Effects of Polydispersity on the Order−Disorder Transition in Block Copolymer Melts

We prepared controlled-polydispersity poly(ethylene-alt-propylene)-b-poly(dl-lactide) (PL) and polystyrene-b-polyisoprene (SI) block copolymers to study the effects of the molecular weight distribution on the order−disorder transition at controlled composition and overall molecular weight. Small-angle X-ray scattering and rheological measurements were carried out to characterize the changes in the order−disorder transition in these self-assembled materials as a function of their composition and polydispersity index. We determined that in both asymmetric PL and SI systems increasing polydispersity in the minority component resulted in a decrease in the segregation strength at the order−disorder transition, whereas increasing polydispersity in the majority component results in an increase in the segregation strength at the order−disorder transition. We speculate that increasing PDI in the majority domain weakens the effective potential holding the phase-separated microdomains ordered on a lattice.