Tina Hecksher

Shear-modulus investigations of monohydroxy alcohols: evidence for a short-chain-polymer rheological response.

In addition to the ubiquitous structural relaxation of viscous supercooled liquids, monohydroxy alcohols and several other hydrogen-bonded systems display a strong single-exponential electrical low-frequency absorption. So far, this so-called Debye process could be observed only using dielectric techniques. Exploiting a combination of broad-band and high-resolution rheology experiments for three isomeric octanols, unambiguous mechanical evidence for the Debye process is found. Its spectral signature is similar to the viscoelastic fingerprint of small-chain polymers, enabling us to estimate the effective molecular weight for the supramolecular structure formed by the studied monohydroxy alcohols. This finding opens the venue for the application of further non-dielectric techniques directed at unraveling the microscopic nature of the Debye process and for an understanding of this phenomenon in terms of polymer concepts.

Mechanical spectra of glass-forming liquids. II. Gigahertz-frequency longitudinal and shear acoustic dynamics in glycerol and DC704 studied by time-domain Brillouin scattering

This paper presents and discusses the temperature and frequency dependence of the longitudinal and shear viscoelastic response at MHz and GHz frequencies of the intermediate glass former glycerol and the fragile glass former tetramethyl-tetraphenyl-trisiloxane (DC704). Measurements were performed using the recently developed time-domain Brillouin scattering technique, in which acoustic waves are generated optically, propagated through nm thin liquid layers of different thicknesses, and detected optically after transmission into a transparent detection substrate. This allows for a determination of the frequency dependence of the speed of sound and the sound-wave attenuation. When the data are converted into mechanical moduli, a linear relationship between longitudinal and shear acoustic moduli is revealed, which is consistent with the generalized Cauchy relation. In glycerol, the temperature dependence of the shear acoustic relaxation time agrees well with literature data for dielectric measurements. In DC704, combining the new data with data from measurements obtained previously by piezo-ceramic transducers yields figures showing the longitudinal and shear sound velocities at frequencies from mHz to GHz over an extended range of temperatures. The shoving models prediction for the relaxation times temperature dependence is fairly well obeyed for both liquids as demonstrated from a plot with no adjustable parameters. Finally, we show that for both liquids the instantaneous shear modulus follows an exponential temperature dependence to a good approximation, as predicted by Granatos interstitialcy model.

Experimental studies of Debye-like process and structural relaxation in mixtures of 2-ethyl-1-hexanol and 2-ethyl-1-hexyl bromide

Binary solutions of 2-ethyl-1-hexanol (2E1H) with 2-ethyl-1-hexyl bromide (2E1Br) are investigated by means of dielectric, shear mechanical, near-infrared, and solvation spectroscopy as well as dielectrically monitored physical aging. For moderately diluted 2E1H the slow Debye-like process, which dominates the dielectric spectra of the neat monohydroxy alcohol, separates significantly from the α-relaxation. For example, the separation in equimolar mixtures amounts to four decades in frequency. This situation of highly resolved processes allows one to demonstrate unambiguously that physical aging is governed by the α-process, but even under these ideal conditions the Debye process remains undetectable in shear mechanical experiments. Furthermore, the solvation experiments show that under constant charge conditions the microscopic polarization fluctuations take place on the time scale of the structural process. The hydrogen-bond populations monitored via near-infrared spectroscopy indicate the presence of a critical alcohol concentration, x(c) ≈ 0.5-0.6, thereby confirming the dielectric data. In the pure bromide a slow dielectric process of reduced intensity is present in addition to the main relaxation. This is taken as a sign of intermolecular cooperativity probably mediated via halogen bonds.

Communication: Identical temperature dependence of the time scales of several linear-response functions of two glass-forming liquids

The frequency-dependent dielectric constant, shear and adiabatic bulk moduli, longitudinal thermal expansion coefficient, and longitudinal specific heat have been measured for two van der Waals glass-forming liquids, tetramethyl-tetraphenyl-trisiloxane (DC704) and 5-polyphenyl-4-ether. Within the experimental uncertainties the loss-peak frequencies of the measured response functions have identical temperature dependence over a range of temperatures, for which the Maxwell relaxation time varies more than nine orders of magnitude. The time scales are ordered from fastest to slowest as follows: Shear modulus, adiabatic bulk modulus, dielectric constant, longitudinal thermal expansion coefficient, and longitudinal specific heat. The ordering is discussed in light of the recent conjecture that van der Waals liquids are strongly correlating, i.e., approximate single-parameter liquids.

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

We present dynamic shear and bulk modulus measurements of supercooled tetraphenyl-tetramethyl-trisiloxane (DC704) and 5-phenyl-4-ether over a range of temperatures close to their glass transition. The data are analyzed and compared in terms of time-temperature superposition (TTS), the relaxation time, and the spectral shape parameters. We conclude that TTS is obeyed to a good approximation for both the bulk and shear moduli. The loss-peak shapes are nearly identical, while the shear modulus relaxes faster than the bulk modulus. The temperature dependence of this decoupling of time scales is constant over the temperature range explored here. In addition, we demonstrate how one can measure reliably the DC shear viscosity over ten orders of magnitude by using the two measuring techniques in combination.

