Melt viscoelastic properties of peroxide cured polypropylene-ethylene octene copolymer thermoplastic...

Melt Viscoelastic Properties of Peroxide CuredPolypropylene-Ethylene Octene CopolymerThermoplastic Vulcanizates

R. Rajesh Babu, Nikhil K. Singha, Kinsuk NaskarRubber Technology Centre, Indian Institute of Technology, Kharagpur 721302, India

The dynamic viscoelastic properties of uncrosslinkedand dynamically crosslinked blends of polypropylene(PP) and ethylene octene copolymer (EOC) wereinvestigated in the melt state to study the mecha-nism of reinforcement, influence of particle size, andkinetics of modulus recovery. Dynamic vulcanizationwas performed by coagent assisted peroxide cross-linking system. Addition of peroxide in PP/EOC blendinvolves two major competing reactions: crosslinkingin EOC and degradation of PP by b chain-scission. Inthis article, morphological and melt rheological prop-erties of the TPVs were studied with special refer-ence to the effect of mixing protocol. Three differentmixing techniques were investigated. They are: (i)conventional or preblending method—melt mixing ofPP and EOC followed by dynamic vulcanization (ii)phase mixing method—curative master batch of EOCadded on the molten PP (iii) split addition of PP—preblending method followed by addition of half partof PP (dilution procedure). The type of mixing proto-col has a significant influence on the rheologicalbehavior. Rheological properties have been evaluatedat the processing temperature (1808C) in a RubberProcess Analyzer (RPA 2000). A variety of rheologicalobservations such as Payne effect, modulus recoveryand shear rate sensitivity were studied by carryingout frequency and strain amplitude sweeps. POLYM.ENG. SCI., 50:455–467, 2010. ª 2009 Society of PlasticsEngineers

INTRODUCTION

Thermoplastic vulcanizates (TPVs) are prepared by the

dynamic vulcanization process, where crosslinking of an

elastomer takes place during its melt mixing with a

thermoplastic polymer under high shear. The resulting

morphology consists of micron sized finely dispersed

crosslinked rubbery particles in a continuous thermoplas-

tic matrix. TPVs have rubber like properties but can be

processed as thermoplastic using conventional thermo-

plastic processing equipments [1–3]. An extensive study

of the TPVs based on various rubber-plastic blends were

examined by Coran and Patel [4, 5]. In the present study,

polypropylene (PP) and ethylene octene copolymer

(EOC) based TPVs were prepared by coagent assisted

peroxide crosslinking system. With the addition of perox-

ide two competing reactions take place simultaneously:

crosslinking in EOC and chain scission of PP. The latter

causes lowering of molecular weight and narrow down

the molecular weight distribution, hence melt becomes

more Newtonian and viscoelasticity decreases. On the

contrary, crosslinking and chain branching occur in EOC

phase, which considerably change the viscoelastic proper-

ties. The mechanical and rheological properties of the

blends depend not only on the constituent polymers but

also on the morphology of the blend. TPVs consist of

densely packed crosslinked rubber particles in thermo-

plastic matrix and their rheological properties can be

compared to that of highly filled polymers [6]. Further-

more, rheological study provides valuable information

about the flow properties of the compound in the melt

processing equipments.

Goettler et al. [7] was the first to study about the

technical importance of the TPVs in terms of rheological

characteristics using a capillary rheometer. Han et al. [8]

described the comparative rheological study of PP, PP/

ethylene propylene diene rubber (EPDM) uncrosslinked

and dynamically crosslinked blends using various rheo-

logical instruments to measure steady shear flow, uniax-

ial extension, and oscillation flow properties of the com-

pounds. Steeman et al. [9] reported that TPVs have a

yield stress for flow and the value increases with

increase in elastomer component in TPVs. An extensive

study of rheological response of PP/EPDM blends and

the TPVs with phenolic resin as curing agent was

explored by Jain et al. [10]. Rheological properties of

various rubber-plastic uncrosslinked and dynamically

crosslinked blends with special reference to the effect of

blend ratio, shear rate, melt elasticity, and degree of

crosslinking have been interpreted and described by sev-

Correspondence to: Kinsuk Naskar; e-mail: [email protected]

Contract grant sponsor: Council of Scientific and Industrial Research

(CSIR), New Delhi, India.

DOI 10.1002/pen.21553

Published online in Wiley InterScience (www.interscience.wiley.com).

VVC 2009 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2010

eral authors [11–13]. Katbab et al. [14, 15] discussed the

formation of morphology and correlated with the

mechanical and rheological properties for the nonrein-

forced and reinforced (carbon black and silica fillers)

TPVs. Chatterjee and Naskar [16] analyzed the rheologi-

cal response of nanosilica filled PP based TPVs. Rheo-

logical behavior of dynamically and statically vulcanized

blends of PP and styrene butadiene styrene triblock co-

polymer (SBS) with reference to the effect of various

curing agents was studied by Ichazo et al. [17]. The lin-

ear viscoelastic properties of PP/styrene ethylene butadi-

ene styrene block copolymer (SEBS) blends and PP/

EPDM TPVs are described in terms of composition ratio

and also discussed about the oil diffusion and distribu-

tion in the blend components by Sengers et al. [18].

