Kinetics and mechanisms of water sorption in hydrophobic, ionizable copolymer gels

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Abstract
The aqueous kinetic swelling properties of a class of cross‐linked hydrophobic polyamine copolymer gels based on n‐alkyl esters of methacrylic acid (nAMA) and N,N‐dimethylaminoethyl methacrylate (DMA) have been studied as a function of solution ionic composition (pH, ionic strength, and buffer content), gel composition, and temperature. Water uptake and swelling in these gels are driven by ionization of the DMA amine groups, which overcomes the hydrophobic tendency of these gels to exclude water in the unionized state. Sorption kinetics in initially glassy gel disks are generally biphasic, characterized by an initial phase of relatively slow water uptake followed by an accelerated phase during which significant volume expansion of the gel occurs. This sorption/swelling behavior strongly suggests a moving penetrant front mechanism. The initial rate of water sorption increases markedly as (1) solution pH decreases, (2) gel nAMA comonomer content decreases, (3) gel nAMA side‐chain length decreases, and (4) temperature increases. Furthermore, the initial phase of sorption in initially glassy gels is generally non‐Fickian and approaches zero‐order behavior as (1) pH increases, (2) nAMA content increases, and (3) temperature decreases. In direct contrast, sorption in initially dry, rubbery gels is monophasic, but non‐Fickian, and approaches zero‐order behavior as temperature increases. This behavior is contrary to the Fickian sorption behavior normally observed in polymers above their glass transition temperatures. Finally, sorption kinetics critically depend upon the nature of the ions in solution: Kinetics are significantly faster in the presence of weak electrolytes than that of strong electrolytes. We discuss the importance of the rate of ion transport in determining the overall sorption kinetics and how sorption kinetics can be non‐Fickian in rubbery gels. Also, we propose a mechanism for the observed enhanced kinetics in the presence of weak electrolytes.