Highly stretchable calcium ion/polyacrylic acid hydrogel prepared by freezing–thawing

Hydrogel is a promising soft matter attracting intensive study in the field of biomaterial, flexible electronics and soft robotics. The creation of a crosslinking polymer network is vital in fabricating hydrogel. A new method of fabrication brings more choices to the field. In this work, we show that the freezing–thawing method can be applied to prepare high-performance calcium–polyacrylic acid hydrogel. We polymerize acrylic acid in calcium chloride solution and obtain a soft and weak hydrogel; after the freezing–thawing treatment, the performance of the hydrogel improves profoundly. The freeze–thawed hydrogel can sustain strain up to 1100% and fracture at a stress of 0.6 MPa. This hydrogel also shows low hysteresis less than 5% at 200% strain and good recovery up to 800% strain. The hydrogel is strengthened by the increase of ionic bonds between cation and polymer chain during the freezing treatment. The same strategy can be applied to increase the inter-chain interaction of other hydrogels and thus provides more possibility in the design of hydrogel network.