Multicritical Phenomena and Microphase Ordering in Random Block Copolymers Melts

A mean-field analysis of microphase separation and phase behavior is described for melts of linear A-B multiblock copolymers, each chain consisting of a stochastic sequence of flexible segments (each with M monomers), A_M and B_M, for a total of Q ≫ 1 segments per chain. We find a phase diagram similar to that for statistical A-B copolymers (M = 1) but with an isotropic Lifshitz point that should be accessible for a wide variety of materials. Due to the broad distribution of sequence lengths of like monomers, the microphases are predicted to lack long-range order and to exhibit unusual temperature sensitivity at the order-disorder transition. This may open the way to a novel class of polyurethanes and thermoplastic elastomers.