Dr Michael G Allan

Dr Thomas Pichon

Christine Cavazza

Dr M F Kühnel

Dr J A Mccune

Dr A Le Goff

Enzyme Catalysis

Keywords: Deep Eutectic Solvents, Hydrogen, Hydrogenase, Oxygen Tolerance, Photocatalysis

This work showcases the performance of [NiFeSe] hydrogenase from Desulfomicrobium baculatum for solar-driven hydrogen generation in a variety of organic-based deep eutectic solvents. Despite its wellknown sensitivity towards air and organic solvents, the hydrogenase shows remarkable performance under an aerobic atmosphere in these solvents when paired with a TiO 2 photocatalyst. Tuning the water content further increases hydrogen evolution activity to a TOF of 60 � 3 s À 1 and quantum yield to 2.3 � 0.4 % under aerobic conditions, compared to a TOF of 4 s À 1 in a purely aqueous solvent. Contrary to common belief, this work therefore demonstrates that placing natural hydrogenases into non-natural environments can enhance their intrinsic activity beyond their natural performance, paving the way for full water splitting using hydrogenases.

Photocatalytic water splitting is viewed as a favourable method of producing green H 2 to combat global energy challenges without requiring large investments into electrolysers and power grids. [1] Suitable systems for efficient solar H 2 production should focus on materials which are robust, cheap, and readily available. [START_REF] Lewis | Proc. Natl. Acad. Sci[END_REF] A research-intensive area in this field is the development of non-precious co-catalysts for the hydrogen evolution half-reaction (HER) and its reverse hydrogen oxidation reaction (HOR). [3] [NiFe], [FeFe] and [NiFeSe] hydrogenases (H 2 ases) are biological catalysts which can reversibly convert protons and electrons into H 2 at low overpotentials without being based on precious metals. [4] Solar-driven H 2 generation has been demonstrated using a range of H 2 ase-photocatalyst combinations [5,6] with [NiFeSe] H 2 ases shown to be particularly active. [7] An interesting aspect of [NiFe] and [NiFeSe] H 2 ases is their O 2 tolerance, [7g, 8] whereby they show only a partial and reversible decrease in catalytic activity under aerobic conditions. Photocatalytic H 2 evolution in high levels of O 2 is an important property as catalytic components in photoreactors may be exposed to O 2 via in situ O 2 formation resulting from water oxidation or through leakage. Despite this, O 2tolerant H 2 ases show performances and lifetimes for H 2 evolution much lower in aerobic environments versus an inert environment. [7d] Solving the O 2 sensitivity is considered a key step towards industrial application of hydrogenases. [9] We recently reported on a novel approach to enabling O 2 -tolerant H 2 evolution through solvent design. [10] Efficient photocatalytic H 2 evolution under aerobic conditions was achieved using a Pt/carbon nitride photocatalyst employing deep eutectic solvents (DESs) with a low O 2 solubility and diffusivity as a reaction medium. DESs have attracted attention in recent years as an alternative class of ionic liquids, as they possess low toxicities and can be prepared from cheap and readily available precursors. [11] However, H 2 ases have so far not been employed outside of conventional aqueous solvents for solar H 2 evolution due to incompatibility with different environments, particularly organic solvents. [12] In the past, chemical modifications to H 2 ases have been investigated to allow them to function in organic solvents. [13] Protection of H 2 ase from O 2 has been achieved by integration with hydrogels [14] and redox-active films. [15] In this work, we showcase the applicability of solvent engineering to the photocatalytic H 2 evolution at a [NiFeSe] H 2 ase with TiO 2 as a light-absorber (Figure 1). We highlight for the first time not only the stability and catalytic activity of [NiFeSe] H 2 ase in organic-based solvents for photocatalytic H 2 production, but also a remarkable enhancement in O 2 tolerance induced in these non-natural solvents without any enzyme modification or catalyst redesign.

First, we assessed the general effect of DESs on the activity of H 2 ases to investigate their potential for biophotocatalytic hydrogen evolution in non-conventional solvents. Previous work has shown that DESs can stabilise airsensitive species [16] and possess a high degree of biocompatibility [17] with an ability to stabilise proteins. [18] We chose [NiFeSe] H 2 ase from Desulfomicrobium baculatum (Db) as a HER catalyst for this work, due to its previously reported suitability for use as a co-catalyst with various photocatalysts. [7] The photocatalytic performance for H 2 generation was investigated in a heterogeneous TiO 2 -[NiFeSe] catalyst system comprised of TiO 2 (2.5 mg mL À 1 ) and Db[NiFeSe] H 2 ase (21 pmol) in a variety of DES-water mixtures. Aqueous TEOA was used as an electron donor, with the pH adjusted to 7.0 prior to mixing with the DES (Table S1). Samples were irradiated with simulated solar light (AM 1.5G) at 40 °C under a continuous purge of N 2 , and H 2 evolution was quantified by gas chromatography (see Supporting Information for full details).

In all solvents tested, we clearly observed photocatalytic H 2 evolution, thus proving that Db[NiFeSe] H 2 ase retains its catalytic HER activity in DES-based solvents. H 2 generation by the TiO 2 -[NiFeSe] photocatalyst system was sustained for > 24 h in all solutions containing the DES glyceline (choline chloride:glycerol 1 : 2). The reaction rate increased upon increasing the water content in the solvents (Figure 2). In an 80 % vol. aq. glyceline solution, TiO 2 -[NiFeSe] generated 24.10 � 0.55 μmol H2 (TON > 1 080 000 � 100 000) after 24.9 h irradiation whereas the same photocatalyst in a 20 % vol. aq. glyceline solution showed a TON > 4 350 000 � 500 000 (91.44 � 11.96 μmol H2 ) with an apparent quantum yield (AQE) of 2.3 � 0.2 % (Table S2). Comparable performance was also observed in other DESs such as ethaline (choline chloride:ethylene glycol, Figure S1). This compares favourably with an aqueous solvent, in which TiO 2 -[NiFeSe] exhibited a TON > 3 500 000 � 31 000 after 24.1 h under otherwise identical conditions. Remarkably, the activity in 20 % vol. aq. glyceline exceeds that observed in a purely aqueous environment by approx. 19 % under otherwise identical conditions. Negligible amounts of H 2 were produced when TiO 2 was irradiated in 60 %. vol. aq. glyceline without added Db[NiFeSe] H 2 ase (Figure S2). To the best of our knowledge, this is the first report of a natural H 2 ase functioning in an organic solvent for solar H 2 productionand even slightly better than in purely aqueous conditions in which it evolved. It also exceeds most previous reports on photocatalytic H 2 evolution using a [NiFeSe] H 2 ase (Table S3).

