Critical phenomena are grouped into classes depending on the spatial dimension (D) and the dimension of the order parameter (N). In the vicinity of the critical temperature Tc, the properties of the systems with the same (D,N) scale with the reduced temperature t=(T-Tc)/Tc with identical critical indices for each property. It has long been appreciated that solutions of polymers in liquid organic solvents belong to the same universality class as simple fluids and small molecule mixtures, and their behavior near Tc is described by the critical x-indices of the three-dimensional Ising model (N=1) [1]. More recently it was found that the region where polymer solutions behave universally with temperature (>>Rg) terminates at T=Tx when the correlation length \ of the concentration fluctuations becomes equal to the radius of gyration Rg of the polymer [2,3]. At T>Tx (^<Rg) the solutions enter the © domain where their properties are described by the mean field T-indices.

Recent experiments have demonstrated that the © condition may be also induced in polymer solutions in supercritical fluids (SCFs) by varying the temperature and/or pressure [4]. A SCF is a substance at a pressure and temperature above the liquid-vapor critical point where the coexisting liquid and vapor phases become indistinguishable. The physical properties of SCFs are similar to those of dense gases, although when highly

Managed by Lockheed Martin Energy Research Corporation under contract DE-AC05-96OR-22464 for the U. S. Department of Energy.

compressed, their density may be comparable to sub-critical liquids. Despite recent progress in many areas of SCF science and technology (see e.g. [5] and references therein), significant challenges remain in developing the same level of understanding of polymer/SCF solutions as has been reached for solutions of polymers in traditional organic solvents. This article will focus on small angle neutron scattering (SANS) and dynamic light scattering (DLS) studies of mixtures of strongly interacting (overlapping) polymers in liquid and supercritical solvents. We have observed an unexpected universality in both thermodynamic and dynamic properties of these seemingly different physical systems.

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