SYNTHESIS AND CHARACTERIZATION OF IONICALLY BONDED DIBLOCK COPOLYMERS. A Dissertation. Presented to. The Graduate Faculty of The University of Akron

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1 SYNTHESIS AND CHARACTERIZATION OF IONICALLY BONDED DIBLOCK COPOLYMERS A Dissertation Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy Lei Feng December, 2013

2 SYNTHESIS AND CHARACTERIZATION OF IONICALLY BONDED DIBLOCK COPOLYMERS Lei Feng Dissertation Approved: Accepted: Advisor Dr. Kevin Cavicchi Department Chair Dr. Robert Weiss Committee Chair Dr. Hendrik Heinz Dean of the College Dr. Stephen Cheng Committee Member Dr. Mark Soucek Dean of the Graduate School Dr. George R. Newkome Committee Member Dr. Jie Zheng Date Committee Member Dr. Mary Ann Meador ii

3 ABSTRACT Block copolymers consist of two or more incompatible polymer chains linked by covalent bonds. These block copolymers can separate into nanometer sized domains whose morphology depends upon the size of the block and interactions between them. The properties of block copolymers can be modified and potentially improved by introducing noncovalent interactions to replace covalent linkages between blocks to form supramolecular block copolymers. These kinds of materials combine the microphase separation inherent to block copolymers with the facile synthesis of supramolecular materials, thereby affording new and unique materials. This dissertation focuses on synthesis and characterization of PS-b-PMA block copolymers with ion-pair junctions. Firstly, the chain-end sulfonated polystyrene (ω-sulfonated PS) was synthesized by reversible addition fragmentation chain transfer (RAFT) polymerization and postpolymerization modification. In the postpolymerization modification two methods were investigated. In the first one, the polymer was converted to a thiol-terminated polymer by aminolysis. Then a sulfonic acid end-group was produced by oxidation of the thiol end-group with m-chloroperoxybenzoicacid (m-cpba). In the second method, the RAFT-polymerized polymer was directly sulfonated by oxidation with m-cpba. After purification by column chromatography, ω-sulfonated PS was obtained by both methods with greater than 95% end-group functionality as measured by titration. The sulfonic acid end-group could be neutralized with various ammonium or imidazolium iii

4 counter ions through acid base or ionic metathesis reactions. These polymers with ionic end-groups can be used as model supramolecular building blocks. Secondly, ammonium end functionalized polymethylacrylate (PMA) was synthesized directly by RAFT polymerization using a functional RAFT agent. Then chain-end sulfonated polystyrene and ammonium end functionalized polymethylacrylate (PMA) were used to synthesize A-B block copolymers by two different methods: the first method was to mix two oppositely charged end group functionalized polymers; the second method was to ionically bond a RAFT agent to the chain end of an end sulfonated polymer to generate a supramolecular macro RAFT agent. Then an A-B block copolymer was prepared by RAFT polymerization using supramolecular macro-raft agent. The polymerization kinetics were investigated and the molecular weight and the chemical structure of the block copolymers were characterized by 1 H-NMR and SEC. The results show that the ion-bonded supramolecular block copolymers, PS-b-PMA, have been successfully prepared with controlled molecular weight and narrow distribution. Thirdly, the morphology of the ion-bonded supramolecular PS-b-PMA diblock copolymers were investigated by small-angle X-ray scattering (SAXS) and rheological techniques. Several covalently bonded PS-b-PMA block copolymers were synthesized by RAFT polymerization and their microdomain structures and rheology behaviors were also investigated. The results showed that the electrostatic interactions between the end ion groups are able to overcome the thermodynamic repulsion of the two blocks resulting in the formation of diblock copolymers with behaviors and morphology similar to those of traditional covalent bonded diblock copolymers and their microdomain structures remain to high temperatures. iv

5 ACKNOWLEDGEMENTS First, I would like to thank my advisor, Dr. Kevin Cavicchi for his guidance, encouragement, and financial support during my studies. I would also like to thank colleagues and friends from Dr. Cavicchi s group, past and present, for their assistance, support and friendship, Second, I would like to thank my committee members in my dissertation defense: Dr. MaryAnn Meador, Dr. Hendrik Heinz, Dr. Mark Soucek and Dr. Jie Feng who have been generous with their time and assistance throughout my graduate studies. Third, I should thank all faculties and staffs in the college of polymer science and Engineering. Their thoughts, suggestions and support have also been an integral part of the successful completion of my work here at the University of Akron. Finally, I would like to dedicate this dissertation to my family for their sacrifices, patience, and support during my graduate studies. v

6 TABLE OF CONTENTS Page LIST OF TABLES... xii LIST OF FIGURES... xiii LIST OF SCHEMES... xviii CHAPTER I. INTRODCTION... 1 II. BACKGROUND Introduction Supramolecular Polymers and Self-Assembly in Block Copolymer Non-covalent Interactions in Supramolecular Polymer Van der Waals Interaction Hydrogen bonds Metal coordination Coulombic interaction Main-Chain and Side-Chain Supramolecular Polymers vi

7 2.4.1 Side-Chain Supramolecular Polymers Main-Chain Supramolecular Polymer Main-Chain Supramolecular Homopolymer and Alternating Copolymers Main-Chain Supramolecular Block Copolymers Main-Chain Supramolecular Block Copolymers via hydrogen bonding and metal coordination Main-Chain Supramolecular Block Copolymers via coulombic interaction Synthesizing End Functionalized Telechelic Polymers RAFT polymerization Rheology of Block Copolymers III. EXPERIMENTAL Materials Chemicals used as received Purification Synthesis of ω-sulfonated polystyrene via reversible addition fragmentation chain transfer polymerization and postpolymerization modification RAFT agent Dibenzyl trithiocarbonate (DBTC) synthesis vii

8 3.2.2 RAFT Polymerization of Polystyrene (PS-RAFT-PS) Thiol-Terminated PS (PS-SH) ω-sulfonated Polystyrene (PS-SO 3 H) (from PS-SH) ω-sulfonated Polystyrene (PS-SO 3 H) (from PS-RAFT-PS) ω-sodium Sulfonate Polystyrene (PS-SO 3 Na) ω-ammonium Sulfonate Polystyrene (PS-SO 3 NH 4 ) ω-trialkyl Ammonium Sulfonate Polystyrene (PS-SO 3 HNR 3 ) ω-tetraalkyl Ammonium Sulfonate Polystyrene (PS-SO 3 NR 4 ) ω-1-ethyl-3-methylimidazolium Sulfonate Polystyrene (PS-SO 3 EMIM) Synthesis of Polymethyl acrylate (PMA-N+Br) Synthesis of N-(4-((dodecylthiocarbonothioylthio)methyl)benzyl)-N,N,Ntriethyl ammonium bromide RAFT Polymerization of Polymethyl acrylate (PMA-N + Br) Synthesis of PS-b-PMA Supramolecular Block Copolymer Synthesis of PS-b-PMA Block Copolymer by mixing PS-SO 3 Na and PMA- N + Br Synthesis of PS-b-PMA Block Copolymer from Supramolecular Macro-RAFT Agent Synthesis of Supramolecular Macro-RAFT Agent (PS-N + -RAFT) viii

9 RAFT Polymerization of PS-b-PMA Block Copolymer from Supramolecular Macro-RAFT Agent Synthesis of Covalent Bonded PS-b-PMA Block Copolymer Synthesis of Benzyl Dodecyl Trithiocarbonate RAFT Polymerization of Polystyrene (PS ) RAFT Polymerization of PS-b-PMA Block Copolymer Characterization Nuclear magnetic resonance (NMR) characterization Matrix-assisted laser desorption/ionization (MALDI) characterization Size Exclusion chromatography (SEC) characterization Differential scanning calorimetry (DSC) characterization Fourier Transform Infrared Spectroscopy (FTIR) characterization Acid-Base Titration characterization Sample preparation for morphology study Small angle x-ray scattering (SAXS) characterization Measurement of dynamic viscoelastic properties IV. SYNTHESIS OF Ω-SULFONATED POLYSTYRENE VIA REVERSIBLE ADDITION FRAGMENTATION CHAIN TRANSFER POLYMERIZATION AND POSTPOLYMERIZATION MODIFICATION ix

10 4.1 Introduction Result and discussion Synthesis of ω-sulfonated PS (PS-SO 3 H) from thiol-terminated PS ω-sulfonated Polystyrene (PS-SO 3 H) (from PS-RAFT-PS) Ion-Exchange of the -SO 3 H Terminated Polymers Conclusion V. SYNTHESIS OF PS-B-PMA BLOCK COPOLYMERS WITH ION-PAIR JUNCTIONS Introduction Result and discussion Synthesis of N-(4-((dodecylthiocarbonothioylthio)methyl)benzyl)-N,N,Ntriethyl ammonium bromide Synthesis of ω-sodium Sulfonate Polystyrene (PS-SO 3 Na) and Polymethyl acrylate (PMA-N + Br) Synthesis of PS-b-PMA Block Copolymer by mixing PS-SO 3 Na and PMA- N + Br Synthesis of Supramolecular Macro-RAFT Agent (PS-N + -RAFT) RAFT Polymerization of PS-b-PMA Block Copolymer from Supramolecular Macro-RAFT Agent Conclusion VI. PHASE BEHAVIOR AND RHEOLOGY STUDY OF PS-B-PMA BLOCK COPOLYMERS WITH ION-PAIR JUNCTIONS x

11 6.1 Introduction Result and discussion Synthesis of Benzyl Dodecyl Trithiocarbonate Synthesis of PS-b-PMA Block Copolymer by Benzyl Dodecyl Trithiocarbonate Rheological behavior of Symmetric or Nearly Symmetric Block Copolymers System Conclusion VII. CONCLUSION REFERENCES xi

12 LIST OF TABLES Table Page 4.1 Characteristics of the PS-RAFT-PS, PS-SH, and PSSO 3 H Polymers Measured by SEC Calculated and Observed Masses for MALDI-TOF Spectra of PS-SO 3 H from the Oxidation of PS-SH Calculated and Observed Masses for MALDI-TOF Spectra of PS-SO 3 H from Direct Oxidation of PS-RAFT-PS Calculated and Observed Peaks for MALDI-TOF Spectra form negative mode and positive mode with sodium trifluoroacetate (NaTFA) Glass Transition Temperatures of PS Polymers Characteristics of the PS-SO 3 Na and PMA-N + Br Polymers Measured by SEC Results of synthesis of block copolymer by the mixing method Results of polymerizations in the presence of the ion-bonded RAFT agent Summary of the molecular characteristics of the block copolymers investigated Glass transition temperatures (Tg) of homopolymers and block copolymers xii

13 LIST OF FIGURES Figure Page 2.1 The composition dependence of the microdomain structures: spheres, cylinders, gyroids, and lamellae, in AB-type diblock copolymers Self-assembly of discotic molecules with the different aggregates as a function of concertration Supramolecular polymers formed by π-π stacking or arene-arene interaction: (a) triphenylenes; (b) phthalocyanines; (c) helicenes; (d) m-phenyleneethynylene oligomers Liquid crystalline supramolecular polymers based on triple hydrogen bonds developed by Lehn Metal coordination complexes and its mode of polymerization Examples of side-chain supramolecular polymers based on hydrogen bonding reported by Fréchet and Kato (a) and Rotello (b) Example of side-chain supramolecular polymers of poly(4-vinylpyridine) and sulfonic acids based on ionic interactions Examples of main-chain supramolecular polymers based on (a) hydrogen bonding, (b) metal coordination and (c) π-π interactions and coulombic interaction Formations of amphiphilic supramolecular miktoarm star copolymers by ion interactions Selected RAFT agents for the synthesis of end-functional polymers Modification of the thiocarbonylthio end group Log G versus log ω plots of various phases of diblock copolymers Log G versus log G plots for SI-9/9 during heating at various temperature (ºC): ( ) 80, ( ) 90, ( ) 100, ( ) 110, ( ) 120, ( ) 130, ( ) 133, ( ) 136, and ( ) xiii

