ordering in both the pre-recrystallization and post-recrystallization meaning little to

no periodicity of the hard segments exist but that the domains have simply thickened by

remaining isothermal at the transition temperature (Figure 5.5b). The corresponding AFM

images indicate an increase in HS domains during annealing, which suggests that a further

increase of the hard segment concentration results in an increase in the size of hard domains

during annealing. This is likely due to coalescence of some of these domains into longer,

irregular domains. The result is a broad distribution of hard domain sizes, which is reflected

as smaller angles and less pronounced SAXS pattern peak. Similar results showing the

effect of increasing the HS content in SAXS behavior were also observed in earlier

investigations.[77, 96]

Analysis of data collected from WAXS experiments (exemplified in Figure 5.6 for the

36.8 wt. % sample) indicates a transition from an amorphous material to a crystalline

material during the isothermal cycle and thus gives further support to changes occurring

during the phase transition. In fact, all three compositions were initially amorphous and

transitioned to a crystallinity value of 7.4%, 16.2%, and 39.3% (samples containing 26.4

wt. %, 36.8 wt. %, and 55.6 wt. % respectively).

These findings are confirmed by the rheological data collected after 120 minutes of

isothermal annealing for TPUs with 36.8 and 55.6 wt. % HS, which is shown in Figure 5.7.

By analyzing the frequency sweeps before and after the isothermal annealing, it is possible

to observe a significant increase in both moduli. Furthermore, the storage modulus is

129

almost independent of the frequency after the isothermal annealing, which is indicative of

a structural change. The effect of the annealing on the uniaxial elongational viscosity was

also investigated (Figure 5.8).

The results indicate an increase in base extensional viscosity of 10-20 times and strain-

softening behavior for all strain rates. The behavior was classified as true strain softening

since the effect happened prior to the observation of any rupture through real-time video.

Any data collected after any evidence of rupture onset was disregarded. A brittle solid like

rupture is observed in Figure 5.9 (still image from video during experiment) when

compared to the typical of phase-separated viscoelastic melt. The results shown for the

55.6 wt. % HS TPU indicated more pronounced strain-softening behavior than the TPUs

containing 26.4 wt. % and 36.8 wt. % HS which suggests a higher degree of embrittlement.

This was also observed in the frequency sweep with the higher modulus values as well. It

was also observed that increasing the HS content caused the transition to shift to higher

temperatures, which affected the viscoelastic properties of the amorphous phase for lower

HS percentages. The change in behavior to strain-softening has been attributed in studies

relating uniaxial flow and SAXS to a complex rearrangement and disordering of micro

domain structures during deformation.[96] While this behavior is likely the root cause of

the drastic extensional transition observed in this work, many factors related to

morphology, increased phase separation, crystal size and type, to name a few, are present

in these systems. Therefore, further studies are necessary to determine the specific origin

observed within this work. When exposed to temperatures near this transition range the

physical properties of the material changes drastically and transitions to entirely different

130

viscoelastic behavior which could be detrimental to the function of these systems in

application.

Our results reveal a direct influence of the HS content in the rheological properties

before, during, and after exposure to isothermal conditions near the phase transition

temperature. This effect is attributed to changes in crystallinity over time while maintaining

a temperature near the endothermic transition noted in DSC, which became more

pronounced as HS content was increased. The isothermal study demonstrated that these

TPUs go through a transition from the melt state to solid-like behavior at the same

temperature, which is attributed to crystallization and is enhanced with increasing HS

content. The resulting highly crystalline hard domains (as opposed to the amorphous hard

domains before melting and recrystallization) gave rise to very different rheological

behavior under both shear and extensional conditions. DSC results support the shift in

crystallinity as well as a shift of the melting point to higher temperatures, suggesting a new

ordered and more uniform hard domain structure. This transition is more pronounced with

the increase of HS content due to the increased amount of crystallizable material present

in the system. SAXS and AFM results indicate a phase separation for pre-crystallized

TPUs. After melting and recrystallization have occurred, the hard domains grow in size

further supporting the drastic changes in behavior stem from these changes. The structures

formed and the differences in HS content suggest that as the fraction of HS within the

system increases the drastic increase in crystallinity strongly affect rheological properties

and overall material behavior.

Conclusions

131

In this work, the effect soft-to-hard segment ratio on the phase transition of model

thermoplastic polyurethanes and its influence on the structure and rheological properties

before and after isothermal recrystallization at temperatures near the material’s transition

temperature was investigated.

The rheological properties of TPUs after polymerization show a phase transition at high

temperatures that is directly related to the content of hard segments. The extensional

viscosity data indicates a strain-hardening behavior that becomes less pronounced with the

increase of hard segment content, suggesting this specific extensional behavior is governed

by the soft segments, as expected.

In order to study the phase transition occurring at the dissociation point, isothermal

experiments, i.e., recrystallization, were conducted near the phase transition temperatures

of each material. The results from DSC present significant changes in the thermograms,

which show that the high-temperature endotherm peak narrowed and shifted to higher

temperatures over time, suggesting a more ordered and crystalline-like structure. SAXS

and AFM results indicate a phase separated system for the recrystallized TPUs, which

confirms the DSC findings. After exposure to isothermal conditions the hard domain sizes

increased, supporting that the increase in crystallinity was occurring in the hard domains

rather than the soft domains. The AFM data even indicated that at the highest HS

percentage (> 50 wt. %) the system appears to move toward a more phase mixed scenario.

The rheological behavior after recrystallization shows a significant increase in both

moduli, with a transition from a viscoelastic fluid behavior to a gel-like behavior, i.e., much

higher storage modulus than loss modulus, with the former showing almost no dependence

on frequency. The uniaxial elongation viscosity increases between 10 and 20 times

132

relatively to the non-crystallized samples, and the behavior changed to strain-softening at

all strain rates. Brittle rupture was observed when compared to the expected rupture mode

typical of a phase-separated viscoelastic melt as observed in the samples before

recrystallization.

Overall, the structural development of TPUs annealed at temperatures near their

dissociation temperature has been shown to significantly alter the crystallinity of the

system and ultimately the properties of the TPUs. Understanding this relationship provides

an avenue to tailor TPU performance through post-processing and expands the broad range

of structure and properties available within these complex systems.

133

References
















[62] N. G. Gaylord and N. V. Seeger, "Polyurethanes: chemistry and technology, Part

I. (High Polymers, Vol. XVI). J. H. SAUNDERS and K. C. FRISCH,

Interscience, New York, 1962. ," Journal of Applied Polymer Science, vol. 8, pp.

1498-1499, 1964.

[63] G. n. Oertel and L. Abele, Polyurethane handbook : chemistry, raw materials,

processing, application, properties, 2nd ed. Munich ; New York

Cincinnati: Hanser ;

Hanser/Gardner distributor, 1994.

[64] S. B. Clough and N. S. Schneider, "Structural studies on urethane elastomers,"

Journal of Macromolecular Science, Part B, vol. 2, pp. 553-566, 1968/12/01

1968.

[65] G. M. Estes, R. W. Seymour, and S. L. Cooper, "Infrared Dichroism of

Segmented Polyurethane Elastomers - Poly," Abstracts of Papers of the American

Chemical Society, pp. 26-&, 1970.

[66] J. A. Koutsky, N. V. Hien, and S. L. Cooper, "Some Results on Electron

Microscope Investigations of Polyether-Urethane and Polyester-Urethane Block

Copolymers," Journal of Polymer Science Part B-Polymer Letters, vol. 8, pp.

353-&, 1970.

[67] A. Frick and A. Rochman, "Characterization of TPU-elastomers by thermal

analysis (DSC)," Polymer Testing, vol. 23, pp. 413-417, 6// 2004.

[68] M. A. Hood, B. Wang, J. M. Sands, J. J. La Scala, F. L. Beyer, and C. Y. Li,

"Morphology control of segmented polyurethanes by crystallization of hard and

soft segments," Polymer, vol. 51, pp. 2191-2198, 5/4/ 2010.

[69] T. R. Hesketh, J. W. C. Van Bogart, and S. L. Cooper, "Differential scanning

calorimetry analysis of morphological changes in segmented elastomers,"

Polymer Engineering & Science, vol. 20, pp. 190-197, 1980.

134

[70] H. N. Ng, A. E. Allegrezza, R. W. Seymour, and S. L. Cooper, "Effect of segment

size and polydispersity on the properties of polyurethane block polymers,"

Polymer, vol. 14, pp. 255-261, 1973/06/01 1973.

