Optimisation and Application of Polyolefin Branch Quantification by Melt‐State ¹³C NMR Spectroscopy

Abstract

Summary: Quantitative branch determination in polyolefins by melt‐state NMR has been investigated paying particular attention to sensitivity per unit time. Comparison of spectra obtained using spectrometers operating at 700, 500 and 300 MHz 1H Larmor frequency, with 4 and 7 mm MAS probeheads, showed that the best sensitivity was achieved at 500 MHz using a 7 mm 13C1H optimised high‐temperature probehead. For materials available in large quantities static melt‐state NMR, using large‐diameter detection coils at 300 MHz, was shown to produce comparable results to melt‐state MAS measurements in less time. Artificial line broadening, introduced by FID truncation, was reduced by the use of π pulse‐train heteronuclear dipolar‐decoupling. This decoupling method, when combined with a higher duty‐cycle, allowed for the whole FID to be acquired. Optimised methods have been applied to the characterisation of short‐chain branching (SCB) in polyethylene‐ and poly(propylene)‐co‐α‐olefins with varying comonomer incorporation. Long‐chain branch (LCB) concentrations of 8 branches per 100 000 CH2 were quantified for an industrial ‘linear’ polyethylene in 13 h, with a signal‐to‐noise ratio of 10 for the α branch site used. The use of J‐coupling mediated polarisation transfer techniques were also shown to be viable for branch quantification in the melt‐state.An example of the time efficient quantification of very low branch contents in polyethylene using optimised 13C melt‐state NMR under magic‐angle spinning. Concentrations of 7–8 branches per 100 000 CH2 groups were determined in only 13 h.magnified imageAn example of the time efficient quantification of very low branch contents in polyethylene using optimised 13C melt‐state NMR under magic‐angle spinning. Concentrations of 7–8 branches per 100 000 CH2 groups were determined in only 13 h.

Analytical Rheology of Polymer Melts: State of the Art

“…In particular, it cannot distinguish between branches with more than 10 carbon atoms in the side chain. This length is far below the threshold of around 100-200 carbon atoms in "long-"chain branching [42,[86][87][88][89][90][91]. In summary, both g-ratio analysis and NMR are tedious and expensive and require specialized training in carrying out the necessary experiments.…”

Evidence of intra-chain phase separation in molten short-chain branched polyethylene

“…Table 1) was measured by solution NMR using the WALTZ-16-program [16]. For L4, melt-state NMR was used [36,37]. Molar mass measurements were carried out by means of a high temperature size exclusion chromatograph (Waters, 150C) equipped with refractive index (RI) and infra-red (IR) (PolyChar, IR4) detectors.…”

Effect of Branch Length on 13C NMR Relaxation Properties in Molten Poly[ethylene‐co‐(α‐olefin)] Model Systems

“…These effects have previously been proven to exist for molten polyethylene systems. [16,21,23] It remains unclear, however, as to why the saturation block did not remove the effects of NOE completely, with further experimentation needed. Thus, due to these ill-understood effects, all T C 1 relaxation times determined by saturation-recovery are rendered less reliable than those obtained with inversion-recovery, irrespective of the SD reported for each site.…”

“…Compensation for MAS frictional heating and decompression-cooling was undertaken using temperature calibration data measured on lead nitrate. [19][20][21] All measurements were conducted at v r /2p ¼ 3 kHz MAS. Shimming was undertaken on the 1 H resonance of the molten sample with a full-width at half-maximum (FWHM) 25 Hz being deemed acceptable.…”

“…For further details of the optimised melt-state MAS method the reader is directed to the literature. [15,21,22] Melt-state spin-lattice relaxation times were measured using both inversion-recovery and saturation-recovery experiments. A combination of 16 dummy scans and 64 real scans were used per t 1 delay to compensate for NOE and saturation effects.…”