3D Orientational Order in a Homologous Series of LC Polyesters With Azobenzene Side Groups and Different Lengths of the Alkylene Spacer in the Main Chain

Abstract

The spatial orientational order of films of a series of LC polymalonates with 4-hexyloxy-4′-nitroazobenzene side groups which differ in the length of the alkylene spacer in the main chain are studied using null ellipsometry and UV/Vis spectroscopy. In the spin-coated films the azobenzene fragments strongly prefer a random in-plane alignment. Uniaxial as well as biaxial order of the azobenzene chromophore were detected in these films irradiating with linearly polarized UV light. Biaxiality is observed in the intermediate stages of irradiation whereas an uniaxial order is realized in the saturated state. Finally, it results in an in-plane structure with its optic axis perpendicular to the electric field vector of the incident light. The components of the tensor of the order parameter were estimated for each homologue as a function of the exposure dose. The scalar order parameters of the photosaturated states decrease monotonously with the increasing length of the alkylene spacer of the LC polymers. Variation of the azobenzene concentration, modification of the liquid crystallinity and peculiarities of the aggregation with the spacer length are considered as the reasons of the effect.

Key Words:

• LC polymers
• azobenzene
• photoisomerization
• photoorientation
• photoinduced anisotropy
• aggregation