Robust, self-healing hydrogels synthesised from catechol rich polymers†
Coordinative interactions between polymer-bound catechols and metal ions are the basis for numerous bio-inspired soft materials. Here, we demonstrate rapid access to catechol rich polymers through reductive amination (RA) strategy. We employed chitosan to exemplify the utility of this protocol. Controlled grafting of catechol pendants (from as low as 18 mol% to as high as 80 mol%) onto chitosan was readily achieved in aqueous medium under ambient conditions by RA protocol. Because of the high density of catechol units grafted onto chitosan, we could accomplish the gelation of water even in acidic medium in the presence of transition metal ions, or on addition of chemical oxidant such as NaIO4. Increasing the mol% of catechol in polymer decreased the amounts of Fe(III) or NaIO4 required to yield gels. UV-vis and Raman studies indicated the presence of mono-complex between Fe(III) and catechol in the gels formed through coordinative crosslinking. Highly ductile hydrogels exhibiting excellent load bearing ability were obtained under these conditions. Electrostatic repulsions between the cationic polymer chains presumably prevented the gel to collapse upon the application of load. These gels were also completely self-healing due to the reversible nature of their coordinative interactions. Gels formed at higher pH are brittle and less resilient compared to those formed at lower pH. Ductility and self-healing ability of coordinative cross-linked gels are superior to those formed by oxidative crosslinking. We conclude that RA strategy offers rapid and easy access to catechol-rich systems, and retention of basic amine functionalities allows the preparation of robust bio inspired soft materials.