Nonmonotonic behavior of the nematic tilt angle in a temperature-induced surface transition

We report on a nonmonotonic temperature behavior of the pretilt in cells containing polymers as alignment layers. The latter are very promising since they enable control of the anchoring of the liquid crystal. We show that such a behavior cannot be interpreted by the standard Landau or mean-field theory. We propose a generalization of the mean-field model, including the temperature dependence of the anchoring due to the polymer layer that enables a description of the nonlinear behavior. The agreement between the predictions of the model and the experimental data is good.

Figure 1

Temperature dependence of the tilt angle ${\varphi }^{\ast }$ in a nematic cell oriented by a rubbed polymer. The nonmonotonic trend of ${\varphi }^{\ast }$ vs $T$ is connected with the presence of an orienting effect of the polymer on the liquid crystal depending on the degree of order of the polymer itself. The term in the surface energy related to this effect has been chosen proportional to the scalar formed with the tensor order parameters of the two ordered media in contact. (a) Liquid crystal MJ05963 in contact with the polymer P1. (b) Liquid crystal MJ05963 in contact with the polymer P2 with more mesogenic side groups.