Isothermal crystallization study on aqueous solution of poly(vinyl methyl ether) by DSC method

Abstract

A study on the isothermal crystallization of water in aqueous solutions of poly(vinyl methyl ether) (PVME) was carried out by the differential scanning calorimetry (DSC). The influence of PVME concentration (49.5, 44.5 and 39.5 v%) and the crystallization temperature (Tc) on crystallization rate G, crystallization enthalpy (ΔHc) and melting enthalpy (ΔHm) was investigated. Avrami equation cannot be used to describe the crystallization process of water in aqueous PVME solution. Within the measured temperature range, the crystallization rate G increases with the crystallization temperature Tc and with the decreasing PVME content. The crystallization enthalpy ΔHc linearly increases with the degree of supercooling. The influence of Tc on the ΔHc becomes more marked with increasing PVME concentration. For 49.5 and 44.5 v% PVME solutions, the amount of water arrested in solution during the isothermal crystallization and the final concentration of PVME-rich phase increase linearly with the Tc, whereas for 39.5 v% PVME solution, these two values almost do not change with Tc. The amount of frozen water in the subsequent cold crystallization is approximately proportional to the initial Tc. The approximately constant ΔHm for a given concentration at the different initial isothermal crystallization temperatures suggests that the total amount of ice from the first isothermal crystallization and the second cold crystallization is same. The quantitative relation of the amount of frozen water in the cold crystallization and the initial Tc demonstrates that PVME/water complexes are thermodynamically unstable.