14 4.1 Amount of -SO3H groups determined by titration versus time (m-cpba:ps-sh molar ratios: - -1:1, - - 1:3, - - 1:6,- - 1: :15) SEC traces of (a) PS-RAFT-PS, (b) PS-SH, (c, d) PSSO3H. Traces (a c) were eluted with THF. Trace (d) was eluted with THF + 2 wt % SEC traces of PS-SH: (a) 1 h, under nitrogen, (b) 24 h, under nitrogen, (c) 1 h, under air, (d) 24 h, under nitrogen H-NMR spectra of (a) PS-RAFT-PS, (b) PS-SH, (c) crude PS-SO 3 H derived from PS-SH, (d) PS-SO 3 H cyclohexane fraction derived from PS-SH, (e) PS-SO 3 H acetone:methanol fraction derived from PS-SH MALDI-TOF spectra of (a) PS-SO 3 H cyclohexane fraction and (b) PS-SO 3 H acetone:methanol fraction. The insets display an expanded view of the spectrum. S, M, and V refer to the structures in Scheme 4.2 and Table 4.2 The floating spectrum in the inset of (a) is of the low intensity peaks and the floating spectrum in the inset of (b) is the calculated spectrum for the PS-SO 3 AgAg + (peak S) Figure 4.6 SEC traces of PS-SO 3 H obtained from direct oxidation of PS-RAFT-PS; (a) eluted in THF, (b) eluted in THF + 2 wt% TOA Figure 4.7 MALDI-TOF spectra of (a) crude PS-SO3H and (b) PS-SO3H acetone:methanol fraction. The insets display an expanded view of the spectrum. S, M, and V refer to the structures in Scheme 4.2 and Table 4.3. The floating spectrum in the inset of (a) is of the low intensity peaks and the floating spectrum in the inset of (b) is the calculated spectrum for the PS-SO 3 AgAg + (peak S) Figure 4.8 PS-SO3H (cyclohexane fraction) derived from PS-SH. Negative Mode Figure 4.9 PS-SO 3 H (cyclohexane fraction) derived from PS-SH. Positive Mode, Na Figure 4.10 PS-SO 3 H (acetone:methanol fraction) derived from PS-SH. Negative Mode Figure 4.11 PS-SO 3 H (acetone:methanol fraction) derived from PS-SH. Positive Mode, Na Figure 4.12 PS-SO 3 H (crude) derived from PS-RAFT-PS. Negative Mode PS-SO 3 H (crude) derived from PS-RAFT-PS. Positive Mode, Na PS-SO 3 H (purified) derived from PS-RAFT-PS. Negative Mode xiv

15 4.15 PS-SO 3 H (purified) derived from PS-RAFT-PS. Positive Mode, Na FTIR spectra of (a) PS-RAFT-PS, (b) PS-SH, (c) PS-SO3H and (d) PS-SO3Na. The sulfonated polymers were prepared by oxidation of PS-SH NMR spectra of the ion-exchanged PS-SO 3 X polymers DSC traces of PS-RAFT-PS, PS-SH, and PS-SO 3 X polymers. The PS-SO 3 X polymers are all derived from PS-SH except those denoted (PS-RAFT-PS), which are derived from the direct oxidation of the PS-RAFT-PS H NMR spectra of N-(4-((dodecylthiocarbonothioylthio)methyl)benzyl)-N,N,Ntriethyl ammonium bromide SEC trace of PS-SO 3 Na (a) and PMA-N + Br (b) SEC trace of mixing PS-SO 3 Na (Mn, 5.3kDa) and PMA-N + Br (Mn, 3.9kDa) block (molar ratio PS/PMA (1)1:3, (2)1:1.5, (3)1:1, (4)3:1) SEC trace of mixing PS-SO 3 Na and PMA-N + Br a) group 1; b) group 2; c) group 3; d) group H-NMR spectra of macro RAFT agent synthesized from PS-SO 3 Na SEC trace of PS-SO 3 Na 1(a) and supramolecular macro-raft agent (b) H-NMR spectra of block copolymers synthesized from macro RAFT agent (1) PS- SO 3 Na 1 (a) reaction time 0 hour (starting polymer) (b) reaction time 1 hour (c) reaction time 2 hour (d) reaction time 3 hour (e) reaction time 4 hour; (2) PS-SO 3 Na 2 (a), (b), (c), (d), (e) same as (1); (3) PS-SO 3 Na 3 (a), (b), (c), (d), (e) same as (1) SEC traces of block copolymers synthesized from macro RAFT agent (1) PS-SO 3 Na 1 (a) reaction time 0 hour (starting polymer) (b) reaction time 1 hour (c) reaction time 2 hour (d) reaction time 3 hour (e) reaction time 4 hour; (2) PS-SO 3 Na 2 (a), (b), (c), (d), (e) same as (1); (3) PS-SO 3 Na 3 (a), (b), (c), (d), (e) same as (1) a) Semilogarithmic kinetic plot of methyl acrylate conversion versus polymerization time 1a) PS-N + -RAFT 6k, 2a) PS-N + -RAFT 20k, 3a) PS-N + -RAFT 33k; and, b) the relationship between molecular weight M n (GPC) and polydispersity with methyl acrylate conversion 1b) PS-N + -RAFT 6k, 2b) PS-N + -RAFT 20k, 3b) PS-N + -RAFT 33k Phase diagrams of AB-type diblock copolymer in terms of χn versus f, which are based on self-consistent mean-field theory, in which χ denotes the Flory Huggins interaction parameter, N denotes the number of segments in the block copolymer, f denotes the volume fraction of one of the blocks, S denotes spherical microdomains, C xv

16 denotes cylindrical microdomains, G denotes gyroids, and L denotes lamellar microdomains H NMR spectra of Benzyl Dodecyl Trithiocarbonate SEC traces of covalently bonded PS -b-pma H NMR spectra of covalent bonded PS -b-pma, block copolymers (1) PS -PMA 1; (2) PS -PMA 2; (3) PS -PMA DSC traces of PS-b-PMA block copolymers with ion-pair junctions Temperature sweep of PS-PMA Temperature sweeps of three different Mw block copolymer with similar PS VOL% Frequency sweeps of the complex viscosity of PS-PMA at different temperatures Plots of (a) log G versus log ω and (b) log G versus log ω for PS-PMA during heating at various temperatures Log G versus log G plots for PS-PMA during heating at various temperatures SAXS profiles from PS-PMA Plots of log G versus log ω and log G versus log ω for PS-PMA 6-11(a), (c) and PS-PMA 33-44(b), (d) during heating at various temperatures Log G versus log G plots for PS-PMA 6-11(a) and PS-PMA 33-44(b) during heating at various temperatures SAXS profiles from PS-PMA 6-11(a) and PS-PMA 33-44(b) Plots of log G versus log ω for covalent bonded block copolymers (a) PS -b-pma 7-12, (b) PS -b-pma and (c) PS -b-pma Plots of log G versus log ω for covalent bonded block copolymers (a) PS -b-pma 7-12, (b) PS -b-pma and (c) PS -b-pma Log G versus log G plots for covalent bonded block copolymers (a) PS -b-pma 7-12, (b) PS -b-pma and (c) PS -b-pma xvi

17 6.18 SAXS profiles from covalent bonded block copolymers (a) PS -b-pma 7-12, (b) PS -b-pma and (c) PS -b-pma Temperature sweep of PS-PMA Plots of (a) log G versus log ω and (b) log G versus log ω for PS-PMA 20-6 during heating at various temperatures Log G versus log G plots for PS-PMA 20-6 during heating at various temperatures SAXS profiles from PS-PMA Temperature sweep of PS-PMA Plots of (a) log G versus log ω and (b) log G versus log ω for PS-PMA during heating at various temperatures Log G versus log G plots for PS-PMA during heating at various temperatures SAXS profiles from PS-PMA Plots of log G versus log ω and log G versus log ω for PS-PMA 6-4(a), (b) and PS-PMA 6-17(c), (d) during heating at various temperatures Log G versus log G plots for PS-PMA 6-4(a) and PS-PMA 6-17(b) during heating at various temperatures Plots of log G versus log ω and log G versus log ω for PS-PMA33-14(a), (b) and PS-PMA 33-69(c), (d) during heating at various temperatures Log G versus log G plots for PS-PMA 6-4(a) and PS-PMA 6-17(b) during heating at various temperatures SAXS profiles from PS-PMA 6-4(a), 6-17(b), 33-14(c) and 33-67(d) xvii

18 LIST OF SCHEMES Scheme Page 2.1 Mechanism of RAFT polymerization Synthesis of sulfonic acid-terminated PS (PS-SO 3 H) Structure of PS with sulfonic acid (S), vinyl (V), and methylene (M) end-groups Ion-exchange reactions with tertiary amine, quaternary ammonium, and imidazolium compounds Synthesis of A-B block copolymer by two different methods Synthesis of N-(4-((dodecylthiocarbonothioylthio)methyl)benzyl)-N,N,N-triethyl ammonium bromide Synthesis of Benzyl Dodecyl Trithiocarbonate.109 xviii

19 CHAPTER I INTRODCTION In a block copolymer, two or more chemically distinct polymer chains (blocks) are connected by covalent bonds to form a single macromolecule. The covalent linkages between these individual blocks prevent macroscopic phase separation even when the polymer blocks are thermodynamically incompatible. Instead, the individual blocks can microphase separate to form domains with sizes comparable to the dimensions of nano scale. Because individual blocks can be selected to have distinct chemical or physical properties, block copolymers have found extensive industrial applications including use as structural plastics, blend stabilizers and emulsifiers[1]. And also many applications rely upon the ability of block copolymers to form numerous periodic nano pattern structures. These block copolymers may be of use for selective membranes, catalysts, porous electrodes, photonic structures[2]. For polymer chemists, controlled and living polymerization techniques such as anionic, cationic, controlled radical, group transfer[3-6] are the tools in order to obtain block copolymers. However, all these techniques, relying on the use of covalent bonds connecting all the monomer units, do show limitations with the choice of monomers or the sequential order of the different blocks as well as the desired chain lengths. 1

20 In contrast to covalent bonding, supramolecular polymers rely on the use of noncovalent interactions as tools to assemble several individual molecules into a perfectly defined supramolecular structure. They exhibit some distinct advantages over covalent counterparts, such as fast and facile functionalization, reversibility, and self-healing. As far as macromolecules are concerned, the polymer chains can be end-functionalized with specific functional groups which are able to further self-assemble by supramolecular organization of macromolecules. This research developed from the goal of developing facile methods to synthesize supramolecular block copolymers with well-defined structures. When compared to covalent bonded block copolymers, these supramolecular block copolymers offer some unique advantages such as 1) the rapid generation of numerous block combinations, 2) the preparation of block copolymers consisting of blocks that can only be prepared separately by using different methods, 3) the functionality, reversibility, and responsibility to environment that non-covalent interactions could bring to the new materials. To ensure microphase separation of supramolecular block copolymers and to couple different blocks, functional units that link the different blocks together should ideally exhibit high stability, strength and non-self-complementary, and at the same time the linkages should be kinetically reversible. Among all the non-covalent interactions such as hydrogen bonding, ionic interactions and metal coordination, ionic interactions are the strongest[7]. Metal coordination are relatively stable but are kinetically more inert, while most hydrogen-bonded functional units have low binding energy. The versatility of the RAFT process toward functional groups allows for the introduction of a wide range of chain-end functionalities. Moreover, the structure of the thiocarbonylthio compounds in 2

21 RAFT agent provides even more approaches to control the functionality of a polymer chain end. Chapter 4 showed that ω-sulfonated PS polymers can be prepared by RAFT polymerization and postpolymerization modification. Capping of these polymers with tertiary amines, quaternary ammonium, and an imidazolium ionic liquid is also demonstrated through acid base neutralization and ionic metathesis reactions. Given the utility of RAFT polymerization to produce a wide range of different polymer chemistry and polymer chain architectures, this approach will widen the range of possible (hemi)- telechelic sulfonated polymers as well as building blocks of supramolecular block copolymers In Chapter 5 the ionically end functionalized polymer from Chapter 4 was used to synthesize A-B block copolymers by two different methods: the first method was by mixing two oppositely charged end group functionalized polymers; the second method was using ion exchange procedure demonstrated in Chapter 4 to ionically bond a RAFT agent, to the chain end of an end sulfonated polymer to generate a supramolecular macro RAFT agent then an A-B block copolymer was prepared by RAFT polymerization in the presence of the supramolecular macro-raft agent. The polymerization kinetics was investigated and the molecular weight and the chemical structure of the resulting block copolymers were characterized by 1 H-NMR and SEC techniques. Block copolymer melts in a microphase-separated state exhibit complex flow behavior. In short terms the specific phase are determined by the block composition, molecular weight, temperature and interactions between the blocks[8]. When a block copolymer is exposed to temperature changes and shear forces transitions between the phases can occur. It have been reported that the transition temperatures can be estimated 3

