Synthesis and Morphologies of Block Copolymer , poly(styrene)-block- poly(4-vinyl pyridine) with Different Architectures.

　　Amphiphilic block copolymers have stimulated significant scientific interest since they offer the possibilities to nanoscale materials with tunable morphology and properties. Living polymerization, such as living ionic and controlled free radical methods are generally employed to synthesize block copolymers with well defined compositions, molecular weights and structures of very elaborate architectures. In this study, the synthesis of amphiphilic block copolymers of polystyrene-block-poly(2-vinyl pyridine) (PS-b-P2VP) and polystyrene-block-poly(4-vinyl pyridine) (PS-b-P4VP) with different architectures were explored. Then, the morphologies of linear, heteroarm, and blockarm PS-b-P4VP were studied with different solvent quality.

In the synthesis of polystyrene-block-poly(2-vinyl pyridine) (PS-b-P2VP), the issues of controlling arm numbers was investigated by the following three factors: (1) the chain length of living PSLi; (2)reaction temperature; (3)[PSLi]/[DVB] molar ratio. The experimental results suggested that the larger chain length of living PSLi led to the low arm number of heteroarm PS-b-P2VP due to the steric effect. The arm numbers was elevated rapidly at a high temperature. The higher [PSLi]/[DVB] molar ratio results in high probability of living PSLi chains reacted with the DVB monomers, which led to the enhancement of arm numbers of star copolymers. The control on the arm number of star PS-b-P4VP had a similar conclusion as that of PS-b-P2VP. Relatively low PDI on the synthesized block copolymers could be obtained through the GPC fractionation.
　　The morphology of the prepared PS-b-P4VP copolymers was studied on the following issues: (1) the effect of selective solvent (MeOH or H2O)on crew-cut aggregates：The results of static light scattering (SLS) showed that linear PS-b-P4VP copolymers aggregated easier than the heteroarm analog since the linear architecture moved faster; (2) the effect of the common solvent (DMF, dioxane, or THF) on the micelle morphology: The experimental results suggested that spherical micelles were observed in all three architectures of PS-b-P4VP regardless on the variation of the DMF/methanol. Aggregation morphology could also be changed with different common solvent quality based on their solubility with PS. The observed morphology include sphere、vesicle、connected sphere、Large compound micelles. The morphology transformation for the linear architecture is faster than that of the star architecture. (3) The effect of pH value on the aggregation morphology: the TEM images showed that the aggregation morphology of the PS-b-P4VP copolymers changed with different pH condition. For example, vesicle morphology was observed at pH=8 but interpenetrating morphology was shown at pH equal to 6. Reduced aggregation size was exhibited fro all three kinds of block copolymers due to the increased repulsion between P4VP chain and solvent.