Further insight into the origin of the varying H 2 ase activity depending on the DES content of the medium was sought from electrochemical measurements of Db[NiFeSe] H 2 ase adsorbed onto an adamantane-modified multi-walled carbon nanotube electrode (MWCNT) [19] in varying concentrations of glyceline. Protein film electrochemistry [20] shows that the enzyme retains its reversible activity towards both HER and HOR with near zero-overpotential in the DES (Figure 3a). Upon increasing the glyceline content in the solvent, the HER current increases before gradually decreasing (Figure 3b). The presence of an HOR current arises from the production of H 2 in the MWCNT layer upon proton reduction. Its decrease upon increasing the glyceline content might not only be caused by a decrease in HER activity but may be amplified by a decrease in H 2 solubility with higher DES concentrations, caused by the "salting out" effect of solutions with high ionic strengths. [21] A lowered H 2 solubility in turn would mitigate the well-documented inhibition of H 2 ase by H 2 , [22] thus allowing for a higher HER activity in DESs. The initial increase in HER current is consistent with the observed increase in photocatalytic H 2 generation reflecting the solvent effect on increasing the activity of the enzyme for proton reduction.

To exploit the impact of the DES content in the solvents on the stability of Db[NiFeSe] H 2 ase under demanding conditions, the TiO 2 -[NiFeSe] photocatalyst system was tested in the same solvents under a constant purge of air (21 % O 2 ). In an aqueous aerobic sample, the photocatalyst produced 2.73 � 1.3 μmol H2 after 24.1 h irradiation (TON = 130 000 � 60 000) corresponding to only 3.7 � 1.7 % of the amount produced under inert but otherwise identical conditions (Figure 4), owing to the known O 2 inhibition of H 2 ase. In the DES-based solutions however, a high level of oxygen tolerance is exhibited without making changes to the photocatalyst or enzyme, particularly at higher percentages of DES. In an 80 % vol. aq. glyceline solution, the O 2 tolerance was near 90 % within experimental error with a total H 2 production of 21.34 � 3.03 μmol H2 (TON = 1 020 000 � 144 000) after > 24 h. Similarly, the AQE of H 2 evolution in an aerobic 60 % vol. aq. glyceline solution was determined to be 2.3 � 0.4 % after 1 h, identical to the value determined under anaerobic conditions. After 5.2 h irradiation in air, the AQE was still at 1.4 � 0.3 %, more than 70 % of the activity in inert conditions (Table S2). While higher water contents in the solvent increased the photocatalytic activity under inert conditions (vide supra), the associated decrease in DES content lowers the O 2 tolerance. This decrease in O 2 tolerance is also observed when other DESs are used as the reaction medium (Figure S3). The optimum compromise is observed at 60 % vol. aq. glyceline whereby TiO 2 -[NiFeSe] continuously generates H 2 during 72 h irradiation at a remarkable overall TOF of 60 � 3 s À 1 in air, and a total TON of > 1 800 000 � 180 000 after the 72-h period (Figure S4). This retention of photocatalytic activity represents a major improvement over previous work showing that even O 2 -tolerant H 2 ases in water undergo considerable inactivation during photocatalytic H 2 evolution in the presence of air (Table S4). A [NiFe] H 2 ase from D. vulgaris shows 65 % retention of activity in air vs. N 2 when embedded in a nanoporous glass plate. [23] In addition, an engineered [NiFe] H 2 ase from E. Coli irradiated with a carbon nitride-TiO 2 photocatalyst system retains 20 % of its activity in air. [24] [NiFe] H 2 ase from T. Roseopersicina covalently bound to a Ru photosensitiser was reported to maintain 11 % of its initial rate in the presence of air. [5e] The Db[NiFeSe] H 2 ase used here has also been shown to exhibit photocatalytic performance in air when used with dyesensitised TiO 2 as the photoabsorber in water, with a lowered H 2 production rate even after just 30 minutes of exposure to air. [7e] To rationalise the dependence of O 2 tolerance on the solvent composition, we determined the O 2 solubilities c(O 2 ) and diffusion coefficients D(O 2 ) in glyceline and water using stepped-potential microwire chronoamperometry (Figure S5). [25] Using the Krichevsky [26] and Wilke-Chang [27] equations, we estimated c(O 2 ) × D(O 2 ) for the different DES/water mixtures. Table S5 shows that as the water content increases from 0 to 100 %, D(O 2 ) increases approx. 800-fold, while c(O 2 ) decreases only marginally. In line with our previously reported model for the O 2 tolerance in which O 2 intolerance is treated as diffusion-controlled O 2 reduction at a spherical photocatalyst particle, [10a] we observe a good correlation between c(O 2 ) × D(O 2 ) and the O 2 tolerance when c(O 2 ) × D(O 2 ) is small, i.e. when the local concentration of O 2 at the photocatalyst is mass transport limited and it competes with the much faster proton diffusion (Figure S6).

In addition to TiO 2 , photocatalytic H 2 evolution at Db[NiFeSe] H 2 ase was further investigated using Eosin Y (EY) as a photoabsorber in aerobic and inert conditions to test the suitability of solvent tuning on a homogeneous photocatalytic system and thus the generality of this approach. In a previous report, it has been shown that EY-Db[NiFeSe] is active for H 2 evolution under visible light irradiation in the presence of atmospheric levels of O 2 , however the photoreactor was not subject to resupply of O 2 and the activity retained in air was only 11 % relative to an inert atmosphere. [7d] We observed in glyceline-based solvents that increasing the DES content led to an increase in O 2 tolerance of EY-Db[NiFeSe] H 2 ase (Figure S7) similar to the aforementioned heterogeneous system, however the overall activity of the photocatalyst system was low. The intrinsically low activity in DESs is in line with a previous report, where photocatalytic H 2 production at EY with synthetic HER catalysts was lower in DESs versus water but showed increased relative activity in air. [10a] To investigate the origin of decreased activity at higher glyceline concentrations, solutions containing TiO 2 and Db-[NiFeSe] H 2 ase were subject to centrifugation, with the supernatant subsequently decanted (see Supporting Information for details). The thus obtained TiO 2 -[NiFeSe] pellet was resuspended in a fresh solution of the solvent in question without added H 2 ase and then irradiated, and the H 2 production was compared to a TiO 2 -[NiFeSe] pellet resuspended in the originally decanted supernatant. H 2 production performance of the TiO 2 -[NiFeSe] in the recycled supernatant was similar to the performance in a fresh solution (Figure 4d). This indicates that the solvent does not hinder the adsorption of the hydrogenase enzyme to the TiO 2 surface, allowing for efficient charge transfer from the photocatalyst to H 2 ase.