22 by rheological measurements. Several mechanical properties such as the storage modulus, loss modulus and complex viscosity can be measured by rheological techniques. The measurements can be conducted in different ways such as varying or constant temperature, angular frequency. The rheological behavior was studied for both ion bonded and covalent bonded block copolymer systems with similar chemical structure and molecular weight in Chapter 6. It will be investigated how different the connections between two blocks affect the rheological behavior in the block copolymer systems. Also SAXS experiments were conducted to reveal the type of microdomain structure of these block copolymer systems. Finally, the results of these studies and potential future directions are discussed in Chapter 7 4

23 CHAPTER II BACKGROUND 2.1 Introduction This chapter introduces supramolecular polymers and self-assembly of supramolecular polymers and block polymer and the role they place in polymer science. In the past a few decades, the study of supramolecular polymers has seen a rapid growth since supramolecular polymers have some significant advances over covalent bonded polymers, for examples, their fast and facile synthesis route, reversibility and responsibility to stimuli. [9]The formation of microphase in block copolymers system is also a classic example of self-assembly. The covalent bonding between immiscible blocks prevents macrophase separation caused by the mixing of polymer blends. Instead, the blocks of copolymer self-assemble into domains of dimensions on nanoscale and can form several specific periodic patterns. [10, 11] Supramolecular block copolymers as novel materials combine the characteristic features of block copolymers such as microphase separation with the reversibility and tunability of supramolecular materials. And among all the non-covalent interactions, such as van der Waals, hydrogen bonding, metal coordination and ionic interactions[12], ionic interaction is stronger than others. This is important because as mentioned above, in order to form microdomains the link 5

24 between two immiscible blocks has to be strong enough to overcome macrophase separation. This chapter introduces the motivations and concepts of a synthetic toolbox based on ionic bonded block copolymer. A brief review on the methods for synthesizing end functionalized telechelic polymers is also presented. And since the presence of microdomains gives unique mechanical properties to block copolymers[13], rheological measurements have become a valuable tool for morphology study of block copolymer. The following chapters will develop and expand the understanding of these concepts and therefore provide a facile synthetic method of supramolecular ionic block copolymers and their phase behavior study. 2.2 Supramolecular Polymers and Self-Assembly in Block Copolymer Traditional molecular chemistry is the chemistry based on the covalent bond, while supramolecular chemistry focuses on the weaker and reversible noncovalent interactions. It involves investigation of molecular systems that the main components are held together by noncovalent bonds. The most important feature of supramolecular chemistry is that the individual components can be considered as building blocks held together by noncovalent attractions. Supramolecular polymers emerged sometime in the early 1990s [14]. It combines supramolecular chemistry and polymer science as supramolecular polymer chemistry that has the ability to improve the properties of conventional polymers and overcoming the shortcomings such as high melt viscosities of conventional polymers. Supramolecular polymers are a novel class of macromolecules where reversible non covalent bonds or interactions linked the individual unit together. The most distinctive feature of supramolecular polymers is that they combine the characteristics of 6

25 conventional polymers with the properties which the reversibility of non-covalent bonding brings into the materials and therefore their properties and functions can be switched on or off through association/dissociation of non-covalent bonds. Their strength can be presented by degree of polymerization, and functions can be prepared through their monomeric units [15]. Unlike the conventional covalent polymers prepared by the polymerization of low molar mass repeat units (monomers), supramolecular polymers are assembles of low or high molar mass molecules reversibly self-assembled through noncovalent interactions [14]. As mentioned above a lot of types of non-covalent interactions can be used to produce supramolecular polymers: van der Waals interactions, H-bonding, metal coordination and electrostatic forces. These interactions are usually weaker than covalent bonds, which makes supramolecular polymers thermodynamically less stable, dynamically more flexible and kinetically more unstable than traditionally covalent bonded polymers [12]. However, it is worth mentioning that the concept of supramolecular polymers is not limited to the polymer chains consist of repeat units held together by non-covalent bonds, it can also be applied to the polymers which are self-assembled of conventional polymers which utilize non covalent interaction to influence their properties. For examples, there are some intermolecular atttractions in conventional polymers such as polyamides (H-bonding), polyesters (dipole-dipole interaction), or polyethylenes (dispersion interaction). Block copolymers are also an example of the self-assemble of polymer systems which leads to nano-structured materials. In short terms this phenomenon is driven by chemical incompatibilities and the connection between the blocks. The incompatibility 7

26 causes the microphase separation whilst the connection prevents the separation on a macroscale. The classical ordered phases in block copolymer systems are; spheres, cylinders, gyroid and lamellae[16], as seen in Figure 2.1. And these block copolymer formed by non-covalent interaction provide a facile method to obtain well defined nanostructured materials. Figure 2.1 The microdomain structures in AB-type diblock copolymers: spheres, cylinders, gyroids, and lamellae, 2.3 Non-covalent Interactions in Supramolecular Polymer Typical non-covalent interactions in supramolecular polymer are: hydrogen, van der Waals bonds and metal ion coordination and coulombic interactions Van der Waals Interaction The binding energy of Van der Waals interaction is about 0.1 to 1 kj/mol [12]. Therefore Van der Waals interactions are the weakest among all the non-covalent interactions. Induced dipoles, charge transfer (π-π stacking), dispersive or London forces and anisotropic attractions are the examples of Van der Waals interactions [14]. 8

27 Supramolecular polymers formed by π-π stacking or arene-arene interaction are highly ordered polymers that are usually formed in solution. Typical examples are discotic (disc-shaped) liquid crystalline polymers[12]. These kinds of polymers can be generated via the initial formation of disc like molecules which have a planar aromatic core and side alkyl chains. Discotics molecules can form rodlike polymers by aggregating in solvent because of the strong π-π (or arene-arene) interactions of their cores. Generally, the interdisc π-π stacking interaction between aromatic cores is several orders of magnitude larger than the intercolumnar van der Waals interactions caused by flexible alkyl side chains. At higher concentration, as these intercolumnar van der Waals interactions become more prominent, gelation or liquid crystalline phase in the bulk occurs (Figure 2.2) [17, 18]. Figure 2.2 Self-assembly of discotic molecules with the different aggregates as a function of (Reprinted with permission from ref [12] Copyright 2001 American Chemical Society) 9

28 The first discotic molecules shown to be liquid crystalline were alkoxy substituted triphenylenes.[19] Phthalocyanines have a much larger core than triphenylenes, in principle they will generate stronger arene-arene interactions [20, 21]. Furthermore, their optical and electrical properties can be easily modified by changing the metal incorporated within the core [22]. Other supramolecular polymers formed by π-π stacking or arene-arene interaction include helicenes and m-phenyleneethynylene oligomers. Figure 2.3 Supramolecular polymers formed by π-π stacking or arene-arene interaction: (a) triphenylenes[19]; (b) phthalocyanines[20-22]; (c) helicenes[23, 24]; (d) m- phenyleneethynylene oligomers[25] (Reprinted with permission from ref [12]. Copyright 2001 American Chemical Society) 10

29 The mechanical properties of the supramolecular polymers formed by discotic moleculars are relatively weak but they have high electronic mobility so it is suitable for them to make plastic transistors and photovoltaics [26-29]. Fibers can also be obtained by the association of some columnar structures Hydrogen bonds Hydrogen bonding is an important directional interaction existing in both synthetic molecules and in nature. The H-bond occurs between a proton donor (C-H, O-H, N-H, F-H) and a proton acceptor (O, N, S) [30]. The binding energy of H-bonding is around between 10 and 50 kj/mol [30]. Hydrogen bonding has been used widely for supramolecular polymers assemble due to their directionality and versatility. Supramolecular polymers based on hydrogen bonding may have single or multiple hydrogen bonds. A very important class of supramolecular polymers formed by H-bonding with well-defined structures are liquid crystalline (LC) polymers. Lehn et al. first introduced supramolecular LC systems in 1990[31]. As it is shown in Figure 2.3, supramolecular polymer chains were formed by triple hydrogen bonding between difunctional diaminopyridines and difunctional uracil derivatives Metal coordination Another class of noncovalent interactions has motivated supramolecular chemistry is metal coordination [32, 33]. While hydrogen bonding is relatively weak, metal coordination is generally considered a much stronger binding interaction[34]. Coordination polymers are interesting because of their magnetic, electronic, or photonic 11

30 properties; however, due to their lack of flexibility metal coordination supramolecular polymers often resemble covalent polymers[35]. When metal coordination supramolecular polymers were dissolved in a strongly coordinating solvent the bonds within these polymers can be cleaved[36]. Figure 2.3 Liquid crystalline supramolecular polymers based on triple hydrogen bonds developed by Lehn (Reprinted with permission from ref [31]. Copyright 1990 WILEY- VCH) Coordination complexes of Cu(I) and Ag(I) with phenanthrolin ligands were the first well-defined metal coordination supramolecular reported in 1996[37]. As shown in Figure 2.4 when the solvent used cannot act as a competitive ligand for the metal the rigid bis(phenanthrolinyl) ligand and Cu(I) can form high molecular weight polymers. 12

31 Figure 2.4 Metal coordination complexes and its mode of polymerization[37] Coulombic interaction Coulombic interactions are the interactions between permanent charges or dipoles that may be of the ion-ion (ion pair), ion-dipole or ion-quadrupole type[38]. They are the interactions between fixed and complementary ionizable groups and are adjusted by coand counter-ions. It is a non-directional interaction when compared to hydrogen bonding and metal coordination, but is the strongest of all non-covalent interactions[39]. Depending on the solvent and ion solution, the binding energy of normal ionic bonding is between kj/mol [40]. However the study of ionic interactions in supramolecular chemistry is relatively limited in opposition to metal coordination and hydrogen bonding because of the relatively weak binding strength in solution since when the ionic pairs are 13

32 solvated in polar solvent it usually leads to lower binding energy or dissociation of the pairs[41]. 2.4 Main-Chain and Side-Chain Supramolecular Polymers Depending on the number and locations of the non-covalent functional unit, supramolecular polymers may be main-chain, side-chain, branched or cross-linked network[42], Side-Chain Supramolecular Polymers When recognition non-covalent units are in the side-chains of a polymer, sidechain supramolecular polymers are obtained. The first noncovalently side-chain supramolecular polymers were prepared by hydrogen bonding and were reported by Fréchet and co-workers in the early 1990s [43-52]. The common noncovalent interactions using in side-chain supramolecular polymers are hydrogen bonding, metal coordination and coulombic interaction. Fréchet and Kato et al [47, 52] developed a method which is attaching liquid crystalline mesogens onto polysiloxane and polyarcylate polymer backbones by the use of hydrogen bonding and were able to form a variety of liquid crystalline mesophases. The Rotello group [53-57] reported the synthesis of triazine- or diaminopyridine containing side-chain polymers and then further assembly via hydrogen bonding. Figure 2.5 shows some examples of side-chain supramolecular polymers based on hydrogen bonding developed by Fréchet and Kato (a) and Rotello (b) 14

33 Figure 2.5 Examples of side-chain supramolecular polymers based on hydrogen bonding reported by Fréchet and Kato (a) and Rotello (b) Metal coordination is able to form significantly stronger interactions than the hydrogen bonding and has also been used for the synthesis of side-chain functionalized polymers[58]. The two most common classes of metal coordination polymers are based on either pyridine or pincer based ligands.[59-65] The third class of non-covalent interactions employed in side-chain supramolecular polymer assembly is the ionic interaction. The most common type of ionic interaction that can be found in side-chain supramolecular polymers is so called polyelectrolyte-surfactant complexes which include a charged polymer backbones and 15

