Field-cycling 31P and 1H NMR relaxometry studying the reorientational dynamics of glass forming organophosphates

We apply field-cycling (FC) P-31 nuclear magnetic resonance (NMR) to access the reorientational susceptibility of two glass formers, m-tricresyl phosphate (m-TCP) and tri-butyl phosphate (TBP). Although FC P-31 studies are still instrumentally demanding, together with FC H-1 data, they provide site-resolved information. A crossover from dipolar relaxation at low frequencies to relaxation determined by chemical shift anisotropy at high frequencies and probed by conventional NMR is identified. A comparison is made between dielectric (DS) and depolarized light scattering (DLS) relaxation spectra demonstrating similar behavior close to T-g, including an excess wing contribution for m-TCP. The time constants of P-31 NMR and DLS, probing the molecular core, agree. The H-1 data monitoring the dynamics of the phenyl groups yield slightly shorter correlation times. At high temperatures, the DS relaxation spectra show a bimodal character: a fast component in agreement with H-1 data, and a slow component much slower than P-31 NMR and DLS suggest. We discuss the possible origins of the slow component. All time constants tend to merge toward T-g. Hence, we propose that site-specific dynamics disappear and a common alpha-relaxation establishes near T-g. In addition, we compare the diffusion coefficient D(T) determined by FC and static field gradient H-1 NMR. Concerning TBP, we present FC P-31 data of both alpha- and beta-processes. Regarding the latter, we compare the DS and NMR susceptibility on absolute scale, yielding a significantly stronger beta-relaxation in the P-31 NMR spectra.

Automated summary

This study applies field-cycling P-31 nuclear magnetic resonance (NMR) to investigate the reorientational susceptibility of two glass formers. The results reveal different relaxation behaviors at different frequencies and demonstrate similar behavior close to the glass transition temperature. Additionally, the diffusion coefficient is compared using different techniques.