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-alt-propylene)-b-styrene) (PS-PEP-PS) triblock copolymer (Mn=50,000 g/mol and 50.8 wt% PS) were characterized by transmission electron microscopy (TEM) and dynamic rheology. The microstructure texture at room temperature and up to about 125°C consisted of alternating PS and PEP lamellae (LAM). Oscillatory shear experiments revealed thermally-reversible post-Tg transitions at 125–130°C and 275°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) microstructure of alternating PS and PEP lamellae with hexagonal-packed PS connectors 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.