What Can the Concept of a Perfect Chemoelastic Solid Tell us about the Mechanical and Thermodynamic Behaviour of a Solid ?

The elastic energy stored in a solid is typically small compared to the energy involved in a phase change. As a consequence, it is relatively easy to generate elastic stresses by a modification of the composition andor a phase transformation. Conversely, the influence of stresses on phase changes is only of importance in special situations. What kind of effects can we expect and what are the order of magnitude of the phenomena is the question which is addressed in this communication. Using a model of a perfect chemoelastic crystalline solid, developed in collaboration with J.W. Cahn many years ago, the various facets of the interactions between stresses and thermodynamics are reviewed. Besides a purely energetic action, the long range nature of the elastic forces and their anisotropy have unusual consequences for the material scientist. Thus because the global elastic energy in a self-stressed system undergoing a phase change is shape dependant, there is a coupling between elastic and surface effects, both contributing to the evolution of the shape of precipitates. The thin film geometry is very useful for the exploration of the new phenomena induced on equilibrium and kinetic evolution of multiphase solids by the presence of such stresses. Limitations of this model due to interactions with cracks or dislocations are briefly discussed

Structure of the lamellar and L2 phases in the system sodium n-octylbenzene sulfonate/n-pentanol/water

The structure of the isotropic phase L2 occurring in the ternary system sodium octylbenzene sulfonate/n-pentanol/water is discussed on the basis of small angle X-ray scattering data. Experimental results are consistent with a local lamellar structure with a fraction of the alcohol expelled from the lamellar regions. The correlation length of this structure is about ten times smaller than is found in lamellar liquid crystals.La structure de la phase isotrope L2 apparaissant dans le système octylbenzène sulfonate de sodium/n-pentanol/eau est discutée à partir de résultats obtenus en diffusion centrale des rayons X. Ces résultats sont compatibles avec une structure locale lamellaire où une partie de l'alcool serait rejetée de la structure lamellaire. La longueur de corrélation d'une telle structure est à peu près dix fois plus faible que dans le cristal liquide lamellaire