Thermo-sensitive swelling behavior in crosslinked N-isopropylacrylamide networks: Cationic,anionic, and ampholytic hydrogels

Crosslinked polymer networks of N-isopropylacrylamide (NiPAAm) containing small amounts of either anionic or cationic comonomers, or mixtures of both were fabricated and characterized in terms of their aqueous swelling and critical behavior. These gels demonstrate critical transition temperatures in aqueous media between a highly solvated, swollen gel state and a collapsed, dehydrated network over temperature ranges comparable to that of pure NiPAAm, with modifications of gel critical points and respective temperature ranges dependent upon comonomer type and content. Copolymer gel swelling ratios are significantly larger than those reported for pure homopolymer NiPAAm gels, even when only 0.5 mol % of comonomer is incorporated. At temperatures exceeding the collapse transition point, all copolymer gels collapse to a state of nearly complete dehydration, demonstrating short-time collapsed-state swelling ratios far lower than those of pure NiPAAm networks. Collapse kinetics for the ionomeric gels are much more rapid than those of pure NiPAAm, achieving collapse state equilibrium on time scales of seconds. Swelling behavior as a function of pH, buffer type, ionic strength, crosslinking, and temperature is detailed over a range of copolymer compositions. © 1993 John Wiley & Sons, Inc.