Enthalpy Relaxation of Liquid Crystalline Polymer with Cyanobiphenyl Group in the Side Chain: Analysis based on Configurational Entropy Model with the AG Expression

Abstract

The enthalpy relaxation of poly(cyanobipheny hexylacrylate) has been studied by differential scanning calorimetry. The experimental results of the heat capacity (c p ) were compared with the prediction of a phenomenological model for the process of structural relaxation based on the configurational entropy (S c ) and the relaxation time expressed by the Adam-Gibbs equation. This model follows the evolution of the configurational entropy of the sample during the whole thermal history including an isothermal annealing. The parameters of the model were calculated by simultaneous fits to the experimental results. With the calculation result, the peak position appeared in DSC curve of c p was interpreted using the plot of temperature dependence of S c . The plot showed that S c rapidly approached to the configurational entropy of equilibrium (S c eq ) in the narrow temperature range around T g , which is often referred to as the recovery of enthalpy, and thus the recovery of S c , in interpreting the enthalpy relaxation. As the measure of structural relaxation upon the isothermal ageing, the value of relaxation function () was considered, which estimates the departure from the equilibrium. The correlation between of S c and of enthalpy is also presented. The physical properties of the liquid crystalline polymer with cyanobiphenyl groups in the side chain (PCBA) have been studied with respect to the effects of molecular weight and spacer length.1,2 In the previous paper, the enthalpy relaxation measured using Differential Scanning Calorimetry (DSC) was analysed with the activation energy spectrum (AES) model. 3This model provides information about thermally activated processes which are available to contribute to observed changes in the properties upon physical ageing. For the enthalpy relaxation of PCBA6 (PCBA in which the spacer length is 6), the analysis gives the relation between the process number and the activation energy, in other words, the distribution of the relaxation process.In general, the decrease in enthalpy upon the isothermal ageing at temperature below the glass transition temperature T g , is closely related to the structural relaxation toward equilibrium in the glassy state. Concerning the relaxation of PCBA6, the optical texture was shown to vary in the previous paper, as an evidence of structural change. The change in optical texture is being considered as the coarsening dynamics of the disclinations. That is to say, the dynamics of PCBA6 molecules bring about the change in structure.On the other hand, the modelling approaches for enthalpy relaxation of polymer have been reported, which attempts to reproduce the measured DSC curve using quantities of state. The quantities of state employed are the configurational entropy, S c , and the fictive temperature, T f . 4,5 It has been shown that the main features of the relaxation process can be modelled on the basis of a distribution of relaxation times that depends both on the temperature and on the structure of the material represented by the value o...

Structural Relaxations in Liquid Crystalline Formation and Glass Transition for Comb‐Like Polymer: Activation Energy Spectrum Analysis

“…Because the structural relaxation is usually accompanied by dynamics in molecular scale, it attracts a lot of interest from the viewpoint of the combination between the structure and properties. The experimental results shown in this study are concerned with the variations in optical texture and heat capacity . Some relaxation models for these physical variations are applied to interpret in both molecular level and macroscopic level.…”

“…SEIKO DSC200 was used to obtain c p ( T ) data from the DSC curve of d q /d t vs. temperature. See references in detail for experiments and analyses …”

Time-temperature superposition in the enthalpy relaxation study of polystyrene

“…The heating scan was carried out at a rate of 5°C/min between 30°C and 160°C in order to measure the amount of enthalpy relaxed during isothermal aging, and thus that recovered during the scan. The experiment of t A = 0, i.e., that without the isothermal step of an oil bath before the scan, was conducted for the reference scan, the result of which was also used to calculate the heat capacity curve, c p ( T ), with reference to the heat capacity data of Indium of 200, 250, 298, 400, and 500K . Furthermore, the H – T relation given for discussion was calculated from the c p ( T ) data of t A = 0.…”

“…was used as the decay function to analyze the relaxation: where τ and β are the relaxation time and shape parameter, respectively. Other details are described in previous works .…”

Relaxations in relation to Liquid Crystalline Formation and Glass Transition for Comb-like Polymer: Structural Relaxation as expressed by Entropy Model and Activation Energy Spectrum

“…The experimental results shown in this study are concerned with significant variations in optical texture and heat capacities as the relaxation phenomena. (1) Detailed models with the configurational entropy for those variations are applied to interpret in both molecular level and macroscopic level.…”