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Vitrimers with 1 : 1 to 2 : 1 epoxy/acid ratio and TBD show increased stiffness and gradual transition from an exchangeable to non-exchangeable network.

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Epoxy/dicarboxylic acid vitrimer was prepared by the solvent-free reaction of diglycidyl ether of bisphenol A (DGEBA) and 1,4-cyclohexane dicarboxylic acid (CHDA) with the addition of monobutyltin oxide (Sn) as a catalyst. By tailoring the catalyst content (≥5 mol %), an effective conversion of functional groups during cure demonstrated the network polymerization mechanisms and a sequence of the side reactions. Indeed, the manufactured vitrimers exhibit creep and full stress relaxation thanks to catalytic transesterifications. By changing the epoxy/diacid ratio, the thermo-mechanical properties and mechanical behavior of the epoxy/acid vitrimers can be tuned while keeping self-healing ability. At high epoxy excess, both glass-transition temperature (T g) and solid–liquid viscoelastic transition temperature (T v) shift to a higher temperature. At vitrimer formulations 1:0.6 and 1:0.5 (epoxy/acyl), a remarkable improvement of fracture toughness (K Ic) is observed, indicating the transition from stiff to relatively ductile materials at 1:0.6. This is attributed to the altered network structures due to etherification and epoxy homopolymerization. The rough fracture surface suggests more energy dissipation during crack propagation in vitrimer with a high excess epoxy. After healing, welded vitrimers still exhibit good fracture toughness with only a slight reduction (<10%) in K Ic. We believe that these vitrimer formulations are promising as matrices in the composite fields.

Nanoparticle dispersion is widely recognised as a challenge in polymer nanocomposites fabrication. The dispersion quality can affect the physical and thermomechanical properties of the material system. Qualitative transmission electronic microscopy, often cumbersome, remains as the ‘gold standard’ for dispersion characterisation. However, quantifying dispersion at macroscopic level remains a difficult task. This paper presents a quantitative dispersion characterisation method using non-contact infrared thermography mapping that measures the thermal diffusivity (α) of the graphene nanocomposite and relates α to a dispersion index. The main advantage of the proposed method is its ability to evaluate dispersion over a large area at reduced effort and cost, in addition to measuring the thermal properties of the system. The actual resolution of this thermal mapping reaches 200 µm per pixel giving an accurate picture of graphene nanoplatelets (GNP) dispersion. The post-dispersion treatment shows an improvement in directional thermal conductivity of the composite of up to 400% increase at 5 wt% of GNP. The Maxwell-Garnet effective medium approximation is proposed to estimate thermal conductivity that compare favourably to measured data. The development of a broadly applicable dispersion quantification method will provide a better understanding of reinforcement mechanisms and effect on performance of large scale composite structures.

Graphene nanoplatelet (GNP) modified epoxy nanocomposites are becoming attractive to aerospace due to possible improvements in their mechanical, electrical and thermal properties at no weight cost. The process of obtaining reliable material systems provides many challenges, especially at larger scale (a volume effect). This paper reports on the main fabrication stages of GNP-based epoxy composites, namely (i) pre-dispersion, (ii) dispersion, and (iii) post-dispersion. Each stage is developed to show the interest and potential it delivers for property enhancement. Chemical modification of GNP is presented; functionalisation by Triton X-100 shows elastic modulus improvements of the epoxy at low particle content (≤3%). The post-dispersion step as an alignment of GNP into the epoxy by an electrical field is discussed. The electrical conductivity is below the simulated percolation threshold and an improvement of the thermal diffusivity of 220% when compared to non-oriented GNP epoxy sample is achieved. The work demonstrates how the addition of functionalised graphene platelets to an epoxy resin will allow it to act as electrical and thermal conductor rather than as insulator

Commercially available lipase from Pseudomonas stutzeri (Lipase TL) is investigated as biocatalyst for the formation of an acid-epoxy chemical network. Molecular model reactions are performed by reacting 2-phenyl glycidyl ether and hexanoic acid in bulk, varying two parameters: temperature and water content. Characterizations of the formed products by 1 H NMR spectroscopy and GC-MS combined with enzymatic assays confirm that lipase TL is able to simultaneously promote acid-epoxy addition and transesterification reactions below 100°C and solely the acid-epoxy addition after denaturation at T > 100°C. A prototype biobased chemical network with β-hydroxyester links was obtained using resorcinol diglycidyl ether and sebacic acid as monomers and the lipase TL as catalyst. DSC, ATR-IR, and swelling analysis confirm gelation of the network.

The preparation and reprocessing of an epoxy vitrimer material is performed in a fully biocatalyzed process wherein network formation and exchange reactions are promoted by a lipase enzyme. Binary phase diagrams are introduced to select suitable diacid/diepoxide monomer compositions overcoming the limitations (phase separation/sedimentation) imposed by curing temperature inferior than 100 °C, to protect the enzyme. The ability of lipase TL, embedded in the chemical network, to catalyze efficiently exchange reactions (transesterification) is demonstrated by combining multiple stress relaxation experiments at 70−100 °C and complete recovery of mechanical strength after several reprocessing assays (up to 3 times). Complete stress relaxation ability disappears after heating at 150 °C, due to enzyme denaturation. Transesterification vitrimers thus designed are complementary to those involving classical catalysis (e.g., using the organocatalyst triazabicyclodecene) for which complete stress relaxation is possible only at high temperature.