Hydrogen-bonding effects on the dynamic swelling of P(N-iPAAm-co-MAA) copolymers.: A case of autocatalytic swelling kinetics

The swelling kinetics of a series of P(N-iPAAm-co-MAA) copolymers and their P(N-iPAAm) and P(MAA) homopolymers previously soaked under different acidic and neutral conditions have been studied at pH 7 to probe the hydrogen-bonding ability of comonomers. Experimental data analysis has revealed that although the state due to the previous soaking pH does not exert any influence on the equilibrium swelling value of copolymers, it exerts a strong influence on their swelling kinetics. Copolymers gels ranging in composition between 30 and 70 mol % of N-iPAAm. previously soaked under acidic conditions exhibit, when subsequently immersed in water at a neutral pH, sigmoidal swelling curves. This behavior has been interpreted according to an autocatalytic mechanism, suggesting that cooperativity on the hydrogen bond disruption controls the swelling behavior. The dynamic swelling of these hydrogels under pH 7 after soaking at this same pH is in very good agreement with a second-order kinetics. This fact corroborates that anomalous features found on the swelling dynamics of these copolymers are related to the strong hydrogen-bonding arrangement between both comonomeric units as well as to the hydrophobicity strongly enhanced by intra- or/and interchain hydrogen bonding.