SOLVENT SWELLING OF ROLL-CAST TRIBLOCK COPOLYMER FILMS

Polystyrene-polybutadiene-polystyrene triblock copolymers were roll-ca st from toluene solutions to form globally oriented films. As-processe d films, containing process-related residual stresses, were exposed to solvent vapours. Three solvents were used in this study: toluene, whi ch is a non-preferential solvent for polystyrene and polybutadiene; me thyl-ethyl-ketone, which is a preferential solvent for the polystyrene blocks; and hexane, which is a preferential solvent for the polybutad iene block. Microstructural changes accompanying the solvent swelling of films with cylindrical and lamellar morphology were monitored using two-dimensional small angle X-ray scattering. Solvent swelling signif icantly improved the symmetry of the hexagonal packing of the cylindri cal domains, which was initially distorted due to the roll-casting flo w field. Solvent swelling was also found to improve the long range ord er in roll-cast film with lamellar morphology. Various phenomena were found to accompany the swelling and deswelling of films with both cyli ndrical and lamellar morphology with the three different solvents used . Especially intriguing results were observed for the case of swelling both morphologies in hexane. For films with a lamellar morphology, af ter 1 h of swelling and 2 h of subsequent deswelling the d-spacing dec reased by 18%. For films with cylindrical morphology, a similar decrea se of 9% was observed. Molecular models are presented to explain these microstructural changes, that are closely linked to the mobility of t he glassy polystyrene blocks, the relaxation of process-related stress and the ability of the polybutadiene-polystyrene junctions at the int erfaces to reposition and accommodate volume changes. (C) 1997 Elsevie r Science Ltd. All rights reserved.