Optical anisotropy and orientational elastic constants of polymer high temperature and reentrant nematics

Orientational elastic deformations of polymer high temperature and reentrant nematics in a magnetic field were studied using the method of Freedericksz transitions. Experiments were performed on series of fractions (molecular weight M from 16000 to 91000) of a comblike polymer. The temperature dependences of orientational elastic constants K-1 and K-3 in the nematic and reentrant nematic phases and birefringence Delta n over the entire range of existence of liquid-crystalline phases were studied. It was shown that at phase transition points N --> S-A, S-A --> N-Re and N-Re --> S-F the optical anisotropy Delta n and, correspondingly, the orientational order parameter S do not undergo a pronounced jump. The study of mechanical equilibrium characteristics did not reveal any differences in the behaviour of high temperature and reentrant nematic phases of the polymer. For the lowest molecular weight fraction, an anomalous "bell-shaped" temperature dependence of the bend constant K-3 was found. This dependence can be explained by the formation of small domains with smectic ordering in the nematic phase. In the molecular weight range investigated, both optical anisotropy and equilibrium mechanical characteristics are independed of M.