Abstract
The straightforward synthesis of a urea polymer network is presented. Commercially available monomers are polymerized using light-induced polymerization, resulting in networks crosslinked by hindered urea molecules. These moieties are reversible and, thus, can be converted into the starting compounds (that is, isocyanate and amine) by a simple thermal treatment. This process is monitored using differential scanning calorimetry as well as Raman and infrared spectroscopy. Furthermore, the self-healing ability of these polymer networks is investigated using scratch-healing tests as well as bulk-healing investigations using tensile testing. The resultant materials have a high E-modulus, are able to heal scratches at temperatures above 70 °C multiple times and their mechanical properties can be partially regenerated. The underlying healing mechanism is based on the reversible opening of the urea bonds and exchange reactions between two functional groups, which were confirmed from a spectroscopic analysis. In summary, these new materials are a new type of intrinsically healable polymers and provide a first step toward hard and healable polymers.
Highlights
The limited availability of natural resources is gradually causing modern society to develop more sustainable uses of oil-based materials
To obtain dynamic networks based on urea bonds, a specific monomer was synthesized
N,N'-di-tert-butylethylenediamine, as a sterically hindered amine, was converted with either hexyl isocyanate, resulting in model compound 1, or with 2-isocyanate ethyl methacrylate, resulting in monomer 2—the reversible crosslinker. Both compounds were characterized via NMR and Raman spectroscopy (Supplementary Information; Supplementary Figures S4) to confirm the formation of urea bonds
Summary
The limited availability of natural resources is gradually causing modern society to develop more sustainable uses of oil-based materials. Extrinsic self-healing materials are based on the encapsulation of a healing agent (for example, in microcapsules) within the polymer matrix, which is released after cracking.[3,4,5] In contrast, intrinsic self-healing materials can recover their properties due to the presence of specific reversible chemical bonds, which is beneficial due to the possibility of multiple healing steps at the same location.[6] These binding motifs can be composed of either dynamic covalent bonds,[7,8,9] for example, the Diels–Alder reaction[10,11,12] or radical-based systems,[13] or supramolecular interactions, such as hydrogen or halogen bonds,[14,15,16] ionic interactions,[17] π–π interactions,[18] host–guest interactions[19] or metal–ligand interactions.[20,21,22,23] Often there is a trade-off between the mechanical properties and the healing efficiency (Figure 1). High healing efficiencies are mostly only possible for low E-moduli; for E-moduli above 107–108 Pa, the number of reported examples decreases significantly, as well as the achieved healing efficiencies
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