Using dielectric spectroscopy, we demonstrate that confinement-induced changes in the glass transition dynamics, as observed for polymethylphenylsiloxane in alumina nanopores, reveal a pronounced ...nonequilibrium nature. Our results indicate that glass formers confined to nanopores are able to recover their bulklike mobility. We found that the characteristic time constant of such an equilibration process correlates with an extremely slow viscous flow rate in cylindrical channels of nanometer size. Thus, all the way to equilibrium, confinement effects seen in faster segmental dynamics are released through the viscous flow which eventually helps to eliminate surplus volume gained by nanoconstrained polymers upon cooling.
Although the first-order liquid-liquid phase transition (LLT) has been reported to exist in various systems (i.e., phosphorus, silicon, water, triphenyl phosphite, etc.), it is still one of the most ...challenging problems in the field of physical science. Recently, we found that this phenomenon occurs in the family of trihexyl(tetradecyl)phosphonium P
based ionic liquids (ILs) with different anions (Wojnarowska et al in Nat Commun 13:1342, 2022). To understand the molecular structure-property relationships governing LLT, herein, we examine ion dynamics of two other quaternary phosphonium ILs containing long alkyl chains in cation and anion. We found that IL with the anion containing branched -O-(CH
)
-CH
side chains does not reveal any signs of LLT, while IL with shorter alkyl chains in the anion brings a hidden LLT, i.e., it overlaps with the liquid-glass transition. Ambient pressure dielectric and viscosity measurements revealed a peculiar behavior of ion dynamics near T
for IL with hidden LLT. Moreover, high-pressure studies have shown that IL with hidden LLT has relatively strong pressure sensitivity compared to the one without first-order phase transition. At the same time, the former exposes the inflection point indicating the concave-convex character of logτ
(P) dependences.
Abstract
The studies of molecular dynamics in the vicinity of liquid–glass transition are an essential part of condensed matter physics. Various experimental techniques are usually applied to ...understand different aspects of molecular motions, i.e., nuclear magnetic resonance (NMR), photon correlation spectroscopy (PCS), mechanical shear relaxation (MR), and dielectric spectroscopy (DS). Universal behavior of molecular dynamics, reflected in the invariant distribution of relaxation times for different polar and weekly polar glass-formers, has been recently found when probed by NMR, PCS, and MR techniques. On the other hand, the narrow dielectric permittivity function ε*(f) of polar materials has been rationalized by postulating that it is a superposition of a Debye-like peak and a broader structural relaxation found in NMR, PCS, and MR. Herein, we show that dielectric permittivity representation ε*(f) reveals details of molecular motions being undetectable in the other experimental methods. Herein we propose a way to resolve this problem. First, we point out an unresolved Johari–Goldstein (JG) β-relaxation is present nearby the α-relaxation in these polar glass-formers. The dielectric relaxation strength of the JG β-relaxation is sufficiently weak compared to the α-relaxation so that the narrow dielectric frequency dispersion faithfully represents the dynamic heterogeneity and cooperativity of the α-relaxation. However, when the other techniques are used to probe the same polar glass-former, there is reduction of relaxation strength of α-relaxation relative to that of the JG β relaxation as well as their separation. Consequently the α relaxation appears broader in frequency dispersion when observed by PCS, NMR and MR instead of DS. The explanation is supported by showing that the quasi-universal broadened α relaxation in PCS, NMR and MR is captured by the electric modulus
M
*(
f
) = 1/ε*(
f
) representation of the dielectric measurements of polar and weakly polar glass-formers, and also
M
*(
f
) compares favorably with the mechanical shear modulus data
G
*(
f
).
Dielectric relaxation studies for model glass-forming liquids confined to nanoporous alumina matrices were examined together with high-pressure results. For confined liquids which show the deviation ...from bulk dynamics upon approaching the glass transition (the change from the Vogel-Fulcher-Tammann to the Arrhenius law), we have observed a striking agreement between the temperature dependence of the α-relaxation time in the Arrhenius-like region and the isochoric relaxation times extrapolated from the positive range of pressure to the negative pressure domain. Our finding provides strong evidence that glass-forming liquid confined to native nanopores enters the isochoric conditions once the mobility of the interfacial layer becomes frozen in. This results in the negative pressure effects on cooling. We also demonstrate that differences in the sensitivity of various glass-forming liquids to the "confinement effects" can be rationalized by considering the relative importance of thermal energy and density contributions in controlling the α-relaxation dynamics (the E(v)/E(p) ratio).
