Healable, transparent coatings based on dynamic covalent acylsemicarbazides

Transparent coatings that can be repaired when damaged are important to increase the lifetime of devices. Here, we fabricate easily accessible, two-component formulations for healable, transparent coatings based on the dynamic covalent acylsemicarbazide (ASC) bond and polydimethylsiloxane (pDMS). Combining dynamic covalent and non-covalent chemistries through ASCs represents an efficient approach to improve the mechanical and dynamical properties of healable materials. Current ASC-based materials suffer from lengthy synthesis requiring high-boiling solvents, and the effects of crosslink density and phase separation on the material properties are poorly understood. To this end, telechelic pDMS of three different lengths end-capped with hydrazides are reacted with a triisocyanate, to form transparent films and coatings fast and straightforwardly. The processing conditions dictate the degree to which nanophase-separated ordering occurs, which results from the hydrogen-bonding interactions of the ASC motifs. The mechanical properties of the networks vary greatly with the length of the pDMS chain, with smaller pDMS chains affording the strongest materials, as a result of higher crosslink and hydrogen-bonding densities. Stress relaxation experiments reveal efficient stress relaxation for all networks, with increasingly complex relaxation behavior upon increasing the pDMS length. The polymer films are optically transparent and show efficient scratch healing after damage. Moreover, the recovery of both mechanical properties and optical transparency of coatings is achieved after healing. Our work highlights that the interplay of dynamic covalent and supramolecular interactions allows to obtain easily accessible, healable coatings. This improves the lifetime of materials that rely on optical transparency and/or mechanical properties, such as in optical lenses for cameras or in medical appliances.