Deviations from the Stress−Optical Rule in Telechelic Associative Polymer Solutions

Abstract

The stress−optical rule is known to be valid for a wide range of polymeric fluids, as long as the chains are only slightly extended. In solutions of associative polymers, strain hardening and shear thickening are known to occur, and non-Gaussian chain stretching has been proposed as an explanation for these phenomena. Hence, the stress−optical rule could be expected to break down for these materials. Here, a rheooptical investigation of a telechelic associative polymer is presented. Various nonlinear flow fields have been applied. When strain hardening or shear thickening occurs, it is shown that the stress−optical rule is indeed not valid anymore. An analysis of the rather small values of the strains at the onset of the nonlinearities suggests that the bridging chains are probably in a stretched state even in the absence of flow. On the other hand, the birefringence data indicate that the local deformation of the polymeric chains is more complex. The final result is that the mechanical behavior can be described quite well by a transient network model, assuming initially strained chains. The rheooptical data, however, prove that the actual structural changes in associative polymers are even more complex, possibly because of the presence of micelles.