Temperature-Dependent Alternating α- or β-Transcrystalline Layers in Coextruded Isotactic Polypropylene Multilayered Films

Continuous and orderly α- or β-transcrystallinity plays a significant role in achieving excellent performance for isotactic polypropylene (iPP). However, this special structure is difficult to obtain through conventional methods. Here, continuous and orderly α- or β-transcrystallinity was constructed in an iPP layer of an eight-layer iPP/α-iPP sample using a layered design and temperature control. The crystalline structure and morphology of the iPP layer were greatly influenced by the cooling rate. The iPP layer contained a large amount of β-transcrystallinity and a small amount of α-transcrystallinity when the sample was cooled to 130 °C for isothermal crystallization at a cooling rate of 50 °C/min. However, only α-transcrystallinity was formed in the iPP layer when the cooling rate was 1 °C/min. The different crystallization mechanisms are discussed in this paper. In addition, the experimental results showed that β-transcrystallinity could simultaneously improve both the tensile and impact properties.