Ultraslow molecular motions in crystalline polyoxymethylene. A complete elucidation using two-dimensional solid state NMR

The slow magic angle spinning 2D exchange NMR experiment applied to semicrystalline polyoxymethylene shows the existence of ultraslow molecular rotations in crystalline regions of the material. By simulating the two-dimensional spinning sideband spectra, which are characteristic for the molecular motion involved, it is found that a rotation of the polyoxymethylene helical chains over 200° is responsible for the observed spectra. The activation energy is 20 kcal/mol in good agreement with values obtained for the α relaxation in dynamic mechanical and dielectric studies.