Investigation of the shear-mechanical and dielectric relaxation processes in two monoalcohols close to the glass transition

Abstract

Shear-mechanical and dielectric measurements on the two monohydroxy (monoalcohol) molecular glass formers 2-ethyl-1-hexanol and 2-butanol close to the glass-transition temperature are presented. The shear-mechanical data are obtained using the piezoelectric shear-modulus gauge method covering frequencies from 1 mHz to 10 kHz. The shear-mechanical relaxation spectra show two processes, which follow the typical scenario of a structural (alpha) relaxation and an additional (Johari-Goldstein) beta relaxation. The dielectric relaxation spectra are dominated by a Debye-type peak with an additional non-Debye peak visible. This Debye-type relaxation is a common feature peculiar to monoalcohols. The time scale of the non-Debye dielectric relaxation process is shown to correspond to the mechanical structural (alpha) relaxation. Glass-transition temperatures and fragilities are reported based on the mechanical alpha relaxation and the dielectric Debye-type process, showing that the two glass-transition temperatures differ by approximately 10 K and that the fragility based on the Debye-type process is a factor of 2 smaller than the structural fragility. If a mechanical signature of the Debye-type relaxation exists in these liquids, its relaxation strength is at most 1% and 3% of the full relaxation strength of 2-butanol and 2-ethyl-1-hexanol, respectively. These findings support the notion that it is the non-Debye dielectric relaxation process that corresponds to the structural alpha relaxation in the liquid.

Slow Debye-type peak observed in the dielectric response of polyalcohols

“…There is also a smaller, and broader, peak at higher frequencies, which is to be identified as the ␣-relaxation. 7,9 We also show the spectra obtained at the same temperature using a thin ͑5 m͒ Teflon sheet covering the upper electrode. We see that, apart from a general reduction of the response, contributions from dc conductivity are reduced without significantly affecting the rest of the spectrum.…”

“…This further explains why the Debye-like process is not readily observable in other techniques such as e.g., mechanical spectroscopy. 9 The shape of the Debye-like peak in the polyalcohols is well described with a Cole-Cole function ͓Eq. ͑3͒ with ␤ =1͔ with only a slight symmetric broadening, partly because of the Teflon film ͑as discussed above͒.…”

Effect of high hydrostatic pressure on the dielectric relaxation in a non-crystallizable monohydroxy alcohol in its supercooled liquid and glassy states

“…24 One of the monohydroxyl alcohols 2-ethyl-1-hexanol has been studied extensively in the literature mainly with a view to resolving as to which of the two processes I or II, is responsible for the structural relaxation. [18][19][20][29][30][31] In particular, dielectric features have been compared with the observations made from calorimetry, 29 mechanical spectroscopy, 29 and recently NMR. 31 The effects of high pressure on dielectric properties have been investigated by three groups.…”

Communication: Slow supramolecular mode in amine and thiol derivatives of 2-ethyl-1-hexanol revealed by combined dielectric and shear-mechanical studies

“…[11][12][13] This is a slow mode which is often very intense in the dielectric signal sometimes completely covering the signal of the α-relaxation. 12,[14][15][16][17] Experimental observations have revealed that this mode has an exponential character, hence the name Debye relaxation. The prominent dielectric amplitude of the Debye relaxation in monohydroxy alcohols cannot be explained exclusively from the molecular dipole moment, but it is due to hydrogen bonding associations (chains, rings, or other units) that induce changes in the orientational correlations of dipoles.…”