NMR study of the energy difference and population of the gauche and trans conformations in solid polyethylene

High-resolution, solid-state C-13 NMR spectra have been obtained for four p olyethylene samples, at temperatures from 20 to 100 degreesC. The samples v aried in crystallinity, lamellar thickness and molecular weight. From the c hemical shift of the peak corresponding to the amorphous phase, the fractio n 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 +/- 0.5 kJ mol(-1). This is higher t han values found previously for polyethylene and alkane chains in melt and solution states, and can be attributed to effects of the semi-crystalline m orphology. 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 c hain folding at lamellar surfaces. The variation in the value of the gauche state degeneracy shows an understandable trend with changes in crystallini ty and lamellar thickness. (C) 2000 Elsevier Science Ltd. All rights reserv ed.