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Kinetic assembly of block copolymers in solution helical cylindrical micelles and patchy nanoparticles

Dissertation
Author: Sheng Zhong
Abstract:
There is always an interest to understand how molecules behave under different conditions. One application of this knowledge is to self-assemble molecules into increasingly complex structures in a simple fashion. Self-assembly of amphiphilic block copolymer in solution has produced a large variety of nanostructures through the manipulation in polymer chemistry, assembly environment, and additives. Moreover, some reports suggest the formation of many polymeric assemblies is driven by kinetic process. The goal of this dissertation is to study the influence of kinetics on the assembly of block copolymer. The study shows kinetic control can be a very effective way to make novel polymeric nanostructures. Two examples discussed here are helical cylindrical micelles and patchy nanoparticles. Helical cylindrical micelles are made from the co-assembly of amphiphilic triblock copolymer poly(acrylic acid)-block -poly(methyl acrylate)- block -polystyrene and organoamine molecules in a mixture of tetrahydrofuran (THF) and water (H2 O). This system has already shown promise of achieving many assembled structures. The unique aspects about this system are the use of amine molecules to complex with acid groups and the existence of cosolvent system. Application of amine molecules offers a convenient control over assembled morphology and the introduction of PMA-PS selective solvent, THF, promotes the mobility of the polymer chains. In this study, multivalent organoamine molecules, such as diethylenetriamine and triethylenetetramine, are used to interact with block copolymer in THF/water mixture. As expected, the assembled morphologies are dependent on the polymer architecture, selection and quantity of the organoamine molecules, and solution composition. Under the right conditions, unprecedented, multimicrometer-long, supramolecular helical cylindrical micelles are formed. Both single-stranded and double-stranded helices are found in the same system. These helical structures share uniform structural parameters, including the width of the micelles, width of the helix, and the pitch distance. There is no preference to the handedness, and both handednesses are observed, which is understandable since there are no chiral molecules or specific binding effects applied during the assembly. The helical structure is a product of kinetic process. Cryogenic transmission electron microscopy has been employed to monitor the morphological transformation. The study indicates there are two complicated but reproducible kinetic pathways to form the helices. One pathway involves the stacks of bended cylinders at early stages and the subsequent interconnection of these bended cylinders. Spherical micelles bud off of the interconnected nanostructure as the final step towards a defect-free helix. Another kinetic pathway shows very short helices are formed at first and aligned via head-to-tail style in the solution, and the subsequent sequential addition of these short helices results in prolonged mature helices. By using a ninhydrin-staining technique, amine molecules within the micellar corona are visualized and confirmed to preferentially locate in the inner side of the helical turns. The aggregation of amine molecules provides a strong attraction force due to electrostatic association between oppositely charged amine and acid groups and accumulation of hydrogen bonding among amine molecules to coil the cylindrical micelles into helical twisting features which are stabilized by the repulsion forces due to the chain packing frustration within the hydrophobic core, steric hindrance of amine molecules as well as the Coulomb repulsion of the excess charged amine groups. The formation mechanism of the helix offers the feasibility to manipulate the helical pitch distance and formation kinetics. The helical pitch distance can be enlarged or shrunk by varying the type and amount of amine molecules used in assembly, introducing inorganic salts, and changing pH. Luckily, the helical structure can be preserved permanently by inducing the amide reaction between amine and carboxylic acid groups. The kinetics of the helix is also subject to many factors, including used amine molecules, inorganic salts and preparation procedure. The aging time for the helix can be either reduced or prolonged. Furthermore, even though the helical formation is pathway-dependent, helical formation can still be triggered from extended cylindrical micelles or stacks of disklike micelles as long as a right condition is applied. Another strategy for kinetic assembly of block copolymer is presented as well. A novel patchy nanoparticle has been produced following this strategy. The patches are formed on the surface of polymeric colloids due to the phase separation of two chemically unlike segments. Certain level of mobility of the polymer chains is required for the blocks to segregate into patches. More importantly, the number and distribution geometry of the patches are related to the particle size. Future efforts are needed to control the particle size in order to manufacture uniform nanoparticles with desired patch patterns for the applications in nanotechnology, drug delivery and nanodevices.