34 ionic functionalized side chain. For example, Poly(4-vinylpyridine)[66] (Figure 2.6) and other pyridine-containing polymers[67] are often used as cationic polymer backbone and molecules containing anionic group such as sulfonic acid can be organized into side chain supramolecular polymers via ionic interactions. Similar structures can also be obtained by combining polyelectrolyte with sulfonic acids[68] and poly styrenesulfonate or poly acrylic acid with quaternary ammonium salts.[69] Tiitu et al[70] reported the formation of hairy tubes by functionalization the highly charged and electrically conducting protonated polyaniline with a series of carboxylic acids. This example demonstrates that the nanostructures of conducting materials can be highly controlled by some simple modifications on the molecular level. Figure 2.6 Example of side-chain supramolecular polymers of poly(4-vinylpyridine) and sulfonic acids based on ionic interactions Main-Chain Supramolecular Polymer Main-chain supramolecular polymers have received the most attention in the literature and all commercialized supramolecular polymers are related to this area [71]. There are two ways in which main-chain supramolecular polymers can be synthesized. The first approach is similar to a typical condensation polymerization where noncovalent 16

35 interactions are applied between monomer units to form homopolymers. The second approach is to use telechelic polymers as the building blocks which conventional polymers are modified with supramolecular recognition units at their chain-ends. Selfassembly of these monotelechelic polymers forms supramolecular diblock copolymers and ditelechelic polymers or telechelic polymers connected by some noncovalent linkers form more complex structures Main-Chain Supramolecular Homopolymer and Alternating Copolymers Conventional synthetic polymers are synthesized by monomeric units held together by covalent bonds into long chains. The length and entanglement of these chains therefore result in polymeric properties [72]. Main-chain supramolecular homopolymers and alternating copolymers are assembled from monomeric units held together by noncovalent interactions and also show polymeric properties. The first example of mainchain supramolecular polymers was reported by Lehn et al and was synthesized by mixing 1:1 ratio of a bis-uracil monomer and a 2,6-diaminopyridine monomer.[31] A number of non-covalent interactions can be applied to form main-chain supramolecular polymers such as hydrogen bonds, metal coordination, π-π interactions and coulombic interaction[73]. Figure 2.7 shows some examples of main-chain supramolecular polymers based on hydrogen bonding, metal coordination, π-π interactions and coulombic interaction. 17

36 Figure 2.7 Examples of main-chain supramolecular polymers based on (a) hydrogen bonding[74], (b) metal coordination[75] and (c) π-π interactions and coulombic interaction[76] Due to non-covalent interactions between the monomer units and the polymerization process these supramolecular polymers show temperature or environment dependent and switchable properties. 18

37 Main-Chain Supramolecular Block Copolymers The second type of main-chain supramolecular polymers is main-chain supramolecular block copolymers. They are obtained by using telechelic polymers with recognition units at their chain-ends as the building blocks. This type of main-chain supramolecular polymers have some advantages over small molecule monomer-based supramolecular polymers: firstly, it combines the characteristic features of block copolymers such as microphase separation with the reversibility and tunability of supramolecular polymer; secondly, a large variety of block copolymers with tunable properties can be assembled easily without the complicated synthesis routes. However, introduction of the noncovalently functionalized recognition units into telechelic polymers chains is much harder than into the small molecules, which requires careful design of synthetic strategies. The common noncovalent interactions such as hydrogen bonding [77], metal coordination[78] and coulombic interaction[79] have been used to assemble main-chain supramolecular block copolymers: Main-Chain Supramolecular Block Copolymers via hydrogen bonding and metal coordination When used as the noncovalent linkage, hydrogen bonding appears to be the most suitable interaction with its tunable reversibility and the resulting polymers may behave as thermoplastic elastomers. Metal coordination interactions are stronger than hydrogen bonding resulting in more stable but lower flexibility. 19

38 Long and co-workers [77]reported the synthesis of UPy-functionalized monotelechelic polymers which can dimerize to form di or homoblock copolymers, In their study, the well-defined monotelechelic PS, PI, and PS-b-PI block copolymers are prepared by anionic living polymerization followed with an end-capping reaction. The hydrogen bonding of two incompatible monotelechelic polymers leads to the formation of supramolecular AB diblock copolymers with various ordered nano scale morphologies. To form AB diblock copolymers with two incompatible homopolymers, the complementary hydrogen bonding recognition units require being very strong. Li and coworkers [80] reported that a tautomer of the UPy unit forms a strong and selective heterocomplex with Napy by quadruple hydrogen bond arrays. This complementary UPy- Napy hydrogen bonding has been shown to be effective in the preparation of main-chain supramolecular polymers through the self-assembly of telechelic polymers. Another major class of noncovalent interactions employed in supramolecular block copolymers is metal coordination [78, 81]. Terpyridine and pincer ligands are the most common used metal coordination interaction because of their significantly strong, directional, and also reversible interactions. Schubert et al reported the synthesis of terpyridine-terminated polymers such as PS, PEG, or poly(ethylene-co-butylene) (PEB) that can form stable complexes through a variety of transition metal ions[82] Main-Chain Supramolecular Block Copolymers via coulombic interaction The use of ionic interactions to form supramolecular AB type diblock copolymers is first investigated by Jérôme et al.[79, 83-87] Their aim was to improve the compatibility of immiscible polymer blends, such as polystyrene and polyisoprene. By mixing of carboxylic acid end-functionalized polystyrene with tertiary amine end- 20

39 functionalized polyisoprene the resulting materials show similar properties that resemble to the covalently-bonded PS-b-PI block copolymer. Russell et al.[79] studied on the same block-copolymer-type supramolecular self-assembly of same system by mixing two telechelic polymers: one is polyisoprene end-capped with a tertiary amine and the other is polystyrene that is terminated with sulfonate or carboxylate moieties. They found out that the sulfonate groups form stronger ionic bondings with the tertiary amino groups than the carboxylate ones by the temperature dependence of the SAXS and these types of block copolymers appear to undergo a classic order-disorder transition characteristic of block copolymers. Also the width of the interface remains sharp in these copolymers formed by ionic interactions when the order-disorder transition is approached in contrast to covalently-bonded block copolymers. Haraguchi et al.[42, 88] studied the aggregate formation of NH 2 -terminated polystyrene and SO 3 H-terminated polyethyleneglycol in toluene and the morphologies of these polymer blends with different ionic pairs such as -NH 2 and -SO 3 H, -NH 2 and COOH. The thermal tunability of lamellar spacing in block copolymer-type supramolecules from NH 2 -terminated polyisoprene and telechelic SO 3 H-terminated polystyrene in bulk has also been investigated by Huh et al.[89] they discovered that diblock- and triblock-like supramolecular system shows that the lamellar microphase swells to almost 300% upon heating, which cannot be accomplished in block copolymer system of the same material with covalent bond and therefore provide another method to control microphase dimensions. Moreover, Pispas et al.[90] reported the formation of a miktoarm block copolymer of polystyrene with a terminal trifunctional group of dimethyl amine and polyisoprene terminated with SO 3 H. More recently, Lu et al. [91-95]developed 21

40 two different strategies for synthesis of amphiphilic supramolecular miktoarm star copolymers by ion bond: the first one is by using carboxy groups and amino groups end functionalized polymer chains as building blocks, amphiphilic supramolecular miktoarm star copolymers were assembled by ion interaction; the second one is by synthesized the ionbonded supramolecular macro-raft agent first and then amphiphilic supramolecular miktoarm star copolymers were synthesized by RAFT polymerization using the macro- RAFT agent. Figure 2.8 shows the formation of amphiphilic supramolecular miktoarm star copolymers by these methods. The obtained supramolecular polymers can be dissociated in dilute acid solution at room temperature. Noro et al.[96] investigated the stoichiometric effects on nanophase-separated structures of block- and graft-type supramolecular copolymers using a pair of SO 3 H and NH 2 moieties and Qian et al.[97] reported the formation of supramolecular graft copolymers from poly (4-vinylpyridine) and a COOH group terminated poly(n-vinylpyrrolidone). Not only are ionic interactions useful to main chain supramolecular block copolymers, but they are also a great tool to form supramolecular polymer networks. Shibata et al.[98] prepared novel supramolecular polymer networks based on poly(4- vinylpyridine) and disulfonic acids. They found out that the formation of supramolecular polymer network based on ionic interaction can improve the heat resistance and the higher degree of ionic interaction of the system get the higher Tg of the system is. 22

41 Figure 2.8 Formations of amphiphilic supramolecular miktoarm star copolymers by ion interactions[91, 92] 2.5 Synthesizing End Functionalized Telechelic Polymers The term telechelic is originated from the Greek words telos (far) and chelos (claw). This term was used to describe a polymer chain having a claw to grip something at its far end[99]. A telechelic polymer is a polymer containing one or more reactive end groups at the end of the polymer chains and a supramolecular telechelic polymer s end groups contains a noncovalent recognition unit that can undergo selfassembly. Telechelic polymers as important building blocks provide a simple method for forming a wide range of block copolymers with different backbones and/or 23

42 functionalities via self-assembly. There are many methods for making telechelic polymers. These include functional initators, functional chain-terminating agent, chain transfer agent and post polymerization modification. There are also a lot of polymerization techniques that can be used to synthesize telechelic polymers as well as block copolymers such as anionic, cationic, controlled radical, group transfer polymerization. However, there are still some limitations with these synthesis methods that have been developed which include: the choice of monomer, limited control during the polymerization, and poor yields and harsh conditions associated with postpolymerization modification RAFT polymerization As the most recent controlled radical polymerization, reversible additionfragmentation chain transfer (RAFT) polymerization was first reported in 1998 by Chiefari et al.[100] It uses a thiocarbonylthio compound as a chain transfer agent to control over the molecular weight and polydispersity during a free radical polymerization. RAFT polymerization can be can be used with a large variety of monomers and is tolerant of a wide range of unprotected functional groups and reaction conditions and also does not require highly rigorous removal of impurities. It can be performed by just adding a suitable RAFT agent to a conventional free radical polymerization usually with the same monomers, initiators, solvents and reaction conditions. When compared to other competitive polymerization technologies, it is easy to implement and inexpensive. The mechanism for RAFT polymerization is given in Scheme 2.1.[101] 24

43 Scheme 2.1 Mechanism of RAFT polymerization (Reprinted with permission from Moad et al.[101] Copyright Society of Chemical Industry) As shown in Scheme 2.1, a RAFT polymerization includes a number of steps: initiation, chain transfer (pre-equilbrium), re-initiation, main equilibrium, propagation and termination. Polymerization begins when radicals are usually generated by decomposition of a peroxide or azo type free radical initiator. The initiating radical reacts with the monomer and follows by the pre-equilibrium step, the radicals on the ends of the propagating chains quickly attack the reactive C=S bonds of the chain transfer agent to produce a carbon centered intermediate radical. This species release the R group as a radical fragment and leave the polymer chain capped with the dithioester. The R radical released is free to initiate new chains by attacking monomer or they may attack back on 25

44 the dithioester capped chain. The capped dithioester are the dormant species in a RAFT reaction and the general form and function of the dormant dithioester is the same as that of the original chain transfer agent. The dormant polymer chain replaces the R group. The pre equilibrium continues until all initiator is consumed and all R groups are released as radicals to initiate more chains. At this point, the main equilibrium begins and is controlled by the same mechanism of radical attack on the C=S bond followed by β- scission of the resulting intermediate radical. However, in the main equilibrium stage this process takes place solely between propagating chains and macro-ctas, resulting in a rapid exchange of the dithioester cap. This rapid exchange ensures each chain has the same probability of growth and termination reactions are minimized therefore the living polymerization characteristics are observed. When polymerization is finished, the chains remain in the capped state and can be re-initiated to form more complex molecules as a macro RFAT agent. The RAFT polymerization is suitable for synthesis of end functional telechelic polymers and also block copolymers through the selection of a RAFT agent. The highly tolerance of the RAFT polymerization toward functional groups allows for the introduction of a wide range of chain-end terminal functionalities. Furthermore, the structure of the thiocarbonylthio compounds in a RAFT agent provides two ways to introduce the functional group to a polymer chain end: 1. By modifying the chain transfer agent with functional groups (via the R and/or the Z group) 26

