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.
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
Although the last decade has brought self-healing materials to the forefront of scientific interests, combining repair and sensing attributes into one material entity have not been addressed. 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. Mechanical nano-indentation analysis combined with molecular modeling and spectroscopic measurements confirm this behavior. The enclosed video clip illustrates molecular repair processes induced by visible light monitored by in situ Raman imaging spectroscopy. These materials may find numerous future applications, where coupling of simultaneous color changes and reversible self-repair responses may lead to new technological paradigms.
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.
Department of Materials Science and Engineering
Clemson University, SC 29634 firstname.lastname@example.org