Temperature and Deformation-Induced Changes in the Mechanical Properties of the Amorphous Regions of Semicrystalline Polypropylene

This work is focused on the impact of temperature and deformation on the mechanical properties, specifically the elastic modulus (E_a) of the amorphous regions in semicrystalline polymers, using polypropylene as a case study. It has been shown that increasing temperature results in an E_a decrease due to the enhanced mobility of polymer chains, triggered by the activation of α relaxation processes within the crystalline component. Consequently, rising temperature reduces the "stiffening" effect of the crystalline regions on the interlamellar layers. Temperature decrease close to the glass transition causes a significant increase in the E_a value, reaching nearly 70 MPa. Next, the effects of crystalline/amorphous component orientation and undisturbed crystallite length on E_a were examined in materials deformed using a channel die at various compression ratios. At low compression ratios, E_a decreases nearly 4-fold, primarily due to the fragmentation of lamellar crystals in the absence of, or with relatively low, orientation of the crystalline and amorphous components. Conversely, at higher compression ratios, with minimal crystal fragmentation, increased orientation of both crystalline and amorphous regions along the deformation direction (E_a measurement direction) leads to a substantial increase in E_a. Ultimately, the material with the highest used compression ratio exhibited an E_a value approximately 20% higher than that of the undeformed material.