Covalent Adaptable Networks through Dynamic N,S-Acetal Chemistry: Toward Recyclable CO2-Based Thermosets.

peer reviewedFinding new chemistry platforms for easily recyclable polymers has become a key challenge to face environmental concerns and the growing plastics demand. Here, we report a dynamic chemistry between CO2-sourced alkylidene oxazolidones and thiols, delivering circular non-isocyanate polyurethane networks embedding N,S-acetal bonds. The production of oxazolidone monomers from CO2 is facile and scalable starting from cheap reagents. Their copolymerization with a polythiol occurs under mild conditions in the presence of a catalytic amount of acid to furnish polymer networks. The polymer structure is easily tuned by virtue of monomer design, translating into a wide panel of mechanical properties similar to commodity plastics, ranging from PDMS-like elastomers [with Young's modulus (E) of 2.9 MPa and elongation at break (εbreak) of 159%] to polystyrene-like rigid plastics (with E = 2400 MPa, εbreak = 3%). The highly dissociative nature of the N,S-acetal bonds is demonstrated and exploited to offer three different recycling scenarios to the thermosets: (1) mechanical recycling by compression molding, extrusion, or injection molding─with multiple recycling (at least 10 times) without any material property deterioration, (2) chemical recycling through depolymerization, followed by repolymerization, also applicable to composites, and (3) upcycling of two different oxazolidone-based thermosets into a single one with distinct properties. This work highlights a new facile and scalable chemical platform for designing highly dynamic polymer networks containing elusive oxazolidone motifs. The versatility of this chemistry shows great potential for the preparation of materials (including composites) of tuneable structures and properties, with multiple end-of-life scenarios.The " Non-Isocyanate Polyurethanes - European Joint Doctorate " [ NIPU-EJD

Emerging polyhydroxyurethanes as sustainable thermosets: a structure–property relationship

peer reviewedPolyhydroxyurethanes (PHU), obtained from CO2-based cyclic carbonates (CC) and polyamines, are known as greener and safer alternatives to conventional polyurethanes. Interestingly, the hydroxyurethane moieties present along the PHU’s backbone offer unexplored opportunities in terms of enhanced adhesion and mechanical properties that could be a major breakthrough in many structural applications. Furthermore, PHUs have shown thermomechanical recyclability arising from the ability of hydroxyurethane moieties to participate in reversible exchange reactions. However, the relationship between the macromolecular structure, the processability, and the final properties of these materials have not been evaluated to a sufficient extent to establish a comprehensive overview of these emerging thermosets. In this sense, this work aims to address this research gap by investigating the rheological and thermomechanical performances of PHU thermosets and opening an unexplored door for future sustainable engineered structural applications. A special emphasis was put on PHU thermosets formulated using potentially biobased monomers. The rheological behavior during cross-linking of the PHU formulations was studied and highlighted the importance of the number of CC functionalities in the viscosity and gel time, ranging from 10 min to nearly 3 h. Moduli superior to 2 GPa and glass transition over 50 °C were obtained for short multifunctional CC. Finally, the dynamic network behavior of these PHUs was also demonstrated through stress-relaxation and reprocessing. High temperatures (over 150 °C) and pressure lead to a good recovery of the thermomechanical properties. Such materials appear to be an interesting platform for structural applications, particularly fiber-reinforced polymers, that can overcome many sustainability challenges.The " Non-Isocyanate Polyurethanes - European Joint Doctorate " [ NIPU-EJD

