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

At present, diblock copolymer phase behavior in the long-chain limit is considered to be governed by three factors: the volume fraction of one block, φ; the segregation strength of the diblock, χN; and the conformational asymmetry parameter, ε. This implies that the phase diagrams for polymers of different chemistry but similar molecular weight should evolve simply and smoothly as ε is varied. We present here partial phase diagrams (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 weights. Comparison of the S/I, VCH/EP, and S/EP phase diagrams reveals that the phase behavior does not vary simply with ε. In particular, the shape and extent of the gyroid region differ greatly between the three chemistries; in the S/EP system, this region disappears into a cusp at moderate segregation strengths (χN≈24). Our findings indicate that factors other than φ, χN, and ε do influence the free energy of diblock copolymer mesophases and their regions of stability.