Dino Leporini

Obstruction model of the fractional Stokes–Einstein relation in glass-forming liquids

Diffusion of tracer molecules in glass-forming liquids is modeled in terms of diffusion in an obstructed space. The obstructions, which model heterogeneities in supercooled liquids, are taken to be spherical and frozen in position for simplicity. This idealized model leads to the widely observed “fractional Stokes–Einstein (FSE) equation” (D∝(η/T)−ξ,0<ξ⩽1) relating the tracer particle diffusion coefficient D and the fluid viscosity η. The spherical obstruction model indicates the exponent value ξ=3/5, but ξ is predicted to vary somewhat with obstruction shape. Experimental values of ξ are summarized for comparison with our model.

ESR evidence for 2 coexisting liquid phases in deeply supercooled bulk water

Using electron spin resonance spectroscopy (ESR), we measure the rotational mobility of probe molecules highly diluted in deeply supercooled bulk water and negligibly constrained by the possible ice fraction. The mobility increases above the putative glass transition temperature of water, Tg = 136 K, and smoothly connects to the thermodynamically stable region by traversing the so called “no mans land” (the range 150–235 K), where it is believed that the homogeneous nucleation of ice suppresses the liquid water. Two coexisting fractions of the probe molecules are evidenced. The 2 fractions exhibit different mobility and fragility; the slower one is thermally activated (low fragility) and is larger at low temperatures below a fragile-to-strong dynamic cross-over at ≈225 K. The reorientation of the probe molecules decouples from the viscosity below ≈225 K. The translational diffusion of water exhibits a corresponding decoupling at the same temperature [Chen S-H, et al. (2006) The violation of the Stokes–Einstein relation in supercooled water. Proc Natl Acad Sci USA 103:12974–12978]. The present findings are consistent with key issues concerning both the statics and the dynamics of supercooled water, namely the large structural fluctuations [Poole PH, Sciortino F, Essmann U, Stanley HE (1992) Phase behavior of metastable water. Nature 360:324–328] and the fragile-to-strong dynamic cross-over at ≈228 K [Ito K, Moynihan CT, Angell CA (1999) Thermodynamic determination of fragility in liquids and a fragile-to-strong liquid transition in water. Nature 398:492–494].

Universal divergenceless scaling between structural relaxation and caged dynamics in glass-forming systems.

On approaching the glass transition, the microscopic kinetic unit spends increasing time rattling in the cage of the first neighbors, whereas its average escape time, the structural relaxation time tau(alpha), increases from a few picoseconds up to thousands of seconds. A thorough study of the correlation between tau(alpha) and the rattling amplitude, expressed by the Debye-Waller factor, was carried out. Molecular-dynamics simulations of both a model polymer system and a binary mixture were performed by varying the temperature, the density rho, the potential and the polymer length to consider the structural relaxation as well as both the rotational and the translation diffusion. The present simulations, together with MD studies on other glassformers, evidence the scaling between the structural relaxation and the caged dynamics. An analytic model of the master curve is developed in terms of two characteristic length scales a(2) (1/2) and sigma(a(2) ) (1/2), pertaining to the distance to be covered by the kinetic unit to reach a transition state. The model does not imply tau(alpha) divergences. The comparison with the experiments supports the numerical evidence over a range of relaxation times as wide as about eighteen orders of magnitude. A comparison with other scaling and correlation procedures is presented. In particular, the density scaling of the length scales a(2) (1/2), sigma(a(2) ) (1/2) proportional to rho(-1/3) is shown to be not supported by the present simulations. The study suggests that the equilibrium and the moderately supercooled states of the glassformers possess key information on the huge slowing-down of their relaxation close to the glass transition. The latter, according to the present simulations, exhibits features consistent with the Lindemann melting criterion and the free-volume model.

Jump reorientation of a molecular probe in the glass transition region of o-terphenyl

The reorientation process of a molecular probe dissolved in the fragile glass former o-terphenyl is studied via electron spin-resonance spectroscopy. Owing to the good resolution of the ESR lineshape in the so-called slow-motion regime, we are able to discriminate between diffusive and jump rotations. In the region of the glass transition ( the results are at variance with the diffusion models and are fairly well reproduced by assuming that the probe jumps with steps . The temperature dependence of the correlation time is well described, in the temperature range investigated, by a double Arrhenius law broken at . The findings are compared with previous NMR, dielectric, fluorescence and molecular dynamics studies.

