Morphological Transitions in an I₂S Simple Graft Block Copolymer:  From Folded Sheets to Folded Lace to Randomly Oriented Worms at Equilibrium

A new equilibrium morphology consisting of randomly oriented wormlike micelles dispersed in a continuous matrix is observed in a neat, strongly segregated I 2 S simple graft block copolymer. The equilibrium nature of the worm phase is determined via a set of selective solvent casting and prolonged annealing experiments. Transmission electron microscopy (TEM) experiments on quenched samples allow a unique opportunity to directly observe the transition of a kinetically trapped, nonequilibrium folded-layer morphology, formed by casting the sample with a solvent selective for polyisoprene (PI), into the equilibrium, randomly oriented worm phase through an intermediate folded-lace morphology. The folded-lace intermediate is similar to the “mesh” structure previously observed by Hashimoto et al. in starblock/homopolymer blends. 1 The simple graft block copolymer, formed by grafting a single polystyrene (PS) chain onto the center of a polyisoprene backbone, introduces a 2:1 PI/PS arm number asymmetry in the microphase separated state. The 0.81 volume fraction of the PS graft is theoretically predicted 2 to be the first volume fraction of graft large enough to force the two PI arms per molecule to the concave side of the PI/PS interface in the microphase separated state. This unique volume fraction, coupled with the novel graft architecture, seems to frustrate the system from choosing a lattice during the microphase separation process.