Broadening the range of vesicle formation & self-assembly of vesicles into stacked bowls
Date of Issue2016
School of Physical and Mathematical Sciences
The thesis summarizes my MSc. research work on broadening the range of vesicle formation from glassy block copolymers, and the fabrication of linear vesicle assemblies and eccentric onion-like micelles from a crew-cut block copolymer. The fundamentals of these works are the understanding of amphiphilic block copolymer self-assembling process in solution, including the classification of block copolymers, the general micelle preparation methods, the thermodynamic and kinetic aspects of micelle formation and evolution, the accessible morphologies, etc. To be specific, amphiphilic block copolymers can assemble into a great variety of morphologies in solution, mainly including spheres, cylinders, lamellas, and vesicles. While the micelles of glassy block copolymers can be readily prepared by the water addition method with organic co-solvent, the experimental conditions for vesicle formation are usually restricted to a small range by kinetic limitations. As high temperature can increase the kinetic energy of the system, it was found that heating could significantly promote the micelle evolution of various types of block copolymers, and hence broaden the range of vesicle formation. Besides, for some specific type of block copolymers, it was observed that heating could also induce the linear assembly of single vesicles in solution followed by their transformation into “onion” structures. In detail, we made a systematic comparison between the heating assisted method and the conventional water addition method under similar conditions (e.g. polymer concentration, solvent ratio, etc.) (Chapter 2). First, we used a crew-cut anionic copolymer, PS192-b-PAA13, as a model system to compare the two methods’ dependence on different parameters (i.e. polymer concentration and water content). Then, the comparison was extended into other copolymer systems, including crew-cut cationic (i.e. PS222-b-P4VP43), neutral (i.e. PS1442-b-PEO795), and even “long-hair” ionic (i.e. PS154-b-PAA49 and PS230-b-P4VP90) copolymers. The results show that for the commonly used neutral or crew-cut block copolymers, the heating method is feasible in preparing under a much broader range than the water addition method. Most importantly, under optimized experimental conditions, vesicles of long-hair ionic copolymers can also be obtained by the heating method, while they are not attainable in the water addition method. During the heating period, a lot of the vesicles formed from PS222-b-P4VP43 were observed to assemble into linear chains, and the phenomenon was studied in greater detail (Chapter 3). It is found that under proper conditions in THF/H2O mixtures, the polymer chains first formed polydispersed vesicles, and then the vesicles assembled into linear chains. After prolonged incubation, the vesicle chains slowly transformed into eccentric “onion” micelles through chains of bowl-like intermediates. By comparing our “onion” formation with similar structures in the literature, it was concluded that the morphological transformation in our system was achieved through a unique mechanism, and the understanding may contribute to the development of new synthetic strategies towards sophisticated nano devices.