Presence of global and local α-relaxations in an alkyl phosphate glass former

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 m_d and the stretching exponent β_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 m_H and stretching exponent β_H. The large difference between the dielectric m_d and the enthalpic m_H, as well as between β_d and β_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 α-relaxations, one global and one local, with a large difference in dynamics is revealed for the first time in a molecular glass former.