Chain dynamics in partially cross‐linked polyethylene by combined rheology and NMR‐based molecular rheology

Abstract

In recent years, 1 H double-quantum NMR (DQ NMR) was established as a suitable molecular-rheology technique to elucidate chain dynamics and to determine entanglement or crosslink densities in linear entangled polymer melts and permanent as well as transient networks. In this work, industrial grade high-density polyethylene, partially cross-linked via electron beam irradiation in the semicrystalline state, is probed in the melt state by low-field DQ NMR and shear rheology. The DQ NMR data is analyzed by two approaches, one assuming the presence of a permanent network and the other considering the potentially complex relaxation spectrum of the studied inhomogeneous systems. A correlation between the DQ NMR results and extent of crosslinking is found. By direct comparison of the rheological results and the NMR-based segmental orientation autocorrelation functions (OACF) via timetemperature superposition (TTS), qualitative consistency between the microscopic and macroscopic observables is established. In this way, the frequency range of shear rheology can be extended by about two decades into the 10 krad/s range. The NMR method is thus a valuable extension of the toolbox of characterization techniques, where gel content measurements by solvent extraction proved to be the least sensitive.

Chain Dynamics of Partially Disentangled UHMWPE around Melting Point Characterized by 1H Low-Field Solid-State NMR

“…While the number of network-like protons of PE0.5M samples began to level off due to the recovery of entanglements caused by the fast reptation of molecular chains. It agreed well with what was observed in partially cross-linked HDPE [50].…”

“…From Figure S4 (see in Supplementary Materials), it could be seen that the number of network-like protons of PE1.1M samples continued to decrease, which was consistent with the trend at 140 • C. While the number of network-like protons of PE0.5M samples began to level off due to the recovery of entanglements caused by the fast reptation of molecular chains. It agreed well with what was observed in partially cross-linked HDPE [50].…”

“…The residual dipolar coupling was contributed only by inter-segment proximity, including topological entanglements. The chain segments moved faster and could free themselves from physical topological constraints as the temperature rose [50]. This meant a decrease in network-like protons was expected, as shown in Figure S4 (see in Supplementary Materials).…”

“…1 H DQ NMR was robust to elucidate residual dipolar coupling, determining entanglement or crosslink densities in linear entangled polymer melts [50]. To further investigate the microscopic mechanism of the PE melting process, we tracked the entanglement network during PE melting with the help of 1 H DQ NMR.…”

Introducing “MEW2” Software: A Tool to Analyze MQ-NMR Experiments for Elastomers

“…Another improvement for the future would be the addition of more than one way to obtain D res , for instance, by fitting an Abragam-like function or through simultaneous fitting of two sets of data, as has been done in other works [34]. Further future developments of MEW2 could include different options for the analysis of various pulse sequences applied to elastomeric materials.…”