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

Periodic pattern formation at the Frederiks transition in the magnetic field has been studied for dilute solutions of a polymethacrylate wit h mesogenic side groups in a nematic solvent, pentylcyanobiphenyl (5CB ). For this purpose, a combination of dielectric measurements and dire ct pattern observations was used to determine the twist elastic consta nt K-2, the rotational viscosity gamma(1), and the wavelength of the p eriodic distortion lambda. For polymer concentrations 0 less than or e qual 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 be ta approximate to 1/2, where B-c is the critical field for formation o f periodic patterns. A strong effect of the polymer molecules on B-c h as 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 c onstructed analogous to the Landau theory for second order phase trans itions, and uses an expansion of the growth rate of the patterns in po wers of the wavenumber kappa = 2 pi/lambda, we describe correctly the observed power law.