Title: Simultaneous Analysis of Optical and Mechanical Properties of Cross-linked Azobenzene-containing Liquid-crystalline Polymer Films

Author: Aki Shimamura, Arri Priimagi, Jun-ichi Mamiya, Tomiki Ikeda*, Yanlei Yu, Christopher J. Barrett, Atsushi Shishido*

Journal: ACS Appl. Mater. Interfaces, 2011, 3(11), 4190-4196

Abstract:

The photomechanical behavior of cross-linked azobenzene-containing liquid-crystalline polymer films was investigated by means of simultaneous measurement of their optical and mechanical properties. The connection between photoisomerization of the azobenzene moieties, photoinduced change in molecular alignment, photoinduced stress generation, and macroscopic bending was analyzed. Upon UV irradiation, the films exhibited bending due to gradient in cis-azobenzene content, and subsequent unbending when cis-azobenzene content became uniform throughout the film. The maximum photoinduced stress was generated in the same time scale as the time required to reach photostationary state in the cis-azobenzene concentration. The maximum values of photogenerated stress strongly depended on the crosslinker concentration, even if the azobenzene concentration and the cis-azobenzene content in the photostationary state were similar for all the polymer films. The stress is connected to the initial Young’s modulus and also to the photoinduced change in birefringence of the polymer films. In addition, a significant photoinduced decrease in Young’s modulus was for the first time observed in cross-linked azobenzene-containing liquid-crystalline polymers, which is likely to be an important factor in dictating their photomechanical behavior.

Fulltext Link: https://pubs.acs.org/doi/abs/10.1021/am200621j

The interrelation between photoisomerization, photoinduced change in molecular alignment, photoinduced stress, and macroscopic bending in cross-linked azobenzene-containing liquid-crystalline polymers were investigated in the paper. We found that the production of cis-azobenzenes induces a change in birefringence, which is directly connected to the generation of stress within the films. Moreover, by studying samples of different crosslinker concentration, we point out that low Young’s modulus is favorable for macroscopic deformation, whereas high modulus promotes the generation of high photoinduced stress into the material system. Finally, we observed a significant decrease in Young’s modulus of cross-linked azobenzene-containing LC polymers upon UV irradiation, which we propose to be related to generation of “defects” (cis-isomers) into the homogeneously aligned cross-linked LC polymers.