[71] S. L. Samuels and G. L. Wilkes, "Comments on the multiple endothermic

behavior observed in segmented urethane systems," Journal of Polymer Science:

Polymer Physics Edition, vol. 11, pp. 807-811, 1973.

[72] R. W. Seymour and S. L. Cooper, "DSC studies of polyurethane block polymers,"

Journal of Polymer Science Part B: Polymer Letters, vol. 9, pp. 689-694, 1971.

[73] R. W. Seymour and S. L. Cooper, "Thermal Analysis of Polyurethane Block

Polymers," Macromolecules, vol. 6, pp. 48-53, 1973/01/01 1973.

[74] J. W. C. Van Bogart, D. A. Bluemke, and S. L. Cooper, "Annealing-induced

morphological changes in segmented elastomers," Polymer, vol. 22, pp. 1428-

1438, 1981/10/01 1981.

[75] G. L. Wilkes, S. Bagrodia, W. Humphries, and R. Wildnauer, "The time

dependence of the thermal and mechanical properties of segmented urethanes

following thermal treatment," Journal of Polymer Science: Polymer Letters

Edition, vol. 13, pp. 321-327, 1975.

[76] G. L. Wilkes and J. A. Emerson, "Time dependence of small‐angle x‐ray

measurements on segmented polyurethanes following thermal treatment," Journal

of Applied Physics, vol. 47, pp. 4261-4264, 1976.

[77] G. L. Wilkes and R. Wildnauer, "Kinetic behavior of the thermal and mechanical

properties of segmented urethanes," Journal of Applied Physics, vol. 46, pp.

4148-4152, 1975.

[78] J. T. Koberstein and A. F. Galambos, "Multiple melting in segmented

polyurethane block copolymers," Macromolecules, vol. 25, pp. 5618-5624, 1992.

[79] A. J. Ryan, C. W. Macosko, and W. Bras, "Order-disorder transition in a block

copolyurethane," Macromolecules, vol. 25, pp. 6277-6283, 1992.

[80] B. Chu, T. Gao, Y. Li, J. Wang, C. R. Desper, and C. A. Byrne, "Microphase

separation kinetics in segmented polyurethanes: effects of soft segment length and

structure," Macromolecules, vol. 25, pp. 5724-5729, 1992.

[81] J. T. Koberstein and T. P. Russell, "Simultaneous SAXS-DSC study of multiple

endothermic behavior in polyether-based polyurethane block copolymers,"

Macromolecules, vol. 19, pp. 714-720, 1986.

[82] W. Hu and J. T. Koberstein, "The effect of thermal annealing on the thermal

properties and molecular weight of a segmented polyurethane copolymer,"

Journal of Polymer Science Part B: Polymer Physics, vol. 32, pp. 437-446, 1994.

[83] J. T. Koberstein, I. Gancarz, and T. C. Clarke, "The effects of morphological

transitions on hydrogen bonding in polyurethanes: Preliminary results of

simultaneous DSC–FTIR experiments," Journal of Polymer Science Part B:

Polymer Physics, vol. 24, pp. 2487-2498, 1986.

[84] L. M. Leung and J. T. Koberstein, "Small‐angle scattering analysis of hard‐microdomain structure and microphase mixing in polyurethane elastomers,"

Journal of Polymer Science: Polymer Physics Edition, vol. 23, pp. 1883-1913,

1985.

135

[85] D. Nichetti and N. Grizzuti, "Determination of the phase transition behavior of

thermoplastic polyurethanes from coupled rheology/DSC measurements,"


[86] R. A. Phillips and S. L. Cooper, "Morphology development during solid‐state

annealing of poly (ether‐ester) multiblock copolymers," Journal of Polymer

Science Part B: Polymer Physics, vol. 34, pp. 737-749, 1996.

[87] J. Silva, R. Andrade, R. Huang, J. Liu, M. Cox, and J. M. Maia, "Rheological

behavior and structure development in thermoplastic polyurethanes under uniaxial

extensional flow," Journal of Non-Newtonian Fluid Mechanics, vol. 222, pp. 96-

103, 2015.

[88] J. Silva, D. Meltzer, J. Liu, M. Cox, and J. Maia, "The influence of thermo‐mechanical history on structure development of elastomeric and amorphous glass

thermoplastic polyurethanes," Polymer Engineering & Science, vol. 54, pp. 1383-

1393, 2014.

[89] Y. Xu, Z. Petrovic, S. Das, and G. L. Wilkes, "Morphology and properties of

thermoplastic polyurethanes with dangling chains in ricinoleate-based soft

segments," Polymer, vol. 49, pp. 4248-4258, 2008.

[90] S. Cossar, D. Nichetti, and N. Grizzuti, "A rheological study of the phase

transition in thermoplastic polyurethanes. Critical gel behavior and microstructure

development," Journal of Rheology (1978-present), vol. 48, pp. 691-703, 2004.

[91] D. Nichetti, S. Cossar, and N. Grizzuti, "Effects of molecular weight and

chemical structure on phase transition of thermoplastic polyurethanes," Journal of

Rheology (1978-present), vol. 49, pp. 1361-1376, 2005.

[92] P. J. Yoon and C. D. Han, "Effect of thermal history on the rheological behavior

of thermoplastic polyurethanes," Macromolecules, vol. 33, pp. 2171-2183, 2000.

[93] S. Yamasaki, D. Nishiguchi, K. Kojio, and M. Furukawa, "Effects of aggregation

structure on rheological properties of thermoplastic polyurethanes," Polymer, vol.

48, pp. 4793-4803, 2007.

[94] M. L. Sentmanat, "Miniature universal testing platform: from extensional melt

rheology to solid-state deformation behavior," Rheologica Acta, vol. 43, pp. 657-

669, 2004.

[95] V. C. Barroso, J. A. Covas, and J. M. Maia, "Sources of error and other

difficulties in extensional rheometry revisited: commenting and complementing a

recent paper by T. Schweizer," Rheologica acta, vol. 41, pp. 154-161, 2002.

[96] R. Mao, E. M. McCready, and W. R. Burghardt, "Structural response of an

ordered block copolymer melt to uniaxial extensional flow," Soft matter, vol. 10,

pp. 6198-6207, 2014.

136

5.8 Figures

Figure 5.1 -DSC curves of TPUs with the endothermic peak become more pronounced

and shifted to higher temperatures with increase of HS.

137

Figure 5.2 – Dynamic moduli versus time of TPUs at different temperatures: A) 26.4 wt.

% HS, B) 36.8 wt. % HS, and C) 55.6 wt. % HS where closed symbols represent G’, and

open symbols represent G” at each respective temperature.

139

Figure 5.3 - Transient elongational viscosity pre-annealing data for A) TPU with 26.4%

HB at 100 °C, B) TPU with 36.8% HB at 150 °C, and C) TPU with 55.6% HB at 195 °C.

The dashed lines represent the linear viscoelastic envelope.

140

Figure 5.4 - DSC curves for TPU samples before and after annealing.

141

Figure 5.5- SAXS patterns a) Comparison of differernt HS contentment and b) 36.8% HS

before and after annealing.

142

Figure 5.6 - WAXS before and after recrystallization of the 36.8 wt. % HS TPU.

143

Figure 5.7 - Frequency sweep data after isothermal annealing for A) TPU with 26.4% HS

at 70 °C B) TPU with 36.8% HS at 150 °C, and C) TPU with 55.6% HS at 195 °C.

144

Figure 5.8- Transient elongational viscosity data after isothermal annealing for A) TPU

with 36.8% HB at 150 °C, and B) TPU with 55.6% HB at 195 °C. The dashed lines

represent the linear viscoelastic envelope.

145

Figure 5.9- Top: Images of viscoelastic rupture under extensional flow (left) and a brittle-

like (right) for TPU with 36.8% HS. Bottom: Images of TPU with 36.8% after cessation

of flow before recrystallization (right) and after (left).

146

5.9 Tables

Sample Mn Mw Đ

26.4% HS - Pre 1.50 X 104 2.85 X 104 1.90

26.4% HS - Post 2.10X104 4.06 X 104 1.93

36.8% HS – Pre 1.71 X104 3.34 X 104 1.96

36.6% HS - Post 2.43 X 104 4.68 X 104 1.93

55.6% HS - Pre 1.88 X 104 3.83 X 104 2.03

55.6% HS - Post 2.64X 104 5.41 X 104 2.04

Table 5.1– Molecular weight information of TPU before and after annealing.

