Chain Trajectory of Semicrystalline Polymers As Revealed by Solid-State NMR Spectroscopy.

Over the last half century, a chain-folding structure of semicrystalline polymers has been debated in polymer science. Recently, C-C double quantum (DQ) NMR spectroscopy combined with C selective isotope labeling has been developed to investigate re-entrance sites of the folded chains, mean values of adjacent re-entry number ⟨⟩ and fraction ⟨⟩ of semicrystalline polymers. This viewpoint highlights the versatile approaches of using solid-state (ss) NMR and isotope labeling for revealing (i) chain trajectory in melt- and solution-grown crystals, (ii) conformation of the folded chains in single crystals, (iii) self-folding in the early stage of crystallization, and (iv) unfolding of the folded chains under stretching.