arXiv subjects

C. Alba-Simionesco

Publications and source records attributed to C. Alba-Simionesco.

11 recordsLinked to original sources

arXiv2015

A systematic study of the isothermal crystallization of the mono-alcohol n-butanol monitored by dielectric spectroscopy

Isothermal crystallization of the mono-hydroxyl alcohol n-butanol was studied with dielectric spectroscopy in real time. The crystallization was carried out using two different sample cells at 15 temperatures between 120 K and 134 K. For all temperatures, a shift in relaxation times to shorter times was observed during the crystallization process, which is characterized by a drop in relaxation strength. The two different sample environments induced quite different crystallization behaviors, consistent and reproducible over all studied temperatures. An explanation for the difference was proposed on the background of an Avrami and a Maxwell-Wagner analysis. Both types analysis suggest that the morphology of the crystal growth changes at a point during the crystallization. The differences between the cells can be explained by this transition taking place at different times for the two cells.

cond-mat.soft↗

arXiv2010

Connection between slow and fast dynamics of molecular liquids around the glass transition

The mean-square displacement (MSD) was measured by neutron scattering at various temperatures and pressures for a number of molecular glass-forming liquids. The MSD is invariant along the glass-transition line at the pressure studied, thus establishing an ``intrinsic'' Lindemann criterion for any given liquid. A one-to-one connection between the MSD's temperature dependence and the liquid's fragility is found when the MSD is evaluated on a time scale of approximately 4 nanoseconds, but does not hold when the MSD is evaluated at shorter times. The findings are discussed in terms of the elastic model and the role of relaxations, and the correlations between slow and fast dynamics are addressed.

cond-mat.soft↗

arXiv2009

The consequence of excess configurational entropy on fragility: the case of a polymer/oligomer blend

By taking advantage of the molecular weight dependence of the glass transition of polymers and their ability to form perfectly miscible blends, we propose a way to modify the fragility of a system, from fragile to strong, keeping the same glass properties, i.e. vibrational density of states, mean-square displacement and local structure. Both slow and fast dynamics are investigated by calorimetry and neutron scattering in an athermal polystyrene/oligomer blend, and compared to those of a pure 17-mer polystyrene considered to be a reference, of same Tg. Whereas the blend and the pure 17-mer have the same heat capacity in the glass and in the liquid, their fragilities differ strongly. This difference in fragility is related to an extra configurational entropy created by the mixing process and acting at a scale much larger than the interchain distance, without affecting the fast dynamics and the structure of the glass.

cond-mat.soft↗

arXiv2004

Scaling out the density dependence of the $α$ relaxation in glassforming polymers

We show that the density and temperature dependences of the $α$-relaxation time of several glassforming polymers can be described through a single scaling variable $X=e(ρ)/T$, where $e(ρ)$ is well fitted by a power law $ρ^x$, $x$ being a species-specific parameter. This implies that ``fragility'' is an intrinsic, density-independent property of a glassformer characterizing its super-Arrhenius slowing down of relaxations, and it leads us to propose a modification of the celebrated Angell plot.

cond-mat.soft↗

arXiv2003

Disentangling density and temperature effects in the viscous slowing down of glassforming liquids

We present a consistent picture of the respective role of density and temperature in the viscous slowing down of glassforming liquids and polymers. Specifically, based in part upon a new analysis of simulation and experimental data on liquid ortho-terphenyl, we conclude that a zeroth-order description of the approach to the glass transition should be formulated in terms of a temperature-driven super-Arrhenius activated behavior rather than a density-driven congestion or jamming phenomenon. The density plays a role at a quantitative level, but its effect on the viscosity and the structural relaxation time can be simply described via a single parameter, an effective interaction energy that is characteristic of the high temperature liquid regime; as a result, density does not affect the ``fragility'' of the glassforming system.

cond-mat.soft↗

arXiv2003

Methyl group dynamics in a confined glass

We present a neutron scattering investigation on methyl group dynamics in glassy toluene confined in mesoporous silicates of different pore sizes. The experimental results have been analysed in terms of a barrier distribution model, such a distribution following from the structural disorder in the glassy state. Confinement results in a strong decreasing of the average rotational barrier in comparison to the bulk state. We have roughly separated the distribution for the confined state in a bulk-like and a surface-like contribution, corresponding to rotors at a distance from the pore wall respectively larger and smaller than the spatial range of the interactions which contribute to the rotational potential for the methyl groups. We have estimated a distance of 7 Amstrong as a lower limit of the interaction range, beyond the typical nearest-neighbour distance between centers-of-mass (4.7 Amstrong).

cond-mat.soft↗

arXiv2002

What can be learned from the schematic mode-coupling approach to experimental data ?

