Rubbers exhibit enormous extensibility up to several hundred per cent, compared with a few per cent for ordinary solids, and have the ability to recover their original shape and dimensions on release of stress. Rubber elasticity is a property of macromolecules that are either covalently cross-linked or connected in a network by physical associations such as small glassy or crystalline domains, ionic aggregates or multiple hydrogen bonds. Covalent cross-links or strong physical associations prevent flow and creep. Here we design and synthesize molecules that associate together to form both chains and cross-links via hydrogen bonds. The system shows recoverable extensibility up to several hundred per cent and little creep under load. In striking contrast to conventional cross-linked or thermoreversible rubbers made of macromolecules, these systems, when broken or cut, can be simply repaired by bringing together fractured surfaces to self-heal at room temperature. Repaired samples recuperate their enormous extensibility. The process of breaking and healing can be repeated many times. These materials can be easily processed, re-used and recycled. Their unique self-repairing properties, the simplicity of their synthesis, their availability from renewable resources and the low cost of raw ingredients (fatty acids and urea) bode well for future applications.

Reinforced silica–epoxy vitrimer nanocomposites have been made in a solvent-free, easily processable, and economical way with filler contents up to 40 wt %. Increasing silica content leads to higher modulus materials in both glassy and rubbery regions. These nanocomposites are insoluble like permanent cross-linked networks but can completely relax stresses by thermoactivated exchange reactions that rearrange the network topology. Furthermore, the surface functionalization of particles improves the dispersion of fillers and accelerates the relaxation process. Similar results were obtained with industrial precipitated silica, which would allow vitrimer nanocomposites to be produced on an industrial scale.