The Dynamics of Freedericksz Transitions in Nematic Solutions of Mesogenic Polymers

Abstract

The dynamics of Freedericksz transitions for solutions of polymethacrylate with mesogenic side groups in a low-molecular-mass nematic liquid crystal cyanobiphenyl was examined. Longitudinal bend and twist Freedericksz transitions were considered. It was found that, in both cases, polymer chains have a damping effect on the backflow that arises when the director rotates in the magnetic field. The suppression of backflow is explained by the fact that the contribution of macromolecules to shear viscosity is greater than that to the coefficient of rotational viscosity. It was shown that the backflow effect is the greater, the higher the molecular mass of polymer. For the twist Freedericksz transition, periodic structures, which are observed during the transition in high fields, were examined. Experimental plots of the wavelength of periodic distortion versus field intensity were perfectly described by the exponential λ = λ₀(h - h_c)^-β, where h_c is the critical field intensity separating the uniform and periodic regimes of Freedericksz transition, and β = 1/2. The presence of polymer chains in solution resulted in a dramatic increase in h_c, that is, the macromolecules stabilized the uniform regime of Freedericksz transition. Phenomenological model that explains the experimentally observed lambda versus h plots was suggested. The model rests on the analogy between the transition from periodic regime to the uniform regime and equilibrium second-order phase transitions.