The complexation between poly (N,N-diethylacrylamide) (PDEA) and poly(acrylic acid) (PAA) in aqueous solution was studied by viscometric, potentiometric, and fluorescence techniques. It was found that an interpolymer complex formed between the two polymers through hydrogen bonding interactions with the stoichiometry of r=0.6 (r is unit molar ratio of PAA/PDEA), and the complex formation show the dependence on pH values. The phase behaviour studies showed that the lower critical solution temperature of the PDEA-PAA aqueous solution gradually increased with the increasing of r from 0.01 to 0.15, until a soluble system in the whole temperature region was obtained, which remained in the range of r=0.15-0.3. At higher PAA concentrations, when r is above 0.3, the system appeared phase separation, and almost no temperature dependence was observed. Based on these conclusion and structure characteristics of PDEA and PAA, a model containing only short sequences of monomer residues was proposed for the structure of PDEA–PAA complex.

A series of pH-and temperature-responsive (N,N-diethylacrylamide-co-methacrylic acid) (DEAM-MAA) random copolymers were synthesized by free-radical copolymerization techniques, and their lower critical solution temperatures (LCSTs) at different pH values and their critical phase-transition pH values at different temperatures were determined by transmittance measurements, respectively. The LCSTs of DEAM-MAA copolymers depend on the MAA content in the copolymers and are significantly affected by the pH of the aqueous solution. Similarly, the critical phase-transition pH values also depend on the MAA content of the DEAM-MAA copolymers and are significantly affected by the temperature of the aqueous solution. The LCST of DEAM-MAA copolymers increases with an increase in the MAA content. The critical phase-transition pH increases with increasing MAA content in the copolymer and also increases with increasing aqueous solution temperature.