Annealing of Oriented PP/PE Double-layer Film within the Melting Range of PE: the Role of Partial Melting and Self-nucleation.

Due to the mechanical stability of the PP layer, the oriented PP/PE double-layer film with a row-nucleated crystalline structure can be annealed at a higher temperature than the PE monolayer film. In this work, the effects of annealing temperature within the melting range of PE on the crystalline structure and properties of PP/PE double-layer films were studied. When the annealing temperature is between 100 and 130 °C, below the melting point of PE, the crystallinity, the long period, lateral dimension and orientation of the lamellae in the PE layer increase with the annealing temperature due to the melting of thin lamellae and the self-nucleated effect of partially-melted melts during annealing. With the annealing temperature further increasing to 138 °C, near the melting ending point of PE, since the lamellae melt completely and the melt memory becomes weak during annealing, some spherulite structures are formed in the annealed sample, resulting in a decrease of orientation. In contrast, the annealing only causes the appearance of a low-temperature endothermic plateau in the PP layer. The improved size and orientation of lamellar structure in the PE layer increase the pore arrangement and porosity of the stretched PP/PE microporous membrane. This study successfully applies the self-nucleation effect of partially-melted polymer melt into the practical annealing process, which is helpful to guide the production of high-performance PP/PE/PP lithium batteries separator and the annealing process of other multilayer products. [ABSTRACT FROM AUTHOR]