Role of molecular architecture in amphiphilic dispersions

Abstract

The self-assembly of amphiphilic block copolymers and other amphiphilic into structures such as spherical micelles, cylindrical micelles, and vesicles in water is a rich area of study in current polymer science. Many of these structures are of great interest for biomedical applications such as drug delivery or non-invasive monitoring. Improving their effectiveness is tied directly to understanding how to influence their geometry. This thesis represents an investigation into the effect that changes in molecular architecture have upon these self-assembled structures. Premixed blends of two vesicle-forming poly(ethylene oxide)-poly(butadiene) (OB) block copolymers of differing molecular weight but uniform composition were found to self-assemble into vesicles in water using cryogenic transmission electron microscopy (cryo-TEM) until a critical polydispersity was reached, above which cylindrical micelles were also formed. The nonlinearity in membrane thickness with respect to composition was described with a modified theory previously applied to bulk lamellae. A similar OB binary blend system was utilized in a collaborative effort towards the creation of peptide-conjugated vesicles with improved bioactivity. Local clustering of peptide-polymer aggregates was observed and attributed to the attractive interactions between peptide ligands. Molecular architecture was also explored as a means for controlling vesicle size via blending of poly(ethylene oxide)-poly(styrene)-poly(butadiene)-poly(ethylene oxide) tetrablocks with chemically similar diblock copolymers prior to dispersion. No positive effect was found, and many non-equilibrium structures were visualized with cryo-TEM. Finally, the aqueous phase behavior of a novel class of amphiphilic dendrimers was examined with cryo-TEM. These alternatives to block copolymer vesicles displayed fascinating self-assembly behavior with a wide variety of aggregate structures, including multilamellar vesicles, long tubular vesicles, disk micelles, ribbons, and cubosomes.