Sequence of the rubbing-induced reorientations of polymer chain segments in nanofilms of a well-defined brush polyimide with a fully rodlike backbone as determined by polarized FTIR spectroscopy and two-dimensional correlation analysis

To characterize the reorientations of molecular segments in polymer films treated by rubbing, which is critical information for understanding the effects of the rubbing process, the surface structures of rubbed, nanoscaled films of polyp-phenylene 3,6-bis[4-(n-octoxy)phenyloxy]pyromellitimide} (C8-PMDA-PDA PI), a well-defined brush polyimide composed of aromatic-aliphatic bristles set into a fully rodlike polymer backbone (two bristles per repeating unit), were extensively investigated as a function of rubbing density by polarized infrared (IR) spectroscopy in transmission mode and by two-dimensional correlation analyses of these IR spectra. The rubbing process was found to strongly cause local reorientations of the rodlike polymer backbones as well as of the bristles on the film surface. On the surface of a rubbed film, the polymer backbones and the aliphatic n-octyl end groups of the bristles are preferentially reoriented along the rubbing direction, but the phenyloxy units of the bristles are preferentially reoriented perpendicular to the rubbing direction; these preferential reorientations are enhanced as the rubbing density increases. The polymer main chains are more strongly influenced by the rubbing process than are the bristles. This study is the first time that the ordering of the rubbing-induced reorientations of the polymer segments in a rubbed film has been determined; the rubbing-induced reorientations were found to occur in the following sequence: PDA phenyl ring, imide ring, phenyloxy unit, imide C-N bond, and aliphatic n-octyl group. Moreover, it was found that in the rubbing process the imide rings (which make up the polymer main chain) undergo biaxial reorientation (i.e., in-plane reorientation along the rubbing direction and out-of-plane reorientation). In particular, the out-of-plane reorientation of the imide rings is enhanced by increasing the rubbing density.