Solvent swelling of roll-cast triblock copolymer films

Abstract

Polystyrene-polybutadiene-polystyrene triblock copolymers were roll-cast from toluene solutions to form globally oriented films. As-processed films, containing process-related residual stresses, were exposed to solvent vapours. Three solvents were used in this study: toluene, which is a non-preferential solvent for polystyrene and polybutadiene; methyl-ethyl-ketone, which is a preferential solvent for the polystyrene blocks; and hexane, which is a preferential solvent for the polybutadiene 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 significantly improved the symmetry of the hexagonal packing of the cylindrical domains, which was initially distorted due to the roll-casting flow field. Solvent swelling was also found to improve the long range order in roll-cast film with lamellar morphology. Various phenomena were found to accompany the swelling and deswelling of films with both cylindrical 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, after 1 h of swelling and 2 h of subsequent deswelling the d-spacing decreased by 18%. For films with cylindrical morphology, a similar decrease of 9% was observed. Molecular models are presented to explain these microstructural changes, that are closely linked to the mobility of the glassy polystyrene blocks, the relaxation of process-related stress and the ability of the polybutadiene-polystyrene junctions at the interfaces to reposition and accommodate volume changes.