Comparison of Dielectric and Mechanical Relaxation Processes in Glycerol—n-Propanol Mixtures

Dielectric and shear mechanical relaxation data are reported for mixtures of glycerol and n‐propanol. Measurements of the dielectric permittivity were made at −60° over a frequency range from 50 cps to 600 kc/sec and indicate a distribution of dielectric relaxation times of the Davidson—Cole form. This distribution was observed to narrow with increasing n‐propanol content. Shear mechanical measurements in the temperature range +10° to −150°C and frequency range 10 to 130 Mc/sec indicate a broad distribution of times which is essentially independent of the relative concentration of the two components of the mixture. The width of the dielectric distribution is shown to be a function of the ratio of the average dielectric to shear relaxation times, ${\mathrm{\tau ̄}}_D{\mathrm{/\tau ̄}}_S$ , becoming narrow as this ratio becomes large. These results are in accord with the concepts of McDuffie and Litovitz who proposed that the origin of the distribution of dielectric relaxation times is related to a cooperative breakup of the liquid structure and that the effects of this structural breakup on the dielectric process are lessened when the dielectric reorientation process takes a much longer time than the structural breakup. In pure n‐propanol it is found that ${\mathrm{\tau ̄}}_D{\mathrm{/\tau ̄}}_S$ is temperature dependent in contrast to a temperature‐independent ratio in glycerol. In order to explain these results, it is suggested that the local free volume in n‐propanol needed for dielectric orientation is larger than that needed for the shear process. Using the free‐volume theory of Macedo and Litovitz it is estimated that in pure n‐propanol the critical local free volume for dielectric orientation is 40% larger than the critical local free volume for shear flow.