Aging, rejuvenation and memory : the example of spin glasses

In this paper, we review the general features of the out-of-equilibrium dynamics of spin glasses. We use this example as a guideline for a brief description of glassy dynamics in other disordered systems like structural and polymer glasses, colloids, gels etc. Starting with the simplest experiments, we discuss the scaling laws used to describe the isothermal aging observed in spin glasses after a quench down to the low temperature phase (these scaling laws are the same as established for polymer glasses). We then discuss the rejuvenation and memory effects observed when a spin glass is submitted to temperature variations during aging, and show some examples of similar phenomena in other glassy systems. The rejuvenation and memory effects and their implications are analyzed from the point of view of both energy landscape pictures and of real space pictures. We highlight the fact that both approaches point out the necessity of hierarchical processes involved in aging. We introduce the concept of a slowly growing and strongly temperature dependent dynamical correlation length, which is discussed at the light of a large panel of experiments.Comment: To appear in the proceedings of the summer school "Ageing and the Glass Transition" (University of Luxembourg, sept. 2005), Springer Lecture Notes in Physic

Influence of suspension viscosity on Brownian relaxation of filler particles

Brownian relaxation caused by Brownian movement of particles in suspensions can macroscopically be probed by small-amplitude oscillatory shear experiments. Phenomenological considerations suggest a direct proportionality between suspension viscosity and Brownian relaxation times. To verify this relation experimentally, a set of nanocomposite suspensions with viscosities varying over five decades is presented. The suspensions are chosen in a way to ensure that particle-particle interactions and average particle-particle distances are identical so that they can be used as a model system to study the mere influence of suspension viscosity on Brownian relaxation. The suggested linear relationship between suspension viscosity and Brownian relaxation time can be confirmed. Moreover, a verification of a recently introduced characteristic timescale for Brownian relaxation is presented

ELECTROOPTIC PROPERTIES OF Rb//2ZnCl//4 IN THE INCOMMENSURATE AND FERROELECTRIC PHASES

The electrooptic coefficients for light propagation directions along three crystallographic axes were measured in the temperature range from 100 to 220 K. A Landau-type free energy expansion describes temperature dependence

Mobility restrictions and glass transition behaviour of an epoxy resin under confinement

Confinement can have a big influence on the dynamics of glass formers in the vicinity of the glass transition. Already 40 to 50 K above the glass transition temperature, thermal equilibration of glass formers can be strongly influenced by the confining substrate. We investigate the linear thermal expansion and the specific heat capacity c(p) of an epoxy resin (diglycidyl ether of bisphenol A, DGEBA) in a temperature interval of 120 K around the glass transition temperature. The epoxy resin is filled into controlled pore glasses with pore diameters between 4 and 111 nm. Since DGEBA can form H-bonds with silica surfaces, we also investigate the influence of surface silanization of the porous substrates. In untreated substrates a core/shell structure of the epoxy resin can be identified. The glass transition behaviours of the bulk phase and that of the shell phase are different. In silanized substrates, the shell phase disappears. At a temperature well above the glass transition, a second transition is found for the bulk phase - both in the linear expansion data as well as in the specific heat capacity. The c(p) data do not allow excluding the glass transition of a third phase as being the cause for this transition, whereas the linear expansion data do so. The additional transition temperature is interpreted as a separation between two regimes: above this temperature, macroscopic flow of the bulk phase inside the porous structure is possible to balance the mismatch of thermal expansion coefficients between DGEBA and the substrate. Below the transition temperature, this degree of freedom is hindered by geometrical constraints of the porous substrates. Moreover, this second transition could also be found in the linear expansion data of the shell phase

Thermal and mechanical properties of styrene butadiene rubber/alumina nanocomposites

SBR is a rubber material with high technical relevance. In order to enhance the mechanical properties of the rubber one method consists e. g. in dispersing inorganic nanoparticles in the polymer matrix. By doing so, the properties of a given composite can be tuned either by changing the nanoparticle concentration or by modifying the surface properties of the fillers. Both interventions have indeed the potential to take influence on the structure and properties of the interphases emerging between the fillers and the polymer matrix. In this contribution we report on the glass transition behavior of SBR/alumina nanocomposites when the concentration and surface properties of the Al2O3 nanoparticles are changed. We also discuss the influence of the fillers on the shear stiffness of the rubber material. The samples were investigated by temperature modulated differential scanning calorimetry (TMDSC) and dynamic mechanical analysis (DMA). TMDSC investigations shed light on a surprising behavior of the glass transition temperature when the nanoparticle concentration is changed: at low filler contents the quasi-static glass transition temperature Tg passes through a minimum. While further increasing the nanoparticle content Tg increases to finally saturate at high concentrations. One of the main DMA results is that increasing of the Al2O3 concentration induces a quasi-solid-like frequency-independent response of the nanocomposites in the low frequency regime