Observation of Chain Branching in Polyethylene in the Solid State and Melt via ¹³C NMR Spectroscopy and Melt NMR Relaxation Time Measurements

Abstract

Branch contents in sparsely short-chain branched polyethylenes (<C28) and end-group concentrations in linear polyethylenes were investigated by solid- and melt-state 13C 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 1H 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 1H) 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. 13C T 1 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 1 ρ, 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.

Molar Mass and Molecular Weight Distribution Determination Of UHMWPE Synthesized Using a Living Homogeneous Catalyst

“…This observation is in agreement with Fujita et al (See ref 2.) Chain branching in these polymers is investigated following the solid-state NMR method of Pollard et al 23 The studies performed in the melt state of the disentangled polyethylenes show the nonexistence of branches even up to the 10 000 C atoms, with the upper limit of the method applied. Therefore, it could be stated that the method used for polymerization in this publication yields linear polyethylenes that are free of branches.…”

A Comprehensive Review on Controlled Synthesis of Long‐Chain Branched Polyolefins: Part 3, Characterization of Long‐Chain Branched Polymers

“…[30] A good resolution of the carbon signal at branching points, as well as a large signal to noise ratio, are required in 13 C NMR quantitative analyses. [15,[31][32][33] Achieving such a high-quality 13 C NMR spectrum, however, is very difficult due to the low natural abundance of the 13 C isotope. Specifically, this low abundancy results that the signal intensity of branched carbons is a factor of 100-1000 lower than that of the backbone carbons.…”

“…An early study was reported by Hatfield et al using a 200 MHz spectrometer ( 1 H resonance), [34,35] followed by intensive works of Wilhelm, Kaminsky and their co-workers. [33,36,37] Compared (5 of 20) 1600012 to the solution NMR, which has a sample concentration less than 20 wt%, the melt and solid-state 13 C NMR generate a higher sample filling factor, resulting in a higher sensitivity towards the presence of sparsely branched side chains and end groups in POs. This technique currently allows the characterization of branch lengths up to 16 carbons.…”

Molecular Dynamics of Neutral Polymer Bonding Agent (NPBA) as Revealed by Solid-State NMR Spectroscopy

“…The simple spectral features also suggest that the sample is very unlikely to show large inhomogeneity. Based on the characteristic values and studies of polymers containing similar functional segments [47,48], all peaks are assigned. The peaks at 29.6 and 36.1 ppm were respectively attributed to the methine and methylene groups present in the main chain.…”