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

We apply field-cycling (FC) P 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 studies are still instrumentally demanding, together with FC H data, they provide site-resolved...

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Bibliographic Details
Published in:The Journal of chemical physics 2022-02, Vol.156 (7), p.074502
Main Authors: Becher, M, Flämig, M, Rössler, E A
Format: Article
Language:English
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Summary:We apply field-cycling (FC) P 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 studies are still instrumentally demanding, together with FC H 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 , including an excess wing contribution for m-TCP. The time constants of P NMR and DLS, probing the molecular core, agree. The H 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 data, and a slow component much slower than P NMR and DLS suggest. We discuss the possible origins of the slow component. All time constants tend to merge toward T . Hence, we propose that site-specific dynamics disappear and a common α-relaxation establishes near T . In addition, we compare the diffusion coefficient D(T) determined by FC and static field gradient H NMR. Concerning TBP, we present FC P data of both α- and β-processes. Regarding the latter, we compare the DS and NMR susceptibility on absolute scale, yielding a significantly stronger β-relaxation in the P NMR spectra.
ISSN:1089-7690