TABLE OF CONTENTS LIST OF TABLES ..................................................................................................... x LIST OF FIGURES ................................................................................................... xi ABSTRACT............................................................................................................ xix

Chapters

1 Introduction .................................................................................................... 1

1.1 Motivation ............................................................................................. 1 1.2 Background ............................................................................................ 2

1.2.1 Polymer Synthesis: ATRP and RAFT ......................................... 6 1.2.2 Block Copolymer in Bulk ......................................................... 10 1.2.3 Block Copolymer in Solution ................................................... 13

1.2.3.1 Sample Preparation for Micelle Solution .................... 13 1.2.3.2 Micellar Morphology ................................................. 14 1.2.3.3 Self-Assembly of Nonionic Polymer .......................... 16 1.2.3.4 Self-Assembly of Ionic Polymer ................................. 19 1.2.3.5 Previous Work on Self-Assembly of PAA-b- PMA-b-PS ................................................................. 21

1.3 Overview ............................................................................................. 22 1.4 References ........................................................................................... 23

2 Materials and Characterization ...................................................................... 31

2.1 Introduction ......................................................................................... 31 2.2 Synthesis of Triblock Copolymers........................................................ 31 2.3 Preparation of Sample Solution ............................................................ 35 2.4 Transmission Electron Microscopy (TEM) ........................................... 35 2.5 Cryogenic Transmission Electron Microscopy (cryo-TEM) .................. 37 2.6 Fourier Transform Infrared Spectroscopy (FTIR) ................................. 38 2.7 Dynamic Light Scattering (DLS) .......................................................... 38 2.8 Atomic Force Microscopy (AFM) ........................................................ 39

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2.9 Small Angle Neutron Scattering (SANS) .............................................. 39 2.10 Small Angle X-ray Scattering (SAXS) ................................................. 40 2.11 References ........................................................................................... 40

3 Formation of Helical Cylindrical Micelles via Self-Assembly of Triblock Copolymers in Solution .................................................................. 42

3.1 Introduction ......................................................................................... 42 3.2 Effect of the Amount of TETR2 ........................................................... 45 3.3 Effect of the Type of Organoamine Molecules ..................................... 48 3.4 Effect of the Solution Composition ...................................................... 51 3.5 Effect of Block Length of PS ............................................................... 55 3.6 Cryo-TEM and AFM Study on Helical Cylindrical Micelles ................ 58 3.7 Conclusion ........................................................................................... 61 3.8 References ........................................................................................... 61

4 Kinetic Study for Helical Formation ............................................................. 66

4.1 Introduction ......................................................................................... 66 4.2 Kinetic Pathway One ........................................................................... 71 4.3 Kinetic Pathway Two ........................................................................... 79 4.4 Stability of Helix in Solution ................................................................ 82 4.5 Defects Formed During the Kinetics .................................................... 84 4.6 Discussion............................................................................................ 86

4.6.1 Balance of Forces in Helical Structures .................................... 86 4.6.2 Assembly Conditions for Helix ................................................. 87 4.6.3 Molecular Diffusion During Aging ........................................... 89

4.7 Conclusion ........................................................................................... 90 4.8 References ........................................................................................... 91

5 Manipulation of Helical Structure and Its Kinetics ........................................ 94

5.1 Introduction ......................................................................................... 94 5.2 Manipulation of Helical Conformation ................................................. 97

5.2.1 Effects of Organoamines on Helix ............................................ 97 5.2.2 Effects of Salts on Helix ..........................................................100 5.2.3 Fixation of Helical Cylindrical Micelles ..................................105

5.3 Manipulation in Helical Formation ......................................................108

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5.3.1 Effects of Salts on Helical Formation ...................................... 108 5.3.2 Effects of Fast Water Addition on Helical Formation ..............112 5.3.3 Effects of Solution Composition Shift on Helical Formation ................................................................................115

5.4 Helix Induced from Other Morphologies .............................................118

5.4.1 Stacking Disks to Helix ...........................................................118 5.4.2 Cylindrical Micelles to Helix ...................................................121

5.5 Conclusion ..........................................................................................123 5.6 References ..........................................................................................124

6 polymeric patchy nanopartilces via kinetic assembly ...................................127

6.1 Introduction ........................................................................................127 6.2 Sample Preparation and Characterization ............................................130

6.2.1 Sample Preparation..................................................................130 6.2.2 Transmission Electron Microscopy (TEM) ..............................130 6.2.3 Dynamic Light Scattering (DLS) .............................................131

6.3 Experimental and Results ....................................................................132

6.3.1 Examination of Patches ...........................................................132 6.3.2 Solution Composition on Patchy Formation .............................134 6.3.3 DLS study on Polymer Aggregation ........................................138 6.3.4 Kinetics for Patchy Formation .................................................139

6.4 Discussion...........................................................................................144 6.5 Conclusion ..........................................................................................150 6.6 References ..........................................................................................151

7 Conclusion, Future Work and Collaborations ...............................................154

7.1 Conclusion ..........................................................................................154 7.2 Future Work ........................................................................................157

