Rubbing-Induced Molecular Alignment on an Orienting Layer of Polyimide with and without Alkyl Side Chains

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Abstract
The rubbing-induced molecular alignment on a surface of polyimide with and without branched alkyl side chains was investigated with optical phase retardation measurement, atomic force microscopy (AFM), and second-harmonic generation (SHG). Our AFM results suggest that the rubbing-induced microgrooves play a minor role in aligning liquid crystal (LC) molecules. From the SHG studies, the detailed in-plane distribution of a LC monolayer on rubbed polymers was deduced, and the polar angle of the LC molecules was found to vary with the rubbing strength exerted on an alkyl-branched polymer. The polar angle of the LC monolayer on an alkyl-branchless main-chain polyimide film was observed to be independent of the rubbing strength. We propose a simple molecular model to reconcile these experimental results.