45 There are various RAFT agents that could be used to introduce α and/or ω functionality in polymers. Figure 2.10 demonstrates some common functional groups and their corresponding RAFT agents. Figure 2.4 Selected RAFT agents for the synthesis of end-functional polymers [ ] 2. By modifying the thiocarbonylthio chain-end group via post polymerization 27

46 Functional groups can also be introduced by the modification or transformation of the thiocarbonylthio group through post polymerization. The modification of thiocarbonylthio groups is shown in Figure Figure 2.5 Modification of the thiocarbonylthio end group (reprint with permission from reference [107] 2011 Society of Chemical Industry) There are a lot of options to remove or transform the thiocarbonylthio end group. Such an end group provides a fantastic way of further chemical modification, which allows for more functional group incorporated into the polymer end chains via post polymerization. 28

47 2.6 Rheology of Block Copolymers The incompatibility of two blocks in block copolymer causes the microphase separation and the presence of microdomains gives some unique mechanical properties to block copolymers. There are three common rheological features of all block copolymer melts with long-range ordered phases that are in contrast to ordinary flexible polymer melts[108]: 1. The low-frequency linear viscoelastic response does not exhibit a terminal relaxation behavior when the material behaves just like a simple viscoelastic liquid. 2. The critical yield strain is smaller at lower frequencies. 3. The rheological properties are very sensitive to deformation history. Figure 2.12 shows the storage modulus G at low frequency for disordered and ordered microphase structure. It is worth noting that long range order is crucial since for the systems lacking long range order none of the three characteristic features are observed. The most typical example is graft block copolymers: these polymers are micro phase separated, but the order is very local. Graft block copolymers are simple viscoelastic liquids at low frequencies, with a wide range of strain at lower frequencies, and are not sensitive to deformation history [109]. The rheology behaviors of graft block copolymers are similar to flexible polymers due to lacking long-range order in their micro phase separated state. 29

48 Figure 2.6 Log G versus log ω plots of various phases of diblock copolymers (reprint with permission from reference [110] 1999 Society of Rheology) Not only have rheological measurements become a valuable tool for phase identification but also they are useful to determine phase transition temperatures of a block copolymer. One of the most intriguing aspects of block copolymers is that phase transitions occur from an ordered state to a disordered state (ODT) or from one ordered state to another ordered state (OOT) when heating. When these transitions occur, there also will be some dramatic viscoelastic characteristics changing. Indication of an OOT can be given by a drop in G [111, 112] and a significant loss in G at a certain temperature can also be related to an ODT. A more common method for finding the ODT has been reported by Han et al. [ ]. The method demonstrates that plots of the loss modulus G vs. the storage modulus G for a block copolymer system with an ordered phase show a temperature dependence while a disordered one is independent of temperature. 30

49 Figure 2.13 shows the log G versus log G plots for SI-9/9(polystyrene-bpolyisoprene diblock copolymer)[82]. The TEM image of this sample shows that SI-9/9 has lamellar microdomains. The log G versus log G plot with a slope much less than 2 in the terminal region maintains almost on a single correlation between 80 and 120 ºC, but makes a sudden shift downward with a slope still less than 2 in the terminal region from 120 to 130 ºC, and when the temperature increased from 130 to 133 ºC, the log G versus log G plots of SI-9/9 is shifted downward further and then remains there as the temperature increased from 136 to 140 ºC with a slope of 2 in the terminal region. From the temperature sweep experiment, it can also be seen that the value of G begins to decrease rapidly at approximately 130 ºC 31

50 Figure 2.7 Log G versus log G plots for SI-9/9 during heating at various temperature (ºC): ( ) 80, ( ) 90, ( ) 100, ( ) 110, ( ) 120, ( ) 130, ( ) 133, ( ) 136, and ( ) 140 (Reprinted from reference [116] Copyright 2000, with permission from the American Chemical Society) Because of the existence of the microdomains a block copolymer in an ordered phase has a higher melt viscosity than in the disordered state. Therefore a Newtonian behavior is not usually seen in an ordered phase, but it can be seen for low frequency in the disordered state. Han et al. [115] have also shown that plots of complex viscosity η* vs. frequency ω give further indication that completely Newtonian behavior may not be 32

51 seen in a polymer melt in an ordered phase. However this method is not as convincing as the G vs. G plots since it has been shown that at temperature lower than the ODT the system can show Newtonian behavior at sufficiently low frequency even when ordered structures still remain[117]. In order to identify the present specific phase additional methods such as small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) are usually used.[118, 119] 33

52 CHAPTER III EXPERIMENTAL 3.1 Materials Chemicals used as received Tri-n-ethylamine (Acros, HPLC grade), tri-n-octylamine (Alfa Aesar, 98%), ammonium hydroxide solution (Sigma-Aldrich, 1 M), n-octadecyl trimethylammonium bromide (Sigma-Aldrich), tetra-n-butylammonium bromide (Sigma-Aldrich), 1-ethyl-3- methylimidazolium chloride (Sigma-Aldrich), m-chloroperoxybenzoicacid (Sigma- Aldrich), n-butylamine (Sigma-Aldrich), chloroform (Fisher Scientific), hexane (EMD), toluene (EMD, ACS grade), methanol (MeOH, Fisher Scientific, reagent grade), tetrahydrofuran (THF, EMD, ACS grade), carbon disulfide (CS 2, Aldrich, ACS reagent, 99.9%), 1-dodecanethiol (Aldrich, 98%), NaOH 50% aqueous solution, p-xylylene dibromide (Aldrich), benzyl chloride (Aldrich), silica gel (EM Science, mesh), phenolphthalein Purification Styrene (99%, stabilized, Acros) and Methyl acrylate (99%, stabilized, Aldrich) were passed through a column of activated basic alumina to remove the inhibitor immediately prior to use. 34

53 methanol. 2, 2-azobisisobutyronitrile (AIBN, 98%, Sigma-Aldrich) was recrystallized from 3.2 Synthesis of ω-sulfonated polystyrene via reversible addition fragmentation chain transfer polymerization and postpolymerization modification RAFT agent Dibenzyl trithiocarbonate (DBTC) synthesis Dibenzyl trithiocarbonate (DBTC) was synthesized via the reaction of equal amount of alkyl halide and carbon disulfide with cesium carbonate in polar aprotic solvents according to previously reported procedure. [120] The product was purified by passing through silica gel column eluted with hexane RAFT Polymerization of Polystyrene (PS-RAFT-PS) Styrene (20 g), dibenzyltrithiocarbonate (0.435 g, mol), and a stir bar were added to a 50-mL round bottom flask sealed with a rubber septum. The solution was purged with dry nitrogen for 15 min, followed by heating to 130 C for 12 h in a thermostated reaction block. The flask was quenched to 0 C, then the PS was isolated by precipitation of the reaction mixture into methanol, filtered and washed three times with methanol, and dried in a vacuum oven at room temperature for 24 h. Mn=8.8 kda, Ð=1.17 (SEC calibrated with PS standards) Thiol-Terminated PS (PS-SH) PS-RAFT-PS (2 g) was dissolved in 20 ml of anhydrous toluene. The solution was purged with dry nitrogen for 15 min and a 20-fold molar excess n-butylamine was 35

54 added and then stirred for 1 h at room temperature. PS-SH was isolated by precipitation of the reaction mixture into methanol, filtered and washed three times with methanol, and dried in a vacuum oven at room temperature for 24 h. Mn=4.0 kda, Ð=1.21 (SEC calibrated with PS standards) ω-sulfonated Polystyrene (PS-SO 3 H) (from PS-SH) PS-SH (2 g) was dissolved in 2 ml of toluene and m-cpba at various molar ratios to the thiol end-group was added to the solution. The reaction was stirred at 0 C for 1 6 h; then PSSO 3 Hwas isolated by precipitation of the reaction mixture into methanol, filtered and washed three times each with methanol and reverse osmosis water. PS-SO 3 H was purified by column chromatography using silica gel (EM Science, mesh). For purification of a 1 g sample of crude PS-SO 3 H, 125g of silica gel was placed in a 2 50 cm 2 column and washed with three column volumes of cyclohexane. The polymer was dissolved in cyclohexane, introduced to the column, and eluted with cyclohexane followed by acetone/methanol (V/V=4/1). Some sulfonated polymer was eluted in cyclohexane resulting in less than 100% mass yield expected from titration of the crude polymer. 96% end-group functionality by titration, yield 50 55% by mass; Mn=4.0 kda, Ð=1.22 (THF), Mn=3.6 kda, Ð=1.22 (THF t 2 wt % TOA; SEC calibrated with PS standards); MALDI-TOF MS (m/z) calcd n [M+Ag + ], found n ω-sulfonated Polystyrene (PS-SO 3 H) (from PS-RAFT-PS) PS-RAFT-PS (2 g) was dissolved in 2 ml toluene and m-cpba at molar ratios of 36

55 30:1 to the trithiocarbonate group was added to the solution. The reaction was stirred at 0 C for 6 h and the polymer was isolated by precipitation of the reaction mixture into methanol. PS-SO 3 H was purified by column chromatography using the same procedures as used for the PS-SO 3 H derived from the PS-SH. 97% end-group functionality by titration, yield 84% by mass; Mn=3.7 kda, Ð=1.20 (THF t 2 wt % TOA; SEC calibrated with PS standards);maldi-tof MS [m/z] calcd n [M+Ag + ], found n ω-sodium Sulfonate Polystyrene (PS-SO 3 Na) Purified PS-SO 3 H (1 g) was dissolved in 5 ml of toluene and0.5 ml of a 0.1 M sodium hydroxide in methanol solution was added. The reaction was stirred at room temperature for 2 h; then PS-SO 3 H was isolated by precipitation of the reaction mixture into methanol, filtered and washed three times with methanol, and dried in a vacuum oven at 120 C for 24 h ω-ammonium Sulfonate Polystyrene (PS-SO 3 NH 4 ) Purified PS-SO 3 H (1 g) was dissolved in 5 ml of THF and 1mL of 1 M ammonium hydroxide was added to the THF solution. The reaction was stirred at room temperature for 2 h; then PS-SO 3 NH 4 was isolated by precipitation of the reaction mixture into methanol, filtered and washed three times with methanol, and dried in a vacuum oven at 120 C for 24 h. 37

56 3.2.8 ω-trialkyl Ammonium Sulfonate Polystyrene (PS-SO 3 HNR 3 ) Purified PS-SO 3 H (0.1 g) was dissolved in 2 ml of toluene and triethylamine (TEA) or TOA at 1.1:1 molar ratio to-so3h end-groups was added to the solution. The reaction was stirred at 60 C for 24 h; then the polymer was isolated by precipitation of the reaction mixture into methanol, filtered and washed three times with methanol, and dried in a vacuum oven at 120 C for 24 h. NMR: PS-SO 3 HTEA (300 MHz, CDCl 3, CDCl ppm) polymer: (broad, 5H, aromatic), (broad, 3H,-CH2-CH-backbone); endgroup: 1.06(q, 9H, N-CH 2 -CH 3 ), 2.74(t, 6H, N-CH 2 -CH 3 ); PS-SO 3 HTOA (300 MHz,CDCl 3, CDCl ppm) polymer: (broad, 5H, aromatic), (broad, 3H, -CH 2 -CH-backbone); end-group:0.88(t, 9H, N-(CH 2 ) 7 -CH 3 ), 1.25(s, 30H N-CH 2 - CH 2 -(CH 2 ) 5 -CH 3 ), 2.46(t, 6H, N-CH 2 ) ω-tetraalkyl Ammonium Sulfonate Polystyrene (PS-SO 3 NR 4 ) Purified PS-SO 3 Na (0.1 g) was dissolved in 2 ml of tolueneand n-octadecyl trimethylammonium bromide (ODTMA-Br)or tetra-n-butylammonium bromide (TBA-Br) at 1.1:1 molar ratio to the -SO 3 Na end-groups was added to the solution. The reaction was stirred at 60 C for 24 h; then the polymer was isolated by precipitation of the reaction mixture into methanol, filtered and washed three times each with methanol and water, and dried in a vacuum oven at 120 C for24 h. For tetramethylammonium chloride (TMA-Cl), 0.1 g ofps-so 3 NH 4 was dissolved in 2 ml of acetone and TMA-Cl at a 1.1:1 molar ratio to the -SO 3 NH 4 end-groups was added to the solution. The reaction was stirred at 50 C for 24 h; then the polymer was isolated by precipitation of the 38

