(692f) Decrystallization of Semi-Crystalline Polyethylene

Crystallization of polyethylene, key to the development of desirable mechanical properties, is well-studied, whereas decrystallization is not. To address this knowledge gap, we pursue a joint experimental and modeling approach to investigate the decrystallization of semi-crystalline polyethylene (PE) that is induced by heating (melting) or solvent (dissolution).

A temperature-controlled Fourier Transform-Infrared (FT-IR) setup has been designed and assembled for in situ PE film melting and dissolution experiments. The time evolution of different infrared regions in HDPE, LDPE, and LLDPE undergoing melting elucidated the crystalline-amorphous interfacial region, and lead to a new FT-IR-based method for the in situ quantification of PE degree of crystallinity. From this study’s dissolution experiments, PE’s highly-mobile gauche conformers were found to shift to higher or lower IR frequency when encapsulated by the solvation shell of p-xylene. Dissolution of LDPE films revealed the terminal CH3 groups of dangling cilia that unravel from crystalline lamellae.

A phenomenological model has been developed that captures the events taking place in the dissolution of semi-crystalline polyethylene, 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, were obtained from fits to experimental data on the time evolution of dissolved mass and degree of crystallinity. With this model validated, a parametric sensitivity analysis was carried out to assess the impact of decrystallization rate constant and disentanglement rate on the solvent-swelling and dissolution of semi-crystalline polyethylene.

The insights obtained from this study facilitate the design of efficient solvent systems and processing conditions for the molecular recycling of polyolefins via dissolution/precipitation. Through this method, specific polymer types can be separated from mixtures or blends, purified from additives, and recovered for further processing, without negatively affecting the properties of the original polymers.