Morphology Re-Entry in Asymmetric PS-PI-PS' Triblock Copolymer and PS Homopolymer Blends

Here, we report the morphology variation in a series of PS-b-PI-b-PS' asymmetric triblock copolymer and PS homopolymer (hPS) blends, where PS' and PS are polystyrene blocks with a molecular weight ratio of approximately 0.11 and PI is poly(isoprene). We find that adding a small amount of hPS results in significant order-order transition (OOT) boundary deflection toward higher PS volume fractions f(PS), which is accompanied by morphology re-entry. For example, the neat triblock copolymer with a PS+PS' volume fraction of f(PS)=0.38 exhibits a lamellar microphase; adding a small amount of hPS reverts the morphology into a hexagonal phase with PS cylinders, while further increasing the hPS fraction leads to normal OOTs from PS cylinders to lamellae, to PI cylinders and finally to spheres. The morphology variation reported here is significantly different from that reported in binary blends of diblock or symmetric triblock copolymer with homopolymer. While the domain features of the LAM structure can be correctly reproduced by self-consistent field theory (SCFT), the observed morphology re-entry is absent in the theoretical SCFT phase diagram. (C) 2015 Wiley Periodicals, Inc.