Low-field nuclear magnetic resonance (NMR) method for studying the molecular dynamics of polymers—crosslinking density, aging process, and fillers

In both industrial production and research laboratories, there is a need for a fast, effective, and simple method to evaluate crosslink density. Low-field NMR is particularly suited for detecting crosslink density changes in production settings. It is easy to use and can serve as a quality control tool during polymer production. Additionally, low-field NMR is highly sensitive to the molecular dynamics of polymers, making it ideal for multi-scale molecular dynamics research, providing reliable data for polymer modification, formulation, aging, and performance evaluation—making it an essential tool for scientific research.

The primary detection target for low-field NMR is hydrogen nuclei (1H). Due to the varying environments of hydrogen atoms on different polymer chain segments, the hydrogen spin magnetic moments (nuclear spins) differ. When a radiofrequency pulse is applied, the relaxation behavior observed as the spin system returns to thermal equilibrium differs based on these environments. The differences in relaxation times provide molecular dynamics information of the polymer system. This molecular dynamics information is directly related to the crosslink density, aging, and fillers in the polymer.

Intermolecular and intramolecular hydrogen proton dipole interactions produce nuclear magnetic resonance (NMR) transverse relaxation. When the temperature is much higher than the polymer’s glass transition temperature, these dipole interactions in the polymer network are considered to average the thermal molecular motion. Because hydrogen protons in the polymer single chain serve as probes for NMR measurements, a modified single-chain model has been introduced to explain the polymer’s transverse relaxation.

Low-Field NMR Can Be Used to Study:

 

1. Determination of activation energy;

2. Crosslink density testing of natural rubber (NR);

3. The impact of sulfur content on rubber crosslinking;

4. The influence of accelerator type and dosage on rubber crosslinking;

5. The effect of zinc oxide and stearic acid content on rubber crosslinking;

6. The evolution of corresponding magnetic resonance model parameters during the rubber vulcanization process;

7. The effect of mixing time on magnetic resonance model parameters;

8. The impact of nanoclay content on rubber crosslinking.