Broadband dielectric spectroscopy on benzophenone: alpha relaxation, beta relaxation, and mode coupling theory

TL;DR

This study provides a comprehensive dielectric analysis of benzophenone, revealing alpha and beta relaxations, and demonstrates that the alpha relaxation aligns well with mode coupling theory predictions, while the beta relaxation shows Johari-Goldstein characteristics.

Contribution

It offers the first detailed broadband dielectric characterization of benzophenone, linking experimental findings to mode coupling theory and comparing dielectric and optical relaxation features.

Findings

Alpha relaxation agrees with mode coupling theory predictions.

Detection of Johari-Goldstein beta relaxation near Tg.

Observation of an excess wing developing from beta relaxation.

Abstract

We have performed a detailed dielectric investigation of the relaxational dynamics of glass-forming benzophenone. Our measurements cover a broad frequency range of 0.1 Hz to 120 GHz and temperatures from far below the glass temperature well up into the region of the small-viscosity liquid. With respect to the alpha relaxation this material can be characterized as a typical molecular glass former with rather high fragility. A good agreement of the alpha relaxation behavior with the predictions of the mode coupling theory of the glass transition is stated. In addition, at temperatures below and in the vicinity of Tg we detect a well-pronounced beta relaxation of Johari-Goldstein type, which with increasing temperature develops into an excess wing. We compare our results to literature data from optical Kerr effect and depolarized light scattering experiments, where an excess-wing like feature was observed in the 1 - 100 GHz region. We address the question if the Cole-Cole peak, which was invoked to describe the optical Kerr effect data within the framework of the mode coupling theory, has any relation to the canonical beta relaxation detected by dielectric spectroscopy.