In summary, we have shown that DESs can act as alternative reaction media to water for the photocatalytic hydrogen evolution using natural hydrogenase enzymes. By tuning the water content in the DESs, both activity and stability of the H 2 evolution activity can be increased to match and even outcompete pure water as a solvent for H 2 evolution. The H 2 evolution activity of TiO 2 -[NiFeSe] in aerobic conditions is drastically improved in DESs, with nearly 90 % activity retained in air, whereas H 2 evolution in water is almost completely quenched in air. This work shows the first instance of Db[NiFeSe] H 2 ase employed in organic solvents for the H 2 evolution reaction and thus highlights the potential of solvent engineering as a novel, highly effective approach to improve natural enzyme performance. Further studies into the influence of temperature and pH on H 2 ase activity and O 2 tolerance as well as their correlation with H 2 and O 2 solubilities in the respective solvent mixtures will provide key factors for designing tailored solvents in the future that can achieve a high O 2 tolerance without lowering the H 2 ase activity.

Figure 2 .

 2 Figure 2. Photocatalytic H 2 generation using a photocatalyst system based on TiO 2 and Db[NiFeSe] H 2 ase in various glyceline-water mixtures. (a) H 2 generation over time and (b) turnover number after 24 h irradiation in solvents of varying glyceline content under inert conditions. Conditions: TiO 2 (5.0 mg), Db[NiFeSe] H 2 ase (21 pmol), 2.0 mL solvent, TEOA (0.4 M), AM 1.5G, 1 sun, 40 °C, constant N 2 purge.

Figure 3 .

 3 Figure 3. Protein film electrochemistry of Db[NiFeSe] H 2 ase adsorbed on a MWCNT electrode in glyceline-water mixtures of varying composition. (a) Cyclic voltammograms and (b) observed current density at À 0.75 V vs. SHE. Conditions: Db[NiFeSe] H 2 ase-MWCNT working electrode, TEOA (0.4 M), pH 7.0, 25 °C, 0.01 V s À 1 scan rate, de-aerated solution.

Figure 4 .

 4 Figure 4. Oxygen tolerance of the photocatalytic H 2 generation using a photocatalyst system based on TiO 2 and Db[NiFeSe] H 2 ase. (a) Photocatalytic H 2 generation in water and (b) in 80 % vol. aq. glyceline under inert conditions and atmospheric levels of O 2 . (c) Oxygen tolerance of TiO 2 -[NiFeSe] determined from total H 2 produced in inert and aerobic conditions after > 24 h irradiation in solvents of varying concentrations of glyceline in water. (d) H 2 produced by TiO 2 -[NiFeSe] in fresh and resuspended solutions containing varying concentrations of glyceline after 9.3 h irradiation under N 2 . Conditions: TiO 2 (5.0 mg), Db[NiFeSe] H 2 ase (21 pmol), 2.0 mL solvent, TEOA (0.4 M), AM 1.5G, 1 sun, 40 °C, constant N 2 or air purge.

Acknowledgements

This work was supported by EPSRC through a DTA studentship to M.G.A. (EP/R51312X/1), and a capital investment grant to M.F.K. (EP/S017925/1), by Universities Wales through the Global Wales International Research Mobility Fund (UNIW/RMF-SU/07) and by HEFCW through the Research Wales Innovation Fund Collaboration Booster. We thank Swansea University for providing start-up funds to M.F.K. and support through the Swansea-Grenoble Collaboration Fund. This work was supported by the Agence Nationale de la Recherche through the LabEx ARCANE program (ANR-11-LABX-0003-01), the Graduate School on Chemistry, Biology and of Univ. Grenoble Alpes CBH-EUR-GS (ANR-17-EURE-0003) and by the CFR PhD program-CEA (PhD funding for T.P.). We thank Dr. Oliver Lenz and Dr. Stefan Frielingsdorf (TU Berlin) and Prof. Frank Marken (Bath) for helpful discussions.

Data Availability Statement

The data that support the findings of this study are available in the Supporting Information of this article. Additional raw data is freely available from the Zenodo repository at https://doi.org/10.5281/zenodo.7573371.

Conflict of Interest

The authors declare no conflict of interest.


Quentin-Arthur Poutrel
email: quentin-arthur.poutrel@espci.fr

Laurence Cornillon
email: laurence.cornillon@thalesaleniaspace.com

Jolanta Hamerlak
email: j.hamerlak@scntpl.pl

Ludovic Chichignoud

Simon Appel
email: simon.appel@esa.int

Ugo Lafont
email: ugo.lafont@esa.int

Shumit Das
email: shumit.das@esa.int

François Tournilhac
email: françois.tournilhac@espci.fr

Olivier Damiano
email: olivier.damiano@thalesaleniaspace.com

New perspectives in fibres reinforced polymer composites for aerospace structures

Keywords: Composites, vitrimers, Thin Plies

The composite field brings constant progress and development with design of new materials and technologies. These aim to reduce cost of manufacture and maintenance, but also, to increase their performances combined with tailorable properties. Thales Alenia Space, always looking for innovative development in composites, is now taking part in two ESA projects focusing on vitrimers materials and thin plies technology. Vitrimers are an emerging class of polymer materials developed in 2011 by Leibler and coworkers. Initially, based on the traditional chemistry of thermosets, they possess their chemical resistances and mechanical strength. Nonetheless, their covalent bonds are exchangeable upon trigger (e.g. heat, UV, etc.). This property is giving them the ability to be weld, reprocessed, or recycled, behaviour usually owned by thermoplastic materials. This covalent bond exchange mechanism makes vitrimers insoluble, yet, weldable onto themselves rendering them highly attractive for the composite field. On the other hand, thin plies technology have been posited as a great alternative to traditional fibres reinforced polymer (FRP) composites. This technique allows to produce lighter and stronger composites while being cost effective. Already used in sports applications, the aerospace field could highly benefits from their development to a larger scale.