Recently, Leblanc [19] studied the nonlinear viscoelastic

property of the various commercially available TPVs

with varying hardness by a Fourier transform rheometer

(FT-rheo) to get insight into the morphology developed

and the subtle role played by the extractible content such

as oil and plasticizers. Li and Kontopoulou [20] studied

the evolution of morphology and rheological response of

PP/EOC TPVs with peroxide as curing agent and corre-

lated with the gel point of the EOC phase. Generally,

coagents are multifunctional vinyl monomers, which are

highly reactive toward free radical either by addition

reaction and/or by hydrogen abstraction. Chain scission

also could be retarded by stabilizing the PP macroradi-

cals by addition reaction across the double bond in the

coagent. Our recent work had revealed the effect of dif-

ferent peroxides [21] and different coagents [22] on fixed

blend ratio of PP/EOC TPVs. Parent et al. [23] studied

the variation of molecular weight and branching distribu-

tion in PP obtained by the treatment of coagent assisted

peroxide. Furthermore, the obtained product architecture

was analyzed by the melt rheological characterization

such as oscillatory shear and extensional deformation.

Generally, the flow properties of the TPVs are influenced

by various factors such as polymer characteristics (mo-

lecular weight, molecular weight distribution, degree of

branching), crosslinked dispersed particles (size, degree

of dispersion, crosslink density), interfacial interaction

and the processing conditions.

In our previous paper [24], mechanical properties

and morphology of the TPVs prepared by three different

mixing protocols were investigated in detail. The

present article aims to understand the melt viscoelastic

property and modulus recovery in molten TPVs

prepared by three different methods. The morphology

of various TPVs was correlated with the observed

dynamic rheological response. Initially, the discussion is

focused on the fixed concentration (optimized concentra-

tion: 2 phr) of curative in TPVs in three different mixing

methods. Furthermore, to get a better insight in the

reinforcement mechanism, the dependence of dynamic

functions was studied by varying the concentrations of

curative.

EXPERIMENTAL

Materials

The general purpose polyolefin elastomer Exact 5371

[specific gravity, 0.870 g/cm3 at 238C; comonomer

(octene) content 13%; melt flow index (MFI), 5.0 g/10

min @1908C/2.16 kg], was obtained from Exxon Mobil

Chemical Company. Polypropylene (PP; specific gravity,

0.9 g/cm3 at 238C; melt flow index, 3.0 g/10 min @

2308C/2.16 kg) was obtained from IPCL, India. Dicumyl

peroxide (DCP; Perkadox-BC-40B-PD) having an active

peroxide content of 40% was obtained from Akzo Nobel

Chemical Company, The Netherlands. Its half life time

(t1/2) at 1388C is 1 hr and specific gravity is 1.53 g/cm3

at 238C. The coagent, triallyl cyanurate SR 507A; (spe-

cific gravity, 1.12 g/cm3 at 238C) was obtained from Sar-

tomer Company.

Preparation of TPVs

All PP-EOC TPVs were prepared by melt blending of

the components in the Haake Rheomix 600 s at a temper-

ature of 1808C with a rotor speed of 70 rpm. The compo-

sition of PP/EOC TPVs employed in this study are shown

in Table 1. TPVs were prepared by three different mixing

methods, which were systematically represented in Fig. 1.

In all the mixing methods, time to perform the dynamic

vulcanization process was maintained for 4 min.

Prepared TPVs were designated as follow: the first let-

ter denotes the method by which the TPVs are prepared,

followed by the second letter and a number indicating the

type of peroxide and concentration of corresponding per-

oxide, respectively. For example, PD2 corresponds to the

composition of 2 phr concentration of dicumyl peroxide

prepared by phase mixing method. After mixing, the

blends were removed from the Haake chamber at hot con-

dition and sheeted out in a two roll mill at room tempera-

ture (258C) after a single pass. Sheets were then cut and

pressed in a compression molding machine (Moore Press,

Birmingham, UK) at 1908C for 4 min at 5 MPa pressure.

Aluminum foils were placed between the mold plates.

The molded sheets were then cooled down to room tem-

perature under the same pressure.

TABLE 1. Composition of TPVs.

Component Phr

Ethylene octene copolymer (EOC) 100

Polypropylene (PP) 50

Dicumyl peroxide (DCP) 0–5a (varied)

Triallyl cyanurate (TAC) 2.0

a Concentration of DCP was optimized at 2.0 phr [21], which corre-

sponds to 3 milliequivalents concentration.

456 POLYMER ENGINEERING AND SCIENCE—-2010 DOI 10.1002/pen

Gel Content

To estimate the degree of crosslinking in the EOC

phase in the TPVs prepared, the samples were subjected

to a series of solvent extraction. All the measurements

were done according to ASTM D 2765. About 2 g of

TPV sample were first extracted for 24 h in hot xylene at

808C to remove the uncrosslinked EOC phase and low

molecular weight PP fragments. It seemed to be a qualita-

tive estimation because of the restricted extraction of

EOC phase in the presence of PP phase. The samples was

completely dried after extraction and weighted. Then the

initial loss can be calculated from the

Xcð%Þ ¼ Wl

W0

� �� 100 (1)

where

Xc ¼ Percentage of residue after xylene extraction at

808CW0 ¼ Initial weight of the specimen and

Wl ¼ Weight after xylene extraction.

The dried sample was further extracted for 72 h in

boiling xylene at 1408C to completely remove the PP and

soluble EOC phase. The gel content can be calculated as

Gelð%Þ ¼ 100� ðWl �WhÞ �WPP

WEOC

� 100

� �(2)

where

Wl¼weight of the sample after xylene extraction at 808CWh ¼ weight of the sample after xylene extraction at

1408CWPP and WEOC ¼ weights of PP and EOC in the PP/

EOC TPVs, respectively.

