Vitrimers are covalently cross-linked polymeric materials that can be thermally processed in a liquid state without losing their network integrity. In this work, novel vitrimers based on the dynamic amine exchange reaction of vinylogous urea moieties are introduced. Following a systematic exploration of different vinylogous acyl compounds (urethanes, amides and urea), vinylogous urea clearly emerge as showing the fastest intrinsic exchange kinetics. The combination of these networks with a simple acid catalyst (0.5 mol% pTsOH) resulted in materials with good mechanical properties (Tg ~ 110°C, E ~ 2.2 GPa) and remarkably short relaxation times above Tg, in the order of a few seconds when heated. This attractive combination of properties made it possible to prepare vinylogous urea based composites as enduring prepregs. We demonstrate that the dynamic material properties give fully cured composites that still allow an efficient thermal fusion of multiple layers as well as thermoforming. Finally, we also demonstrate fiber-recycling by a simple chemical treatment.

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Today, fiber-reinforced polymer composites are a standard material in applications where a high stiffness and strength are required at minimal weight, such as aerospace structures, ultralight vehicles, or even flywheels for highly efficient power storage systems. Although fiber-reinforced polymer composites show many advantages compared to other materials, delamination between reinforcing plies remains a major problem limiting further breakthrough. Traditional solutions that have been proposed to toughen the interlaminar region between reinforcing plies have already reached their limit or have important disadvantages such as a high cost or the need for adapted production processes. Recently, electrospun nanofibers have been suggested as a more viable interlaminar toughening method. Although the expected benefits are numerous, the research on composite laminates enhanced with electrospun nanofibrous veils is still very limited. The work that has been done so far is almost exclusively focused on interlaminar fracture toughness tests with different kinds of nanofibers, where typically a trial and error approach has been used. A thorough understanding of the micromechanical fracture mechanisms and the parameters to obtain toughened composites has not been reported as of yet, but it is crucial to advance the research and design highly damage-resistant composites. This article provides such insight by analyzing the nanofiber toughening effect on three different levels for several nanofiber types. Only by combining the results from different levels, a thorough understanding can be obtained. These levels correspond to the hierarchical nature of a composite: the laminate, the interlaminar region, and the matrix resin. It is found that each level corresponds to certain mechanisms that result in a toughening effect. The bridging of microcracks by electrospun nanofibers is the main toughening mechanism resulting in damage resistance. Nevertheless, the way in which the nanofiber bridging mechanism expresses itself is different for each scale and dependent on parameters linked to a certain scale. The multiscale analysis of the toughening mechanisms reported in this paper is therefore crucial for understanding the behavior of nanofiber toughened composites, and as such allows for designing novel, damage-resistant, nanofiber-toughened materials.

This work presents a straightforward strategy to introduce highly dynamic and adaptable cross-links into common epoxy resin formulations. For this, an oligomeric amine-based curing agent containing vinylogous urethane (VU) bonds was developed. This novel polyfunctional amine curing agent can be used as a drop-in solution for existing epoxy resin technologies, resulting in transparent, rigid and, at the same time, highly reprocessable catalyst-free epoxy vitrimers. The oligomeric VU curing agents are prepolymerised via a straightforward condensation reaction between acetoacetates extended with different classical amine monomers and epoxy hardeners. It is found that vitrimer properties can be readily introduced into these epoxy formulations by converting less than 50 mol% of the hardener's amine functionalities into dynamic vinylogous urethane bonds. In this way, epoxy vitrimers can be obtained with material properties comparable to ones of the VU-free epoxy formulations. In addition, remarkably short processing times are observed in the absence of any catalyst, and the material displayed very short stress relaxation times and good recyclability, actually representing the most performant VU-based vitrimers so far. Furthermore, a proof of concept for its use in obtaining glass fiber-reinforced epoxy composites is also presented.

Mechanoluminescence (ML), a general term for the phenomenon in which light emission occurs during any mechanical action on a solid, can be divided roughly into two classes: destructive ML and non-destructive ML. For practical use in high-end applications (e.g. pressure sensors), materials with non-destructive ML properties are preferred. This paper reports on the strong non-destructive ML in BaSi 2 O 2 N 2 :Eu. When irradiated in advance with UV or blue light, this phosphor shows intense blue-green light emission upon mechanical stimulation such as friction or pressure. The ML has an emission band peaking at 498 nm, which is about 4 nm red-shifted compared to the steady state photoluminescence. The origin of the ML is discussed and related to the persistent luminescence of BaSi 2 O 2 N 2 :Eu. The same traps are responsible for both the phenomena.Based on the occurrence of ML in this phosphor, we were able to derive that the predominant crystallographic structure of BaSi 2 O 2 N 2 :Eu belongs to space group Cmc2 1 .