Fracture behavior of heat treated liquid crystalline polymers

Abstract

Thermotropic polymers are thermally treated in air at temperatures T a , where T =T a -T s n =40°C, and T s n is the solid-to-nematic transition. Samples are extruded thin films of a series of thermotropic random copolyesters termed B-N, COTBP and RD1000. The thermal treatment produces a second endotherm without changing T s n for B-N and RD1000. However, for COTBP T s n is significantly increased. Regardless of the complex thermal behavior exhibited by the thermotropes, the thermal treatment produces a significant increase in Young's modulus, more than 30% for B-N and over 100% for COTBP. The increase in mechanical modulus is correlated with a thermally-induced fiber-like morphology.

Nanostructure reorganization in a thermotropic copolyester. A simultaneous WAXS and SAXS study

“…Recently, we have reported on the thermal transitions in heat‐treated extruded tapes of several thermotropic LCPs, including B–N, when heat treatment was carried out, some 40°C below the respective T s→n . The production of secondary endotherms was a predominant feature in extruded tapes of B–N, confirming reports by Cheng and Chung et al .…”

“…In addition to the synthesis of LCPs, emphasis has been placed on the fundamental understanding of the origin and modification of physical and thermal properties and the analysis of the corresponding structure and property relationship during post‐treatments . The thermal treatment in polymeric fibers is a vital factor to stabilize the molecular chains and improve mechanical (tenacity and modulus) and chemical properties.…”

“…It has been reported for thermotropic LCPs that thermal treatment near the solid‐to‐nematic transition temperature ( T s→n ) (typically 20–40°C below T s→n , for a prolonged time in an inert atmosphere) leads to increase in molecular weight as well as to an increase in the degree of crystallinity. Therefore, chemical resistance, melting point, thermal retention of tensile properties and end‐use temperature also increase . That is, by careful manipulation of thermal treatment processes, the structure/morphology of thermotropic LCPs can be tuned.…”