Self-Repairing Materials

Self-Repairable Polyurethane Networks by Atmospheric Carbon Dioxide and Water

Sugar moieties were incorporated into cross-linked polyurethane (PUR) networks in an effort to achieve self-repairing in the presence of atmospheric carbon dioxide (CO2) and water (H2O). When methyl-α-d-glucopyranoside (MGP) molecules are reacted with hexamethylene diisocyanate trimer (HDI) and polyethylene glycol (PEG) to form cross-linked MGP-polyurethane (PUR) networks, these materials are capable of self-repairing in air. This process requires atmospheric amounts of CO2 and H2O, thus resembling plant behavior of carbon fixation during the photosynthesis cycle. Molecular processes responsible for this unique self-repair process involve physical diffusion of cleaved network segments as well as the formation of carbonate and urethane linkages. Unlike plants, MGP-PUR networks require no photo-initiated reactions, and they are thus capable of repair in darkness under atmospheric conditions.

UV-induced self-repairing polydimethylsiloxane–polyurethane (PDMS–PUR) and polyethylene glycol–polyurethane (PEG–PUR) Cu-catalyzed networks

UV-induced self-repairing polydimethylsiloxane–polyurethane (PDMS–PUR) crosslinked networks capable of repairing mechanical damage upon UV exposure were developed. Induced by the presence of the copper chloride (CuCl2) catalyst, the network remodeling and bond reformation are achieved by the formation of Cu–O coordination complexes, covalent Si–O–Si hydrolysis with subsequent bond reformation. Upon UV exposure, Cu–O complexes undergo tetrahedral-to-distorted tetrahedral rearrangements which parallel the Si–O bond reformation. When PDMS was substituted with OH-terminated polyethylene glycol (PEG) to form PEG–PUR crosslinked networks, square planar-to-tetrahedral rearrangements occur during the damage–repair cycle. Alkyl backbone distortions and segmental motions induced by the local Cu–O symmetry changes result in volume changes of the metal–ligand complex center. These studies show that a combination of supramolecular and covalent bonds facilitates self-repairing.

Self-Repairing Polymeric Nano-Materials

We are in the process of developing thermoplastics and thermosetting polymeric systems that are capable of self-repairing upon exposure to various physico-chemical conditions. Molecular level understanding of network remodeling events resulting from damage and self-repair will lead to a new generation of sustainable materials.Oxetane-Chitosan-Polyurethane Self-Repairing Networks.

UV-healable supramolecular polymer networks

This study focuses on the development of facile polyethylenimine–copper (C2H5N–Cu) supramolecular polymer networks which upon mechanical damage are capable of reversible UV-induced self-repairs by the reformation of Cu–N coordination bonds. The chemical changes responsible for self-healing that leads to network remodeling include the formation of C2H5N–Cu complexes which, upon UV absorption, induce charge transfer between σ(N) bonding and dx2–y2(Cu) antibonding orbitals. The primary structural component responsible for network remodeling is the C2H5N–Cu coordination complex center that, upon UV exposure, undergoes square-planar-to-tetrahedral (D4h → Td) transition. The energy difference between dx2–y2 and dxz/yz orbitals during process change decreases significantly, enabling the σ(N) → dx2–y2(Cu) charge transfer and leading to energetically favorable Cu–N geometries. Manifested by virtually no temperature changes during UV-initiated self-healing, a unique feature of this network is the high efficiency of the damage–repair cycle resulting from the reversible conversion of electromagnetic radiation to chemical energy.

Self-Repairable Copolymers that Change Color

These studies report the development of poly(methyl methacrylate/n-butylacrylate/2-[(1,3,3-trimethyl-1,3-dihydrospiro[indole-2,3′-naphtho[2,1-b][1,4]oxazin]-5-yl)amino]ethyl-2-methylacrylate) [p(MMA/nBA/SNO)] copolymer films that upon mechanical scratch undergo color changes from clear to red in the damaged area, but upon exposure to sunlight, temperature and/or acidic vapors, the damaged area is self-repaired and the initial colorless appearance is recovered. The process is reversible and driven by the ring-opening-closure of spironapthoxazine (SNO) segments to form merocyanine (MC), which are recovered back to the SNO form. Upon mechanical damage, SNO segments of the neighboring copolymer segments form inter-molecular H-bonding that stabilizes copolymer backbone, that remains in an extended conformation. External stimuli, such as light, temperature, or acidic environments cause a dissociation of the H-bonded MC pairs, which are converted back to SNO. This process is associated with the p(MMA/nBA/SNO) backbone collapse, thus pulling entangled neighboring copolymers to fill removed mass and repair a scratch.

Repairing Polymers Using Oscillating Magnetic Field

Highly controllable microwave plasma reactions on surfaces of polymeric materials offer a platform of covalent bonding of acid groups via reactions and hydrolysis of maleic anhydride. Such functional groups can be further used for surface reactions leading to stimuli-responsive polymeric surfaces that are biologically active or may exhibit antibacterial, antithrombotic, or antifouling attributes. This process can be applied to almost any thermoplastic polymer. Several examples below show covalent attachment of various antibiotic molecules that, through a molecular spacer (PEG), exhibit antimicrobial properties.