Behavior of poly(N-vinylcaprolactam-co-methacrylic acid) macromolecules in aqueous solution:: Interplay between Coulombic and hydrophobic interaction

Dynamic light scattering measurements have been performed for aqueous solutions of linear copolymer of N-vinylcaprolactam (VCa) and methacrylic acid (MA) with variable composition (i.e., charge degree of polymer chain). The swelling behavior of macromolecules was investigated as a function of both the solution pH value and fraction of methacrylic acid groups. It was shown that these copolymers are thermosensitive, and intermacromolecular aggregation takes place at the temperature increase. They exhibit compositionally dependent swelling behavior as a function of both pH and temperature. The behavior of studied copolymer was explained by the partial formation of ion pairs, The dependence of hydrodynamic diameter on pH of the solution is nonmonotonic. In acidic media at room temperatures we observed the intermolecular aggregation of copolymers accompanied by a sharp increase of the light scattering intensity. In alkaline media intermacromolecular aggregation is observed at higher temperatures. The effect of oppositely charged surfactant (cationic cetylpyridinium chloride, CPC) on the swelling behavior of PVCa-MA macromolecules was investigated under various conditions of pH and temperature. The association of PVCa-MA macromolecules was demonstrated to be a function of pH and CPC concentration. The equilibrium size of copolymer-CPC aggregates was found. It was shown that the size of intermacromolecular aggregates containing copolymers and CPC depends only weakly on the variation of ambient conditions (pH, surfactant, or copolymer concentration). The effect of temperature on the stability of copolymer/surfactant complexes was studied. This result is compared with recent theoretical predictions.