Crystallization and multiple melting behavior of oriented thick lamellae in isotactic polypropylene

Abstract

The crystallization and melting of semicrystalline polymers are one of the most fundamental issues of polymer physics. However, how crystal orientation and thickness relate to the multiple melting behavior is not well understood. Here, well-oriented thick lamellae (22–37 nm) have been produced in isotactic polypropylene (iPP) by crystallization under the combined effects of flow (30 s⁻¹) and pressure (50, 75, 100, 125 MPa). The melting of these oriented thick lamellae is followed by real-time wide- and small-angle X-ray scattering measurements. The results show that the melting of oriented thick lamellae of iPP follows the sequential melting model at the early stage, and then the melting of the entire lamellar stacks at high temperature. More importantly, the temperature dependence of the reciprocal lamellar thickness of these oriented thick lamellae is shown to follow the multistage model. A common melting line is obtained which coincides with that of the unoriented lamellae crystallized under quiescent conditions, giving an equilibrium melting temperature of 183 °C and a folded surface free energy of $\left(86-122\right)\times {10}^{-7}$ J/cm². Somewhat unexpectedly, the oriented thick lamellae thicken continuously on the heating, whereas the relatively thin lamellae obtained from quiescent crystallization melt directly without thickening. This phenomenon is discussed in terms of the irreversible thermodynamic process of lamellar thickening. The present work sheds light on the crystallization and melting behavior of oriented thick lamellae in iPP, and generalizes the multistage model to iPP crystallized under external fields.