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Effect of Molecular Weight on the Surface Morphology, Molecular Reorientation, and Liquid Crystal Alignment Properties of Rubbed Polystyrene Films
journal contribution
posted on 2003-12-30, 00:00 authored by Seung Woo Lee, Jinhwan Yoon, Hak Chul Kim, Byeongdu Lee, Taihyun Chang, Moonhor ReeThe molecular reorientations and surface morphologies of rubbed films formed from atactic
polystyrene (PS) samples with various molecular weights
(weight-average molecular weight) in the
range 2700−83 000 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
: 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 interactionsthose with the reoriented main chain segments and those with the reoriented
side groups.