Morphological behavior bridging the symmetric AB and ABC states in the poly(styrene-b-isoprene-b-ethylene oxide) triblock copolymer system

We report on the characterization of the morphological behavior expressed b etween the symmetric AB diblock and ABC triblock copolymer states. Experime ntally, a gradual transformation between these states was achieved through systematic addition of PEO to a single symmetric PS-PI diblock copolymer (M W similar to 18 800 g/mol), resulting in a series of 10 poly(styrene-b-isop rene-b-ethylene oxide) triblock copolymers varying only in the amount of PE O added. Final compositions ranged from 2.9 to 33.2 vol % (experimentally s ymmetric) PEO, with the remaining volume necessarily divided equally betwee n the PS and PI segments. The molecular weight of the parent diblock was ch osen such that resulting triblocks would undergo order-disorder transitions (ODTs) at experimentally accessible temperatures over much of the composit ion range studied. Our results focus on the detection and characterization of six morphologies observed to form between these two symmetric states and some of the kinetic and directional (heating vs cooling) characteristics o f two thermally induced order-order transitions (OOTs) observed. We conclus ively identify the formation of two- and three-domain lamellar structures, hexagonally packed core-shell cylinders, and the pentacontinuous core-shell gyroid morphology and speculate on probable assignments for two untypical structures also observed. Finally, we discuss some of the general behavior characteristics of this system in terms of their dependence on block connec tivity and chi interaction parameter sequencing.