On the origin of time-aging-time superposition.

Time-aging-time superposition and the concept of single-parameter aging refer to the experimentally verified scenario in which the relaxation profile is shifted as a whole along the logarithmic time or frequency scale during physical aging, i.e., without changing the shape of the susceptibility spectrum or decay function. This homogeneous aspect of aging and structural recovery appears to contrast the heterogeneous nature of structural relaxation in equilibrium. A picture is proposed in which both structural recovery and relaxation are heterogeneous, but lacking a local correlation of time constants. This scenario is consistent with time-aging-time superposition and single-parameter aging, as well as with recovery and relaxation processes being subject to practically the same time constant dispersion.

To age or not to age: Anatomy of a supercooled liquid's response to a high alternating electric field.

Physical aging and structural recovery are the processes with which the structure of a system approaches equilibrium after some perturbation. Various methods exist, that initiate structural recovery, such as changing the temperature or applying a strong, external static field. This work is concerned with high alternating electric fields and their suitability to study structural recovery and aging. The present work demonstrates that rationalizing the nonlinear dielectric response of a supercooled liquid to high-amplitude ac-fields requires multiple fictive temperatures. This feature is in stark contrast to structural recovery after a temperature down-jump or a considerable increase in the static electric field, for which a single parameter, the fictive temperature or material time, describes the structural change. In other words, the structural recovery from a high ac-field does not adhere to time aging-time superposition, which is so characteristic of genuine aging processes.

Comparing two sources of physical aging: Temperature vs electric field.

Physical aging is the process of a system evolving toward a new equilibrium, and thus the response to a change in external parameters such as temperature T, pressure p, or static electric field E. Using a static electric field has been shown to access physical aging above the glass transition temperature Tg, in the regime of milliseconds or faster, but the relation to its temperature jump counterpart has not been investigated to date. This work compares temperature and field induced physical aging in the limit of small perturbations for supercooled tributyl phosphate. It is found that both structural recovery dynamics are very similar, and that they match the collective reorientational dynamics as observed by dielectric relaxation. The results facilitate expanding the range of aging experiments to well above Tg, where a comparison with structural relaxation in equilibrium is straightforward, thus improving models of structural recovery and physical aging.

Fast vs slow physical aging of a glass forming liquid.

Using electric fields to initiate the process of physical aging has facilitated measurements of structural recovery dynamics on the time scale of milliseconds. This, however, complicates the interesting comparison with aging processes due to a temperature jump, as these are significantly slower. This study takes a step toward comparing the results of field and temperature perturbations by providing data on field-induced structural recovery of vinyl ethylene carbonate at two different time scales: 1.0 ms at 181 K and 33 s at 169 K, i.e., 4.5 decades apart. It is found that structural recovery is a factor of two slower than structural relaxation in equilibrium, with the latter determined via dielectric relaxation in the limit of linear response. The relation between recovery and relaxation dynamics remains temperature invariant across the present experimental range.

Engineering the glass structure of a discotic liquid crystal by multiple kinetic arrests.

X-ray scattering has been used to characterize the columnar packing and the π stacking in a glass-forming discotic liquid crystal. In the equilibrium liquid state, the intensities of the scattering peaks for π stacking and columnar packing are proportional to each other, indicating concurrent development of the two orders. Upon cooling into the glassy state, the π-π distance shows a kinetic arrest with a change in the thermal expansion coefficient (TEC) from 321 to 109 ppm/K, while the intercolumnar spacing exhibits a constant TEC of 113 ppm/K. By changing the cooling rate, it is possible to prepare glasses with a wide range of columnar and π stacking orders, including zero order. For each glass, the columnar order and the π stacking order correspond to a much hotter liquid than its enthalpy and π-π distance, with the difference between the two internal (fictive) temperatures exceeding 100 K. By comparison with the relaxation map obtained by dielectric spectroscopy, we find that the δ mode (disk tumbling within a column) controls the columnar order and the π stacking order trapped in the glass, while the α mode (disk spinning about its axis) controls the enthalpy and the π-π spacing. Our finding is relevant for controlling the different structural features of a molecular glass to optimize its properties.

One experiment makes a direct comparison of structural recovery with equilibrium relaxation.

For a molecular glass-former, propylene glycol, we directly compare the equilibrium fluctuations, measured as "structural" relaxation in the regime of linear response, with structural recovery, i.e., field induced physical aging in the limit of a small perturbation. The two distinct correlation functions are derived from a single experiment. Because the relaxation time changes only 2% during structural recovery, no aging model is needed to analyze the results. Although being conceptually different processes, dielectric relaxation and recovery dynamics are observed to be identical for propylene glycol, whereas single-particle dynamics as seen by photon correlation spectroscopy are significantly faster. This confirms the notion that structural recovery and aging are governed by all modes observed by dielectric spectroscopy, i.e., including cross correlations, not only by single-particle dynamics. A comparison with analogous results for other materials suggests that the relation between relaxation and recovery time scales may be material specific rather than universal.

