Polymer solutions: Equilibrium clusters versus shear clusters

Polymer solutions are inhomogeneous on mesoscopic scales as a result of chemical bonds linking their monomeric units. This situation leads to polymer clusters within which the polymer concentration c(cluster), is only a small fraction of the overall concentration c. The ratio c/c(cluster) (overlap parameters Omega) quantifies the number of clusters that need to overlap to yield c. Equilibrium clusters (minimization of Gibbs energy) and shear clusters (minimization of entropy production) differ fundamentally where Omega(equ)(il) >= Omega(sh)(ear). Only in the vicinity of the glass transition temperature and at high concentration the opposite is the case. Experimental information on Omega(equ)(il) as a function of phi, the volume fraction of polymer, yields coil-overlap and cross-over concentrations in agreement with the results of scattering studies; analogous information on Omega(s)(hear) (phi) gives access to cross-over concentrations under shear. Theoretical aspects and questions of practical interest arising from the observed differences between equilibrium and shear clusters are being discussed.