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 13 C CH 3 15% labeled isotactic polypropylene (iPP) in melt-grown crystals at two crystallization temperatures (T c s) of 100 and 150°C were investigated using 13 C− 13 C Double quantum (DQ) NMR and spin-dynamics simulation. The former provided the α 1 packing structure while the latter lead to the α 2 -rich sample with an α 2 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 α 1 and α 2 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 T c .

Chain Trajectory, Chain Packing, and Molecular Dynamics of Semicrystalline Polymers as Studied by Solid-State NMR

“…It is worthwhile to note that recent MD simulation of coarse grained Poly(vinyl alcohol) (PVA) model predicted < n > to be ~2.4 in the melt-grown and < n > is independent of T c [73,74]. These simulation results are very consistent with recent ssNMR observations [52,53,54,55,56].…”

“…This fact indicates that the 13 C selective isotopic labeling approach is applicable to chain-folding analysis in both melt- and solution-grown crystals over a wide T c range. The same conclusions were obtained in i PP [53] and PLLA systems [58].…”

“…The important findings falsify the well-established LH theory [3,4], and point out importance of primary nucleation in polymer crystallization [6,17]. The folded chain structures accessible by ssNMR [52,53,55,56,57,58,62] are consistent with recent views on primary nucleation in amino acids [87], metal ions [88,89], and proteins [90], among others. Combining atomic scale information by ssNMR with direct observations of morphology, as well as visualization of chains by computations, will significantly contribute to understanding in the research field of crystallization.…”

“…The simulated DQ curve based on the shortest 13 C– 13 C interstem distance of 4.0 Å and relaxation value of T 2 = 8.3 ms can reproduce the experimental data (red curves in Figure 1e,f). Similar packing analysis was conducted on i PP [53,54] and PLLA crystals [58,63]. In all systems, the internuclear distances determined by NMR are consistent with those by WAXD and electron diffraction (ED) within 5% errors.…”

“…Recently, 13 C– 13 C double quantum (DQ) NMR spectroscopy has been applied to elucidate chain trajectory and packing structures for 13 C selectively isotopic enriched isotactic -Poly(1-butene) ( i PB1), isotactic -poly(propylene) ( i PP), and poly(L-lactic acid) (PLLA) in the melt- and solution-grown crystals [52,53,54,55,56,57,58,59,60,61,62,63]. 13 C– 13 C dipolar interaction is governed by internuclear distance, spin topology, and spin number.…”