Polyampholyte hydrogels formed via electrostatic and hydrophobic interactions

Abstract

We introduce here a novel strategy to produce supramolecular polyampholyte hydrogels exhibiting pH sensitivity and anti-polyelectrolyte effect. The hydrogels were synthesized by photopolymerization of N,N-dimethylacrylamide (DMA) with equimolar amounts of the ionic monomers acrylic acid (AAc) and 4-vinylpyridine (4VP) under solvent-free condition. Instead of a chemical cross-linker, stearyl methacrylate (C18) was included into the comonomer feed to create hydrophobic associations. Both the electrostatic and hydrophobic interactions produce intermolecular linkages between the polymer chains acting as physical cross-link zones that are stable in water. Polyampholyte hydrogels are in a swollen state at pH<4 and pH>6 while they undergo a swelling-to-collapse transition between these pH values by adopting a collapsed conformation over a certain range of pH including their isoelectric points. This swelling behavior is a result of the pH difference between the inside and outside of the hydrogel, as demonstrated by the theory of swelling equilibrium. Rheological measurements indicate the reversible nature of the cross-link zones with finite lifetimes. Polyampholyte hydrogels containing 80–92% water exhibit Young’s moduli between 18 and 58kPa and sustain tensile strains up to 560%, while those prepared using a chemical cross-linker are brittle in tension. Cyclic mechanical tests show a large mechanical hysteresis and the existence of reversibly and irreversibly broken bonds under large strain.