Most covalent adaptable networks give highly interesting properties for material processing such as reshaping, recycling and repairing. Classical thermally reversible chemical cross-links allow for a heat-triggered switch between materials that behave as insoluble cured resins, and liquid thermoplastic materials, through a fully reversible sol–gel transition. In 2011, a new class of materials, coined vitrimers, was introduced, which extended the realm of adaptable organic polymer networks. Such materials have the remarkable property that they can be thermally processed in a liquid state without losing network integrity. This feature renders the materials processable like vitreous glass, not requiring precise temperature control. In this mini-review, an overview of the state-of-the-art in the quickly emerging field of vitrimer materials is presented. With a main focus on the chemical origins of their unique thermal behavior, the existing chemical systems and their properties will be discussed. Furthermore, future prospects and challenges in this important research field are highlighted.

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Vitrimers: permanent organic networks with glass-like fluidity

Vitrimers are a new class of polymeric materials with very attractive properties, since they can be reworked to any shape while being at the same time permanently cross-linked. As an alternative to the use of transesterification chemistry, we explore catalyst-free transamination of vinylogous urethanes as an exchange reaction for vitrimers. First, a kinetic study on model compounds reveals the occurrence of transamination of vinylogous urethanes in a good temperature window without side reactions. Next, poly(vinylogous urethane) networks with a storage modulus of approximate to 2.4 GPa and a glass transition temperature above 80 degrees C are prepared by bulk polymerization of cyclohexane dimethanol bisacetoacetate, m-xylylene diamine, and tris(2-aminoethyl) amine. The vitrimer nature of these networks is examined by solubility, stress-relaxation, and creep experiments. Relaxation times as short as 85 s at 170 degrees C are observed without making use of any catalyst. In addition, the networks are recyclable up to four times by consecutive grinding/compression molding cycles without signifi cant mechanical or chemical degradation.

Vinylogous Urethane Vitrimers

Aromatic disulfide metathesis has been reported as one of the very few dynamic covalent chemistries undergone at room-temperature. Here, bis(4-aminophenyl) disulfide is effectively used as a dynamic crosslinker for the design of self-healing poly(urea–urethane) elastomers, which show quantitative healing efficiency at room-temperature, without the need for any catalyst or external intervention.

Catalyst-free room-temperature self-healing elastomers based on aromatic disulfide metathesis

It is likely that a half-century ago even enthusiastic and optimistic proponents of the synthetic polymer industry (Mr. McGuire included) could not have predicted the massive scale on which synthetic polymers would be manufactured and used today. Ultimately, the future success of this industry will rely on the development of sustainable polymers—materials derived from renewable feedstocks that are safe in both production and use and that can be recycled or disposed of in ways that are environmentally innocuous. Meeting these criteria in an economical manner cannot be achieved without transformative basic research that is the hallmark of this journal. In this Perspective we highlight five research topics—the synthesis of renewable monomers and of degradable polymers, the development of chemical recycling strategies, new classes of reprocessable thermosets, and the design of advanced catalysts—that we believe will play a vital role in the development of sustainable polymers. We also offer our outlook on sev...

https://par.nsf.gov/servlets/purl/10026121

50th Anniversary Perspective: There Is a Great Future in Sustainable Polymers

Advances in 4D Printing: Materials and Applications

Fiber-reinforced polymer composites (FRPCs) made of a “dynamic” epoxy resin possess good mechanical properties, while showing reprocessability, reparability and recyclability. The FRPCs are easily synthesized from readily available materials and could therefore be easily implemented in transportation, energy or construction industries, among others.

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Epoxy resin with exchangeable disulfide crosslinks to obtain reprocessable, repairable and recyclable fiber-reinforced thermoset composites

Recently we have shown the self-healing ability of a poly(urea-urethane) elastomer reversibly cured with aromatic disulfide bridges. Here we show that although being chemically crosslinked, such materials can be easily reprocessed by applying heat and pressure to obtain any desired form. This offers a unique paradigm towards thermoset processing and recycling, as well as for isocyanate-free manipulation of polyurethanes.

The processability of a poly(urea-urethane) elastomer reversibly crosslinked with aromatic disulfide bridges

4-Aminophenyl disulfide (4-AFD) has been recently reported as a hardener for the design of reprocessable, repairable and recyclable epoxy networks. Now we show that such a hardener is also responsible for a mechanochromic effect, due to an intermediate radical species formed upon damage, as confirmed by theoretical studies.

Roberto Martin

Papers

Catalyst-free room-temperature self-healing elastomers based on aromatic disulfide metathesis

Epoxy resin with exchangeable disulfide crosslinks to obtain reprocessable, repairable and recyclable fiber-reinforced thermoset composites

The processability of a poly(urea-urethane) elastomer reversibly crosslinked with aromatic disulfide bridges

Transient mechanochromism in epoxy vitrimer composites containing aromatic disulfide crosslinks

Room temperature self-healing power of silicone elastomers having silver nanoparticles as crosslinkers