Photoinduced Deformation And The Stability Of Nematic Liquid Crystal Elastomers

Nematic liquid crystal elastomers are kinds of new materials which incorporate the elasticity of rubbers and anisotropy of liquid crystals. Monodomain nematic elastomers has uniaxial orientation and can suffer a deformation as mush as 400%. When added with photosensitive molecules, such as azobenzene molecules, the nematic orientation order will be disrupted due to the photoisomerization of azobenzene molecules which result the phase transition from trans-state to cis-state. In the viewpoint of macroscopic, the phase transition will cause the spontaneous contraction along the nematic axis, and expansion in other two directions perpendicular to the nemaic axis. Based on the optical-mechanical effects, this thesis studied the photoinduced deformation under different conditions and checked the stability of the elastomers. With different illumination patterns (Striped, annular and interference pattern) and different nematic orientation, we give out the variation properties of photoinduced surface topography of nematic by Green Function approach. We performed the surface profiles after deformation with numerical computation, and we find out that the properties of photoinduced surface profiles greatly depend on the the shape of illumination and the nematic orientation. Furthermore, when studied the case of interference illumination, a surface relief grating appeared in the surface of the elastomer. We can easily manipulate the intensity of the grating by controlling the incident angle of the interference lights without contacting the sample. In the further work, we study the photoinduced deformation of films of nematic elastomers with finite thickness. For simplicity, we assume the films floating on a certain liquid, and only the normal forces on the interface are considered. When the films are correspondingly thin, the whole films deformed strongly. With the increasing of the density of the liquid, the surface topography of grating has little change, but the deformation of the interface become smaller and smaller. When the films are quite thick, the influence of the liquid can be ignored and the main factor impacts the photoinduced deformation is the inhomogeneous elastic deformation. By analyzing, we find out this deformation is unstable. With the same normalized parameter a (related with photostrain), the smaller the thickness of the film and the density of the liquid, the easier of the surface relief grating to lose stability. When the films are quite thick (in the paper we denote the normalized thickness as 300), the photoinduced deformation is quite stable no matter how large of the density of the liquid.