Strong increase of correlations in liquid glycerol observed by nonlinear dielectric techniques.

Nonlinear dielectric measurements are an important tool to access material properties and dynamics concealed in their linear counterparts, but the available data are often intermittent and, on occasion, even contradictory. Employing and refining a recently developed technique for high ac field dielectric measurements in the static limit, we ascertain nonlinear effects in glycerol over a wide temperature range from 230 to 320 K. We find that the temperature dependence of the Piekara factor a, which quantifies the saturation effect, changes drastically around 290 K, from ∂a/∂T = +1.4 to -130 in units of 10-18 V2 m-2 K-1. These high values of |a| quantify not only elevated dielectric saturation effects but also indicate a temperature driven increase in higher-order orientational correlations and considerable correction terms with respect to the central limit theorem. No signature of this feature can be found in the corresponding low field data.

Structures of glasses created by multiple kinetic arrests.

X-ray scattering has been used to characterize glassy itraconazole (ITZ) prepared by cooling at different rates. Faster cooling produces ITZ glasses with lower (or zero) smectic order with more sinusoidal density modulation, larger molecular spacing, and shorter lateral correlation between the rod-like molecules. We find that each glass is characterized by not one, but two fictive temperatures Tf (the temperature at which a chosen order parameter is frozen in the equilibrium liquid). The higher Tf is associated with the regularity of smectic layers and lateral packing, while the lower Tf with the molecular spacings between and within smectic layers. This indicates that different structural features are frozen on different timescales. The two timescales for ITZ correspond to its two relaxation modes observed by dielectric spectroscopy: the slower δ mode (end-over-end rotation) is associated with the freezing of the regularity of molecular packing and the faster α mode (rotation about the long axis) with the freezing of the spacing between molecules. Our finding suggests a way to selectively control the structural features of glasses.

A liquid with distinct metastable structures: Supercooled butyronitrile.

The dielectric relaxation behavior of the molecular glass former butyronitrile is revisited by measuring both bulk samples cooled from the melt and samples obtained by physical vapor deposition. We find that the dielectric constant in the viscous regime of the bulk liquid is much higher than reported previously, reaching εs = 63 at T = 103 K, i.e., just above the glass transition temperature Tg = 97 K. By contrast, varying the deposition temperature and rate of vapor-deposited samples leads to dielectric constants in a range between 4.5 and 63 at T = 103 K. Values much below εs = 63 persist for thousands of seconds, where the dielectric relaxation time is about 0.1 s. The observations can be interpreted by the formation of clusters in which pair-wise anti-parallel dipole orientation is the preferred state at temperatures well below the glass transition. These non-crystalline clusters are long-lived even above Tg, where the remaining volume fraction is in the state of the equilibrium polar liquid.

AC versus DC field effects on the crystallization behavior of a molecular liquid, vinyl ethylene carbonate (VEC).

Using electric fields to control crystallization processes shows a strong potential for improving pharmaceuticals, but these field effects are not yet fully explored nor understood. This study investigates how the application of alternating high electric fields can influence the crystallization kinetics as well as the final crystal product, with a focus on the possible difference between alternating (ac) and static (dc) type fields applied to vinyl ethylene carbonate (VEC), a molecular system with field-induced polymorphism. Relative to ac fields, static electric fields lead to more severe accumulation of impurity ions near the electrodes, possibly affecting the crystallization behavior. By tuning the amplitude and frequency of the electric field, the crystallization rate can be modified, and the crystallization outcome can be guided to form one or the other polymorph with high purity, analogous to the findings derived from dc field experiments. Additionally, it is found that low-frequency ac fields reduce the induction time, promote nucleation near Tg, and affect crystallization rates as in the dc case. Consistency is also observed for the Avrami parameters n derived from ac and dc field experiments. Therefore, it appears safe to conclude that ac fields can replicate the effects seen using dc fields, which is advantageous for samples with mobile charges and the resulting conductivity.

Polyamorphism in vapor-deposited 2-methyltetrahydrofuran: A broadband dielectric relaxation study.