7.2.1 Mechanical Property of Helix ..................................................157 7.2.2 Size Control of Patchy Nanoparticles.......................................159

7.3 Collaborations .....................................................................................160

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7.3.1 Enzymatically Triggered Micellization .................................... 160 7.3.2 Biodegradable Diblock Copolymers ........................................163 7.3.3 Skin-Penetration for Liposomes ...............................................167 7.3.4 PEG-Peptide Conjugations ......................................................169

7.4 References ..........................................................................................172

Appendix

A FTIR study ..................................................................................................173

A.1 Introduction ........................................................................................173 A.2 Charged Status for PAA in Solution ....................................................173 A.3 Effect of Amines on Acid ....................................................................176 A.4 FTIR Spectrum for TETR2 .................................................................179 A.5 Discussion...........................................................................................182 A.6 References ..........................................................................................182

B Reprint Permission ......................................................................................183

B.1 Reprint Permission for Figure 1.1 ........................................................183 B.2 Reprint Permission for Figure 1.2 ........................................................184 B.3 Reprint Permission for Figure 1.3 ........................................................185 B.4 Reprint Permission for Figure 1.4 ........................................................186

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LIST OF TABLES Table 2.1 Block copolymers used in the study of helical cylindrical micelles. ...... 32 Table 2.2 Chemical structures of organoamine molecules .................................... 33 Table 6.1 Block copolymers used in the study of patchy nanoparticles................131

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LIST OF FIGURES Figure 1.1 Schematic representation of diversity vs complexity in nature and synthetic polymers. 12 ............................................................................. 5 Figure 1.2 Polymer architecture available through controlled/―living‖ polymerization. 22 .................................................................................. 7 Figure 1.3 Phase diagram for linear AB diblock copolymers, comparing theory and experiment. (a) Self-consistent mean field theory predicts four equilibrium morphologies: spherical (S), cylindrical (C), gyroid (G) and lamellar (L). (b) Experimental phase portrait for poly(isoprene-styrene) diblock copolymers. The resemblance to the theoretical diagram is remarkable. 41 ........................................... 11 Figure 1.4 Different geometries formed by block copolymers in selective solvent conditions. 1 ............................................................................. 15 Figure 2.1 Synthesis procedure of PAA-b-PMA-b-PS .......................................... 34 Figure 2.2 Molecular structure of PAA-b-PMA-b-PS ........................................... 34 Figure 2.3 Reaction of ninhydrin with ammonia. The benzene rings increase the in situ electron density, leading to the darker contrast under TEM than the place where has less amine groups. ............................... 37 Figure 3.1 Conventional TEM images for AMS88 with TETR2 at different molar ratio of amine : acid = (A) 1 : 1, nanoparticles with internal stacking substructures; (B) 5 : 1, stacking disks; (C) 10 : 1, helix; (D) 15 : 1, single- and double-stranded helices. H 2 O volume in THF was 67% in THF and after 20-day aging. The contrast of polymeric structures was enhanced by uranyl acetate staining. ............ 47

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Figure 3.2 Conventional TEM images for AMS88 with different amine molecules at molar ratio of amine : acid = 10 : 1. (A) amine = DI2, spherical micelles; (B) amine = TRI2, helix; (C) amine = TETR2, helix; (D) amine = HEXA2, stacking disks and toroids (arrow point at). H 2 O volume in THF was 67% and after 20-day aging. The contrast of polymeric structures was enhanced by uranyl acetate staining. .................................................................................. 50 Figure 3.3 Conventional TEM images for AMS88 with amine molecules with the same functionality but different amine spacers. (A) amine = TRI3 which has three methyl groups between two amine groups, short cylindrical micelles; (B) amine = TRI2 which has two methyl groups between two amine groups, helix. Molar ratio of amine : acid = 10 : 1. H 2 O volume in THF was 67% and after 20- day aging. The contrast of polymeric structures was enhanced by uranyl acetate staining. ........................................................................ 51 Figure 3.4 Conventional TEM images for AMS88 at different solution compositions. H 2 O volume in THF was (A) 40%, stacking disks; (B) 50%, stacking cylinders; (C) 67%, helix; (D) 80%, unwounded cylinders and spherical micelles. Amine = TETR2, molar ratio of amine : acid = 10 : 1, and after 20-day aging. The contrast of polymeric structures was enhanced by uranyl acetate staining. .............................................................................................. 54 Figure 3.5 Conventional TEM images for triblock copolymers with different PS block length. (A) PS = 28, bundle aggregates; (B) PS = 44, stacking toroids; (C) PS = 88, helix; (D) PS = 101, bulky aggregates. Amine = TETR2, molar ratio of amine : acid = 10 : 1, H 2 O volume in THF = 67%, and after 20 day aging. The contrast of polymeric structures was enhanced by uranyl acetate staining. ........ 57 Figure 3.6 Cryo-TEM images of (A) single- and (B) double-stranded helices in solution. (C) Schematic of helical cylindrical micelle. .................... 59 Figure 3.7 AFM images for helical cylindrical micelles. We found both left- (B) and right (C) handed helices in the same system............................ 60 Figure 4.1 Cryo-TEM images of assembled triblock copolymer PAA-b- PMA-b-PS with TETR2, and amine : acid molar ratio = 10 : 1 in the mixture of 67% volume H 2 O in THF after aging for (A) 0 day, showing stacks of bended short cylindrical micelles; and (B) 20 days, showing mature helical cylindrical micelles. .............................. 68

