Abstract
In this paper, we demonstrate and thoroughly analyze the activation volumetric properties of selected liquid crystals in the nematic and crystalline E phases in comparison with those reported for glass-forming liquids. In the analysis, we have employed and evaluated two entropic models (based on either total or configurational entropies) to describe the longitudinal relaxation times of the liquid crystals in the density scaling regime. In this study, we have also exploited two equations of state: volumetric and activation volumetric ones. As a result, we have established that the activation volumetric properties of the selected liquid crystals are quite opposite to such typical properties of glass-forming materials, i.e., the activation volume decreases and the isothermal bulk modulus increases when a liquid crystal is isothermally compressed. Using the model based on the configurational entropy, we suggest that the increasing pressure dependences of the activation volume in isothermal conditions and the negative curvature of the pressure dependences of isothermal longitudinal relaxation times can be related to the formation of antiparallel doublets in the examined liquid crystals. A similar pressure effect on relaxation dynamics may be also observed for other material groups in case of systems, the molecules of which form some supramolecular structures.
Highlights
Using the model based on the configurational entropy, we suggest that the increasing pressure dependences of the activation volume in isothermal conditions and the negative curvature of the pressure dependences of isothermal longitudinal relaxation times can be related to the formation of antiparallel doublets in the examined liquid crystals
It should be added that the compounds with the CN group show a tendency to forming antiparallel doublets in the Liquid crystalline (LC) phases50 The experimental dielectric and pVT data examined by us have been earlier measured and reported for the nematic phase of 7CB51,52, 7PCH53,54, 8PCH55,56, and 8OCB18,57 as well as the crystalline E phase of 6BT58,59 and 8BT60,61
From the inspection of the isothermal pressure dependences of the longitudinal dielectric relaxation times τ|| of the LC systems subjected to compression, one can spot an intriguing behavior of the isothermal dependences τ|| (p), which increase with increasing pressure as those established for the glass forming (GF) systems but reveal an untypical curvature that is negative in contrast to the positive one commonly observed for the GF systems
Summary
Taking into account the fundamentals of the MYEGA model32,23 (which considers the network constraints using a two-state pattern according to that they are either intact or broken with the energy difference represented by the parameter AM), we can suggest that the negative values of this parameter obtained for the investigated LC systems can be related to forming antiparallel doublets in these materials as already mentioned.
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