Abstract
High-resolution, solid-state 13C NMR spectra have been obtained for four polyethylene samples, at temperatures from 20 to 100°C. The samples varied in crystallinity, lamellar thickness and molecular weight. From the chemical shift of the peak corresponding to the amorphous phase, the fraction of gauche conformers was determined. By assuming a Boltzmann distribution of trans and gauche conformations, the energy difference between the trans and gauche states was found to be 6.4±0.5kJmol−1. This is higher than values found previously for polyethylene and alkane chains in melt and solution states, and can be attributed to effects of the semi-crystalline morphology. The degeneracy of the gauche state was found to be significantly greater than the value of 2 expected for equilibrium in the melt. This is explained in terms of an additional number of gauche conformations due to chain folding at lamellar surfaces. The variation in the value of the gauche state degeneracy shows an understandable trend with changes in crystallinity and lamellar thickness.
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