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 me asured for Langmuir-Blodgett (LB) films of cis- and trans-6Az1-PVA [po ly(vinyl alcohol) (PVA) derivative having azobenzene (Az) side chains; Figure 1B] which receive keen interest as a highly photoreactive laye r for the surface photoregulation of nematic liquid crystal (LC) align ment. Vibrational frequencies of CH2 antisymmetric and symmetric stret ching bands show that the hydrocarbon chain in the multimonolayer film s contains gauche conformers. Probably, the disorder of the hydrocarbo n chain increases its mobility, making the contact between the LC mole cule and 6Az1-PVA feasible. Comparison of band intensities between the IR transmission and RA spectra indicates that the hydrocarbon chain a nd -OCH2COO- linkage assume rather randomly oriented states. This conc lusion is in good agreement with the suggestion by Seki et al. (Langmu ir 1993, 9, 211) that the Az side chains do not have highly ordered st ructure prior to the deposition of LC molecules, and that the normal o rientation of Az side chains with respect to the substrate plane is ca used by an induced fitting between the two-dimensional Az host layer a nd 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-PV A are laid on the surface of the substrate with its long axis perpendi cular to the polarized UV light irradiated. The LC molecules on the LB film of trans-6Az1-PVA form homeotropic orientation with respect to t he substrate surface. The efficiency of the polarization on the anisot ropy of the LB film of cis-6Az1-PVA has been studied by irradiating po larized UV lines with their polarization planes perpendicular and para llel to the dipping direction in turn. It has been found that the alte rnation of dichroism occurs without the irradiation of a blue line, an d that the polarized lines have different efficiency. The polarized IR spectra also suggest that the anisotropy induced by light illuminatio n is superior to that imposed during the dipping process.