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 I2S 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.