147

Chapter 6 : Contribution of Inflow Conditions on

Residence Time Output in Extrusion Adapter Analysis


Journal article:

J. L. Gadley and J. M. Maia To Be Submitted (Journal of Polymer Engineering)

148

6.1 Abstract

Simulations using finite element methods (FEM) are commonly used when

designing extrusion components and processes. However, oversimplification of these

techniques provides a significant potential for generating misleading information. These

techniques are most commonly applied to extrusion dies and have also been used to study

flow within extruder barrels. This work focuses on the adapter located between these

components which is commonly over looked. The focus of this study was to understand

the effect of inflow conditions into the adapter on the residence time within the adapter. To

accomplish this, a symmetric adapter was designed and studied with different inflow

boundary conditions. Through using a fully developed inflow, an inflow condition with

rotational flow, and an imbalanced inflow condition, this work indicated a drastic change

in the residence time predicted. These results demonstrate the potential for misleading

information from simulation and that careful considerations must be made depending on

the system of interest.

149

6.2 Introduction

Understanding the influence of industrial processing conditions on the final product

characteristics is crucial to efficient conversion of polymeric materials into a useful form.

In order to bridge the gap between processing conditions, processing stability, and product

quality computational studies are commonly used. Many simulations have been performed

for a multitude of reasons to troubleshoot and optimize the extrusion process. Even with

the aid of powerful computing techniques, simulating the entire process of extrusion is

impractical therefore these efforts are generally focused on a specific extrusion

component.[97, 98] Optimizing the flow within the die of an extruder for example is

commonly performed to meet the requirements of a particular product goal and is utilized

to troubleshoot the cause of defects within an extrudate.[99-102] There are many different

variables which effect the material forming process during extrusion. The thermal history

and flow conditions to which polymers are exposed vary drastically between different

stages of the extrusion process. For example, the pressures and shear rates present within

the extruder barrel affect the material much differently than flowing through the die and

forming into the final product. However, the flow behavior in any specific portion of the

extrusion process is dependent on all these factors occurring simultaneously throughout all

regions of the equipment, adding to the complexity of accurately treating simulation of

these systems. Since extrusion is very commonly used in industry to continually produce

products at high throughput, increasing efficiency of the process and reducing the number

of defects produced provides potential to drastically reduce cost. Because of the high

throughput potential of industrial extrusion processes, using experience based trial and

error techniques to optimize the process is a very time and resource intensive endeavor. In

150

order to alleviate these difficulties, simulation techniques are commonly used to aid in

identifying and avoiding processing inefficiencies.

Computational investigation of many different aspects of a twin screw extruder

have been carried out under a multitude of conditions and studies seeking the most accurate

technique for capturing realistic behavior in these systems. Of these studies, many have

been focused on understanding the complex flow behavior which exists within the barrel

of the extruder itself. [103-112] These studies have been performed across a wide array of

approaches and complexity but have shown that the capturing of proper flow characteristics

is very important to understanding the process albeit a difficult endeavor. Understanding

how these flow conditions develop and influence material characterizations have been

approximated many different ways particularly concerning the mixing characteristics

within kneading sections of the screw. [103-108, 110-114] While understanding how

materials are affected by processing conditions within the barrel is very important

information, these simulations alone are insufficient for many product issues due to flow

through other downstream influences on the material. It is well known that proper design

of an extrusion die is crucial to achieving the desired results within an extrusion process.

Properly balancing flow behavior and mitigating stagnant areas has been subjected to a

plethora of simulation studies depending on particular goals. Previous studies have shown

a significant influence of die design on the characteristics of the extrudate. However, die

design remains more of an art tailored to a specific need. [115] Through experience and

more recent simulation work, many die design guidelines have been implemented to

excellent effect in order to produce the targeted extrudate characteristics. Through the

simulations of screw elements and dies for thermally sensitive material, residence time and

151

thermal history to which the material is exposed are generally the primary focus. While

finite element methods (FEM) have been used extensively to model flow behavior in the

barrel and die of extrusion equipment, less effort has been focused on understanding how

flow between the extruder screws and entrance into the die affects the process, particularly

in thermally sensitive materials.

The focus of this particular work was to utilize FEM simulation to understand how

the flow through an adapter between the extruder barrel and the die itself could promote

degradation in thermally sensitive materials. When performing any FEM simulation as

previously noted, achieving a result which is physically representative of the actual process

is of the utmost importance. Capturing this behavior presents a challenge when dealing

with transient behavior which exists when material exits from the tips of the screws. While

every type of extrusion process would require a different treatment of this flow depending

on the flow characteristics of the system, this work focused on a low speed counter-rotating

twin screw extruder. Under these conditions, the melt in the extruder barrel tends to

preferentially travel along the top of the screws which results in entering the adapter under

unbalanced conditions. Most commonly a setup such as this would be used with an adapter

which symmetrically contracts the flow of the material down from the size of the extruder

barrel at the exit to the inflow geometry of the die for profile extrusion. The primary scope

of this work was to understand how altering these inflow conditions affected the residence

time within the adapter and to help understand the most feasible approach to capturing this

type of inflow condition within the FEM software. Specifically, this investigation was

focused on detecting the presence of stagnation zones within a die adapter under flow

conditions consistent with counter rotating twin screw extruders. In order to accomplish

152

this task, simulations were performed on the same adapter model using the same material

conditions. While these parameters were kept constant, the treatment of the inflow

boundary conditions were changed to investigate the influence of different adapter entry

flows on stagnation points within the adapter.

6.3 Experimental

6.3.1 Simulation Setup and Parameters

The simulations in this work were performed using ANSYS® POLYFLOW® CFD

(computational fluid dynamics) FEM software. Initially the desired flow channel

dimensions were modeled using computer-aided design (CAD) software (SolidWorks).

The imported flow geometry was then meshed using tetrahedral elements in order to

generate a 3D model of the extrusion adapter. This particular adapter geometry shown in

Figure 6.1, was utilized to eliminate the potential for introducing geometric imbalances

into the simulation through maintaining a symmetric design. The adapter geometry was

specifically designed as symmetric in order to eliminate any unnecessary complex behavior

resulting from geometric influences. The adapter contraction was also designed

representative of what would be used in practice to transfer polymer melt from an extruder

barrel to an extrusion die. Using this geometry, the mesh was generated through using

POLYFLOW® and was refined to ensure adequate resolution was attainable through the

simulation without creating too high of a mesh density which would lead to prohibitively

high computational cost as shown in Figure 6.2A. These simulations consisted of using

identical geometry for all conditions through only varying boundary conditions at the

(6.1)

153

inflow boundary. Across all simulation iterations performed, the mesh was generated using

identical conditions and resulted dividing the model into approximately 300,000 elements.

A gradient was applied to the mesh density to ensure the inflow boundary and thinner flow

channel regions contained adequate resolution during the simulations. Utilizing this

computational mesh, isothermal flow simulations were carried out under the assumption

that the fluid within the channel was incompressible. In order to accurately capture the

material behavior within the flow channel, the polymer was assumed to adhere to the Phan-

Thien-Tanner (PTT) model. Generally, Newtonian conditions were used initially to

troubleshoot these simulations and in the case of more complex inflow conditions, these

Newtonian conditions were used as comparison to aid in understanding the effect of the

inflow conditions on different in-channel material behaviors. A single mode PTT model

was used as shown in Equation 6.1. The PTT representation was used because the model

incorporates an exponential relaxation coefficient which enables the prediction of biaxial

elongation and step shear during flow while maintaining accuracy near startup values and

is known to reliably fit experimental data. [116] Specifically, this equation is represented

in POLYFLOW® using the form shown in Equation 6.2.

Evaluating this form the total extra stress tensor, τ, is divided into two separate components

representing viscous and extensional behavior. The first term of Equation 6.2 contains the

material parameter, ξ, which represents the shear thinning characteristics of a fluid. The

second term in the equation represents the contribution of extensional stress in the system.

This exponential term was chosen due to the known agreement with experimental results

which contains the material parameter, ε. This material parameter is used to correct for the

(6.2)

154

extensional stress component during flow of complex materials such as the polymers used

in this case. Through treating the materials with the PTT model in this manner, the

influence of flow on the material behavior was accounted for in the most realistic way

possible for the simulations in this work.

