Self-Healing Poly(acrylic acid) Hydrogels: Effect of Surfactant

Physical poly(acrylic acid) gels were prepared by micellar copolymerization of hydrophilic monomer acrylic acid with large hydrophobic monomer stearyl methacrylate (C18) in solutions of worm-like anionic surfactant (sodium dodecyl sulfate, SDS) and cationic surfactant (cetyltrimethyl ammonium bromide, CTAB) micelles. Hydrophobe content of the monomer mixtures was 2 mol.% and total monomer concentration in gels was 20 w/v%. Gelation reactions were monitored by rheometry using oscillatory deformation tests. Hydrogel samples were also subjected to uniaxial elongation and compression tests after preparation and swelling. The physical gels with SDS and CTAB showed similar properties as long as they are at the state of preparation, such as insoluble in water and exhibit time-dependent dynamic moduli, high Young's modulus, high fracture stress, high elongation ratios at break. Also self-healing was evidenced by mechanical measurements. Physical gels formed in SDS solution were stable, exhibited high equilibrium swelling ratios and SDS progressively extracted from the network in water. On the other hand, gels with CTAB formed a complex and resulted in shrinkage after gels were immersed in a large excess of water because both electrostatic interactions between the charged components and hydrophobic interactions between the polymer backbone and the alkyl chains of the surfactant are important in driving the self-assembly of molecules to form ordered structures. These structures of the polyelectrolyte surfactant complexes have unusual mechanical properties. Also, the hydrogel samples with CTAB self-healed via heating and surfactant treatment of the damaged areas withstand up to 1.05 MPa stresses and rupture at a stretch of 800%.