Thermodynamic Modeling of the Solid-Liquid Phase Transition in Polyethylene Copolymer-Solvent Systems Based on Continuous Thermodynamics and Lattice Cluster Theory

In this work, a thermodynamic model is developed based on continuous thermodynamics and lattice cluster theory to describe solid-liquid equilibria of polymer-solvent systems, where the polymer shows a certain molecular architecture, semi-crystallinity, and a continuous molecular weight distribution. The new thermodynamic model is validated by predicting the solid-liquid phase behavior of ethylene/1-hexene copolymer-1,2,4-trichlorobenzene mixtures for different short-chain branchings, degrees of crystallinities, and molecular weight distributions. It turned out that this thermodynamic model is capable of capturing the solid-liquid transition zone, where a manifold of solid-liquid equilibria exists, due to the continuous character of the molecular weight distribution. For the first time, the coexistence region of the solid-liquid transition of a polyethylene-solvent system is predicted based on a thermodynamic consistent model. Further model calculations show how the polydisperse nature of the polymer influences the coexistence region in a complex and nonlinear manner, especially in the low-molecular-weight regime. This gives new insights into the solid-liquid phase behavior of polydisperse polymer-solvent mixtures and provides valuable information on the field of polymer crystallization.