EFFECTS OF CHAIN-LENGTH AND ITS DISTRIBUTION ON THERMOTROPIC MESOPHASE STRUCTURE OF TRI-O-HEPTYL CELLULOSE

A relatively low-molecular weight tri-O-heptyl cellulose was prepared and fractionated by use of GPC to obtain several fractions of fairly narrow distribution. The fractions and the original (unfractionated) sample were examined with their thermotropic liquid crystalline properties. They were found to form a cholesteric mesophase and undergo a thermally induced inversion of helicoidal sense at a characteristic temperature T(N). This phenomenon can be understood as a continuous change with temperature of the twisting angle between the adjacent pseudonematic layers in the cholesteric structure. Both T(N) and the isotropization temperature T(i) were found to be an increasing function of chain length, a phenomenon common to many cellulosic cholesterics. The chain length distribution has an important effect on the mesophase structure. It was suggested that T(N) and T(i) are better characterized by the z-average chain length than by the number-and weight-averages.