Abstract
Polymer chain ends mostly confined to crystallite surfaces have been detected by versatile isotope labeling and advanced NMR.
A comprehensive
13
C 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
13
COO-acetyl groups and their dynamics probed by
13
C 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
1
H spin diffusion from the amorphous regions shows confinement of chain ends to the crystallite surface, corroborated by fast
13
C 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.