Miscibility Enhancement in Polyisoprene-Polyolefin Block Copolymers via Styrene Incorporation*

Polydienes like polyisoprene (PI) and polyolefins like hydrogenated mid-vinyl polybutadiene (hPB) generally show limited compatibility (high interaction energy density, X). Both the regular mixing model and the copolymer equation suggest that styrene units (S) should boost inter-block miscibility when incorporated in small amounts into hPB, up to 40 wt% S, via random copolymerization. Mixing thermodynamics in symmetric polydiene-polyolefin block-random copolymers composed of PI and a selectively saturated random copolymer of mid-vinyl polybutadiene and styrene (hSBR) were investigated via location of the order-disorder transition (ODT) temperatures. Block and “block-random” copolymers were prepared by anionic polymerization, followed by selective saturation of the butadiene units. ODT measurements show that X is 0.74 MPa at 0 wt% S, and exhibits the predicted parabolic variation with wt% S, with a minimum in X near 40 wt% S. Near the minimum, one-phase and two-phase blends of an hSBR with PIs of two different molecular weights, revealed that 0.10 < X < 0.16 MPa, indicating that the copolymer equation provides a better description of the mixing thermodynamics than the regular mixing model (for which X = 0 at the minimum).