A phase diagram of neutral polyampholyte - from solution to tough hydrogel

Our recent study has revealed that neutral polyampholytes form tough physical hydrogels above a critical concentration C_m,c by forming ionic bonds of wide strength distribution. In this work, we systematically investigate the behavior of a polyampholyte system, poly(NaSS-co-DMAEA-Q), randomly copolymerized from oppositely charged monomers, sodium p-styrenesulfonate (NaSS) and acryloyloxethyltrimethylammonium chloride (DMAEA-Q) without and with a slight chemical cross-linking. A phase diagram of formulation has been constructed in the space of monomer concentration C_m and cross-linker density C_MBAA. Three phases are observed for the as-synthesized samples: homogeneous solution at dilute C_m, phase separation at semi-dilute C_m, and homogenous gel at concentrated C_m. Above a critical C_m,c, the polyampholyte forms a supramolecular hydrogel with high toughness by dialysis of the mobile counter-ions, which substantially stabilizes both the intra- and inter chain ionic bonds. The presence of the chemical cross-linker (C_MBAA > 0) brings about a shift of the tough gel phase to lower C_m,c. The tough polyampholyte gel, containing ∼50 wt% water, is highly stretchable and tough, exhibits fracture stress of σ_b∼ 0.4 MPa, fracture strain of ε_b∼ 30, and the work of extension at fracture W_ext∼ 4 MJ m^-3. These values are at the level of most tough soft materials. Owing to the reversible ion bonds, the poly(NaSS-co-DMAEA-Q) gels also exhibit complete self-recovery (100%) and high fatigue resistance upon repeated large deformation.