Tough hydrogel based on covalent crosslinking and ionic coordination from ferric iron and negative carboxylic groups

In this work, carboxylic groups were introduced to latex microspheres (LMs) to obtain poly(styrene-butyl acrylate) (P(St-BA)-COOH) LMs with negative charges by a soap-free emulsion polymerization method. Then P(St-BA)-COOH LMs were added into an aqueous solution composed of acrylamide (AAm) and acrylic acid (AAc). The P(AAm-co-AAc)-MBA/LMs hydrogel was fabricated after adding redox initiators and N,N’-methylene diacrylamide (MBA) as covalent crosslinker by a free radical copolymerization. Subsequently, P(AAm-co-AAc)-MBA/LMs hydrogels were soaked into a ferric solution to achieve a tough hybrid hydrogel with covalent crosslinking and ionic coordination interaction between ferric ions and negative carboxylic groups arising from LMs and P(AAm-co-AAc) chains. The resultant P(AAm-co-AAc)-MBA/LMs-Fe3+ hydrogels possessed high fracture stress of 1440 kPa, large strain of 1140% and fracture energy of 6983 kJ/m3. This would help us to deeply understand the relationship between the structure and performance of LMs-reinforced hydrogels.