T-junction grain boundaries in block copolymer-homopolymer blends

T-junction grain boundaries were studied in a blend of polyisoprene homopolymer and a single graft, block copolymer I2S with two equal length blocks of polyisoprene and one arm of polystyrene linked at a common junction point. The overall polyisoprene Volume fraction in the blend was 0.52, and its equilibrium morphology was lamellar. While T-junctions were previously observed to be quite rare compared to other tilt grain boundary morphologies such as chevrons and omegas, they were found in abundance in the blend used in the current study. The T-junctions in the blend show a number of distinctive characteristics including enlarged semicylindrical end caps terminating polystyrene lamella and an increase in the spacing of the lamella as they near their termination at the T-junction. Simple free energy calculations show that the homopolymer present in the blend stabilizes the cylindrical curvature of the end caps, rendering the T-junction morphology more stable in blends than in neat block copolymers. This agrees with the observed greater frequency of occurrence of the T-junctions in our blend sample than in neat block copolymers.