The boundary conditions were applied to the model as shown in Figure 6.2 B-D.

The inflow boundary condition was maintained at an industrially relevant flow rate of 360

kg/hr which was applied across the face of the inflow surface of the adapter. These inflow

conditions were adjusted to include different inflow behavior to mimic material exiting

from the extruder screws. Configurations used to help understand this effect included a

fully developed inflow along the entire inflow face, rotational inner walls with fully

developed inflow, and a systematically imbalanced inflow surface. Using these inflow

conditions, generic PTT parameters were selected and are shown in Table 6.1

Relaxation Time (sec)

Viscosity (Pa s)

ξ ε Flow Rate Kg/hour

1.0 50,000 0.30 0.20 360

Table 6.1 - PTT Input Parameters for Simulations

The PTT parameters used for this study were selected as generic values which are

synonymous with processing amorphous polymers which would act as a representative

model for industrially processed materials with high thermal sensitivity such as poly(vinyl

chloride) (PVC). [98, 117]


The overall scope of this work was to use FEM to understand the importance of

capturing the complex conditions present as polymeric material exits the extruder screws

and enters a production component, namely a die adapter. More specifically the goal was

155

to understand the effect of varying these conditions on the residence time within the adapter

and to determine which inflow conditions most accurately mimic the production

conditions. The first simulation was performed under the condition which assumes the

material enters the adapter equally across the whole surface and under fully developed

conditions. While these assumptions are highly simplified, these types of assumptions are

often made when simulating industrial processes and in many cases turn out to adequately

predict behavior and troubleshoot simple issues. This method also holds the advantage of

requiring minimal computational load.

The initial part of this study focused on a simple inflow condition where the

material was assumed to be flowing under fully developed flow conditions from between

the tip of the extruder screws and the outside wall of the barrel and adapter. The images

shown in Figure 6.3 demonstrate the inflow boundary which was used with an evenly

disturbed flow rate and the resultant residence time and velocity plots. Through

examination of the velocity plots, the observation was made that the highest velocity

regions are in the center where all the entering material passes the screw tips and travels

throughout the remainder of the adapter as would be expected. The lowest velocity regions

are mainly focused at the contraction to the smallest channel dimension for entry into a die

downstream. These low velocity regions are confirmed by the residence time plots through

showing the material dwells longest in these areas. Through comparison of the velocity

and residence time plots it becomes immediately apparent that the contraction is having the

largest effect coupled with the flow length, where the longest flow length passes near the

walls at the contraction, contributing to the increased residence time. However, under these

conditions there was no evidence of recirculation which would lead to a stagnation zone

156

and the highest residence time present was predicted at 353 sec. While in some very

thermally sensitive materials this could certainly cause an issue the volume of material in

this area is very small and the average residence time in the adapter was less than 30 sec.

These results indicate the constriction through changing cross sections is partially

contributing to higher residence times, even in a symmetric adapter, but is likely amplified

by conditions missed due to oversimplification.

The second iteration simulated in this study focused on adding a rotational

component to the inflow condition in order to better imitate flow conditions expected as a

result of screw tip rotation during exit of the extruder barrel. A visualization of this inflow

is shown in Figure 6.4A. Due to limitations with convergence of the simulation, the

rotational component could not be added directly to the inflow face. Instead, the inflow

conditions were maintained from the previous simulation and the fluid surface at inner wall

of the adapter which is representative of where the screw tips contact the fluid, were

rotated. The fluid surface applied a rotational velocity component of 10 RPM as is

commonly used in counter rotating industrial applications. Additionally, for simplification

this particular simulation was run using a Newtonian fluid in order to ensure model

convergence. Under these flow conditions the velocity and residence times responded by a

slightly exaggerated region of lower velocity and increased residence times. Interestingly,

the addition of the rotational component to the surface of the screws provided an imbalance

as one would expect as material exits the extruder screws during counter rotational

operation even with a fully developed inflow on the entire starting surface. Further

investigation of the residence time and velocity plots show the behavior due to the

contraction was retained in this case similarly to the initial simplified simulation. However,

157

particularly the velocity values calculated show the tendency for backflow to form near the

screw tips on the opposite side to the high velocity region. Due to the rotation in the flow

this low flow area then tends to end up in the increased residence time areas near the

contraction as well. Through these compounding effects the residence time was increased

by approximately 60 sec but a larger volume of material was exposed to these conditions

than predicted under the initial simplified conditions. In order to make a more direct

comparison to the simplified inflow conditions the same rotational inflow conditions were

performed using a PTT model fluid.

Using rotational flow coupled with the PTT model confirmed that the effect caused

by rotation also existed under real material behavior conditions. Although, after adding the

material model the residence time in the channel was slightly affected as shown in Figure

6.5, but the simulation time increased to 82 hours. These results indicate the material’s

property dependence on the flow conditions caused an increase in maximum residence time

but a decrease in the average time for the fluid to pass through the adapter. While the

change is relatively small this provides two points to further understand stagnation zones

in a symmetric adapter. First, the inflow conditions significantly affect the residence time

and velocity profiles expected in the system. Secondly, using the proper material model is

of known importance to accurate results but in this case has a minimal effect compared to

the inflow condition chosen. It should be noted that the approximate 60 sec further increase

in maximum residence time with the PTT model is important to further understanding how

a stagnation zone behaves from a compounding effect standpoint. Although these results

are useful in understanding how high residence time regions form in a very simple flow

158

channel during processing, it is likely the fully developed inflow boundary surface was still

dominating the behavior within the adapter causing deviation from realistic behavior.

The final simulation was carried out in an attempt to emulate the expected flow

imbalance while minimizing computational time and eliminating the dominance of the

fully developed system. To accomplish this, a unique inflow condition was added to the

model to force an imbalance without the computational complexity of adding rotation as

shown in Figure 6.6. Concentrating 65% of the material in to the smallest region, 25%

through the moderate region, and the remaining 10% in the largest region of inflow was

based on counter rotation behavior from literature and through visual observation of

counter rotating twin screw extruders with the screw the die and adapter removed. The

simulation results of residence time and flow velocity under these conditions are depicted

in Figure 6.7. These results indicate a similar imbalance that was predicted when using the

rotational surface conditions. However, these conditions removed the bias that was noted

previously by imposing high velocities along the entire inflow face. This means a similar

extent of imbalance was captured but the imbalances in velocity were also accounted for

with this configuration. Qualitatively these results aligned much better with what was

observed from experience with the twin screw extruder. Quantitatively these results

showed a significant increase in residence time with a stagnation zone near the screw tips,

near the barrel outlet opposite the side of high inflow and at the contraction as well. Under

these conditions the maximum residence time increased to 607 sec with an average

residence time of 58 sec. These results align physically with what is observed and indicates

that with some material systems this component of the extrusion process could cause an

unwanted high residence time area.

159

Overall, a comparison was formed between the different inflow approaches which

could potentially be used to simulate an extrusion adapter and is displayed in Figure 6.8.

The plot shown here indicates that a generic fully developed inflow surface under predicts

the total residence time in the system, even with a realistic material model. While adding

rotation to the system moved towards more realistic behavior, the residence time

distribution shown in Figure 6.9 suggests the presence of the fully developed inflow surface

still had a dominant effect on the results. In order to capture the results nearest to those

experienced in industry the imbalanced inflow approach appeared to provide the most

useful insight to flow though the adapter. It is apparent that regardless of the simulation

setup, the first 70% of the material passes through the adapter very similarly. However,

when considering stagnation regions, particularly for thermally sensitive systems, the few

percent remaining for extremely long times would be the most troublesome. Through

providing an observation based imbalance the maximum expected residence time increased

from 363 sec to 607 sec and the average residence time increased from 28 sec to 58 sec.

Additional, 95% of the adapter volume cleared the channel in 140 sec in the simplified case

versus 249 sec in the most complex case. This work showcases the importance of looking

at every component of the extrusion process, not only the flow behavior in the screws and

die as is commonly done. Furthermore, while it is well known that accurate simulation

results are highly dependent on all simulation inputs, this work specifically shows how

important matching inflow conditions with realistic behavior could affect results and

ultimately design decisions.

160

6.5 Conclusions

Understanding the interaction between components along an extrusion line is very

important to eliminating production based imperfections in the extrudate. In order to

understand these effects, simulations are commonly used to identify processing or

component design issues. The focus of this work was to understand the influence of flow

through an extrusion adapter on residence time within the process. To understand this

behavior different inflow conditions were investigated to elucidate how to efficiently study

extrusion adapters and study the effect of inflow conditions on these measurements.