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Figure 4.2 Representative cryo-TEM images of micellar structures at different transformation stages of kinetic pathway one: (A) starting with stacks of short bended cylindrical micelles; (B) interconnecting short cylinders; (C) perfecting helical strands; (D) mature helical cylindrical micelles. ..................................................... 72 Figure 4.3 Convent ional TEM images for t he ninhydr in- st ained nanostructures at (A) 1-day aging, stacks of branched cylinders were the dominant morphology and (B) 20-day aging, single- stranded (arrow 1) and double-stranded (arrow 2) helices were presented. The image contrast came from the concentration of in situ ninhydrin moieties after specific reaction with organoamine molecules. The dark contrast is proportional to the aggregation of amine molecules, indicating the segregation of amine molecules along within the helical micelle corona. The assembly system was AMS88 with TETR2 at molar ratio of amine : acid = 10 : 1 in the solution of 67% volume H 2 O in THF............................................. 76 Figure 4.4 Schematic demonstration of preferential aggregation of amino molecules around the cylindrical micelles and the consequent helical structure. .................................................................................. 77 Figure 4.5 Representative cryo-TEM images of micellar structures at different transformation stages of kinetic pathway two: (A) starting with stacks of short bended cylindrical micelles; (B) forming short helical micelles; (C) interconnecting short helical micelles; (D) mature single-stranded helix. ......................................... 81 Figure 4.6 Representative cryo-TEM images of helical structures during prolonged aging in solution. (A) Helix lost coiling feature at end; (B) helical coils were loosened; (C) helix and extended cylindrical micelles; (D) extended cylindrical micelles without helical features. .............................................................................................. 83 Figure 4.7 Representative TEM images of defects formed during helical formation kinetics: (A) remnant of bended cylindrical micelles; (B) spheres attached to helix; (C) toroids formed along with helix; (D) disklike micelles formed along with helix. .................................... 85

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Figure 5.1 Cryo-TEM images of helical cylindrical micelles using different amines: (A) amine = TETR2, amine : acid molar ratio = 10 : 1; (B) amine = TETR2, amine : acid molar ratio =10 : 1, high mage images, pitch distance = 45 nm; (C) amine = TRI2, amine : acid molar ratio = 10 : 1, pitch distance = 64 nm; (D) amine = TETR2, amine : acid molar ratio = 15 : 1, pitch distance = 58 nm. .................... 99 Figure 5.2 Cryo-TEM images of (A) helical cylindrical micelles, amine = TETR2 at amine : acid molar ratio = 10 : 1; (B) helical structure after adding NaCl = 0.1 M; (C) SAXS measurement of free helix, helix with 0.1 M NaCl, and helix in acidic solution. The peaks are due to scattering from regular helical conformation. The positions for peaks moved when solution condition was changed. ......102 Figure 5.3 Helical micelles (A) aging 7 days after adding MA at amine : acid = 1 : 1; (B) aging 3 months after adding monoamines at amine : acid = 1 : 1; (C) aging 7 days after adding monoamines at amine : acid = 5 : 1; (B) aging 3 months after adding monoamines at amine : acid = 5 : 1.............................................................................104 Figure 5.4 TEM images for helical micelles were turning into spherical micelles when (A) aging 7 days after adding MA at molar ratio amine : acid = 1 : 1 and (B) aging 7 days after adding MA at molar ratio amine : acid = 5 : 1. ..........................................................105 Figure 5.5 Cryo-TEM images for (A) free helical cylindrical micelles made from TETR2 at amine : acid molar ratio = 10 : 1 in the mixture of 67% volume H 2 O in THF; (B) and (C) separate fixations of helix by adding DPEM; (D) SAXS measurement for free helix and fixed helices in the their original solution. ..........................................107 Figure 5.6 TEM images for micelles when MA was added. of amine : acid molar ratio = 1 : 1 after aging (A) 9 days, (B) 15 days, and (C) 3 months; MA at amine : acid molar ratio = 5 : 1 after aging (A) 9 days, (B) 15 days, and (C) 3 months. It also had TETR2 of amine : acid molar ratio = 10 : 1 in the mixture of 67% volume H 2 O in THF. ......................................................................................111

