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

Abstract

Articles you may be interested inDielectric relaxation and crystallization of nanophase separated 1-propanol-isoamylbromide mixture 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, TD/T 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 TD/T 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.

Mechanical spectra of glass-forming liquids. I. Low-frequency bulk and shear moduli of DC704 and 5-PPE measured by piezoceramic transducers

“…There are reports of constant time-scale indices in measurements, [23][24][25][26] as well as reports of a temperaturedependent time scale indices. [27][28][29] Sometimes the relaxation times (or peak frequencies) of different probes are simply compared in an Arrhenius plot (log τ or log ω max as a function of inverse temperature) of different probes without explicitly evaluating the time scale index, see, e.g., Schröter and Donth, 30 who collected data on glycerol from dielectric, heat capacity, shear retardation, and nuclear magnetic resonance measurements. However, none of these studies compare quantities measured under identical experimental conditions, which is critical when comparing, e.g., relaxation times which are extremely temperature dependent.…”

Coupling of the electrical conductivity to the structural relaxation, absence of physical aging on the time scale of the Debye process, and number of correlated molecules in the supercooled monohydroxy alcohol 2-ethylhexanol

“…However, despite numerous recent dielectric studies directed at unraveling the nature of the Debye-like response in monohydroxy alcohols [14][15][16][17], its microscopic origin is still not generally agreed upon. Adding to the puzzle is the finding that no significant signature of the Debye-like relaxation process could be detected in mechanical [18,19] and in specific heat spectroscopy [13,20]. It should be noted that earlier differential scanning calorimetric data suggested that the Debye process could add a few percent to the change of the heat capacity accompanying the glass transition [21].…”

Liquid 1-propanol studied by neutron scattering, near-infrared, and dielectric spectroscopy

“…[20][21][22][23] Monohydroxy alcohols, just like all other liquids, show a structural relaxation process also called α-relaxation. The αprocess is resolved by a number of experimental techniques such as dielectric [24][25][26] and mechanical spectroscopy 27 as well as calorimetry 28 and light scattering. 29 For monohydroxy alcohols the α-process has been shown to be related to the motion of the molecular alkyl chain.…”