Crosslink Density

Equilibrium solvent swelling measurements were car-

ried out on the PP/EOC TPVs to determine the crosslink

density of the EOC in presence of PP. The overall cross-

link density was calculated using the modified Flory-

Rehner equation [21, 25, 26]. From the degree of swelling,

the overall crosslink density was calculated relative to the

(EOC þ PP) phases and expressed as (m þ PP). The latter

was done in order to avoid the need to correct for a part of

the PP, being extracted as amorphous PP. A circular piece

of 2 mm thickness was made to swell in cyclohexane for

about 48 h to achieve equilibrium swelling condition. Ini-

tial weight, swollen weight, and de-swollen or dried

weight were measured and substituted in the Flory-Rehner

equation to get the values of crosslink density.

Morphology

Phase morphology of the cryofractured samples of var-

ious blends was investigated by a JEOL JSM 5800 Digital

FIG. 1. Schematic diagram of the preparation of thermoplastic vulcanizates by various methods.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2010 457

Scanning Electron Microscope (SEM). All the blends

were cryofractured in liquid nitrogen to avoid any possi-

bility of phase deformation during cracking process. For

uncrosslinked PP/EOC blends, low melting temperature

EOC component was preferentially extracted by treating

in hot xylene at 708C for 30 min. For dynamically cross-

linked PP/EOC TPVs, uncrosslinked PP was preferentially

extracted by treating in hot xylene at 1208C for 60 min.

All the treated samples were dried under vacuum oven

at 708C for 5 h to remove the traces of solvents. The

dried samples were then gold sputtered and examined

under SEM.

Rheological Measurements

Melt rheological behavior of the TPVs were analyzed

in a Rubber Process Analyser (RPA 2000, Alpha Technol-

ogies) equipped with biconical dies. All the specimens

were directly loaded between the dies maintained at

1808C, and the test were carried out in both frequency

sweep and strain sweep mode.

Frequency Sweep. The frequency was logarithmically

increased from 0.33 to 33 Hz at a constant strain ampli-

tude of 5%. A strain of 5% was selected to ensure that

the rheological behavior was located in the linear visco-

elastic region.

Strain Sweep. The strain amplitude sweep was

performed from 1 to 1200% at a constant frequency of

0.5 Hz.

Modulus Recovery. To determine the recovery

kinetics, each sample underwent the following test

sequences: (a) frequency sweep, (b) strain sweep, (c)

relaxation time of 2 min, (d) frequency sweep, (e) strain

sweep, (f) relaxation time of 2 min, (g) frequency

sweep, and (h) strain sweep. All the tests were carried

at a temperature of 1808C. Average of three measure-

ments was reported for each sample. Similar sequences

were pursued by Osman and Atallah [27] in characteriz-

ing the viscoelastic property of calcium carbonate filled

polyethylene.

RESULTS AND DISCUSSION

Gel Content and Overall Crosslink Density

Variation in the gel content values and the overall

crosslink density values of TPVs with same concentration

of peroxide prepared by three different methods are

shown in the Table 2. Gel content was measured in two

stages for the samples prepared. In the first stage, PP was

extracted at 808C by hot xylene. It is believed that this

can extract uncrosslinked EOC phase along with very low

molecular weight PP fractions. During low temperature

extraction (808C) process, PP remains almost insoluble in

xylene (may be very low molecular weight species were

extracted out). However, at high temperature (1408C)soluble PP fractions are removed leaving only crosslinked

EOC gels. Maximum value of gel content is observed for

phase-mixed TPV and the least for preblended TPV at

both stages of extraction. The differences can be roughly

compared to the extent of crosslinking in the EOC phase.

It can be concluded that, higher gel content in the PD2

indicates that crosslinking reaction of the EOC proceeds

predominantly with less influence on the PP phase. Over-

all crosslink density values are in complete agreement

with the gel content measurements. In equilibrium solvent

swelling measurements, a significant contribution of both

the crosslinking in EOC phase and the degradation in PP

phase are coupled to give the values. This further con-

firms the limited extent of degradation in the PP phase

with the higher crosslinking degree in EOC phase for

PD2 TPV.

Morphology

A schematic representation of morphology evolution of

50/100 PP/EOC uncrosslinked and dynamically vulcan-

ized blends with same amount of peroxide prepared by

three different methods is shown in Fig. 2 and corre-

sponding SEM photomicrographs were shown in Fig. 3. It

is interesting to note that in uncrosslinked blend, EOC

forms the matrix phase with PP as dispersed one. During

dynamic vulcanization, phase inversion occurs, that is, PP

forms the matrix and crosslinked EOC form the dispersed

TABLE 2. Gel content and crosslink density of TPVs.

Compound name Xc(%)a Gel (%)bOverall crosslink

density (m þ PP) 3 1024 (mol/ml)cCrosslink

density (m) 3 1024 (mol/ml)d

CD2 78.3 6 0.6 39.3 6 1.1 2.27 6 0.25 1.55 6 0.2

PD2 97.2 6 0.8 60.8 6 1.2 5.24 6 0.3 1.55 6 0.2

SD2 90.5 6 1.0 55.5 6 1.0 3.57 6 0.2 1.55 6 0.2

a Percentage of residue after Xylene extract of TPVs at 808C.b Gel content after boiling xylene extract of TPVs at 1408C (on samples previously extracted at 808C temperature).c Equilibrium solvent swelling measurements in PP/EOC TPVs in cyclohexane at room temperature for 72 hr.d Crosslink density of only EOC with the same amount of peroxide as that in TPVs.


