On the correlation between fragility and stretching in glassforming liquids

We study the pressure and temperature dependences of the dielectric relaxation of two molecular glassforming liquids, dibutyl phtalate and m-toluidine. We focus on two characteristics of the slowing down of relaxation, the fragility associated with the temperature dependence and the stretching characterizing the relaxation function. We combine our data with data from the literature to revisit the proposed correlation between these two quantities. We do this in light of constraints that we suggest to put on the search for empirical correlations among properties of glassformers. In particular, argue that a meaningful correlation is to be looked for between stretching and isochoric fragility, as both seem to be constant under isochronic conditions and thereby reflect the intrinsic effect of temperature

Transition vitreuse et hétérogénéités dynamiques dans les liquides moléculaires et les polymères

Afin de progresser dans la compréhension des causes du ralentissement visqueux, nous avons travaillé parallèlement sur la caractérisation de longueurs associées aux hétérogénéités dynamiques à l'approche de la transition vitreuse, et sur les propriétés des liquides surfondus et des verres qui en découlent. La particularité de notre étude est l'utilisation de paramètres extérieurs supplémentaires pour mieux comprendre la transition vitreuse, tels que la pression, la longueur de la chaîne dans les polymères, ou l'étude de mélanges. Dans la première partie de ce travail, nous avons utilisé une nouvelle méthode permettant d'accéder à la taille des hétérogénéités dynamiques, en définissant le nombre de molécules dynamiquement corrélées à travers les susceptibilités dynamiques d'ordre supérieur. Nous avons pu montrer que ce nombre augmente à l'approche de la transition vitreuse de manière relativement semblable pour de nombreux systèmes moléculaires. Dans un second temps, nous avons appliqué cette nouvelle méthode d'analyse à des fondus de polymères. Pour ces systèmes, nous nous sommes intéressés aux propriétés de l'état vitreux, en particulier à leur sensibilité à la longueur de la chaîne du polymère considéré. Cette étude nous a permis de nous interroger notamment sur les relations entre la fragilité et d'autres propriétés. Les corrélations entre dynamique rapide et dynamique lente proposées dans la littérature ne semblent globalement bien vérifiées que pour l'étude comparée de différents systèmes à Patm et semblent systématiquement mises en défaut sous pression ou pour différentes Mw.Trying to understand the reasons of the viscous slowing down of the dynamics while approaching the glass transition temperature, we used temperature, pressure, length of the chains in the case of polymeric systems or mixtures as complementary external parameters to influence glass transition. In this work, we studied in parallel length scales associated to dynamical heterogeneities near the glass transition and the properties of the supercooled liquid and the glass they form. In a first part, we use a new approach using multi-points susceptibilities to get a number of dynamically correlated molecules. We show that this number is increasing in a similar way for various molecular liquids. In a second part we apply this new approach to an other class of systems : polymer melts that have a behavior very similar to that of molecular liquids close to the glass transition. For these systems, we study some properties of the glassy state and their sensitivity to the length of the chain of the polymer, in particular in the idea of comparing them with the fragility. We show that the correlations between slow and fast dynamics proposed in the literature do seem to work well only at atmospheric pressure while comparing different molecular liquids. In the case of studies using mixtures, or pressure or different chain lengths for polymers, the correlations do not seem to hold. The chain length or composition dependence of the fragility seems to be completely independent of the one of the fast dynamics. It could explain the systematic failure of the correlations we find.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Dynamics of dilute colloidal suspensions in modulated potentials

The dynamics of dilute suspensions of charged polystyrene spheres have been studied in a sinusoidal potential. We investigated experimentally and theoretically the effect of the wavelength and amplitude of this potential as well as the particle size on the mean square displacement, the distribution of displacements and the non-Gaussian parameter. These properties scale with the radius of the particles and the magnitude of the wavevector of the potential as expected from a dimensional analysis. In contrast, they show a non-trivial dependence on the amplitude of the potential, which determines the barrier height encountered in long-distance motions and also the intermediate-time dynamics in the potential well. The periodic potential leads to particle dynamics which resemble the self dynamics of various systems approaching their glass transition. In particular, we found that the time-dependent mean square displacements determined in our system are surprisingly close to the ones in a quasi twodimensional colloidal supercooled liquid. In this case, the role of the colloid volume fraction is played by the amplitude of the potential. The similarity of the mean square displacements is particularly striking since an individual colloidal particle in a periodic potential represents a considerably simpler situation than a highly concentrated multi-particle system