Synchrotron scattering and thermo-mechanical properties of high performance thermotropic polymer. A multi-scale analysis and structure-property correlation

A multi-scale analysis, at the μm → nm → Å-scale on the influence of thermal treatment on the microstructure of the thermotropic polymer based on 60 mol% (1,4)-hydroxybenzoic acid (B), 5 mol% (2,6)-hydroxynaphthoic acid (N), 17.5 mol% terephthalic acid (T) and 17.5 mol% biphenol (BP) -named COTBP- was carried out. Extruded tapes 30 μm thick were heat treated at 300 °C for up to 300 min. Wide-angle X-ray scattering of as-extruded tapes revealed high macromolecular alignment. Small-angle X-ray scattering exhibited diamond-shaped diffuse scattering along the equatorial axis and, strikingly, meridional scattering revealed long-range order of ca. 43.9 nm periodicity. Heat treatment reduced surface roughness, increased the birefringence, and sharpened the meridional and main interchain reflections indicating an increase of crystallinity, molecular register and alignment. On the other hand, SAXS intensity decreased in 2θ and became azimuthally narrower evidencing lateral compression of scattering features. The width of scattering objects decreased from 8.7 to 6.6 nm, whereas their length increased from 20.9 to 22.8 nm, and the misorientation angle B_ϕ decreased from 2.1° to 0.5°. Because heat treatment was carried out without applied tension, the results also revealed a self-reinforcing effect. The structural re-organization correlated with significant increase of thermal properties, i.e. the changes relative to as-extruded tape of degradation and melting temperatures were ΔT_dec∼20 K, ΔT_s→ñ75 K, and the change of Young's modulus was ΔE∼50 GPa.