Dynamic Measurements on Polymer Liquid Crystals

II. Thermotropic Mesophase of Hydroxypropyl Cellulose

Abstract

The sample of hydroxypropyl cellulose used in this study forms a mesophase at temperatures from about 130°C to about 175°C. The microscopic examination shows that the isotropic phase begins to appear at about 175°C and grows with temperature to become prominent at 190°C. At about 225°C the sample becomes optically isotropic. The DSC curve shows a very broad endothermic area spreading from 130°to 250°C having shallow bottoms at about 175°C and 225°C. The former may be related to the first appearance of isotropic phase and the latter to the transition to the completely isotropic state. In connection with change in state, the steady-flow viscosity decreases with temperature until a minimum is reached at 173°C and then tends to increase to make a peak at 190°C. In the frequency region from 10^-2 to 10² radian·sec^-1, the thermotropic mesophase at 175°C exhibits very; high storage and loss shear moduli of the order of magnitude of 10⁵-10⁶ Pa, in contrast with the magnitude of 10-10³ Pa of the lyotropic mesophase. The logarithmic plot of storage shear modulus against frequency gives a flat curve with a small slope like the plateau zone of rubberlike consistency. The dynamic data of the thermotropic mesophase of hydroxypropyl cellulose at 175°C resemble in dimension the dynamic data at the plateau zone of lightly cross-linked amorphous polymers such as lightly vulcanized Hevea rubber at 25°C. At 211°C the dynamic shear moduli are reduced to one-hundredth of the values at 175°C, indicating that the network is strongly loosened by the preponderance of isotropic phase. At 250°C the dynamic properties change radically and exhibit the characteristics of isotropic liquids. The effect of temperature on the dynamic properties is interpreted in terms of the resistance of network to the oscillatory deformation at different temperatures.