Waterborne polymer nanogels non-covalently crosslinked by multiple hydrogen bond arrays

Abstract

Synthetic colloidal hydrogel particles of sub-micron dimensions, often referred to as microgels and/or nanogels, generally consist of water-soluble polymers held together through chemical crosslinking by covalent bonds in order to preserve the distinct colloidal particle identity. Here we demonstrate the synthesis of non-covalently crosslinked nanogel particles in which the crosslinking through covalent bonds is replaced by physical crosslinking induced by strong self-complementary quadruple hydrogen bond interactions. The multiple hydrogen bond (MHB) arrays were introduced in the form of a 2-ureido-4[1H]pyrimidinone (UPy) functionalized polyethylene glycol methacrylate (PEGMA) comonomer, which was employed in the synthesis of colloidal nanogels made from N-isopropylacrylamide (NIPAm) or a mixture of 2-(2-methoxyethoxy)ethyl methacrylate and oligo(ethylene glycol) methacrylate (MEO₂MA-co-OEGMA) following conventional free radical polymerization routes. The temperature-dependent swelling properties of the non-covalently crosslinked nanogels with differences in UPy loadings were studied, clearly demonstrating that MHB arrays can work as crosslinking moieties warranting the colloidal particle identity of the prepared hydrogels.