phase. With the addition of peroxide, EOC gets cross-

linked and PP undergoes degradation. This leads to

increase in viscosity ratio, which plays an important role

in driving the morphology of TPVs. The less viscous PP

encapsulates the more viscous crosslinked EOC phase to

minimize the mixing energy. In this particular blend sys-

tem, proportion of EOC is twice than that of PP (50/100

PP/EOC), which results to form densely packed three-

dimensional network structure. These crosslinked EOC

particles are dispersed in the form of aggregates and/or

agglomerates in the PP matrix. Furthermore, these EOC

aggregates or agglomerates are embedded in the PP mac-

romolecules via joint shell mechanism or segmental inter-

diffusion mechanism [14, 15, 28, 29, 30]. Addition of

curative (peroxide and coagent) in PP/EOC blends results

to give several reactions: crosslinking in EOC, degrada-

tion in PP, generation of in situ graft copolymer, limited

extend of crosslinking in PP phase and formation of

branched PP chains. However different methods have

their own influence on the aforementioned reactions

which had been discussed in detail in one of our earlier

communications [24]. Among the three different methods

employed at equal concentration of curative dosage, the

efficiency of crosslinking in the EOC phase apparently

has an important effect on the evolution of morphology,

i.e., the particle size. The finest particle size (better mor-

phology) was observed for PD2 whilst CD2 exhibits the

largest rubber particles (see Fig. 3). This may be attrib-

uted due to the high degree of crosslinking in EOC with

minor effect on PP phase in the PD2. In contrast, smaller

particle size is associated with higher surface area having

higher probability of adsorption of PP chains on their sur-

face. The PP chains are adsorbed on the surface of cross-

linked EOC domains via physical and/or chemical bond-

ing. Within measurement accuracy, no clear distinction

between the particle size of PD2 and SD2 is seen. How-

ever, PD2 seems to exhibit somewhat lower particle size

than SD2 at this particular concentration of peroxide. The

morphological evolutions are in agreement with the gel

content and overall crosslink density values.

Rheological Properties

Strain Dependence of Viscoelastic Behavior. Strain

sweep test was carried out to characterize the strain de-

FIG. 2. Schematic representation of morphology evolution of dynami-

cally vulcanized blends prepared by different methods.

FIG. 3. Scanning electron photomicrograph of uncrosslinked and dynamically vulcanized blends at 2 phr

concentration of DCP. (a) Uncrosslinked. (b) Preblending method. (c) Phase mixing method. (d) Split addi-

tion method.


pendence of viscoelastic properties of the samples and

to determine the linear viscoelastic region (LVE). Figure

4a–c display the variation of storage modulus (G0), lossmodulus (G00) and loss factor (tan d ¼ G00/G0) respectively,as a function of strain amplitude for 50/100 PP/EOC

uncrosslinked and dynamically vulcanized blends. Irre-

spective of the different methods attempted, dynamically

vulcanized blends show increased dynamic modulus and

dynamic viscosity (also called as dynamic functions) than

uncured blends. In general, both the modulus (storage and

loss modulus) are increased by the process of dynamic

vulcanization. However, increase in storage modulus was

larger than loss modulus throughout the strain range. Since

shear stress develops almost linearly with the strain ampli-

tude, complex modulus-stress plot look quite similar to

those given in Fig. 4a. Storage modulus of the sample

(uncured and dynamically cured) exhibit a linear region at

low strain and nonlinear region at high strain amplitude.

Uncrosslinked blend shows a longer linear region up to

around 70% strain followed by a nonlinear region. But a

progressive nonlinear behavior, that is, decrease of

dynamic functions with the increase of strain amplitude

can been clearly seen for TPVs. Such behavior resembles

to the so-called Payne effect of rubber highly filled with

active fillers like carbon black and silica [31, 32]. Princi-

pally, rheology of TPV can be analogically compared with

that of the rubber filled with active fillers [6]. According

to Payne, nonlinearity is due to the disintegration of sec-

ondary agglomerates filler network structure. However, in

case of TPVs, the secondary structure corresponds to the

crosslinked EOC domains dispersed in the PP matrix in

the form of aggregates and/or agglomerates. Indeed nonli-

nearity of TPVs can then be imagined to be associated

with both the mechanism: disintegration of agglomerated

EOC domains and debonding of crosslinked EOC domains

from the PP matrix phase. In uncrosslinked blends, the

nonlinearity at higher strain level is due to the disintegra-

tion or breakdown of trapped molecular entanglement net-

work and/or debonding of macromolecular chains of PP

anchored to the continuous EOC phase.

To understand the behavior of TPVs in the strain

sweep test, different rheological parameters were observed

and tabulated in Table 3. Among the three different meth-

ods practiced, PD2 has a high modulus at low strain with

a very short linear viscoelastic range and CD2 shows pro-

longed flat region, indicating that higher strain is required

for the onset of nonlinearity than the others. Storage mod-

ulus at low strain amplitude (2% strain) had the following

order: phase [ split [ preblend and the reverse is true

for modulus at high strain amplitude (1200% strain). It is

expected that better interfacial interaction due to cross

curing tendency of the PP-EOC phase leads to give higher

modulus value at high strain amplitude for preblending

method. It seems that main reason for this behavior lies

FIG. 4. Variation of viscoelastic properties as a function strain amplitude of uncrosslinked and dynamically

vulcanized blends at 1808C. (a) Storage modulus. (b) Loss modulus. (c) tan d. (d) G-plot (G00 vs. G0).


in the morphology developed and the particle size of the

dispersed phase. In case of filled polymer system, there is

a direct consequence of the particle size and filler net-

working (magnitude of aggregates and agglomerates).

