INFLUENCE OF SURFACTANTS ON THE RHEOLOGY OF ASSOCIATING POLYMERS IN SOLUTION

The influence of low molecular weight surfactants on the rheology of a queous solutions of model hydrophobic ethoxylated urethane (HEUR) asso ciative thickeners (AT) under dynamic shear has been studied. Solution s of HEUR-AT behave as Maxwell fluids with a well-defined single relax ation time. It is shown that surfactants profoundly influence the rheo logical properties displayed. The type of effect observed is dependent on the polymer concentration. For example, in the case of the plateau modulus, at low polymer concentrations, the surfactant produces an in itial rise in modulus, which then peaks and gradually falls as the sur factant concentration is increased. At higher polymer concentrations t he initial rise and peak are not observed. In the case of the viscoela stic relaxation time, ostensibly similar trends are observed at low po lymer concentrations as are observed for modulus. Increasing the polym er concentration, however, does not result in the disappearance of the peak. The relaxation time is also observed to broaden at higher surfa ctant concentrations. The results described are compared with the pred ictions of an elementary statistical mechanical model, supported by Mo nte Carlo simulation, describing the distribution of chain states in r elation to the concentration of micelles. The micellar concentration i s shown to determine the ratio of chains in looped conformations to th ose present as bridges. When the effect of surfactant on the total num ber of micelles is included in the model, the predictions of the theor y are found to account for the observed experimental results.