Therefore, Thales Alenia Space ESA projects have started to explore simultaneously the potential of both this new class of polymer and technology as follow: (i) a one year TDE project focusing on the development of vitrimer prepreg and laminate for space applications. So far, a vitrimer resin with appropriate properties (Tg, outgassing, latency) for impregnation, lamination and welding has been selected. Further work in the prepreg development is expected to result in vitrimer integration with low investments on the manufacturing lines. (ii) An ongoing Thales Alenia Space/SCNTPL project (TRP) for which materials (resin and fibres) choice, manufacturing and use case trade off resulted in small scale prototypes (2019). Further characterisations and design of full scale prototypes have ensured the continuity of the project during 2020 and first prototypes (full scale) are expected in 2021. This TRP project shows the possibility to apply thin plies technology to real scale aerospace applications.

INTRODUCTION

Composite materials have been widely used throughout human history from plywood (3400 B.C.) to the development of modern fibres reinforced polymer (FRP) in the 1900's. Since their development, FRP composites have positively impacted several industries such as automotive, building, energy production, or aerospace [START_REF] Mangalgiri | Composite materials for aerospace applications[END_REF]- [START_REF] Bello | Epoxy resin based composites, mechanical and tribological properties: A review[END_REF].

The combination of high strength fibres with polymer resin matrices leads to lightweight materials (compared to traditional ones, such as metal), having high mechanical performances while giving a high freedom in designing complex structures or shapes [START_REF] Bello | Epoxy resin based composites, mechanical and tribological properties: A review[END_REF].

Thermoset resins have traditionally been a favoured choice as matrices due to their high mechanical properties and chemical resistance [START_REF] Jin | Synthesis and application of epoxy resins: A review[END_REF]. However, their covalently bonded network (giving them these properties) has also its drawbacks. Once the network is cured, processes, repairs, and assembly of different parts become a challenge, often involving high cost interventions [START_REF] Diamanti | Structural health monitoring techniques for aircraft composite structures[END_REF], [START_REF] Jin | Synthesis and application of epoxy resins: A review[END_REF], [START_REF] Staszewski | Ultrasonic/Guided Waves for Structural Health Monitoring[END_REF]. Moreover, the environmental impact of these FRP has risen concerns as recycling is often difficult, involving harsh chemicals and high energy costs [START_REF] Oliveux | Current status of recycling of fibre reinforced polymers: Review of technologies, reuse and resulting properties[END_REF]. Recently, thermoplastic matrices have attracted attention of both industrials and researchers. Indeed, various properties of these polymer types are appealing such as near-infinite shelf life, recyclability, and weldability. Nevertheless, thermoplastics processing and manufacturing is often not adapted for composites production which are used in the aerospace industry (e.g. infusion, resin transfer moulding, etc.). This fact is currently limiting their applications to wider scales in this area.

In 2011, a third class of resins was developed by Leibler and coworkers, namely, vitrimers [START_REF] Montarnal | Silica-Like Malleable Materials from permanent organic network[END_REF]. Prototypes of vitrimers were based on epoxy formulations and crosslinked (i.e. as a thermoset), while having the inherent property to exchange their covalent bonds under application of heat. The bond exchange relied on transesterification mechanisms which are associative. Hence, vitrimers are reprocessable, healable, recyclable, yet, insoluble and not subject to structural integrity losses [START_REF] Montarnal | Silica-Like Malleable Materials from permanent organic network[END_REF]. Since the first vitrimers epoxy formulations, various chemistries have been explored to produce different vitrimer networks. Some composites manufacturing involving different chemistries such as transesterification, transiminification or disulphide exchanges have been studied [START_REF] Oliveux | Current status of recycling of fibre reinforced polymers: Review of technologies, reuse and resulting properties[END_REF]. Despite these studies and outstanding potential of vitrimer as matrices for composites (crosslinked network able to be weld or reprocessed), their use as prepregs for aerospace applications has been barely investigated. Therefore, Thales Alenia Space carried out an initial survey in collaboration with the ESPCI Paris to validate the possibility to produce these prepregs at laboratory scale. In late 2019, following this survey, the European Space Agency (ESA) funded the continuity of the project to further develop manufacture and characterisation of these FRP. This led to the definition of the required resin properties (choice of chemicals: resin, hardener and catalyst) which would require the least investment on scaling up vitrimers uses in aerospace applications.

In the meantime, thin plies composites development was also explored to reach wider scale applications. The plies thicknesses allow to optimise and tailor the FRP properties whilst delaying (or suppressing) crack initiation [START_REF] Galos | Thin-ply composite laminates: a review[END_REF]. Moreover, when compared to traditional prepregs, their surface weight can be reduced by 2 to 10, allowing production of even lighter composites [START_REF] Arteiro | Thin-ply polymer composite materials: A review[END_REF]. The better freedom of fibres orientation gives also a wider range of liberties to optimise and design composites. Following this goal, Thales Alenia Space found promising results for various specific applications in aerospace. These results led to two main developments, presented in this work: solar array substrates and integral 3D sandwich structures.

This work is presented as follow: (i) presentation of vitrimer project (general considerations, vitrimers for composites in aerospace applications, preliminary results for resin formulation, and future perspective of this project). (ii) Presentation of the thin plies development including general considerations, their applications in aerospace industry, and a focus on solar array substrates and 3D sandwich structures. (iii) Finally, the potential to combine both of these projects to produce FRP composites for the aerospace field is considered.

VITRIMERS

GENERAL CONSDIDERATIONS and BACKGROUND

Internal survey to evaluate the feasibility of prepreg material with a vitrimer resin started in 2018. Initially the project looked at the capacity to integrate vitrimers as matrix for FRP composite and to use their properties akin to thermoplastics (weldability). The rationale behind vitrimer networks rather than thermoplastic ones was the possibility to adapt traditional preparation processes (at workshop level) used for epoxy thermoset. This would, firstly, limit investments; in the meantime, the welding capacity of vitrimer would facilitate the equipment phase on the platforms. Secondly, other properties of vitrimers motivated research efforts such as self-healing and recyclable properties.

