Thermoanalytical study of the cure characteristics of a main-chain liquid crystalline oligoester: Model-free approach to kinetic analysis of non-isothermal data

Abstract

The curing behavior of a liquid crystalline oligoester (LCO), based on 4,4′-diacetoxybiphenyl and sebacic acid in presence of a mixture 75:25 of diglycidyl terephthalate and triglycidyl trimellitic acid ester (D&TGE), as a curing agent, was studied by non-isothermal differential scanning calorimetry (DSC). The analysis of non-isothermal data was carried out using the model-free isoconversional method. Activation energy as a function of extent of conversion, α, have been calculated by the Friedman technique. It shows a constant value of 95.5 ± 2.5 kJ/mol, in the 0.27 < α < 0.68 range. The activation energy, out of the above-mentioned interval, is strongly dependent on the degree of extent of the curing reaction. The y( α) and z( α) plots, which were obtained through transformation of DSC data, are not completely identical at a variety of heating rates. It indicates that the kinetic process dose not follow a single-step curing reaction model and a multi-step process controls it during curing. To approve the non-isothermal analysis result, we carried out further, depolarizing transmittance measurements and optical microscopy observations on the stoichiometric mixture of LCO/D&TGE during curing, at various heating rates. The results of these experiments exhibit that the kinetic process of the investigated system is strongly influenced by altering the mesomorphism features of the sample.