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Sequential Coating of Nanoporeswith Charged Polymers: A GeneralApproach for Controlling PoreProperties of Self-AssembledBlock Copolymer Membranes

Doctoral Thesis

Author(s):Baettig, Julia

Publication date:2014

Permanent link:https://doi.org/10.3929/ethz-a-010385318

Rights / license:In Copyright - Non-Commercial Use Permitted

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DISS.-NO. ETH 22337

Sequential Coating of Nanopores with Charged

Polymers: A General Approach for Controlling

Pore Properties of Self-Assembled Block

Copolymer Membranes

A dissertation submitted to the

ETH ZÜRICH

for the degree of

Doctor of Sciences

presented by

Julia Baettig

M. A., State University of New York at Buffalo, U.S.A.

born May 14th

, 1980

citizen of the United States of America

accepted on the recommendation of

Prof. Dr. Dieter A. Schlüter, examiner

Prof. Dr. Anders Egede Daugaard, co-examiner

Prof. Dr. Lucio Isa, co-examiner

Dr. Anzar Khan, co-examiner

2014

Acknowledgements

I would like to thank several people for their time and effort, without which this dissertation

work would not have been possible. First, I would like to thank Prof. Dr. Dieter Schlüter for

allowing me to join the research group and giving me generous support, which was both

necessary and extraordinarily helpful, to complete my PhD.

For being the best supervisor I could ever imagine or ask for, I shall always be indebted to

Dr. Anzar Khan, whose constant guidance and help was the foundation for the work

contained here. For both the freedom to explore my own ideas and the direction to keep

working on a logical path, I will be forever grateful.

I thank my labmates, Dr. Jingyi Rao, Dr. Ikhlas Gadwal, Dr. Nergiz Cengiz, Selmar Binder,

and all of the other coworkers in the Schlüter group who have made the working environment

a happy place to be every day.

Special thanks go to Ms. Daniela Zehnder, for all of the help with administrative problems

and difficulties, whose support was invaluable. Also, to Dr. Thomas Schweizer for all of the

training and help making a glovebox, as well as the kind help with my GPC results.

An enormous amount of thanks goes to my husband, Pio Bättig, for the construction of all the

electronic equipment that I needed to perform my thesis work and the two years of being a

stay-at-home dad, without which I am certain this would not have been possible.

Finally, I would like to thank my committee members, Prof. Dr. Lucio Isa, Prof. Dr. Anders

Egede Daugaard, and Prof. Dr. Hans Christian Öttinger, who made my defense both a

constructive and relatively painless experience.

Table of Contents

Summary ................................................................................................................................................. ii

Zusammenfassung .................................................................................................................................. iii

Chapter 1: Introduction and Background ................................................................................................ 1

Aim and Organization of this Thesis ................................................................................................ 17

Chapter 2: Development and Testing of a Pressure Sensor Platform ................................................... 23

2.1 Amplification Data Collection Box Development ................................................................... 24

2.2 Software ................................................................................................................................... 28

2.3 Sensor Testing and Calibration ................................................................................................ 29

References ......................................................................................................................................... 32

Chapter 3: Membrane Formation and Layer-by-Layer Polyelectrolyte Deposition ............................. 33

3.1 Introduction, Membrane Formation and Characterization ...................................................... 33

3.2 Base Layer Deposition: Poly(Acrylic Acid) and Introduction to Pore Characterization through

Pressure Measurements ..................................................................................................................... 43

3.3 Layer-by-Layer Deposition of a Weak and a Strong Polyelectrolyte inside the Nanopores ... 51

3.4 Layer by layer Deposition: Weak-Weak and Strong-Strong Polyelectrolytes ........................ 67

3.5 Multilayer Testing and Characterization ................................................................................. 70

3.6 Conclusions ............................................................................................................................. 74

References ......................................................................................................................................... 77

Chapter 4: Separations Using Polyelectrolyte Multilayers in Nanostructured Phase-Inversion

Membranes ............................................................................................................................................ 81

4.1 Dye Separation ........................................................................................................................ 81

4.2 Size separation: Sieving curves ............................................................................................... 89

Chapter 5: Experimental Details ........................................................................................................... 99

General Conclusions ........................................................................................................................... 103

Appendix ............................................................................................................................................. 105

Symbols and Abrreviations ............................................................................................................. 105

Curriculum Vitae ................................................................................................................................ 107

i

ii

Summary

ecent advances in membrane technology have led to the development of block

copolymer membranes with an ordered isoporous structure formed by self-

assembly. The functionalization of such membranes imparts them with huge

potential in the fields of catalysis, water treatment, enzyme immobilization, and

chemical separations. Membranes can be functionalized by covalent modification or by the

addition of polymers that bind to the outermost membrane material, through ionic

interactions, hydrogen bonding, or hydrophobic interactions.

In this thesis, progress was made towards the development of a general strategy for the

functionalization of block copolymer membranes. Constructed from poly(styrene)-b-poly(4-

vinylpyridine) (PS-b-P4VP), these polymeric phase-inversion membranes possess a

polystyrene core and a surface entirely coated with P4VP. The initial functionalization of this

membrane was performed by using poly(acrylic acid), which has the unique property of being

able to both hydrogen bond with the 4-vinylpyridine groups and also simultaneously present

negatively charged carboxylate groups to the interior of the nanopore. Therefore, PAA was

used for a base layer from which to begin layer-by-layer assembly. This was then followed by

the layering of oppositely charged polyelectrolyte chains onto the pore wall by pushing

polyelectrolyte solutions in sequence through the membrane. A method was developed for

depositing polyelectrolyte multilayers onto and in the pores of a polystyrene-b-poly(4-

vinylpyridine) membrane. By using a strong and a weak polyelectrolyte, poly(styrene

sulfonate) and protonated poly(ethylene imine), we looked at layer development as it relates

to pH, and correspondingly charge of the poly(ethyleneimine). Weak-weak and strong-strong

polyelectrolyte combinations were also explored. A pressure sensor was used to monitor the

change in pore size during the layer deposition, providing valuable insight into the layer-by-

layer process inside of the nanopore. Using charged polyelectrolytes we made inroads to

control the pore size and chemistry. The pore size and chemistry was then explored with dye

separation and sieving curve analyses.

R

iii

Zusammenfassung

eit kurzem ist es möglich, Copolymermembranen herzustellen, bei denen eine

geordnete isopore Struktur durch Selbstorganisation erzielt wird. Werden diese

Membranen zusätzlich noch funktionalisiert, können sie in den Bereichen Katalyse,

Wasserbehandlung, Enzymimmobilisierung oder chemischer Trennung eingesetzt werden.

Diese Funktionalisierung geschieht entweder durch kovalente Verknüpfung oder durch das

Hinzufügen von Polymeren an die äusserste Membranschicht, wobei die Polymere entweder

durch ionische oder hydrophobe Wechselwirkungen oder durch Wasserstoffbrücken

gebunden werden.

Diese Dissertation dokumentiert die Fortschritte, die bei der Entwicklung einer allgemeinen

Strategie zur Funktionalisierung von Blockcopolymermembranen erzielt wurden. Ausgehend

von Polystyrol-b-poly(4-vinylpyridin) (PS-b-P4VP), besitzen diese durch Phasenumkehr

hergestellten Membranen einen Polystyrol-Kern und eine vollständig mit P4VP bedeckte

Oberfläche. Die erste Funktionalisierung dieser Membran wurde mit Polyacrylsäure (PAA)

durchgeführt. Diese hat die einzigartige Eigenschaft, einerseits Wasserstoffbrücken mit den

4-Vinylpyridin-Gruppen zu bilden und andererseits im Inneren der Nanoporen negativ

geladene Carboxylatgruppen zur Verfügung zu stellen. Aus diesen Gründen ist PAA ein

idealer Unterbau, um eine schichtweise Selbstorganisation zu beginnen. Auf die PAA folgen

mehrere Lagen von entgegengesetzt geladenen Polyelektrolytketten, die durch abwechselndes

Spülen der Membran mit Polyelektrolytlösungen an die Porenwände angebracht wurden. Es

wurde eine Methode entwickelt, um mehrlagige Polyelektrolyte auf und in die Poren einer

Polystyrol-b-poly(4-vinylpyridin)-Membran anzubringen. Ein starkes und ein schwaches

Polyelektrolyt wurden eingesetzt, d.h. Natriumpoly(styrolsulfonat) und protoniertes

Polyethylenimin, und der Schichtaufbau und seine pH-Abhängigkeit und damit die Ladung

des Polyethylenimins wurden untersucht. Darüber hiraus galt es schwach-schwache und

stark-starke Polyelektrolytkombinationen zu untersuchen. Mittels eines Drucksensors konnte

die Veränderung der Porengrösse während der Ablagerung der Schichten untersucht werden.

Dies ergab wertvolle Erkenntnisse über den schichtweisen Aufbau im Inneren der

S

iv

Nanoporen. Durch die Verwendung von geladenen Polyelektrolyten erzielten wir Fortschritte

in der Steuerung der Porengrösse und Porenchemie. Mit den so erhaltenen Membranen

wurden dann Trennungsexperimente an Farbstoffmischungen sowie Siebkurvenanalysen

durchgeführt.

v

1

Chapter 1: Introduction and Background

Membranes

The use of membranes in everyday life, both in and outside of a lab environment, has become

pervasive. In a water purification environment, advanced membranes provide reliable low-

fouling water purification through long lasting membranes. Pore sizes for these membranes

range from 3-5 Å in reverse osmosis membranes to several microns in microfiltration

membranes. In the chemistry lab as well, membranes provide easy filtration through syringe

filtration, ultrafiltration, and dialysis membranes. Choice of material, surface modification,

and pore size are each critical for the development of next-generation membranes – aiming to

increase selectivity, flux, and functionality.1

Membranes have also been used as supports for designed functionality – as catalytic supports

in membrane reactors,2 or to separate specific molecules by size or ionic character.

3 In the

biomedical realm as well, they have been suggested for uses in drug delivery and tissue

engineering.4

In general, a membrane performs as a barrier, in that it is designed to allow certain entities to

pass while preventing others from doing the same. The separation can be based on size,

shape, or chemical properties and used to perform such separation processes as filtration,

ultrafiltration, or dialysis (based on pore size), adsorption, absorption, or ion exchange (based

on affinity or charge).5 Considerations are taken for surface properties such as hydrophilicity

and for bulk properties – an inorganic membrane made of silica or alumina may be brittle but

considerably stronger than a polymeric membrane for example. For commercial membranes,

the most important properties are selectivity and flux – how well something can be separated

at what rate.5 For academic research on membranes, the goals are more far-ranging – from

catalysis6 to drug delivery

7 to tissue engineering,

8 adding new functionality and potential for

specific applications is key.

2

Membrane formation

A number of different methods are used for the formation of membranes. Electrospinning,

for example, can quickly produce fibers that are pressed into membranes of differing pore

sizes. Track-etched membranes consist of monodisperse pores that are etched into a solid

polymer disc, although the pore density is relatively low. Immersion precipitation occurs

when a polymer solution is spread in a nonsolvent and allowed to coagulate. By varying the

solvent, temperature, and time it is possible to gain a reasonable amount of control over pore

size and structure. Another method that is used is interfacial polymerization, which is

polymerizing a monomer at the interface of two incompatible liquids (such as oil and water).

This method is the most important for the fabrication of reverse osmosis membranes and

allows for a huge variety of monomers to be polymerized, forming membranes with

controlled functionality and hydrophilicity.1

Phase inversion membranes are formed when a polymer in a good solvent is immersed in a

miscible nonsolvent. Initially, these membranes were made with a single polymer and it was

observed that the types of voids that formed within the membrane were controllable by the

rate of solvent mixing, the bath temperature, and the evaporation rate and time of solvent

before entering the nonsolvent bath.9 For example, in the image shown in Figure 1, the

authors were able to significantly change the membrane structure simply by changing the

method of applying the nonsolvent to the polymer mixture.10

3

Figure 1-1: From Reference 10, changes in phase inversion membrane structure due to changes

in the nonsolvent application method; a: water bath, b: flowing water, c: 5mL of water, d: 5 mL

of 25 wt% NMP water.

Isoporous membranes

While membranes have become more controlled in pore size, they are still relatively

disordered - there are limited options when looking for a membrane with truly uniform pore

size. Track-etched membranes are one of these options, although the pore density is not

extremely high. Another option has been the anodization of alumina. These membranes can

be made in a wide range of pore sizes and form extremely ordered morphologies with

uniform pore size (Figure 2).11

On the other hand, these membranes are extremely brittle,

which can make them difficult to work with. Additionally, they need some either sort of

4

chemical modification or reliance of hydrophobic forces in the case of layer-by-layer self-

assembly in order to functionalize the surface.

Figure 1-2: From Reference 11, structure and pore size of porous anodized aluminum oxide membrane.

Finally, the recent development in phase-inversion membrane technology has provided a

breakthrough in block copolymer phase-inversion membranes. The paper of Peinemann et.al.

demonstrated the discovery of a phase-inversion method using polystyrene-b-poly(4-vinyl

pyridine) (PS-b-P4VP) which provided a well-ordered surface (Figure 3) reminiscent of the

structures found in solvent annealed block copolymer thin films, sitting atop a less-ordered

typical phase inversion membrane structure.12

5

Figure 1-3: From Reference 12, structure and pore size of PS-b-P4VP ordered phase inversion membrane, a: cross

section and b: top view. Scale bar is 500 nm.

Since the development of this membrane, a variety of additives have been probed in order to

improve the structure of the membranes, including magnesium acetate,13

2-(4'-

hydroxybenzeneazo) benzoic acid,14

and carbohydrates.15

Membrane Functionalization Overview

Because membrane functionalization is such an expansive topic, the background here has

been limited to the areas closest to the subject matter of the thesis – the functionalization and

modification of isoporous membranes and the important points of layer-by-layer self-

assembly in membranes of all types. A summary of the important points of layer-by-layer

self-assembly, which has been studied in extraordinary depth on surfaces, will be covered

later in the introduction.

Membrane Functionalization – Track-etched Membranes

Membranes can be functionalized by including a functional polymer in the membrane

formation process or by modifying the surface of a membrane with a small molecule. For

track-etched membranes, a few groups have used different techniques to achieve

modifications of these membranes.

Armstrong et.al. used single and multiple polyelectrolyte multilayers of poly(styrene

sulfonate) and poly(allylamine hydrochloride) to separate K+ from Mg

2+ ions in aqueous

solution.16

The initial layer of poly(styrene sulfonate) was bound to the track-etched

6

polycarbonate membrane through hydrophobic interactions. The charged environment

created by the layer inside of the 50 or 30 nm track-etched pore (Figure 4) led to a selectivity

of K+/Mg

2+ >10 at 8 mM ionic strength. Interestingly, it was found that an increasing number

of layers led to a reduction in the separation, implying that the surface charge decreases with

an increase in the number of multilayers.

It is important to mention here, with the discussion of polyelectrolyte multilayers in

membranes, that in no cases have polyelectrolytes been deposited selectively into pores. In

one case discussed later, for example, when polyelectrolytes are deposited into a pore to form

a nanotube, the surface of the membrane must be physically wiped of polyelectrolyte

multilayer before dissolution in order to have a nanotube and not some sort of structure

connected by a surface layer.17

This is important, for the most part, for visualization purposes

when it comes to functionalization of a membrane.

Figure 1-4: From Reference 16, track-etched membrane with a. 50 nm and b. 30 nm pores.

The Thayumnavan group took a different approach towards the functionalization of track-

etched membrane pores. Using a commercial track-etched polycarbonate membrane with a

thin layer of poly(vinyl pyrrolidone) (PVP) on top, the group used tin chloride to

7

functionalize the membrane surface with Sn2+

ions and then deposited anionic polymers

inside of the membrane to create different functionalities. Using only one or two layers of

polyelectrolyte, they were able to separate mixtures, through diffusion of water, of anionic

and cationic dyes (Figure 5c,d), mixtures of differently sized molecules with similar charges,

and mixtures of differently sized and charged proteins.18

Especially of interest was the way

that they were able to monitor the membrane pressure using a low-cost sensor (Figure 5a,b) –

this became the inspiration for the system design discussed later.

Figure 1-5: From Reference 18, a: sensor calibration using track-etched membrane pores, b: Sensor pressure

response from a change in membrane pore size, c and d: separation of positively charged rhodamine dye (red) from

negatively charged calcein dye (orange) with pores containing a positively charged (c) polymer and a negatively

charged (d) polymer.

