Contrasting two different interpretations of the dynamics in binary glass forming mixtures

Quantifying Concentration Fluctuations in Binary Glass-Forming Systems by Small- and Wide-Angle X-ray Scattering

Functionality of amorphous multicomponent systems largely depends upon the miscibility among components, especially in systems such as amorphous drugs and electrolytes. An in-depth understanding of mixing behaviors of various constituents is necessitated. Here, we applied the small- and wide-angle X-ray scattering (SWAXS) technique to monitor the mixing behaviors in three typical glass-forming binary systems imposed by varied heat of mixing. It is found that the Porod invariant (Q) determined at the glass transition temperature is remarkably enhanced as the concentration fluctuation becomes intensified. Meanwhile, the deviation of Q from the ideal mixing law is markedly weaken at elevated temperatures. The results unambiguously suggest that the degree of concentration fluctuations in mixing systems can be accurately quantified by the structural property, allowing the link to mixing thermodynamics.

Reorientational dynamics in highly asymmetric binary low-molecular mixtures-A quantitative comparison of dielectric and NMR spectroscopy results

Previously, we scrutinized the dielectric spectra of a binary glass former made by a low-molecular high-T_g component 2-(m-tertbutylphenyl)-2'-tertbutyl-9,9'-spirobi[9H]fluorene (m-TPTS; T_g = 350 K) and low-T_g tripropyl phosphate (TPP; T_g = 134 K) [Körber et al., Phys. Chem. Chem. Phys. 23, 7200 (2021)]. Here, we analyze nuclear magnetic resonance (NMR) spectra and stimulated echo decays of deuterated m-TPTS-d₄ (²H) and TPP (³¹P) and attempt to understand the dielectric spectra in terms of component specific dynamics. The high-T_g component (α₁) shows relaxation similar to that of neat systems, yet with some broadening upon mixing. This correlates with high-frequency broadening of the dielectric spectra. The low-T_g component (α₂) exhibits highly stretched relaxations and strong dynamic heterogeneities indicated by "two-phase" spectra, reflecting varying fractions of fast and slow liquid-like reorienting molecules. Missing for the high-T_g component, such two-phase spectra are identified down to w_TPP = 0.04, indicating that isotropic reorientation prevails in the rigid high-T_g matrix stretching from close to T_g ^TPP to T_g1 w_TPP. This correlates with low-frequency broadening of the dielectric spectra. Two T_g values are defined: T_g1 (w_TPP) displays a plasticizer effect, whereas T_g2 (w_TPP) passes through a maximum, signaling extreme separation of the component dynamics at low w_TPP. We suggest understanding the latter counter-intuitive feature by referring to a crossover from "single glass" to "double glass" scenario revealed by recent MD simulations. Analyses reveal that a second population of TPP molecules exists, which is associated with the dynamics of the high-T_g component. However, the fractions are lower than suggested by the dielectric spectra. We discuss this discrepancy considering the role of collective dynamics probed by dielectric but not by NMR spectroscopy.

Unveiling the strong dependence of the α-relaxation dispersion on mixing thermodynamics in binary glass-forming liquids

Structural α-relaxation dispersion in binary molecular glass forming mixtures with distinct mixing enthalpy ΔHmix was investigated using enthalpic and dielectric relaxation measurements across the entire composition range. This study focused on the dependence of the relaxation dispersion on the mixing thermodynamics by determining the non-exponential exponent β, and its composition dependence. The β values determined by the enthalpic and dielectric relaxations agree well. Remarkably, it is found that the systems with positive enthalpy of mixing (exothermic, ΔHmix >0) have positive deviations in the composition dependence of β from the linear averaging of the two β values of the pure components, while negative deviations are observed for the systems with negative enthalpy of mixing (endothermic, ΔHmix <0). Furthermore, the relation between the non-exponential behaviors and entropy of mixing is discussed, revealing that the positive or negative deviation of β in its composition dependence on mixing is accompanied by the same sign of the excess entropy of mixing relative to the ideal one.

