One polyethylene and nine ethene/α-olefin copolymers differing in amount (0.4−2.9 mol %) and
molar mass of the comonomer were characterized by NMR, SEC-MALLS, and rheology. Samples were
polymerized using a [Ph2C(2,7-di-t-BuFlu)(Cp)]ZrCl2/MAO catalyst, with octene, octadecene, and hexacosene
as comonomers, resulting in polymers of M
w ≈ 190 kg/mol. The comonomer content was determined by melt-state NMR. For the homopolymer 0.37 and 0.30 LCB/molecule were found by NMR and SEC-MALLS,
respectively. Rheological quantities, such as the zero shear rate viscosity (η0), increased with LCB as compared
to linear samples of the same M
w. The shape of the viscosity function and the linear steady-state elastic compliance
(
) showed a dependence on comonomer content and length. These findings are used to elucidate the various
long-chain branching architectures. The highest comonomer content samples behaved like typical linear polymers
in rheological experiments, while those with less comonomer contents were found to be long-chain branched.
Besides the comonomer content, the type of comonomer has an influence on the branching structure.
Branch contents in sparsely short-chain branched polyethylenes (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.
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 13C1H 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.
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