Shear and dielectric responses of propylene carbonate, tripropylene glycol, and a mixture of two secondary amides

Propylene carbonate and a mixture of two secondary amides, N-methylformamide and N-ethylacetamide, are investigated by means of broadband dielectric and mechanical shear spectroscopy. The similarities between the rheological and the dielectric responses of these liquids and of the previously investigated tripropylene glycol are discussed within a simple approach that employs an electrical circuit for describing the frequency-dependent behavior of viscous materials. The circuit is equivalent to the Gemant-DiMarzio-Bishop model, but allows for a negative capacitive element. The circuit can be used to calculate the dielectric from the mechanical response and vice versa. Using a single parameter for a given system, good agreement between model calculations and experimental data is achieved for the entire relaxation spectra, including secondary relaxations and the Debye-like dielectric peak in the secondary amides. In addition, the predictions of the shoving model are confirmed for the investigated liquids.

Communication: Supramolecular structures in monohydroxy alcohols: Insights from shear-mechanical studies of a systematic series of octanol structural isomers

A recent study [C. Gainaru, R. Figuli, T. Hecksher, B. Jakobsen, J. C. Dyre, M. Wilhelm, and R. Böhmer, Phys. Rev. Lett. 112, 098301 (2014)] of two supercooled monohydroxy alcohols close to the glass-transition temperature showed that the Debye peak, thus far mainly observed in the electrical response, also has a mechanical signature. In this work, we apply broadband shear-mechanical spectroscopy to a systematic series of octanol structural isomers, x-methyl-3-heptanol (with x ranging from 2 to 6). We find that the characteristics of the mechanical signature overall follow the systematic behavior observed in dielectric spectroscopy. However, the influence from the molecular structure is strikingly small in mechanics (compared to roughly a factor 100 increase in dielectric strength) and one isomer clearly does not conform to the general ordering. Finally, the mechanical data surprisingly indicate that the size of the supramolecular structures responsible for the Debye process is nearly unchanged in the series.

Broadband dynamics in neat 4-methyl-3-heptanol and in mixtures with 2-ethyl-1-hexanol

The relatively small dielectric Debye-like process of the monohydroxy alcohol 4-methyl-3-heptanol (4M3H) was found to depend slightly on the intramolecular conformation. Proton and deuteron nuclear magnetic resonance demonstrate that the hydroxyl dynamics and the overall molecular dynamics take place on similar time scales in contrast to the situation for the structural isomer 2-ethyl-1-hexanol (2E1H) [S. Schildmann et al., J. Chem. Phys. 135, 174511 (2011)]. This indicates a very weak decoupling of Debye-like and structural relaxation which was further probed using volume expansivity experiments. Shear viscosity as well as diffusometry measurements were performed and the data were analyzed in terms of the Debye-Stokes-Einstein equations. In mixtures of 4M3H with 2E1H the Debye-like process becomes much stronger and for 2E1H mole fraction of more than 25% the behavior of this alcohol is rapidly approached. This finding is interpreted to indicate that the ring-like supramolecular structures in 4M3H become energetically unfavorable when adding 2E1H, an alcohol that tends to form chain-like molecular aggregates. The concentration dependence of the Kirkwood factor in these mixtures displays a high degree of similarity with experimental results on monohydroxy alcohols in which the pressure or the location of the OH group within the molecular structure is varied.

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

In this paper, we present results of dielectric and shear-mechanical studies for amine (2-ethyl-1-hexylamine) and thiol (2-ethyl-1-hexanethiol) derivatives of the monohydroxy alcohol, 2-ethyl-1-hexanol. The amine and thiol can form hydrogen bonds weaker in strength than those of the alcohol. The combination of dielectric and shear-mechanical data enables us to reveal the presence of a relaxation mode slower than the α-relaxation. This mode is analogous to the Debye mode seen in monohydroxy alcohols and demonstrates that supramolecular structures are present for systems with lower hydrogen bonding strength. We report some key features accompanying the decrease in the strength of the hydrogen bonding interactions on the relaxation dynamics close to the glass-transition. This includes changes (i) in the amplitude of the Debye and α-relaxations and (ii) the separation between primary and secondary modes.

Communication: Linking the dielectric Debye process in mono-alcohols to density fluctuations

We provide the first evidence that the puzzling dielectric Debye process observed in mono-alcohols is coupled to density fluctuations. The results open up for an explanation of the Debye process within the framework of conventional liquid-state theory. The spectral shape of dynamical bulk modulus of 2-ethyl-1-hexanol is nearly identical to that of the shear modulus, and thus the supramolecular structures believed to be responsible for the slow dielectric Debye process are manifested in the bulk modulus in the same way as in the shear modulus.This work provides the first direct evidence that the puzzling dielectric Debye process observed in mono-alcohols is coupled to density fluctuations. The results open up for an explanation of the Debye process within the framework of conventional liquid-state theory. The spectral shape of the dynamical bulk modulus of the two studied mono-alcohols, 2-ethyl-1-hexanol and 4-methyl-3-heptanol, is nearly identical to that of their corresponding shear modulus, and thus the supramolecular structures believed to be responsible for the slow dielectric Debye process are manifested in the bulk modulus in the same way as in the shear modulus.