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-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-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-c. The tough polyampholyte gel, containing similar to 50 wt % water, is highly stretchable and tough, exhibits fracture stress of sigma(b) similar to 0.4 MPa, fracture strain of epsilon(b) similar to 30, and the work of extension at fracture W-ext similar to 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.