In this paper, we investigate the molecular dynamics and ions transport properties of polymerized imidazolium-based protic ionic liquid HSO3–BVImOTfa new material with potential applications in ...energy storage and electrochemical devices. The results of dielectric measurements, analyzed in modulus M*(f) and conductivity σ*(f) formalisms, combined with temperature-modulated differential scanning calorimetry experiments, have revealed a fundamental difference between the conducting properties of the examined polymer membrane and its low-molecular weight counterpart. Our findings indicated a strong decoupling between conductivity relaxation times τσ (related to the ions migration through the polymer matrix) and segmental dynamics when the ionic transport is controlled by fast proton hopping through the dense hydrogen-bond network. Finally, we also discuss, for the first time, the effect of water content on the glass transition temperature value, relation between the charge and mass diffusion, reflected in the decoupling phenomenon, and the conductivity mechanism of examined polyHSO3–BVImOTf.
Low physical stability is the main reason limiting the widespread use of amorphous pharmaceuticals. One approach to overcome this problem is to mix these drugs with various excipients. In this study ...coamorphous drug–drug compositions of different molar ratios of ezetimib and indapamid (i.e., EZB 10:1 IDP, EZB 5:1 IDP, EZB 2:1 IDP, EZB 1:1 IDP and EZB 1:2 IDP) were prepared and investigated using differential scanning calorimetry (DSC), broadband dielectric spectroscopy (BDS), and X-ray diffraction (XRD). Our studies have shown that the easily recrystallizing ezetimib drug can be significantly stabilized in its amorphous form by using even a small amount of indapamid (8.8 wt %). DSC experiments indicate that the glass transition temperature (T g) of the tested mixtures changes with the drug concentration in accordance with the Gordon–Taylor equation. We also investigated the effect of indapamid on the molecular dynamics of the ezetimib. As a result it was found that, with increasing indapamid content, the molecular mobility of the binary drug–drug system is slowed down. Finally, using the XRD technique we examined the long-term physical stability of the investigated binary systems stored at room temperature. These measurements prove that low-molecular-weight compounds are able to significantly improve the physical stability of amorphous APIs.
In this study, we employed dielectric spectroscopy to investigate the effect of temperature and pressure on the ion dynamics of phosphonium ionic liquids (ILs) differing by the length of an alkyl ...chain, P666,n TFSI (n = 2, 6, 8, 12). We found that both temperature and pressure dependence of dc-conductivity (σdc) determined for all examined ILs herein exhibit unique characteristics, unusual for aprotic ILs. Two regions differing by ion self-organization have been identified from the derivative analysis of σdc(T –1) data. On the other hand, isothermal measurements performed at elevated pressure revealed a unique concave–convex character of σdc(P) dependences, resulting in a clear minimum in the pressure behavior of activation volume. Such an inflection point characterizing the pressure dependence of σdc in P666,n TFSI ILs can be considered an inherent feature of ion dynamics governed by structural self-assembly. Our results offer a unique perspective to link the ion mobility at various T–P conditions to the nanostructural organization of ionic systems.
Supercooled liquids are characterized by relaxation times that increase dramatically by cooling or compression. From a single assumption follows a scaling law according to which the relaxation time ...is a function of h(ρ) over temperature, where ρ is the density and the function h(ρ) depends on the liquid in question. This scaling is demonstrated to work well for simulations of the Kob-Andersen binary Lennard-Jones mixture and two molecular models, as well as for the experimental results for two van der Waals liquids, dibutyl phthalate and decahydroisoquinoline. The often used power-law density scaling, h(ρ)∝ργ, is an approximation to the more general form of scaling discussed here. A thermodynamic derivation was previously given for an explicit expression for h(ρ) for liquids of particles interacting via the generalized Lennard-Jones potential. Here a statistical mechanics derivation is given, and the prediction is shown to agree very well with simulations over large density changes. Our findings effectively reduce the problem of understanding the viscous slowing down from being a quest for a function of two variables to a search for a single-variable function.
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