The simulations completed within this work demonstrated that the technique used

to emulate material entering the adapter from the screws is very influential on the

simulation results. These results suggest that while assuming the a fully developed flow

field exists at this transition mitigate simulation time, these inflow conditions tend to

grossly under predict residence time results due to over simplification. A hybrid approach

was taken to implement the addition of screw rotation to the interface to help offset the

fully developed flow condition applied. While these conditions lead to a more realistic flow

profile, the fully developed inflow boundary condition tends to dominate the residence time

results by under predicting residence times while requiring prohibitively high

computational cost. In order to further refine the inflow conditions to the adapter, a gradient

in flow rate was introduced based on known counter rotating twin screw extruder flow

behavior preferentially along one side of the screws. This approach lead to significantly

higher residence times and lower computation times. The behavior captured through this

technique provide insight to why extrusion adapters appear to play a significant role in

increased residence time which is problematic for processing thermally sensitive materials.

161

Even with a very simple symmetric design, relatively high residence times may result from

the nature of material flowing out of the extruder screws and into downstream components.

While every simulation must be evaluated on a case by case basis, this work

demonstrates the importance of considering how to accurately imitate the conditions

experienced in practice. Through simply treating the same simulation with different inflow

boundary conditions, it was demonstrated that a drastic change in response variable was

measured. These observations make evident how easily component design or processing

changes may be misled based on over simplified simulation conditions. Although

simulations are powerful tools, their use must be carefully considered to ensure accuracy.

6.6 References

[97] O. S. Carneiro and J. M. Nobrega, "Recent developments in automatic die design

for profile extrusion," Plastics Rubber and Composites, vol. 33, pp. 400-408,

2004.

[98] P. D. Gujrati and A. I. Leonov, Modeling and simulation in polymers. Weinheim:

Wiley-VCH, 2010.

[99] J. F. T. Pittman, "Computer-aided design and optimization of profile extrusion

dies for thermoplastics and rubber: a review," Proceedings of the Institution of

Mechanical Engineers Part E-Journal of Process Mechanical Engineering, vol.

225, pp. 280-321, Nov 2011.

[100] I. Szarvasy, J. Sienz, J. F. T. Pittman, and E. Hinton, "Computer aided

optimisation of profile extrusion dies - Definition and assessment of the objective

function," International Polymer Processing, vol. 15, pp. 28-39, Mar 2000.

[101] J. M. Nobrega, O. S. Carneiro, P. J. Oliveira, and F. T. Pinho, "Flow balancing in

extrusion dies for thermoplastic profiles Part I: Automatic design," International

Polymer Processing, vol. 18, pp. 298-306, Sep 2003.

[102] J. M. Nobrega, O. S. Carneiro, F. T. Pinho, and P. J. Oliveira, "Flow balancing in

extrusion dies for thermoplastic profiles - Part III: Experimental assessment,"

International Polymer Processing, vol. 19, pp. 225-235, Sep 2004.

[103] H. Cheng and I. Manas‐Zloczower, "Study of mixing efficiency in kneading discs

of co‐rotating twin‐screw extruders," Polymer Engineering & Science, vol. 37, pp.

1082-1090, 1997.

162

[104] H. F. Cheng and I. Manas-Zloczower, "Distributive mixing in conveying elements

of a ZSK-53 co-rotating twin screw extruder," Polymer Engineering and Science,

vol. 38, pp. 926-935, Jun 1998.

[105] A. S. Fard and P. D. Anderson, "Simulation of distributive mixing inside mixing

elements of co-rotating twin-screw extruders," Computers & Fluids, vol. 87, pp.

79-91, Oct 25 2013.

[106] A. S. Fard, M. A. Hulsen, H. E. H. Meijer, N. M. H. Famili, and P. D. Anderson,

"Tools to Simulate Distributive Mixing in Twin-Screw Extruders,"

Macromolecular Theory and Simulations, vol. 21, pp. 217-240, May 2012.

[107] T. Ishikawa, T. Amano, S. I. Kihara, and K. Funatsu, "Flow patterns and mixing

mechanisms in the screw mixing element of a co-rotating twin-screw extruder,"

Polymer Engineering and Science, vol. 42, pp. 925-939, May 2002.

[108] T. Ishikawa, S. I. Kihara, and K. Funatsu, "3-D non-isothermal flow field analysis

and mixing performance evaluation of kneading blocks in a co-rotating twin srew

extruder," Polymer Engineering and Science, vol. 41, pp. 840-849, May 2001.

[109] T. Kajiwara, Y. Nagashima, Y. Nakano, and K. Funatsu, "Numerical study of

twin-screw extruders by three-dimensional flow analysis - Development of

analysis technique and evaluation of mixing performance for full flight screws,"

Polymer Engineering and Science, vol. 36, pp. 2142-2152, Aug 1996.

[110] B. Vergnes, G. Della Valle, and L. Delamare, "A global computer software for

polymer flows in corotating twin screw extruders," Polymer Engineering and

Science, vol. 38, pp. 1781-1792, Nov 1998.

[111] H. H. Yang and I. Manas‐Zloczower, "Flow field analysis of the kneading disc

region in a co‐rotating twin screw extruder," Polymer Engineering & Science, vol.

32, pp. 1411-1417, 1992.

[112] X. M. Zhang, L. F. Feng, W. X. Chen, and G. H. Hu, "Numerical Simulation and

Experimental Validation of Mixing Performance of Kneading Discs in a Twin

Screw Extruder," Polymer Engineering and Science, vol. 49, pp. 1772-1783, Sep

2009.

[113] T. Avalosse, Y. Rubin, and L. Fondin, "Non-isothermal modeling of co-rotating

and contra-rotating twin screw extruders," Journal of Reinforced Plastics and

Composites, vol. 21, pp. 419-429, 2002.

[114] H. E. H. Meijer and P. H. M. Elemans, "The Modeling of Continuous Mixers .1.

The Corotating Twin-Screw Extruder," Polymer Engineering and Science, vol.

28, pp. 275-290, Mar 1988.

[115] H. F. Giles, J. R. Wagner, and E. M. Mount, Extrusion : the definitive processing

guide and handbook. Norwich, NY: William Andrew Pub., 2005.

[116] R. G. Larson, Constitutive equations for polymer melts and solutions. Boston:

Butterworths, 1988.

[117] T. Glomsaker, E. L. Hinrichsen, F. Irgens, and P. Thorsteinsen, "Numerical

simulation of extrusion of S-PVC formulations in a capillary rheometer,"

Rheologica Acta, vol. 39, pp. 80-96, Jan 2000.

163

6.7 Figures

Figure 6.1 – 3D Model rendering of extrusion adapter used in simulation studies.

164

Figure 6.2 – Extrusion adapter geometry showing meshed flow channel (A), inflow

boundary condition (B), zero wall velocity boundary surface (C), and outflow boundary

surface (D).

165

Figure 6.3 – Volume rendering of residence time (A), simplified inflow condition (B),

and velocity streamline plot (C).

166

Figure 6.4 – Helical inflow condition with Newtonian material characteristics (A) used to

generate a volume rendering of residence time (B), and velocity streamline plot (C).

167

Figure 6.5 - Helical inflow condition with PTT model material (A) used to generate a

volume rendering of residence time (B), and velocity streamline plot (C).

168

Figure 6.6 – Depiction of purposely imbalanced inflow conditions based on material

exiting a counter-rotating twin screw extruder.

169

Figure 6.7 – Residence time volume rendering plot (A) shown with velocity streamline

plot (B) of an adapter with imbalanced inflow boundary conditions.

170

Figure 6.8– Comparison plot of material volume remaining versus residence time

between different adapter inflow boundary conditions.

0 100 200 300 400 500 6000

20

40

60

80

100

Vo

lum

e %

Re

ma

inin

g

Time (sec)

Simple Inflow

Complex Inflow

Rotational

Rotational PTT

171

1E-4 0.001 0.01 0.1 1-50

0

50

100

150

200

250

300

350

Volu

me

Ele

me

nt

Co

unts

t/tmax

Simple-PTT

Rot-Newtonian

Rot-PTT

Complex-PTT

1E-4 0.001 0.01 0.1 1

Offset V

olu

me E

lem

ent C

ount

t/tmax

Complex-PTT

Rot-PTT

Rot-Newtonian

Simple-PTT

Figure 6.9 - Residence time distribution plots of various inflow conditions and material

models.

