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 Tg 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. [Display omitted] •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 Tg and elastic modulus E’.•changes of activation energy appear a condition to enable molecular rearrangements and thermo-mechanical reinforcement.