T-junction grain boundaries in block copolymer-homopolymer blends

T-junction grain boundaries were studied in a blend of polyisoprene homopol ymer and a single graft, block copolymer I2S with two equal length blocks o f 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 e quilibrium morphology was lamellar. While T-junctions were previously obser ved to be quite rare compared to other tilt grain boundary morphologies suc h 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 distincti ve characteristics including enlarged semicylindrical end caps terminating polystyrene lamella and an increase in the spacing of the lamella as they n ear their termination at the T-junction. Simple free energy calculations sh ow that the homopolymer present in the blend stabilizes the cylindrical cur vature 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 nea t block copolymers.