Fabrication Of Polymeric Hollow Spheres And Their Applications In Encapsulation & Release Of Nanocrystals

As an efficient approach to nanoscale materials with well-defined structures, self-assembly has generated broad interest in chemistry and materials science. It has been extensively studied that block and graft copolymers self-assembled into micelles with well-defined structures. The obtained polymeric micelles could be in spheres, vesicles, worm-like cylinders and other novel structures. Recently, self-assembly of block copolymers consisting of rigid and flexible components received special interests in that inherent tendency of the rod block to form orientational order greatly affects the details of molecular packing. In fact, it was reported that self-assembly of rod-coil block copolymers resulted in some unprecedented morphologies both in solution and in the solid state.Among the targeting materials from self-assembly of polymers, hollow spheres on nanometer and submicrometer scales have attracted much attention owing to their potentials for serving as carriers of catalysts, enzymes and drugs etc. Wooley et al. and Liu et al. produced hollow spheres via self-assembly of block copolymers into core-shell micelles followed by crosslinking of the shell and degradation of the core. ''Layer-by-layer (LBL)" technique using alternative depositions of oppositely charged species on various templates, and subsequent sacrificing the core can produces diverse hollow spheres. In addition, vesicles and particles used as templates for in-situ polymerization to produce hollow spheres have been reported. Rod-coil block copolymers were found to be able to form large-size hollow spheres directly in their selective solvent.Recently, research efforts of Jiang's group have been devoted to developing a block-copolymer-free strategy to fabricate micelles based on homopolymer pairs. This novel approach results in "non-covalently connected micelles'' (NCCM), in which only hydrogen bonds rather than chemical bonds exist between the shell and core. As a typical example, the NCCM composed of poly(4-vinyl pyridine) (PVPy) shell and hydroxyl-containing polystyrene (PS(OH)) core was realized in a selective solvent for PVPy due to hydrogen bonding between the hydroxyl groups and pyridine units. Furthermore, subsequent cross-linking of the PVPy shell and dissolution of the PS(OH) core moiety led to hollow spheres.When we used a rigid rod polyimide (PI) with carboxyl ends as a building block to fabricate NCCM with PVPy in their common solvent chloroform, it was unexpected that hollow spheres were obtained. Comparing with the existing procedures for producing micelles and hollow spheres, this process of using rod-coil homopolymer pairs in their common solvent seemed much simpler and more straightforward. As we demonstrated in the previous paper, this unusual phenomenon can be attributed to two main factors: the rigid character of PI and its 'grafting' to the PVPy chains. The propensity to parallel packing of the rod blocks is believed to play an important role in such unusual assembly behavior.The content of this thesis could be divided into two parts.First, structural factors of the rigid and coil polymer pairs influencing their self-assembly in their common solvent were investigated. Several pyridine-unit-containing polymers with different structures, i.e. poly(4-vinyl pyridine) (P4VP), poly(2-vinyl pyridine) (P2VP) ,the copolymers of styrene and 4- vinyl pyridine (SVP) and PS-b-P2VP block copolymer were used. Its counterparts, the rigid proton-donating polymers, carboxyl-ended polyimide and poly(amic acid) ester were used. All the rigid-coil polymer pairs could self-assemble into hollow aggregates in submicrometer size. The results show a common trend that the hydrodynamic radius of the assembled hollow spheres decrease with increasing the ratio of the rigid proton-donating polymer to the flexible proton-accepting polymer. It was also found, at the same weight ratio, the size of hollowspheres obtained from PAE/P4VP(P2VP) is much smaller than that of PI/P4VP(P2VP), this may be attributed to the double ends in PAE. Comparing with the mono-...