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, stron gly segregated I2S simple graft block copolymer. The equilibrium natur e of the worm phase is determined via a set of selective solvent casti ng and prolonged annealing experiments. Transmission electron microsco py (TEM) experiments on quenched samples allow a unique opportunity to directly observe the transition of a kinetically trapped, nonequilibr ium folded-layer morphology, formed by casting the sample with a solve nt selective for polyisoprene (PI), into the equilibrium, randomly ori ented worm phase through an intermediate folded-lace morphology. The f olded-lace intermediate is similar to the ''mesh'' structure previousl y 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 b e 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 mi crophase separated state. This unique volume fraction, coupled with th e novel graft architecture, seems to frustrate the system from choosin g a lattice during the microphase separation process.