This is due to the fact that the smaller the particle, the

smaller is the inter-aggregate distance and hence higher is

the probability towards the formation of filler network.

It was observed that deviation from the linear visco-

elastic behavior of TPVs starts at lower strain than

uncrosslinked blends. Transition point in the region where

the linear to nonlinear viscoelastic behavior starts is

defined as critical strain amplitude (cc) and for uniformity

it may be taken as the strain value at the storage modulus

equal to 90% of the initial storage modulus (0.9 G00). It is

clear from the Fig. 4a and Table 3, phase mixed TPV

shows lower critical strain amplitude followed by split

addition and preblending TPV. It can be stated generally

that presence of secondary clusters increase the modulus

and enhance the nonlinearity.

Strain dependency of loss modulus (viscous modulus)

of uncrosslinked and dynamically crosslinked blends is

presented in Fig. 4b. In contrast to the G0 behavior of

TPVs, G00 shows a maximum at the critical strain ampli-

tude region whereas uncrosslinked blends is indeed linear

at low strain region followed by decrease in loss modulus

value with increasing deformation. Dependence of loss

modulus on strain amplitude depends on the rates of net-

work breakdown and reformation as well as sliding of

macrochains at the domain surface. The magnitude of loss

maxima are substantially emphasized for the phase mixed

TPVs. It may be related to the energy dissipation pro-

duced by the breakdown of crosslinked EOC aggregates

[32].

By definition, loss factor tan d is determined by both

loss and storage moduli (tan d ¼ G00/G0). Uncrosslinkedblends exhibit higher tan d value in the entire strain am-

plitude range, which is due to the dominance of viscous

nature. However, TPVs exhibit more elastic behavior at

low strain region, that is, G0 [ G00, with increasing defor-

mation TPVs become progressively more viscous and two

curves intersect at tan d ¼ 1 (G0 ¼ G00; Fig. 4c). The

effect of smaller particle size and high degree of cross-

linking in the EOC phase are manifested by a substantial

shift of the intersection point to the low strain region. G-

plot (G00 vs. G0) of the strain sweep data is shown in Fig.

4d. Generally, G0 is related to the extent of filler network

formation and G00 to the breakdown and reformation of

these structures. In uncrosslinked blends, there is a linear

relationship between G0 and G00 in the strain region stud-

ied. In case of TPVs, a semicircular arc is observed which

may be a direct manifestation of morphology developed.

Similar semicircular arc is observed for the carbon black

filled SBR rubber and smaller particles exhibited more

pronounced arc [33]. Phase mixed TPVs show more pro-

nounced and bigger arc followed by preblended and split

added TPVs. Variation in the TPVs prepared by different

methods is associated with the different state and magni-

tude of crosslinked EOC network (aggregates and/or

agglomerates) formation and reformation.

Frequency Dependence of Viscoelastic Properties. Dy-

namic frequency sweep tests were performed at fixed low

strain amplitude (selected from linear viscoelastic region)

to further study the network formation and microstruc-

tural changes. Figure 5a shows the logarithmic plot of

dynamic functions (complex modulus G*, complex vis-

cosity g*) as a function of frequency (x) for the uncros-

slinked (TPO) and dynamically crosslinked blends

(TPVs). As shown, g* decreases with increase in x for

all the blends (TPO and all TPVs), which clearly demon-

strates the pseudoplastic behavior and significant non-

Newtonian properties (shear thinning behavior). Irrespec-

tive of different methods, TPVs exhibit higher viscosity

values in the entire range of frequency than the uncros-

slinked blends and a more prominent increase of g* in

the low frequency region and the curve tends to converge

at the higher frequency.

Zero shear viscosity (g�0) values, obtained experimen-

tally at the x ¼ 0.1 Hz are shown in Table 4. The higher

g�0 value is observed for phase mixed TPVs. Moreover,

viscosity curve at low frequency can be fitted in the

Power law equation.

TABLE 3. Rheological parameters and values observed from the strain sweep experiments.

Comp

First run Second run Third run

G00a G01

b ccc tan d ¼ 1d DG0e G0

0 G01 cc tan d ¼ 1 DG0 G00 G01 cc tan d ¼ 1 DG0

PP-EOC 2.0 0.13 51.0 – 1.87 2.5 0.13 40.2 – 2.37 2.8 0.13 15.0 – 2.67

CD2 65.8 1.44 17.2 350.8 64.3 55.3 1.21 11.5 88.9 54.1 52.8 1.11 10.5 75.0 51.7

PD2 90.5 0.86 14.2 150.6 89.6 79.5 0.81 8.1 80.5 78.7 80.1 0.79 7.2 80.4 79.3

SD2 73.0 1.09 15.0 220.1 71.9 72.7 1.01 5.5 73.7 71.7 73.7 1.0 4.5 62.3 72.7

Average of three measurements were reported.a Low strain storage modulus (modulus at 2% strain; kPa).b High strain storage modulus (modulus at 1200% strain; kPa).c Critical strain amplitude [strain at 90% G0

0; 0.9 G00 (%)].

d Strain amplitude at G0 ¼ G00 (crossover point; %).e DG0 ¼ G01 2 G0

0 (change in storage modulus value with strain amplitude sweep).