57 reaction mixture into methanol, filtered and washed three times each with methanol and water, and dried in a vacuum oven at 120 C for 24 h. NMR: PS-SO 3 TMA (300 MHz, CDCl 3, CDCl 3 = 7.24 ppm) polymer: (broad, 5H, aromatic), (broad, 3H,-CH 2 -CH-backbone); end-group: 3.02(t, 12H, N-CH 3 ); PSSO 3 TBA(300 MHz, CDCl 3, CDCl ppm) polymer: (broad, 5H, aromatic), (broad, 3H, -CH 2 -CH-backbone); end-group: 0.97(t, 12H, N-(CH 2 ) 3 - CH 3 ), 1.68(t,16H, N-(CH 2 ) 2 -CH 2 -CH 3, 3.27(t, 8H, N-CH 2 -); PSSO 3 ODTMA(300 MHz, CDCl3, CDCl ppm) polymer: (broad, 5H, aromatic), (broad, 3H -CH 2 -CH-backbone); end-group: 0.87(t, 3H, N-(CH 2 ) 17 -CH 3 ), 1.24(s, 30H N-CH 2 -CH 2 - (CH 2 ) 15 -CH 3 ), 2.03(t, 2H,N-C-CH 2 ), 2.92(t, 9H, N-CH 3 ), 3.05(t, 2H, N-CH 2 -CH 2 -) ω-1-ethyl-3-methylimidazolium Sulfonate Polystyrene (PS-SO 3 EMIM) Purified PS-SO 3 NH 4 (0.1 g) was dissolved in 2 ml of acetone and 1-ethyl-3- methylimidazolium chloride (EMIMCl) at1.1:1 molar ratio to -SO 3 NH 4 end-groups was added to the solution. The reaction was stirred at 50 C for 24 h; then the polymer was isolated by precipitation of the reaction mixture into methanol, filtered and washed three times each with methanol and water, and dried in a vacuum oven at 120 C for 24 h. NMR: PS-SO 3 EMIM (300 MHz, CDCl 3, CDCl 3 =7.24 ppm)polymer: (broad, 5H, aromatic), (broad, 3H-CH 2 -CH-backbone); end-group: 1.51(t, 3H, C- CH 3 ),3.95(t, 3H, N + -CH 3 ), 4.26(t, 2H, N-CH 2 ), 10.47(s,1H,HC=N + ). 39

58 3.3 Synthesis of Polymethyl acrylate (PMA-N+Br) Synthesis of N-(4-((dodecylthiocarbonothioylthio)methyl)benzyl)-N,N,N-triethyl ammonium bromide The synthetic route involved reactions of a thiol with carbon disulfide and an aromatic dihalides under basic conditions to obtain a trithiocarbonate followed by quaternzation of the halide to introduce a quaternary ammonium group mol 1- dodecanthiol (3.015g), mol Aliquat 336 (0.24g) and 100ml toluene were added into a flask. The solution was stirred under nitrogen gas in an ice bath. After 15mins, mol NaOH 50% aqueous solution (1.215g) was injected into the reactor. The solution became white. Then 15mins later, 0.015mol carbon disulfide (1.71g, dissolved in 5ml toluene) was injected into the solution. The color of solution changed to yellow, and the viscosity increased. 15mins later, 0.015mol p-xylylene dibromide (3.96g, dissolved in 40ml toluene) was injected into the solution. The solution was stirred for 12 h at room temperature under nitrogen. The reaction was terminated by adding deionized water into the reactor, and the organic layer was washed by deionized water 3 times. The organic layer was collected in a separation funnel, dried with anhydrous Na 2 SO 4, and the solvent was removed by rotational evaporation. The product was crystallized in a freezer, and a yellow solid was obtained. The solid was dissolved in CH 2 Cl 2 with the conc. of 10%, and a 1.2 molar excess of triethylamine was added into the reactor. The reactor was heated to 50 C for 12 h, cooled in water bath, and the solvents were removed by rotational evaporation. 40

59 To purify N-(4-((dodecylthiocarbonothioylthio)methyl)benzyl)-N,N,N-triethyl ammonium bromide, the RAFT agent was mixed with methanol and heated to 60 C for 2 h. There was some insoluble solid, which are likely the didecyltrithiocarbonate; the RAFT agent was dissolved in methanol. The mixture was filtered, and the solution was collected. After drying, a yellow solid was recovered. Then the RAFT agent was dissolved in acetone, there was some insoluble solid, which should be p-xylene dibromide quaternized by TEA. The insoluble solid was filtered and the RAFTammonium salt was recrystallized. NMR: N-(4-((dodecylthiocarbonothioylthio)methyl)benzyl)-N,N,N-triethyl ammonium bromide (300 MHz, CDCl 3, CDCl 3 = 7.24 ppm) 7.54 (1H, aromatic), 7.36 (1H, aromatic), 4.86(2H, N + -CH 2 -Ar), 4.58(2H, Ar-CH 2 -S), 3.43(6H, N + -CH 2 -CH 3, 2H, S-CH 2 -C 11 H 23 ), 1.67(2H, S-CH 2 -CH 2 -C 9 H 19 ), 1.46(9H, N + -CH 2 -CH 3 ), 1.26(18H, - (CH 2 ) 3 -CH 3, 0.87(6H, -(CH 2 ) 11 -CH 3 ) RAFT Polymerization of Polymethyl acrylate (PMA-N + Br) Methyl acrylate (10 g), N-(4-((dodecylthiocarbonothioylthio)methyl)benzyl)- N,N,N-triethyl ammonium bromide (0.225 g, mol), azobisisobutyronitrile(0.0131g, mol) and a stir bar were added to a 50-mL round bottom flask sealed with a rubber septum. The solution was purged with dry nitrogen for 15 min, followed by heating to 60 C for 4 h in a thermostated reaction block. The flask was quenched to 0 C, and then the PMA was isolated by precipitation by adding the reaction mixture into hexane, dissolved in toluene and reprecipitated into hexane again, and dried in a vacuum oven at room temperature for 24 h. 41

60 3.4 Synthesis of PS-b-PMA Supramolecular Block Copolymer Synthesis of PS-b-PMA Block Copolymer by mixing PS-SO 3 Na and PMA-N + Br Different molar ratios of PS-SO 3 Na and PMA-N + Br and 10 ml of toluene were added to a 20 ml vial. The reaction was stirred at 60 C for 24 h; then the polymer was isolated by precipitation of the polymer into hexane, dissolved in toluene and reprecipitated into hexane again, and dried in a vacuum oven at room temperature for 24 h Synthesis of PS-b-PMA Block Copolymer from Supramolecular Macro-RAFT Agent Synthesis of Supramolecular Macro-RAFT Agent (PS-N + -RAFT) PS-SO 3 Na (0.5 g) was dissolved in 10 ml of toluene and N-(4- ((dodecylthiocarbonothioylthio)methyl)benzyl)-n,n,n-triethyl ammonium bromide at a 1.1:1 molar ratio to -SO 3 Na end-groups was added to the solution. The reaction was stirred at 60 C for 24 h; then the polymer was isolated by precipitation of the reaction mixture into methanol, filtered and washed three times each with methanol and water, and dried in a vacuum oven at 120 C for 24 h RAFT Polymerization of PS-b-PMA Block Copolymer from Supramolecular Macro-RAFT Agent An example reaction is as follows. Methyl acrylate (4 g), PS-N + -RAFT (0.5 g, mol, M n,pst =6,530, PDI =1.18), azobisisobutyronitrile(0.0025g, mol) 42

61 and a stir bar were added to a 50-mL round bottom flask sealed with a rubber septum. The solution was purged with dry nitrogen for 15 min, followed by heating to 60 C for 1-4 h in a thermostated reaction block. The flask was quenched to 0 C, and then the copolymer was isolated by precipitation of the reaction mixture into hexane, dissolved in toluene and reprecipitated into hexane again, and dried in a vacuum oven at room temperature for 24 h. 3.5 Synthesis of Covalent Bonded PS-b-PMA Block Copolymer Synthesis of Benzyl Dodecyl Trithiocarbonate The synthetic route involved the reaction of a thiol with carbon disulfide and an alkyl halide under basic conditions to obtain a trithiocarbonate mol 1-dodecanthiol (3.015g), mol Aliquat 336 (0.24g) and 100ml toluene were added into a flask. The solution was stirred under nitrogen gas in an ice bath. After 15mins, mol NaOH 50% aqueous solution (1.215g) was injected into the reactor. The solution became white. Then 15mins later, 0.015mol carbon disulfide (1.71g, dissolved in 5ml toluene) was injected into the solution. The color of solution changed to yellow, and the viscosity increased. 15mins later, 0.015mol benzyl chloride (1.89g) was injected into the solution. The solution was stirred for 12 h at room temperature under nitrogen. The reaction was terminated by adding deionized water into the reactor, and the organic layer was washed by deionized water for 3 times. The organic layer was collected in a separation funnel, dried with anhydrous Na 2 SO 4, and the solvent was removed by rotational evaporation. The product was crystallized in freezer, and a yellow solid was obtained. Then the RAFT agent was dissolved in chloroform and recrystallized 3 times. 43

62 NMR: Benzyl Dodecyl Trithiocarbonate (300 MHz, CDCl 3, CDCl 3 = 7.24 ppm) 7.53 (1H, aromatic), 7.36 (1H, aromatic), 7.24 (1H, aromatic), 4.52(2H, Ar-CH 2 -S), 3.21(2H, S-CH 2 -C 11 H 23 ), 1.77(2H, S-CH 2 -CH 2 -C 9 H 19 ), 1.41(2H, -CH 2 -CH 3 ), 1.26(16H, - (CH 2 ) 3 -CH 3, 0.87(3H, -(CH 2 ) 11 -CH 3 ) RAFT Polymerization of Polystyrene (PS ) Styrene (20 g), benzyl dodecyl trithiocarbonate (0.662 g, mol), and a stir bar were added to a 50-mL round bottom flask sealed with a rubber septum. The solution was purged with dry nitrogen for 15 min, followed by heating to 130 C for 8 h in a thermostated reaction block. The flask was quenched to 0 C, then the PS was isolated by precipitation of the reaction mixture into methanol, filtered and washed three times with methanol, and dried in a vacuum oven at room temperature for 24 h. Mn=6.8 kda, Ð=1.18 (SEC calibrated with PS standards) RAFT Polymerization of PS-b-PMA Block Copolymer An example reaction is as follows. Methyl acrylate (4 g), PS-RAFT (0.53 g, mol, M n,pst =6.8kDa, PDI =1.18), azobisisobutyronitrile(0.0025g, mol) and a stir bar were added to a 50-mL round bottom flask sealed with a rubber septum. The solution was purged with dry nitrogen for 15 min, followed by heating to 60 C for 1-4 h in a thermostated reaction block. The flask was quenched to 0 C, and then the copolymers was isolated by precipitation of the reaction mixture into hexane, dissolved in toluene and reprecipitated into hexane again, and dried in a vacuum oven at room temperature for 24 h. 44

63 3.6 Characterization Nuclear magnetic resonance (NMR) characterization 1 H NMR spectra were measured on a Varian Mercury-300MHz spectrometer. Samples were dissolved in deuterated chloroform (CDCl3, 99.8%D, Cambridge Isotope laboratories) at concentrations of 15 mg/ml. The 1 H NMR spectra were referenced to the residual protons peak of CDCl3 at 7.27 ppm. The relaxation time was 5s Matrix-assisted laser desorption/ionization (MALDI) characterization MALDI-TOF mass spectra were acquired on a BrukerUltraflex-III TOF/TOF mass spectrometer (Bruker Daltronics, Billerica, MA) equipped with a Nd:YAG laser (at 335 nm). All spectra were measured in either positive or negative reflectron mode. The instrument was calibrated prior to each measuremen twith nonyl phenol ethoxylate sulfate as a negative external standard or poly(methyl methacrylate) as a positive external standard. DCTB served as the matrix and was dissolved in chloroform at a concentration of 20 mg/ml. PSSO 3 Hsamples were dissolved in chloroform at a concentration of 10 mg/ml. Either sodium-tfa or AgTFA served as a cationizing agent to aid in the ionization process while running in positive reflectron mode. The cationizing salt solutions were dissolved in methanol at a concentration of 10 mg/ml. Matrix, sample (or standard), and salt solutions were mixed in a ratio of 10:2:1 (v/v), respectively. MALDI- TOF samples were prepared by depositing 1-lL aliquots of the mixture in wells of a 384- well ground-steel target plate; after evaporation of the solvent, the plate was inserted into the MALDI source. The attenuation of the Nd:YAG laser was adjusted to minimize 45

