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.