Dynamic mechanical analysis of thermotropic copolyester—short glass fiber composites

Abstract

AbstractThe influence of short glass fibers (SGF) on the dynamic mechanical properties and microstructure of thermotropic copolyesters based on 1,4‐hydroxybenzoic acid and 2,6‐hydroxynaphthoic acid (denoted B‐N) has been investigated. The mechanical relaxations of the reinforced B‐N/SGF composites were studied under torsion mode and the results were correlated with the degree of molecular alignment and degree of crystallinity as revealed by wide‐angle X‐ray scattering (WAXS). Addition of SGFs to B‐N liquid crystalline polymer (LCP) produced an increase in the solid‐to‐nematic transition temperature as well as an increase in the decomposition temperatures. Dynamic mechanical analysis (DMA) showed that the local motions associated with the β‐transition (∼61°C) are enhanced as the frequency of oscillation increases, i.e. the tan δ maximum increases at higher frequencies. On the other hand, the cooperative motions associated with the α‐transition (∼100°C) are decreased at higher frequencies. The addition of SGF reduced significantly the strength of the α‐transition. Thus, the cooperative molecular motions involving segments of the molecular chains associated with the α‐transition were compromised by the presence of SGFs resulting in an increase of the storage modulus, melting, and degradation temperatures. X‐ray scattering showed that the SGFs disrupted the degree of molecular alignment (as quantified by the order parameter $\bar P_2 $), $\bar P_2 $, and the degree of crystallinity in the composites decreased as the concentration of SGFs increased. Thus, it is suggested that the increase in storage modulus in the composites is attributed to the contribution of the SGFs. Copyright © 2008 John Wiley & Sons, Ltd.