Elucidation of the Chain-Folding Structure of a Semicrystalline Polymer in Single Crystals by Solid-State NMR

Despite tremendous efforts over the last half-century to elucidate the chain-folding (CF) structure of semicrystalline polymers, the re-entrance sites of folded chains, the successive CF number n, and the adjacent re-entry fraction F have not been well characterized due to experimental limitations. In this report, 13 C– 13 C double-quantum (DQ) NMR was used to determine for the first time the detailed CF structure of 13 C CH 3 -labeled isotactic poly(1-butene) ( i PB1) in solution-grown crystals blended with nonlabeled i PB1 across a wide range of crystallization temperatures ( T c s). Comparison of the results of DQ experiments and spin dynamics simulations demonstrated that the majority of individual chains possess completely adjacent re-entry structures at both T c = 60 and ∼0 °C, as well as indicated that a low polymer concentration, not kinetics, leads to cluster formations of single molecules in dilute solution. The changes in crystal habits from hexagonal shapes at T c = 60 °C to rounded shapes at ∼0 °C (kinetic roughness) are reasonably explained in terms of kinetically driven depositions of single molecule clusters on the growth front.