Approaching smaller size ranges, the Martin group has performed extraordinary separations

using track-etched membranes with 30nm pores into which either gold or silver was

electroplated onto the interior pore wall. Controlling the thickness of the metallic film

allowed the pore to be reduced to molecular dimensions, allowing the separation of a larger

molecule like quinine from the relatively smaller pyridine.19

In another earlier study, polyelectrolyte multilayers were formed in track-etched membranes

using polypeptides (PLGA/PLL) and synthetic polyelectrolytes (PSS/PAH) to form

multilayers within a 200 nm pore size track-etched membrane. The membrane was then

studied for ion selectivity.20

One study that looked at pairs of strong polyelectrolytes in track-etched membrane pores of

different sizes, polystyrene sulfonate and poly(vinylbenzyl ammonium chloride) made layers

as thick as 50 nm inside of nanopores, filling even a 200 nm pore after just a few layers.21

a b c d

8

The potential applicability of layer-by-layer assemblies in track-etched membranes was

shown when in 2003 the Caruso group made layer-by-layer nanotubes from the pores of 400

nm track-etched membranes. Depositing positively charged PEI and then successive layers of

positively and negatively charged PAH and PSS or PAA on the surface of a polycarbonate

track etched membrane through immersion in polyelectrolyte solution and then dissolving the

polycarbonate membrane in DCM gave, in most cases, nanotubes which could then be

imaged through SEM.17

Hydrogen bonded polyelectrolyte multilayer nanotubes made in 200 nm pores were later

published, made from layers of poly(4-vinylpyridine) and poly(acrylic acid).22

These were

able to be made porous by dissolving the PAA layer in a NaOH solution.

In a study that provided insight into the differences between layer-by-layer assembly in

nanopores as opposed to on surfaces, the Jonas group made PAH/PSS multilayers in 100,

200, and 500 nm track-etched pores and then estimated the pore size using gas-flow

porometry. By observing pore diameters, it was determined that two “regimes” exist for

polyelectrolyte multilayer buildup in pores – a regime where layers behave like typical

multilayers (Regime 1 in Figure 6) and a regime in which a gel fills the pore due to

polyelectrolyte entanglement (Regime 2 in Figure 6). After drying, these layers shrink, as

they contain 20-40 percent water, to form the polyelectrolyte multilayer pore.23

9

Figure 1-6: From Reference 23, depiction of two layer-by-layer regimes in nanopores – Regime 1 where layer-by-

layer assembly proceeds as it does on surfaces, and Regime 2 where a more dense interwoven structure is present.

Membrane functionalization – Alumina membranes

Also published in 2003, the Li group made flexible polyelectrolyte nanotubes using an

alumina membrane as opposed to a track-etched one. The advantage of an alumina membrane

is that it can be dissolved easily in a NaOH solution. The PAH/PSS multilayers here were

extremely thick as well, with three layer-pairs being 50-80 nm thick. Another advantage of

using the alumina membrane was the pore density, which is much higher than that of track

etched membranes, and the group was able to create large numbers of nanotubes with

diameters of approximately 200 nm and lengths of up to 60 µm (Figure 7).24

10

Figure 1-7: From Reference 24: polyelectrolyte nanotubes at different magnifications, created from deposition in

alumina membranes followed by membrane dissolution. A, B, C, and D represent different views and magnifications

of the nanotubes.

The Bruening group has published by far the most literature on layer-by-layer assembly in the

pores of and on the surface of alumina membranes, exploring the area systematically and

exploiting the assembly to create new functional membranes. In one of the most elegant

examples, gold colloid nanoparticles were deposited onto PAA/PAH multilayers in 200 nm

pores (Figure 8), and then used to catalyze the reduction of 4-nitrophenol with extreme

efficiency in a flow-through reactor.25

This study also highlighted the potential utility of

layer-by-layer assembly in nanopores.

11

Figure 1-8: From Ref. 25, a) bare alumina membrane with 200 nm pore diameter and b) gold nanoparticles

immobilized onto one PAA/PAH multilayer.

Another innovative approach to layer-by-layer assembly in nanopores was taken this year by

the Nakashima group.26

Instead of using two polyelectrolytes, the group used a combination

of poly(acrylic acid) and Fe3+

ions in alternating layers to form nanotubes from 35 nm pores

in an alumina membrane.

Membrane functionalization – Highly Ordered Phase-Inversion Membranes

The Abetz group has already made some progress towards functionalizing the pore interior

by applying a layer of polydopamine, which can then be functionalized with an atom transfer

radical polymerization (ATRP) initiator or with another polymer.27

By prefunctionalizing

poly(dopamine) with 2-bromoisobutyryl bromide, a typical ATRP initiator, they were then

able to deposit the initiator onto the membrane surface with hydrogen bonding interactions

and grow hydroxyethyl methacrylate from the membrane surface, introducing new

functionality to the membrane.28

Using a different tactic for functionalization, the Stamm group functionalized PS-P4VP phase

inversion membranes by using either diiodobutane vapors or propane sultone to quaternize

the P4VP, resulting in a charged antifouling membrane surface.14

However, the

12

functionalization process reduced the flux, and most likely also was not a complete

functionalization of the surface as the pore remained open.

In general though, this type of isoporous membrane remains relatively unexplored as far as

functionalization is concerned. It presents an advantage in that the pore density is as high as

the alumina membrane but the membrane is much less brittle. The phase inversion process is

very simple and effective method of forming ordered membranes.

Layer-by-layer deposition in other types of pores

It is important to mention a couple of additional studies in order to understand the

mechanisms underlying layer-by-layer assembly in nanopores. First is the work of the

Azzaroni group, who deposited polyelectrolyte multilayers in a single conical nanopore, then

measured the current voltage curve to obtain information about surface charge within the

nanopore. What they found was that as layer-by-layer deposition proceeded the surface

charge inside of the pore decreased.29

In a second study, the Azzaroni group studied P4VP brushes inside of a solid state nanopore.

Especially relevant to the current project was the observation that the pH behavior of the

P4VP inside the nanopore was much different than the bulk. The ionic character of the P4VP

inside the nanopore reached into a much wider range of pH, especially as the pore was closed

to as little as 5 nm in size. What this meant was that a P4VP lined nanopore at pH 6 was

much more charged than the corresponding bulk P4VP would be. This charge difference

disappeared above approximately pH 7. 30

Ordered Nanoporous Thin Films – The Connection

The inspiration for the project came from the recent advances in nanoporous thin film

functionality. Highly ordered nanoporous thin films with pores lined with P4VP from blends

of PS-b-P4VP and poly(vinyl phenol)-b-poly(ethylene oxide) (PVPh-b-PEO) were

developed,31

and the next logical step was to find a way to functionalize and utilize these thin

films, perhaps as a membrane-type material. However, the manufacture of such a large area

13

of thin film without finding holes and defects is extraordinarily difficult. It is difficult enough

to float a thin film on water for examination with TEM, but to have an area the size of a

membrane would be performing the near-impossible. Membranes require highly uniform

surfaces in order to ensure complete filtration. In addition, starting from a pore diameter of 17

nm seemed to be quite limiting. So, as the Hillmyer group did when they moved from

polystyrene-polylactide (PS-PLA) block copolymer thin films to making PS-PLA

membranes,32

we looked towards the PS-P4VP membrane as a beginning for a membrane

system.

Because these highly ordered phase-inversion membranes are positioned at the juncture of the

two areas of block copolymer self-assembly and membrane science, these highly ordered

phase-inversion membranes seemed an ideal starting point for studying the potential for

modification – the membranes were relatively robust when compared to a block copolymer

thin film and the 4-vinylpyridine interior offered functionality with which to modify the

membrane. However, as the Nunes group had discovered when studying the pH responsive

character of the membrane, the protonation of the 4-vinylpyridine decorated pore interior

causes the polymer to swell and the pore to close.33

While this is potentially appealing for

some applications, it removes the most straightforward method for functionalization of the

interior of the uppermost layer of pores.

The goal of the project, then, became the combination of PS-P4VP membrane

functionalization with the enormous variety and potential found within layer-by-layer self-

assembly. By examining closely the formation of multilayers within the pore, it would be

potentially possible to control both the pore size and the pore chemistry, an appealing target

for advanced membranes. The PS-P4VP membrane would be advantageous, as it contains

inherent functionality in the nature of the P4VP polymer, which is both protonatable and an

excellent hydrogen bonding receptor. The isoporous nature of the membrane would help

ensure that the pore size was being reduced uniformly across the membrane.

Layer-by-layer Self-Assembly Overview

14

The world of polyelectrolyte multilayers was introduced by Decher, who first introduced the

idea of producing ultrathin polyelectrolyte films through the alternating deposition of layers

of alternately charged polyelectrolytes.34

As depicted in Figure 9, this extraordinarily simple

method of making thin organic films opened up an entire new arena of scientific research, to

which a large number of studies have been devoted over the past 20 years.

Figure 1-9: Depiction of polyelectrolyte multilayer buildup with poly(styrene sulfonate) and poly(allylamine

hydrochloride); from Reference 34.

It was later verified that the layer buildup is caused by an overcharging of the surface layer

after deposition, resulting in a positively or a negatively charged surface which is then

available for further deposition.35

Since the amount of research in polyelectrolyte multilayers is so enormous, this introduction

will cover only the necessary points for the present research. The first important message in

the body of research covering these systems is that every detail is important. The charge of

the polymer, the charge density of the polymer, the added electrolyte, the amount of time

spent in solution – even seemingly minute changes can lead to a change in the resulting layer

buildup.36

15

The type of polyelectrolyte used in the formation of multilayers plays an enormous role in the

buildup of the film. For example, polyelectrolytes poly(glutamic acid) and poly(allylamine

hydrochloride) have been shown to grow exponentially on surfaces.37

By simply switching

the poly(glutamic acid) to poly(styrene sulfonate), a strong polyelectrolyte, the growth was

changed to a linear process rather than an exponential one.38

Polyelectrolyte multilayers are deposited from water, and so the hydration level of the

multilayers is also a significant area of research. 39

This relates in particular to the Jonas work

mentioned earlier (Reference 23). Water content in multilayers is usually measured by some

type of reflectivity – either X-ray or neutron, and differences between dry thickness and

hydrated thickness show a hydration level between 20 and 50 percent of the film, depending

on whether D2O or H2O (as they have differing densities) is used in the study. However, it is

clear that water is a significant portion of any polyelectrolyte assembly. 39

The significant message of the research into polyelectrolyte multilayers is that every factor is

important and must be considered, and that conditions must be carefully controlled when

depositing multilayers.

Conclusion

A large amount of work has been done in the field of membrane functionalization. Recent

developments in membrane technology has allowed the formation of “isoporous” membranes

in which pore size is ordered and uniform. This thesis, following up on previous work done in

PS-b-P4VP nanoporous thin films, will explore possibilities of performing layer-by-layer

self-assembly in the pores of PS-b-P4VP phase inversion membranes. While layer-by-layer

assembly of two polyelectrolytes has been performed in larger pores (generally around 100-

200 nm) before, it has not been shown in a pore under 50 nm in size to the best of our

knowledge. Therefore, the development of polyelectrolyte multilayers in these membranes

will explore new “nano-scale” layer-by-layer deposition of polyelectrolytes and explore the

lower limits of deposition in nanopores. This will be made possible by the use of controlled

water flow and a pressure sensor as a real-time monitoring device to gain insight into the

deposition rates and layer thickness. Finally, some exploration of the potential of size and

16

charge based separations will be performed using dye (charge) and polymer (size) solutions.

With the combination of visualization (electron microscopy) techniques, separation

experiments, and pressure measurements, we will be able to gain new understanding into

polyelectrolyte multilayer formation in nanopores.

17

Aim and Organization of this Thesis

Modification of a membrane to control the chemistry and pore size is a challenging subject,

and the present thesis seeks to move towards the control of these by performing layer-by-

layer self-assembly inside the nanopores of a self-assembled polystyrene-b-poly(4-

vinylpyridine) phase inversion membrane. The thesis will develop a method both for

examining the layer-by-layer assembly as the process takes place using pressure

measurements and explore the possibilities of controlling the layer-by-layer assembly process

in a membrane with sub-50 nm pores. The potential for separations due to chemical

properties or size will then be explored.

Chapter 2

This chapter describes the development of a sensor platform that will allow a piezoelectric

pressure sensor to function as an in-line and real-time pore size monitoring system.

Chapter 3

This chapter will describe the layer-by-layer self-assembly of poly(styrene sulfonate) and

protonated poly(ethylene imine).

Chapter 4

This chapter describes the attempts at separations due to chemical properties such as charge

and hydrophilicity and size separation through sieving curves.

Chapter 5

This chapter describes the experimental details of the processes described herein.

18

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applications. Chem Eng Process 2009, 48 (1), 17-28.

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supported and hybrid liquid membranes. Sep Purif Technol 2001, 22-3 (1-3), 551-558.

4. Stamatialis, D. F.; Papenburg, B. J.; Girones, M.; Saiful, S.; Bettahalli, S. N. M.;

Schmitmeier, S.; Wessling, M., Medical applications of membranes: Drug delivery, artificial

organs and tissue engineering. J Membrane Sci 2008, 308 (1-2), 1-34.

5. Mulder, M., Basic principles of membrane technology. 2nd ed.; Kluwer Academic:

Dordrecht ; Boston, 1996; p 564 p.

6. Xu, J.; Bhattacharyya, D., Fe/Pd nanoparticle immobilization in microfiltration

membrane pores: Synthesis, characterization, and application in the dechlorination of

polychlorinated biphenyls. Ind Eng Chem Res 2007, 46 (8), 2348-2359.

7. Tao, S. L.; Desai, T. A., Microfabricated drug delivery systems: from particles to

pores. Adv Drug Deliver Rev 2003, 55 (3), 315-328.

8. Hadjizadeh, A.; Mohebbi-Kalhori, D., Porous hollow membrane sheet for tissue

engineering applications. J Biomed Mater Res A 2010, 93A (3), 1140-1150.

9. Guillen, G. R.; Pan, Y. J.; Li, M. H.; Hoek, E. M. V., Preparation and

Characterization of Membranes Formed by Nonsolvent Induced Phase Separation: A Review.

Ind Eng Chem Res 2011, 50 (7), 3798-3817.

10. Shao, X.; Dong, D. H.; Parkinson, G.; Li, C. Z., Microstructure control of oxygen

permeation membranes with templated microchannels. J Mater Chem A 2014, 2 (2), 410-417.

11. Lee, W.; Ji, R.; Gosele, U.; Nielsch, K., Fast fabrication of long-range ordered porous

alumina membranes by hard anodization. Nat Mater 2006, 5 (9), 741-747.

12. Peinemann, K. V.; Abetz, V.; Simon, P. F. W., Asymmetric superstructure formed in

a block copolymer via phase separation. Nat Mater 2007, 6 (12), 992-996.

19

13. Gallei, M.; Rangou, S.; Filiz, V.; Buhr, K.; Bolmer, S.; Abetz, C.; Abetz, V., The

Influence of Magnesium Acetate on the Structure Formation of Polystyrene-block-poly(4-

vinylpyridine)-Based Integral-Asymmetric Membranes. Macromol Chem Physic 2013, 214

(9), 1037-1046.

14. Tripathi, B. P.; Dubey, N. C.; Choudhury, S.; Simon, F.; Stamm, M., Antifouling and

antibiofouling pH responsive block copolymer based membranes by selective surface

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V., Carbohydrates as Additives for the Formation of Isoporous PS-b-P4VP Diblock

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layer coating of porous membrane templates. Adv Mater 2003, 15 (21), 1849-1853.

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23. Roy, C. J.; Dupont-Gillain, C.; Demoustier-Champagne, S.; Jonas, A. M.; Landoulsi,

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24. Ai, S. F.; Lu, G.; He, Q.; Li, J. B., Highly flexible polyelectrolyte nanotubes. J Am

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using layer-by-layer adsorption of polyelectrolyte/metal nanoparticle films in porous

supports. Nano Lett 2006, 6 (10), 2268-2272.

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Cation and Anionic Polymer in Nanoporous Anodic Aluminum Oxide with 35 nm Pore.

Chemistry Letters 2014, 43 (9), 1478-1480.

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Abetz, V., Double Stimuli-Responsive Isoporous Membranes via Post-Modification of pH-

Sensitive Self-Assembled Diblock Copolymer Membranes. Adv Funct Mater 2013, 23 (6),

731-738.

