Daniele Prevosto

Many-Body Nature of Relaxation Processes in Glass-Forming Systems

Most glass-forming systems are composed of basic units interacting with each other with a nontrivial anharmonic potential. Naturally, relaxation and diffusion in glass formers is a many-body problem. Results from recent experimental studies are presented to show the effects of many-body relaxation and diffusion manifested on the dynamic properties of glass formers. Considering that the effects are general and critical, the problem of glass transition will not be solved until the many-body nature of the relaxation process has been incorporated fundamentally into any theory.

Interdependence of Primary and Johari-Goldstein Secondary Relaxations in Glass-Forming Systems

We report evidence from broadband dielectric spectroscopy that the dynamics of the primary alpha- and secondary Johari-Goldstein (JG) beta-processes are strongly correlated in different glass-forming systems over a wide temperature T and pressure P range, in contrast with the widespread opinion of statistical independence of these processes. The alpha-beta mutual dependence is quantitatively confirmed by (a) the overall superposition of spectra measured at different T-P combinations but with an invariant alpha-relaxation time; (b) the contemporary scaling of the isothermal-pressure and isobaric-temperature dependences of the alpha-and beta-relaxation times as plotted versus the reduced variable Tg(P)/T where Tg is the glass transition temperature. These novel and model-independent evidences indicate the relevance of the JG relaxation phenomenon in glass transition, often overlooked by most current theories.

Thermodynamic scaling of α-relaxation time and viscosity stems from the Johari-Goldstein β-relaxation or the primitive relaxation of the coupling model

By now it is well established that the structural α-relaxation time, τ(α), of non-associated small molecular and polymeric glass-formers obey thermodynamic scaling. In other words, τ(α) is a function Φ of the product variable, ρ(γ)/T, where ρ is the density and T the temperature. The constant γ as well as the function, τ(α) = Φ(ρ(γ)/T), is material dependent. Actually this dependence of τ(α) on ρ(γ)/T originates from the dependence on the same product variable of the Johari-Goldstein β-relaxation time, τ(β), or the primitive relaxation time, τ(0), of the coupling model. To support this assertion, we give evidences from various sources itemized as follows. (1) The invariance of the relation between τ(α) and τ(β) or τ(0) to widely different combinations of pressure and temperature. (2) Experimental dielectric and viscosity data of glass-forming van der Waals liquids and polymer. (3) Molecular dynamics simulations of binary Lennard-Jones (LJ) models, the Lewis-Wahnström model of ortho-terphenyl, 1,4 polybutadiene, a room temperature ionic liquid, 1-ethyl-3-methylimidazolium nitrate, and a molten salt 2Ca(NO(3))(2)·3KNO(3) (CKN). (4) Both diffusivity and structural relaxation time, as well as the breakdown of Stokes-Einstein relation in CKN obey thermodynamic scaling by ρ(γ)/T with the same γ. (5) In polymers, the chain normal mode relaxation time, τ(N), is another function of ρ(γ)/T with the same γ as segmental relaxation time τ(α). (6) While the data of τ(α) from simulations for the full LJ binary mixture obey very well the thermodynamic scaling, it is strongly violated when the LJ interaction potential is truncated beyond typical inter-particle distance, although in both cases the repulsive pair potentials coincide for some distances.

Dynamics of supercooled and glassy dipropyleneglycol dibenzoate as functions of temperature and aging: Interpretation within the coupling model framework

Dielectric relaxation measurements of a typical small molecular glassformer, dipropyleneglycol dibenzoate show the presence of two secondary relaxations. Their dynamic properties differ in the equilibrium liquid and glassy states, as well as the changes during structural recovery after rapid quenching the liquid to form a glass. These differences enable us to identify the slower secondary relaxation as the genuine Johari-Goldstein (JG) beta-relaxation, acting as the precursor of the primary alpha-relaxation. Agreement between the JG beta-relaxation time and the independent relaxation time of the coupling model leads to predicted quantitative relations between the JG beta-relaxation and the alpha-relaxation that are supported by the experimental data.

Two secondary modes in decahydroisoquinoline: which one is the true Johari Goldstein process?

Broadband dielectric measurements were carried out at isobaric and isothermal conditions up to 1.75 GPa for reconsidering the relaxation dynamics of decahydroisoquinoline, previously investigated by Richert et al. [R. Richert, K. Duvvuri, and L.-T. Duong, J. Chem. Phys. 118, 1828 (2003)] at atmospheric pressure. The relaxation time of the intense secondary relaxation tau(beta) seems to be insensitive to applied pressure, contrary to the alpha-relaxation times tau(alpha). Moreover, the separation of the alpha- and beta-relaxation times lacks correlation between shapes of the alpha-process and beta-relaxation times, predicted by the coupling model [see for example, K. L. Ngai, J. Phys.: Condens. Matter 15, S1107 (2003)], suggesting that the beta process is not a true Johari-Goldstein (JG) relaxation. From the other side, by performing measurements under favorable conditions, we are able to reveal a new secondary relaxation process, otherwise suppressed by the intense beta process, and to determine the temperature dependence of its relaxation times, which is in agreement with that of the JG relaxation.

Local dielectric spectroscopy of nanocomposite materials interfaces

Local dielectric spectroscopy is performed to study how relaxation dynamics of a polyvinyl-acetate ultrathin film is influenced by inorganic nanoinclusions of a layered silicate (montmorillonite). Dielectric-loss spectra are measured by electrostatic-force microscopy in the frequency-modulation mode in ambient air. Spectral changes in both shape and relaxation time are evidenced across the boundary between pure polymer and montmorillonite sheets. Dielectric-loss imaging is also performed, evidencing spatial variations of dielectric properties near nanostructures with nanometer-scale resolution.

