Electric and magnetic field induced anisotropy in polymer films for liquid crystal alignment

Abstract

This study describes the development of new ways to introduce anisotropy in polymer films for nematic liquid crystal alignment using electric and magnetic fields. The methods used to achieve the anisotropies and the underlying mechanisms are presented. The liquid crystal alignment on these anisotropic surfaces is characterized and explained using various experiments. Anisotropy of a photopolymer film was obtained by poling the cinnamoyl side groups of polyvinylcinnamate near the glass transition temperature of the polymer, followed by photoreaction of the side group using unpolarized or linearly polarized UV light. The electric field induced anisotropy shows a strong anomaly at a temperature T$_c$ that lies about 10 degree below the polymer glass transition. This behavior is explained by assuming an antiferroelectric-like ordering of the side chains that show an electric field driven phase transition. The anisotropy of the poled film was stabilized via the photoreactions. The anisotropy induced by the combined poling and photoreaction using linearly polarized UV light was enhanced compared to that induced by the photoreaction only. Nematic liquid crystal alignment observed on such an electrically induced polar anisotropy shows a good stability and a pretilt angle. The alignment mechanism is explained based on the film structure consisting of unreacted cinnamates and the photoproducts that mainly consist out of b-truxinate. Cinnamates contribute mainly to a stable flow alignment, while b-truxinate is mostly responsible for the azimuthal anchoring and pretilt angle. The anisotropy of a capillary condensed film of nematic liquid crystals on a polymer surface was induced via aligning this film by a high magnetic field at elevated temperatures. The surface isotropic-nematic transition appears to be uniquely related to the specific substrate. The transition is clearly of first order on a glass substrate, while on polymer coated glass, it shows a mixed first a...