Soluble-insoluble-soluble transitions of aqueous poly(N-vinylacetamide-co- acrylic acid) solutions

Several poly(N-vinylacetamide-coacrylic acid)s with various copolymer compositions have been synthesized, and their unique phase-transition behavior in aqueous salt (Na₂SO₄ or NaCl) solutions was investigated. Copolymers containing more than 51 mol % N-vinylacetamide (NVA) show reentrant soluble-insoluble-soluble transitions with increasing temperature. The soluble-insoluble transition temperature (T_p1) increased linearly with increasing NVA content, whereas the insoluble-soluble transition temperature (T_p2) was almost constant irrespective of the NVA content. Potentiometric titration of the copolymer solutions suggested that the acrylic acid (AA) carboxyl groups form hydrogen bonds with the NVA amide groups even under soluble conditions. Dehydration of the NVA amides and their consequent hydrogen bonding with the AA carboxyl groups during the soluble-insoluble transition process was indicated by FTIR measurements. Addition of salt (Na₂SO₄ or NaCl) to the aqueous media reduces the solvent quality and enhances the intra- and interchain interactions of the copolymers. Thus, T_p1 was observed to decrease and T _p2 was observed to increase with increasing salt concentration. However, the addition of urea to the media reverses the concentration dependence of T_p1 and T_p2 by disturbing the intra- and interchain interactions of the copolymers.