EVALUATION OF THERMODYNAMIC THEORIES TO PREDICT INTERFACIAL-TENSION BETWEEN POLYSTYRENE AND POLYPROPYLENE MELTS

The commonly used thermodynamic theories (mean field theory and the sq uare gradient theory) to predict interfacial tension between polymers have been modified. The results of these theoretical developments have not yet been fully tested and compared to experimental data. In this paper, experimental data for the effects of temperature, molecular wei ght, and molecular weight dispersity on interfacial tension for polypr opylene/polystyrene polymer pairs are compared with the predictions of the new versions of the above theories. To evaluate these theories, i t is necessary to know the Flory-Huggins interaction parameter for the polymer pairs studied. The relation correlating the Flory-Huggins int eraction parameter to the Hildebrand solubility parameter was not suit able for evaluating the theoretical predictions of interfacial tension . Instead, the Flory Huggins interaction parameter was expressed as th e sum of an enthalpic contribution, chi(H), and entropic contribution, chi(S). In the absence of reliable experimental values, a method was developed to evaluate the two contributions, based on interfacial tens ion data. The procedure provided an interaction parameter that is allo wed to depend on molecular weight. When this approach was used, the pr edictions of only the new version of the square gradient theory were i n good agreement with the experimental data for the influence of tempe rature and molecular weight on interfacial tension. However, the theor y predicted the effect of polydispersity on interfacial tension only a t high temperatures.