Data driven approach for coupled electro-mechanical problems

International audienceThe classical approach in the computational mechanics considers the formulation of material modelswhose development rely on the data usually collected either through experimental measurements ornumerical simulation. The constitutive model should describe the data set as faithfully as possible,keeping at the same time it’s mathematical structure as simple as possible. Those two opposite demandsusually lead to material model that contains a number of simplifications and adjustments leading toempirical model which is less rich than the data used for it’s formulation. To overcome this informationloss, Kirchdoerfer and Ortiz [1] recently proposed a new approach which avoids the need of constitutivemodel in computational mechanics, replacing it by material data.In this work, we propose an application of the data driven approach (DDA) to the coupled, electrome-chanical behaviour. More precisely, we focus on the linear piezo-electric continuum whose governingequations are related to 1. mechanical (M) sub-problem described by elasticity, related kinematics andboundary conditions; 2. electrical (E) sub-problem related to the electrostatics, Gauss and Maxwell lawand related boundary conditions; 3. electro-mechanical coupling definedonly through the constitutive re-lations. As opposed to ’standard’ DDA related to elasticity [1] where the local state of the system in everymaterial point is characterized by set of stress-strain couples, here the material response is characterizedby 4-tuples composed of strain, stress and electrical field and electrical displacement. In DDA the solu-tion is characterized by electro-mechanical points that are ’the closest’ to material points, we propose anextension of the norm to metrize the distance. In addition, we give the details of the DDA algorithm forcoupled piezoelectric behaviour. Analogously as in standard DDA, the objective here is to findelectro-mechanical states(4-tuples) such that: (i) the strain and electrical fields are derived from displacementand electrical potential fields respecting the mechanical compatibility equations and Maxwell’s law and(ii) that the stress and electrical displacement verify the corresponding mechanical equilibrium as wellas Gauss law (dielectric equivalent). The coupling being only in terms of constitutive model, which weabandon, the great advantage of applying DDA approach to a class of coupled multiphysics problems isthat it results in twodecoupledsub-problems, the mechanical and dielectric problem. The performanceof the proposed extension is shown on few illustrative examples

Data driven approach in multiphysics framework: application to coupled electro-mechanical problems

International audienceSolving coupled multiphysics problems using finite elements and conventional modeling approach requires building, from coupled finite element formulation often in the same code, the monophysical operators and the multiphysics coupling operators. In this work the data driven, model-free computational approach is fitted in the multiphysics framework adapted for 'smart' materials. Thus, in this work we ought to expand the phase space and propose a new norm for the distance based data driven solver adapted to the problem at hands. Given the good material database which naturally encodes the coupling interactions, we show that proposed application of data driven approach enables to avoid the coupling tangent terms altogether. In other words, the data driven approach permits to decouple the different 'physics' and to manage the coupling from the data. We illustrate the performance and robustness of the approach on two examples related to lattice model composed of planar, piezoelectric truss network and of finite element discretized linear piezoelectric solids. The numerical tests show a good convergence properties related to the number of data points, as well as the choice of material and weighting parameters

Towards sustainable reprocessable structural composites: benzoxazines as biobased matrices for natural fibers

In this work, we synthesized and investigated three fully biobased benzoxazine matrices containing exchangeable ester bonds for natural fiber composites. The thermoset properties were investigated and the transesterification behavior was assessed. The obtained polymers show high tunability. Using isosorbide as the starting building block, the thermoset exhibits a glass transition of 130 °C, a bending modulus of 2.5 GPa, and thermal stability leading to degradation occurring after 270 °C with 31% char at 800 °C. All formulations stress relax under catalyst-free conditions within an hour with properties recovery superior to 80%. Finally, flax composites were manufactured. We highlight strong affinities between the matrices and the fibers through high mechanical performances with a modulus over 30 GPa and stress at break of 400 MPa in the longitudinal direction. 5 GPa modulus and 47 MPa stress at break were found in the transverse direction. Excellent fire retardancy properties, with self-extinguishment and UL-94 V1 classification were obtained for the isosorbide-based/flax composite. The obtained composites were able to be welded with comparable results to glued ones, paving the way to processable laminates and stable cured prepreg perfectly suited for transportation-engineered applications

Towards in-situ acoustic emission-based health monitoring in bio-based composites structures: Does embedment of sensors affect the mechanical behaviour of flax/epoxy laminates ?

