Dynamic mechanical and dielectric relaxation in a series of main chain thermotropic liquid crystalline polyesters

Abstract

The effects of structural changes on the frequency dependent dynamic mechanical (d.m.a.) and dielectric relaxation behaviour in a series of wholly aromatic thermotropic main chain liquid crystalline (LC) polyesters were investigated. The polymers were side-group substituted poly(hydroquinone-terephthalates), modified with biphenols and 4-hydroxybenzoic acid. The side groups, R, were varied as R = methyl, phenyl, or t-butyl. Data were also obtained for Vectra A950. These LC polymers exhibit three prominent relaxation processes. Some of the LC polymers in this series are ‘non-crystalline’ with strong and narrow glass transitions which are shown to vary as much as 80°C depending on the choice of substitution of the hydroquinone group. Tertiary butyl substitution on the hydroquinone units was found to lead to the highest glass transition ( ca 185°C). Another common feature of most of these LCPs is a sharp but weak second ‘glass transition’ observed at lower temperatures than the main glass transition. This is attributed to motions of non-substituted aromatic ester species including 4-hydroxybenzoic acid (HBA) due to their relatively low barriers to rotation. Comparisons are made with the wholly aromatic, HBA-rich, LC polymer, Vectra®. Three of the LC polymers exhibit essentially the same subglass γ relaxation at ca 60°C (1Hz) which is attributed to local motion involving the non-substituted aromatic units such as HBA. Adding methyl side group substituents to the ortho position of the biphenol group restricts the subglass γ process in a kinetic sense, shifting the process to higher temperatures. The effect of substitution of the hydroquinone group on the γ relaxation was also systematically investigated.