MAGIC ANGLE SPINNING NUCLEAR-MAGNETIC-RESONANCE STUDIES OF MOLECULAR-MOTION IN CROSS-LINKED POLYETHYLENE

Magic angle spinning nuclear magnetic resonance spectroscopic investigations were performed on solid state high density polyethylene (HDPE) and low density polyethylene (LDPE) crosslinked by gamma-irradiation using doses up to 40 Mrad. The free induction decay was measured at various spinning frequencies in order to obtain information on the influence of crosslinks and branches on molecular mobility in the amorphous regions of the semicrystalline material. The anisotropy of motion caused by crosslinks and entanglements is characterized by the anisotropy parameter, a, and the correlation time of anisotropy fluctuations tau-ca. It was found that in HDPE the values of a are larger and those of tau-ca are smaller than in LDPE. In HDPE crosslinking leads to a shift of the spectrum of a to larger values whereas in LDPE no change is observed upon irradiation. However, if the material is molten, upon irradiation the molecular mobility is markedly decreased in both HDPE and LDPE. The results are discussed comparing densities of crosslinks and entanglements.