NONEQUILIBRIUM THERMAL-BEHAVIOR OF 2 MAIN-CHAIN THERMOTROPIC POLYMERS

The effect of thermal treatment on the thermodynamic properties and mo rphology of two nematic, thermotropic main-chain polymers with mesogen ic groups containing 3,3'-biphenylene units and flexible spacers of di fferent lengths has been studied by differential scanning calorimetry and X-ray scattering. An appreciable difference in the thermal behavio ur 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 t he isotropic state, whereas BF8 samples did not crystallize even at ve ry slow cooling rates and possessed a glassy, nematic structure at roo m temperature. The enthalpy of the nematic-isotropic transition and th e increase in the specific heat capacity at the glass transition for B F8 samples were greatly influenced by the rate used to cool the isotro pic melt. This result is tentatively explained in terms of an improvem ent of the nematic ordering during cooling.