Photoalignment of liquid crystals using a covalently attached self-assembled ultrathin film fabricated from diazoresin and cinnamate polyelectrolyte

Abstract

Layer-by-layer films have been fabricated from diazoresin (DR) and poly[4-sulfonyl phenoxy(4-acryloyloxy)cinnamate, sodium salt] (PACSS) in aqueous solution via electrostatic attraction. The ionic bonds between the diazonium ions and sulfonate ions in the film were converted into covalent bonds by irradiation with 365 nm UV light. It was also confirmed that the vinylene units of the cinnamoyl group in PACSS did not change in this process. Covalently attached self-assembled ultrathin films containing photosensitive cinnamoyl groups could align liquid crystals (LC) uniformly with good stability after irradiation with linearly polarized ultraviolet light (LPUVL). In this work, polarized UV-vis spectroscopy was utilized to investigate the photochemical process of the covalent film. It was found that cinnamoyl moieties parallel to the polarization direction of the LPUVL were consumed by the photoreaction faster than those perpendicular to the polarization direction. It can be concluded that the selective photoreaction induced the anisotropy of the films. Experimental results revealed that the degree of photoreaction and the dichroic ratio of the films depended on the number of adsorbed layers and the irradiation time. These results were correlated with the LC homogeneous alignment behavior in the parallel LC cell. The reorientation behavior of the LC molecules was found to be associated with the covalent film thickness and irradiation time.