Immobilized ¹³C-labeled polyether chain ends confined to the crystallite surface detected by advanced NMR

A comprehensive ¹³C nuclear magnetic resonance (NMR) approach for characterizing the location of chain ends of polyethers and polyesters, at the crystallite surface or in the amorphous layers, is presented. The OH chain ends of polyoxymethylene are labeled with ¹³COO-acetyl groups and their dynamics probed by ¹³C NMR with chemical shift anisotropy (CSA) recoupling. At least three-quarters of the chain ends are not mobile dangling cilia but are immobilized, exhibiting a powder pattern characteristic of the crystalline environment and fast CSA dephasing. The location and clustering of the immobilized chain ends are analyzed by spin diffusion. Fast ¹H spin diffusion from the amorphous regions shows confinement of chain ends to the crystallite surface, corroborated by fast ¹³C spin exchange between chain ends. These observations confirm the principle of avoidance of density anomalies, which requires that chains terminate at the crystallite surface to stay out of the crowded interfacial layer.