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Solid-State Nanochannels: When Nanoconfinement Really Matters. J Am Chem Soc 2010,

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31. Rao, J. Y.; Ma, H.; Baettig, J.; Woo, S.; Stuparu, M. C.; Bang, J.; Khan, A., Self-

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to porous materials with reactive nanochannel chemistry. Soft Matter 2014, 10 (31), 5755-

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32. (a) Querelle, S. E.; Jackson, E. A.; Cussler, E. L.; Hillinyer, M. A., Ultrafiltration

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Inter 2013, 5 (11), 5044-5050; (b) Zalusky, A. S.; Olayo-Valles, R.; Wolf, J. H.; Hillmyer,

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Hillmyer, M. A.; Cussler, E. L., Self-Assembled Block Copolymer Thin Films as Water

Filtration Membranes. Acs Appl Mater Inter 2010, 2 (3), 847-853.

33. Nunes, S. P.; Karunakaran, M.; Pradeep, N.; Behzad, A. R.; Hooghan, B.; Sougrat, R.;

He, H. Z.; Peinemann, K. V., From Micelle Supramolecular Assemblies in Selective Solvents

to Isoporous Membranes. Langmuir 2011, 27 (16), 10184-10190.

34. (a) Decher, G., Fuzzy nanoassemblies: Toward layered polymeric multicomposites.

Science 1997, 277 (5330), 1232-1237; (b) Decher, G.; Hong, J. D.; Schmitt, J., Buildup of

Ultrathin Multilayer Films by a Self-Assembly Process .3. Consecutively Alternating

Adsorption of Anionic and Cationic Polyelectrolytes on Charged Surfaces. Thin Solid Films

1992, 210 (1-2), 831-835; (c) Decher, G.; Hong, J. D., Buildup of Ultrathin Multilayer Films

by a Self-Assembly Process .2. Consecutive Adsorption of Anionic and Cationic Bipolar

Amphiphiles and Polyelectrolytes on Charged Surfaces. Ber Bunsen Phys Chem 1991, 95

(11), 1430-1434; (d) Decher, G.; Hong, J. D., Buildup of Ultrathin Multilayer Films by a

Self-Assembly Process .1. Consecutive Adsorption of Anionic and Cationic Bipolar

Amphiphiles on Charged Surfaces. Makromol Chem-M Symp 1991, 46, 321-327.

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Surface A 1998, 137 (1-3), 253-266.

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growth regime of polyelectrolyte multilayer films: Changing from exponential to linear

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22

39. Schonhoff, M.; Ball, V.; Bausch, A. R.; Dejugnat, C.; Delorme, N.; Glinel, K.;

Klitzing, R. V.; Steitz, R., Hydration and internal properties of polyelectrolyte multilayers.

Colloid Surface A 2007, 303 (1-2), 14-29.

23

Chapter 2: Development and Testing of a Pressure

Sensor Platform

Introduction

A measurement setup was developed in order to allow the in-situ measurement of layer-by-

layer deposition in the membrane. This would ideally be measured with a real time output to

monitor the layer as it was deposited, then recorded for further later data analysis. For ease

of calibration purposes, the sensor PX26-001DV from Omega Engineering was used. This

same sensor was also used in the Savariar et.al. paper on molecular discrimination inside of

nanotubules.1 This calibration would be done using a 70 cm high water column, providing an

easy method of measuring and calibrating the sensor with each set of measurements. For

ease of calculation, the effect on pressure due to altitude was neglected. This sensor has been

used as well for the use of hydraulic measurements in soil science using a commercial data

logger and a INA125 instrumentation amplifier setup.2 The circuit was developed prior to

knowledge of this publication.

The overall setup was envisioned as such: a syringe pump would provide a constant and

adjustable flow of water to the membrane holder, connected to the pressure transducer

through a T-piece. The pressure transducer would be connected to the amplification/data

collection box via the provided connector (Omega Engineering). The scale would provide

flow rate data through a serial cable, connected to the amplification/data collection box,

which would then provide output data to the computer, which would act as a data logger

(Figure 1). A photo of the actual setup is also provided in Figure 1. This provided a basic

setup for low-cost membrane measurements with easy calibration and as little introduction of

new variables as possible. The major development performed was in the construction of the

instrumentation amplifier/data collection box, which will be described further in this chapter.

24

Figure 2-1: Schematic (top) and picture (bottom) of the sensor and filter-holder setup.

2.1 Amplification Data Collection Box Development

This section describes the development and layout of the amplification and data collection

box used to gather membrane pressure data from the PX26-001DV sensor. The circuit board

layout can be found in Figure 11, and the electrical schematic can be found in the appendix.

25

Figure 2-2: Layout of data collection box circuit board.

Power supply and stabilization circuitry

An external supply with 9-12 volts was fed to linear regulator (7805), which furnishes a

stabilized 5V for the digital electronics. For the sensor, instrumentation amplifier and op-amp

(operational amplifier), higher positive and negative supply voltages are needed. For this

purpose a Traco Power TMA0512D Dual output +/- 12 V DC/DC converter was used. The

voltages are stabilized with LC (inductance-capacity) low pass filters (L1-C14 for +12 V and

L2-C15 for -12 V) with a cutoff frequency of about 5.9 kHz (Fcut=1/(2*Pi*Sqrt(L*C))). This

reduces noise in the output signal. As the output voltage of the unloaded DC-DC-converter is

higher than 12 V, it is then limited to +/- 12 V over two Zener-Diodes (D3 and D4).

Microcontroller and Support Circuitry

An Atmel ATmega1284p was chosen as microcontroller (MCU) to control the circuitry,

interface to the scale, provide the reference voltage and digitize the sensor output signal. The

ATmega1284p was chosen for the following reasons:

It is easily programmable with the Arduino (www.arduino.cc) environment.

It has a second serial port that can be used to interface with a scale, the first serial port

is used for interfacing with the computer via a UART-USB converter.

It is available reasonably cheaply in single quantities (about 9.25 CHF)

26

The programming of the bootloader requires a 6-pin ISP (In System Programming) interface.

Clock generation for the MCU is provided via a 16 MHz crystal and two 22 pF load

capacitors. Jumper JP4 interfaces to two buttons (with 2 10 kOhm pullup resistors R16 and

R17) and two LEDs via 2 270 Ohm current limiting resistors (R14 and R15) to prevent the

LEDs from pulling too much current and burning up.

There are two I2C interfaces provided on the board. One is used by the 2*16 character

HD44780 display via a PCF8574 port expander, the second one is for possible additional

peripherals but was not needed in this case. A reset switch with its 10 kOhm pullup resistor is

provided. All power supply pins are connected to the 5V VCC rail via 100 nF bypass

capacitors. These stabilize the rail for transient load and “short circuit” noise to ground,

which serve to stabilize the supply network.

USB interface

Once the bootloader is written to the microcontroller via an external programmer, the

ATmega1284p can be easily programmed via its’ built in serial port. Also, once in operation,

a way to get the data from the microcontroller to the computer is needed, and conveniently

provided by the same port. In order to interface this port to a computer a USB to serial

converter is required.

This interface is provided by a FTDI FT231X USB-UART converter in a TSSOP-20

package, a USB-to-serial interface. Circuitry follows the data sheet for a FT231X in Self

Powered Configuration. Another possibility would have been to forgo the external supply and

use the power supplied via USB from the computer but that would have necessitated more

stabilization circuitry.

The FT231X connects to the ATmega1284P via the serial input (RXD0) and output (TXD0)

pins as well as to the reset pin via capacitor C.

Scale Interface

In lieu of a dedicated flowmeter, a Mettler Toledo DeltaRange PG4002-S was used. The scale

provides a RS-232 serial interface through which the current weight on its load cell can be

27

exported. As the RS-232 protocol sends the strings of bits (0 or 1) by alternating between

logic 0: [3..15 V RS-232] and logic 1: [-15..-3 V RS-232] and the serial interface of the MCU

works with 0 and 5 V for logic 0 and 1 respectively, a converter chip was needed, provided in

the form of a STMicroelectronics ST232C with four external capacitors for the charge

pumps. On the PCB a 3-pole header is provided which is connected to a DB9 connector at the

back of the interface. The ST232 is connected to the second serial port (RX1/TX1 on the

MCU).

Sensor Interface

In order to measure the pressure at the membrane, a differential pressure sensor Omega

PX26-001DV was used. One port is connected to the water circuit, the other port is left open

to the atmospheric air pressure. The sensor outputs a differential volrFW between

MOUTSENS and POUTSENS proportional to the applied pressure (0: 0 V, 1 Psi: 16.7 mV)

for an input voltage of 10.0 volts. As the sensor is ratiometric, its output is also proportional

to the applied supply voltage; a fact that we could turn to our advantage by producing the

supply voltage from the ADC reference voltage. Any fluctuation of the full-scale reference

would thus be the same for the sensor and the ADC and cancel out.

The ADC of the ATmega1284p can either be supplied with an external analog reference

voltage, use the 5 V supply voltage or generate “Selectable 2.56V or 1.1V ADC Reference

Voltage”[1] via bandgap references. In the case that the internal references are used, they are

provided at the AREF- pin (analog Reference) of the MCU.

It was decided to use the 2.56 V reference from the MCU and multiply it by four to 10.24 V

using a STMicroelectronics LM324 Op-Amp (operational amplifier) in non-inverting voltage

amplifier configuration.

The LM324 is a quad Op-Amp (contains four individual op amps). IC2C is used for the

multiplicaton. The multiplication factor is set as 4 by two 0.1% tolerance precision resistors

R22 (10 kOhm) and R23 (30 kOhm). The amplification is calculated via the following

equation:

Amp=(R22+R23)/R22

28

The op-amp serves the purpose of having a high input impedance which prevents a voltage

drop of the source that may have become problematic.

The 10.24 Volt from the op-amp are then provided:

As supply voltage for the pressure sensor.

To the zero reference input for the instrumentation amplifier via a 10 kOhm precision

resistor and a 200 Ohm trimmer so the zero output can be adjusted from 0 to about

200 mV (200 Ω* 10.24 V/10200 Ω) via a second Op Amp in voltage follower

configuration. For digitalization of the signal with the ADC it is advantageous that the

zero point be set slightly above (the noise at) 0V.

The signal from the pressure amplifier is directly fed into the differential inputs of the

instrumentation amplifier (U3, Texas Instruments INA126), which is “basically” constructed

from two operational amplifiers and precision trimmed resistors that determine the voltage

gain with the external resistor RG following this relation:

G=5+(80 kΩ/RG)

In our case we use a 0.1% tolerance 680 Ohm resistor, resulting in an amplification factor of

122.647.

The maximum output voltage from the sensor at 1 psi is calculated as follows:

10.24 V*16.7 mV/10.0 V = 17.1008 mV

while the maximum output voltage after instrumentation amplifier INA126 is:

17.1mV*122.65=2097.3628 mV

The output voltage from the INA126 is fed into the ADC via a 4.7 kOhm resistor and a diode

from ground both of which serve to protect the ADC input. We are using the 2.56 V internal

reference of the microcontroller as the full scale comparison voltage for the ADC.

2.2 Software

A program running on the microcontroller serves to interface with the computer by outputting

the data via serial interface on the microcontroller, which is output to the computer via a USB

to serial interface. The program runs through its initialization routine where the interface to

29

the scale and to the computer are set up, the display is initialized, the analog reference voltage

for the ADC (2.56 V) is set up. Simultaneously, the timer function is started.

The user is then asked if he wants to calibrate the sensor with 2 water pressures (0 and 70 cm)

and if the scale should be tared to 0. Otherwise, the measuring mode is started and runs

forever in a loop.

In the measurement mode, the function runacqdisp is run at an interval of one second

(triggered by an interrupt). That function outputs a counter value, the weight from the scale,

the raw sensor data, and the calculated pressure (from raw data using the calibration values)

to the serial port and starts a function displaying the weight and the raw sensor data on the

display. After that, the function runmeasure is called which gets the measurements for the

next cycle. The current weight is polled off the scale and 256 measurements of the sensor

data are called every 2 ms. The calibration routine asks for the application of 0 pressure (no

water level above the sensor input) and for the application of 703 mm H2O (corresponding to

the full measurement scale of the sensor. These values are measured and stored to the

EEPROM (nonvolatile memory in the microcontroller).

2.3 Sensor Testing and Calibration

The sensor was tested and calibrated using a 1m high water column made of PVC tubing. A

buret was not used due to problems with air bubbles. Water levels of 10 cm difference were

recorded; the output can be found in Figure 12.

30

Figure 2-3: Sensor testing raw data output for different water levels. Bottom graph is a subset of top graph to show

data points.

As can be seen in the graph, there is no visible noise while pressure is being recorded, and the

pressure measurements can be taken over long periods of time without change (200 minutes

total for the graph shown). The major unavoidable source of noise was the syringe pump,

which contains a stepper motor that produces a periodic noise seen in Figure 13. While this

did produce a relatively large noise spectrum, it could be averaged out and did not

significantly increase the standard deviation of the results.

31

Figure 2-4: Graph of periodic noise from syringe pump.

32

References

1. Savariar, E. N.; Krishnamoorthy, K.; Thayumanavan, S., Molecular discrimination

inside polymer nanotubules. Nat Nanotechnol 2008, 3 (2), 112-117.

2. Gnecchi, J. A. G., Instrumentation for Measurement of Laboratory and In-Situ Soil

Hydraulic Conductivity Properties. Self 4 (7.0), 11.2.

33

Chapter 3: Membrane Formation and Layer-by-

Layer Polyelectrolyte Deposition

3.1 Introduction, Membrane Formation and Characterization

3.1.1 Introduction

The most studied layer-by-layer self-assemblies on surfaces have been through ionic

interactions between layers,1 and this seemed an ideal place to begin for layer-by-layer

deposition in nanopores. As mentioned earlier as well, the Bruening group utilized these

electrostatic interactions to perform layer-by-layer self-assembly in larger-pored aluminum

oxide membranes and were able to immobilize gold nanoparticles within the pores

successfully.2 The goal of this chapter was to successfully perform layer-by-layer deposition

in isoporous membranes, the formation of which (as described in the introduction) was

introduced by Peinemann et.al3 and further developed by Nunes et.al.

4 By doing so, it would

be possible to both control the pore size within the membrane and open up new possibilities

for additional functionalization within the membrane pores.

3.1.2 Isoporous Phase-Inversion Membrane Formation and Characterization

The membrane was formed by the phase inversion method using 1:1:1 DMF:THF:dioxane as

a solvent mixture using a commercially available block copolymer. These membranes consist

entirely of a single block copolymer, PS-b-P4VP of high molecular weight: PS(145 kDa)-b-

P4VP(50 kDa), Mw/Mn=1.07 (Figure 1).

34

Figure 3-1: Structure of the block copolymer, poly(styrene)-b-poly(4-vinylpyridine).

This high molecular weight is necessary for the fast phase separation that occurs in the

membrane and for the relatively (compared to typical block copolymer thin films) large pore

size. The polystyrene block composes the majority of the membrane, with the interior of the

pores, both in the ordered and disordered areas of the membrane, are coated with a P4VP

brush, the minor portion of the block copolymer. The top ~500 nm of the membrane are an

ordered PS-b-P4VP structure reminiscent of block copolymer self-assembled cylindrical thin

films, while the bottom portion of the membrane is a typical disordered phase-inversion

membrane, with a pore size much larger than the nanoporous top layer.

The ordered morphology (Figure 2) on the surface of the membrane is caused by the fast

evaporation of THF in a similar manner to block copolymer self-assembled cylindrical

morphology that has been demonstrated in thin films upon evaporation.5 During the “phase

inversion” process the more hydrophilic DMF remains in the P4VP block, allowing the

evaporation of the THF to form ordered cylinders while keeping the P4VP cylinder swollen

with solvent since DMF has a much higher boiling point. Membrane thickness was measured

by examining a cross-section by SEM. While the phase inversion membrane was formed

using a doctor blade with a gate height of 250 μm, once the solvent had evaporated the

membrane height had been reduced to 65 μm in size, as measured by SEM (Figure 2).

35

Figure 3-2: SEM images of surface and bottom (top left and right), cross section (middle) and wide top views

(bottom) of formed PS-b-P4VP membrane.

36

It was possible to see the pore morphology through cross-section images of the initial

membrane as well. From these images (Figure 3) it was determined that the pore was straight,

up to 750 nm in length, and possessed the same diameter as the surface pores (between 40

and 50 nm). As seen in the right image in Figure 3, a less-ordered portion of the surface

served to add cohesion to the structure and gave better pore cross-section images.

Figure 3-3: SEM cross-sections showing straight pores through the surface layer of the membrane.

