Effects of photo-isomerizable side groups on the phase and mechanical properties of main-chain nematic elastomers

Abstract

The polymer design of nematic liquid crystal elastomers is critical for controlling its nematic order and responses to stimuli such as heat and light. The nematic order is closely associated with thermal phase properties and unique mechanical properties, such as soft elasticity and viscoelasticity. Furthermore, it affects the actuator performance. Here, we focus on the effects that relatively short side groups branched from the main chain of nematic elastomers have on the nematic order. As side groups, a series of monomers were synthesized and tethered to the main chains via azobenzene moieties. Light irradiation was used to explore the effects of the tethered angles on the nematic states after photo-isomerization of the azobenzenes. Their phase and mechanical properties showed that the side groups in the trans azobenzene form lower the nematic order in comparison to that of the chains without side groups. This lowering of the nematic order depends on the length and flexibility of the side groups in a complex manner. After ultraviolet (UV) irradiation, the cis-rich states tended to slightly lower the nematic order. It was also inferred that the trans-to-cis isomerization is suppressed when the side groups are rigid. The results serve as a benchmark for tuning the basic properties of main-chain nematic elastomers and their photoresponses through the design of polymer networks.