We propose a detailed investigation of the schematic mode-coupling approach to experimental data, a method based on the use of simple mode-coupling equations to analyze the dynamics of supercooled liquids. Our aim here is to clarify different aspects of this approach that appeared so far uncontrolled or arbitrary, and to validate the results obtained from previous works. Analyzing the theoretical foundations of the approach, we first identify the parameters of the theory playing a key role and obtain simple requirements to be met by a schematic model for its use in this context. Then we compare the results obtained from the schematic analysis of a given set of experimental data with a variety of models and show that they are all perfectly consistent. A number of potential biases in the method are identified and ruled out by the choice of appropriate models. Finally, reference spectra computed from the mode-coupling theory for a model simple liquid are analyzed along the same lines as experimental data, allowing us to show that, despite the strong simplification in the description of the dynamics it involves, the method is free from spurious artifacts and provides accurate estimates of important parameters of the theory. The only exception is the exponent parameter, the evaluation of which is hindered, as for other methods, by corrections to the asymptotic laws of the theory present when the dynamics is known only in a limited time or frequency range.

cond-mat.dis-nn↗

arXiv2002

Locally Preferred Structure and Frustration in Glassforming Liquids: A Clue to Polyamorphism?

We propose that the concept of liquids characterized by a given locally preferred structure (LPS) could help in understanding the observed phenomenon of polyamorphism. ``True polyamorphism'' would involve the competition between two (or more) distinct LPS, one favored at low pressure because of its low energy and one favored at high pressure because of its small specific volume, as in tetrahedrally coordinated systems. ``Apparent polyamorphism'' could be associated with the existence of a poorly crystallized defect-ordered phase with a large unit cell and small crystallites, which may be illustrated by the metastable glacial phase of the fragile glassformer triphenylphosphite; the apparent polyamorphism might result from structural frustration, i. e., a competition between the tendency to extend the LPS and a global constraint that prevents tiling of the whole space by the LPS.

cond-mat.stat-mech↗

arXiv2001

"de Gennes" narrowing in supercooled molecular liquids : Evidence for center-of-mass dominated slow dynamics

The density correlation function F(q,t) of the two similar substituted aromatic liquids, Toluene and m-Toluidine, is studied by coherent neutron spin-echo and time-of-flight scattering for wave vectors q around the maximum q_max of the total static structure factor S_m(q) in the supercooled i.e. high density state far away from the normal fluid state. The wave-vector dependence of the mean structural relaxation time tau shows in both liquids a very pronounced de Gennes-like narrowing centered around q_0 < q_max, where q_0 corresponds to the first maximum in the center-of-mass static structure factor S_COM}(q). We find that the narrowing can be described quantitatively by using S_COM(q)/q^2 instead of S_m(q)/q^2 indicating that at the corresponding molecular length scales the relaxation of F(q,t) is dominated by purely translational motion.

cond-mat.dis-nn↗

arXiv2000

Quantitative investigation of the mean-field scenario for the structural glass transition from a schematic mode-coupling analysis of experimental data

A quantitative application to real supercooled liquids of the mean-field scenario for the glass transition ($T_g$) is proposed. This scenario, based on an analogy with spin-glass models, suggests a unified picture of the mode-coupling dynamical singularity ($T_c$) and of the entropy crisis at the Kauzmann temperature ($T_K$), with $T_c>T_g>T_K$. Fitting a simple set of mode-coupling equations to experimental light-scattering spectra of two fragile liquids and deriving the equivalent spin-glass model, we can estimate not only $T_c$, but also the static transition temperature $T_s$ corresponding supposedly to $T_K$. For the models and systems considered here, $T_s$ is always found above $T_g$, in the fluid phase. A comparison with recent theoretical calculations shows that this overestimation of the ability of a liquid to form a glass seems to be a generic feature of the mean-field approach.

cond-mat.dis-nn↗

arXiv1997

Study of the Depolarized Light Scattering Spectra of Supercooled Liquids by a Simple Mode-Coupling Model

By using simple mode coupling equations, we investigate the depolarized light scattering spectra of two so-called "fragile" glassforming liquids, salol (phenylsalicylate) and CKN (Ca_{0.4}K_{0.6}(NO_3)_{1.4}), measured by Cummins and coworkers. Nonlinear integrodifferential equations for the time evolution of the density-fluctuations autocorrelation functions are the basic input of the mode coupling theory. Restricting ourselves to a small set of such equations, we fit the numerical solution to the experimental spectra. It leads to a good agreement between model and experiment, which allows us to determine how a real system explores the parameter space of the model, but it also leads to unrealistic effective vertices in a temperature range where the theory makes critical asymptotic predictions. We finally discuss the relevance and the range of validity of these universal asymptotic predictions when applied to experimental data on supercooled liquids.

cond-mat.stat-mech↗