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 ccluster is only a small fraction of the overall concentration c. The ratio c/ ccluster (overlap parameters Ω) 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 Ωequil ≥ Ωshear. Only in the vicinity of the glass transition temperature and at high concentration the opposite is the case. Experimental information on Ωequil as a function of φ, the volume fraction of polymer, yields coil-overlap and cross-over concentrations in agreement with the results of scattering studies; analogous information on Ωshear (φ) 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. [Display omitted] •Polymer solutions are inhomogeneous on mesoscopic scales; they form clusters.•Segmental clusters (resulting from chain connectivity) prevail under equilibrium conditions.•Molecular clusters (caused by entropy minimization) form under Newtonian shear.•The polymer concentration in equilibrium clusters is normally much less than in shear clusters.•Opposite behavior: Near glass transition T and at high polymer concentrations.