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.

Anisotropic jump model of the rotational dynamics in glasses

Anisotropic jump reorientation occurs in glasses, polymers, and plastic crystals. A general theoretical frame to describe such processes is presented. It generalizes previous work by Ivanov [Sov. Phys. JETP 18, 1041 (1964)]. A tractable model is given by a simple phenomenological assumption on the jump distribution. Analytical results and fast numerical methods to evaluate the relevant quantities are derived. The model is validated by comparing the predictions with ESR experiments on stiff, cylindrical tracers dissolved in the glassformer o-terphenyl.

Scaling between structural relaxation and caged dynamics in Ca0.4K0.6(NO3)1.4 and glycerol: free volume, time-scales and implications for pressure–energy correlations

The scaling of slow structural relaxation with fast caged dynamics is seen in the molten salt Ca0.4K0.6(NO3)1.4 (CKN) over about 13 decades of the structural relaxation time. Glycerol scaling has been analysed in detail. In glycerol, the short-time mean-square displacement , a measure of the caged dynamics, is contributed by the free volume. It is seen that, in order to see the scaling, the observation time of the fast dynamics must be shorter than the time-scales of the relaxation processes. Systems with both negligible (like CKN, glycerol and network glassformers) and high (like van der Waals liquids and polymers) pressure–energy correlations exhibit scaling between the slow relaxation and the fast caged dynamics. According to the available experiments, an isomorph-invariant expression of the master curve of the scaled data is indistinguishable from a simpler non-invariant expression. Instead, the latter agrees better with the simulations on a wide class of model polymers.

Nonlinear electron spin resonance techniques for the study of inhomogeneously broadened spectra

The application of nonlinear multiple irradiation electron spin resonance techniques to the case of inhomogeneously broadened spectra is studied. A detailed theoretical analysis within a unified method shows that the longitudinally detected electron spin resonance (LODESR) and the double modulation electron spin resonance (DOMESR) techniques represent two different aspects of the same physical effect and, under the same conditions, both give information on the longitudinal relaxation time T1 of the single spin packet. Previous papers, giving different interpretation for the double modulation spectrum, are critically reviewed. The usefulness of the two techniques in the case of inhomogeneously broadened lines is put into evidence by experiments with dextrose chars pyrolyzed at different temperatures. The results are in excellent agreement with theoretical results. The optimum application ranges of these nonlinear techniques are discussed and compared.

Pressure and temperature dependence of structural relaxation dynamics in polymers: a thermodynamic interpretation

We analyse the slowing down of the structural relaxation dynamics of polymers in terms of the Adam and Gibbs theory. We consider a previously derived general relation between the configurational and the excess entropy, which was used to derive an analytical equation for the dependence of the structural relaxation time from the pressure and temperature. The model proved to successfully fit the relaxation dynamics of poly(methyl methacrylate), poly(propylene glycol) and poly(propylene glycol dimethylether), of different molecular weights, over a wide region of temperature and pressure values above the glass transition.

Signatures of the fast dynamics in glassy polystyrene: First evidence by high-field Electron Paramagnetic Resonance of molecular guests

The reorientation of one small paramagnetic molecule (spin probe) in glassy polystyrene (PS) is studied by high-field electron paramagnetic resonance spectroscopy at two different Larmor frequencies (190 and 285 GHz). Two different regimes separated by a crossover region are evidenced. Below 180 K the rotational times are nearly temperature independent with no apparent distribution. In the temperature range of 180-220 K a large increase of the rotational mobility is observed with the widening of the distribution of correlation times which exhibits two components: (i) a deltalike, temperature-independent component representing the fraction of spin probes w which persist in the low-temperature dynamics; (ii) a strongly temperature-dependent component, to be described by a power distribution, representing the fraction of spin probes 1-w undergoing activated motion over an exponential distribution of barrier heights g(E). Above 180 K a steep decrease of w is evidenced. The shape and the width of g(E) do not differ from the reported ones for PS within the errors. For the first time the large increase of the rotational mobility of the spin probe at 180 K is ascribed to the onset of the fast dynamics detected by neutron scattering at T(f)=175+/-25 K.