(647g) Dissolution of Semicrystalline Polyethylene: Experiments and Modelling

Dissolution/precipitation is a low-GHG emission methodology for recycling waste plastic that cannot be mechanically recycled. Through this method, specific polymer types can be separated from mixtures or blends, purified from additives or fillers, and recovered for further processing, without negatively affecting the properties of the original polymers. Surprisingly, there is little information publicly available regarding the phase behavior of common types of plastics, and even less on their dissolution kinetics. To address this knowledge gap, we pursue a joined experimental and modeling approach to investigate the dissolution of semicrystalline polyethylene. A phenomenological model is developed that captures the phenomena governing the dissolution of semicrystalline polymers, e.g., solvent diffusion in the solid polymer, transformation from crystalline to amorphous domains, specimen swelling with solvent, polymer chain untangling in the specimen, and polymer diffusion into the solvent. The two key parameters of the model, decrystallization rate constant and disentanglement rate, are obtained from fits to experimental data on the time evolution of dissolved mass and degree of crystallinity. The former is obtained from a material balance. Decrystallization of the polyethylene films is quantified in real-time using a temperature-controlled Fourier Transform-Infrared (FT-IR) liquid cell. Specifically, crystallinity changes are quantified by evaluating the factor group splitting phenomenon corresponding to the bending methylene (-CH₂-) vibrational modes between 700 and 750 cm^-1. With our model validated for the case of polyethylene dissolution in good solvents, a parametric sensitivity analysis is carried out to assess the impact of decrystallization rate constant, disentanglement rate, and solvent diffusivity on the swelling and dissolution of semicrystalline polyethylene films. The insights obtained from this study would facilitate the design of efficient solvent systems and processing conditions for the dissolution/precipitation-based “molecular” recycling of polyolefins from mixed plastic waste.