Effect of Chemical Structure on the Isobaric and Isochoric Fragility in Polychlorinated Biphenyls

TL;DR

This study investigates how chemical structure, specifically chlorine content, influences the relaxation dynamics and fragility of polychlorinated biphenyls under different pressure and temperature conditions.

Contribution

It demonstrates that increasing chlorine content enhances the density's role in relaxation times and fragility, revealing the impact of molecular bulkiness on glassy dynamics.

Findings

Relaxation times are more density-dependent with higher chlorine content.

Fragility at constant volume decreases as chlorine content increases.

Density scaling effectively superimposes relaxation data across conditions.

Abstract

Pressure-volume-temperature data, along with dielectric relaxation measurements, are reported for a series of polychlorinated biphenyls (PCB), differing in the number of chlorine atoms on their phenyl rings. Analysis of the results reveals that with increasing chlorine content, the relaxation times of the PCB become governed to a greater degree by density, rho, relative to the effect of temperature, T. This result is consistent with the respective magnitudes of the scaling exponent, gamma, yielding superpositioning of the relaxation times measured at various temperatures and pressures, when plotted versus rho^gamma/T. While at constant (atmospheric) pressure, fragilities for the various PCB are equivalent, the fragility at constant volume varies inversely with chlorine content. Evidently, the presence of bulkier chlorine atoms on the phenyl rings magnifies the effect density has on the relaxation dynamics.