FT-IR STUDY OF MOLECULAR-ORIENTATION AND ANISOTROPY IN COMMAND SURFACES FORMED BY LANGMUIR-BLODGETT-FILMS OF AZOBENZENE AMPHIPHILIC POLYMERS

FT-IR transmission and reflection-absorption (RA) spectra have been measured for Langmuir-Blodgett (LB) films of cis- and trans-6Az1-PVA [poly(vinyl alcohol) (PVA) derivative having azobenzene (Az) side chains; Figure 1B] which receive keen interest as a highly photoreactive layer for the surface photoregulation of nematic liquid crystal (LC) alignment. Vibrational frequencies of CH2 antisymmetric and symmetric stretching bands show that the hydrocarbon chain in the multimonolayer films contains gauche conformers. Probably, the disorder of the hydrocarbon chain increases its mobility, making the contact between the LC molecule and 6Az1-PVA feasible. Comparison of band intensities between the IR transmission and RA spectra indicates that the hydrocarbon chain and -OCH2COO- linkage assume rather randomly oriented states. This conclusion is in good agreement with the suggestion by Seki et al. (Langmuir 1993, 9, 211) that the Az side chains do not have highly ordered structure prior to the deposition of LC molecules, and that the normal orientation of Az side chains with respect to the substrate plane is caused by an induced fitting between the two-dimensional Az host layer and guest LC molecules. Polarized FT-IR spectra have been obtained for LC molecules photoregulated by the LB films of cis- and trans-6Az1-PVA. The spectra show that the LC molecules on the LB film of cis-6Az1-PVA are laid on the surface of the substrate with its long axis perpendicular to the polarized UV light irradiated. The LC molecules on the LB film of trans-6Az1-PVA form homeotropic orientation with respect to the substrate surface. The efficiency of the polarization on the anisotropy of the LB film of cis-6Az1-PVA has been studied by irradiating polarized UV lines with their polarization planes perpendicular and parallel to the dipping direction in turn. It has been found that the alternation of dichroism occurs without the irradiation of a blue line, and that the polarized lines have different efficiency. The polarized IR spectra also suggest that the anisotropy induced by light illumination is superior to that imposed during the dipping process.