A5036725105- Material Dynamics and Properties
- Thermodynamic properties of mixtures
- Phase Equilibria and Thermodynamics
- Glass properties and applications
- Polymer crystallization and properties
- Polymer Nanocomposites and Properties
- Liquid Crystal Research Advancements
- Theoretical and Computational Physics
- Advanced Sensor and Energy Harvesting Materials
- Ionic liquids properties and applications
- Photochemistry and Electron Transfer Studies
- Rheology and Fluid Dynamics Studies
- Conducting polymers and applications
- Dielectric materials and actuators
- Metallic Glasses and Amorphous Alloys
- Analytical Chemistry and Sensors
- Force Microscopy Techniques and Applications
- Spectroscopy and Quantum Chemical Studies
- Ferroelectric and Piezoelectric Materials
- Electrochemical Analysis and Applications
- Advanced Materials and Mechanics
- Lipid Membrane Structure and Behavior
- Chemical Thermodynamics and Molecular Structure
- NMR spectroscopy and applications
- Structural Response to Dynamic Loads
United States Naval Research Laboratory
2015-2024
Naval Research Laboratory Chemistry Division
2011-2020
Division of Chemistry
2020
United States Navy
2019
United States Department of the Navy
2019
Tokyo Institute of Technology
2010
George Mason University
2001-2009
University of Cambridge
2008
University of Pisa
1994-2006
Institute of Physics
2005
An intriguing problem in condensed matter physics is understanding the glass transition, particular dynamics equilibrium liquid close to vitrification. Recent advances have been made by using hydrostatic pressure as an experimental variable. These results are reviewed, with emphasis insight provided into mechanisms underlying relaxation properties of glass-forming liquids and polymers.
Classification of glass-forming liquids based on the dramatic change in their properties upon approach to glassy state is appealing, since this most conspicuous and often-studied aspect glass transition. Herein, we show that a generalized scaling, log tau proportional T^(-1)V^(-gamma), where gamma material-constant, yields superpositioning for ten glass-formers, encompassing van der Waals molecules, associated liquids, polymers. The exponent reflects degree which volume, rather than thermal...
Upon decreasing temperature or increasing pressure, a noncrystallizing liquid will vitrify; that is, the structural relaxation time, τα, becomes so long system cannot attain an equilibrium configuration in available time. Theories, including well-known free volume and configurational entropy models, explain glass transition by invoking single quantity governs The dispersion of (i.e., function) is either not addressed derived as parallel consequence (or afterthought) thus independent τα. In...
Viscosities η and their temperature T volume V dependences are reported for seven molecular liquids polymers. In combination with literature viscosity data five other liquids, we show that the superpositioning of relaxation times various glass-forming materials when expressed as a function TVγ, where exponent γ is material constant, can be extended to viscosity. The latter usually measured higher temperatures than corresponding times, demonstrating validity thermodynamic scaling throughout...
The recently discovered scaling law for the relaxation times, tau(T,upsilon) = I(Tupsilon(gamma)), where T is temperature and upsilon specific volume, derived by a revision of entropy model glass transition dynamics originally proposed Avramov [J. Non-Cryst. Solids 262, 258 (2000)]. In this modification calculated an alternative route. resulting expression variation time with shown to accurately fit experimental data several glass-forming liquids polymers over extended range encompassing...
The importance of glass formation and the transition is linked to their universality, embracing many classes materials: metallic, inorganic, organic. There no agreement on what drives this phenomenon; moreover, experiments are challenging due nonequilibrium nature glassy state. We present a new approach that provides information about very slow structural relaxation in state reveals important role secondary relaxation. Structural ($\ensuremath{\alpha}$) times for polyvinylethylene were...
Abstract High‐temperature flexible polymer dielectrics are critical for high density energy storage and conversion. The need to simultaneously possess a bandgap, dielectric constant glass transition temperature forms substantial design challenge novel polymers. Here, by varying halogen substituents of an aromatic pendant hanging off bicyclic mainchain polymer, class high‐temperature olefins with adjustable thermal stability obtained, all uncompromised large bandgaps. Halogens substitution...
Master curves of the relaxation time, $\ensuremath{\tau}$, or viscosity, $\ensuremath{\eta}$, versus ${T}^{\ensuremath{-}1}{V}^{\ensuremath{-}\ensuremath{\gamma}}$, where $T$ is temperature, $V$ specific volume, and $\ensuremath{\gamma}$ a material constant, are used to deduce effect pressure on dynamic crossover fragility. The determined from change in slope derivative plots times viscosities. We confirm our previous findings that value $\ensuremath{\tau}$ $\ensuremath{\eta}$ at independent...