Z� ¼ kon (3)

where Z* is a complex viscosity, k is sample exponential

factor, o is the oscillation frequency and n is the shear

thinning exponent. The value of n can be determined

from the slope of straight line obtained in the low fre-

quency region in the log (Z*) vs. log (o) plot. The value

of n increased drastically for the dynamically vulcanized

blends, that is, more pseudoplastic behavior is obtained

by dynamic vulcanization (Table 4). Higher moduli in the

low shear rate region and more shear thinning behavior

with increasing shear rate can afford to give better proc-

essing characteristics. This can be attributed due to the

state and mode of three-dimensional network structure

formed by the dispersed crosslinked EOC particles and

the overlapping of the matrix phase, that is, PP macromo-

lecules absorbed by physical and/or chemical crosslinking

on the surface of domains. Thereby interface becomes

less mobile, which leads to increase in the shear viscosity

at low frequency region. As the frequency increases, the

network structure, mainly aggregates/agglomerates tends

to collapse and deform to exhibit higher shear thinning

behavior [34]. This implies that the crosslinked EOC

domains could behave as the filler in the PP matrix. It

was previously reported that, TPVs with high amount of

rubber have been shown to behave like highly filled mol-

ten thermoplastic [7, 8, 14].

Frequency sweep at small amplitude does not signifi-

cantly deform the microstructure of the complex fluid; it

allows understanding the reinforcing effect. Figures 5a–c

show the dependence of G0, G00, and tan d as a function

of angular frequency. Both the moduli increase with

increase in x for all the blends. In TPOs, G00 is larger

than G0 in the low frequency region, demonstrating vis-

cous nature of the compound. However, with increase in

frequency the two moduli cross each other called cross-

over frequency (xc), that is, G0 ¼ G00, denotes the transi-

tion from viscous to rubbery response. In uncrosslinked

blend xc observed at higher frequency (30 Hz). But in

case of TPVs, G0 becomes large than G00 over the whole

range of experimental frequency, hence no xc is

observed. Therefore the TPVs can be described as ‘‘visco-

elastic solid.’’ In other words, dynamically vulcanized

samples show high elasticity in the experimental fre-

quency range. In TPOs, loss factor tan d is high and

decays very fast with increase in x. But TPVs show very

low values in the entire range of frequency applied and

the curve becomes almost flat, reflecting the presence of

three-dimensional network structures and their influence

on the moduli. In a nutshell, in contrast to TPOs, TPVs

show a gradual increase in both the modulus and decrease

in their x dependency. In general, dynamic functions at

low frequency is strongly depend on the physical network

formation (hydrodynamic and secondary structure) by the

inclusion, whereas at high frequency is dominated by

matrix (along with hydrodynamic effect).

Modulus Recovery. Experiments on recovery of

dynamic functions after the application of large strain am-

plitude perturbation were performed for all the samples

investigated. Figure 6 shows that the comparative subse-

quent strain sweep results performed immediately after a

relaxation time of 2 min and a frequency sweep (for

clarity only phase mixed TPV is shown in Fig. 6). The

typical qualitative feature of Payne effect, that is, decrease

FIG. 5. Variation of viscoelastic property as a function of angular fre-

quency for uncrosslinked and dynamically crosslinked blends. (a) Com-

plex modulus and complex viscosity. (b) Storage modulus and loss mod-

ulus. (c) tan d.


of G0 and the appearance of G00 max in the subsequent

strain sweep test is observed. However, linear range

decreased (cc at lower strain amplitude) and both the mod-

uli (G0 and G00) decrease with the formation of G00 max at

lower strain amplitude in the subsequent strain sweep test.

The variation of modulus value at low and high strain am-

plitude in all the three strain sweeps are shown in Table 3.

These evolutions are associated with the nonreversal de-

formation of the network formed by the disintegration of

crosslinked EOC aggregates and/or agglomerates and also

de-wetting of the EOC domains from the PP matrix as

well as rupture of chain entanglements and chains con-

necting aggregates. This corresponds to the so called phe-

nomenon Mullins effect or stress softening effect [35]. It

is important to note that all the dynamic stress softening is

achieved in the second sweep (2nd run) and only minor

effects take place in the subsequent sweep (3rd run). Fur-

thermore, it is observed from the figures, that the linear

complex modulus (modulus at 2% strain) of TPVs is

reformed to a large extent (more than 90% of initial or lin-

ear modulus is recovered) in the subsequent strain sweep.

In the literature, it had been shown that, when the calcium

carbonate filled Polyethylene (highly loaded particulate

filled polyethylene) is subjected to subsequent strain sweep

[27], the onset of nonlinearity shifted to the higher defor-

mation (strain %) and a drastic decrease was observed in

the initial storage modulus. However, in case of TPVs the

reverse is observed, that is, initial storage modulus is

regained to large extent and the onset of linearity is shifted

to the lower deformation.

Dependence of dynamic functions as a function of fre-

quency for both the uncrosslinked and dynamically cross-

linked blends, before and after strain sweep followed by

relaxation time is plotted in Fig. 7a,b. A marginal reduc-

tion in the dynamic functions is observed only in the low

frequency region and the curves converge at the higher fre-

quency. It is worth noting that no Mullins effect or strain

history effects occur between the 2nd and 3rd runs. The

dynamic functions are almost similar in both 2nd and 3rd

runs. These results strongly suggest that the EOC domains

formed energetically elastic network structure. In other

words, the network is not significantly affected by the

strain sweep or all the strain effects are recovered during

the 2-min relaxation time between the subsequent sweeps.

