Determination of Chain-Folding Structure of Isotactic Polypropylene in Melt-Grown α Crystals by 13C–13C Double Quantum NMR and Selective Isotopic Labeling

Abstract

Chain-folding (CF) patterns, successive CF number ⟨n⟩ , and fraction ⟨F⟩ of 13C CH3 15% labeled isotactic polypropylene (iPP) in melt-grown crystals at two crystallization temperatures (Tcs) of 100 and 150 °C were investigated using 13C–13C Double quantum (DQ) NMR and spin-dynamics simulation. The former provided the α1 packing structure while the latter lead to the α2-rich sample with an α2 fraction of ca. 57%. It was demonstrated that the iPP chains adopt small cluster structures consisting of 6–8 stems connected via adjacent re-entry as mean structures in both α1 and α2 phases. There was no kinetic effect on ⟨F⟩ and ⟨n⟩ values, while kinetics did influence the folding directions of the chains under the assumptions of configurational and packing constraints. It was concluded that the CF process occurs at the growth front and dominates the chain-packing process of iPP α phases in a wide Tc.