Surface morphology, molecular reorientation, and liquid crystal alignment properties of rubbed nanofilms of a well-defined brush polyimide with a fully rodlike backbone

Abstract

Nanoscaled films of poly{p-phenylene 3,6-bis[4-(n-butoxy)phenyloxy]pyromellitimide} (C4-PMDA-PDA PI), a well-defined brush polyimide (PI) composed of aromatic-aliphatic bristles set into a fully rodlike polymer backbone (two bristles per chemical repeat unit of the polymer backbone), were studied by atomic force microscopy, optical retardation, prism coupling, and linearly polarized IR spectroscopy before and after mechanical rubbing with velvet fabric, and their liquid crystal (LC) aligning abilities were investigated. Uniform, homogeneous LC alignment was achieved at the rubbed film surfaces of PIs with positively birefringent characteristics. Surprisingly, however, the LC alignment director for this PI is perpendicular to the rubbing direction as well as to the reorientation direction of the polymer main chains. This is the first time that LCs on a PI surface have been induced to align in the direction perpendicular to the rubbing direction, a significant departure from the LC alignment observed for all other PI materials reported so far, for which LC alignment is always parallel to the rubbing direction. This unusual LC alignment is attributed to the strength of the anisotropic intermolecular interactions of the LC molecules with the short bristles attached perpendicularly to the polymer chain, which override the interactions with the main polymer chains and with the microgroove lines with fine grooves that are created along the rubbing direction. The LC pretilt angle is expected to be 25-55degrees with respect to a direction perpendicular to the rubbing direction, although this angle could not be determined due to the limitations of the crystal-rotation apparatus employed. In particular, the n-butyl end group of the bristle was found to play a critical role in the pretilting of LC molecules, as well as in the perpendicular LC alignment. In addition, the surface morphology of the rubbed PI films was investigated in detail, with particular consideration being given to the polymer property characteristics, the structure of the velvet fabric fibers, and the rubbing conditions.