Effect of Chemical Structure on the Isobaric and Isochoric Fragility in   Polychlorinated Biphenyls
  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. 
C.M. Roland and R. Casalini