ILLUMINATION-INDUCED MODIFICATIONS OF LANGMUIR-BLODGETT-FILMS CONSISTING OF A DEUTERATED POLY(VINYL ALCOHOL) HAVING AN AZOBENZENE SIDE-CHAIN TRANSFERRED AT VARIED PACKING DENSITY

Langmuir-Blodgett (LB) films consisting of an amphiphilic azobenzene ( Az) polymer having a deuterated poly(vinyl alcohol) were prepared at v aried lateral Az packing density, and their initial and light-modulate d structures were evaluated by X-ray reflectometry and Fourier-transfo rm infrared (FTIR) spectroscopy. X-ray reflectometry indicated that th e layer structure of the LB film is strongly dependent on the lateral packing density. The more densely packed film was characterized by the better-defined structure in terms of the order of the local lamellar patterns and surface smoothness of the total film. The overall film th ickness decreased and increased upon exposure to UV and visible light, respectively; however, the lamellar structure was irreversibly disord ered by the first exposure to UV light. The deuterated PVA chain was f ound to have no appreciable orientational preference in the inplane di rection, as proven by polarized transmission FT-IR, although a nematic liquid crystal (LC) placed on these LB films exhibited a definitive h omogeneous in-plane alignment depending on the Az packing density [Sek i, T.; et al. Thin Solid Films 1994, 243, 675]. UV light (365 nm) irra diation on the LB films induced a tilt of the alkyl part in the Az sid e chain from the surface normal. The time course observation of the UV light induced changes in the IR signals revealed that the motion of t he alkyl part in the most densely packed LB film was retarded from tha t of the Az aromatic rings. The IR signals from the alkyl part reverte d to the original state on subsequent visible light (436 nm) illuminat ion, but reproducibility of the signals from the aromatic ring was poo r. Finally, an increase in the Az packing density substantially retard ed the response of the UV light induced homeotropic --> planar alignme nt change of a nematic LC placed on these Az LB films. The retarding e ffect was more manifest in this intermolecularly driven system.