Thermosensitive phase-separation behavior of poly(acrylic acid)-graft-poly(N,N-dimethylacrylamide) aqueous solution

To investigate the influence of molecular architecture of poly(acrylic acid) (PAAc)-graft- poly(N,N-dimethylacrylamide) (PDMAAm) (PAAc-g-PDMAAm) on its solution properties in aqueous media, graft copolymers were synthesized by free radical copolymerization of AAc with PDMAAm macromonomer. PAAc-g-PDMAAms were water-soluble at lower temperatures, while they showed a liquid-liquid phase separation from water above the characteristic transition temperatures. The phase transition behavior was affected by the copolymer compositions and pH of the solution. Below the transition temperature, hydrogen-bonding formation between AAc and DMAAm units was observed in the PAAc-g-PDMAAm aqueous solution by FT-IR-ATR measurement. Craft copolymers were thus formed spherical associates with a diameter of ca. 150 nm estimated from the dynamic light scattering measurement. At higher temperatures, the dehydrated regions and the weak dehydration of the PDMAAm chains induced cooperatively the hydrophobic aggregation, resulting in thermosensitive phase separation from water. Randomly copolymerized poly(AAc-co-DMAAm) was a water-soluble polymer which demonstrated no phase transition. The mixture of two linear polymeric chains, PAAc and PDMAAm, strongly resulted in interpolymer complexes via hydrogen bonding in aqueous solution, causing precipitation independent of temperatures. The graft-type copolymers demonstrate characteristic solution properties in an aqueous milieu, despite the fact that all polymer systems in this study had AAc and DMAAm units.