Journal
JOURNAL OF CHEMICAL PHYSICS
Volume 147, Issue 13, Pages -Publisher
AMER INST PHYSICS
DOI: 10.1063/1.4994868
Keywords
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Funding
- National Basic Research Program of China (973 Program) [2015CB856805]
- National Natural Science Foundation of China (NSFC) [11474247, 51131002]
- Natural Science Foundation of Hebei Province [A2014203260]
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The dynamics of a molecular glass former, tributyl phosphate (TBP), with an alkyl phosphate structure (three alkyl branches emanating from a polar core of PO4) is studied in the supercooled regime by dielectric and thermal (or enthalpic) relaxations. The dielectric fragility index md and the stretching exponent beta(d) of the Kohlrausch-Williams-Watts correlation function are determined. Analyses of the enthalpic relaxation data by the Tool-Narayanaswamy-Moynihan-Hodge formalism yield the enthalpic fragility index mH and stretching exponent fi H. The large difference between the dielectric md and the enthalpic mH, as well as between beta(d) and beta(H), is a remarkable finding. The differences are interpreted by the formation of molecular self-assemblies. The interpretation is supported by the quite comparable fragility determined by viscosity and the enthalpic relaxation. The Kirkwood factor calculated at low temperatures is also consistent with the interpretation. The results suggest that the enthalpic relaxation involving the motions of all parts of TBP is global, while the dielectric relaxation detects the local rotation, which might originate from the rotation of the dipole moment of the core. The presence of two structural alpha-relaxations, one global and one local, with a large difference in dynamics is revealed for the first time in a molecular glass former. Published by AIP Publishing.
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