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

Recent advances in post-stretching processing of polymer films with in situ synchrotron radiation X-ray scattering

The stretch-induced structural evolution mechanism is a long-standing scientific question in the post-stretching processing of polymer films. X-ray scattering, especially a combination of small- and wide-angle X-ray scattering (SAXS/WAXS), provides a powerful method to study the hierarchical structure of polymer films. Recent advances in synchrotron radiation (SR) light sources and detection techniques allow one to measure the structural evolution of polymer films during post-stretching processing in real time with ultrahigh time resolution, which benefits the understanding on this topic. This review summarizes some recent investigations on post-stretching processing of polymer films, which combine in situ X-ray scattering techniques with purposely designed tensile apparatus in terms of three aspects: uniaxial stretching, biaxial stretching and stretching with chemical reactions. Concerning the polymer bulk, traditional deformation mechanisms like stretch-induced crystallization (SIC), crystal slipping, phase transition and melting-recrystallization are discussed for the uniaxial and biaxial post-stretching of polymer films. New deformation models have been developed to focus on the structural evolution on the length scale of lamellar stacks, which consider the potential microphase separation of the interlamellar amorphous phase and microbuckling. For solution systems, the coupled effects of the mechanical work from external force and the chemical potential from possible chemical reactions are taken into account for the structural evolution during stretching in solution. Roadmaps of structural and morphological evolution in the processing parameter space (i.e., temperature, stress, strain and the concentration of additive in the bath solution) are eventually constructed for precursor films. The accumulation of a structural evolution database for post-stretching processing of polymer films can be expected to provide a helpful guide for industrial processing for high-performance polymers in the near future.

Unfolding of Isotactic Polypropylene under Uniaxial Stretching

Despite numerous investigations on polymer processing, understanding the deformation mechanisms of semicrystalline polymer under uniaxial stretching is still challenging. In this work, ¹³C-¹³C Double Quantum (DQ) NMR was applied to trace the structural evolution of ¹³C-labeled isotactic polypropylene (iPP) chains inside the crystallites stretched to an engineering strain (e) of 21 at 100 °C. DQ NMR based on spatial proximity of ¹³C labeled nuclei proved conformational changes from the folded chains to the locally extended chains induced by stretching. By combining experimental findings with literature results on molecular dynamics, it was concluded that transportation of the crystalline chains plays a critical role to achieve large deformability of iPP.