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Figure 5.7 TEM images for assembled micelles when (A) added MgSO 4 = 0.05 M aging for 1 day; (B) added NaSO 4 = 0.05 M aging for 10 days; and (C) added NaCl = 0.05 M aging for 10 days. Original micellar solution was amine = TETR2 of amine : acid molar ratio = 10 : 1 and in the mixture of 67% volume H 2 O in THF. All the salts were introduced into the system at the beginning of the process. .............................................................................................. 112 Figure 5.8 TEM images for assembled micelles after aging for (A) 0 day, (B) 9 days, (C) 65 days and (D) 3 months. The assembly solution was amine = TETR2 of amine : acid molar ratio = 10 : 1 and in the mixture of 67% volume H 2 O in THF. The water was titrated at a rate of 2 ml/hour which was doubled from original titration rate of 1 ml/hour. ..........................................................................................114 Figure 5.9 TEM images for assembled micelles (A) in 50% volume H 2 O in THF; and (B) aging 20 days, and (C) and (D) aging 8 months after adding water into 50% volume H 2 O in THF to shift solution composition to 67% volume H 2 O in THF. Single- and double- stranded helical structures were found after months aging. Original solution was TETR2 of amine : acid molar ratio = 10 : 1. .....117 Figure 5.10 TEM images for assembled micelles (A) in 80% volume H 2 O in THF; after aging (B) 0 day and (C) 21 days after introducing THF in 80% volume H 2 O in THF to shift solution composition to 67% volume H 2 O in THF. ..........................................................................118 Figure 5.11 Cryo-TEM images (A) and (B) for stacking disks formed in condition of amine = TETR2 of amine : acid molar ratio = 5 : 1 in the mixture of 67% volume H 2 O in THF. The disks can be induced into branched bended cylinders and finally into helical cylindrical micelles when amine : acid molar ratio increased to 10 : 1. The process was shown in bended cylinders (C) and (D), and helix (E). .....................................................................................120 Figure 5.12 Cryo-TEM images for (A) extended cylindrical micelles made from AMS101 with TETR2 of amine : acid molar ratio = 20 : 1; and (B) helical structures formed after adding 0.1 M NaCl into the cylindrical micelle solution. The solution was the mixture of 67% volume H 2 O in THF. ..........................................................................122

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Figure 6.1 The chemical structure of the block copolymer and the schematic for the patchy nanoparticle. Nanoparticles of 100 nm in diameter were characterized under conventional TEM. The surface patches show darker contrast from the main body of particles as well as the background. ................................................................................. 133 Figure 6.2 HAADF images and EDX analysis for patches. (A) and (B) showed the elements for the background. (C) and (D) indicated high concentration of iodide elements in the patches. .........................134 Figure 6.3 TEM images for (A) homogenous particles from SMeI53 in 25% volume H 2 O in THF; (B) multicompartment particles from SMeI53 in 20% volume H 2 O in dioxane; (C) patchy nanoparticles from SMeI53 in 25% volume H 2 O in dioxane; (D) nanoparticles with dark region inside from SMeI53 in 50% volume H 2 O in dioxane. .............................................................................................135 Figure 6.4 Schematic of (A) a typical core-shell micelle and (B) a polymeric colloid. The diameter of the micelle was calculated based on block length of SMeI53 and the diameter of the colloid was measured from TEM for SMeI53. ......................................................137 Figure 6.5 DLS shows the hydrodynamic radius (R H ) of the polymeric particles was inversely proportional to the stirring in time. (A) RH of particles was larger in dioxane than in THF. (B) The longer the block length, the larger the aggregated particles. ................139 Figure 6.6 Representative TEM images for patch growth using SMeI207. (A) colloids without patches, 0-day ageing; (B) patches appeared on the surface of particles, 10-day ageing; (C) Patches grew in size and number, ageing 15 days; (D) patches formed after ageing 50 days; and (E) stable for 3 months; (E) the polymer chemistry for this kinetic study. ...............................................................................141 Figure 6.7 Representative TEM images for patch growth using SMeI53. (A) colloids without patches, 0-day ageing; (B) patches appeared on the surface of particles, 60-hour ageing; (C) patches started disappearing, 4-day ageing; (D) colloids with homogenous block mixing, 2-month ageing. ....................................................................142 Figure 6.8 A plot of the ageing time needed for patch growth as a function of the block length of P4VPMeI. It was clear that the ageing time was increased with the length of P4VPMeI blocks. ............................143

