Effect of molecular weight on the surface morphology, molecular reorientation, and liquid crystal alignment properties of rubbed polystyrene films

The molecular reorientations and surface morphologies of rubbed films formed from atactic polystyrene (PS) samples with various molecular weights Mw- (weight-average molecular weight) in the range 2700-83000 and narrow polydispersity were investigated in detail by atomic force microscopy, optical retardation analysis, and linearly polarized infrared spectroscopy. We have first discovered previously unknown surface topography features in rubbed films of PS with > 9800 Mw: submicroscale groovelike meandering structures composed of fine groovelike pebbles in tens of nanometers are present, oriented perpendicular to the rubbing direction. The unusual surface morphology is a significant departure from the surface topographies observed so far for rubbed PS and other polymer films, for which grooves are usually only found parallel to the rubbing direction. These surface morphology variations might be due to the different deformation responses of the different molecular weight PS films to the shear forces caused by contact of fibers with the film surfaces during the rubbing process. On all the rubbed PS films, the vinyl main chains, however, were preferentially reoriented along the rubbing direction, and the planes of the phenyl side groups were preferentially reoriented perpendicular to the rubbing direction with para directions that were positioned nearly normal to the film plane. Nematic liquid crystal (LC) molecules were found to always align on the rubbed PS surfaces along the orientation direction of the submicroscale grooves generated by rubbing. However, the anchoring energies of the LC alignments were very low depending on the molecular weight of the sample, and these LC alignments all had limited stability. We conclude that the alignment of LCs with only weak molecular interactions with rubbed PS film surfaces is governed by their interactions with the submicroscale grooves in cooperation with one of the two other important interactions-those with the reoriented main chain segments and those with the reoriented side groups.