64 unwanted polymer fragmentation and to maximize the sensitivity Size Exclusion chromatography (SEC) characterization The molecular weight and molecular weight distribution of the polymers were characterized by SEC using a Waters Breeze system with three Styragel columns at 35 C and are fractive index detector with THF or THF + 2 wt% TOA as the mobile phase. The molecular weight versus elution time was calibrated using low-molecular weight dispersity PS standards Differential scanning calorimetry (DSC) characterization DSC measurements were carried out using a Q200 DSC by TA Instruments. The DSC measurements were performed at10 C/min heating rate under a N 2 atmosphere. The samples were first heated to 150 C at 10 C/min and subsequently cooled to 0 C/min at 10 C/min (first heating/cooling). The Tg was obtained from a second heating cycle under the same conditions. Tg was assigned to the inflection point of the transition region in each heating trace Fourier Transform Infrared Spectroscopy (FTIR) characterization FTIR spectroscopy was performed using a Thermoscientific Nicolet 380 FTIR spectrometer. Samples were prepared by drop casting 20 mg/ml solutions of polymer in chloroform onto KBr plates. Spectra were obtained at a resolution of 4 cm -1 and 32 scans Acid-Base Titration characterization The degree of -SO 3 H functionality of the polymer was characterized by titration. 46

65 In a standard experiment, a solution of 0.5 g polymer in mixed solvents of toluene/methanol (v/v=9:1) was titrated with a 0.01 M NaOH solution in toluene/methanol (v/v=9:1) using phenolphthalein as an indicator. For comparison, the same samples were measured by excessive titration by 1 ml of 0.01 M NaOH, and reverse titration with 0.01 M benzoic acid in toluene/methanol 9/1 (v/v) after stirring for 1 min Sample preparation for morphology study Samples for morphology study were prepared by first dissolving a predetermined amount of block copolymer into toluene and then slowly evaporating the solvent. The evaporation of toluene was carried out initially in open air at room temperature for 3-5 days and then in a vacuum oven at 60 C for several days until there was no further change in weight. Finally, the samples were annealed in a vacuum oven at 130 C for 3 days Small angle x-ray scattering (SAXS) characterization The measurements were performed on a Rigaku small-angle X-ray scattering system. The instrument uses Cu Kα radiation (λ = 1.54 Å) and operates at a current of 0.88 ma and a potential difference of 45 kv. Measuring time for all samples was 30 min. Scattering patterns were averaged azimuthally to give the one-dimensional form of intensity I (arbitrary units) as a function of the magnitude of the scattering wave vector q =q=4πλ-1sin(θ/2), where λ and θ are the radiation wavelength (λ = nm) and scattering angle, respectively. 47

66 3.6.9 Measurement of dynamic viscoelastic properties An advanced rheometric expansion system (ARES, ARES-G2, TA Instruments) equipped with a force-rebalanced transducer was used in the oscillatory mode. The samples were prepared on the lower plate of the 8 mm diameter parallel plate geometry setup and were heated under a nitrogen atmosphere until flow. Subsequently, the upper plate was brought into contact, the gap thickness was adjusted to 1 mm and the sample was slowly cooled to the desired starting temperature. The elastic (G ) and loss (G ) moduli were monitored in different types of experiments. First, the linear and nonlinear viscoelastic ranges were identified. In all subsequent experiments strain amplitudes within the linear viscoelastic range were used (typically below 10%). These experiments involved: (1) Isothermal frequency sweeps with a ω ranging from 100 to 0.1 rad/s. The temperature varied from 40 to 220 C depending on the samples. The same sample was measured at several temperatures. (2) Temperature sweeps at a constant frequency (5 rad/s) and a temperature increase of 5 C/min. In order to compare the results, the same frequency and temperature rate have been used for all samples. The temperature was varied from 40 to 220 C depending on the sample. The samples were only used for one measurement. 48

67 CHAPTER IV SYNTHESIS OF Ω-SULFONATED POLYSTYRENE VIA REVERSIBLE ADDITION FRAGMENTATION CHAIN TRANSFER POLYMERIZATION AND POSTPOLYMERIZATION MODIFICATION[121] 4.1 Introduction Polymers terminated with a sulfonate end group (ω-sulfonated polymers) are useful as model ionomers, modifiers for thin films and surfaces, and supramolecular building blocks for linear and graft block copolymers [88, ]. A widely used method to produce these polymers is through anionic polymerization by termination with propane sultone. [129] While this approach generates well-defined polymers with quantitative yields of functional end-groups, it has limits to the range of polymers that can be produced due to the high purity reaction conditions and chemical requirements of the polymerization technique. [130] The advent of controlled radical polymerization techniques has provided new routes for the synthesis of well-defined polymer systems. For example, nitroxidemediated radical polymerization, atom transfer radical polymerization, and reversible 49

68 addition fragmentation chain transfer (RAFT) polymerization have been used to synthesize polymers containing sulfonate groups. [ ] These provide a range of options for producing sulfonated polymers when considered in concert with other approaches, such as postpolymerization modification. [136, 137] RAFT polymerization has also proven useful for producing polymers with welldefined functional groups. [138] The key to the polymerization is the addition of a dithioester RAFT agent, which is able to undergo reversible chain transfer reactions with the growing polymer chain to achieve controlled polymerization conditions. As a consequence of the chain transfer reactions, the dithioester group is incorporated into the polymer chain. These polymers are converted to thiol-terminated polymers by treatment with primary amines. [139, 140] These thiol-terminated polymers can be further functionalized or used as building blocks, for example, through thiol-ene chemistry or disulfide formation. [ ] This chapter reports the synthesis of ω-sulfonated polystyrene (PS-SO 3 H) through two methods. The first is the synthesis of PS by RAFT polymerization, aminolysis of the subsequent polymer to produce thiol-terminated PS, and subsequent oxidation of the thiol-terminated polymer with m-chloroperoxybenzoicacid (m-cpba). The oxidation reaction of thiols is commonly used in small molecule organic synthesis and has been used to introduce sulfonate groups along polymer backbones. [ ] The second method is the direct oxidation of the RAFT polymerization-prepared PS with m-cpba. This route was based on a previous observation by Zagorevskii et al. [152] where ω- sulfonated PS was detected by matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization mass spectrometry in negative mode after the PS samples 50

69 prepared by RAFT polymerization were stored in tetrahydrofuran (THF). This was attributed to the formation of peroxides in the unstabilized THF and oxidation of the trithiocarbonate end-group to produce a sulfonate end-group. Longer storage in THF was reported to produce further chemical transformations. The oxidation of the thiolterminated PS was found to produce more side products than the direct oxidation of the PS prepared by RAFT polymerization. Purification of the crude PS-SO 3 H from both methods by column chromatography was found to produce nearly quantitatively functionalized polymers (>95%) as measured by end-group titration. Capping of these polymers with tertiary amines, quaternary ammonium, and an imidazoliumionic liquid is demonstrated through acid base neutralization and ionic metathesis reactions. The influence of different ammonium sulfonates on the physical properties of the polymers is demonstrated by differential scanning calorimetry (DSC) measurements. RAFT polymerization is a straightforward method to produce ω-sulfonated polymers. These polymers with ionic end-group can be used as model supramolecular building blocks, which will be discussed in the following chapters. 4.2 Result and discussion Synthesis of ω-sulfonated PS (PS-SO 3 H) from thiol-terminated PS The scheme for the synthesis of ω-sulfonated PS (PS-SO 3 H) from thiolterminated PS is shown in Scheme 4.1. Briefly, a PS polymer prepared by RAFT polymerization with the RAFT agent dibenzyltrithiocarbonate (PS-RAFT-PS) was treated with n-butylamine to prepare thiol terminated PS (PS-SH). 51

70 Scheme 4.1 Synthesis of sulfonic acid-terminated PS (PS-SO3H) The PS-SH was then treated with m-cpba to produce the sulfonic acidterminated PS (PS-SO 3 H). Different molar equivalents of m-cpba to the thiol end-group and the reaction time were investigated to optimize the reaction conversion. The concentration of -SO 3 H groups in the crude polymer was characterized by titration and reported as themol-so 3 H/g polymer in Figure 4.1. As shown in Figure 4.1, the reactions began to level off after 1 h and no gain in conversion was found from m-cpba:ps-sh molar ratios above 10:1. 52

71 Figure 4.1 Amount of -SO3H groups determined by titration versus time (m-cpba:ps- SH molar ratios: - -1:1, - - 1:3, - - 1:6,- - 1: :15) Figure 4.2 shows the size exclusion chromatography (SEC) traces of the PS- RAFT-PS, PS-SH in THF and PS-SO 3 H in THF and THF +2 wt % trioctylamine (TOA). TOA has previously been used as an additive to limit interaction between the -SO 3 H groups and the column. [133] Table 4.1 lists the number average molecular weight, M n, and molecular weight dispersity, Ð, values calculated based on PS standards for each polymer. 53

72 Figure 4. 2 SEC traces of (a) PS-RAFT-PS, (b) PS-SH, (c, d) PSSO 3 H. Traces (a c) were eluted with THF. Trace (d) was eluted with THF + 2 wt % The molecular weight is halved from PS-RAFT-PS to PS-SH due to formation of two thiol-terminated chains from the parent trithiocarbonate-containing polymer. However, little change in the molecular weight dispersity is observed either during the aminolysis or sulfonation. Oxidative coupling of the thiol end-groups to produce disulfides was not observed under these reaction conditions, although this side reaction has been previously observed. [140, 153]The absence of disulfide formation is attributed to the short reaction time (1 h) and anaerobic reaction conditions. If the same reaction was run without nitrogen sparging, a broadened peak with tailing at lower elution volume 54

73 (higher molecular weight) indicative of coupling was observed by SEC as shown in Figure 4.3. When the aminolysis was run for 24 h with or without nitrogen sparging, an elution peak shifted to lower elution volume was observed by SEC indicative of formation of a significant amount of coupled polymer. Table 4.1 Characteristics of the PS-RAFT-PS, PS-SH, and PSSO 3 H Polymers Measured by SEC Polymer M n (kda) Ð PS-RAFT-PS a PS-SH a PS-SO 3 H a PS-SO 3 H b a With THF as the mobile phase. b With THF + 2 wt % TOA as the mobile phase. 55

74 Figure 4.3 SEC traces of PS-SH: (a) 1 h, under nitrogen, (b) 24 h, under nitrogen, (c) 1 h, under air, (d) 24 h, under nitrogen Based on the SEC-determined molecular weight of the crude PS-SO 3 H (4 kda), an -SO 3 H end-group concentration of 0.25mmol/g polymer would be expected. This value is greater than what was observed by titration indicating that there is unfunctionalized polymer and/or reaction side products in the crude PS-SO 3 H. Running the reaction at room temperature using a 10:1 mole ratio of m-cpba to PS-SH resulted in a higher -SO 3 H end-group concentration of mmol/g polymer. Annealing a crude PS-SO3H sample (0 C, 6 h 15:1m-CPBA:PS-SH, mmol/g -SO 3 H, g) under vacuum overnight caused noticeable yellowing of the polymer, a reduction in the sample mass to g, and an -SO 3 H end-group concentration of mmol/g polymer was determined by titration. This result indicates that some volatile impurities were present in the crude PS-SO 3 H. These impurities in the crude PS-SO 3 H were difficult to identify. 56

