ON THE INFLUENCE OF THE FRANK ELASTICITY ON THE MAGNETIC REORIENTATION OF NEMATIC POLYMERS

We study the influence of the anisotropy of the Frank elastic constant s on the magnetic reorientation of the nematic phase of polymer liquid crystals. In the magnetic reorientation following a 90 degrees direct or rotation with respect to an aligning magnetic held, a pattern of in version walls develops which depends on the relative magnitude of the elastic constants and the magnetic coherence length. We show how this dependence can be experimentally studied by proton NMR. The transition from a homogeneous director reorientation to a distorted director reo rientation is theoretically studied as a function of the rotation angl e alpha. A critical angle of rotation alpha(c) shows up, and we study its dependence on the anisotropies K-3/K-1 and K-3/K-2. Depending on t hese ratios and on the wavelength of the distortion, critical angles 4 5 degrees < alpha(c) < 90 degrees are predicted for materials with pos itive anisotropy of the magnetic susceptibility chi(a) and 0 degrees < alpha(c) < 45 degrees (mod pi/2) for materials with chi(a) < 0. Withi n the frame of a phase transition analogy, a Landau-like theory predic ts the transition to be second order.