Two Chain-Packing Transformations and Their Effects on the Molecular Dynamics and Thermal Properties of α-Form Isotactic Poly(propylene) under Hot Drawing: A Solid-State NMR Study

Abstract

The chain packing, crystal thickness, molecular dynamics, and melting temperature of α-form isotactic polypropylene (iPP) drawn uniaxially at high temperatures of 100–150 °C were investigated using solid-state (SS) NMR and DSC. Two types of iPP samples with disordered (α1) and relatively ordered (α2-rich) packing structures were prepared via different thermal treatments and drawn up to an engineering strain (e) of approximately 20. High-resolution 13C NMR detected continuous α2 → α1 transformations in the original α2-rich samples over the entire deformation range at all drawing temperatures (Tds). A sudden α1 → α2 transformation was found to occur in the original α1 sample in the small e range of approximately 3–7 at Td = 140 °C. Then, in the late stage, the newly grown α2 structure reversely transformed into α1 structure with further increase in e, as observed in the original α2-rich sample. These results indicate that at least two different processes are involved in large deformations. On the basis of c...