Reinforcement Mechanism. The factors which affect

the rheological characteristics of the TPVs are size of the

dispersed domain, state of dispersion, intrinsic characteris-

tics of the blend components, volume fraction, interfacial

interaction, and processing conditions [2, 7]. Melt rheo-

logical characteristics of the TPVs can be analogically

compared with the filled polymer composites [8]. In gen-

eral, active fillers (such as carbon black, silica) have the

tendency to form agglomerates, especially at higher load-

ing, leading to the formation of cluster or chain-like struc-

ture or network structure. Depending on the activity of

the filler surface, the network could be of direct contact

mode or joint shell mode [32]. In case of fillers, well wet-

ted by the polymer molecules (carbon black in hydrocar-

bon rubber) the filler network might be formed by a joint

shell mechanism [32, 33]. A schematic representation of

joint shell mechanism is shown in Fig. 8. Because of the

better interfacial interaction between the polymer and fil-

ler, the polymers molecules are adsorbed onto the surface

of the filler either chemically or physically [33]. Thereby

the segmental mobility of the polymer molecules are re-

stricted and the modulus might increase in the vicinity of

TABLE 4. Rheological parameters and values observed from the frequency sweep experiments.

Comp

First run Second run Third run

G00a g00

b nc G00 g00 n G0

0 g00 n

PP-EOC 3006 0.944 20.3132 3490 1.097 20.3362 4630 1.40 20.3735

CD2 139,000 43.58 20.8210 126,000 39.56 20.7931 116,000 36.54 20.7829

PD2 235,000 73.92 20.8941 190,000 59.75 20.8594 183,000 57.61 20.8556

SD2 169,000 53.11 20.8412 158,000 49.62 20.8051 153,000 48.02 20.8051

Average of three measurements is reported.a Complex modulus at low frequency (0.05Hz; kPa).b Zero shear viscosity (complex viscosity at low frequency; Pa s).c Shear thinning component (slope of the straight line obtained by plotting log (g*) and log (x) between the frequency 0.1 and 1.0 Hz).

FIG. 6. Storage and loss modulus of phase mixed TPVs plotted as a

function of strain amplitude. (Full symbol represents for first run and

open symbol for second measurement after relaxation and frequency

sweep.)


the filler particle. Smaller particles have higher tendency

to form filler network and have more immobilized rubber

shell in comparison with the larger filler particle. Simi-

larly, in TPVs, the difference between the surface energy

of the PP matrix and the crosslinked EOC particles leads

to the adsorption of the PP molecule onto the surface of

the crosslinked EOC particles through segmental interfu-

sion mechanism [28, 29]. For better understanding and to

avoid complication, discussion is narrowed down to only

preblended and phase mixed TPVs.

To explore the reinforcement characteristics of the

TPVs prepared, the concentration of the curative (perox-

ide) was varied in both preblending and phase mixing

method and the corresponding gel content and overall

crosslink density values are shown in Fig. 9. The modulus

evolution and viscosity curve for the phase mixed TPVs

are presented in Fig. 10a,b. We observe a decrease in

dynamic functions (G* and g*) at high concentration of

peroxide (4.6 phr). As previously mentioned, phase mix-

ing method involves the addition of EOC curative master

batch to the molten PP to form TPVs. From the method

of preparation, it can be visualized that high concentration

of peroxide have more number of reactive species in the

EOC phase which readily undergo crosslinking reaction

before the optimum dispersion of EOC domains during

mixing with molten PP. Furthermore, tightly crosslinked

domains restrict the disintegration of crosslinked EOC

particles which results to form coarsest crosslinked EOC

particles. In case of preblending method, particle size con-

tinuously decreases with increase in concentration of per-

oxide combined with the significant lowering of molecular

weight in the PP phase (b-chain scission). A speculative

model of morphology development in dynamically vul-

canized blends prepared by preblending and phase mixing

method as a function of concentration of peroxide is

shown in Fig. 11. The flow behavior of the polymer

blends can be well illustrated by the Power law model

(Eq. 3). The Power law constants such as flow behavior

index (n) and consistency index (k) have been calculated

from the linear fit of the log–log plot of complex viscos-

ity and the frequency (Fig. 10b) for the preblended and

phase mixed TPVs as a function of peroxide concentra-

tion is shown in Table 5. All the samples show pseudo-

plastic behavior, that is, n value is less than one for all

the samples. However, n value is found to increase with

increase in concentration of peroxide. It is well known

that higher n value indicates a less pseudoplastic behav-

ior. When compared at equal concentration of peroxide,

preblended TPVs show more pseudoplastic nature than

phase mixed TPVs. For example, n value of CD2 and

PD2 was found to be 0.82 and 0.89, respectively. The

consistency index is defined as the viscosity value at unitFIG. 7. Dependence of viscoelastic property as a function of angular

frequency. (Full symbols for first frequency sweep and open symbols for

second sweep after an strain amplitude sweep and a relaxation time of

2 min. (a) Complex viscosity (g*). (b) Complex modulus (G*).)

FIG. 8. Joint shell model: describing the adsorption of rubber molecu-

lar chains or polymer segments on the surface of carbon black.


shear rate. It is observed that k value increases with an

increase in peroxide concentration for the preblended

TPVs. But, in case of phase mixed TPVs, k value

increases with increase in concentration of peroxide up to

3.6 phr, beyond which k value tends to decrease. This

indicates that PD4.6 sample shows lower resistance to

flow.