Indeed, thermosets based FRP are known to be sensible to internal damages [START_REF] Tallman | Damage detection via electrical impedance tomography in glass fiber/epoxy laminates with carbon black filler[END_REF], [START_REF] Ferri Aliabadi | Structural health monitoring for advanced composite structures[END_REF]. The stiff polymer matrix, despite its high mechanical strength and chemical resistance, is often brittle. When FRP composites undergo shocks or fatigue, cracks and delamination are likely to occur which could result in catastrophic failures [START_REF] Mangalgiri | Composite materials for aerospace applications[END_REF]. Detection, monitoring and repairs of these damages are time consuming and difficult. Often repairs (with bolts and joints) can induce new damages during the repairing step which reduces the composites lifespan [START_REF] Ferri Aliabadi | Structural health monitoring for advanced composite structures[END_REF]. Vitrimers, via bond exchanges, have demonstrated the capability to heal cracks with either temperature (e.g. 60 min at 180°C [START_REF] Yang | A fully bio-based epoxy vitrimer: Selfhealing, triple-shape memory and reprocessing triggered by dynamic covalent bond exchange[END_REF]) or light (e.g. 10 s under IR light [START_REF] Yang | Carbon nanotube-vitrimer composite for facile and efficient photowelding of epoxy[END_REF]) triggering. Consequently, their use as FRP composite matrix could also reduce maintenance cost and increase their lifespan. Finally, vitrimers have been shown to be recyclable either by reprocessing of the polymer (crushing samples into pieces followed by a moulding step [START_REF] Montarnal | Silica-Like Malleable Materials from permanent organic network[END_REF]) or by dissolution in an appropriate solvent -chosen accordingly for the bond exchange mechanism -followed a possible repolymerisation with solvent evaporation [START_REF] Hamel | Reaction-Diffusion Model for Thermosetting Polymer Dissolution through Exchange Reactions Assisted by Small-Molecule Solvents[END_REF].

One of the rising concern coming with the increasing use of FRP composites is the end-of-life of these products. Traditional thermoset resins require either application of high temperature or use of harsh chemicals to be removed from the fibre reinforcements. These operations make impossible to re-use the polymer and also severely damage the fibres [START_REF] Oliveux | Current status of recycling of fibre reinforced polymers: Review of technologies, reuse and resulting properties[END_REF]. In fact, the damages often lead shorter fibre with lowered properties which can only be re-used in applications requiring lower mechanical properties. Several efforts have been made in order to lower the impact of matrix elimination from fibres, and vitrimer matrices were reported as one of the most adequate candidates [START_REF] Oliveux | Current status of recycling of fibre reinforced polymers: Review of technologies, reuse and resulting properties[END_REF]. The ability to dissolve and repolymerise in "soft" solvent at relatively low temperature (< 200°C) would limit fibre damages but also recycling costs. Moreover, resins repolymerisation for further use would bring a new dimension to the recycling of composites with the possibility to achieve close-loop recyclability on the whole composite's components [START_REF] Yu | Carbon Fiber Reinforced Thermoset Composite with Near 100% Recyclability[END_REF]. Some examples in literature have shown the possibility to prepare vitrimer composites, even using dry prepreg [START_REF] Ruiz De Luzuriaga | Epoxy resin with exchangeable disulfide crosslinks to obtain reprocessable, repairable and recyclable fiber-reinforced thermoset composites[END_REF]. However, wet prepreg fitting industry requirements, to the authors' knowledge, has not yet be demonstrated. Therefore, NTPT (prepreg manufacturer) and Thales Alenia Space (end user of the materials) set a list of requirements to find a resin producing high performance composites with optimum fibre/resin ratio: (i) Low viscosity at impregnation temperature (≈ 100°C). (ii) Latency at ambient temperature or cold (-18°C) temperature, for storing and transport before manufacturing. (iii) High temperature stability after curing. Tg ≈ 120°C.

The results of this preliminary study were satisfying:

(i) Latency of the hardener/catalyst mix is reasonable. (ii) Tg ≈ 100°C. (iii) Performances obtained with common "Novolac" resin.

At the end of the internal survey, the main concern was the slightly high viscosity of the resin, not yet suitable for prepreg preparation.

VITRIMERS IN FRP FOR AEROSPACE APPLICATIONS -TDE

End 2019, the ESPCI Paris was awarded a contract with ESA ("Effect of space environment on Vitrimer based CFRP ") to evaluate, with Thales Alenia Space, the feasibility of such a prepreg for aerospace applications. Multiple interests motivated and defined this research effort, similarly to that of the initial survey in 2018:

(i) The 3R (Recyclability, Repairability, Reprocessability) characteristics of vitrimers.

(ii) Welding of vitrimers onto themselves, like for thermoplastics. (iii) Vitrimers, in a prepreg form, should have a transformation process very similar to these of thermoset. For example, the prepreg should be tacky enough to allow manual or automatized layup, and its cure cycle should not force industrials to heavy investments.

Set objectives of the project were:

(i) Define a resin with appropriate properties for prepregs impregnation and aerospace applications (Tg, outgassing, latency, viscosity at impregnation temperature, tensile and welding properties) (ii) Manufacture prepreg with a very common reinforcement (fabric and not UD tape, common industrial grade carbon fiber). (iii) Evaluation of process limitations and constraints. (iv) Mechanical testing (tensile properties). (v) Welding of plates followed by shear and peel tests.

To reach these objectives, the choice of vitrimer network (i.e. bond exchange mechanism, and hardener type) was carefully considered. As aforementioned, first illustration of the vitrimer concept, a number of different systems were investigated based on transesterification [START_REF] Montarnal | Silica-Like Malleable Materials from permanent organic network[END_REF] and other exchange mechanisms [START_REF] Ruiz De Luzuriaga | Epoxy resin with exchangeable disulfide crosslinks to obtain reprocessable, repairable and recyclable fiber-reinforced thermoset composites[END_REF]- [START_REF] Stukenbroeker | Polydimethylsiloxane quenchable vitrimers[END_REF]. In early works, the prototypes of vitrimers have been formulated on the basis of epoxy thermoset mixes [START_REF] Montarnal | Silica-Like Malleable Materials from permanent organic network[END_REF]. More recently, it has been shown that vitrimer materials can also be implemented with benefit through modification of thermoplastics [START_REF] Röttger | High-performance vitrimers from commodity thermoplastics through dioxaborolane metathesis[END_REF]. Thus, new development of vitrimers has been published through modification of commodity thermoplastics, in which bond can exchange by the metathesis of dioxaborolanes. Recently, improvement of heatresistance of poly-butylene terephthalate (PBT), an engineering thermoplastics was reported, using Zn(II) catalysed epoxy-vitrimer chemistry [START_REF] Demongeot | Cross-Linking of Poly(butylene terephthalate) by Reactive Extrusion Using Zn(II) Epoxy-Vitrimer Chemistry[END_REF], [START_REF] Farge | Solid Rheological Properties of PBT-Based Vitrimers[END_REF]. However, for reasons mentioned earlier, the use of thermoplastics based networks is not favored for this project.

