Abstract

Daily damage as scratches or fractures on the most polymeric material are inevitable, which shorten the lifespan, change/weaken the original integrity, and sometimes lead to the catastrophic failure of the materials. Self-healing or self-repairing is a desired property in the design and development of high-performance materials with their built-in ability to repair physical damage for various applications such as surface coatings, tissue engineering, and sensors.
Intrinsic self-healing utilizing dynamic chemistry is a promising method that allows for the development of effective self-repairing polymeric materials. This method involves the incorporation of non-covalent bonds through physical interactions such as π-π stacking, ionic interaction, metal binding, and hydrogen bonding. However, the use of physical bonding has a major drawback—small mechanical properties of prepared compounds due to the nature of the weak physical interaction. Another method utilizes reversible covalent bonds such as the Diels-Alder/retro-Diels-Alder reaction, alkoxyamine recombination, urea chemistry, and disulfides. Although these dynamic covalent bonds can provide higher mechanical properties compared to the physical interactions, the self-healing behavior often can be limited and require severe external stimuli to achieve a complete self-repairing procedure.
My Masters’ research aims to explore the advantages and disadvantages of covalent and supramolecular (physical) networks. Two novel self-healable networks were developed; one network designed with dynamic disulfide linkages and the other with both disulfide and supramolecular metal-ligand associations. Dynamic disulfide linkages are excellent candidates to explore in developing self-healable polymeric materials since they can be readily cleaved/disturbed to thiols or thiyl radicals in response to external stimuli, and then subsequently rebounded to induce self-repair of the damaged parts. In a similar way, the metallo-complex/ionic links are widely incorporated in forming self-healable polymeric networks because of the dynamic linkages between the ionic crosslinkers and their counter-ions.
For the first network, we explored having poly(methacrylate)-based crosslinked materials for which self-healing is based only on dynamic disulfide-thiol chemistry. Such materials were prepared by the extent oxidation of excess thiols in the lightly crosslinked networks through sulfide linkages. The second system consists of a multiblock copolymer with self-healable blocks and a middle block. The self-healable blocks are poly(methacrylate)-based units with pendant disulfides linkages and/or another pendant carboxylic acids groups. The presence of two different pendant dynamic linkages enables the formation of polymeric crosslinked materials—with dual self-repairing units—through disulfide-thiol exchange and metallo-complex with metal ions. It is believed that these unique designs along with their tunable self-healing kinetics demonstrate well the versatility of our methods to prepare self-healable polymeric crosslinked networks that have a promising potential for the development of multifunctional industrial applications.