The relaxational dynamics in metallic glasses (MGs) is investigated by using mechanical spectroscopy. spectra show that MGs there are two relaxations: (i) the α relaxation, linked to glass transition, as observed other classes of amorphous materials; and (ii) β well below with an intensity strongly dependent on MG composition, nature which has been local microstructure MGs. For we find relaxation time depends, a reproducible fashion, thermal history samples. During aging experiments,...
Flexible large bandgap dielectric materials exhibiting ultra-fast charging-discharging rates are key components for electrification under extremely high electric fields. A polyoxafluoronorbornene (m-POFNB) with fused five-membered rings separated by alkenes and flexible single bonds as the backbone, rather than conjugated aromatic structure typically conventional high-temperature polymers, is designed to achieve simultaneously thermal stability bandgap. In addition, an asymmetrically...
Structural relaxation of fragile glass-forming liquids in the vicinity glass transition were measured by dielectric spectroscopy, as a function temperature and pressure. From volume dependence times, we show that effects thermal free fluctuations are comparable. The implication is theoretical approaches which intermolecular barrier heights related to local density essential formulating theory structural dynamics supercooled liquids.
Broadband dielectric spectroscopy, heat capacity spectroscopy (3ω method), and viscosimetry have been used to study the dynamic glass transition of two glass-forming epoxy resins, poly [(phenyl glycidyl ether)-co-formaldehyde] diglycidyl ether bisphenol-A. In spite their rather simple molecular structure, dynamics these systems is characterized by well-separated crossover regions where relaxation times main secondary relaxations β γ approach each other. The has three parts: a process at high...
Dielectric measurements are reported for the α-relaxation in polystyrene (PS) of varying molecular weights. Although segmental relaxation dispersion was essentially invariant to Mw, Tg-normalized temperature dependence (fragility) increases systematically with weight. The latter result corroborates dynamic mechanical and light scattering studies, but is at odds (T−Tg) superpositioning shift factors recoverable creep compliance PS. failure dielectric relaxations time superimpose when...
Glass formers are defined as "type B" when they exhibit a distinct Johari-Goldstein (JG) relaxation, but lack an excess loss ("excess wing," EW) in their structural relaxation peak. By studying the dielectric spectra of well-known type-B glass former under high pressure, we unequivocally show existence EW, simultaneously with JG relaxation. Moreover, at very pressures (0.6 GPa), EW becomes peak, although correlated The implication is that rather than higher frequency ascribed to process,...
The dynamics of diglycidyl ether bisphenol-A (DGEBA) was studied by employing broadband dielectric spectroscopy over a wide temperature range extending from below to above the glass-transition temperature. Dielectric spectra reveal existence two relaxation processes: structural relaxation, slowing down for decreasing and freezing at ${T}_{g}$, secondary process present in both liquid glassy phase. Above glass transition behavior time is properly described Vogel-Fulcher-Tamman equation with...
In this paper, an interpretation of the high-frequency crossover, observed for glass formers at temperature far above transition, is described in terms large change heterogeneous character dynamics. For five prototypical formers, dielectric relaxation data spanning many decades were analyzed. Unlike behavior (structural) times ${\ensuremath{\tau}}_{\ensuremath{\alpha}},$ which deviate from a Vogel-Fulcher-Tamman (VFT) relationship, non-cooperative times, ${\ensuremath{\tau}}_{0},$ calculated...
The effect of pressure on the dielectric relaxation two polyhydroxy alcohols is examined by analysis existing data glycerol, together with new measurements xylitol. fragility, or Tg-normalized temperature dependence, changes for low pressures, but becomes invariant above 1 GPa. When compared at temperatures which α-relaxation times are equal, there no (<1 GPa) shape α dispersion higher temperatures. However, nearer Tg, broadens peak, consistent expected correlation fragility breadth...
Dielectric data on the local segmental relaxation of amorphous poly(vinyl acetate) (PVAc) are analyzed. At atmospheric pressure, fragility (Tg-normalized temperature dependence) is invariant to molecular weight. Since also independent combined and pressure dependences times can be analyzed using Avramov model structural relaxation. This entropy gives a satisfactory description yields an expression for dependence dynamic glass transition identical empirical Andersson equation. Although in...
The dielectric spectra of most simple liquids are characterized by two relaxation processes: (i) the α-process, an intense, broad non-Debye with a non-Arrhenius temperature dependence and (ii) β process, evident mainly below glass transition having nearly Arrhenius behavior. However, monoalcohols show three that resemble those normal third very intense Debye peak at lower frequencies, which is non-Arrhenius. Interestingly, this process not observed other techniques such as light scattering...