Micellar Synthesis and Properties of Hydrophobically Associating Polyampholytes

Water-soluble polymers containing a small amount of hydrophobic groups have been prepared in aqueous medium by radical polymerization of acrylamide, two oppositely charged monomers (sodium 2-acrylamido-2-methylpropanesulfonate and [2-(methacryloyloxy)ethyl] trimethylammonium chloride) and a hydrophobic monomer (N,N-dihexylacrylamide or N-(4-ethylphenyl)acrylamide). The synthesis has been achieved by using a micellar process, in which the use of a surfactant ensures the solubilization of a hydrophobic monomer in aqueous solution. The rheological properties in aqueous solution depend on the amount and nature of the hydrophobes, the proportion of charged monomer units, the polymer concentration, and the addition of salt. In the semi-dilute regime, the hydrophobically modified low charge density polyampholytes show a very significant enhancement in the viscosity, even in high salinity media. Examination of the viscosity both as a function of shear rate and as a function of time (fixed shear rate) shows these solutions to have shear thinning, shear thickening and rheopectic behavior. Fluorescence measurements show the existence of hydrophobic domains even at high dilution. The aggregation is a gradual process without a distinct critical aggregation concentration.