Liquid–Liquid Equilibrium in Xylene Solubles (XS) Analysis of Polypropylene

A multicomponent Flory-Huggins model is implemented and utilized to describe the liquid–liquid equilibrium phenomenon in mixtures of polypropylene and xylene, in the context of the well-known xylene solubles (XS) test. The XS experiment is a common procedure in many polymer laboratories, used to determine the percentage of xylene solubles in samples of polypropylene, which provides an approximate measure of the atactic and oligomeric chains. Despite the importance of the test, the literature lacks a thermodynamic perspective regarding the description of this extraction phenomenon. In the present study, the Flory-Huggins interaction parameter is adjusted in a multicomponent framework to ensure that equilibrium chain length distributions calculated with the proposed model best match experimental distributions. It is shown that the experimental data obtained from XS analyses can be accurately fitted by the proposed model and that the estimated Flory-Huggins interaction parameter is more sensitive to the polymer average molar mass than to the degree of tacticity, when a particular catalyst is considered.