Non-equilibrium thermal behaviour of two main-chain thermotropic polymers

The effect of thermal treatment on the thermodynamic properties and morphology of two nematic, thermotropic main-chain polymers with mesogenic groups containing, 3,3′-biphenylene units and flexible spacers of different lengths has been studied by differential scanning calorimetry and X-ray scattering. An appreciable difference in the thermal behaviour of polymer samples with hexamethylene (BF6) and octamethylene (BF8) spacers was observed, despite identical thermal treatments. We found that BF6 samples had a crystalline structure after slow cooling from the isotropic state, whereas BF8 samples did not crystallize even at very slow cooling rates and possessed a glassy, nematic structure at room temperature. The enthalpy of the nematic-isotropic transition and the increase in the specific heat capacity at the glass transition for BF8 samples were greatly influenced by the rate used to cool the isotropic melt. This result is tentatively explained in terms of an improvement of the nematic ordering during cooling.