Application of processes of photoisomerization in the alignment of liquid crystals

Author(s)

Leila Thomazelli Thieghi

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto de Física (IF/SBI)
• Defense date: 2003-02-13

Examining board members

Elisabeth Andreoli de Oliveira; Luiz Roberto Evangelista; Yoshio Kawano; Osvaldo Novais de Oliveira Junior; Helena Maria Petrilli

• Advisor: Elisabeth Andreoli de Oliveira; Jean Jacques Bonvent

Abstract

The aim of this work is to study the superficial interactions of liquid crystals in an interface liquid Crystal-polymer, with the use of polymer films containing azo-dyes groups as side chain. In this study a photopolymer was used, where the ordering of the side group can be promoted along a preferential direction through a trans-cis isomerization process. Through the irradiation of the polymeric film with polarized light it is possible to promote the formation of an optical anisotropy in the film. The intensity of this effect can be controlled through the incident energy and the concentration of the azo-dye side groups presents in the polymer. In this work a systematic study of the orientation induced to the nematic liquid crystal by a polymeric film containing azo-dye groups in the side group was performed. The investigated aspects are: -The thermal and temporary stability of the alignment induced to the liquid crystal; -The competition among two orientation effects induced to the liquid crystal, the rubbing and the photo alignment; -The influence of the concentration of the side groups and of the irradiation energy. To characterize the interactions between the liquid crystal and the polymeric film we used optical techniques that allowed the determination of the azimuthal and zenithal anchoring strengths as a function of the concentration of side groups and of the irradiation energy. We showed to be possible an azimuthal anchoring that can have its intensity controlled by the concentration of the side groups, and the irradiation time with polarized light, or still for the power of the excitation laser, and a zenithal anchoring quite weak, that practically is not affected by the process of photo isomerization. (AU)