Phase behavior of styrene-isoprene diblock derivatives with varying conformational asymmetry

At present, diblock copolymer phase behavior in the long-chain limit is con sidered to be governed by three factors: the volume fraction of one block, phi; the segregation strength of the diblock, chi N; and the conformational asymmetry parameter, epsilon. This implies that the phase diagrams for pol ymers of different chemistry but similar molecular weight should evolve sim ply and smoothly as epsilon is varied. We present here partial phase diagra ms (covering the cylinder-gyroid-lamella region) for two derivatives of the well-known styrene-isoprene diblocks: styrene-(ethylene-alt-propylene) and vinylcyclohexane-(ethylene-alt-propylene), all of similar molecular weight s. Comparison of the S/I, VCH/EP, and S/EP phase diagrams reveals that the phase behavior does not vary simply with epsilon. In particular, the shape and extent of the gyroid region differ greatly between the three chemistrie s; in the S/EP system, this region disappears into a cusp at moderate segre gation strengths (chi N approximate to 24). Our findings indicate that fact ors other than phi, chi N, and epsilon do influence the free energy of dibl ock copolymer mesophases and their regions of stability.