Depositing a simple organic molecular glass-former 2-methyltetrahydrofuran (MTHF) onto an interdigitated electrode device via physical vapor deposition gives rise to an unexpected variety of states, as revealed by dielectric spectroscopy. Different preparation parameters, such as deposition temperature, deposition rate, and annealing conditions, lead, on the one hand, to an ultrastable glass and, on the other hand, to a continuum of newfound further states. Deposition below the glass transition temperature of MTHF leads to loss profiles with shape parameters and peak frequencies that differ from those of the known bulk MTHF. These loss spectra also reveal an additional process with Arrhenius-like temperature dependence, which can be more than four decades slower than the main structural relaxation peak. At a given temperature, the time constants of MTHF deposited between 120 K and 127 K span a range of more than three decades and their temperature dependencies change from strong to fragile behavior. This polyamorphism involves at least three distinct states, each persisting for a duration many orders of magnitude above the dielectric relaxation time. These results represent a significant expansion of a previous dielectric study on vapor deposited MTHF [B. Riechers et al., J. Chem. Phys. 150, 214502 (2019)]. Plastic crystal states and the effects of weak hydrogen bonding are discussed as structural features that could explain these unusual states.

Organic glasses with tunable liquid-crystalline order through kinetic arrest of end-over-end rotation: the case of saperconazole.

Liquid crystals (LCs) undergo fast phase transitions, almost without hysteresis, leading to the notion that it is difficult to bypass LC transitions. However, recent work on itraconazole has shown that a nematic-to-smectic phase transition can be frustrated or avoided at moderate cooling rates. At each cooling rate, the highest smectic order obtained is determined by the kinetic arrest of the end-over-end molecular rotation. We report that the same phenomenon occurs in the system saperconazole, an analog of itraconazole where each of the two Cl atoms is replaced by F. Saperconazole has a wider temperature range over which smectic order can develop before kinetic arrest, providing a stronger test of the previous conclusion. Together these results indicate a general principle for controlling LC order in organic glasses for electronic applications.

Ultrastable and polyamorphic states of vapor-deposited 2-methyltetrahydrofuran.

This work reports results gained from dielectric spectroscopy on the organic molecular glass-former 2-methyltetrahydrofuran (MTHF), which was deposited onto an interdigitated electrode device by physical vapor deposition. By a suitable selection of preparation parameters (deposition temperature, deposition rate, and annealing conditions), various states of MTHF could be created: ultrastable glass, a liquid state with unusual dielectric properties, or the ordinary liquid state as obtained by supercooling. Observations on kinetic stability as well as on the suppression of dielectric loss in the ultrastable state resemble previous findings for other molecular glass-formers. Remarkably, after annealing just above Tg, all vapor-deposited films of MTHF display a static dielectric constant in the liquid state (εs) that is up to a factor of two below that of the ordinary bulk liquid. A structural transition to the ordinary liquid-cooled state of MTHF occurs at temperatures far above its conventional Tg, indicative of polyamorphism: the formation of an unusual structure that is achieved by physical vapor deposition and that differs from the ordinary liquid state obtained by supercooling. The present results also reveal that the dielectric constant of the as deposited glass (ε∞) is reduced to practically the value of the squared refractive index, n2.

Dielectric properties of vapor-deposited propylbenzenes.

Dielectric susceptibility data of vapor-deposited films of iso-propylbenzene (IPB) and n-propylbenzene (NPB) have been recorded across a wide range of deposition temperatures, Tdep, mostly below the glass transition temperature, Tg. The results for the real and imaginary components of dielectric susceptibility are compared with recently published results for 2-methyltetrahydrofuran (MTHF). Common to all three systems are the following: (i) increased kinetic stability seen as higher onset temperature for the transformation to the liquid state for Tdep ≈ 0.85Tg; (ii) the reduction of the dielectric loss (χ″) for as-deposited glasses, a signature of increased packing density that is maximal for Tdep ≈ 0.85Tg; and (iii) a reduced level of the storage component (χ') for as-deposited glasses, an effect that is almost deposition temperature invariant for Tdep < Tg. Material specific behavior is observed when heating the as-deposited films to 1.2Tg: IPB and NPB transform directly into the ordinary liquid state if judged on the basis of dielectric susceptibility, whereas MTHF has been reported to enter an unusual liquid state prior to a liquid-liquid transition at higher temperatures. These results are discussed in the context of the curious scattering results reported by Ishii et al. for some benzene derivatives, which hint at a liquid-liquid transformation.

Rate exchange rather than relaxation controls structural recovery.

We observe structural recovery after an electric field step by probing the dielectric loss profile near its maximum, which displays a field-induced shift towards lower frequencies. These dynamics display time aging-time superposition (TaTS) for the majority of relaxation modes, thus implying homogeneous recovery dynamics. Although assumed by generally accepted models, the same modes can not be responsible for structural relaxation and for structural recovery, as the former is heterogeneous and the latter is homogeneous regarding the nature of the dynamics. This conflict is resolved by proposing that structural recovery is governed by rate exchange, a process that refers to the homogeneous fluctuations of rate constants in equilibrium and restores ergodicity more slowly than the relaxation observed as a simple correlation decay. This recognition has wide-ranging consequences on how aging and nonlinear dynamics such as scanning calorimetry should be modeled.

