Evidence for a thermally reversible order-order transition between lamellar and perforated lamellar microphases in a triblock copolymer

The microstructure and microphase behavior of a poly(styrene-b-(ethylene-al t-propylene)-b-styrene) (PS-PEP-PS) triblock copolymer (M-n = 50,000 g/mol and 50.8 wt% PS) were characterized by transmission electron microscopy (TE M) and dynamic rheology. The microstructure texture at room temperature and up to about 125 degrees C consisted of alternating PS and PEP lamellae (LA M). Oscillatory shear experiments revealed thermally-reversible post-T-g tr ansitions at 125-130 degrees C and 275 degrees C that are attributed to an order-order transition (OOT) of the mesophase texture and an order-disorder transition (ODT), respectively. Evidence for a perforated lamellar (PL) mi crostructure of alternating.PS and PEP lamellae with hexagonal-packed PS co nnectors perforating the PEP phase above the ODT and a thermally-reversible LAM <-> PL OOT was obtained from TEM of quenched and slowly cooled samples . Unlike previous reports of reversible OOT transitions in block copolymers , the reversibility of the OOT for these block copolymers does not require shearing the sample. (C) 1999 Elsevier Science Ltd. All rights reserved.