Coupling of Caged Molecule Dynamics to JG β-Relaxation: I.

The paper (Sibik, J.; Elliott, S. R.; Zeitler, J. A. J. Phys. Chem. Lett. 2014, 5, 1968-1972) used terahertz time-domain spectroscopy (THz-TDS) to study the dynamics of the polyalcohols, glycerol, threitol, xylitol, and sorbitol, at temperatures from below to above the glass transition temperature Tg. On heating the glasses, they observed the dielectric losses, ε″(ν) at ν = 1 THz, increase monotonically with temperature and change dependence at two temperatures, first deep in the glassy state at TTHz = 0.65Tg and second at Tg. The effects at both temperatures are most prominent in sorbitol but become progressively weaker in the order of xylitol and threitol, and the sub-Tg change was not observed in glycerol. They suggested this feature originates from the high-frequency tail of the Johari-Goldstein (JG) β-relaxation, and the temperature region near 0.65Tg is the universal region for the secondary glass transition due to the JG β-relaxation. In this paper, we first use isothermal dielectric relaxation data at frequencies below 10(6) Hz to locate the second glass transition temperature Tβ at which the JG β-relaxation time τJG reaches 100 s. The value of Tβ is close to TTHz = 0.65Tg for sorbitol (0.63Tg) and xylitol (0.65Tg), but Tβ is 0.74Tg for threitol and 0.83Tg for glycerol. Notwithstanding, the larger values of Tβ of glycerol are consistent with the THz-TDS data. Next, we identify the dynamic process probed by THz-TDS as the caged molecule dynamics, showing up in susceptibility spectra as nearly constant loss (NCL). The caged molecule dynamics regime is terminated by the onset of the primitive relaxation of the coupling model, which is the precursor of the JG β-relaxation. From this relation, established is the connection of the magnitude and temperature dependence of the NCL and those of τJG. This connection explains the monotonic increase of NCL with temperature and change to a stronger dependence after crossing Tβ giving rise to the sub-Tg behavior of ε″(ν) observed in experiment. Beyond the polyalcohols, we present new dielectric relaxation measurements of flufenamic acid and recall dielectric, NMR, and calorimetric data of indomethacin. The data of these two pharmaceuticals enables us to determine the value of Tβ = 0.67Tg for flufenamic acid and Tβ = 0.58Tg or Tβ = 0.62Tg for indomethacin, which can be compared with experimental values of TTHz from THz-TDS measurements when they become available. We point out that the sub-Tg change of NCL at Tβ found by THz-TDS can be observed by other high frequency spectroscopy including neutron scattering, light scattering, Brillouin scattering, and inelastic X-ray scattering. An example from neutron scattering is cited. All the findings demonstrate the connection of all processes in the evolution of dynamics ending at the structural α-relaxation.

Is the Johari-Goldstein β-relaxation universal?

The Johari-Goldstein (JG) β-relaxation is supposedly a universal feature of glassy dynamics and has strong connections to structural α-relaxation in all glass-formers. On the other hand, some small molecular glass-formers give no indication that JG relaxation is present, despite numerous investigations using a variety of experimental techniques. These exceptions cast doubt on universality and fundamental importance. Theoretical considerations suggest that the JG β-relaxation is present but unresolved in these glass-formers because it is sandwiched between the more intense α-relaxation and a faster, but non-JG, secondary γ-relaxation. We report representative data on two glass-formers, benzophenone and dimethyl phthalate. We show that by dissolving either glass-former in a host with a much higher glass transition temperature, we were able to move the α-relaxation further from the γ-relaxation. We also report, for the first time, JG β-relaxation in two glass-formers of current research interest.

Equilibrated polyethylene single-molecule crystals: molecular-dynamics simulations and analytic model of the global minimum of the free-energy landscape

The crystalline state of a single polyethylene chain with N = 500 monomers is investigated by extensive MD simulations. The polymer is folded in a well defined lamella with ten stems of approximately equal length, arranged into a regular, hexagonal pattern. The study of the microscopic organization of the lamella, which is in an equilibrium condition, evidences that the two caps are rather flat, i.e. the loops connecting the stems are short. An analytic model of the global minimum of the free energy, based on the assumption that the entropic contribution is mainly due to the combinatorics of the stems and loops and neglecting any conformational contribution, is presented. It provides for the first time a quantitative explanation of the MD results on the equilibrium geometry of single-chain crystals.

Correlation of structural and Johari–Goldstein relaxations in systems vitrifying along isobaric and isothermal paths

The effect of isobaric cooling (over the range 190-350 K) and isothermal compression (up to 700 MPa) on structural α- and secondary β-relaxations has been studied for low molecular weight glass-forming systems. The shape of the α-loss peak was found to change with temperature T and pressure P but to be constant for a combination of T and P giving the same T α (T, P). The invariance of shape at constant T α (T, P) involved also the excess wing, i.e. the process showing up at the high-frequency tail of the α-loss peak in systems with no well-resolved β-process. Likewise, systems where the excess wing evolved to a well-resolved β-peak showed that the timescale of the β-process was strongly related to that of the α-peak. Also in this case, once a given value T α (T, P) was fixed, a corresponding value Tp(T, P) was found for different T and P. Same results were found also for a binary mixture of a polar rigid molecule dissolved in an apolar solvent, i.e. a model system for Johari-Goldstein intermolecular relaxation. These evidences imply that a strong correlation exists between structural α- and Johari-Goldstein relaxation over a wide interval of temperature and density.