Preparation Of Highly Stretchable And Tough Hydrogels Based On The Properties Of Condensed Tannins

Hydrogels are defined as aggregations of chemically crosslinked polymer networks and water.In the last three decades,polymeric hydrogels have attracted lots of scientific interest due to its potential application in sensing,¹ vehicles for drug delivery,²,3,3 actuation,⁴ scaffolds for tissue engineering,⁵etc.However,the chemically crosslinked hydrogels exhibit brittle,low-stretchable and poor mechanical properties which limited its extensive use for practical application.In the process of fabrication of conventional crosslinked hydrogel,the crosslinker take part in the polymerization process to form the crosslinked network in random way.Therefore,the chain lengths between crosslinking points always had a broad distribution and structural inhomogeneity always occurred with high concentration of crosslinker.⁶ Therefore,stress concentration point is easily formed in the network which makes the hydrogel broken easily?Figure 1a?.The effective utilization of traditional chemically crosslinked hydrogels as load-bearing structural materials is seriously hampered by the fragility and weakness arising from an inhomogeneous network structure.During the past decades,various types of tough hydrogels has been fabricated by introducing effective energy dissipation mechanism or designing distinctive structures,such as a topological gel,⁷nanocomposite gel synthesized by polymerizing with nano materials,^8-13double-network hydrogels,^14-16macromolecular microsphere composite?MMC?hydrogels,¹77 polyampholytes hydrogels,¹88 hydrogels with transformable domains,¹99 microgel-reinforced hydrogels²⁰ and others.^21-24All of these methods are based on the same idea that new components were incorporated into the network of polyacrylamide to improve the homogenous degree of network for effective energy dissipation.However,almost no research work has been reported to improve the homogeneous degree of the network from the mechanism based on radical polymerization.Inspired by the function of condensed tannins in protecting tissue from the attack of free radicals,we designed a simple and novel method to prepare chemically crosslinked and highly tough hydrogels with ideally homogeneous network with the presence of condensed tannins.During the process of polymerization,condensed tannins could capture free radicals from the initiator and become the starting point for the gro h of polymer.By this way,condensed tannin could act as both co-initiator and crosslinking center.As a result,these hydrogels could be stretched up to2000%with a maximal fracture energy 3.51MJ/m³.SEM observation indicated that this novel hydrogel has homogeneous network structure which is quite different from the heterogeneous network structure of conventional chemically crosslinked hydrogels.Such good homogeneous network could effectively transfer the load among the network to endow the hydrogel with high toughness.Inspired by using the condensed tannins as co-initiator and crosslinker to prepare stretchable hydrogel?TIC-gel?and by tannins'ability to preapre tannins reduced graphene oxide?TGO?at 80?,this paper designs a simple and effective method with a thermal treatment process for the mixture of tannin and graphene oxide?GO?to prepare super extensible TGO composite TIC-gel?TGO-gel?.After the thermal treatment process,graphene oxide is reduced by tannin and its surface becomes orderly which leads to strong?–?stacking interaction between tannins and graphene sheets.By this way,the interaction between graphene sheets and polymer chains is improved.As a result,these composite hydrogels have high tensile strength?0.5-1.2MPa?and extremely high elongations?3100-4800%?which is much higher than those of GO composites TIC-gel?TA/GO-gel?without thermal-treated process and those of normal graphene composite hydrogel.