Example of composites with vitrimer as matrix are scarce in literature but few examples can be reported. Chabert et al. [START_REF] Chabert | Multiple welding of long fiber epoxy vitrimer composites[END_REF]. showed the possibility to implement composite materials from vitrimers by using current composite techniques. In this work, vitrimer matrices with a Tg value > 100°C were identified and adapted for procesing by resin transfert molding (RTM). Vitrimer composites containing more than 50 vol% of continuous glass fibers showed high bonding strength and repeated welding ability while their mechanical properties were comparable to those of non-vitrimer control samples. Odriozola and coworkers [START_REF] Ruiz De Luzuriaga | Epoxy resin with exchangeable disulfide crosslinks to obtain reprocessable, repairable and recyclable fiber-reinforced thermoset composites[END_REF] at CIDETEC (Spain) also investigated reprocessing, reparation and recycling of glass-fiber reinforced composites using a matrix made of and epoxy resin with exchangeable disulfide crosslinks. In these works, the starting point composite plates were manufactured from low viscosity reactive mixtures using the RTM technique. Odriozola additionally reports the assembly of multiple layers by compaction of individual prepreg sheets from fully cured dynamic epoxy resin. Although illustrating the original properties of vitrimers, fully cured prepreg sheets have disadvantage of no tack and therefore, this process does not allow precise positioning comparable with handling of prepregs.

Repairable and recyclable woven carbon fiber composites were also reported using malleable polyimine networks [START_REF] Taynton | Repairable woven carbon fiber composites with full recyclability enabled by malleable polyimine networks[END_REF]. These materials, can show a Tg up to 135°C and are sometimes also called vitrimers. These are however networks made up of small molecules (aldehydes and primary amines) joined to each other by imine bridges. The constitutive molecules, amines and aldehydes are likely to degrade into respectively carbamate and carboxylic acid by contact with atmospheric CO2 and O2. The imination reaction produces water that has to be stripped out during curing and reversely water initiates depolymerisation into volatile compounds. For these reasons, this type of material is not considered potentially suitable for the project.

Therefore, the choice was set onto epoxy vitrimers based on transesterification reactions as they can be easily applied to the epoxy/acid and epoxy/anhydride curing systems which are currently used in the industry of thermoset materials. However, most reported epoxy vitrimers show a low glass transition temperature, Tg (< 100 °C), which limits their applications [START_REF] Montarnal | Silica-Like Malleable Materials from permanent organic network[END_REF], [START_REF] Chabert | Multiple welding of long fiber epoxy vitrimer composites[END_REF]- [START_REF] Demongeot | Coordination and Catalysis of Zn 2+ in Epoxy-Based Vitrimers[END_REF] . To achieve adequate molecular mobility at high temperatures, the cross-linked structures of epoxy vitrimers are mostly constructed with soft epoxy monomer and curing agent, which eventually result in low Tg. Although a rigid building block structure and a high cross-link density can increase the value of Tg [START_REF] Liu | Catalyst Selection, Creep, and Stress Relaxation in High-Performance Epoxy Vitrimers[END_REF], they tend to limit the mobility of the polymer chains and compromise the reparability and reprocessability [START_REF] Woo | Cure kinetics of epoxy/anhydride thermosetting matrix systems[END_REF].

Efforts to increase the glass transition temperature of epoxy vitrimers date back to the first publication on vitrimers; a brief review is presented below. To allow comparisons, we denote Tg when the measurement is done by differential scanning calorimetry (DSC) and Tα when done by dynamic mechanical analysis (DMA). Thus, in reference, using the same epoxy resin, Di-Glicidyl Ether of Bisphenol A (DGEBA) cured with either a long chain (C36) fatty diacid or short chain (C5) glutaric anhydride. It was shown the possibility to rise Tg from about 15°C ( Tα = 23°C) to above 60-70°C ( Tα = 98°C) [START_REF] Montarnal | Silica-Like Malleable Materials from permanent organic network[END_REF], [START_REF] Capelot | Metal-catalyzed transesterification for healing and assembling of thermosets[END_REF], [START_REF] Capelot | Catalytic control of the vitrimer glass transition[END_REF]. Incorporation of 40 wt % GLYMO-silica in the same system was also found to enhance Tα by a few degrees [START_REF] Legrand | Silica-Epoxy Vitrimer Nanocomposites[END_REF]. Further on, vitrimers based on industrial epoxy monomers (Araldite® LY564 / Aradur® 917CH) were investigated to apply vitrimers through the resin transfer molding (RTM) technique [START_REF] Chabert | Multiple welding of long fiber epoxy vitrimer composites[END_REF]. Vitrimer matrices thus synthesised exhibited a Tg of 103°C, (T α = 112°C). An epoxy/anhydride infusion system (Epikote® MGS RIM 145) was used in a screening of catalysts for vitrimer application (Tg not reported) [START_REF] Liu | Catalyst Selection, Creep, and Stress Relaxation in High-Performance Epoxy Vitrimers[END_REF]. Use of sebacic acid / cyclohexane dicarboxylic acid mixtures as curing agent of DGEBA was reported to bring Tg between 30°C and 80°C [START_REF] Chen | Reprocessible Epoxy Networks with Tunable Physical Properties: Synthesis, Stress Relaxation and Recyclability[END_REF]. Whereas, use of the same sebacic acid component with various epoxies gave rise to a glass transition temperature of Tg = 55°C [START_REF] Pei | Mouldable liquidcrystalline elastomer actuators with exchangeable covalent bonds[END_REF], [START_REF] Poutrel | Dicarboxylic acid-epoxy vitrimers : Influence of offstoichiometric acid content on cure reactions and thermo-mechanical properties[END_REF]. Recently, a biobased epoxy resin featuring the triphenylmethane rigid core was proposed to impart a glass temperature as high as Tα = 187°C in vitrimer chemistry based materials [START_REF] Liu | A Self-Healable High Glass Transition Temperature Bioepoxy Material Based on Vitrimer Chemistry[END_REF].

