Thermoset polymers and fiber-reinforced polymer composites possess the chemical, physical, and mechanical properties necessary for energy-efficient vehicles and structures, but their energy-inefficient manufacturing and the lack of end-of-life management strategies render these materials unsustainable. Here, we demonstrate end-of-life deconstruction and upcycling of high-performance poly(dicyclopentadiene) (pDCPD) thermosets with a concurrent reduction in the energy demand for curing via frontal copolymerization. Triggered material deconstruction is achieved through cleavage of cyclic silyl ethers and acetals incorporated into pDCPD thermosets. Both solution-state and bulk experiments reveal that seven- and eight-membered cyclic silyl ethers and eight-membered cyclic acetals are incorporated efficiently with norbornene-derived monomers, permitting deconstruction at low comonomer loadings. Frontal copolymerization of DCPD with these tailored cleavable comonomers enables energy-efficient manufacturing of sustainable, high-performance thermosets with glass transition temperatures of >100 °C and elastic moduli of >1 GPa. The polymers are fully deconstructed, yielding hydroxyl-terminated oligomers that are upcycled to polyurethane-containing thermosets having a higher glass transition temperatures than that of the original polymer upon reaction with diisocyanates. This approach is extended to frontally polymerized fiber-reinforced composites, where large-fiber volume fraction composites (Vf = 65%) containing a cleavable comonomer are deconstructed and the reclaimed fibers are used to regenerate composites via frontal polymerization that display properties nearly identical to those of the original. This work demonstrates that the use of cleavable monomers, in combination with frontal manufacturing, provides a promising strategy to address sustainability challenges for high-performance materials at multiple stages of their lifecycle.
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Recyclable and malleable thermosets enabled by activating dormant dynamic linkages
Chemical recycling of polymers is critical for improving the circular economy of plastics and environmental sustainability. Traditional thermoset polymers have generally been considered permanently crosslinked materials that are difficult or impossible to recycle. Herein, we demonstrate that by activating ‘dormant’ covalent bonds, traditional polycyanurate thermosets can be recycled into the original monomers, which can be circularly reused for their original purpose. Through retrosynthetic analysis, we redirected the synthetic route from forming conventional C–N bonds via irreversible cyanate trimerization to forming the C–O bonds through reversible nucleophilic aromatic substitution of alkoxy-substituted triazine derivatives by alcohol nucleophiles. The new reversible synthetic route enabled the synthesis of previously inaccessible alkyl-polycyanurate thermosets, which exhibit excellent film properties with high chemical resistance, closed-loop recyclability and reprocessing capability. These results show that ‘apparently dormant’ dynamic linkages can be activated and utilized to construct fully recyclable thermoset polymers with a broader monomer scope and increased sustainability.
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- Award ID(s):
- 1901807
- NSF-PAR ID:
- 10471304
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Nature Chemistry
- Volume:
- 14
- Issue:
- 12
- ISSN:
- 1755-4330
- Page Range / eLocation ID:
- 1399 to 1404
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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The need to minimize the amount of polymeric waste entering landfills and oceans has led to several research avenues in the field of polymer science. Particularly, the development of intrinsically self-healing and reprocessable thermoset polymers containing dynamic crosslinks has garnered significant interest in the recent years. Reversible B–O bonds in certain orgonoboron compounds have shown great versatility and promise as dynamic crosslinks for the design of self-healing and reprocessable bulk polymer networks. This review provides an overview of the chemistry of organoboron species with dynamic B–O bonds amenable to the design of healable/reprocessable thermosets. Recent developments in this fairly young and interesting research topic are highlighted, along with a critical commentary on the scope and future challenges in designing robust dynamic materials with reversible B–O bonds.more » « less
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Abstract Thermally cured thermoset polymers such as epoxies are widely used in industry and manufacturing due to their thermal, chemical, and electrical resistance, and mechanical strength and toughness. However, it can be challenging to 3D print thermally cured thermosets without rheological modification because they tend to flow and not hold their shape when extruded due to cure times of minutes to hours. 3D printing inside a support bath addresses this by allowing the liquid polymer to be held in place until the thermoset is fully cured and expands the structures that can be printed as extrusion is not limited to layer‐by‐layer. Here, the use of Freeform Reversible Embedding (FRE) to 3D print off‐the‐shelf thermoset epoxy into lattice structures using nonplanar extrusion is reported. To do this, the authors investigate how extrusion direction in 3D space impacts epoxy filament morphology and fusion at filament intersections. Furthermore, the advantages of this approach are shown by using nonplanar printing to produce lattice geometries that show ≈ four times greater specific modulus compared with lattice structures printed using other materials and printing techniques.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1038/s41557-022-01046-4
