The functionality of associative polymer networks: The association behavior of hydrophobically modified urethane-ethoxylate (HEUR) associative polymers in aqueous solution

We re-examine the rheology of aqueous solutions of a hydrophobically modified urethane ethoxylate associative polymer with M(n)=51000 containing C16H33 end groups. Fluorescence data indicate that association commences at a very low concentration (about 10 ppm). Over a wide range of concentrations, the system forms spherical flower-like micelles consisting of about 10 looped chains. We determine the chain-end aggregation number to be 22 +/- 2 end groups per micelle, independent of polymer concentration. Analysis of the rheology data indicates that larger structures are formed through secondary aggregation of the polymer micelles when the polymer concentration c(pol) > 0.2 wt.%. Treating the micelles as the fundamental building blocks of the secondary association, we calculate from the plateau modulus (at c(pol) > 1.2 wt.%) the effective functionality of the network and the fraction of chains that bridge neighboring micelles. Both of these parameters increase as the c(pol) increases.