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 β 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 β 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 β phase