Gina L. Fiore

Optically healable supramolecular polymers

Polymers with the ability to repair themselves after sustaining damage could extend the lifetimes of materials used in many applications. Most approaches to healable materials require heating the damaged area. Here we present metallosupramolecular polymers that can be mended through exposure to light. They consist of telechelic, rubbery, low-molecular-mass polymers with ligand end groups that are non-covalently linked through metal-ion binding. On exposure to ultraviolet light, the metal–ligand motifs are electronically excited and the absorbed energy is converted into heat. This causes temporary disengagement of the metal–ligand motifs and a concomitant reversible decrease in the polymers’ molecular mass and viscosity, thereby allowing quick and efficient defect healing. Light can be applied locally to a damage site, so objects can in principle be healed under load. We anticipate that this approach to healable materials, based on supramolecular polymers and a light–heat conversion step, can be applied to a wide range of supramolecular materials that use different chemistries.

Optically healable polymers

Polymers that can easily be repaired after being damaged are attractive as this characteristic can improve the reliability, functionality, and lifetime of many products. In the last decade, researchers have thus developed new approaches to create stimuli-responsive polymer systems, which have the ability to autonomously heal or can be repaired upon exposure to an external stimulus. This review summarizes the current knowledge of optically healable or photo-healable polymers. The use of light as a stimulus for healing offers several attractive features, including the ability to deliver the stimulus locally, which opens up the possibility of healing the material under load, as well as the ability to tailor the wavelength of light to selectively address a specific component of the material, e.g. only the damaged parts. So far, two main classes of optically healable polymers have been explored, which are structurally dynamic polymers and mechanically activated reactive systems.

Optically responsive supramolecular polymer glasses.

The reversible and dynamic nature of non-covalent interactions between the constituting building blocks renders many supramolecular polymers stimuli-responsive. This was previously exploited to create thermally and optically healable polymers, but it proved challenging to achieve high stiffness and good healability. Here we present a glass-forming supramolecular material that is based on a trifunctional low-molecular-weight monomer ((UPyU)3TMP). Carrying three ureido-4-pyrimidinone (UPy) groups, (UPyU)3TMP forms a dynamic supramolecular polymer network, whose properties are governed by its cross-linked architecture and the large content of the binding motif. This design promotes the formation of a disordered glass, which, in spite of the low molecular weight of the building block, displays typical polymeric behaviour. The material exhibits a high stiffness and offers excellent coating and adhesive properties. On account of reversible dissociation and the formation of a low-viscosity liquid upon irradiation with ultraviolet light, rapid optical healing as well as (de)bonding on demand is possible.

Supramolecular Cross-Links in Poly(alkyl methacrylate) Copolymers and Their Impact on the Mechanical and Reversible Adhesive Properties

Hydrogen-bonded, side-chain-functionalized supramolecular poly(alkyl methacrylate)s were investigated as light- and temperature-responsive reversible adhesives that are useful for bonding and debonding on demand applications. Here, 2-hydroxyethyl methacrylate (HEMA) was functionalized with 2-ureido-4[1H]pyrimidinone (UPy) via a hexamethylenediisocyanate (HMDI) linker, to create a monomer (UPy-HMDI-HEMA) that serves to form supramolecular cross-links by way of forming quadruple hydrogen bonded dimers. UPy-HMDI-HEMA was copolymerized with either hexyl methacrylate or butyl methacrylate to create copolymers comprising 2.5, 5, or 10 mol % of the cross-linker. The mechanical properties of all (co)polymers were investigated with stress-strain experiments and dynamic mechanical analysis. Furthermore, the adhesive properties were studied at temperatures between 20 and 60 °C by testing single lap joints formed with stainless steel substrates. It was found that increasing the concentration of the UPy-HMDI-HEMA cross-linker leads to improved mechanical and adhesive properties at elevated temperatures. Concurrently, the reversibility of the bond formation remained unaffected, where rebonded samples displayed the same adhesive strength as regularly bonded samples. Debonding on demand abilities were also tested exemplarily for one copolymer, which for light-induced debonding experiments was blended with a UV-absorber that served as light-heat converter. Single lap joints were subjected to a constant force and heated or irradiated with UV light until debonding occurred. The necessary debonding temperature was comparable for direct heating and UV irradiation and varied between 28 and 82 °C, depending on the applied force. The latter also influenced the debonding time, which under the chosen conditions ranged from 30 s to 12 min.

