Observation of chain branching in polyethylene in the solid state and melt via 13C NMR spectroscopy and melt NMR relaxation time measurements

Branch contents in sparsely short-chain branched polyethylenes (<C-28) and end-group concentrations in linear polyethylenes were investigated by solid- and melt-state C-13 NMR using model copolymers of 1-octene, 1-octadecene, 1-hexacosene, and linear PE standards. Benchmark measurements were established for single-pulse acquisition under MAS on these melts at 423 K, using 7 mm ceramic rotors and commercially available solid-state probes at 300 and 500 MHz H-1 Larmor frequency. Signal improvement by a factor of 3-4 was observed, corresponding to a factor 10 reduction in measurement time, when compared to a reference solution-state (400 MHz H-1) measurement. Transient NOE effects due to proton decoupling during FID acquisition were found to affect significantly the accuracy of branch content determination and end-group analysis in the melt spectra, even at moderate pulse repetition rates. (CT1)-C-13 and NOE buildup rates and intensity variations were established at 423 K and correlated to the local packing of the branched chain architecture. CP/MAS spectra at and below room temperature on the solid-state polyethylenes showed good agreement with reference spectra, providing an alternative method for sensitivity improvement, with about a factor of 10 saving in measurement time as compared to the melt measurements. Using an optimized parameter set, molecular weight determination of a linear polyethylene standard with M-n > 100 000 g/mol was shown to be feasible in both solid-state and melt measurements in less than a one-day measurement, obtained on a 500 MHz spectrometer and 4 mm rotor. Using this enhanced signal intensity, NMR relaxation times were investigated in the melt with respect to their inherent sensitivity to the branching architecture. These measurements included T-1rho, T-1, and T-1(NOE). It was found that T-1(NOE) seems to have the best sensitivity to determine the approximate length of the side chain branches for n > 6.