NMR CHARACTERIZATION OF POLYETHYLENE WITH EMPHASIS ON INTERNAL CONSISTENCY OF PEAK INTENSITIES AND ESTIMATION OF UNCERTAINTIES IN DERIVED BRANCH DISTRIBUTION NUMBERS

A comprehensive n.m.r. characterization of a low-density polyethylene (LDPE) sample with respect to the distribution of short branches, satu rated and unsaturated long branch/chain ends is presented. High precis ion carbon chemical shift values of all assigned resonance peaks with standard deviations of less than 0.005ppm (at 403K and 75MHz carbon re sonance frequency) were obtained by mathematical deconvolution of the carbon spectrum, thus enabling a differentiation not only of short bra nches (C-n<6) but also hexyl-, oxtyl- and longer branches). Nuclear Ov erhauser Enhancement (NOE) measurements revealed that less than 50% of the non-equivalent carbon nuclei experienced full NOE of 2.98 while m ore than 25% of the corresponding carbon nuclei showed a NOE of less t han 2.75. Based on spin-lattice relaxation time (T-1) and NOE measurem ents a statistical evaluation showed that internal quantitative consis tency between peak intensities existed only under no-NOE conditions. D erived branch distribution numbers from a set of 100 synthetic carbon n.m.r. spectra of the sample, revealed that each branch could be chara cterized by a Gaussian or normal distribution function. Average values and standard deviations of the branch numbers are presented. (C) 1997 Elsevier Science Ltd.