Iron Tris(bipyridine) PEG Hydrogels with Covalent and Metal Coordinate Cross-Links

Spontaneous gel formation of iron(II) tris(bipyridine)-centered poly(ethylene glycol) methacrylate ([Fe{bpy(PEG-MA)2}3]2+) was observed without the addition of a cross-linking agent. BpyPEG2 macroligands were first modified with methacrylate groups using methacrylic anhydride and then combined with FeSO4 to produce [Fe{bpy(PEG-MA)2}3]SO4. End group analysis by 1H NMR spectroscopy verified quantitative methacrylation of the PEG hydroxyl chain ends. A series of experiments and control reactions were performed to investigate the conditions required for gel formation. Hydrogels of [Fe{bpy(PEG-MA)2}3]SO4 were produced both in the presence and in the absence of a photoinitiator. Controls using MA-PEG-MA also formed hydrogels in the presence of [Fe(bpy)3]2+; however, the addition of a radical scavenger, TEMPO, prevented formation of a polymer network, suggesting radical involvement. Treatment of preformed hydrogels of bpy(PEG-MA)2 with aqueous solutions of FeSO4, CuBr2, and CoCl2 also produced materials with color changes indicative of complexation.

Luminescent nanoparticles with lanthanide-containing poly(ethylene glycol)-Poly(ε-caprolactone) block copolymers.

Lanthanide-containing nanoparticles have attracted much attention due to their unique optical properties and potential in nanotechnological applications. An amphiphilic block copolymer of poly(ethylene glycol)-b-poly(ε-caprolactone) methyl ether (mPEG-PCL) was functionalized with a dipicolinic acid (dpa) moiety and coordinated to lanthanide ions to afford [Ln(dpa-PCL-PEG-OCH3)3](HNEt3)3 (Ln = Eu(3+), Tb(3+)). Micelle-like nanoparticles of dpa-PCL-PEG-OCH3 macroligand and metal-centered polymers were prepared by solvent displacement methods. Dynamic light scattering analysis (DLS) and cryogenic transmission electron microscopy images confirmed the presence of solid sphere (<47 nm in diameter) and vesicle (>47 nm in diameter) morphologies. The viability and stability of the lanthanide complexes in micelle-like nanoparticles was explored by DLS and luminescence spectroscopy, and found to be stable for several weeks.

Ruthenium tris(bipyridine) complexes with sulfur substituents: model studies for PEG coupling.

Ruthenium polypyridyl complexes are incorporated into polymers for sensing and light emitting materials applications. Coupling reactions between metal complexes and polymers are one route to polymeric metal complexes. In an effort to increase conjugation efficiency, tune materials properties, and introduce a responsive crosslink, ruthenium tris(bipyridine) derivatives with sulfur substituents were synthesized and compared to oxygen analogues. Difunctional thiols, thioesters, thioethers, and disulfides, as well as hexafunctional nonpolymeric model systems, were explored. Upon exposure to oxygen, the thiol derivative was readily oxidized. These studies guided Ru(bpy)3 PEG coupling reactions with disulfide and thioether linkages, which proceeded to approximately 80% and approximately 60% yield, respectively. The luminescence properties of the Ru PEG derivatives and model systems were investigated. The emission spectra and lifetimes for all complexes in CH3CN under an inert atmosphere are comparable to [Ru(bpy)3]Cl2. Lifetime data for nonpolymeric analogues fit to a single exponential decay indicating heterogeneity, suggesting sample homogeneity, whereas data for polymers fit to a multiexponential decay. In contrast to certain [Ru(bpy)3](2+)/thiol mixtures, no intramolecular quenching by the sulfide is observed for [Ru(bpy)2{bpy(CH2SH)2}](PF6)2. Emission spectra red shift and multiexponential decay are noted for the oxidized Ru thiol product. The rates of oxygen quenching are slower for Ru PEG derivatives than those for nonpolymeric analogues, which may be attributed to shielding effects of the polymer chain.