172

Bibliography






pp. 481-487, 2006.



2009.


1999.



1995.




[7] G. G. Odian, Principles of polymerization, 4th ed ed. Hoboken, N.J.: Wiley, 2004.

[8] Z. Wirpsza and T. J. Kemp, Polyurethanes : chemistry, technology, and

applications. Chichester ; New York: E. Horwood, 1993.







[11] S. Ajithkumar, S. S. Kansara, and N. K. Patel, "Kinetics of Castor Oil Based

Polyol-Toluene Diisocyanate Reactions," European Polymer Journal, vol. 34, pp.

1273-1276, 1998.


segemented, high molecular weight, aliphatic poly(ether-urea) copolymers in


5829-5836, 2004.

[13] S. Parnell, K. Min, and M. Cakmak, "Kinetic Studies of Polyurethane

Polymerization with Raman Spectroscopy," Polymer, vol. 44, pp. 5137-5144,

2003.

[14] C. Dubois, S. Desilets, A. Ait-Kadi, and P. Tanguy, "Bulk Polymerization of

Hydroxyl Terminated Polyubutadiene (HTPB) with Toluene Diisocyanate (TD):

A Kinetics Study using 13C-NMR," Journal of Applied Polymer Science, vol. 58,

pp. 827-834, 1995.

[15] D. Kincal and S. Ozkar, "Kinetic Study of the Reaction Between Hydroxyl-

Terminated Polybutadiene and Isophorone Diisocyanate in Bulk by Quantitative

FTIR Spectroscopy," Journal of Applied Polymer Science, vol. 66, pp. 1979-

1983, 1997.

173

[16] V. Sekkar, K. Ambika, and K. Ninan, "Rheo-kinetic Evaluation on the Formation

of Urethane Networks Based on Hydroxyl-Terminated Polybutadiene," Journal of

Applied Polymer Science, vol. 79, pp. 1869-1876, 2001.

[17] O. Gunter, Polyurethane Handbook. Munich: Hanser Publishers, 1985.

[18] L. L. Ferstandig and R. A. Scherrer, "Mechanism of Isocyanate Reactions with

Ethanol," Journal of American Chemical Society, vol. 81, pp. 4838-4842, 1959.

[19] H. G. Wissman, L. Rand, and K. C. Frisch, "Kinetics of Polyether Polyols-

Diisocyanate Reactions," Journal of Applied Polymer Science, vol. 8, pp. 2971-

2978, 1964.

[20] M. Gambiroza-jukic, Z. Gomzi, and H. J. Mencer, "Kinetic Analysis of Bulk

Polymerizatio of Diisocyanate and Polyol," Journal of Applied Polymer Science,

vol. 47, pp. 513-519, 1993.

[21] A. H. Navarchian, F. Picchioni, and L. P. B. M. Janssen, "Rheokinetics and Effect

of Shear Rate on the Kinetics of Linear Polyurethane Formation," Polym. Eng.

Sci., vol. 45, pp. 279-287, 2005.

[22] R. G. Pearson and E. L. Williams, "Interfacial Polymerization of an Isocyanate

and a Diol," Journal of Polymer Science Part a-Polymer Chemistry, vol. 23, pp.

9-18, 1985.

[23] R. G. Pearson and E. L. Williams, "A Comparison of Heterogeneous and

Homogeneous Kinetics in the Formation of a Polyurethane," Journal of Polymer

Science Part a-Polymer Chemistry, vol. 25, pp. 565-573, Feb 1987.

[24] R. W. Seymour, G. M. Estes, and S. L. Cooper, "Infrared Studies of Segmented

Polyurethan Elastomers .1. Hydrogen Bonding," Macromolecules, vol. 3, pp. 579-

&, 1970.

[25] G. M. Estes, S. L. Cooper, and A. V. Tobolsky, "Block Polymers and Related

Heterophase Elastomers," Journal of Macromolecular Science-Reviews in

Macromolecular Chemistry, vol. C 4, pp. 313-&, 1970.





of polyurethane melts. 2. Effect of block incompatibility on the microstructure,"

Macromolecules, vol. 33, pp. 382-394, Jan 25 2000.




[29] A. Kintanar, L. W. Jelinski, I. Gancarz, and J. T. Koberstein, "Complex

Molecular Motions in Bulk Hard-Segment Polyurethanes - a Deuterium Nmr-

Study," Macromolecules, vol. 19, pp. 1876-1884, Jul 1986.




[31] S. Yamasaki, D. Nishiguchi, K. Kojio, and M. Furukawa, "Effects of

polymerization method on structure and properties of thermoplastic

polyurethanes," Journal of Polymer Science Part B-Polymer Physics, vol. 45, pp.

800-814, Apr 1 2007.

174

[32] C. Dubois, S. Desilets, A. Aitkadi, and P. Tanguy, "Bulk-Polymerization of

Hydroxyl Terminated Polybutadiene (Htpb) with Tolylene Diisocyanate (Tdi) - a

Kinetics Study Using C-13-Nmr Spectroscopy," Journal of Applied Polymer

Science, vol. 58, pp. 827-834, Oct 24 1995.

[33] D. Kincal and S. Ozkar, "Kinetic study of the reaction between hydroxyl-

terminated polybutadiene and isophorone diisocyanate in bulk by quantitative

FTIR spectroscopy," Journal of Applied Polymer Science, vol. 66, pp. 1979-1983,

Dec 5 1997.

[34] H. Kothandaraman and A. S. Nasar, "The Kinetics of the Polymerization Reaction

of Toluene Diisocyanate with Polyether Polyols," Journal of Macromolecular

Science-Pure and Applied Chemistry, vol. A31, pp. 339-350, 1994.

[35] S. Parnell, K. Min, and M. Cakmak, "Kinetic studies of polyurethane

polymerization with Raman spectroscopy," Polymer, vol. 44, pp. 5137-5144, Aug

2003.


segmented, high molecular weight, aliphatic poly(ether-urea) copolymers in


5829-5836, Aug 5 2004.



Oct 2009.

[38] E. B. Richter and C. W. Macosko, "Kinetics of Fast (Rim) Urethane

Polymerization," Polymer Engineering and Science, vol. 18, pp. 1012-1018,

1978.

[39] G. Anzuino, A. Pirro, O. Rossi, and L. P. Friz, "Reaction of Diisocyanates with

Alcohols .1. Uncatalyzed Reactions," Journal of Polymer Science Part a-Polymer

Chemistry, vol. 13, pp. 1657-1666, 1975.

[40] M. Cioffi, K. J. Ganzeveld, A. C. Hoffmann, and L. P. B. M. Janssen,

"Rheokinetics of linear polymerization. A literature review," Polymer

Engineering and Science, vol. 42, pp. 2383-2392, Dec 2002.

[41] E. Papadopoulos, M. Ginic-Markovic, and S. Clarke, "Reaction Kinetics of

Polyurethane Formation Using a Commercial Oligomeric Diisocyanate Resin

Studied by Calorimetric and Rheological Methods," Macromolecular Chemistry

and Physics, vol. 209, pp. 2302-2311, Nov 24 2008.

[42] J. S. Martin, J. M. Laza, M. L. Morras, M. Rodriguez, and L. M. Leon, "Study of

the curing process of a vinyl ester resin by means of TSR and DMTA," Polymer,

vol. 41, pp. 4203-4211, May 2000.

[43] A. Y. Malkin and S. G. Kulichikhin, "Rheokinetics of Curing," Advances in

Polymer Science, vol. 101, pp. 217-257, 1991.

[44] J. M. Laza, J. L. Vilas, F. Mijangos, M. Rodriguez, and L. M. Leon, "Analysis of

the crosslinking process of epoxy-phenolic mixtures by thermal scanning

rheometry," Journal of Applied Polymer Science, vol. 98, pp. 818-824, Oct 15

2005.

[45] M. M. Coleman, K. H. Lee, D. J. Skrovanek, and P. C. Painter, "Hydrogen-

Bonding in Polymers .4. Infrared Temperature Studies of a Simple Polyurethane,"

Macromolecules, vol. 19, pp. 2149-2157, Aug 1986.

175

[46] J. Blackwell and K. H. Gardner, "Structure of the Hard Segments in Polyurethane

Elastomers," Polymer, vol. 20, pp. 13-17, 1979.