Further examination of the membrane surface revealed that the edges of the as-formed

membrane contained a less ordered morphology than the central portion (Figure 4, top). Parts

of the formed membrane had defects as well (Figure 4, bottom), which could potentially

affect the final pore size distribution of the membrane. Because of these observations, a

sample of each membrane was examined by SEM prior to use.

37

Figure 3-4: SEM of defects observed in different portions of membranes. The top right image is a magnified view of

the darker regions of the top left image. The bottom images show defective portions near the edge of the membrane

area.

3.1.3 Pore polydispersity

As a beginning measure for characterization of the membrane, it was important to understand

the membrane structure and quantify the “isoporous” nature of the pores. If the pores would

then be slowly closed, then the dispersity of the pores would become increasingly important

as the smallest pores would be blocked completely before larger pores. A crude calculation of

the pore polydispersity could be obtained using Matlab, which is capable of image processing

(program found in Appendix B). A SEM picture containing a 2x2 µm area of the polymer

membrane surface was used for the calculation (Figure 5). The average pore size was found

to be 40.6 nm with a standard deviation of 6.72 nm and a PDI of 1.03.

38

Figure 3-5: Histogram showing pore size distribution (left), and image with pores outlined in red as processed by

MATLAB (right, 2x2 µm).

3.1.4 Flux calculation at low flow rates

Membrane flux is defined as the volume of liquid through a membrane divided by the time

and the area:

J =Q

S

where J is the filtrate flux (L/h*m2) where L is liters, h is hours, and m is meters, Q is the

filtrate flow (L/h), and S is the membrane surface area. When pressure is factored in, the

instantaneous specific flux is found by the equation:

JS =J

ΔP

Where Js is the instantaneous specific flux (L*h-1

*m-2

*bar-1

) and ΔP is the transmembrane

pressure (TMP, bar).6

In general, membrane flux calculations are carried out using commercial equipment at high

pressures (1 bar or more). However, due to the limitations of the sensor (a maximum pressure

of 1 psi (0.069 bar), all of the experiments and flux calculations were performed at a much

lower flow rate and pressure. It was verified that the pressure within the range of the sensor

was linear with the flow rate (Figure 6). With this knowledge, the pressure could be scaled

39

from one flow rate to another for easy comparison when the pressure exceeded the limitations

of the pressure sensor. The exception to this was at very low flow rates and very low

pressure, <0.01471 bar, where the pressure essentially leveled off. At higher pressures, the

flow rate maintained the proper ratio with pressure.

Solution viscosity is also a consideration when calculating membrane flux values. Because of

this, the pressure value was always taken after the layer deposition using buffer solutions

rather than during the addition of the polyelectrolyte, so that viscosity considerations may be

neglected.

Figure 3-6: Pressure vs. flow rate at different syringe pump settings. This allowed the flux to be compared despite

different flow rates during deposition (to maintain a pressure under 1 PSI).

There were some variations in flux values for the membranes as well (Table 1), which can be

attributed to the number of defects in the membrane surface, as was seen in Figure 4. In

addition, the membranes were made by hand with a doctor blade, while commercial

membranes are made using belt-driven membrane forming machines. The measured

membrane flux was close to flux measurements published by the Stamm group using a

commercial membrane setup, which reported 600 Lm-2

h-1

bar-1

.7

40

Table 3-1: Initial pressure and flow rate of membranes.

Flux (Lm-2

h-1

bar-1

) Std. Dev.

Membrane A 547.12 0.593

Membrane B 601.44 0.732

Membrane C 439.99 0.440

Membrane D 792.60 0.565

Membrane E 501.86 0.816

Membrane F 370.02 0.491

Membrane G 415.09 0.393

Some experiments were attempted before the pressure sensor was completed at a much

higher flow rate as well. In these cases, the higher deposition rate seemed to either promote

membrane fouling, to form layers that were so thick that the membrane fouled after only a

few layers, or both. It seemed after these initial experiments that the low flow rate was

necessary for the controlled deposition of polyelectrolyte multilayers.

3.1.5 Flux decrease with 50% methanol/water mixture

Because of the study by Cho et.al concerning layer-by-layer assembly with poly(acrylic acid)

and P4VP where the layers were deposited in 1:1 methanol:water,8 a 50% methanol solution

was tested in the membrane. Unfortunately, due to the solubility of the P4VP chains in

methanol and the insolubility in water, the 50% methanol environment caused the chains to

swell and closed the pore substantially, causing the pressure to increase dramatically, the

corresponding flux decreasing from 507 to 30 L/h*m2*bar. It was for this reason that

methanol was not considered as an option for the deposition of layers within the pores.

41

3.1.6 Flux increase after rinsing with 50% methanol water solution

Fortuitously, after rinsing with methanol and removal of the methanol by flushing with water

for 24 hours, the flux through the membrane was noticeably increased, as observed by a

decrease in the pressure at a constant flow rate. This could be understood as a rearrangement

and collapse of the P4VP chains, which was not possible in water, in which P4VP is

insoluble. The methanol could partially solubilize the P4VP chains and as the methanol was

removed by rinsing with water, the chains would be forced into a collapsed state along the

pore wall. This made an ideal base layer for multilayer deposition, as will be discussed at a

later point.

Solution Flux (Lm-2

h-1

bar-1

) Std. Dev.

Buffer pH 7.4 506.86 0.4533

50/50 MeOH/H2O 30.426 0.6121

Buffer pH 7.4 1584.5 0.9061

Table 3-2: Pressure difference before, during, and after a solution of 50% methanol water.

3.1.7 Flux rate with different pH buffer solutions

To test the effect of pH on pore size and flux, a few different buffer solutions were tested

(Table 3). At the low flow rates used in this set of experiments, and in contrast to the pH

responsive behavior reported by Nunes,4 the pressure at a constant flow rate did not increase

until the pH was dropped to 4, and even then the pressure did not rise dramatically. One

explanation would be that with the low force exerted on the membrane (because of the low

flow rate and low pressure), the hydrophobic P4VP surface screened the interior pyridine

moieties from the acidic solution, and thus only the P4VP at the surface layer was protonated,

causing the pore to close only slightly. As the pressure sensor was limited to approximately

42

70 mbar, and the pH responsiveness of the membrane has been demonstrated before, this was

not explored further.

Table 3-3: Pressure testing with different pH buffer solutions, at a flow rate of 4.6x10-4.

Solution Pressure

(bar)

Std. Dev. Flux

(Lm-2

h-1

bar-1

)

Millipore water 0.0348 0.00130 473

0.1M buffer solution, pH 7.4 0.0325 0.00126 507

0.1M buffer solution, pH 5 0.0328 0.00177 502

0.1M buffer solution, pH 4 0.0357 0.00130 461

Millipore water (after buffer solutions) 0.0287 0.00131 572

3.1.8 Crosslinking results

In an effort to potentially stabilize and charge the P4VP layer, the membrane was crosslinked

using diiodopropane vapors. After three days of exposure to vapors, the membrane could no

longer be dissolved in dichloromethane, indicating successful crosslinking. However, upon

examination with SEM it was seen clearly that the pores had reacted in a similar manner to

the application of low pH aqueous solution and the pores had closed as the 4-vinylpyridine

became positively charged (Figure 7). Some of the pores had become smaller, while many

appeared to have been eliminated entirely. The Stamm group had functionalized the P4VP

portion of PS-b-P4VP membranes by crosslinking with diiodobutane vapors in previous

reports;7 but it is likely that the degree of protonation was lower, and as a decrease in flux

was reported as well it was decided not to pursue this method of functionalization.

43

Figure 3-7: SEM image of the membrane surface after crosslinking with diiodopropane vapor.

3.2 Base Layer Deposition: Poly(Acrylic Acid) and Introduction to Pore

Characterization through Pressure Measurements

3.2.1 Poly(acrylic acid) as a base layer for pore functionalization – interactions between

PAA and P4VP

Because of the limited options for functionalization of P4VP, and the pore-closing effects of

lowering the pH, poly(acrylic acid) was chosen as a base layer for attachment. Poly(acrylic

acid) has the advantage of being able to both hydrogen bond with the P4VP layer and to

present deprotonated and negatively charged carboxylate groups for further ionic deposition

at the interior of the pore surface (Figure 8).

44

Figure 3-8: Depiction of poly(acrylic acid) layer adhesion to P4VP layer inside the pore wall. Counterions are

omitted.

It was critical to understand the interaction between poly(acrylic acid) and P4VP as this base

layer would be the “glue” for the entire assembly above it. While interactions between P4VP

and PAA have been studied before in neutral micelle solutions,9 and indeed used for

polyelectrolyte multilayers from methanol solutions in much larger nanopores,10

it was

critical to have some understanding of the interactions between the two polymers before

attempting to use PAA inside of the membrane pores. Infrared spectroscopy was used to gain

an understanding of the ionization of the poly(acrylic acid) and the way that it interacts in a

neutral aqueous solution. IR spectra were carried out on solid polymer samples, after freeze-

drying in the case of mixing experiments. It was also important to note that the poly(acrylic

acid) used was in its protonated form (not the sodium salt) as received from Sigma-Aldrich.

This is important because in some studies which look at the ionization titrations of PAA, the

anionic poly(acrylic acid) is nearly fully deprotonated at pH 7 and would therefore be

unlikely to hydrogen bond.8 In an alternate study which looked at the ionization of films of

PAA cast from different pH solutions, this number was closer to 60% ionization at pH 7.11

It

was clear from looking at the interaction in solution between poly(acrylic acid) and low

molecular weight P4VP that there is at least some hydrogen bonding present, as the P4VP is

partially solubilized by the poly(acrylic acid), an interaction that takes place neither with

PEI·HCl or PSS, the positively and negatively charged electrolytes used in this chapter

45

(Figure 9). The P4VP, which is insoluble in water, remains unchanged in solutions of pure

water, PSS, and PEI·HCl, while P4VP combined with an aqueous PAA solution in pH 7.4

buffer becomes cloudy.

Figure 3-9: Solutions of polymer mixtures, from left to right: P4VP in water; PAA in water; PAA and P4VP in water;

P4VP and PSS in water; P4VP and PEI·HCl in water. Each solution sat with water-insoluble P4VP (visible in far-left

vial) for 1 week.

This solubilization can be seen by looking at infrared spectra of P4VP, PAA and both PAA

adsorbed onto P4VP and a freeze-dried mixture of P4VP+PAA from pH 7.4 buffer solution

(Figure 10). In the mixture of PAA and P4VP, the PAA concentration is much higher as the

P4VP is only partially solubilized. However, the peak at 1710-1730 cm-1

, which represents

the protonated carboxylic acid group, is visible, despite the much larger deprotonated acid

group at 1590 cm-1

. The amount of PAA vs. P4VP can be seen in the peaks around 2938 cm-1

for PAA which represents the –OH stretching frequency12

– the P4VP peaks have nearly

disappeared, while being overwhelmed by the PAA peak. When PAA is adsorbed onto a solid

piece of P4VP and then washed thoroughly with water, the P4VP peaks are much clearer as it

is the overwhelming majority of the material. However, the interesting feature of this IR

spectrum is the visibility of the peak at 1700 cm-1

. This peak indicates either that the pyridine

nitrogen has been protonated, which is unlikely at pH 7.4, or that the adhered poly(acrylic

acid) is nearly all protonated, as the peak at 1590 cm-1

for deprotonated PAA is not visible.

Therefore, it is clear that the conditions at the surface are much different than conditions in

46

solution. This is supported by a study which looked at the propensity for P4VP to be

protonated when in the presence of a strong anion. The study concluded that, while it is

possible to induce ionic interactions with a strong anion, the interaction between poly(acrylic

acid) and P4VP is predominantly one of hydrogen bonding.13

Overall, in a pH 7.4 solution,

the majority of both P4VP and PAA chains are deprotonated, but when a layer of PAA is

adhered to the surface of P4VP there is an increase in the number of protonated chains,

allowing the layer deposition to take place.

Figure 3-10: IR spectra of solid poly(acrylic acid), solid poly(4-vinylpyridine), poly(acrylic acid) adsorbed onto a solid

piece of P4VP, and a mixture of PAA and P4VP freeze-dried from pH 7.4 buffer solution. Relevant IR peaks:

protonated PAA carbonyl at 1700 cm-1, deprotonated PAA at 1590 cm-1, protonated pyridine nitrogen at1700 cm-1,

deprotonated pyridine nitrogen at 1610 cm-1, PAA –OH group at2930 cm-1.

47

3.2.2 Poly(acrylic acid) Deposition and Pore Characterization through Pressure

Measurements

Deposition of the PAA layer at low flow rates showed little increase in pressure. This was

expected, as pore diameter vs. area does not change in a linear fashion because the pore is

circular, as the basic graph of pore size vs pressure (assuming a constant flow rate) in Figure

11 shows. This can be modeled through the Hagen-Poiseuille equation, which is used in fluid

dynamics to describe a fluid as it passes through a cylindrical pipe:

𝐽 =𝜋∆𝑃𝑟𝑝

4𝜏𝑛𝑝

8𝜂∆𝑥

where J is the solvent flux (not the specific flux, which involves pressure, but the filtrate flux

involving flow rate and surface area), ΔP is the pressure rp is the pore radius,τ is the tortuosity

factor, Δx is the membrane thickness, and η is the viscosity of the solution.14

It has been

noted in the literature that very few membranes follow this equation closely due to their

structure.15

However, using it as a basic model for the expected increase in pressure, and

assuming that all of the other factors are a constant k, we can plot

∆𝑃 = 𝑘 ∗1

𝑟𝑝4

to get a general idea of what the pressure data should look like as a pore closes. As is seen in

Figure 11, we would expect to see a very slow increase in pressure up to a point, and then a

rapid increase as the pore becomes progressively narrow.

Unfortunately, this also becomes problematic when attempting to reach very small pore sizes.

Using the average pore size of 40.6 nm as a starting number, it is possible to roughly

determine pore size using flux data calculated from the pressure data. Rearranging the Hagen-

Poiseuille equation once more and making the factors that are constant into a constant k,

where dp is the diameter of the pore:

𝐽

∆𝑃= 𝑘 ∗ 𝑑𝑝

4

48

so that when comparing two pore sizes, this rearranges again to the equation:

𝑑𝑝1 = (𝐽2 ∗ 𝑑𝑝1

4 ∗ ∆𝑃1

∆𝑃2 ∗ 𝐽1)1 4⁄

and from this it is possible to gain a rough determination of the final pore size. Since J/ΔP is

the definition of the specific flux, this is basically comparing the two specific flux

measurements to get a pore size estimate. This is a rough estimate because the tortuosity of

the pore most likely changes during layer deposition, the surface properties of the pore differ

due to more or less hydrophilic polymers on the surface, and the fact that as the pores

becomes more and more closed the smallest of the pores in the distribution found in Figure 5

may be blocked entirely. A disclaimer must be made here, that since neither the flow rate or

the pressure is held constant for the entire experiment, and just the ratios compared, that it

would be extremely difficult to determine additional information from the calculations here,

for example to examine the other parameters of the Hagen-Poiseuille equation.

Another limitation arises due to the same situation portrayed in Figure 11: as the pressure

increases according to a power function; while the pore size approaches zero, the lowest

possible pore size that can be measured with the sensor limitation of 1 psi and the slowest

setting on the syringe pump would be 10.5 nm. With a pressure sensor able to read 10 psi, the

pore size limitation would be 5.9 nm and with 100 psi 3.3 nm. A 1 nm pore size could nearly

be measured with a 1000 psi sensor. This could also be worked around to a small extent by

using a larger membrane surface area. Overall, this provides an explanation for why

commercial systems work on much larger pressure scales with larger membranes; however,

such a sensor which operated at 1000 psi would not be as sensitive.

49

Figure 3-11: Graph of y=1/x4 as pore size approaches zero.

The layer deposition did increase the pressure though, and then leveled off as the P4VP

groups on the pore surface became saturated with hydrogen-bound PAA chains (Figure 12).

There was no evidence of membrane fouling, which would be seen by a constant increase in

pressure.

50

Figure 3-12: Pressure increase with time as 0.01 M WRT monomer 5k poly(acrylic acid) is added to the pore in

millipore water (around pH 6).

