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Degradation in the mechanical properties of recycled polymer materials has been recently appearing as a big issue in polymer science. The molecular mechanism of the degradation is considered in part due to residual memories of flow experienced during molding processes, and therefore the mechanical recycling through remolding involving melting and recrystallization has been attempted in recent years. In the present paper, the molecular processes of melting and recrystallization are investigated by the molecular dynamics simulation for polyethylene with special interest in the melt memory effects. We also studied the mechanical properties of the recrystalized materials that have undergone different recrystallization processes aiming to discover better recycling strategies. A successive step-by-step approach is adopted to study the loss of the crystal memory during retention in the melt, the effects of the melt memory on the mode of recrystallization, the relation between the recrystallization mode and the resulting higher-order structure, and the mechanical properties controlled by the higher-order structures. It is shown that the melt memory clearly remains in various order parameters that persist over time scales corresponding to the Rouse time, the remaining melt memory markedly affects the crystallization mode leading to distinct crystalline morphologies, and the distinct morphologies obtained give different mechanical responses during large deformations.

We investigated the molecular characteristics of virgin polypropylene and recycled polypropylene that was obtained from byproducts (for example, runners, etc.) at the injection-molding of virgin polypropylene. We also tested the tensile properties of 3 mm-thick specimens made from each sample. The results showed that the two materials had almost the same molecular characteristics and tensile properties. However, their fractured surfaces were very different. Furthermore, a 100 µm-thick film of the recycled sample was very brittle compared to a film made from the virgin polypropylene. The recycled sample had a history of shear deformation at the time of injection-molding, and may have a unique inner structure due to shear deformation. We found a unique peak in an endothermic curve with a rapid heating rate, and this result supports the above assumption. We also found that the dependence of the tensile fracture energy on the duration of UV irradiation was essentially the same in these samples. Based on these results, we propose a new theoretical equation that can estimate the fracture ratio of molecules.