The viscoelastic properties of poly(N-vinyl pyrrolidone-co-dimethylaminopropylmethacrylamide) (copolymer I) and its hydrophobically modified analogue, poly(N-vinyl pyrrolidone-dimethylaminopropylmethacrylamide-dimethylaminopropylmethacr ylamide (circle plus)-C12H25Cl circle minus) (copolymer II), were compared in the concentration range from I to 20% with dynamic and Brookfield rheometers. In addition, gel permeation chromatography/multi-angle laser light scattering light scattering data were obtained to characterize the molecular weights and radii of gyration. for the polymer samples. In general, the hydrophobically modified polymer copolymer II was characterized by a chain configuration that was more compact and a viscosity that was an order of magnitude lower than that of its unmodified counterpart, copolymer I (both polymers were characterized by similar molecular weights), in dilute (1% w/w) and semiconcentrated solutions (<5% w/w). The difference in the chain configurations resulted in significant differences in the measured viscosities of the polymer solutions as a function of polymer concentration and in the presence of added electrolyte. On the basis of dynamic rheological measurements for more concentrated solutions in the range of 5-10% for copolymer I and 5-20% for copolymer 11, the viscous modulus predominated (loss modulus > storage modulus), with crossover points occurring at 10% (w/w) and 10 Hz for copolymer I and 20% (w/w) and 20 Hz for copolymer II. The mixtures of copolymer II with anionic surfactants, such as sodium dodecyl sulfate and polyoxyethylene (2) dodecyl ether sulfate (sodium laureth-2-sulfate), showed strong interactions by exhibiting viscosity maxima and flocculation points corresponding to the surfactant/polymer alkyl group and surfactant/polymer charge concentration ratios in the range of 1-2. Copolymer Il showed network formation by significant viscosity buildup in combination with two nonionic surfactants, which included C14-15 Pareth-7 and C12-13 Pareth-3. On the basis of viscosity measurements, copolymer I was found not to interact with anionic and nonionic surfactants. (C) 2007 Wiley Periodicals, Inc.