Non-equilibrium excess order in the isotropic state of main-chain liquid-crystal-forming polymers

A new non-equilibrium phenomenon has been recognized in main-chain liquid-crystal polymers. Persistence of non-equilibrium residual short-range nematic-like order can be detected in the isotropic phase up to several minutes after the nematic-to-isotropic transition has occurred. The effect was observed by differential scanning calorimetry and is closely related to the pronounced non-equilibrium effects in the nematic state. As we reported previously, annealing in the nematic phase, on the timescale of hours, is required to bring the temperature and enthalpy of isotropization up to its equilibrium value. Now we find that even in the isotropic state annealing is required in order to achieve thermodynamic equilibrium; only upon holding above Ti is the residual excess nematic-type order fully removed, and thus all memory of previous annealings in the nematic state erased. The non-equilibrium effects are particularly pronounced for higher-molecular-weight polymers. In the case of the α-methylstilbene polyether with alkylene spacers, M̄n = 31 000, the time constant of the exponential approach to equilibrium is found to be 2.1 min at T = Ti + 10°C.