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Figure 6.9 A plot of the patch size as a function of the block length of P4VPMeI. It was clear that the size of patches was increased with repeat unit ratio of P4VPMeI to PS blocks. ........................................ 144 Figure 6.10 Geometric distributions of patches on the surface of nanoparticles with different size. .............................................................................149 Figure 6.11 A plot of the diameter of the particle as a function of its patch number. It shows the size of the particle was increased dramatically with the number patches residing on the surface.............150 Figure 7.1 Scheme for synthetic approach for the enzymatic and chemical preparation of amphiphilic diblock copolymers. .................................161 Figure 7.2 TEM images of block copolymer 1a after incubation in buffer pH 5 with the enzyme. .............................................................................162 Figure 7.3 Modular synthesis of diblock copolymers. .........................................165 Figure 7.4 (a) GPC chromatograms of the eluted PEG peak shown with increasing UV exposure times; (b) release of encapsulated biocytin; (c, d) cryo-TEM images of 2NPA vesicles (c) before and (d) after 6 h of UV exposure. Scale bars are 100 nm. .........................166 Figure 7.5 Cryo-TEM images of formulations corresponding to ―rigid‖ and ―flexible‖ liposomes. The former contains 13 mM EPC alone and its images, shown in (a), reveal a number of liposomes that are either unilamellar or bilamellar. The latter is a mixture of 13 mM EPC and 11 mM T80 (molar ratio of T80 = 46%) and its images in (b) reveal a combination of spherical unilamellar liposomes and a number of disklike micelles (marked by arrows). The disks are seen from the side, which is why they show up as dark lines or curves in the images. ..........................................................................168 Figure 7.6 Cryo-TEM images for 17H6 at low mag (A) and high mag (B); and images for PEG5K-c17H6 at low mag (C) and (D). The pH of the solution is 2.3. ..........................................................................171 Figure A.1 FTIR absorbance spectra of PAA in different THF/D 2 O mixtures with volume D 2 O of (A) 0%, (B) 20%, (C) 60%, and (D) 75%. ..........175 Figure A.2 A plot of function of charged status of PAA to the solution content. The PAA tend to be protonated at higher THF content solution. .............................................................................................176

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Figure A.3 Effect of organoamine molecules on the protonation of PAA. ............ 178 Figure A.4 Effect of the THF/D 2 O ratio on the interaction between PAA and amines. The volume ratio of THF in D 2 O was (A) 0%, (B) 20%, (C) 30%, and (D) 50%. ......................................................................179 Figure A.5 IR spectra for (A) pure TETR2 and (B) 2 wt% TETR2 in 67% volume H 2 O in THF. ..........................................................................181

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ABSTRACT There is always an interest to understand how molecules behave under different conditions. One application of this knowledge is to self-assemble molecules into increasingly complex structures in a simple fashion. Self-assembly of amphiphilic block copolymer in solution has produced a large variety of nanostructures through the manipulation in polymer chemistry, assembly environment, and additives. Moreover, some reports suggest the formation of many polymeric assemblies is driven by kinetic process. The goal of this dissertation is to study the influence of kinetics on the assembly of block copolymer. The study shows kinetic control can be a very effective way to make novel polymeric nanostructures. Two examples discussed here are helical cylindrical micelles and patchy nanoparticles. Helical cylindrical micelles are made from the co-assembly of amphiphilic triblock copolymer poly(acrylic acid)-block-poly(methyl acrylate)-block-polystyrene and organoamine molecules in a mixture of tetrahydrofuran (THF) and water (H 2 O). This system has already shown promise of achieving many assembled structures. The unique aspects about this system are the use of amine molecules to complex with acid groups and the existence of cosolvent system. Application of amine molecules offers a convenient control over assembled morphology and the introduction of PMA-PS selective solvent, THF, promotes the mobility of the polymer chains. In this study, multivalent organoamine molecules, such as diethylenetriamine and triethylenetetramine, are used to interact with block copolymer in THF/water mixture.

