Effect of solvent quality on the level of association and encounter kinetics of hydrophobic pendants covalently attached onto a water-soluble polymer

Pyrene-labeled poly(N,N-dimethylacrylamide)s were prepared by radical copolymerization of N V-dimethylacrylamide with N-(1-pyrenylmethyl)acrylamide. The progress of the copolymerization reaction was followed by H-1 NMR to ensure that the pyrene-labeled monomer was homogeneously incorporated into the polymer backbone. Since pyrene is hydrophobic and poly(N,N-dimethylacrylamide) is water-soluble, a set of water-soluble associative polymers was generated. The effect of solvent quality toward these associative polymers was investigated. The level of association and the kinetics of encounter between pendants were determined in N,N-dimethylformamide (DMF), acetone, water, and mixtures of acetone and water. Analysis of the fluorescence decays with a blob model yields quantitative results which agree with the qualitative information retrieved by other techniques (static light scattering, intrinsic viscosity, UV-vis absorption). The level of association between hydrophobic pendants was found to be small in acetone (a good solvent for pyrene). It increases when water is added to the solution, since pyrene is insoluble in water. The level of association is smaller in DMF than in acetone, because DMF is a better solvent than acetone for the polymer, and swelling of the polymer coil results in a decrease of the interactions existing between the pyrene groups.