Correlation between entropy fluctuations and the dielectric relaxation of glass-forming systems: The central role of dipolar–dipolar cross correlations

Abstract

The premise that the dielectric α relaxation has a one-to-one correspondence with entropy fluctuations in equilibrium near the glass transition was experimentally verified in a systematic and quantitative manner for glass-forming systems in general. Validation of this relation was structured at different levels, taking into account various ingredients as the apolar–polar character, macromolecular structure, the presence of hydrogen bonds, or complex structure and dynamics. The results reclaim the suitability of dielectric spectroscopy to echo the primary structural relaxation of glass-forming systems, demonstrating that the dielectric response effectively captures the structural relaxation by reliably correlating with entropy fluctuations. The correlation with entropy fluctuations holds even when the dielectric strength of the systems is high and the dielectric response is narrow and dominated by cross correlations, proving that dipolar intermolecular interactions are fundamental to the structural relaxation and not a particularity of the dielectric probe. This one-to-one correspondence between structural and dielectric α relaxation does not support the existence of a generic spectral shape for the primary structural relaxation valid for all kinds of susceptibility functions.