However, the hydrogen bonding capability of the poly(acrylic acid) could be enhanced by

depositing the layer at pH 5. This led to a somewhat thicker layer, as seen by a pressure

increase from 0.01036 to 0.04578 bar (a 442% increase), compared to the increase at pH 7.4,

from 0.02611 to 0.03672 bar (a 141% increase). This demonstrates that the thickness of the

initial PAA layer is at least partially tunable by the pH of deposition. A possible reason for

this film thickness difference is this: the poly(acrylic acid) at pH 5 is much more protonated

(approximately 50% as opposed to the minimal protonation at pH 7) and therefore interacts

much more with the P4VP, partially solubilizing it and forming a thicker layer of P4VP-PAA

complex in the center of the pore. When a smaller percentage of PAA units are protonated,

the polymer will adhere to the pore wall but not solubilize the P4VP as much, acting more

like a polyelectrolyte layer in the typical “layer by layer” study.

One effect of this first layer of poly(acrylic acid) was that the pores were more visible

compared to the bare membrane under a cross-section by SEM (Figure 13). It is possible that

even this initial layer had a stabilizing effect on the interior of the pores. The pores at this

51

point were more clearly characterizable, with a depth of at least 500 nm and a diameter that

matched the pore size on the surface, approximately 45 nm.

Figure 3-13: Profile views (different views of the same membrane) of membrane pores following one layer of 5k PAA.

3.3 Layer-by-Layer Deposition of a Weak and a Strong Polyelectrolyte inside the

Nanopores

3.3.1 Electrostatic layer-by-layer assembly in nanopores: PSS/ PEI·HCl

Poly(styrene sulfonate) and protonated poly(ethylene imine) were chosen for layer-by-layer

assembly on top of the poly(acrylic acid) layer because low molecular weight samples of

each polymer were commercially available (as opposed to poly(allylamine hydrochloride) for

example), and because the layer-by-layer self-assembly of these two polymers has been

studied in detail on surfaces using a variety of techniques.16

A low molecular weight was

chosen to avoid the risk of the polymer spanning the pore as the layer-by-layer assembly

52

decreases the pore size. This was evidenced by the fast fouling of the membrane with an

attempt to use poly(styrene sulfonate) with a molecular weight of 75kDa during initial tests

(Figure 14). In addition, polyelectolyte degree of polymerization has been shown not to

affect multilayer deposition, rather, differing amounts of salt in the solution will affect the

layer thickness.17

Figure 3-14: Fouling of the membrane after using high molecular weight poly(styrene sulfonate) as a second layer

above 5kPAA.

3.3.2 Surface layer-by-layer assembly study on a silicon wafer

As a first test for the layer-by-layer self-assembly, the experiment was performed on a silicon

wafer that had been spin-coated with P4VP. The experiment was tried first with low

molecular weight P4VP, but it was found that the PAA solution solubilized the P4VP enough

to mostly remove the film. Therefore, high molecular weight P4VP was used as a base layer.

The wafers were coated with PAA, and then successive layers of PSS and PEI·HCl in a pH

7.4 buffer solution (Figure 15). It was clear after 15 layer pairs of deposition that a multilayer

was forming, and that it possessed the same anti-reflective quality found in other multilayer

films.8 It was also observed that before drying the multilayer films possessed an iridescent

53

quality that was removed upon drying, suggesting that the films are swollen with water and

the film is thinned upon removal of the water.

Figure 3-15: Multilayers without (top) and with (bottom) a light source. From left to right, P4VP film, P4VP-PAA,

P4VP-PAA-PEI, P4VP-PAA-PEI-PSS, P4VP,PAA-(PEI-PSS)2.5, PAA-(PEI-PSS)4, PAA-(PEI-PSS)6.5, PAA-(PEI-

PSS)9,PAA-(PEI-PSS)15.

3.3.3 Deposition of Initial Layers – Differences in Preparation

When experimenting with differences between deposition at different pH levels or

with/without collapsing the P4VP chains with methanol, the first layers were the most

dramatically affected by these changes. When all of the layers were deposited in pH 7.4

buffer or water, the PEI·HCl layer caused the pressure to decrease rather than increase – the

only layer that behaved in this manner. This is possibly attributed to a collapse of the mostly-

ionized poly(acrylic acid) against the pore wall when combined with the PEI·HCl layer –

going from a loosely bound brush inside of the pore to a more defined layer. In comparison,

when the PAA layer was deposited at pH 5 and the PEI·HCl layer at pH 6, the pressure

increased dramatically upon addition of the PEI·HCl layer. As was mentioned previously, the

solublization of the P4VP chains by the more protonated PAA made a thicker base layer,

which allowed for considerably more PEI·HCl to be deposited in the pore. The results of

these initial layer depositions can be seen in Table 4. In all cases the pressure increased as

expected with the first PSS layer deposited.

As a note here, it is important to mention that each layer deposition is carried out until the

pressure levels off – ideally this means that the pore is coated to its fullest extent. At the very

least, it means that the pore coating process has ceased the changing of the pore size and has

reached some sort of equilibrium.

54

Table 3-4: Pressure differences before, during, and after rinse with 50/50 methanol/water mixture.

Initial

Pressure (bar)

After PAA

layer (bar)

After PEI

layer 1 (bar)

After PSS

layer 1 (bar)

Membrane A: Deposited with

no methanol

0.0261 0.0367 0.0212 0.0275

Membrane B: Methanol rinse,

all layers in pH 7.4 buffer

0.0167 0.0189 0.0171 0.0195

Membrane C: Methanol rinse,

pH 5 for PAA, pH 6 for

PEI·HCl, pH 7.4 for PSS

0.0104 0.0313 0.1041 0.0473

3.3.4 Layer by Layer Deposition – Pressure Data and Analysis

Subsequent layer-by-layer deposition of polyelectrolyte pairs showed substantial differences

depending on conditions. For example, membrane A, which had layers deposited in Millipore

water without first collapsing the P4VP chains, showed very unusual behavior upon

successive layer deposition. With every successive PSS layer, the pressure would increase,

followed by the PEI·HCl layer, which decreased the pressure (Figure 16). This was attributed

to the remaining P4VP chains in the central portion of the pore, coated with anionic PAA,

which caused a nonlinear layer deposition. The pressure increase in the layers exceeded the

capacity of the sensor, and so the flow rate was changed and then the resulting pressure data

scaled for comparison. The flux data, or the ratio between flow rate and pressure (with no

scaling involved) is therefore also included for an additional comparison.

55

Figure 3-16: Pressure increase (left) and corresponding flux/pore size decrease with layer increase for an as-formed

membrane, all layers deposited in Millipore water. Note the zig-zag pattern of pressure increase with PSS followed by

pressure decrease with PEI.

Upon examination of the SEM images of the membranes (Figure 17), the pores did appear

smaller on both the surface and cross-section, but the pressure data was difficult to interpret

with the nonlinear pattern of pore decrease. After reviewing these data, it was concluded that

the methanol rinse before layer deposition was necessary for the formation of clear

polyelectrolyte multilayers. It was at this point that the water was changed for pH 7.4 buffer

to ensure that it was not the acidity of the poly(acrylic acid) that was closing the pore and that

it was indeed the interaction between PAA and P4VP.

56

Figure 3-17: Cross section (left) and top-view of the membrane following the deposition shown in Figure 16.

At this point, the plan for layer-by-layer deposition was clear – with the low molecular

weight PSS and protonated PEI using a base layer of PAA (Scheme 1), the assembly would

be studied at different pH levels and characterized using pressure and SEM data.

57

Scheme 1: Overall plan for layer-by-layer deposition of polyelectrolytes in PS-b-P4VP membrane pores.

Layers were again deposited, in a buffer solution pH 7.4 for membrane B, but this time with

the initial methanol rinse. Although the pressure increased much faster, nearly 0.16 bar with 9

58

layers for membrane B vs. 0.06 bar with 12 layers for membrane A, the pressure increased

(Figure 18) in a manner similar to the predicted pore-closing model using the simplified

Hagen-Poiseuille equation. Since the other factors are unknown and most likely

unpredictable, as attempts from the Abetz group to measure pore diameters from flux in PS-

P4VP membranes have shown,18

it is not possible to derive the pore diameter from a single

pressure measurement accurately. This is most likely due to a reasonable number of dead-end

pores, combined with some amount of hydrophobic P4VP polymer present in the middle of

the pore rather than around the edge. However, using the Hagen-Poiseuille equation

presented earlier a pore size estimate was made from the flux data. The estimation of pore

size based on an assumed initial pore diameter of 40.6 nm is given in Figure 18. From this

graph, it appears that for the initial layer deposition the pore size changes very little, followed

by a steady decrease in pore size; the rate will be discussed in a subsequent section.

Figure 3-18: Deposition in 0.1M pH 7.4 phosphate buffer solution.

59

Looking at the membrane by SEM (Figure 19) confirmed the pressure data. The pores are

clearly significantly smaller, both on the surface and the cross-section images. Moreover, the

layers appear to have significantly stabilized the pores, as large sections of the pores are now

visible. While there is some uneven deposition within the pores, the pore size has been

reduced to 17-22 nm, compared to the initial 37-47 nm.

Another factor which must be considered here is the permeability of polyelectrolyte

multilayers to water. Polyelectrolytes have been used to build up surface layers on porous

membranes and are still permeable to water.19

Indeed, studies of polyelectrolyte multilayer

hydration have showed water contents of 78% for polyelectrolyte multilayer capsules, while

other studies have found hydration levels between 20 and 40 percent.20

So while the pore

appears to be 17-22 nm by SEM, after drying, it is almost certain that within the pore is a gel-

like network rather than a solid polymer. If this is indeed the case, then the term “pore

reduction” can only be applied after drying in the traditional sense of adding solid layers to a

pore wall. An unknown that is presented here is the amount of water that may be going

through the polyelectrolyte layers as opposed to the pore, and to what extent.

60

Figure 3-19: Cross-section (left) and top-view (right) of the final product of deposition in pH 7.4 buffer solution, as

shown in Figure 18.

A visual inspection of both the layered (Figure 20 bottom) and the beginning structure

(Figure 20 top) serves further to illustrate the large difference between the starting pore size

and the final pore size.

61

Figure 3-20: Direct comparison of cross sections (right) and top views (left). The top pair of images is from the initial

membrane, and the bottom pair of images is from the membrane after layer-by-layer deposition at pH 7.4.

An additional membrane was made later at these (pH 7.4) conditions to test what would

happen if many layers were added at these conditions. Interestingly, after a point that

corresponded to (approximately) the 10th

or 11th

layer, the pressure began to fluctuate with

each layer in a “zig-zag” fashion that could not be explained until afterwards, when observing

the SEM image of the final membrane (Figure 23). At this point it was clearly visible that the

layers had started to be deposited on the surface at some point. This zig-zag pattern can

potentially be explained by the observations of Schwarz and Schoenhoff, who noted an odd-

even effect in poly(allylamine hydrochloride)/PSS multilayers (Figure 21) due to differing

amounts of hydration in the layers after each deposition step.21

However, the conclusion of

this study was that water immobilization was increased during polycation deposition, and

decreased during polyanion deposition. In the case of the membrane shown in Figure 22,

flux decreased after each polyanion layer deposition and increased after each polycation

62

deposition. It appears in this case that there is at least some potential correlation between

water immobilization and flux through a polyelectrolyte multilayer on top of a membrane.

Figure 3-21: From Ref. 21, increase and decrease in relaxation rate of water during deposition of PAH/PSS

multilayers, corresponding to differing amounts of hydration after layer deposition.

In any case, it is presumed that it is this point at layer 10-11 at which the membrane begins

the polyelectrolyte surface layer buildup, first because the 9-layer membrane preceding it

showed no sign of different behavior, and second because of the unusual odd-even behavior

of the membrane. It is also notable that the layer thickness of the deposited multilayers on

top of the membrane after this point is incredibly thick – the entire multilayer thickness on

top of the membrane is at least 10 microns. Also interesting is the fact that, although 10

microns of multilayers were building up on the top of the membrane, the flux did not

decrease faster than it did during the pore-filling segment, in fact, the rate of flux decrease

slowed at this point. It appears from this that the pore-filling vertical multilayers decrease

flux more than the altogether thicker but unconfined horizontal multilayers. This also has a

potential explanation – in multilayers deposited into 800 nm pores of track-etched

63

membranes by the Cohen group, the degree of swelling in confined spaces is lower than that

of free polyelectrolyte multilayers.22

If the layer is less swollen, then the water flow through

the space could be more restricted.

Supporting the idea of a gel-like filled pore, as was originally described by the Jonas group,23

is the SEM image of the pores beneath the 10 microns of polyelectrolyte multilayers (Figure

23). These pores, although also considerably reduced from the initial pore size, still do not

appear completely closed. This would suggest that there is some reduction of the

polyelectrolyte multilayers during the drying process.

Figure 3-22: Deposition of a large number of PSS/PEI·HCl layers at pH 7.4.

64

Figure 3-23: Surface multilayer buildup after deposition of 25 multilayers.

3.3.5 Deposition at varied pH – More Highly Charged Polyelectrolytes

As an additional test, and because in general multilayers are constructed using solutions at a

range of pH values, the layers were deposited using the pH previously reported for PSS-PEI

multilayer films.16a

Each PSS layer was deposited at pH 7.4, and each PEI·HCl layer was

deposited at pH 6, while the base PAA layer was deposited at pH 5. At these pH levels, the

polyelectrolytes are more ionized. The effects were dramatic – the layer thickness for PAA

and PEI·HCl were increased significantly. As is seen in the pressure graph (Figure 24), after

the initial three layers (which were discussed previously), the pressure went up nearly three

times higher than the layers deposited at pH 7.4.

65

Figure 3-24: Layer-by-layer deposition with more charged polyelectrolytes – PAA at pH 5, PEI at pH 6, and PSS at

pH 7.4.

It seemed at this point that it was feasible to have some control over layer deposition

thickness within the pore simply by tuning the pH values of the weak polyelectrolyte in a

weak-strong polyelectrolyte layer-by-layer setup. However, as in the deposition at pH 7.4,

small variations in the conditions caused large variations in the layer thickness. Figure 25

shows three additional depositions, where it was discovered that a variation as small as

rinsing the membrane with a different pH solution before layer deposition caused a

significant difference in the layer thickness. In addition, when a new solution was made of

PEI·HCl between two sets of membrane depositions, the slope of the decrease in layer

thickness changed considerably. So, as is seen for polyelectrolyte multilayers on surfaces,24

a

large number of very small changes can contribute to the layer thickness within the nanopore.

66

The results of thicker layers from more charged PEI is surprising, because the opposite has

been shown to be true on traditional flat-surface polyelectrolyte multilayers. In a study of the

degree of ionization in weak-strong pairs of polyelectrolyte layers, the more charged

polyelectrolyte pair was the thinner rather than the thicker one.11

Figure 3-25: Three additional membrane polyelectrolyte deposition sets (top, middle, bottom), PAA deposited at pH

5, PEI·HCl at pH 6, and PSS at pH 7.4. The bottom membrane had one additional PEI·HCl layer deposited which

was not able to be monitored using the pressure sensor.

Although the membrane showed in Figure 24 was not observed by SEM, as it was then used

to test multilayer deposition reversibility, the membranes shown in Figure 24 were all

examined by SEM. As in the case of many multilayers, the beginning of multilayer buildup

on the surface of the membrane begins after just 1.5 PEI/PSS multilayer pair (Figure 26). The

point of surface deposition for more charged polyelectrolytes in this case is clearly between 1

and 1.5 multilayer pairs.

67

Figure 3-26: Polyelectrolyte buildup on surface of membranes after 0.5 (left, no surface buildup), 1 (middle) and 1.5

(right) layer-pairs deposited at pH 7.4 for PSS and pH 6 for PEI·HCl.

3.4 Layer by layer Deposition: Weak-Weak and Strong-Strong Polyelectrolytes

3.4.1 Layer by Layer Deposition of Two Strong Polyelectrolytes: PSS and qPEI

At this point, a strong and weak polyelectrolyte combination had been tested at different pH

levels, and it was of interest to see how the deposition would work with a combination of two

strong polyelectrolytes. PEI was quaternized using methyl iodide to create a strong

polyelectrolyte and then deposited using the same procedure as the PSS/PEI HCl multilayers.

Assuming that the higher charge of the PEI was responsible for the thicker layer, the

quaternized PEI should represent maximum charge and give the thickest possible layer. The

layers deposited in these membranes were extremely thick, and despite the pressure

decreasing steadily across five layers (Figure 27), showed layer buildup on the top of the

membrane upon examination with SEM (Figure 28). Unusually, though the pressure buildup

seems to be faster with the weak polyelectrolyte PEI at pH 6. Clearly, in this case, charge is

not the only factor involved in layer thickness; it is likely that factors such as hydrophilicity

also play a role.

