Lowfield NMR of Elastomers

Measurements with nuclear magnetic resonance spectroscopy (solid state NMR) allow the non-destructive determination of changes in network structure caused by aging, the plateau modulus, or the swelling grade. Typical values measured in solid state NMR for elastomer characterization are the decrease of magnetization and the spin-spin relaxation time T2. From the adjustment of magnetization decrease, the molar mass of the network arc between two cross-linked sections can be determined.

During the aging of elastomers, thermo-oxidative degradation and formation of new cross-links occur simultaneously. Additionally to chemical cross links, filler materials also form new cross-links. For this reason, changes in filler network (polymer-filler interactions) also need to be examined. A comparison with other characterization methods and vibrational resistance experiments allow conclusions on the effects of aging on the molecular structure.

NMR Measurement of a Sulfur Cross-Linked Rubber

The figure exemplarily shows the decrease in magnetization of a Hahn-Echo sequence of a sulfur cross-linked rubber after a storage time of 24 hours at 100 °C under air atmosphere. The NMR relaxation behavior depends on the structure of the cross-linked elastomer. Through adjustment with theoretical models the network arc length between two cross-links can be determined. The changes of network/cross-link density can be monitored with aging experiments.