Role of the n-alkyl end of bristles in governing liquid crystal alignment at rubbed films of brush polymer rods

Abstract

A series of poly(p-phenylene 3,6-bis(4-(n-alkyloxy)phenyloxy)pyromellitimide)s (Cn-PMDA-PDA PIs) with various lengths of the n-alkyl bristle end group, which are well-defined polymer rods with two bristles per repeat unit of the polymer rod backbone, were studied as thin films using atomic force microscopy, optical retardation, and linearly polarized infrared spectroscopy before and after rubbing with a velvet fabric, and their nematic liquid crystal (LC) aligning abilities were investigated. The LC molecules on the rubbed film surfaces of Cn-PMDA-PDA PIs with n-butyl, n-hexyl, and n-heptyl bristle end groups were found to be aligned perpendicular to the rubbing direction; this is a significant departure from the parallel LC alignment observed for all conventional PI materials. In contrast, rubbing of the film surfaces of Cn-PMDA-PDA PI with n-octyl bristle end groups induced LC alignment parallel to the rubbing direction. Both the perpendicular and parallel LC alignments were determined to have very large anchoring energies (> 1 X 10(-3) J/cm(2)). The rubbing process was found to create microgrooves in the PI films along the rubbing direction and to orient the polymer main chains and the n-alkyl bristle end groups along the rubbing direction but to orient the phenyloxy bristle units perpendicular to the rubbing direction. These results collectively indicate that the n-alkyl bristle end groups play an important role in LC alignment and that this role depends on the length of the n-alkyl chain; specifically, the parallel oriented n-alkyl bristle end groups hinder perpendicular LC alignment because of their van der Waals interactions with the aliphatic tails of the LC molecules. The critical length of the n-alkyl bristle end group for the transition from perpendicular to parallel LC alignment at the rubbed PI films is eight carbons (i.e., the n-octyl end group). In conclusion, LC alignment on the surfaces of rubbed PI films is determined by a play-off between the directionally anisotropic interactions of the LC molecules with (1) the oriented segments of the polymer main chains, (2) the oriented segments of the bristles, and (3) the microgrooves.