68

Figure 3-27: Pressure, pore size, and flux data for deposition of two strong polyelectrolytes, qPEI and PSS.

Figure 3-28: Cross section view of polyelectrolyte multilayer buildup on the qPEI/PSS membrane depicted through

pressure data in Figure 27.

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3.4.2 Layer by Layer Deposition of Two Weak Polyelectrolytes: PAA and PEI·HCl

Two weak polyelectrolytes were then deposited at pH values at which they are reasonably

charged, PAA at pH 7.4 and PEI·HCl at pH 6. Unsurprisingly, following past observations,

the layer thickness was extremely high (Figure 29) and multilayer buildup was shown on the

surface with SEM after only three layers being deposited (Figure 30). More interesting was

that this membrane was the first to show obvious asymmetry where it came to layer

deposition thickness, with the PAA layers being thinner and the PEI·HCl layer being

extremely thick. This may be partially explained by the hydrophilicity of PAA increasing the

water flux through the pores, but without more data it is impossible to draw any conclusions.

Figure 3-29: Deposition of two weak polyelectrolytes; PAA and PEI·HCl, in the membrane pores.

70

Figure 3-30: Picture showing polyelectrolyte multilayer buildup on top and pore cross-section after deposition of

PAA/ PEI·HCl, pressure data found in Figure 29.

3.5 Multilayer Testing and Characterization

3.5.1 Effect of methanol and pH after deposition

The effect of a mixture of methanol and water on the initial membrane was profound – the

P4VP within the membrane swelled to the point that the pressure increased dramatically,

nearly closing the pore. It was therefore of interest to test the effect of methanol post-

deposition of the multilayers. What was observed was that the pressure did increase with the

methanol/water mixture, but not nearly as dramatically as with the initial membrane. (Figure

31) More interestingly, the water flux increased after this rinse with methanol/water,

suggesting a potential method for collapsing the multilayers without drying the membrane,

which might cause defects in the structure. Further layers could be added after this methanol

treatment, although since the pore was at maximum loading already it reached the limit of the

pressure sensor capabilities quickly.

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Figure 3-31: Membrane flux before, during, and after 50/50 MeOH/H2O rinse with layers in the pore.

3.5.2 Deposition rates

As the layer deposition was being monitored in real time, another interesting aspect that

could be explored was the rate of deposition of the layers. The flow rate was known for each

setting of the syringe pump, so by calculating the amount of solution that had passed through

the membrane the deposition rates of layers could be compared. The deposition rate was

determined by looking at the pressure data and taking the beginning and end points of

pressure increase (Figure 32). This gives a view into layer by layer deposition that is unique

to membranes – in order to monitor layer-by-layer deposition rates on surfaces a technique

such as neutron scattering or ellipsometry is required. By using pressure as a sensor, a truly

different glimpse into the deposition of polyelectrolyte multilayers in the nanopore is gained.

72

Figure 3-32: Example of a layer deposition for determination of rates. The amount of time between point A,

beginning of pressure increase and point B, end of pressure decrease, is used with flow rate to determine the amount

of solution used to deposit each layer.

The comparison of the deposition rates between PSS and PEI·HCl at pH 7.4 and the

deposition rates between PSS and PEI·HCl at pH 6 for PEI·HCl showed interesting trends

(Figure 33).

Figure 3-33: Comparison of polyelectrolyte deposition rates between deposition at different pH (left) and deposition

at pH 7.4 (right).

73

The first and most obvious difference is the difference in the deposition rates of poly(acrylic

acid), which takes nearly twice the amount of volume to reach an endpoint at pH 7.4. This

makes sense, as the minimal amount of protonated PAA units present at pH 7.4 would make

the deposition much slower. The layer PSS1, which is where the pressure drops due to a

potential collapse in the layers, appears to be finished much faster than the other layers,

although that may be because the pressure is dropping rather than increasing as well. Most

interesting, though, is that with more highly charged polyelectrolytes the deposition rates of

both cationic and anionic polyelectrolytes appear to slow down with each sequential layer.

With the less charged PEI·HCl solution, however, only the deposition rate of PEI·HCl slows

down whereas the PSS remains approximately the same. It is possible that this relates to the

decrease in charge as layer-by-layer deposition proceeds, which is observed in nanopores by

the Azzaroni group. With more charged polyelectrolytes, the decrease in charge affects the

next layer substantially. However, with fewer charges on PEI·HCl, there could potentially be

less of an effect on the following PSS layer – leading to a constant deposition rate of PSS

without regard to the layer. However, without current-potential experiments that would give

some insight into the pore charge, this is impossible to verify.

3.5.3 Reversibility of deposition

Since the layers are held together by ionic interactions (and hydrogen bonding at the base),

they were presumably able to be reversed by application of a strong base. With the extremely

high pressure, though, it was uncertain whether or not this would be possible, and so a

solution of NaOH at pH 12 was applied to a membrane in which thick layers had been

deposited, most likely having some buildup on the surface as well. However, after a

considerable amount of time, slowly increasing the flow rate as the pressure dropped, the

pressure dropped from 0.36 bar to 0.049 bar, only about twice its initial pressure, a

considerable drop. When examining the SEM images (Figure 34), there appeared to be some

deposits on the surface of the membrane, either from NaOH salts or from polyelectrolyte-salt

buildup. Looking at the cross-section view, it was clear that the pores were once again open

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and that this buildup was the likely cause of the small increase in pressure. Significant as

well is the amount of layers that were initially deposited – five layers deposited at more

highly charged pH means almost certainly that there was a considerable amount of

polyelectrolyte layer buildup on the surface of the membrane, which did not hinder the

reversibility. It is likely that the ionic interactions as well as the hydrogen bonding is

disrupted by extremely high pH, as the removal of the polyelectrolyte multilayers would be

difficult with only the base layer disrupted.

Figure 3-34: Cross-section (left) and top-view (right) of membrane after removal of polyelectrolyte multilayers with a

NaOH solution.

3.6 Conclusions

Exploration of the potential controllability of polyelectrolyte deposition within the nanopores

of ordered block copolymer phase inversion membranes with an average pore diameter of

~41 nm, after a pre-treatment of the brushlike pore wall with a methanol/water mixture in

order to collapse the chains, was performed using different combinations of polyelectrolytes

in order to vary charge. Low molecular weight polyelectrolytes poly(styrene sulfonate),

protonated poly(ethylene imine), quaternized poly(ethylene imine) and poly(acrylic acid)

were tested to determine the deposition properties of strong and weak polyelectrolyte

combinations. In order to guarantee complete charge overcompensation for each layer, the

polyelectrolyte solution was pushed through the membrane pores using a syringe pump while

75

monitoring the pressure at a constant flow rate. This flow rate monitoring allowed the

deposition to be stopped only when polyelectrolyte was unable to be further deposited on the

surface, which ensured equal charge overcompensation through the entire length of the

nanopore and maximum layer thickness. The pressure level after deposition and rinsing with

buffer solution was then recorded while buffer solution was passing through the pore to

ensure a standard solution for each measurement, as viscosity can affect the pressure

measurements.

The first layer, poly(acrylic acid) in all cases, was used to connect the poly(4-vinyl pyridine)

to the rest of the polyelectrolyte layers by hydrogen bonding to the P4VP layer and

presenting a negative charge to the interior of the nanopore, which was then built upon by

positively charged PEI·HCl or quaternized PEI. The layer thickness of the PAA could be

varied by changing the pH of the solution – a lower pH of 5 meant that the PAA was close to

50% protonated, and so the PAA layer was much thicker. Deposition at pH 7.4 where PAA

has extremely few protonated repeat units led to a very thin layer which, nonetheless,

supported additional layers. While in many cases the extraordinarily thick layer deposition

seen in past studies of polyelectrolyte deposition in nanopores was seen in multilayers of

PSS/ PEI·HCl, PSS/qPEI, and PSS/PAA, the combination of PSS/ PEI·HCl in particular

proved to be extremely promising in its controllability, providing very thick layers at pH

levels where the PEI·HCl was more protonated and considerably thinner and better controlled

deposition at a steady pH of 7.4 where PEI·HCl is less protonated, a finding contrary to those

previously reported for layer-by-layer assembly on surfaces. With the thick layers deposited

at varied pH, or with two strong or two weak polyelectrolytes, the pore became filled quickly

and polyelectrolyte multilayers began to build up on the surface of the membrane. These

multilayers were water permeable despite considerable thickness in one case, but behaved

differently in terms of pressure increase – displaying a zig-zag pattern of pressure increase

then decrease as successive layers were added. This may be due to a change in hydration of

the multilayers that has been seen in studies of layer-by-layer assembly on surfaces.

This hydration of polyelectrolyte multilayers, however, makes it extremely difficult to get an

accurate picture of the structure inside the pores. In one attempt to make a large number of

76

layers, the polyelectrolyte began to build up substantially on the surface. However, the

swelling of the polyelectrolyte layers may have some effect on the flow through the pore. By

examination with SEM, however, the layers collapse after drying, leaving a pore diameter

that matches the calculated “diameter” estimated by using the Hagen-Poiseuille equation.

Measurement of pressure as a real-time analytical tool was also useful for looking at rates of

deposition, which were different depending on whether the polyelectrolytes were deposited at

different pH levels (more highly charged PEI·HCl) or at pH 7.4 (less highly charged

PEI·HCl).

Reversibility of the multilayers was demonstrated by nearly completely removing the layers

deposited using an aqueous NaOH solution at pH 12 to disrupt the ionic interactions holding

the multilayers together.

It was also possible to collapse the multilayers by passing through a solution of 50/50

methanol/water and then returning to water, at which point the flux increased. The potential

here is for the system to remain wetted while compacting the layers. This controllable layer-

by-layer deposition has potential for future additions of functionality to nanoporous

membranes, either through reactions of polymers within the pores or by immobilizing

functional objects such as enzymes or nanoparticles within the nanopore.

77

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Multilayer Films by a Self-Assembly Process .2. Consecutive Adsorption of Anionic and

Cationic Bipolar Amphiphiles and Polyelectrolytes on Charged Surfaces. Ber Bunsen Phys

Chem 1991, 95 (11), 1430-1434; (c) Decher, G.; Hong, J. D., Buildup of Ultrathin Multilayer

Films by a Self-Assembly Process .1. Consecutive Adsorption of Anionic and Cationic

Bipolar Amphiphiles on Charged Surfaces. Makromol Chem-M Symp 1991, 46, 321-327; (d)

Decher, G.; Hong, J. D.; Schmitt, J., Buildup of Ultrathin Multilayer Films by a Self-

Assembly Process .3. Consecutively Alternating Adsorption of Anionic and Cationic

Polyelectrolytes on Charged Surfaces. Thin Solid Films 1992, 210 (1-2), 831-835.

2. Dotzauer, D. M.; Dai, J. H.; Sun, L.; Bruening, M. L., Catalytic membranes prepared

using layer-by-layer adsorption of polyelectrolyte/metal nanoparticle films in porous

supports. Nano Lett 2006, 6 (10), 2268-2272.

3. Peinemann, K. V.; Abetz, V.; Simon, P. F. W., Asymmetric superstructure formed in

a block copolymer via phase separation. Nat Mater 2007, 6 (12), 992-996.

4. Nunes, S. P.; Karunakaran, M.; Pradeep, N.; Behzad, A. R.; Hooghan, B.; Sougrat, R.;

He, H. Z.; Peinemann, K. V., From Micelle Supramolecular Assemblies in Selective Solvents

to Isoporous Membranes. Langmuir 2011, 27 (16), 10184-10190.

5. Kim, S.; Briber, R. M.; Karim, A.; Jones, R. L.; Kim, H. C., Environment-controlled

spin coating to rapidly orient microdomains in thin block copolymer films. Macromolecules

2007, 40 (12), 4102-4105.

6. Adham, S. e. a., Optimization of Membrane Treatment for Direct and Clarified Water

Filtration (Subject Area: High-Quality Water). American Water Works Research Foundation:

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7. Tripathi, B. P.; Dubey, N. C.; Choudhury, S.; Simon, F.; Stamm, M., Antifouling and

antibiofouling pH responsive block copolymer based membranes by selective surface

modification. J Mater Chem B 2013, 1 (27), 3397-3409.

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8. Cho, J.; Hong, J. K.; Char, K.; Caruso, F., Nanoporous block copolymer

micelle/micelle multilayer films with dual optical properties. J Am Chem Soc 2006, 128 (30),

9935-9942.

9. Zhang, W. Q.; Shi, L. Q.; An, Y. L.; Wu, K.; Gao, L. C.; Liu, Z.; Ma, R. J.; Meng, Q.

B.; Zhao, C. J.; He, B. L., Adsorption of poly(4-vinyl pyridine) unimers into polystyrene-

block-poly(acrylic acid) micelles in ethanol due to hydrogen bonding. Macromolecules 2004,

37 (8), 2924-2929.

10. Tian, Y.; He, Q.; Cui, Y.; Tao, C.; Li, J. B., Assembly of nanotubes of poly(4-

vinylpyridine) and poly(acrylic acid) through hydrogen bonding. Chem-Eur J 2006, 12 (18),

4808-4812.

11. Choi, J.; Rubner, M. F., Influence of the degree of ionization on weak polyelectrolyte

multilayer assembly. Macromolecules 2005, 38 (1), 116-124.

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Poly(styrene)-block-Poly(4-vinyl pyridine). J Polym Sci Pol Phys 2009, 47 (12), 1192-1202.

13. Shibata, M.; Kimura, Y.; Yaginuma, D., Thermal properties of novel supramolecular

polymer networks based on poly(4-vinylpyridine) and disulfonic acids. Polymer 2004, 45

(22), 7571-7577.

14. Schafer, A., Natural Organics Removal Using Membranes. Technomic Publishing

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16. (a) Singh, S.; Junghans, A.; Waltman, M. J.; Nagy, A.; Iyer, R.; Majewski, J., Neutron

reflectometry characterization of PEI-PSS polyelectrolyte multilayers for cell culture. Soft

Matter 2012, 8 (45), 11484-11491; (b) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T.,

Assembly of Multicomponent Protein Films by Means of Electrostatic Layer-by-Layer

Adsorption. J Am Chem Soc 1995, 117 (22), 6117-6123.

17. Spruijt, E.; Westphal, A. H.; Borst, J. W.; Cohen Stuart, M. A.; van der Gucht, J.,

Binodal Compositions of Polyelectrolyte Complexes. Macromolecules 2010, 43 (15), 6476-

6484.

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18. Rangou, S.; Buhr, K.; Filiz, V.; Clodt, J. I.; Lademann, B.; Hahn, J.; Jung, A.; Abetz,

V., Self-organized isoporous membranes with tailored pore sizes. J Membrane Sci 2014, 451,

266-275.

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membranes for water purification applications. J Hazard Mater 2013, 252, 401-412.

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Klitzing, R. V.; Steitz, R., Hydration and internal properties of polyelectrolyte multilayers.

Colloid Surface A 2007, 303 (1-2), 14-29.

21. Schwarz, B.; Schonhoff, M., Surface potential driven swelling of polyelectrolyte

multilayers. Langmuir 2002, 18 (8), 2964-2966.

22. Lee, D.; Nolte, A. J.; Kunz, A. L.; Rubner, M. F.; Cohen, R. E., pH-induced hysteretic

gating of track-etched polycarbonate membranes: Swelling/deswelling behavior of

polyelectrolyte multilayers in confined geometry. J Am Chem Soc 2006, 128 (26), 8521-8529.

23. Roy, C. J.; Dupont-Gillain, C.; Demoustier-Champagne, S.; Jonas, A. M.; Landoulsi,

J., Growth Mechanism of Confined Polyelectrolyte Multilayers in Nanoporous Templates.

Langmuir 2010, 26 (5), 3350-3355.

24. El Haitami, A. E.; Martel, D.; Ball, V.; Nguyen, H. C.; Gonthier, E.; Labbe, P.;

Voegel, J. C.; Schaaf, P.; Senger, B.; Boulmedais, F., Effect of the Supporting Electrolyte

Anion on the Thickness of PSS/PAH Multilayer Films and on Their Permeability to an

Electroactive Probe. Langmuir 2009, 25 (4), 2282-2289.

