Comparison of A-block polydispersity effects on BAB triblock and AB diblock copolymer melts

Recent experiments on triblock copolymer melts suggest that polydispersity effects are dramatically enhanced when polydisperse blocks are constrained by both ends to the internal interfaces of an ordered morphology. To quantify the relevance of architecture, we compare BAB triblock and AB diblock copolymer melts with polydisperse A blocks and monodisperse B blocks, using self-consistent field theory (SCFT). We do, in fact, find an enhanced shift in the order-order transitions (OOTs) of the triblock copolymer system in good agreement with the experiments, which we attribute to a reduction of entropy in the A-rich domains due to the absence of chain ends. There is also a slightly enhanced dilation of the domains, but not nearly to the same degree as reported by the experiments. Unlike in the experiments, our calculations indicate that the polydispersity-induced shifts in the order-disorder transition (ODT) should be quantitatively similar for both diblocks and triblocks. It is possible that some of the pronounced effects observed in the experiments have more to do with the detailed shape of the molecular-weight distribution than the triblock architecture.