On the influence of thermal annealing on molecular relaxations and structure in thermotropic liquid crystalline polymer

The influence of thermal annealing on mechanical relaxations and microstructure of a high performance thermotropic copolyester was investigated using dynamic mechanical analysis. The copolyester consisted of random units of 73 mol % 1,4-hydroxybenzoic acid (B) and 27 mol % 2,6-hydroxynaphthoic acid (N), denoted B-N 73:27. As-extruded tapes exhibit a mechanical relaxation at ca. 105 °C denoted α and associated to cooperative molecular motions akin to the glass transition, and a mechanical relaxation ca. 60 °C denoted β and associated to motions of the N moiety. Thermal annealing at 240 °C (well above the glass transition) and without tension, enabled molecular rearrangements afforded by an initial reduction and then a steady increase of the activation energy of the α and β relaxations. Then, at the bulk level, the glass transition temperature T g first decreased and then increased during annealing. The elastic tensile modulus E’ was an increasing function of annealing time and the temperature before mechanical failure increased from 200 to 240 °C. Regarding the microstructure, thermal annealing induced a phase transformation from pseudo-hexagonal to orthorhombic phase. Furthermore, the degree of crystallinity (and enthalpy of fusion) increased, the nanovoid width decreased and the correlation of B or N repeat unit matching was reduced as reflected by the broadening of the 002 meridional reflection. The initial reduction of activation energy appears a condition to enable molecular rearrangements leading to the eventual thermo-mechanical reinforcement. Thus, understanding of molecular rearrangements during thermal annealing enables tuning the physical properties by controlling the microstructure of high performance liquid crystal polymers. • The thermal annealing, relaxations and microstructure of a thermotropic polymer were investigated. • Annealing changed cooperative and local relaxations and induced Å- and nm-scale structural changes. • Then, annealing induced higher crystallinity, and increase of T g and elastic modulus E’. • changes of activation energy appear a condition to enable molecular rearrangements and thermo-mechanical reinforcement.