An EPR spin label study of the glass forming liquid dibutyl phthalate

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Abstract
Spectra from a small nearly spherical probe and a large nearly cylindrical probe are studied as a function of temperature from 130–370 K. These spectra are used to calculate a rotational correlation time τ and the temperature dependence of τ is matched, at higher temperatures, to existing viscositymodels such as the energy activation, Vogel–Fulcher, and Grest–Cohen models. The glass temperature is observed at ∼ 180 K and the data indicate a reduction in translational diffusion, but not rotational diffusion, as the glassy state is reached. We also observe that the spherical probe tumbles isotropically while the cylindrical probe tumbles anisotropically. In the latter case, the tumbling can be represented by an axially symmetric rotational diffusiontensor whose axis coincides with the long axis of the molecule. Finally, we present a phenomenological model for the temperature dependence of τ which fits the data well at all temperatures.