Umit Gulyuz, Oguz Okay
A promising strategy to design synthetic hydrogels with the ability to self-heal is to substitute the covalently cross-linked polymer chains by supramolecular ones. Although supramolecular hydrogels generally exhibit rapid self-healing without the need for any stimulus, they suffer from low mechanical strength which prevents them from any stress-bearing applications. Here, we describe a novel way for the production of self-healing hydrogels with shape memory behavior of high tensile strength (0.71.7 MPa) and stretch at break (800900%). Hydrophobically modified poly(acrylic acid) (PAAc) chains with cetyltrimethylammonium (CTA) counterions form the physical network of such hydrogels. They were prepared via micellar copolymerization of acrylic acid with 2 mol % stearyl methacrylate (C18) as the hydrophobic comonomer in an aqueous NaBr solution of cetyltrimethylammonium bromide (CTAB). Extraction of free CTAB micelles from the physical gels results in a drastic increase in their Youngs moduli (from 830 to 180600 kPa) and tensile strengths (from 0.10.2 to 0.71.7 MPa) due to the complex formation between PAAc and CTAB. Loading and unloading cycles conducted on hydrogels both at the state of preparation and at equilibrium in water show a significant hysteresis and good superposition of the successive loading curves, demonstrating damage done during loading is recoverable in nature. The hydrogel samples self-healed via heating and surfactant treatment of the damaged areas withstand up to 1.5 MPa stresses and rupture at a stretch of 600%. Because of the drastic change in the elastic modulus of PAAc hydrogels with a change in temperature, they also exhibit shape memory properties with a recovery ratio of 100%.