Hence, the epoxy-anhydride systems already proposed in prototypes of vitrimers is shown to be suitable to formulate thermosets with a high Tg. In addition, conventional epoxyanhydride formulations always present competition between the anionic homopolymerisation (by opening of the epoxy ring which leads to nonexchangeable ether bonds) and acyl + epoxy addition reactions (which in turn lead to desirable exchangeable ester bonds in vitrimers). However, a high-Tg vitrimer formulation aims at introducing both chemical groups promoting exchange reactions and network elements imparting a high Tg. A problem that arises with acuity is the compatibility of these components. Thus, as shown recently in vitrimerised polyethylene, the final compound has a micro-separated hierarchical structure that may eventually impact stress relaxation ability [START_REF] Ricarte | Linear Viscoelasticity and Flow of Self-Assembled Vitrimers: The Case of a Polyethylene/Dioxaborolane System[END_REF].

VITRIMER FORMULATION AND MATRIX PRELEMINARY RESULTS

Epoxy Novolac resin was chosen as suitable for its high viscosity at ambient temperature. To find a suitable hardener, a set of requirements was set for the polymer: (i) Tα ≥ 130°C, (ii) viscosity suitable for impregnation, (i ii) gel time ≥ 1 h at Timpregnation, (iv) gel time ≥ 1 week at room temperature, and (v) impregnation temperature ≤ 100°C.

DEN439 from Olin Epoxy, combined with Dyhard 100SF hardener (dicyandiamide, nonvitrimer reference) or "ad-hoc" hardener CHDA (cyclohexane dicarboxylic acid) were investigated with various catalysts. Preliminary DMA results show satisfying Tα ≈ 135°C (Fig. 1) for the CHDA.

Figure 1 -DMA characterization of DEN439_CHDA resin (no catalyst)

Further SAXS characterisation showed that the cured network were rather homogeneous (despite the initial difficulty to react epoxy with CHDA). However, it can be seen that the sample, at different temperatures, constantly show the presence of fine crystalline peaks in the wide angle region (around 2p/q = 5 Ǻ, Fig. 2). Thereby confirming the presence of crystallized components. Their intensity decrease at the beginning of the curing process but crystalline peaks do not disappear after cure. Thus, even though DMA data indicate formation of a rather homogeneous network, results of time-resolved morphological analyses reveal that the Zn(II) catalyst remains incompletely dissolved at the end of the reaction.

Figure 2 -SAXS data of the network showing only partial solvation of catalyst was achieved for DEN439_CHDA catalysed by Zn(II)

Once the network was defined as sufficiently satisfying, the project passed to the next stage of composites impregnation and lamination. However, as mentioned earlier, a tradeoff of materials (fibre) was performed for cost and availability purposes. This trade-off was carefully undertaken to choose the most suitable reinforcement. Contrary to the usual unidirectional tape, fabrics can help the resin to flow through their thickness, and thus, ease the impregnation process, whereas the unidirectional tape somehow acts as a barrier. At the same time, too many fibres crossings may force some resin accumulations, resulting in lower amount of resin on the ply surface, jeopardizing the potential for welding. This delicate balance led to select a 1-1 twill pattern fabric. This activity is still in progress, and the final results are expected by mid-2021. Current work include to manufacture the required quantity of laminates, cure them and characterise their properties on two main points: (i) testing properties of laminates to assess aerospatial potential (mechanical [ILSS, tension], outgassing, thermal cycling). (ii) Welding of two plates followed by in plane shearing tests.

PERSPECTIVES FOR VITRIMERS IN AEROSPACE

The outstanding properties of vitrimers set them to be a remarkable candidate for further use in FRP composites. Many drawbacks associated with thermoset resins, such as processing, reparability and recycling could be tackled by swapping to vitrimers matrices. This would allow, not only to reduce manufacturing costs, but also achieving better carbon footprint through the recovery of fibres and polymer whilst being easily adapted to current industry processes. However, they still suffer, just like thermosets or thermoplastics, from the initial chemistry they come from. Therefore, a PhD work on bio-sourced composite material that will include vitrimers is undergoing with the ESA and the University of Nice.

This study ensures continuity of the presented work, nonetheless vitrimer structures have still a lot of checkmarks to validate before use in aerospace applications. Radiation or ATOX behaviour is still to be demonstrated, and the industrialisation of the prepreg is not yet guaranteed. This does not stop effort to combine vitrimer matrices with other technologies. As presented below, thin plies technology is an efficient method to produce complex and lightweight structures for aerospace applications. And it is envisaged that vitrimers could later, easily fit within this approach.

THIN PLIES

GENERAL CONSIDERATIONS

Thin plies (Fig. 3) have been studied in many areas over the years and have demonstrated a wide range of benefits, from mass savings to better material homogeneity or even higher impact resistance. Moreover, the addition of Automated Tape Layering (ATL), combined with the outstanding joining ability of thin plies, is cost efficient for structures assembly, thanks to the reduction of inspection and/or manual compactions required with classical materials.

Therefore, following previous projects focused on performance, a wider approach was used to determine which kind of space component could get the most benefits from this technology. This study look at performance improvement, cost reduction, design simplification or more global change. The TDE project (as mentioned earlier) aims to define the most suitable applications of this technique. It is by itself, embedded in a longer term vision as shown in Fig. 4.

Figure 4 -Thin Ply related projects

THIN PLIES FOR SPACE STRUCTURES -RATIONALE FOR APPLICATIONS SELECTION

To ensure to cover the full potential benefits of thin plies, the approach has been to evaluate this technology in a wide range of applications, considering several factors: (i) Mass savings (thinner sandwich skin while remaining quasi isotropic) (ii) Intrinsic performances (better homogeneity, smoother load paths, etc.) (iii) Cost (use of industrial grade fibres, reduction of inspections and manual operations)

(iv) Design freedom (innovative and optimised joining techniques, capacity to be laid on curved surface with less issues than conventional tape, and higher performance than fabrics) A trade-off has been performed based on the various specificities of the thin plies. Some of these criteria were known at the start of the project (reduction of individual areal weight, better isotropy for thin layups, better ability for complex shapes layup, cost reduction via automatisation, etc.), while some others were hypothetic and needed to be confirmed during the project. Fig. 5 describes the rationale for this trade-off:

Figure 5 -Rationale for applications selection

The conclusion of the applications selection was that Solar Arrays Substrates (SA) and 3D integral structures were the most suitable candidates. The main goals for each applications are resumed below:

(i)

SA system: challenge was to design a substrate with the same properties as the existing one, but at a much lower cost and, if possible, lower weight.

