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Thermodynamic Aspects of Phase Stability
Published in Mary Anne White, Physical Properties of Materials, 2018
An orientationally disordered solid also has been referred to as a plastic crystal because of the ease of deformation of this type of material due to the rotating molecules that make it up. The term “orientationally disordered solid” is now preferred over “plastic crystal,” as the latter is sometimes too limiting, and the former term accurately describes the physical picture.
Crystallization kinetics in the chiral compound showing the vitrification of the smectic CA* phase for moderate cooling rates
Published in Phase Transitions, 2023
Aleksandra Deptuch, Ewa Juszyńska-Gałązka, Małgorzata Jasiurkowska-Delaporte, Anna Drzewicz, Marcin Piwowarczyk, Magdalena Urbańska
Liquid crystals are phases where the long-range orientational order is present while the positional order is lowered in comparison with the solid crystals. In a nematic phase, the positional order is only short-range, as in liquids. In the smectic phases, molecules form layers, which is a quasi-long-range order in one dimension, and within the smectic layers the positional order is short-range [1,2]. There are also smectic phases with a long-range positional order within layers, where the molecules have still a possibility to reorient around their long and short axes. Such ordered smectic phases are closer to plastic crystal phases than to liquid crystals [1,3,4]. The intermediate properties of liquid crystals between these isotropic liquid and a solid crystal, and their sensitivity to an electric field and temperature enable various applications, e.g. in liquid crystal displays, in thermometers or for preparation of glasses with anisotropic properties [5,6].
Mesomorphic behaviour and vibrational dynamics of nCFPB liquid crystalline homologues
Published in Phase Transitions, 2019
E. Juszyńska-Gałązka, W. Zając
Comparing the calorimetric results for 4CFPB, 6CFPB and 8CFPB one can easily notice that there is no cold crystallization in 6CFPB during heating, while the cold crystallization was found for 4CFPB and 8CFPB compounds. 6CFPB and 8CFPB both have glass of crystal phase, while 4CFPB exhibits glass of nematic phase. In contrast to 4CFPB and 8CFPB, the 6CFPB compound has one plastic crystal phase. 8CFPB assumes four crystal phases during cooling and three crystal phases during heating with some kind of disorder of molecules. On the other hand, 4CFPB assumes two crystal phases of CONDIS type during heating. A multitude of crystal phases with some kind of disorder of molecules in 4CFPB or 8CFPB can be a reason why supercooled nematic or crystal phase, respectively, transforms to a more order crystal phase during heating (cold crystallization). The occurrence of the vitrification process of the crystal or nematic phases and the cold crystallization should be carefully checked for the whole homologous series of nCFPB.
From idealised to predictive models of liquid crystals
Published in Liquid Crystals, 2018
Choosing a lattice model means treating a liquid crystal as a sort of plastic crystal, where molecular positions are restricted to a lattice and only orientations change, a choice that may seem crazy at the outset, looking at how very different this is from real liquid crystals, where fluidity is a defining feature. The solution of the paradox is that, when choosing such a lattice model, the aim is not to try and reproduce all of the properties of a real liquid crystal, but rather its orientational ones alone, particularly around the nematic-isotropic transition, where the behaviour is expected not to depend too much on molecular details. Some of the universal characteristics of the nematic-isotropic phase transition are: being of weak first order, with small entropy of transition and small volume change, both roughly 10% of the solid-isotropic liquid one [15], with an orientational scalar order parameter (with the molecule axis and the director) which at the isotropic transition is is just the most important example, of rank of a complete set of order parameters defined as Legendre polynomial averages [10]. The temperature variation of the order parameter in reduced units () is also, to a first approximation, quite similar for different nematic, if not universal. The NI transition also shows strong pretransitional effects in Kerr or Cotton-Mouton experiments [16,17], similar to those expected from a second order transition, diverging at a temperature slightly below , although differences exist for different materials [18], so that the phenomenon does not appear to be truly universal.