INFLUENCE OF POLYMER-MOLECULES ON THE ROTATIONAL VISCOSITY AND BACKFLOW EFFECT AT THE BEND FREEDERIKS TRANSITION IN NEMATIC LIQUID-CRYSTALS

Reorientation of the nematic director at the bend Freederiks transitio n has been studied for dilute solutions of side-chain mesogenic polyme thacrylates in a low molecular weight nematic liquid crystal, pentylcy anobiphenyl (5CB). The temperature dependences of the pure twist visco sity coefficient gamma(1) and bend elastic constant K-3 were obtained for nematic solutions containing polymers with different degrees of po lymerization (N = 34, 88, and 312). The increment of twist viscosity d elta gamma(1) increases with N according to the power law delta gamma( 1) proportional to N-alpha with an exponent 0.67 < alpha < 0.87 for te mperatures of solutions T = 23-33 degrees C. This result corresponds t o nondraining chains in a good solvent. The study of the dynamics of b end deformation shows a more rapid increase in the shear Miesowicz vis cosity coefficient eta(c) in comparison to the twist viscosity coeffic ient gamma 1 with increasing polymer concentration. This observation w as supplemented by measurements of the ratio gamma(1)/(eta c) over a w ide range of the degree of polymerization N. This ratio was found to d epend on N according to the power law eta(c)/gamma(1) proportional to N-0.162.