SALS, WAXS and mechanical properties of heat‐treated thermotropic polymers

Abstract

The influence of heat treatment on the texture, microstructure and tensile mechanical properties of extruded thin films of a series of high-performance thermotropic liquid crystal polymers (LCPs) was investigated. LCPs based on random units of hydroxybenzoic acid (B), hydroxynaphthoic acid (N), terephthalic acid (TA) and biphenol (BP) were kindly supplied by the former Hoechst Celanese Corp as 50 μm thick extruded tapes. The LCPs, denoted B-N, COTBP and RD1000, have B and N as common comonomers and vary the other comonomers. Thus, this study also enables the investigation of the influence of monomer composition on microstructure and mechanical properties. Heat treatments were carried out at temperatures close to the solid-to-nematic transition (T s→n ) for periods up to 5 h, under dry air conditions. The thermal treatment produced either two endotherms or a small increase of T s→n (B-N and RD1000), or increased significantly T s→n (COTBP). Moreover, when heat treatment was carried out approximately 40°C below the respective T s→n , the mechanical Young's modulus, E, along the extrusion axis, increased for all LCPs. Strikingly, for COTBP, E increased over 100% relative to the as-extruded film. The results also showed that the optimum treatment time for improving the Young modulus, under dry air atmosphere, was between 3 and 4 h. Wide-angle X-ray scattering showed a significant sharpening of crystalline reflections and concentration of the 002 meridional reflection as a result of thermal treatment, suggesting the elimination of defects and a better alignment of the molecular chains along the extrusion axis. This would explain the increase in tensile modulus.

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

“…Previously, it was shown that thermal treatment produced secondary endotherms and an initial increase of melting (solid‐to‐nematic, T s→n ) temperature, in agreement with other studies . Thermogravimetric analysis was also carried out to determine the influence of thermal treatment on thermal stability of B–N; the results are shown in Fig.…”

“…Uniaxial tensile testing carried out at room temperature showed that the elastic Young's modulus of B–N increased as a function of heat treatment time, thus correlating with increase of degree of crystallinity. Here, we further investigated the consequences of heat treatment by carrying out microindentation testing to determine the microhardness, H , of B–N copolyester.…”

“…Indentation anisotropy, ∆ H , as defined by was determined from the individual microhardness values, and the results are correlated with radius of gyration ratio (obtained from SAXS), degree of crystallinity (obtained from WAXS), and elastic Young's modulus normalized by the maximum value attained after 5 hr (92 GPa). The combined plot is shown in Fig.…”

“…Preliminary wide‐angle X‐ray scattering results also showed an increase of degree of crystallinity and crystal size, in agreement with Saw et al study on fibers . These molecular scale changes correlated with an increment of tensile modulus …”

“…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.…”