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81

Chapter 4: Separations Using Polyelectrolyte

Multilayers in Nanostructured Phase-Inversion

Membranes

4.1 Dye Separation

4.1.1 Introduction

Dyes are used as model charged compounds in both polyelectrolyte multilayer absorption

studies1 and in membrane separation studies, such as the aforementioned research by Savariar

et.al.2, in which track-etched membranes were functionalized with a layer of a

polyelectrolyte. Passing dyes through a membrane can give valuable information about

surface charge and the interior chemistry of the nanopore that might otherwise be impossible

to obtain. On a flat surface which is built up with polyelectrolyte multilayers, it is possible to

run experiments to find zeta potential, look at IR spectroscopy, ellipsometry, or neutron

scattering measurements to find information about layer thickness and composition,3 and

AFM measurements to look at surface roughness.4 Advances in carbon nanotube membranes,

for example, have shown surprising results and exceptionally high flux due to the interior

properties of the nanotube, such as slip length and hydrogen bonding networks.5 In charged

networks as well, the interior environment of a pore lined with a charged polymer may differ

from that of a charged polymer on a surface.6

The Rubner group at MIT has done several studies on dye absorption into polyelectrolyte

multilayers7 to simulate the absorption of charged drugs. Methylene blue was used as a

model cationic drug and loaded up to 17.3 nm deep into a PAA/PAH multilayer film, and the

slow release of the dye into water was studied in both buffered and nonbuffered solutions.

Interestingly, the release of the model drug into pure water and into buffer solution was

different depending on the conditions. It was also found that buffered solutions led to better

loading of the thin film with the dye.7b

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Dye separation was chosen for these membranes for ease of characterization using UV-Vis

spectroscopy as well as being a visual aid to determine separation, and also as the rhodamine

6g/calcein combination provided an easy comparison to the previous studies of dye

separations using polyelectrolyte lined track-etched membranes. Ion separations are also

typically performed using polyelectrolyte lined membranes, but require mass or flame

spectroscopy for detection.8

Several sets of dyes were considered: rhodamine 6g and calcein, as was previously used by

Savariar et.al. 2, methylene blue which was used by Rubner as a model cationic dye,

7b and

Coomassie brilliant blue due to its relatively large molecular size and use in dye separation

studies done on polyelectrolyte multilayers on disordered membranes9 the list and structures

of which can be found in Figure 1. The dyes were pushed through the membrane using low

flow rates with a syringe pump, and fractions were collected to assess the rate of flow of each

dye through the membrane. This could then be analyzed using UV-Vis spectroscopy but also

visually as the dyes were differently colored enough to be easily seen.

4.1.2 Dye Absorption or Rejection by Multilayers – Unexpected Results

It was expected originally with these membranes that they would behave as the membranes in

the Savariar et.al. 2 paper did under diffusion conditions – separating dyes via charge effects.

Several membranes later, though, it was clear that membrane dye separation was not as

simple with polyelectrolyte multilayers. The membrane in the experiments appeared to be

acting much more as an ion-exchange resin type process and much less with charge

screening. It became clear that dye separation by charge with these membranes was not at all

straightforward.

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Figure 4-1: Structure of dyes used for separation experiments.

A first experiment shed light on two different aspects of these dye separation experiments. A

membrane with 1 pair of thick PSS/PEI·HCl layers was formed and then used to separate

rhodamine and calcein in an aqueous solution which contained equal concentrations of both.

This could be compared with a membrane where the pores were not only filled but large

amounts of multilayers had been deposited on the surface (25 total layers). Interestingly,

because they were made using different methods, the flux rates of the two membranes were

similar. Despite these similar flux rates, which would typically indicate a similar pore size,

the difference was very clear: the more multilayers were present, the more dye could be

absorbed and therefore separated. So there was an inherent limitation in doing layer-by-layer

assembly in the pores as a separation technique which was that a limited number of layers

could be placed without building up on the surface as well. However, when layers were built

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up on the surface they were capable of separating large amounts of dye. This was the first

indication that an absorption process was taking place rather than a charge-based separation.

If the process were charge based, and assuming that the assertion that charge decreases with

an increased number of layers, then the dye separation would be worse with an increased

number of layers and not better.

Figure 4-2: 25-layer membrane (top) vs. 2-layer membrane (bottom) for separation of calcein and rhodamine). The

left portion of the black line represents the dye separation, the right portion represents dye recovered after switching

to water. This is depicted both visually (A) and with UV-vis data (B).

This also gave some answers to size separation limitations as well – if a dye as large as

calcein could pass through the filled membrane, then the amount of space in the layers due to

hydration is quite substantial, especially given that 15 microns of surface multilayer buildup

A

B

85

were present. Another interesting observation was that while the membrane was separating

the dyes, the concentration of calcein emerging on the other side, while reaching a constant

concentration, was still much less than the initial dye concentration of 10-4

M. The indication

here was that either both dyes were being absorbed, but at different rates, or that each dye had

a different rate with which it passed through the multilayer-coated membrane. It is also

possible that both of these are true.

However, this gave no indication of charge-based dye separation. In fact, with all dye

combinations used, no matter what layer was present in the outer portion of the membrane,

PAA, PEI, qPEI, PSS, and no matter which layers were present underneath, the separation

was the same every time (Figure 3). With the thick layers, there was no easily defined

difference between a single layer and three layers in separation.

Figure 4-3: Separation from membranes of different configurations. Top: PAA-PEI, middle: PAA-PEI-PSS-PEI,

bottom: PAA-qPEI-PSS-qPEI (some buildup on surface).

It is important as well to comment that all of the experiments were done in a buffer solution.

This was initially chosen as well to mirror the reported rhodamine/calcein separation

experiments. This may have had the effect of hindering any charge screening while helping

absorption. However, previously reported differences in charge separation between higher

and lower buffer concentrations, while present, were not significant. 2 Because of this, most

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likely a difference between water and buffer may have affected the rate of travel through the

pore some for each dye but probably would not have affected the outcome of the experiment.

4.1.3 Reusability of membranes

It was important before starting to test whether a single membrane could be reused for

separations. There was precedent to the answer – in studies of multilayers and dyes the

Caruso group found that after dyes had been absorbed and released by polyelectrolyte

multilayers that a small portion of dye remained on the surface, but the surface retained its

overall charge and ability to absorb more dye.1 This was verified by doing successive

separations with the same membrane. Layers were still then able to be absorbed into the pore

after separations were complete. At this point, it was clear that Caruso’s assertions that the

majority of a dye is able to be released from polyelectrolyte multilayers, save a small surface

layer,1 held true for these membranes as well, and that they could be used for multiple

separations.

Figure 4-4: Calcein-rhodamine dye separation done sequentially - left 3 vials first, then the membrane was rinsed

with water, then the right 2 vials, membrane with a single layer of PAA.

4.1.4 Addition of more dye pairs

At this point, the question was: how could this dye separation be explained? It was possible

that the extreme hydrophilic nature of the calcein, with its four exterior carboxylate groups,

would cause it to travel in and out of the pores more easily. There was also the question of

87

charge – did it matter that one was negatively charged? It was possible that there was some

overall positive or negative charge of the layers that was affecting the separation.

At this point another pair of dyes were chosen: methyl orange and methylene blue. Using

solubility as a measure of hydrophilicity in this case, the methylene blue is much more

soluble than methyl orange. In this case, the methylene blue was separated out first from the

membrane, no matter which layer was present on the surface.

Figure 4-5: Separation of methylene blue and methyl orange mixture (shown on left) with various amounts of

multilayers. Top: poly(acrylic acid), middle: PAA-PEI, bottom: PAA-PEI-PSS. Left side of black line is dye

separation, right side is dye release after switching to water.

One set of dyes were chosen to see whether the size of the dye (though much larger than the

pore) would affect the separation. Calcein and Coomassie brilliant blue were separated

(Figure 4) and it was clear that the separation was still proceeding with calcein first, but that

brilliant blue followed quickly.

Figure 4-6: Dye separation of mixture of brilliant blue and calcein (shown on left). Layers present are PAA-PEI-PSS.

It seems that, despite the relatively large size of brilliant blue, it has no problem infiltrating

into the polyelectrolyte to become immobilized, allowing the calcein to pass through the

88

membrane first. With the relatively high concentration of dye in the membrane, it is highly

unlikely that the dye is confined exclusively to the surface. Also, the surface of the

membrane was covered with poly(styrene sulfonate), so the anionic charge of the brilliant

blue and the anionic charge of the PSS should not have caused any complexation to occur.

This speaks to a high percentage of hydration in the multilayers, into which large dye

molecules can penetrate.

In all of the membranes the dye which was retained was then released afterwards once the

solution was switched to water or buffer. It did not appear to make a significant difference

which was used, though buffer could potentially have an effect if the ions present were able

to displace the dye more easily in the multilayers.

The phenomena of “concentration polarization” cannot be completely discounted as well –

when charge screening occurs then a buildup of one charged species will appear on the flow

side of the membrane. However, in this case, it seems clear that absorption and/or

hydrophilic effects are overcoming the charge effects. Concentration polarization was

therefore discounted as a reason for the slow release of dye from the membrane after

switching to water.

4.1.5 Conclusions

In the end, the conclusion was that the dye interactions with all of the possible polyelectrolyte

interactions are too complex to be explained completely by simple charge, hydrophilicity,

functional groups, or counterions. However, it seemed clear by the end that absorption-based

processes dominated clearly over a charge separation. The best correlation that could be

drawn from these few dye separations is that the more water-soluble compound seemed to be

the one that would emerge first, no matter the surface layer or charge. The multilayers seem

much too complex to be used as a simple “exchange resin” type process, and it is clear that

more investigation is required to understand fully the process of separation which takes place

in the unique environment of the nanopore. Slow release of the absorbed dye also has

potential, as was seen with the multilayers used by Rubner for model cationic drug release. It

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may be that multilayers in membrane form will find unforeseen uses in this area.

Additionally, membranes could be used as an easy test for polyelectrolyte multilayer

absorption, especially as a preliminary test for absorption by polyelectrolyte-coated particles

or surfaces.

4.2 Size separation: Sieving curves

When membranes are tested for pore size, there are several options. A bubble point test, in

which air is forced upwards through the membrane and the point at which the bubbles appear

on the surface is recorded.10

An alternative method is a “sieving curve”, where a solution

with broad molecular weight distributions of a noninteracting polymer is passed through the

membrane. The curve can then be analyzed by GPC and compared to the curve of other

membranes.11

In this case, it was of interest to test the size reduction of the pore, especially as

the multilayers within the pore seemed to be permeable to large dyes.

Sieving curves are typically done with dextran since it has a broad molecular weight

distribution and with the possible exception of hydrogen bonding has a very low chance of

interacting with any membrane type. In this case, though, we chose poly(ethylene glycol) as

the sieving polymer since dextran is insoluble in chloroform, the GPC solvent. Sieving was

done using a 1 mg/mL solution of four different molecular weights of PEG, which are found

along with their radius of hydration in Table 1.

The goal of using a sieving curve in this project was to explore the structure of the layers

within the nanopore. With two options in mind – one in which the layers are very loosely

joined and fill the nanopore while further layers infiltrate between these layers, increasing the

pressure; and another where the layers are more compact and lining the edges of the

nanopore, a more traditional picture of layer-by-layer self-assembly, the picture of the

loosely-filled nanopore would present a far smaller pore space than the edge-lined layer

picture. So, a sieving curve, in which a broad spectrum of PEG molecular weights is pushed

through the membrane, should give some answers. GPC would provide a method of looking

at the separation of the membrane, provided that each membrane separation was done using

the same flow rate and amount of solution separated.

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Table 4-1: Hydrodynamic radii of PEG molecular weights used, from Ref. 12.

PEG Molecular Weight (kDa) Rh (nm) Diameter (nm)

11.84 3.49 6.98

100 11.95 23.90

300 23.36 46.72

600 32.95 65.90

As is seen in Table 1, the PEG with a radius of about 3.4 nm (10kDa PEG was used as

opposed to the 11.84kDa PEG reported in the literature)12

should pass through the membrane

easily and is included as a point of reference. The 600kDa PEG with a radius of 33 nm

(diameter of 66 nm) should be filtered out by even the bare unmodified membrane. And the

100kDa and 300kDa PEG with diameters of 24 and 46 nm respectively should be the most

conclusive as to how much the pore size has actually changed during layer-by-layer

deposition.

The first step was to get a calibration curve from the PEG samples available in order to have

a reasonable estimate of relative pore size. The four PEG samples were run through the GPC

and peaks were recorded in order to make a crude calibration curve (Figure 7).

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Figure 4-7: Separate GPC curves for the PEG samples used in the sieving experiment. Inset graph shows GPC

calibration for PEG samples.

The three membranes that were tested and the calculated pore sizes based on pressure

measurements are listed in Table 2. The purpose of running sieving curves for these pore

sizes was twofold: one, they would give an idea of whether the pressure data accurately

matched the actual pore size; and two, from this correlation we could gain a better

understanding of the structure within the nanopore.

Table 4-2: List of membranes used for PEG sieving experiment.

Membrane Name Membrane Flux (Lm-2

h-1

bar-1

) Estimated Pore Size

Sieve 1 365 35 nm

Sieve 2 580 40.6 nm (unmodified

membrane)

Sieve 3 162 23.8 nm

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Typically for a sieving curve, an ideal feed solution is one in which the same amount of

polymer is present at each molecular weight. Because of the narrow molecular weight of the

10kDa PEG though, it unfortunately showed a sharp peak at this molecular weight (Figure 8).

However, since no membrane with a pore size below 20nm was tested, this was deemed not

to be problematic.

Figure 4-8: GPC of feed solution for sieving curves. Peak positions from separate GPC curves of each solution are

marked.

If smaller pores were to be tested, though, a different feed solution would need to be used.

Another problem with sieving tests is membrane fouling. With the 40 and 35 nm pore size

membranes, the pressure increased within a measurable quantity during sieving. With the 20

nm pore sizes, the pressure increased beyond the ability of the sensor to measure it during the

filtration. This meant that the high molecular weight PEG was building up on the feed side of

the membrane. While the flow rate that matched that of the unmodified membrane could be

maintained, a pore size any smaller than this would probably have been fouled to the point

where the flow rate could not be maintained, affecting the accuracy of the sieving curves. It

has been shown that the flow rate and the pressure affect rejection coefficients calculated

during sieving measurements.13

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The measure of how well a membrane performs is known as a “rejection coefficient”. This is

defined by the equation, from Ref. 13:

𝑅𝑖 = 1 − 𝐶𝑖,𝑝/𝐶𝑖,0

where R is the rejection coefficient, Ci,p is the permeate concentration and Ci,0 is the feed

concentration. For a partial rejection coefficient, the same equation can be used (also from

Ref. 13):

𝑅 = 1 − 𝐶𝑝/𝐶0

where R is the partial rejection coefficient, Cp is the permeate concentration, and C0 is the

feed concentration.

Figure 4-9: From Ref. 13, sample sieving curve and calculation of partial rejection coefficients.

Measuring concentration was somewhat problematic in this case, as there were some

insoluble salts in the freeze-dried output which affected the weight that was measured.

Therefore, it was decided to use a reference point at the low molecular weight end of the

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10kDa PEG peak to normalize the intensity so that the rejection coefficient could be

calculated. A reference point of 14.7 mL retention volume was chosen and all refractive

index was normalized to this point after being baseline corrected. There was some

inconsistency in the baseline, however, which led to a reasonable amount of error. Because of

this it was difficult to make any conclusions from the graph of the bare membrane and the

graph of just a few layers from looking at only the GPC curve (Figure 9). It is possible,

however, to see clearly the difference between these two sieves and the sieve of the ~25 nm

pore size.

Figure 4-10: GPC curve of the initial feed mixture, and the three sieve membrane tests listed in Table 2.

The rejection coefficients were then plotted, but due to the uneven PEG distribution ended up

being quite uneven (Figure 11). A dip in the rejection curve that occurs halfway through the

graph is not due to a change in the rejection but a low point in the PEG distribution. So,

ignoring this dip, it appears that the pore size estimate only from GPC data for the initial

membrane would be very close to the hydrated diameter of 300 kDa PEG, or 46 nm.

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Figure 4-11: Total sieve curve (top) and portion of interest (bottom) for PEG mixture filtration.

This is further corroborated by the GPC UV data, which is not used for calibration due to

inconsistencies in the molecular weight vs. retention time plot.

Figure 4-12: UV GPC curve for sieving and feed mixture.

