A convenient preparation of photoactive monolayers for liquid crystal photoalignment by the surface adsorption of an aminoalkylated azobenzene on poly(acrylic acid) films

Abstract

Spin-cast poly(acrylic acid) (pAA) films on a substrate plate are immersed in a hexane solution of 4-butyl-4′-(10-aminodecyloxy)azobenzene, resulting in surface-selective adsorption and leading to the formation of photoactive monolayers. The surface adsorption behaviour is critically affected by the concentration of the azobenzene adsorbate solutions, and a 2.5×10^–5 mol dm^–3 solution gives an azobenzene monolayer with a surface density of 2 molecules nm^–2 without any modification of the surface morphology of the pAA film. The surface density of the azobenzene is controlled by a binary system consisting of the azobenzene and n-octadecylamine. Photoirradiation of the surface-modified film with linearly polarised UV light for E-to-Z photoisomerisation results in the orientational transformation of the nematic liquid crystal layer from homeotropic to homogeneous alignment. It has been found that the minimum exposure energy of polarised UV light required for the liquid crystal alignment photocontrol is determined by the surface density of the azobenzene. There is a critical density; no photoresponse is observed for average densities of the azobenzene smaller than ca. 0.7 molecules nm^–2 . With densities larger than this value, the smaller the density of the chromophore on pAA surface is, the faster the photoreorientation of the liquid crystal effectively takes place.