[47] M. Furukawa, Y. Mitsui, T. Fukumaru, and K. Kojio, "Microphase-separated

structure and mechanical properties of novel polyurethane elastomers prepared

with ether based diisocyanate," Polymer, vol. 46, pp. 10817-10822, Nov 21 2005.




[49] M. S. Sanchez-Adsuar, E. Papon, and J. J. Villenave, "Influence of the synthesis

conditions on the properties of thermoplastic polyurethane elastomers," Journal of

Applied Polymer Science, vol. 76, pp. 1590-1595, Jun 6 2000.



Part II. Relationship between the prepolymer and polyurethane properties,"

Journal of Applied Polymer Science, vol. 76, pp. 1602-1607, Jun 6 2000.



Part 1. Prepolymer characterization," Journal of Applied Polymer Science, vol.

76, pp. 1596-1601, Jun 6 2000.

[52] A. Y. Malkin, "Rheology in polymerization processes," Polymer Engineering &

Science, vol. 20, pp. 1035-1044, 1980.

[53] A. Y. Malkin, S. G. Kulichikhin, D. N. Emel'yanov, and I. E. Smetanina,

"Rheokinetics of free-radical polymerization," Polymer, vol. 25, pp. 778-784,

1984.

[54] V. W. A. Verhoeven, M. P. Y. Van Vondel, K. J. Ganzeveld, and L. P. B. M.

Janssen, "Rheo-kinetic measurement of thermoplastic polyurethane

polymerization in a measurement kneader," Polymer Engineering and Science,

vol. 44, pp. 1648-1655, Sep 2004.

[55] S. D. Lipshitz and C. W. Macosko, "Kinetics and energetics of a fast polyurethane

cure," Journal of Applied Polymer Science, vol. 21, pp. 2029-2039, 1977.

[56] S. D. Lipshitz, F. G. Mussati, and C. W. Macosko, "Kinetic and viscosity

relations for urethane network polymerizations," Soc. Plastics Eng. Antec. Tech.

Papers21, pp. 239-241, 1975.

[57] C. S. Schollenberger, K. Dinbergs, and F. D. Stewart, "Thermoplastic

Polyurethane Elastomer Melt Polymerization Study," Rubber Chemistry and

Technology, vol. 55, pp. 137-150, 1982.

[58] B. J. Briscoe, M. B. Khan, and S. M. Richardson, "Rotary injection takes RIM to

new frontiers," Plastics and rubber international, vol. 13, pp. 20-24, 1988.

[59] P. Krol and Z. Wietrzynskalalak, "Study on Structure and Properties of

Carbamates as Model Compounds for Urethane Polymers," European Polymer

Journal, vol. 31, pp. 689-699, Jul 1995.

[60] I. Y. Gorbunova, M. L. Kerber, I. N. Balashov, S. I. Kazakov, and A. Y. Malkin,

"Cure rheokinetics and change in properties of a phenol-urethane composition:

Comparison of results obtained by different methods," Polymer science. Series A,

Chemistry, physics, vol. 43, pp. 826-833, 2001.

176

[61] A. Y. Malkin, S. G. Kulichikhin, and G. K. Shambilova, "Strain Effect on the

Phase State of Polyvinyl Acetate Solutions," Vysokomolekulyarnye Soedineniya

Seriya B, vol. 33, pp. 228-231, Mar 1991.

[62] N. G. Gaylord and N. V. Seeger, "Polyurethanes: chemistry and technology, Part

I. (High Polymers, Vol. XVI). J. H. SAUNDERS and K. C. FRISCH,

Interscience, New York, 1962. ," Journal of Applied Polymer Science, vol. 8, pp.

1498-1499, 1964.

[63] G. n. Oertel and L. Abele, Polyurethane handbook : chemistry, raw materials,

processing, application, properties, 2nd ed. Munich ; New York

Cincinnati: Hanser ;

Hanser/Gardner distributor, 1994.

[64] S. B. Clough and N. S. Schneider, "Structural studies on urethane elastomers,"

Journal of Macromolecular Science, Part B, vol. 2, pp. 553-566, 1968/12/01

1968.

[65] G. M. Estes, R. W. Seymour, and S. L. Cooper, "Infrared Dichroism of

Segmented Polyurethane Elastomers - Poly," Abstracts of Papers of the American

Chemical Society, pp. 26-&, 1970.

[66] J. A. Koutsky, N. V. Hien, and S. L. Cooper, "Some Results on Electron

Microscope Investigations of Polyether-Urethane and Polyester-Urethane Block

Copolymers," Journal of Polymer Science Part B-Polymer Letters, vol. 8, pp.

353-&, 1970.

[67] A. Frick and A. Rochman, "Characterization of TPU-elastomers by thermal

analysis (DSC)," Polymer Testing, vol. 23, pp. 413-417, 6// 2004.

[68] M. A. Hood, B. Wang, J. M. Sands, J. J. La Scala, F. L. Beyer, and C. Y. Li,

"Morphology control of segmented polyurethanes by crystallization of hard and

soft segments," Polymer, vol. 51, pp. 2191-2198, 5/4/ 2010.

[69] T. R. Hesketh, J. W. C. Van Bogart, and S. L. Cooper, "Differential scanning

calorimetry analysis of morphological changes in segmented elastomers,"


[70] H. N. Ng, A. E. Allegrezza, R. W. Seymour, and S. L. Cooper, "Effect of segment

size and polydispersity on the properties of polyurethane block polymers,"

Polymer, vol. 14, pp. 255-261, 1973/06/01 1973.

[71] S. L. Samuels and G. L. Wilkes, "Comments on the multiple endothermic

behavior observed in segmented urethane systems," Journal of Polymer Science:

Polymer Physics Edition, vol. 11, pp. 807-811, 1973.

[72] R. W. Seymour and S. L. Cooper, "DSC studies of polyurethane block polymers,"

Journal of Polymer Science Part B: Polymer Letters, vol. 9, pp. 689-694, 1971.

[73] R. W. Seymour and S. L. Cooper, "Thermal Analysis of Polyurethane Block

Polymers," Macromolecules, vol. 6, pp. 48-53, 1973/01/01 1973.

[74] J. W. C. Van Bogart, D. A. Bluemke, and S. L. Cooper, "Annealing-induced

morphological changes in segmented elastomers," Polymer, vol. 22, pp. 1428-

1438, 1981/10/01 1981.

[75] G. L. Wilkes, S. Bagrodia, W. Humphries, and R. Wildnauer, "The time

dependence of the thermal and mechanical properties of segmented urethanes

177

following thermal treatment," Journal of Polymer Science: Polymer Letters

Edition, vol. 13, pp. 321-327, 1975.

[76] G. L. Wilkes and J. A. Emerson, "Time dependence of small‐angle x‐ray

measurements on segmented polyurethanes following thermal treatment," Journal

of Applied Physics, vol. 47, pp. 4261-4264, 1976.

[77] G. L. Wilkes and R. Wildnauer, "Kinetic behavior of the thermal and mechanical

properties of segmented urethanes," Journal of Applied Physics, vol. 46, pp.

4148-4152, 1975.

[78] J. T. Koberstein and A. F. Galambos, "Multiple melting in segmented

polyurethane block copolymers," Macromolecules, vol. 25, pp. 5618-5624, 1992.

[79] A. J. Ryan, C. W. Macosko, and W. Bras, "Order-disorder transition in a block

copolyurethane," Macromolecules, vol. 25, pp. 6277-6283, 1992.

[80] B. Chu, T. Gao, Y. Li, J. Wang, C. R. Desper, and C. A. Byrne, "Microphase

separation kinetics in segmented polyurethanes: effects of soft segment length and

structure," Macromolecules, vol. 25, pp. 5724-5729, 1992.

[81] J. T. Koberstein and T. P. Russell, "Simultaneous SAXS-DSC study of multiple

endothermic behavior in polyether-based polyurethane block copolymers,"

Macromolecules, vol. 19, pp. 714-720, 1986.

[82] W. Hu and J. T. Koberstein, "The effect of thermal annealing on the thermal

properties and molecular weight of a segmented polyurethane copolymer,"

Journal of Polymer Science Part B: Polymer Physics, vol. 32, pp. 437-446, 1994.

