Structural and orientational relaxation in supercooled liquid triphenylphosphite

Silence, Scott M.; Duggal, Anil R.; Dhar, Lisa; Nelson, Keith A.
April 1992
Journal of Chemical Physics;4/1/1992, Vol. 96 Issue 7, p5448
Academic Journal
Structural and orientational relaxation processes in the glass-forming liquid triphenylphosphite (TPP) were studied by impulsive stimulated scattering, a time-resolved four-wave mixing technique. The α structural relaxation processes which can couple to the longitudinal and shear acoustic modes are analyzed phenomenologically in terms of a distribution of relaxation times fL(τL,T) or fS(τS,T), respectively, in the temperature range of 270–240 K. The two distributions appear to be identical, and undergo marked broadening as the sample is cooled. They are best described by a stretched exponential relaxation function whose exponent decreases with temperature. The molecular orientational relaxation times τor follow an Arrhenius temperature dependence and are longer than the average relaxation times 〈τL> and 〈τS> at high temperature, but converge as the temperature is lowered. We conclude that the observed orientational relaxation is decoupled from the α structural relaxation process and is a manifestation of the β orientational relaxation process. The low frequency intramolecular mode observed in the experiment was found to be essentially independent of temperature. A picture of structural and orientational relaxation processes in supercooled TPP consistent with recent theories of structural relaxation processes in supercooled liquids is proposed.


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