Influence of Liquid-Crystalline Interactions

We investigate light-induced deformation of azobenzene elastomers which can display liquid-crystalline (LC) order. It is shown that photo-mechanical behavior of azobenzene elastomers is determined by the strength of the LC-interactions which is proportional to the density of rod-like azobenzene chromophores. At weak LC-interactions, a uniaxial order and uniaxial deformation of azobenzene elastomers along the polarization vector of the light E is observed. At strong LC-interactions, the light is able to induce a phase transition from the uniaxial to the biaxial state, with two axes being related to the vector E and to a preferable alignment of the chromophores in the plane perpendicular to E. The phase transition can be either of the first or of the second order. Azobenzene elastomers can demonstrate elongation or contraction along the polarization vector E, depending on the orientation distribution of chromophores around the main chains of network strands. The results of the theory are in a qualitative agreement with experiments and computer simulations, which demonstrate biaxial ordering in azo-containing polymers.