It is believed that crosslinked EOC domains are dis-

persed in the form of aggregates and agglomerates. Fur-

thermore, these aggregates and/or agglomerates are con-

nected via joint shell mechanism to form three-dimen-

sional network structure. It is inseparable in the given

experimental window, to determine the network formation

through direct bonding mode or indirect mode (through

polymer chains bridging on the different domains). It is

well known that, smaller the particle size, higher is the

surface area, shorter the interaggregate distance and hence

formation of more developed network via segmental inter-

diffusion mechanism [36, 37]. Larger crosslinked EOC

particles account to the decrease in the G0 and g* in both

strain sweep and frequency sweep tests. This invokes the

possibility of less adsorbed PP molecular chains and also

aid in the roll over of polymer chains over the coarser

crosslinked EOC domains or aggregates when deformed

in the shear mode. The increase in particle size also

reduces the network formation by increasing the distance

between the particles. Although the overall crosslink den-

sity and the gel content of the TPVs increases with

increase in concentration of peroxide in phase mixed

TPVs, dynamic functions decrease at higher concentration

of peroxide. However, in case of preblending method,

FIG. 9. Gel content and overall crosslink density values as a function

of concentration of peroxide for both preblended and phase mixed TPVs.

FIG. 10. Variation of viscoelastic properties of phase mixed TPVs.

(a) Storage modulus vs. Strain amplitude. (b) Complex viscosity vs. Fre-

quency.

FIG. 11. A speculative model of morphology development in dynami-

cally vulcanized blends prepared by preblending and phase mixing

method as a function of concentration of peroxide. C, preblending

method; P, phase mixing method; DCP, dicumyl peroxide.


within the curative concentration limit, initial storage

modulus value increases with increase in curative concen-

tration. Therefore, it can be expected that network forma-

tion overrides the effect of degradation in PP matrix

effect on the rheological measurements. These clearly em-

phasis that the primary factors which determine the melt

viscoelastic characteristics of the TPVs are the particle

size and their network formation via adsorption tendency.

Other important factor for the reinforcement mecha-

nism is the chain bridging effect, that is, influence of ma-

trix molecular weight on the property improvement [38].

When the matrix chain length is sufficiently long, PP

chains are immobilized by chain adsorption at one or sev-

eral points along their chain lengths which affect their

stress response on the dynamic experiments. In other

words, the generated liable bonds alter the mobility and

dynamic stiffness. High molecular weight species or long

chain molecules efficiently transfer the stress from the

crosslinked domains to the matrix. This contributes to the

reduction in the interlayer slip between matrix and dis-

persed domains which gives rise to increase in dynamic

function in frequency sweep and retraction of dynamic

functions after subsequent strain perturbation [38]. A

schematic representation illustrating difference in binding

the particles and the particle size of the TPVs prepared

by preblending TPVs (conventional method) and phase

mixed TPVs at equal concentration of curative (2 phr:

optimized concentration) is shown in Fig. 12. It is very

clear form Table 3 and Fig. 4a that PD2 shows higher

modulus value at low strain and more nonlinear behavior.

It is very clear from the morphological analysis that PD2

exhibits very small particle size whilst CD2 shows the

largest. It can be stated that, Payne effect depends on the

state and degree of network formation. The network

formed via joint shell mechanism would be much less

rigid which may disintegrate at a relatively lower level of

applied strain amplitude [32, 33]. Smaller crosslinked

EOC particles have higher tendency to adsorb the PP

chains on the surface and hence form more trapped PP

chains by connecting the particles. The adsorbed chains

on the surface of the domains via physical and/or chemi-

cal adsorption generally broaden the relaxation spectrum

towards longer time which has been reflected in the

higher dynamic modulus and viscosity in the frequency

and strain sweep experiments [32, 36, 37].

CONCLUSIONS

Rheological and morphological characteristics of perox-

ide cured PP/EOC TPVs have been analyzed with special

reference to the effect of blending sequence or mixing pro-

tocol. Dynamic rheological behavior of the samples was

studied and correlated with the final morphology. Irrespec-

TABLE 5. Flow behavior index (n) and consistency index (k) value for

the preblended and phase mixed TPVs as a function of peroxide

concentration.

Compound ID n k (kPa sn) Compound ID n k (kPa sn)

CD0.6 0.8012 7.24 PD0.6 0.8662 13.1

CD2.0 0.8210 12.02 PD2.0 0.894 15.4

CD3.6 0.8562 14.45 PD3.6 0.905 19.5

CD4.5 0.8819 18.62 PD4.5 0.913 7.76

FIG. 12. Schematic representation of possible structure developed by TPVs illustrating difference in binding

the particles and the particle size of the TPVs prepared by preblended TPVs (conventional method) and

phase mixed TPVs at 2 phr concentration of DCP.


tive of blending sequence used, dynamically cured blends

show higher dynamic modulus and viscosity than uncured

blend. At this particular composition ratio, TPVs respond

as a viscoelastic solid as a result of formation of three-

dimensional network structure. Among the methods

employed, phase mixed TPVs (PD2) shows higher dynamic

modulus and more nonlinear behavior. Interpretation of

results of increase in dynamic functions and the nonlinear-

ity still remains a subject of debate. The concept of modu-

lus recovery and network formation yields a good interpre-

tation. The network structure formed by the dispersed phase

mainly depends on the particle size and their interaction.

Recovery test carried out by subsequent strain sweep and

frequency sweep proved the presence of the memory effect

(Mullins effect) and the existence of the energetically elas-

tic network domains. Both polymer matrix characteristics

and the dispersed phase characteristics are equally impor-

tant in contributing to the overall dynamic properties of the

TPVs. There exists a direct consequence between the parti-

cle size and the degree of crosslinking in the dispersed

phase. The higher the degree of crosslinking in the dis-

persed phase, the smaller is the particle size. Lower the par-

ticle size (higher the surface area) better is the adsorption

on the matrix phase which leads to form better and more

developed network structure. When the matrix chain length

is sufficiently short, lower is the efficiency to transfer the

applied stress or strain between the aggregates and/or

agglomerates and also to the matrix phase.

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