Miscibility Enhancement in Polyisoprene/Polyolefin Block Copolymers and Polymer Blends

Abstract

The growing awareness within the polymer science community that commercially viable materials can be developed by melt mixing already existing ones led to an interest to explore ways that improve miscibility between known polymers. This strategy allows for savings in research and development of new materials with equivalent properties, and for faster development time. This dissertation seeks to assist this endeavor by presenting a simple chemical modification method to enhance compatibility in classic polydiene-polyolefin systems. The foremost commercial application of polyisoprene, a polydiene that is the synthetic equivalent of natural rubber, is tire manufacture. If mixed well, saturated polybutadiene (hPB), a well-known polyolefin, can impart thermo-oxidative stability and improved gas-barrier properties to polyisoprene. However, polyisoprene and hPB are incompatible due to structural differences. Selectively saturated block copolymers prepared via living anionic polymerization showed significant improvement in polyisoprene/hPB compatibility (characterized by >50% decrease in interaction energy density, X) on randomly incorporating small amounts of styrene into the hPB chain. Moreover, the variation of block copolymer interaction energy with styrene content could be accurately predicted by standard mixing models like the regular mixing model and the copolymer equation model, at all levels of styrene incorporation. Analogous polymer blends displayed similar thermodynamic behavior, after correcting for fluctuation and end-group effects that become significant at low molecular weights. This similarity suggested that X is a segment-level parameter, independent of mixture architecture, as predicted by fundamental mixing theories. The polar modifier used for styrene-butadiene random copolymerization was evaluated for efficacy in randomizing the butadiene sequence with other styrenic monomers like 4-methylstyrene, 4-t-butylstyrene, and 4-vinylbiphenyl, to potentially expand the roster of species that could aid in PI/hPB compatibility enhancement. This modifier is effective in randomizing the 4-methylstyrene/butadiene sequence but proved to be ineffective for the others. This result was utilized to investigate polyisoprene-containing block copolymer thermodynamics in systems where 4-methylstyrene was incorporated into the hPB chain, instead of styrene. These block copolymers exhibited >25% decrease in X on incorporating ~25 wt% 4-methylstyrene, but the interaction energy variation with composition could not be accurately predicted by the regular mixing model or the copolymer equation model.