The Dynamics of Hydrated Proteins Are the Same as Those of Highly Asymmetric Mixtures of Two Glass-Formers

Customarily, the studies of dynamics of hydrated proteins are focused on the fast hydration water ν-relaxation, the slow structural α-relaxation responsible for a single glass transition, and the protein dynamic transition (PDT). Guided by the analogy with the dynamics of highly asymmetric mixtures of molecular glass-formers, we explore the possibility that the dynamics of hydrated proteins are richer than presently known. By providing neutron scattering, dielectric relaxation, calorimetry, and deuteron NMR data in two hydrated globular proteins, myoglobin and BSA, and the fibrous elastin, we show the presence of two structural α-relaxations, α1 and α2, and the hydration water ν-relaxation, all coupled together with interconnecting properties. There are two glass transition temperatures T _g ^α1and T _g ^α2 corresponding to vitrification of the α1 and α2 processes. Relaxation time τ_α2(T) of the α2-relaxation changes its Arrhenius temperature dependence to super-Arrhenius on crossing T _g ^α1 from below. The ν-relaxation responds to the two vitrifications by changing the T-dependence of its relaxation time τ_ν(T) on crossing consecutively T _g ^α2 and T _g ^α1. It generates the PDT at T _d where τ_ν(T _d) matches about five times the experimental instrument timescale τ_exp, provided that T _d > T _g ^α1. This condition is satisfied by the hydrated globular proteins considered in this paper, and the ν-relaxation is in the liquid state with τ_ν(T) having the super-Arrhenius temperature dependence. However, if T _d < T _g ^α1, the ν-relaxation fails to generate the PDT because it is in the glassy state and τ_ν(T) has Arrhenius T-dependence, as in the case of hydrated elastin. Overall, the dynamics of hydrated proteins are the same as the dynamics of highly asymmetric mixtures of glass-formers. The results from this study have expanded the knowledge of the dynamic processes and their properties in hydrated proteins, and impact on research in this area is expected.

Reorientational dynamics of highly asymmetric binary non-polymeric mixtures – a dielectric spectroscopy study

Two separated relaxations α₁ and α₂ with different temperature dependences are identified in the mixtures. They are attributed to the dynamics associated with the high-T_g (α₁) and the low-T_g component (α₂) with distinct T_g concentration dependences.

Experimental evidence of co-existence of equilibrium and nonequilibrium in two-glass-transition miscible mixtures

Two glass-transitions have been observed in some miscible molecular mixtures with notable differences in geometry or chemistry of constituents. The explanation of the phenomena has been puzzling with diverse structural models. Here, we present detailed studies on two glass-transition mixtures composed of tripropyl phosphate (TPP) and polystyrene (PS) by using calorimetric and dielectric measurements. We found that ageing between the two transitions always generates endothermic peaks at temperatures ∼4 K higher than the ageing temperatures and, subsequent thermal cycles around the peaks can remove the ageing effect and restore the systems, confirming the co-existence of nonequilibrium and equilibrium states in the regions. We also found that the broad glass transition thermogram is associated with highly stretched relaxation dynamics. The results allow us to draw a conclusion of continuous mobility gradient spanning the two TPP-PS glass-transitions, rather than complete phase separation.

Isochronal Superposition of the Structural α-Relaxation and Invariance of Its Relation to the β-Relaxation to Changes of Thermodynamic Conditions in Methyl m-Toluate

The dielectric spectra of methyl m-toluate (MMT) in supercooled liquid and glassy states were measured over wide ranges of temperature T at ambient and elevated pressures P. We found that the frequency dispersion of the loss peak contributed by the structural α-relaxation is invariant to changes of P and T, while keeping the loss peak frequency f_α(T,P) constant. This isochronal superposition property of the α-relaxation holds for different choices of f_α(T,P). The invariant frequency dispersions for the same f_α(T,P) are also indicated by the fractional exponent β_KWW in the Fourier transform of the Kohlrausch-Williams-Watts (KWW) function. Similarly, the fragility m index of MMT keeps approximately constant on varying pressure, largely different from H-bonded glass formers. The secondary β-relaxation at a frequency higher than f_α(T,P) is found to shift to lower frequencies by elevating pressure in concert with the α-relaxation. The ratio τ_α(T,P)/τ_β(T,P) is approximately unchanged to variations of T and P while keeping τ_α(T,P) constant. These properties observed in MMT offer experimental evidence of the dynamic correlation between α- and β-relaxations in pure small-molecule glass-formers.

