Deuteron n.m.r. study of chain motion in solid polyethylene

The chain motion in the amorphous regions in linear polyethylene is studied by exploiting various ²H n.m.r. pulse methods. The experimental techniques employed are described in considerable detail. In particular, the solid echo- and the spin alignment spectra are recorded over a wide range of temperatures (123–393 K) as a function of the pulse spacings. In addition the spin-lattice relaxation times of both polymerization and alignment are given for the crysralline as well as the amorphous regions. The data are analysed in terms of highly constrained conformational motions which generate an exchange of C-H bond directions between 2, 3, or all 4 tetrahedral sites on a diamond lattice. Thus in the temperature interval from the γ-relaxation up to ～200 K only 2 sites are accessible, whereas a third site is appreciably populated at room temperature. Between 330 K and the melting point the number of conformations accessible to the chain must be high enough to lead to an increasingly isotropic 4 site exchange. The motion remains highly constrained and localized, however, up to the melting region as shown by the analysis of the spin-alignment spectra. The intensity ratio of the deuteron n.m.r. signals from the rigid crystalline and the mobile amorphous regions, respectively, yields the crystallinity of the sample as a function of temperature in remarkable agreement with the results obtained by X-rays. Finally, frequency dependent relaxation of spin alignment due to torsional oscillations of the chains in the crystalline regions is observed and analysed.