Polyelectrolyte and antipolyelectrolyte effects in swelling of polyampholyte and polyzwitterionic charge balanced and charge offset hydrogels

The swelling properties of polyampholytic hydrogels with different charge stoichiometric ratio were characterized by continuous monitoring changes in optical length of 60 μm sized hydrogels with 2 nm resolution using an interferometric readout platform. The hydrogels were synthesized by copolymerizing acrylamide, crosslinker Bis and charged monomers. Anionic–cationic polyampholyte gels consisted of different molar ratio between anionic monomer 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA) and cationic monomer (3-acrylamidopropyl) trimethylammonium chloride (APTAC). The polyzwitterionic hydrogels consisted of zwitterionic monomer [2-(methacryloyloxy)ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide (SBA) with a certain molar of AMPSA or APTAC in addition to the AAM. The observed swelling properties versus the ionic strength in the aqueous immersion solution revealed apparent anti-electrolyte signatures for both types of polyampholytic hydrogels. The swelling kinetics was not affected by charge imbalance within the polyampholyte or polyzwitterionic hydrogels. Polyampholyte hydrogels with balanced charge ratio slightly shrinked and then swelled with the increase of ionic strength, which is at variance with the anionic AMPSA offset hydrogels displaying shrinking in low ionic strength and relatively small swelling at high ionic strength. The swelling properties of imbalanced polyampholyte hydrogels were not equal to simply linear overlap of those of polyampholyte hydrogels with balanced charges, and corresponding polyelectrolyte hydrogels.