Mechanically Resistant Poly(N-vinylcaprolactam) Microgels with Sacrificial Supramolecular Catechin Hydrogen Bonds

Microgels (mu gels) swiftly undergo structural and functional degradation when they are exposed to shear forces, which potentially limit their applicability in, e.g., biomedicine and engineering. Here, poly(N-vinylcaprolactam) mu gels that resist mechanical disruption through supramolecular hydrogen bonds provided by (+)-catechin hydrate (+C) are synthesized. When +C is added to the microgel structure, an increased resistance against shear force exerted by ultrasonication is observed compared to mu gels crosslinked by covalent bonds. While covalently crosslinked mu gels degrade already after a few seconds, it is found that mu gels having both supramolecular interchain interactions and covalent crosslinks show the highest mechanical durability. By the incorporation of optical force probes, it is found that the covalent bonds of the mu gels are not stressed beyond their scission threshold and mechanical energy is dissipated by the force-induced reversible dissociation of the sacrificial +C bonds for at least 20 min of ultrasonication. Additionally, +C renders the mu gels pH-sensitive and introduces multiresponsivity. The mu gels are extensively characterized using Fourier-transform infrared, Raman and quantitative nuclear magnetic resonance spectroscopy, dynamic light scattering, and cryogenic transmission electron microscopy. These results may serve as blueprint for the preparation of many mechanically durable mu gels.