Rubbing-Induced Surface Morphology and Polymer Segmental Reorientations of a Model Brush Polyimide and Interactions with Liquid Crystals at the Surface

Poly{p-phenylene-3,6-bis[[4-(n-octyloxy)phenyl]oxy]pyromellitimide} (C8-PMDA-PDA PI), a model brush polymer with a fully rodlike backbone, was determined to be positively birefringent by prism coupling analysis. Films of the PI were examined in detail by optical retardation and polarized infrared spectroscopy before and after mechanical rubbing with a velvet fabric. In addition, the alignment behavior of liquid crystal (LC) molecules in contact with rubbed films of the model polymer was investigated. Atomic force microscopic imaging revealed that rubbing caused microgrooves, as well as fine grooves (around 100 nm in size) with a surface morphology that resembled ground-beef, to develop parallel to the rubbing direction. The morphologies of these grooves are attributed to the structure of the fabric fibers and the shear deformation characteristics of the polymer. At the rubbed surface, the polymer main chains and the n-octyl end groups of the bristles were determined to be oriented parallel to the rubbing ...