(ii) 3D integral structures: challenge was in the capacity to build such a complex structure with unidirectional tapes, without jeopardizing mass savings. Particular attention was brought on curvature areas where poor material and/or process quality are likely to occur.

The development plan has then been set to cover these two very different kinds of challenges.

SOLAR ARRAYS AND 3D STRUCTURE

SOLAR ARRAY SUBSTRATE

For cost and availability purpose, a compromise was made when selecting the fibre for the SA application. The selected fibre has a Young's modulus 30% lower than the reference, for cost and availability reasons. This choice could influence many properties (mechanicals) of the substrate, but also its local strength (face wrinkling, FW and intracell buckling, IB failure modes are driven by the skin stiffness and are prominent on these large honeycomb cells panels). Such modifications of the structure properties could lead to more reinforcements needed which would go against the weight reduction goal rendering the solution obsolete. However, selected fibre has a 15% higher failure strength, which is beneficial for reinforced and pure compression (PC) modes. To validate this choice, extensive mechanical characterisation has been performed on various layups, defined after a preliminary design (built upon hypothesis). Edgewise compression testing was performed at various temperatures (Min, ambient and max), for different configurations described in Table 1, in a cross-check approach between THALES ALENIA SPACE and SCNTPL. Further tests are currently still in progress, but the first conclusions are very promising for this new generation of solar array substrates, based on an industrial approach. In this particular case, the use of thin plies allowed us to move from 3k high modulus fibres to 12k intermediate modulus fibre, with obvious benefits at cost and procurement levels.

Apart from the possibility to build thinner quasi isotropic skins, the thin ply technology also allows to distribute the fibres more evenly over the plies, while keeping the initial skin thickness. It was expected that this "progressive" layup could bring some improvement, and tests confirmed that trend, as described in Table 2.

Table 2 -Comparative edgewise compression

3D INTEGRAL STRUCTURE

Satellites platforms are mostly built from flat sandwich panels, assembled together via bolted joints and inserts. In some cases, 3D structures, manufactured as a whole sandwich on a shaped mould, are used and show some evident benefits when it comes to mass savings or ease of integration. Our vision is to spread this concept as much as possible, even to large platforms equipped with complex and powerful payload. Nonetheless, this kind of structure rises new challenges, and among them, the ability to layup curved shapes with unidirectional tape is of major importance. Indeed, the classical way to design and build such structure is to use fabrics, known for their ease of use on complex shapes. Keeping optimisation as a priority, building these structures with unidirectional tape is an obvious "direction" to follow. Depending on the shape of the structure, two options may be considered:

(i) Use continuous fibres on several adjacent faces of the structure (ii) Layup each side separately, and connect them via some "fresh" (uncured) doublers locally on the angles.

The first solution induces some fibre orientation issues, as tape across the curvature area cannot be bent. The second solution creates some local reinforcement, jeopardizing the mass savings expected from such a design. Fibre placement could be another option, but is currently out of the scope of this work.

ThinPlies, with their superior flexibility, can offer a smart alternative:

(i) The lower individual ply thickness helps applying the tape onto curved surfaces (ii) The thinner plies make some scarf junctions possible, with very little local mass increase.

In the frame of the "Thin Ply Laminate Composite Structure" TDE project, the integral 3D structure was found to be the second type of applications benefiting the most from thin plies (behind Solar Array Substrate). The concept and design of the 3D structure is shown in Figure 6 Figure 6

-3D integral structure concept

To validate the hypothesis of easier manufacturability and better quality across the radii, some representative samples (Figure 7) have been made and testing will be carried out shortly.

Figure 7 -angles layup with ThinPlies

Both solar arrays substrate and this project are managed in close cooperation between THALES ALENIA SPACE and SCNTPL. The next steps of these projects consist of testing these samples to then build, inspect and test a full scale integral structure that will prefigure the platforms of the future.

CONCLUSION

Vitrimers matrices and thin plies technology are two promising solutions to enhance the FRP composites uses in aerospace applications. On one side, vitrimers show the possibility to be adapted to current manufacture process whilst even having the potential to simplify them (welding of crosslinked networks). Preliminary results demonstrated that epoxy Novolac and CHDA hardener is a potential candidate for the development of resins suitable for infusion process and prepregs preparation.

Proper catalyst selection is still undergoing in order to obtain homogeneous networks with stable properties and the high mixture viscosity remains a possible limitation if this specific network for fibres impregnation. All these parameters are to be defined more accurately through 2021 with the start of vitrimer composites design and manufacture using industrial processes.

Concomitantly, thin plies technology have shown promising results to produce lightweight and strong composites compared to their traditional counterparts.

Throughout study of aerospace components showed that the applications which could mostly benefit from this technology are solar array substrate and 3D integral structures. Both of these applications have been demonstrated to benefit in lighter weight ratio while reducing costs of manufacture.

The continuity of this work will ensure to develop several approaches to design aerospace composites. The possible combination of vitrimers matrices and thin plies technology would create a high synergy to: (i) simplify and reduce cost of manufacture and process, (ii) produce lighter composites with increased lifespan, (iii) have eco-friendly FRP composites able to achieve close-loop recyclability. Moreover, investigation of bio-sourced chemicals would only improve the environmental impact of these new composites.

This work shows a small insight in the possible evolution of the FRP composites for aerospace while using new technologies allowing us to envision the future generation of these materials and structures.

Figure 3

 3 Figure 3 General presentation of Thin Ply (www.thinplytechnology.com)

Table 1 -

 1 Solar array sandwich configurations

	Configuration	Skin thickness	Honeycomb	Failure mode
	Current skin	0,1 mm	9-20	IB
	Current skin	0,1 mm	4-20	FW
	Doubler2	0,9 mm	4-20	PC
	It appears the IB and FW were slightly
	higher than expected values, allowing us to add
	very limited reinforcements, keeping the mass of
	the substrate at a very acceptable level.

5. AKNOWLEDGEMENT This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. We thank Denis T. Keane of the Advanced Photon Source for his help in using beamline DND-CAT 5ID-D. We thank the Aerospace Research Institute and Costantinos Soutis for supports and funding which permitted the preliminary studies on these vitrimers development. The work presented is performed under ESA Contract No. 4000128430 for the Vitrimers project, and ESA Contract No. 4000126111 and No. 4000123840 for the Thin Ply project.