Partially ordered glass-forming phases – relaxation processes and cold crystallization

Abstract

This review paper concerns the dielectric, calorimetric and optical studies of the chosen liquid crystal materials exhibiting the new class of the partially ordered glasses, where the partial order of the molecules in the glass-forming mesophases survives vitrification. Features of dynamics of the polar molecules observed on approaching the glass transition have been analysed: In the liquid-like glass-forming phases of the nematic, cholesteric and ferroelectric smectic order of molecules the super-Arrhenius temperature dependence of the structural α-relaxation observed in isotropic liquids has been confirmed. On the contrary, in the glass-forming solid-like smectic phases and the orientationally and conformationally disordered (ODIC and CONDIS) crystal phases the Arrhenius behaviour of the molecular dynamics has been found. In the most of the glass phases the Arrhenius secondary relaxation related to the librations/intra molecular motions has been evidenced. The kinetics of the isothermal and non-isothermal cold crystallization processes going on after de-vitrification (softening) of the partially ordered glass has been successfully analysed basing on the Avrami and the Ozawa models. The correlation between dynamic and thermodynamic features has been successfully checked for several compounds. In addition, the influence of the partial order of molecules on the phase polymorphism and molecular dynamics is discussed for three chosen liquid crystals under confinement. The findings enhance our understanding of nucleation/vitrification and molecular mobility in anisotropic soft matter systems, what is important for ensuring the thermal and structural stability of functional liquid crystalline materials.