Rubbing-induced surface morphology and polymer segmental reorientations of a model brush polyimide and interactions with liquid crystals at the surface

Abstract

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 direction whereas the phenyloxy units of the bristles were found to be oriented perpendicular to the rubbing direction. When LC molecules were placed in contact with the rubbed PI films, the LC molecules formed a uniformly aligned structure with a pretilt angle ranging from 25 to 87degrees along the rubbing direction, depending on the rubbing density. The tendency to form this structure was attributed to the favorable anisotropic interactions of the LC molecules with the parallel reoriented polymer main chains and n-octyl end groups of the bristles in the rubbed surface, as well as with the microgrooves and fine grooves aligned parallel to the rubbing direction. The large pretilt angles were favored despite the relatively short alkyl side end group of the PI, which contains only half of the 16 carbons generally required to achieve large pretilt angles of LCs. This result suggests that the n-octyl end groups of the bristles play a critical role in the generation of large pretilt angles, most likely through favorable interactions between these groups and the aliphatic tails of the LC molecules.