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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 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.

The friction between two like-charged polyelectrolyte gels in pure water is measured by using a normal strain-controlled rheometer with a parallel-plates geometry. The effects of normal stress, gel elasticity and sample thickness on the velocity dependence of friction between the gels are investigated. The frictional stress demonstrates strong velocity dependence (liquid-like) when the gel is soft and thick, while it demonstrates a weak or even no velocity dependence (solid-like) when the gel is rigid and thin. The former is interpreted by a combined mechanism of boundary lubrication and hydrated lubrication, wherein the thickness of the lubricating layer is velocity-independent, due to the formation of an electric double layer at the soft and repulsive interfaces. On the other hand, the latter is interpreted by a combined mechanism of boundary lubrication and elastohydrodynamic lubrication, wherein the thickness of the lubricating layer is velocity-enhanced by the water entrainment during sliding. The friction of the soft, thick sample is related to micro-contact while that of the rigid, thin sample is related to macroscopic geometric effect. This work may contribute to the science of friction between two soft and repulsive interfaces in water.

Robust bonding of a hydrogel in aqueous environment, either to another hydrogel or to a solid, is one of the major unsolved issues for the practical applications of hydrogels in various fields. Here we report robust bonding between a pair of hydrogel sheets, containing over 90 wt% of water, by 10 applying the double-network (DN) structure. In the optimal condition, the peeling energy of the united gel sheets reaches 1200 J/m 2 , which is comparable to the bulk fracture energy of a normal type of tough DN gels. This hydrogel bonding technique is also applied to form tough bonding between hydrogel and plastic plates. Furthermore, based on this technique, we have developed a facile method to synthesize robust double network hydrogels with any desirable free -shape from 15 micro-gel precursors. These novel techniques will substantially merit the ap plications of the tough hydrogels in various fields, such as an artifical meniscus.

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