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

• Published in: Macromolecular chemistry and physics 2006-02, Vol.207 (4), p.382-395
• Main Authors: Klimke, Katja, Parkinson, Matthew, Piel, Christian, Kaminsky, Walter, Spiess, Hans Wolfgang, Wilhelm, Manfred
• Format: Article
• Language: English
• Subjects: Applied sciences; branch; Exact sciences and technology; melt; NMR; Organic polymers; Physicochemistry of polymers; polyethylene; Properties and characterization; quantification; Structure, morphology and analysis
• ISSN: 1022-1352; 1521-3935
• DOI: 10.1002/macp.200500422

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.