A pH-sensitive, strong double-network hydrogel: Poly(ethylene glycol) methyl ether methacrylates-poly(acrylic acid)

We here describe new double network (DN) hydrogels with excellent mechanical strength and high sensitivity to pH changes. The first polymer network has a bottle brush structure and is formed from oligo-monomers of poly(ethylene glycol) methyl ether methacrylate (PEGMA). Poly(acrylic acid) (PAA) is used as the second network. This double network features strong intermolecular interactions between the neutral poly(ethylene glycol) (PEG) side chains of PPEGMA and the non-ionized carboxylic acid groups of the PAA second network. When immersed in solutions with a pH below ∼4 the DN hydrogels have a low swelling ratio and are opaque as a result of solvent-polymer phase separation driven by the formation of dense hydrogen-bonded clusters. The compression strength (∼8 MPa) is at least 14 times higher than the analogous single networks. When immersed in solutions with a pH >4, the hydrogels are transparent and exhibit a high swelling ratio with a compression strength of ∼1 MPa. The PEG side chain length can be readily controlled without greatly altering the overall DN topology by choosing PEGMA monomers having different PEG side chain lengths. Longer PEG side branches give higher compression and tensile strengths at pH <4 when hydrogen bonded clusters form. The robust nature of these DN gels over a wide pH range may be useful for applications such as artificial muscles and controlled release devices.