[83] J. T. Koberstein, I. Gancarz, and T. C. Clarke, "The effects of morphological

transitions on hydrogen bonding in polyurethanes: Preliminary results of

simultaneous DSC–FTIR experiments," Journal of Polymer Science Part B:

Polymer Physics, vol. 24, pp. 2487-2498, 1986.

[84] L. M. Leung and J. T. Koberstein, "Small‐angle scattering analysis of hard‐microdomain structure and microphase mixing in polyurethane elastomers,"

Journal of Polymer Science: Polymer Physics Edition, vol. 23, pp. 1883-1913,

1985.

[85] D. Nichetti and N. Grizzuti, "Determination of the phase transition behavior of

thermoplastic polyurethanes from coupled rheology/DSC measurements,"


[86] R. A. Phillips and S. L. Cooper, "Morphology development during solid‐state

annealing of poly (ether‐ester) multiblock copolymers," Journal of Polymer

Science Part B: Polymer Physics, vol. 34, pp. 737-749, 1996.

[87] J. Silva, R. Andrade, R. Huang, J. Liu, M. Cox, and J. M. Maia, "Rheological

behavior and structure development in thermoplastic polyurethanes under uniaxial

extensional flow," Journal of Non-Newtonian Fluid Mechanics, vol. 222, pp. 96-

103, 2015.

[88] J. Silva, D. Meltzer, J. Liu, M. Cox, and J. Maia, "The influence of thermo‐mechanical history on structure development of elastomeric and amorphous glass

thermoplastic polyurethanes," Polymer Engineering & Science, vol. 54, pp. 1383-

1393, 2014.

[89] Y. Xu, Z. Petrovic, S. Das, and G. L. Wilkes, "Morphology and properties of

thermoplastic polyurethanes with dangling chains in ricinoleate-based soft

segments," Polymer, vol. 49, pp. 4248-4258, 2008.

178

[90] S. Cossar, D. Nichetti, and N. Grizzuti, "A rheological study of the phase

transition in thermoplastic polyurethanes. Critical gel behavior and microstructure

development," Journal of Rheology (1978-present), vol. 48, pp. 691-703, 2004.

[91] D. Nichetti, S. Cossar, and N. Grizzuti, "Effects of molecular weight and

chemical structure on phase transition of thermoplastic polyurethanes," Journal of

Rheology (1978-present), vol. 49, pp. 1361-1376, 2005.

[92] P. J. Yoon and C. D. Han, "Effect of thermal history on the rheological behavior

of thermoplastic polyurethanes," Macromolecules, vol. 33, pp. 2171-2183, 2000.

[93] S. Yamasaki, D. Nishiguchi, K. Kojio, and M. Furukawa, "Effects of aggregation

structure on rheological properties of thermoplastic polyurethanes," Polymer, vol.

48, pp. 4793-4803, 2007.

[94] M. L. Sentmanat, "Miniature universal testing platform: from extensional melt

rheology to solid-state deformation behavior," Rheologica Acta, vol. 43, pp. 657-

669, 2004.

[95] V. C. Barroso, J. A. Covas, and J. M. Maia, "Sources of error and other

difficulties in extensional rheometry revisited: commenting and complementing a

recent paper by T. Schweizer," Rheologica acta, vol. 41, pp. 154-161, 2002.

[96] R. Mao, E. M. McCready, and W. R. Burghardt, "Structural response of an

ordered block copolymer melt to uniaxial extensional flow," Soft matter, vol. 10,

pp. 6198-6207, 2014.

[97] O. S. Carneiro and J. M. Nobrega, "Recent developments in automatic die design

for profile extrusion," Plastics Rubber and Composites, vol. 33, pp. 400-408,

2004.

[98] P. D. Gujrati and A. I. Leonov, Modeling and simulation in polymers. Weinheim:

Wiley-VCH, 2010.

[99] J. F. T. Pittman, "Computer-aided design and optimization of profile extrusion

dies for thermoplastics and rubber: a review," Proceedings of the Institution of

Mechanical Engineers Part E-Journal of Process Mechanical Engineering, vol.

225, pp. 280-321, Nov 2011.

[100] I. Szarvasy, J. Sienz, J. F. T. Pittman, and E. Hinton, "Computer aided

optimisation of profile extrusion dies - Definition and assessment of the objective

function," International Polymer Processing, vol. 15, pp. 28-39, Mar 2000.

[101] J. M. Nobrega, O. S. Carneiro, P. J. Oliveira, and F. T. Pinho, "Flow balancing in

extrusion dies for thermoplastic profiles Part I: Automatic design," International

Polymer Processing, vol. 18, pp. 298-306, Sep 2003.

[102] J. M. Nobrega, O. S. Carneiro, F. T. Pinho, and P. J. Oliveira, "Flow balancing in

extrusion dies for thermoplastic profiles - Part III: Experimental assessment,"

International Polymer Processing, vol. 19, pp. 225-235, Sep 2004.

[103] H. Cheng and I. Manas‐Zloczower, "Study of mixing efficiency in kneading discs

of co‐rotating twin‐screw extruders," Polymer Engineering & Science, vol. 37, pp.

1082-1090, 1997.

[104] H. F. Cheng and I. Manas-Zloczower, "Distributive mixing in conveying elements

of a ZSK-53 co-rotating twin screw extruder," Polymer Engineering and Science,

vol. 38, pp. 926-935, Jun 1998.

179

[105] A. S. Fard and P. D. Anderson, "Simulation of distributive mixing inside mixing

elements of co-rotating twin-screw extruders," Computers & Fluids, vol. 87, pp.

79-91, Oct 25 2013.

[106] A. S. Fard, M. A. Hulsen, H. E. H. Meijer, N. M. H. Famili, and P. D. Anderson,

"Tools to Simulate Distributive Mixing in Twin-Screw Extruders,"

Macromolecular Theory and Simulations, vol. 21, pp. 217-240, May 2012.

[107] T. Ishikawa, T. Amano, S. I. Kihara, and K. Funatsu, "Flow patterns and mixing

mechanisms in the screw mixing element of a co-rotating twin-screw extruder,"

Polymer Engineering and Science, vol. 42, pp. 925-939, May 2002.

[108] T. Ishikawa, S. I. Kihara, and K. Funatsu, "3-D non-isothermal flow field analysis

and mixing performance evaluation of kneading blocks in a co-rotating twin srew

extruder," Polymer Engineering and Science, vol. 41, pp. 840-849, May 2001.

[109] T. Kajiwara, Y. Nagashima, Y. Nakano, and K. Funatsu, "Numerical study of

twin-screw extruders by three-dimensional flow analysis - Development of

analysis technique and evaluation of mixing performance for full flight screws,"

Polymer Engineering and Science, vol. 36, pp. 2142-2152, Aug 1996.

[110] B. Vergnes, G. Della Valle, and L. Delamare, "A global computer software for

polymer flows in corotating twin screw extruders," Polymer Engineering and

Science, vol. 38, pp. 1781-1792, Nov 1998.

[111] H. H. Yang and I. Manas‐Zloczower, "Flow field analysis of the kneading disc

region in a co‐rotating twin screw extruder," Polymer Engineering & Science, vol.

32, pp. 1411-1417, 1992.

[112] X. M. Zhang, L. F. Feng, W. X. Chen, and G. H. Hu, "Numerical Simulation and

Experimental Validation of Mixing Performance of Kneading Discs in a Twin

Screw Extruder," Polymer Engineering and Science, vol. 49, pp. 1772-1783, Sep

2009.

[113] T. Avalosse, Y. Rubin, and L. Fondin, "Non-isothermal modeling of co-rotating

and contra-rotating twin screw extruders," Journal of Reinforced Plastics and

Composites, vol. 21, pp. 419-429, 2002.

[114] H. E. H. Meijer and P. H. M. Elemans, "The Modeling of Continuous Mixers .1.

The Corotating Twin-Screw Extruder," Polymer Engineering and Science, vol.

28, pp. 275-290, Mar 1988.

[115] H. F. Giles, J. R. Wagner, and E. M. Mount, Extrusion : the definitive processing

guide and handbook. Norwich, NY: William Andrew Pub., 2005.

[116] R. G. Larson, Constitutive equations for polymer melts and solutions. Boston:

Butterworths, 1988.

[117] T. Glomsaker, E. L. Hinrichsen, F. Irgens, and P. Thorsteinsen, "Numerical

simulation of extrusion of S-PVC formulations in a capillary rheometer,"

Rheologica Acta, vol. 39, pp. 80-96, Jan 2000.

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