Block copolymers with controlled interphase width. Effects of interphase and composition on domain dimensions

Ultrathin osmium tetroxid stained sections of solution cast block copolymer films were studied by electron microscopy. Two morphological systems were prepared from diblock copolymers containing 80 wt.‐% styrene and 20 wt.‐% isoprene: Systems with asymmetric lamellar morphology cast from cyclohexane, a preferential solvent for PI; and spherical morphology of rubber spheres in a continuous PS matrix, cast from tetrachloromethane, a non‐selective solvent. For the two morphologies the variation of domain dimensions was studied in dependence of molecular weight in the range from 70 000 to 600 000 g/mol. Two types of block copolymers were investigated: Unmodified block copolymers and block copolymers containing 10 wt.‐% of a random interblock between the pure polystyrene and polyisoprene blocks. For both kinds of morphologies the domain dimensions of the unmodified systems vary with M n 2/3 as predicted by Helfand's theory. In the modified block copolymers a much weaker increase with M n is found for both morphological systems. The results can be understood in terms of equilibrium domain dimensions controlled by an interfacial tension depending on both composition and sequence structure of the block copolymers.