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Recyclable, Strong Thermosets And Organogels Via Paraformaldehyde Condensation With Diamines
Published 2014 · Materials Science, Medicine
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Recyclable Thermoset Polymers The high mechanical strength and durability of thermoset polymers are exploited in applications such as composite materials, where they form the matrix surrounding carbon fibers. The thermally driven polymerization reaction is usually irreversible, so it is difficult to recycle the constituent monomers and to remove and repair portions of a composite part. García et al. (p. 732; see the Perspective by Long) now describe a family of polymers formed by condensation of paraformaldehyde with bisanilines that could form hard thermoset polymers or, when more oxygenated, produce self-healing gels. Strong acid digestion allowed recovery of the bisaniline monomers. A strong polymer formed by heating can be digested with strong acid to recover and recycle its bisaniline monomers. [Also see Perspective by Long] Nitrogen-based thermoset polymers have many industrial applications (for example, in composites), but are difficult to recycle or rework. We report a simple one-pot, low-temperature polycondensation between paraformaldehyde and 4,4ʹ-oxydianiline (ODA) that forms hemiaminal dynamic covalent networks (HDCNs), which can further cyclize at high temperatures, producing poly(hexahydrotriazine)s (PHTs). Both materials are strong thermosetting polymers, and the PHTs exhibited very high Young’s moduli (up to ~14.0 gigapascals and up to 20 gigapascals when reinforced with surface-treated carbon nanotubes), excellent solvent resistance, and resistance to environmental stress cracking. However, both HDCNs and PHTs could be digested at low pH (<2) to recover the bisaniline monomers. By simply using different diamine monomers, the HDCN- and PHT-forming reactions afford extremely versatile materials platforms. For example, when poly(ethylene glycol) (PEG) diamine monomers were used to form HDCNs, elastic organogels formed that exhibited self-healing properties.