xx

As expected, the assembled morphologies are dependent on the polymer architecture, selection and quantity of the organoamine molecules, and solution composition. Under the right conditions, unprecedented, multimicrometer-long, supramolecular helical cylindrical micelles are formed. Both single-stranded and double-stranded helices are found in the same system. These helical structures share uniform structural parameters, including the width of the micelles, width of the helix, and the pitch distance. There is no preference to the handedness, and both handednesses are observed, which is understandable since there are no chiral molecules or specific binding effects applied during the assembly. The helical structure is a product of kinetic process. Cryogenic transmission electron microscopy has been employed to monitor the morphological transformation. The study indicates there are two complicated but reproducible kinetic pathways to form the helices. One pathway involves the stacks of bended cylinders at early stages and the subsequent interconnection of these bended cylinders. Spherical micelles bud off of the interconnected nanostructure as the final step towards a defect-free helix. Another kinetic pathway shows very short helices are formed at first and aligned via head-to-tail style in the solution, and the subsequent sequential addition of these short helices results in prolonged mature helices. By using a ninhydrin-staining technique, amine molecules within the micellar corona are visualized and confirmed to preferentially locate in the inner side of the helical turns. The aggregation of amine molecules provides a strong attraction force due to electrostatic association between oppositely charged amine and acid groups and accumulation of hydrogen bonding among amine molecules to coil the cylindrical micelles into helical twisting features which are stabilized by the repulsion

xxi

forces due to the chain packing frustration within the hydrophobic core, steric hindrance of amine molecules as well as the Coulomb repulsion of the excess charged amine groups. The formation mechanism of the helix offers the feasibility to manipulate the helical pitch distance and formation kinetics. The helical pitch distance can be enlarged or shrunk by varying the type and amount of amine molecules used in assembly, introducing inorganic salts, and changing pH. Luckily, the helical structure can be preserved permanently by inducing the amide reaction between amine and carboxylic acid groups. The kinetics of the helix is also subject to many factors, including used amine molecules, inorganic salts and preparation procedure. The aging time for the helix can be either reduced or prolonged. Furthermore, even though the helical formation is pathway-dependent, helical formation can still be triggered from extended cylindrical micelles or stacks of disklike micelles as long as a right condition is applied Another strategy for kinetic assembly of block copolymer is presented as well. A novel patchy nanoparticle has been produced following this strategy. The patches are formed on the surface of polymeric colloids due to the phase separation of two chemically unlike segments. Certain level of mobility of the polymer chains is required for the blocks to segregate into patches. More importantly, the number and distribution geometry of the patches are related to the particle size. Future efforts are needed to control the particle size in order to manufacture uniform nanoparticles with desired patch patterns for the applications in nanotechnology, drug delivery and nanodevices.

xxii

This dissertation has demonstrated the promise of kinetic control on the self-assembly of block copolymers. Macromolecules exhibit enormous ability to reorganize and interact during assembly to form novel nanostructures if they are exposed to the right conditions. This ability is also closely related to the molecular chemistry. Therefore, the combination of molecular design and kinetic assembly procedure can possibly make significant contribution to the scientific research and industrial development

1

Chapter 1 INTRODUCTION 1.1 Motivation Self-organizing particulate system with defined size and hierarchical structure from simple atoms or molecules to perform specific functions is of eminent interest in nanotechnology. 1, 2 Block copolymer, a nice synthetic analogy to traditional lipids and surfactants, consists of two or more chemically distinctive segments connected by covalent linkage. Recent advances in polymerization chemistry, such as anionic polymerization and living radical polymerization, have enabled a variety of block copolymers to be synthesized with precise control over their architecture, molecular weight, weight polydispersity, block chemistry, functionality and polyelectrolyte property. 3 By taking advantage of the chemistry sophistication and combining unique sample preparation, we can manipulate the interactions amongst the building blocks and consequently drive block copolymers to self-assemble into complex, novel structures. In all polymer self-assembly systems, the observed nanostructures are either in the state of equilibrium or kinetically trapped. Very few nanostructures have been made from the purposeful manipulation of the kinetic assembly of block copolymer in solution. One of the main reasons for this paucity in kinetic processing of nanostructure is because the mobility of the polymer chain in poor solution is

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extremely low and generally makes it difficult to affect the assembled structures with solvent processing. The purpose of this dissertation is to introduce the kinetic effects into the assembly system to produce novel nanostructures that are not available by conventional methods. One example is the helical cylindrical micelles that are formed by kinetic co-assembly of poly(acrylic acid)-block-poly(methyl acrylate)-block- polystyrene (PAA-b-PMA-b-PS) with multivalent organoamine molecules in the mixture of tetrahydrofuran and water. Moreover, novel patchy nanoparticles are produced in another assembly system that involves block copolymer polystyrene- block-poly(methyl-4-vinyl pyridinium iodide) in the cosolvent system of dioxane and water. One common aspect in these two systems is that polymer chains are granted certain mobility which allows them to reorganize into intermediate structures. The mobility for polymer chains to migrate is due to the existence of cosolvents which are able to plasticize the polymer chains. 1.2 Background Materials science is one of the fundamental disciplines that build human society. The development and choice of the materials often define a certain era in human history, such as the Stone Age, Bronze Age, and Steel Age. Materials Science is one of the oldest forms of engineering, starting from ancient human making knife from stones or bowls from mud. However materials science is also such a young scientific area that ―materials‖ first appeared in the word of science during the Cold War. The departments of materials science were not established in US universities until 1960s. 4 In the modern age, materials science is an interdisciplinary field that