96

In this curve, however, there is a clear difference in the high molecular weight portion – the

600kDa PEG shows up at around 12.5 mL in the feed mixture and then disappears in the

portion filtered by the unmodified membrane. So the assertion that the sieving data for the

unmodified membrane is around the hydrated diameter of 300kDa PEG (46 nm) is verified by

the UV curve. The retention coefficient in Figure 11 for Sieve curve 3, the ~25 nm pore size

membrane, makes a sharp increase from around 20% to nearly 100% between 12.6 and 13.5

mL retention time. This would point towards a pore diameter around 27 nm, but as the

sieving curve estimate for the unmodified membrane is higher than the SEM, the

measurements from sieving most likely run high and the measured pore diameter of ~25 nm

would be close to the sieving curve measurement. It is clear from this, however, that there is

no gel-filled pore with water filtering through polyelectrolyte layers. The sieving curve data

shows an open pore which is closed gradually by the addition of polyelectrolyte layers.

Conclusion

A sieving curve is a way to measure pore diameter in a membrane by filtering a broad

molecular weight solution through the membrane and then looking at the permeate with GPC.

Using membranes of three different pore sizes, it was possible to correlate the pore size

calculated by the pressure data to the pore size determined by the sieving curve. This is

valuable information about the interior structure of the nanopore, and verifies that the layer-

by-layer assembly happens along the walls of the nanopore, gradually closing the pore, rather

than filling up the pore with a loosely packed gel.

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References

1. Tedeschi, C.; Caruso, F.; Mohwald, H.; Kirstein, S., Adsorption and desorption

behavior of an anionic pyrene chromophore in sequentially deposited polyelectrolyte-dye thin

films. J Am Chem Soc 2000, 122 (24), 5841-5848.

2. Savariar, E. N.; Krishnamoorthy, K.; Thayumanavan, S., Molecular discrimination

inside polymer nanotubules. Nat Nanotechnol 2008, 3 (2), 112-117.

3. Schonhoff, M., Self-assembled polyelectrolyte multilayers. Curr Opin Colloid In

2003, 8 (1), 86-95.

4. McAloney, R. A.; Sinyor, M.; Dudnik, V.; Goh, M. C., Atomic force microscopy

studies of salt effects on polyelectrolyte multilayer film morphology. Langmuir 2001, 17

(21), 6655-6663.

5. Majumder, M.; Chopra, N.; Andrews, R.; Hinds, B. J., Nanoscale hydrodynamics -

Enhanced flow in carbon nanotubes. Nature 2005, 438 (7064), 44-44.

6. Tagliazucchi, M.; Azzaroni, O.; Szleifer, I., Responsive Polymers End-Tethered in

Solid-State Nanochannels: When Nanoconfinement Really Matters. J Am Chem Soc 2010,

132 (35), 12404-12411.

7. (a) Hiller, J.; Rubner, M. F., Reversible molecular memory and pH-switchable

swelling transitions in polyelectrolyte multilayers. Macromolecules 2003, 36 (11), 4078-

4083; (b) Chung, A. J.; Rubner, M. F., Methods of loading and releasing low molecular

weight cationic molecules in weak polyelectrolyte multilayer films. Langmuir 2002, 18 (4),

1176-1183.

8. Armstrong, J. A.; Bernal, E. E. L.; Yaroshchuk, A.; Bruening, M. L., Separation of

Ions Using Polyelectrolyte-Modified Nanoporous Track-Etched Membranes. Langmuir 2013,

29 (32), 10287-10296.

9. Baburaj, M. S.; Aravindakumar, C. T.; Sreedhanya, S.; Thomas, A. P.; Aravind, U.

K., Treatment of model textile effluents with PAA/CHI and PAA/PEI composite membranes.

Desalination 2012, 288, 72-79.

10. Gribble, C. M.; Matthews, G. P.; Laudone, G. M.; Turner, A.; Ridgway, C. J.;

Schoelkopf, J.; Gane, P. A. C., Porometry, porosimetry, image analysis and void network

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modelling in the study of the pore-level properties of filters. Chem Eng Sci 2011, 66 (16),

3701-3709.

11. (a) Meireles, M.; Bessieres, A.; Rogissart, I.; Aimar, P.; Sanchez, V., An Appropriate

Molecular-Size Parameter for Porous Membranes Calibration. J Membrane Sci 1995, 103 (1-

2), 105-115; (b) Michaels, A. S., Analysis and Prediction of Sieving Curves for Ultrafiltration

Membranes - a Universal Correlation. Separ Sci Technol 1980, 15 (6), 1305-1322; (c) Aimar,

P.; Meireles, M.; Sanchez, V., A Contribution to the Translation of Retention Curves into

Pore-Size Distributions for Sieving Membranes. J Membrane Sci 1990, 54 (3), 321-338.

12. Armstrong, J. K.; Wenby, R. B.; Meiselman, H. J.; Fisher, T. C., The hydrodynamic

radii of macromolecules and their effect on red blood cell aggregation. Biophys J 2004, 87

(6), 4259-4270.

13. Nobrega, R.; Debalmann, H.; Aimar, P.; Sanchez, V., Transfer of Dextran through

Ultrafiltration Membranes - a Study of Rejection Data Analyzed by Gel-Permeation

Chromatography. J Membrane Sci 1989, 45 (1-2), 17-36.

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Chapter 5: Experimental Details

Instrumentation

Membrane structures were characterized using scanning electron microscopy (FEG-SEM,

Zeiss LEO Gemini 1530, Germany) with an in-lens detector. Prior to imaging, the thin films

were coated with platinum in order to avoid charging and to allow imaging at higher

resolutions. NMR spectra were recorded on a Bruker AV300 MHz and AV500 MHz

spectrometers, using CDCl3 as the solvent. Analytical GPC measurements were performed

using a Viscotek GPC system equipped with a pump and a degasser (GPCmax VE2001, flow

rate 1.0 mL/min), a detector module (Viscotek 302 TDA) and three columns (2 × PLGel Mix-

C and 1 × ViscoGEL GMHHRN 18055, 7.5 × 300 mm for each) using chloroform as an

eluent. UV/Vis measurements were carried out on a Lambda 20 double beam UV/Vis-

spectrophotometer from Perkin-Elmer.

Membrane formation

PS(145,000)-b-P4VP(50,000) block copolymer, Polymer Source, was dissolved in 1:1:1 w/w

DMF:THF:dioxane to form a 17 wt% solution. The solution was kept in a sealed jar under

shaking conditions for 3 days to dissolve. A glass substrate was used for membrane

formation with a doctor blade with 250 µm gate height. After spreading of the polymer

solution, the solution was allowed to sit for 20 s before immersion in a 20ºC water bath,

where it sat for several hours. The membrane was transferred to and stored in Millipore

water until use.

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Layer deposition

Layers were deposited using a 10 mL syringe and an Orion Sage syringe pump. Between

each polyelectrolyte solution deposited, buffer solution was allowed to pass through the

membrane for at least 45 minutes.

Poly(acrylic acid), Acros, with a molecular weight of 5k was dissolved in a 0.1M buffer

solution to form a 0.01M PAA solution with respect to the monomer. The solution was

filtered through a 0.02 µM syringe filter before loading into a 10 mL syringe, after which it

was pushed through the membrane at a rate of 0.480 mg/s. This was continued until sensor

data showed that the pressure increase had leveled off.

Linear poly(ethylene imine), Mw=5000, was dissolved in a solution of ethanol and treated

with an excess of 2M HCl until the polymer had completely precipitated. This precipitate

was washed thoroughly with ethanol and dried, then dissolved in a 0.1M buffer solution to

form a 0.01M PEI·HCl solution with respect to monomer. The solution was filtered through

a 0.02 µM syringe filter before loading into a 10 mL syringe, after which it was pushed

through the membrane at a starting rate of 0.480 mg/s for initial layers, and lower rates for

subsequent layers to maintain a differential pressure under 0.07 bar (1.0 PSI). This was

continued until sensor data showed that the pressure increase had leveled off, or in cases

where the pressure did not change, was continued for at least 1 hour.

Poly(styrene sulfonate), Fluka, Mw=4300, was dissolved in 0.1M pH 7.4 buffer solution to

form a 0.01M PSS solution with respect to the monomer. The solution was filtered through a

0.02 µM syringe filter before loading into a 10 mL syringe, after which it was pushed through

the membrane at a starting rate of 0.480 mg/s for initial layers, and lower rates for subsequent

layers to maintain a differential pressure under 6900 Pa (1.0 PSI). This was continued until

sensor data showed that the pressure increase had leveled off.

101

Sensor calibration and data collection

The sensor was calibrated with a 70cm high column of water at two data points, 0 cm and 70

cm. This was then verified by taking data points at each 10cm of water height.

PEG size separation

A 1 mg/mL solution of poly(ethylene glycol) was made by using equal weights of 900 kDa,

600 kDa, 300 kDa, and 10 kDa PEG and diluting to 1 mg/mL with Millipore water. The PEG

solution was then loaded into a fresh 20 mL syringe and pushed through the membrane at a

rate between 0.34 and 1.3 mL/hour, keeping the pressure under 1 bar. A total mass of 2.5g

was collected for each separation.

Dye separation

Dye mixtures were made in 0.1 M phosphate buffer solution, pH 7.4. Each dye had a

concentration of 10-4

M and was pushed through the membrane at a rate of between 0.34 and

1.3 mL/hour. Pressure was not recorded during dye separation. Aliquots between 0.2 and 1.5

mL were collected during dye separation for analysis with UV-Vis.

PEI quaternization

Poly(ethyleneimine) was dissolved in ethanol and added to a 1:1 solution of ethanol/methyl

iodide which was stirred at 50ºC for three hours, after which a solid had formed in the

mixture. The solid was washed with ethanol and air dried. Precursor PEI 1H-NMR (δ, ppm,

300 MHz, CDCl3): 2.5 ppm (s, CH2CH2 backbone); qPEI after methylation 1H-NMR (δ,

ppm, 300 MHz, CDCl3): 2.3-4.8 ppm (multiple broad peaks, CH backbone, NCH3, N(CH3)

102

103

General Conclusions

The following general conclusions can be drawn from the work carried out in this thesis:

1. Layer by layer assembly inside of the ~40 nm nanopores present in polystyrene-b-

poly(4-vinylpyridine) phase inversion membranes is possible using a flow-through

system of layer deposition and polyelectrolytes including a partially hydrogen-

bonding base layer of poly(acrylic acid) and upper layers of poly(styrene sulfonate)

and poly(ethylene imine) HCl.

2. By monitoring the pressure during deposition at constant flow rates, the deposition

process can be monitored to ensure that complete coating of the nanopore is present.

From these pressure data, an approximation of the reduced size of the nanopore can

be made. By using a continuous monitoring process, the deposition rates can also be

studied.

3. The thickness of the PEI/PSS bilayers inside of the nanopore can be controlled to

some extent by changing the pH at which the PEI is deposited. At a lower pH the PEI

is more positively charged and, contrary to layer-by-layer deposition on surfaces,

deposits a thicker layer in the nanopore. The thickness of the initial PAA layer can

also be controlled. In this case, a less ionized PAA has more acid groups available for

hydrogen bonding and provides a thicker base layer.

4. These polyelectrolyte-modified membranes can be used for separations of dyes.

However, the mechanism of separation is not well-understood. The reduced pore size

of the polyelectrolyte-layered membrane can also be seen through sieving curves of a

broad molecular weight distributed solution of poly(ethylene oxide).

104

105

Appendix

Symbols and Abrreviations

PS Polystyrene

P4VP Poly(4-vinylpyridine)

PAA Poly(acrylic acid)

PVP Polyvinylpyrrolidone

PLGA Poly(L-glutamic acid)

PLL Poly(L-lysine)

PSS Poly(styrene sulfonate)

PAH Poly(allylamine hydrochloride)

WRT With respect to

DCM Dichloromethane

SEM Scanning electron microscopy

ATRP Atom-transfer radical polymerization

PVPh Poly(vinyl phenol)

PEO Poly(ethylene oxide)

PEG Poly(ethylene glycol)

DMF Dimethylformamide

THF Tetrahydrofuran

PDI Polydispersity index

TMP Transmembrane pressure

PEI Poly(ethylene imine)

qPEI Quaternized poly(ethylene imine)

GPC Gel permeation chromatography

106

Circuit Diagram Schematic

107

Curriculum Vitae

Personal Information

Full name Julia Ann Baettig

Date of Birth May 14, 1980

Present Address Im Binzacher 12, 8166 Niederweningen, Switzerland

Email jbaettig@mat.ethz.ch ; juliabaettig1@gmail.com

Skills

• Polymer synthesis skills including: Atom Transfer Radical Polymerization (ATRP), Reversible Addition-

Fragmentation Chain Transfer Polymerization (RAFT), Ring-Opening Polymerization (ROP)

• Thin film block copolymer self-assembly through solvent annealing • Experience in monomer synthesis, multi-step organic synthesis and purification • Nanostructure characterization and elucidation techniques: AFM, SEM, TEM, and SAXS

• Polymer and small molecule characterization and separation: NMR, GPC, IR, DSC, MPLC, UV-Vis • Electrochemistry techniques: CV, QCM • Proofreading, editing, and writing of scientific papers

• Software proficiency using Microsoft Office, iWork, ChemDraw, Adobe Illustrator and Photoshop, Igor

Pro, iNMR and MestreNova

Education

ETH Zürich, Zürich, Switzerland November 2014 (expected)

Doctoral studies, Materials Science Advisor: Dr. Anzar Khan

University at Buffalo, The State University of New York, USA July 2011

M.A., Chemistry Advisor: Prof. Dr. Javid Rzayev

State University of New York at Potsdam, USA Jul 2008

B.S., Chemistry Advisor: Prof. Dr. Maria Hepel

B.A., Anthropology Advisor: Prof. Dr. Bethany Usher

List of Publications

• “Self-assembly of an interacting binary blend of diblock copolymers in thin films: a potential route to

porous materials with reactive nanochannel chemistry” J. Rao, H. Ma, J. Baettig, S. Woo, M.C. Stuparu, J.

Bang, & A. Khan. Soft Matter 2014, 10(31), 5755-5762

• “Functionalized Molecular Bottlebrushes” I. Gadwal, J. Rao, J. Baettig, A. Khan. Macromolecules 2014,

47(1), 35-40 • “Interactions and reactivity of Hg(II) on glutathione modified gold electrode studied by EQCN technique”

M. Hepel, J. Dallas, M. D. Noble, J. Electroanal. Chem. 2008, 622, 173-183 • “Multifunctional polypeptide EQCN sensors: probing the cysteamine-glutathione film permeability with

Hg(II) ions” M. Hepel, J. Dallas, Sensors 2008, 8, 7224-7240

108

Experience

ETH Zürich: Doctoral Student Jan 2014-present

Block Copolymer Membrane Development

• Layer-by-layer self-assembly to control the size and chemistry of nanopores in ordered phase-inversion

membranes

• Development of a pressure-monitoring system to observe the rate of deposition during a flow-through

process for the layer-by-layer deposition of polyelectrolyte multilayers in a membrane

Nanostructured Block Copolymer/Homopolymer Thin Films

• Solvent annealing of PEO-b-PMMA thin films to obtain nanostructured morphology • Synthesis and characterization of functional diblock and triblock copolymers for use in block copolymer

self-assembled thin films

State University of New York at Buffalo: Master’s Student August 2008-July 2011

Synthesis and Melt Self-Assembly of Random Bottlebrush Copolymers

• Synthesis of PS-PLA random bottlebrush copolymers • Examination of melt self-assembly using SAXS State University of New York at Potsdam: Bachelor’s Student January 2006-July 2008

Glutathione Self-Assembled Monolayer Thin Films for Mercury Detection

• EQCN and Quartz Crystal Microbalance approach to detection of mercury using glutathione Administrative Assistant: Express Personnel, Kelly Services 2004-2005

Various administrative and secretarial duties including typing, working with Microsoft Office, record-

keeping, receptionist duties

GIS Technician, US Department of Agriculture 2002-2004

Digitized Texas farmland using ArcView GIS software

Presentations

Oral

• Jun 2010, 37th Northeast Regional Meeting (NERM 2010) of the American Chemical Society,

Potsdam, NY, USA

“Synthesis and characterization of polystyrene-polylactide random bottlebrush copolymers and their

self-assembly.”

Awards and Scholarships

• Gordon Harris fellowship at SUNY at Buffalo, 2008 (2000 USD/year)

• Departmental scholar in chemistry at SUNY Potsdam, 2008

• Departmental scholar in anthropology at SUNY Potsdam, 2008

• Canfield scholarship in anthropology at SUNY Potsdam, 2007&2008 (400 USD

Languages

• English (Native language)

• German (Basic knowledge, oral and written)

Teaching Experience

University at Buffalo, The State University of New York, USA Aug 2008 - May 2011

Teaching assistant, general and organic chemistry laboratory