Perspective: Nonlinear approaches to structure and dynamics of soft materials.

Nonlinear dielectric effects have been the subject of research for over 100 years. A recent surge of activities devoted to this field focuses on soft condensed materials, where interactions and collective dynamics play an important role. The hope is to gain deeper insight into the origin of non-exponential responses and super-Arrhenius behavior, and identify dynamical correlations and connections between dynamics and thermodynamic potentials. While relaxation spectroscopy can reveal details of the dynamics across an impressive range of time scales, many interesting features remain obscured if limited to the regime of linear response. This perspective portrays the recent progress in the field of nonlinear dynamics, with focus on the complex dynamics of supercooled liquids with departures not too far from equilibrium. Nonlinear dielectric effects will be highlighted and compared with the mechanical and calorimetric counterparts.

Relaxation time and excess entropy in viscous liquids: Electric field versus temperature as control parameter.

On the basis of adiabatic calorimetry data and results obtained from dielectric relaxation studies in the presence of a high static electric field, the effects of temperature and electric field induced changes of the excess entropy are compared for the same sample: supercooled cresolphthalein dimethylether. A field induced reduction of the excess entropy by 45 mJ K-1 mol-1 at constant temperature increases the structural relaxation time by 0.75%, while the same entropy change originating from lowering the temperature at constant field increases the time constant by 3.5%. Therefore, there is no simple link connecting excess entropy and relaxation time that is independent of the control parameter that is used to modify the entropy. A consequence is that the Adam-Gibbs approach does not provide a quantitative prediction for how the dynamics of liquids depend on the electric field, and, more generally, on excess entropy. This work compares the dynamics for temperature versus field induced changes of isobaric excess entropy, thereby eliminating previous uncertainties arising from isochoric versus isobaric conditions and from unknown relations between thermodynamic, excess, and configurational entropies.

Anomalously large isotope effect in the glass transition of water.

We present the discovery of an unusually large isotope effect in the structural relaxation and the glass transition temperature Tg of water. Dielectric relaxation spectroscopy of low-density as well as of vapor-deposited amorphous water reveal Tg differences of 10 ± 2 K between H2O and D2O, sharply contrasting with other hydrogen-bonded liquids for which H/D exchange increases Tg by typically less than 1 K. We show that the large isotope effect and the unusual variation of relaxation times in water at low temperatures can be explained in terms of quantum effects. Thus, our findings shed new light on water's peculiar low-temperature dynamics and the possible role of quantum effects in its structural relaxation, and possibly in dynamics of other low-molecular-weight liquids.

Non-linear dielectric signatures of entropy changes in liquids subject to time dependent electric fields.

A model of non-linear dielectric polarization is studied in which the field induced entropy change is the source of polarization dependent retardation time constants. Numerical solutions for the susceptibilities of the system are obtained for parameters that represent the dynamic and thermodynamic behavior of glycerol. The calculations for high amplitude sinusoidal fields show a significant enhancement of the steady state loss for frequencies below that of the low field loss peak. Also at relatively low frequencies, the third harmonic susceptibility spectrum shows a "hump," i.e., a maximum, with an amplitude that increases with decreasing temperature. Both of these non-linear effects are consistent with experimental evidence. While such features have been used to conclude on a temperature dependent number of dynamically correlated particles, Ncorr, the present result demonstrates that the third harmonic susceptibility display a peak with an amplitude that tracks the variation of the activation energy in a model that does not involve dynamical correlations or spatial scales.

Communication: Temperature derivative of the dielectric constant gives access to multipoint correlations in polar liquids.

Fluctuations of the dipole moment of a macroscopic dielectric sample are induced by thermal motions. The variance of the sample dipole moment, characterizing the extent of thermal fluctuations, is a decaying function of temperature for many polar liquids. This result is inconsistent with the Nyquist (fluctuation-dissipation) theorem predicting the variance of a macroscopic property to grow linearly with temperature. The reason for a qualitatively different behavior is in strong multi-particle correlations of dipolar orientations. An equation connecting the temperature slope of the dielectric constant to a static three-point correlation function is derived. When applied to experimental data for polar and hydrogen-bonding liquids at normal conditions, the three-point correlations of different liquids fall on a single master curve as a function of the dielectric constant. Static three-point correlation functions can potentially reflect the growing spatial correlation length on approach to the glass transition. However, the measured temperature slope of the dielectric constant of glycerol does not indicate a change in such a lengthscale.