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

In recent years, H-1 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 cross-linking is found. By direct comparison of the rheological results and the NMR-based segmental orientation autocorrelation functions (OACF) via time-temperature 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.