75 Figure 4.4 is the 1 HNMR spectroscopy of the PS-RAFT-PS, PS-SH, and PS-SO 3 H and showed little difference.. Figure H-NMR spectra of (a) PS-RAFT-PS, (b) PS-SH, (c) crude PS-SO 3 H derived from PS-SH, (d) PS-SO 3 H cyclohexane fraction derived from PS-SH, (e) PS-SO 3 H acetone:methanol fraction derived from PS-SH The crude PS-SO 3 H was purified by column chromatography using silica gel and eluted with cyclohexane followed by acetone:methanol (4:1 volume ratio). The polymer 57

76 eluted with each solvent was analyzed by thin layer chromatography (TLC) using chloroform as the mobile phase. The cyclohexane-eluted fraction exhibited two spots at R f values of 0 and 1. The PS-SO 3 H was assigned the R f =0 spot as the acidic -SO 3 H group should slow its elution through the column. The R f =1 spot was assigned to the unsulfonated polymer and reaction byproducts. The acetone:methanol eluted fraction exhibited one peak at R f =0 in CHCl 3, which corresponds to the PS-SO 3 H. After purification, an -SO 3 H end-group concentration of mmol/g polymer was determined by titration, which corresponds to an end-group functionality of 96% based on a PS-SO 3 H molecular weight of 4 kda. The MALDI time-of-flight (MALDI-TOF) mass spectra of the cyclohexaneeluted and acetone:methanol-eluted PS-SO 3 H fractions are shown in Figure 2.5. These spectra were obtained using a matrix of trans-2-[3-(4-tert-butylphenyl)-2-methyl-2- propenylidene]malononitrile (DCTB) and silver trifluoroacetate(agtfa) as the salt. Scheme 4.2 displays the structures corresponding to the peaks labeled S, V, and M in Figure 4.5. Scheme 4. 2 Structure of PS with sulfonic acid (S), vinyl (V), and methylene (M) endgroups 58

77 Figure 4.5 MALDI-TOF spectra of (a) PS-SO 3 H cyclohexane fraction and (b) PS-SO 3 H acetone:methanol fraction. The insets display an expanded view of the spectrum. S, M, and V refer to the structures in Scheme 4.2 and Table 4.2 The floating spectrum in the inset of (a) is of the low intensity peaks and the floating spectrum in the inset of (b) is the calculated spectrum for the PS-SO 3 AgAg + (peak S). 59

78 Table 4.2 lists the calculated mass for each structure and observed mass for each peak of the spectra with the Ag + cation mass subtracted. The most intense peak corresponds to the PS-SO 3 Ag. The calculated peak is shown in the inset of Figure 4.5(b) above the spectrum. A similar peak is observed in the MALDI-TOF spectra using Na as the cation or in negative mode. For the PSSO 3 H, both the Ag + and Na + cations ion exchange with theso 3 H group and add an additional cation. The peaks labeled V and M in Figure 4.5(a) correspond to the PS with a vinyl or methylene end-group. These peaks have previously been observed in the MALDI-TOF spectrum of free-radically polymerized PS and result from the cleavage of the PS chain during the MALDI-TOF measurement. [154] Table 4.2 Calculated and Observed Masses for MALDI-TOF Spectra of PS-SO 3 H from the Oxidation of PS-SH Observed m/z a Peak Formula Calculated m/z Cyclohexane Fraction Acetone:Methanol Fraction S C 7 H 7 -(C 8 H 8 ) n -SO 3 H n n n n b V C 7 H 7 -(C 8 H 8 ) n -C 8 H n n N/A M C 7 H 7 -(C 8 H 8 ) n -C 9 H n n N/A a The mass of the Ag cation ( Da) has been subtracted. b Calculated mass for PS-SO 3 Ag 60

79 As these peaks are only seen using silver cations, one impurity in the PS-SO 3 H is assigned to neutral PS without any PS-SO 3 H groups. This polymer is most likely produced by irreversible termination reactions during the RAFT polymerization by combination and/or disproportionation of two growing PS chains. [155] Two other peaks at higher m/z were observed with periodicity corresponding to a PS repeat unit but could not be identified based on the probable end-group structures of PS. By comparison of Figure 4.5(a) and (b) it is clear that these non-psso 3 H polymer impurities are largely removed by the column purification as they are almost indistinguishable from the baseline intensity in the purified PS-SO 3 H (acetone methanol fraction) ω-sulfonated Polystyrene (PS-SO 3 H) (from PS-RAFT-PS) In the second method of oxidation, the PS-RAFT-PS was directly treated with m- CPBA to produce the PS-SO 3 H. Titration of the crude PS-SO 3 H indicated an -SO 3 H concentration of mmol/g polymer. Annealing the sample under vacuum produced no yellowing or measurable weight change. Assuming no small molecule byproducts this corresponds to 96% PS-SO 3 H functionality. SEC characterization of the PSSO 3 H from direct oxidation showed significant broadening when eluted in pure THF [Fig. 4.6(a)]. This is attributed to interaction of the -SO 3 H groups with the SEC column. When eluted with THF t 2 wt % TOA, a monomodal peak was observed with an M n =3.7 kda and Ð=1.20 [Fig. 4.6(b)]. The crude PS-SO 3 H was also purified by the same method as used for the PS- SO 3 H prepared from the PS-SH. Only spots corresponding to the PS-SO 3 H were observed in the TLC measurements. After purification, a PS-SO 3 H end-group 61

80 concentration of mmol/g polymer was measured by titration corresponding to 97% PS-SO 3 H functionality. The MALDI-TOF mass spectra are shown in Figure 4.7. The same peak sequences are observed as in Figure 4.5. The calculated and observed masses for these two spectra are listed in Table 4.3. Trace non-ps-so 3 H byproducts are evident in the purified polymer in Figure 4.7(b) similar to the purified PS-SO 3 H derived from PS- SH in Figure 4.5(b). MALDI-TOF spectra of the PS-SO 3 H derived from the oxidation of PS-SH and PS-RAFT-PS in negative mode and positive mode with sodium trifluoroacetate (NaTFA) are shown in Figures Figure The calculated peak shape is shown in the inset for the PS-SO3 - or PS-SO3Na +Na+. The calculated and observed masses for the PS-SO - 3 and PS-SO3Na +Na+ are listed in Table 4.4. Negative mode resulted in a larger error between the calculated and observed mass. This is attributed to the low molecular weight of the standard (NPES) for the negative mode measurements making calibration more difficult. 62

81 Figure 4.6 SEC traces of PS-SO 3 H obtained from direct oxidation of PS-RAFT-PS; (a) eluted in THF, (b) eluted in THF + 2 wt% TOA 63

82 Figure 4.7 MALDI-TOF spectra of (a) crude PS-SO3H and (b) PS-SO3H acetone:methanol fraction. The insets display an expanded view of the spectrum. S, M, and V refer to the structures in Scheme 4.2 and Table 4.3. The floating spectrum in the inset of (a) is of the low intensity peaks and the floating spectrum in the inset of (b) is the calculated spectrum for the PS-SO 3 AgAg + (peak S). 64

83 Table 4.3 Calculated and Observed Masses for MALDI-TOF Spectra of PS-SO 3 H from Direct Oxidation of PS-RAFT-PS Observed m/z a Peak Formula Calculated m/z Cyclohexane Fraction Acetone:Methanol Fraction S C 7 H 7 -(C 8 H 8 ) n -SO 3 H n n n n b V C 7 H 7 -(C 8 H 8 ) n -C 8 H n n N/A M C 7 H 7 -(C 8 H 8 ) n -C 9 H n n N/A a The mass of the Ag cation ( Da) has been subtracted. b Calculated mass for PS-SO 3 Ag 65

84 Table 4.4 Calculated and Observed Peaks for MALDI-TOF Spectra form negative mode and positive mode with sodium trifluoroacetate (NaTFA) Spectrum Mode Formula Calculated m/z Observed m/z PS-SO 3 H (PS-SH) (cyclohexane PS-SO 3 H (PS-SH) (cyclohexane PS-SO 3 H (PS-SH) (acetone:methanol PS-SO 3 H (PS-SH) (acetone:methanol fraction) PS-SO 3 H (PS-RAFT- PS) (crude polymer) PS-SO 3 H (PS-RAFT- PS) (crude polymer) PS-SO 3 H (PS-RAFT- PS) (purified) PS-SO 3 H (PS-RAFT- PS) (purified) Negative C 7 H 7 -(C 8 H 8 ) n -SO 3 H n n a Positive, C 7 H 7 -(C 8 H 8 ) n -SO 3 H n n d Na n b Negative C 7 H 7 -(C 8 H 8 ) n -SO 3 H n n a Positive, Na + C 7 H 7 -(C 8 H 8 ) n -SO 3 H n n b n c n d n d Negative C 7 H 7 -(C 8 H 8 ) n -SO 3 H n n a Positive, C 7 H 7 -(C 8 H 8 ) n -SO 3 H n n d Na n b Negative C 7 H 7 -(C 8 H 8 ) n -SO 3 H n n a Positive, Na + C 7 H 7 -(C 8 H 8 ) n -SO 3 H n n a n b n d n d a H + added before calculation b calculated for PS-SO 3Na c calculated for PS-SO 3K d Na + subtracted before calculation 66

85 Figure 4.8 PS-SO3H (cyclohexane fraction) derived from PS-SH. Negative Mode 67

86 Figure 4.9 PS-SO 3 H (cyclohexane fraction) derived from PS-SH. Positive Mode, Na + 68

87 Figure 4.10 PS-SO 3 H (acetone:methanol fraction) derived from PS-SH. Negative Mode 69

88 Figure 4.11 PS-SO 3 H (acetone:methanol fraction) derived from PS-SH. Positive Mode, Na + 70

89 Figure 4.12 PS-SO3H (crude) derived from PS-RAFT-PS. Negative Mode 71

90 Figure 4.13 PS-SO3H (crude) derived from PS-RAFT-PS. Positive Mode, Na + 72

91 Figure 4.14 PS-SO3H (purified) derived from PS-RAFT-PS. Negative Mode 73

92 Figure 4.15 PS-SO3H (purified) derived from PS-RAFT-PS. Positive Mode, Na + 74

93 4.2.3 Ion-Exchange of the -SO 3 H Terminated Polymers The sulfonic acid end-group of the PS-SO 3 H was converted to other sulfonate forms through either acid base neutralization or ionic metathesis reactions as shown in Scheme 4.3. These reaction schemes are based on similar approaches that have been used in the synthesis of small molecule ionic liquids, polyelectrolyte complexes, and anionically polymerized ω-sulfonated PS liquid crystalline ionomers. [69, 123, 133, 156, 157] For the addition of tertiary amines, they could be mixed directly with the PS- SO 3 H. For the addition of quaternary ammonium or imidazolium groups, the PS-SO 3 H was first converted to the sodium sulfonate (PS-SO 3 Na) or ammonium sulfonate (PS- SO 3 NH 4 ) form, respectively. Solvent conditions were chosen to drive the reaction by the precipitation of either the NaCl or NH 4 Cl salt. Figure 4.16 is the FTIR analysis of the PS- SO 3 H and PSSO 3 Na prepared by oxidation of PS-SH shows a shoulder appeared at about 1042 cm -1 for O=S=O stretching in the sulfonated polymers consistent with what has previously been observed in other sulfonated polymers. [150, 158] Scheme 4.3 Ion-exchange reactions with tertiary amine, quaternary ammonium, and imidazolium compounds. 75

94 Figure 4.16 FTIR spectra of (a) PS-RAFT-PS, (b) PS-SH, (c) PS-SO3H and (d) PS- SO3Na. The sulfonated polymers were prepared by oxidation of PS-SH. The ion-exchanged polymers were characterized by 1 H NMR spectroscopy. Figure 4.17 shows the NMR spectrum of the original PS-SO 3 H and the ion-exchanged PS-SO 3 X polymers. 76

95 Figure 4.17 NMR spectra of the ion-exchanged PS-SO 3 X polymers Thermal Properties of PS-SO 3 X Polymers The effect of the counter ion on the sulfonate end-group on the glass transition temperature (T g ) of the polymer was characterized by DSC. Table 4.5 lists the T g values for a series of polymers and the individual DSC traces are shown in Figure