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.

Determination of Local Packing Structure of Mesomorphic Form of Isotactic Polypropylene by Solid-State NMR.

Understanding the local packing structures of a disordered mesomorphic phase is a challenging issue in polymer characterization. In this work, 13C-13C through-space interactions, as well as a molecular dynamics analysis based on the reorientation of chemical shift anisotropy (CSA), were proposed for the evaluation of the local packing structure of the mesomorphic form of isotactic polypropylene (iPP). 13C-13C double quantum (DQ) buildup curves of 13C 15% CH3 selectively labeled iPP and spin-dynamics simulations demonstrated that the local packing structures in the mesomorphic form were very similar to the packing in the ß phase. Moreover, centerband only detection of exchange (CODEX) NMR proved that the correlation time ⟨τc⟩ of the overall stem dynamics in the mesomorphic form followed the same Arrhenius line observed for the ß phase, but it deviated from that for the α phase. Based on both structural and dynamic results, it was concluded that the local packing structure in the mesomorphic form was exceedingly close or the same as that of the ß phase.

Folding of Polymer Chains in the Early Stage of Crystallization.

Understanding the structure formation of an ordered domain in the early stage of crystallization is one of the long-standing issues in polymer science. In this study, we investigate the chain trajectory of isotactic polypropylene (iPP) formed via rapid and deep quenching, using solid-state NMR spectroscopy. Comparisons of experimental and simulated 13C-13C double quantum (DQ) buildup curves demonstrated that individual iPP chains adopt adjacent re-entry sequences with an average folding number ⟨n⟩ = 3-4 in the mesomorphic form, assuming an adjacent re-entry fraction ⟨F⟩ of 100%. Therefore, long flexible polymer chains naturally fold in the early stage of crystallization, and folding-initiated nucleation results in formation of mesomorphic nanodomains.