The molecular weight effect on the mesomorphic behaviour of liquid‐crystalline poly(ester‐β‐sulfide)s

Abstract

AbstractA DSC study is reported on the dependence of thermal properties on molecular weight in two series of thermotropic liquid‐crystalline poly(ester‐β‐sulfide)s, 9S4 and 9S5. The molecular weights range from approximately 1 600 to 14 000. All of the polymers exhibit a nematic phase preceded, at lower temperature, by a smectic A phase which is monotropic in the majority of the cases. The nematic‐isotropic transition temperatures and relevant entropy changes were found to be practically insensitive to molecular weight variations. In contrast, the smectic A‐nematic transition temperatures and entropy changes increase with increasing molecular weight and reach a plateau value at an average chain length of 6–7 repeating units. It is suggested that in the nematic phase of these polymers the flexible segment plays a role of diluent of the rigid groups, whereas at the nematic‐smectic A transition a cooperative increase in chain stiffness takes place.