PERIODIC PATTERN-FORMATION IN SIDE-GROUP POLYMER NEMATIC SOLUTIONS AT THE TWIST FREDERIKS TRANSITION

Periodic pattern formation at the Frederiks transition in the magnetic field has been studied for dilute solutions of a polymethacrylate with mesogenic side groups in a nematic solvent, pentylcyanobiphenyl (5CB). For this purpose, a combination of dielectric measurements and direct pattern observations was used to determine the twist elastic constant K-2, the rotational viscosity gamma(1), and the wavelength of the periodic distortion lambda. For polymer concentrations 0 less than or equal to omega less than or equal to 1% the values of lambda were found to follow a power law lambda proportional to (B - B-c)(-beta) with beta approximate to 1/2, where B-c is the critical field for formation of periodic patterns. A strong effect of the polymer molecules on B-c has been observed, due to the contribution of the macromolecules to the shear Miesowicz viscosity coefficient eta(a) being large compared to the effect on gamma(1). In a simple phenomenological model, which is constructed analogous to the Landau theory for second order phase transitions, and uses an expansion of the growth rate of the patterns in powers of the wavenumber kappa = 2 pi/lambda, we describe correctly the observed power law.