Dynamic mechanical and thermal behavior of liquid-crystalline polybutadiene-diols with mesogenic groups in side chains

Liquid-crystalline polybutadiene-diols (LCPBDs) with the comb-like architecture were synthesized by reaction of a LC thiol with the double bonds of telechelic HO-terminated polybutadiene (PBD). LCPBDs with various initial molar ratios of thiol to double bonds of PBD, R0, in the range from 0.15 to 1, were prepared by the radical reaction at temperature 60°C for 48h. The experimentally obtained degree of modification, Re, after the reaction and purification, was determined from elemental analysis – from the amount of sulphur bounded in LCPBDs, GPC and from 1H NMR spectra. The physical properties were investigated by differential scanning calorimetry and dynamic mechanical spectroscopy. With increasing Re ratio the glass transition temperature of LCPBDs, Tg, increases from ∼−45°C (neat PBD) to ∼20°C (Re∼0.5). LC transition starts at Re∼0.27 (the transition temperature Tm∼27°C). With increasing Re temperature Tm increases and for Re∼0.5 reaches the value Tm∼74°C; at the same time also the change in enthalpy at LC transition increases. The LC transition could be detected also by the dynamic mechanical spectroscopy; especially shape and position of mechanical functions on frequency and free volume parameters strongly depend on degree of modification.