Thermodynamics of polymer mixtures

Abstract

Research into the thermodynamic behaviour of copolymer blends has been stimulated by the increasing number of applications in which these materials can be used. In this work, it was intended to characterise the thermodynamics of mixtures of two industrial copolymers and to review the experimental techniques and theoretical analyses currently used in this field. The copolymers used were poly(ethylene - co-vinyl acetate), with Mn = 3290, and poly(tetradecyl fumarate - co - vinyl acetate), with Mn = 10400. The thermodynamics of these mixtures was studied using Differential Scanning Calorimetry, Inverse Phase Gas Chromatography. Solvent Vapour Sorption, Heats of Mixing Calorimetry and Phase Contrast Optical Microscopy. The results of these experiments were interpreted using Flory-Huggins Lattice fluid theory and Flory-Prigogine equation of state theory. Additionally, the results of the calorimetry and chromatography experiments were used to predict the theoretical phase boundary with the intention of comparing the phase boundaries determined experimentally with those predicted theoretically. Unfortunately this comparison could not be made because none of the techniques listed above located a miscibility limit between 303 and 393K. Although some of the experimental results are in conflict, it has been concluded that these materials are immiscible in all proportions in this temperature range. The theoretically simulated spinodal condition occurs between 5 and 50K and is of little practical use in the absence of its experimental equivalent and its extreme temperature. The free energy change which occurs on mixing these copolymers is dominated by the entropic contribution and the equation of state was concluded to be inadequate to interpret this type of behaviour. It is believed that this is the first work which uses experimental data and a partition function to calculate directly a phase boundary without the inclusion of a fixing parameter.