International audienceMonitoring integrity of operating structures has become a crucialneed in industrial applications while motivation for moresustainable materials pushes forward the development of naturalfibre composites (NFC). The complexity and variability of theirfailure behaviour and mechanical properties still limit theexploitation of their full potential. Acoustic Emission (AE) hasshown promising results to predict remaining service life ofstructures, allowing identification, localisation and assessment ofdamages in composite materials. Thus, embedment of AE sensors couldbe a reliable solution for real-time Structural Health Monitoring(SHM). This paper aims to quantify<br /&gtthe effect of embedded millimetre-sized metallic inserts mimickingsensor integration on material integrity under tensile and bendingloads by means of Digital Image Correlation, AE, and infraredthermography. Results show a limited effect on monotonic tensileproperties, driven by the size of the sensor, while highlighting anincrease of strain and stress at failure under three-point bending.Tensile-tensile fatigue resistance is only slightly affected by theembedment of sensors, with a small increase in the S–N curve slope.Overall results suggest that the embedment of miniature sensors inNFC is possible and could be a suitable solution for damageassessment and health monitoring in such sustainable structures

TOWARDS INTEGRATED HEALTH MONITORING OF BIO-BASED COMPOSITE STRUCTURES: INFLUENCE OF ACOUSTIC EMISSION SENSOR EMBEDMENT ON MATERIAL INTEGRITY

Structural Health Monitoring (SHM) and Natural Fibres Composites (NFC) have livedsince early 2000’s important progress due to the needs of reducing material consumption andweight in operating structures while limiting the environmental footprint and increasing thesafety of such structures. Acoustic Emission (AE) is one of the numerous solutions that appearsto be suitable to predict service life and assess damage in composite structures. NFCs have shownpromising properties for structural applications but their complex behaviour and the limitedknowledge on their long-term durability still hinder their full development. In that sense, AEbasedSHM could allow a more important breakthrough of bio-based composites by monitoringdamage evolution and assessing their remaining life in service conditions. The new generationof AE-sensors offers also opportunities of embedment thanks to their smaller size, leading tobetter protection of the sensors and increased sensitivity. We propose in this work to study theeffect of the embedment of millimetric-size dummy sensors in flax/epoxy laminates on themonotonic tensile behaviour and damage kinetic up to failure through AE methods. Resultshighlight a limited effect of the presence of dummy sensors on the mechanical behaviour andproperties. AE results coupled with X-ray tomography pictures however highlight a change inlocation of damages that are more likely to append in the insert vicinity, and in the onset of themost severe damages, which appears at lower global deformation levels

Covalent adaptable networks through dynamic N,S-acetal chemistry: toward recyclable CO2-based thermosets

Designing easily recyclable polymers with customized properties is a key challenge to face up environmental concerns and the growing plastics demand. Here, we report a dynamic chemistry between CO2-sourced alkylidene oxazolidones and thiols that delivers circular non-isocyanate polyurethane networks embedding N,S-acetal bonds. Oxazolidones are synthesized from cheap reagents and carbon dioxide. The polymer structure is tuned by monomer design, translating in a wide panel of mechanical properties, ranging from PDMS-like elastomers (Young’s modulus (E) = 2.9 MPa and elongation at break (Ebreak) = 159%) to polystyrene-like rigid plastics (E = 2800 MPa, Ebreak = 2%). The dynamic nature of the N,S-acetal bond offers multiple closed- and open-loop recycling options, facilitating repeated thermoset reprocessing into the same material or the production of a different one of distinct properties. The versatility of this chemistry shows great potential for preparing materials (including composites) of tunable properties that can be recycled by multiple scenarios

A novel paradigm approach to design structural natural fiber composites from fully sustainable CO2-derived thermosets with outstanding interfacial strength and circular features

We herein propose capitalizing on strong hydrogen bonding from novel bio-CO2-derived dynamic thermosets to achieve high-performance natural fiber composites (NFC) with circular features. CO2- and biomass-derived polyhydroxyurethane (PHU) thermosets were selected, for the first time of our knowledge, as matrices for their ability to make strong H-bond, resulting in outstanding mechanical properties for NFC. Exploiting this H-bond key feature, exceptional interface bonding between flax and PHU was confirmed by atomic force microscopy and rationalized by atomistic simulation. Without any treatment, an increase of 30% of stiffness and strength was unveiled compared to an epoxy benchmark, reaching 35 GPa and 440 MPa respectively. Related to the thermoreversible nature of hydroxyurethane moieties, cured flax-PHU were successfully self-welded and displayed promising properties, together with recyclability features. This opens advanced opportunities that cannot be reached with epoxy-based composites. Implementing CO2-derived thermosets in NFC could lead to more circular materials, critical for achieving sustainability goals