Dielectric relaxation and proton field-cycling NMR relaxometry study of dimethyl sulfoxide/glycerol mixtures down to glass-forming temperatures

Mixtures of glycerol and dimethyl sulfoxide (DMSO) are studied by dielectric spectroscopy (DS) and by 1H field-cycling (FC) NMR relaxometry in the entire concentration range and down to glass-forming temperatures (170-323 K). Molecular dynamics is accessed for 0 < xDMSO ≤ 0.64, at higher concentration phase separation occurs. The FC technique provides the frequency dependence of the spin-lattice relaxation rate which is transformed to the susceptibility representation and thus allows comparing NMR and DS results. The DS spectra virtually do not change with xDMSO and T, only the relaxation times become shorter. This is in contrast to the non-associated mixture toluene/quinaldine for which strong spectral changes occur. The FC relaxation spectra of glycerol in solution with DMSO or (deuterated) DMSO-d6 display a bimodal structure with a high-frequency part reflecting rotational and a low-frequency part reflecting translational dynamics. Regarding the rotational contribution in the glycerol/DMSO-d6 mixtures, no spectral change with xDMSO and T is observed. Yet, the non-deuterated mixture reveals a broader relaxation spectrum. Time constants τrot(T) probed by the two techniques complement each, a range 10-11 s < τ < 10 s is covered. The glass transition temperature Tg(xDMSO) is determined, yielding Tg = 149.5 ± 1 K of pure DMSO by extrapolation. Analysing the low-frequency FC NMR spectra allows to determine the diffusion coefficient Dtrans. Its logarithm shows a linear xDMSO-dependence as does lg τrot. The ratio Dtrans/Drot is independent of xDMSO and its low value indicates large separation of translation and rotation. The corresponding unphysically small hydrodynamic radius indicates strong failure of Stokes-Einstein-Debye relation. Such anomaly is taken as characteristics of a 3d hydrogen-bonded network. We conclude, although DMSO is an aprotic liquid the molecule is continuously incorporated in the hydrogen network of glycerol. Both molecules display common dynamics, i.e., no decoupling of the component dynamics is found in contrast to quinaldine/toluene with a similar Tg difference of its components.

Dynamically asymmetric binary glass formers studied by dielectric and NMR spectroscopy

We investigate the component dynamics in asymmetric binary glass formers. Focusing on the dielectric spectra of the high-T_g components m-tricresyl phosphate and quinaldine mixed with toluene as low-T_g component, the broadend spectra cannot be described by Kohlrausch or Cole-Davidson (CD) functions. Instead, we apply a generalized CD function which allows to control the width of the susceptibility independently of its high-frequency flank. The spectra show a common broadening and failure of the frequency-temperature superposition with increasing toluene concentration. This is confirmed by stimulated echo experiments showing an increased stretching of the probed orientational correlation function. In analogy to the definition of T_g, we consider "isodynamic points". For each component, a different but linear concentration dependence of 1/T_iso is revealed, indicating different time scales. Qualitativly, we do not find significant differences for the present mixtures with T_g-contrasts of 63-89K compared to those with larger T_g-contrast ( [Formula: see text] K): Whereas the high-T_g component shows relaxation features similar to those of neat glass formers, yet, with "atypical" weak relaxation broadening, the faster low-T_g component displays pronounced dynamic heterogeneities. This is supported by scrutinizing NMR relaxation data of several mixtures investigated previously as a function of concentration. A universal evolution of the dynamics of the high-T_g as well as the low-T_g component is suggested for mixtures with high [Formula: see text]T_g .

Coexistence of two structural relaxation processes in monohydroxy alcohol-alkyl halogen mixtures: Dielectric and rheological studies

Evidence for the existence of two glass transitions is found in binary mixtures of monohydroxy alcohols with an aprotic alkyl halide by means of dielectric spectroscopy and, markedly, also shear rheology. In the mechanical data, an enormous separation of two components becomes obvious for suitable compositions. The observation of bimodal motional heterogeneity is possible despite the fact that the glass transition temperatures of these substances differ by only 40 K. Obviously, the hydrogen-bond driven formation of supramolecular structures in one of the mixture components facilitates the emergence of dynamic contrast which for other binary liquids was so far only observed in the presence of much larger glass transition temperature differences.

Why is the change of the Johari–Goldstein β-relaxation time by densification in ultrastable glass minor?

Coupling-Model-based theoretical explanation of the minor change of JG β-relaxation achieved by ultrastability in contrast to the dramatic change in α-relaxation.