On the nature of the high-frequency relaxation in a molecular glass former: A joint study of glycerol by field cycling NMR, dielectric spectroscopy, and light scattering

C. Gainaru, O. Lips, A. Troshagina, R. Kahlau, A. Brodin, F. Fujara, E. A. Rössler

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Abstract

Fast field cycling H1 NMR relaxometry is applied to determine the dispersion of spin-lattice relaxation time T1 (ω) of the glass former glycerol in broad temperature (75-360 K) and frequency (10 kHz-30 MHz) ranges. The relaxation data are analyzed in terms of a susceptibility ″ (ω) ∞ ω T1 (ω), related to the second rank (l=2) molecular orientational correlation function. Broadband dielectric spectroscopic results suggest the validity of frequency temperature superposition above the glass transition temperature Tg. This allows to combine NMR data of different temperatures into a single master curve ″ (ω τα) that extends over 15 decades in reduced frequency ω τα, where τα is the structural α -relaxation time. This master curve is compared with the corresponding ones from dielectric spectroscopy (l=1) and depolarized light scattering (l=2). At ω τα <1, NMR susceptibility is significantly different from both the dielectric and light scattering results. At ω τα 1, there rather appears a difference between the susceptibilities of rank l=1 and l=2. Specifically, at ω τα 1, where the susceptibility is dominated by the so-called excess wing, the NMR and light scattering spectra (both l=2) rather coincide with each other and are about three times more intense than the dielectric (l=1) spectrum. This is explained by assuming that the high frequency dynamics correspond to only small-angle excursions. Below Tg, dielectric and NMR susceptibility compare well and exhibit an exponential temperature dependence.

Original languageEnglish
Article number174505
JournalJournal of Chemical Physics
Volume128
Issue number17
DOIs
StatePublished - 2008
Externally publishedYes

Funding

The authors thank G. Diezemann and V.N. Novikov for very fruitful discussions. The support by Deutsche Forschungsgemeinschaft (DFG) through Grant Nos. FU 308/9, RO 907/8, and SFB 481 is appreciated.

FundersFunder number
Deutsche ForschungsgemeinschaftFU 308/9, RO 907/8

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