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Abstract: There is always an interest to understand how molecules behave under different conditions. One application of this knowledge is to self-assemble molecules into increasingly complex structures in a simple fashion. Self-assembly of amphiphilic block copolymer in solution has produced a large variety of nanostructures through the manipulation in polymer chemistry, assembly environment, and additives. Moreover, some reports suggest the formation of many polymeric assemblies is driven by kinetic process. The goal of this dissertation is to study the influence of kinetics on the assembly of block copolymer. The study shows kinetic control can be a very effective way to make novel polymeric nanostructures. Two examples discussed here are helical cylindrical micelles and patchy nanoparticles. Helical cylindrical micelles are made from the co-assembly of amphiphilic triblock copolymer poly(acrylic acid)-block -poly(methyl acrylate)- block -polystyrene and organoamine molecules in a mixture of tetrahydrofuran (THF) and water (H2 O). This system has already shown promise of achieving many assembled structures. The unique aspects about this system are the use of amine molecules to complex with acid groups and the existence of cosolvent system. Application of amine molecules offers a convenient control over assembled morphology and the introduction of PMA-PS selective solvent, THF, promotes the mobility of the polymer chains. In this study, multivalent organoamine molecules, such as diethylenetriamine and triethylenetetramine, are used to interact with block copolymer in THF/water mixture. As expected, the assembled morphologies are dependent on the polymer architecture, selection and quantity of the organoamine molecules, and solution composition. Under the right conditions, unprecedented, multimicrometer-long, supramolecular helical cylindrical micelles are formed. Both single-stranded and double-stranded helices are found in the same system. These helical structures share uniform structural parameters, including the width of the micelles, width of the helix, and the pitch distance. There is no preference to the handedness, and both handednesses are observed, which is understandable since there are no chiral molecules or specific binding effects applied during the assembly. The helical structure is a product of kinetic process. Cryogenic transmission electron microscopy has been employed to monitor the morphological transformation. The study indicates there are two complicated but reproducible kinetic pathways to form the helices. One pathway involves the stacks of bended cylinders at early stages and the subsequent interconnection of these bended cylinders. Spherical micelles bud off of the interconnected nanostructure as the final step towards a defect-free helix. Another kinetic pathway shows very short helices are formed at first and aligned via head-to-tail style in the solution, and the subsequent sequential addition of these short helices results in prolonged mature helices. By using a ninhydrin-staining technique, amine molecules within the micellar corona are visualized and confirmed to preferentially locate in the inner side of the helical turns. The aggregation of amine molecules provides a strong attraction force due to electrostatic association between oppositely charged amine and acid groups and accumulation of hydrogen bonding among amine molecules to coil the cylindrical micelles into helical twisting features which are stabilized by the repulsion forces due to the chain packing frustration within the hydrophobic core, steric hindrance of amine molecules as well as the Coulomb repulsion of the excess charged amine groups. The formation mechanism of the helix offers the feasibility to manipulate the helical pitch distance and formation kinetics. The helical pitch distance can be enlarged or shrunk by varying the type and amount of amine molecules used in assembly, introducing inorganic salts, and changing pH. Luckily, the helical structure can be preserved permanently by inducing the amide reaction between amine and carboxylic acid groups. The kinetics of the helix is also subject to many factors, including used amine molecules, inorganic salts and preparation procedure. The aging time for the helix can be either reduced or prolonged. Furthermore, even though the helical formation is pathway-dependent, helical formation can still be triggered from extended cylindrical micelles or stacks of disklike micelles as long as a right condition is applied. Another strategy for kinetic assembly of block copolymer is presented as well. A novel patchy nanoparticle has been produced following this strategy. The patches are formed on the surface of polymeric colloids due to the phase separation of two chemically unlike segments. Certain level of mobility of the polymer chains is required for the blocks to segregate into patches. More importantly, the number and distribution geometry of the patches are related to the particle size. Future efforts are needed to control the particle size in order to manufacture uniform nanoparticles with desired patch patterns for the applications in nanotechnology, drug delivery and nanodevices.