Chain-Folding Structures of a Semi-crystalline Polymer in Bulk and Single Crystals Elucidated by 13C--13C Double Quantum NMR

Semi-crystalline polymers are crystallized as folded chains in thin lamellae of ca. 5-20 nm from random coils in the melt and solution states. However, understanding of detailed chain-folding structure and crystallization mechanism are still challenging issue due to various experimental limitations. We recently developed a new strategy using $^{13}$C--$^{13}$C double-quantum (DQ) NMR with selectively $^{13}$C isotope labeled \textit{isotactic} poly(1-butene) form I to investigate chain-trajectory in solution and melt grown crystals at various \textit{Tc}s. This new method can determine the re-entrance sites, the successive folding number ($n)$, and the fractions ($F)$ of chain-folding in a wide Tc range. In melt grown crystals at \textit{Tc} $=$ 95 $^{\circ}$C, a comparison of experimental and simulated DQ efficiency determined that the polymer chains alternatively change chain-folding directions and the stems tightly pack via intramolecular interactions, and the fraction (F) of adjacent re-entry structure ranges from 70{\%} at n $=$ 4 to 100{\%} at mixed structures of n $=$ 1 and 2. Furthermore, DQ efficiency is independent of Tc in bulk crystals. This means chain-folding do not change in a wide Tcs.