Epoxy Vitrimer Materials by Lipase-Catalyzed Network Formation and Exchange Reactions.

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.

Lipase-Catalyzed Epoxy-Acid Addition and Transesterification: from Model Molecule Studies to Network Build-Up.

Commercially available lipase from Pseudomonas stutzeri (lipase TL) is investigated as a 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 1H NMR spectroscopy and gas chromatography-mass spectrometry 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 bio-based chemical network with ß-hydroxyester links was obtained using resorcinol diglycidyl ether and sebacic acid as monomers with lipase TL as catalyst. Differential scanning calorimetry, attenuated total reflection, and swelling analysis confirm gelation of the network.

Fabrication and characterisation of 3D printed MWCNT composite porous scaffolds for bone regeneration.

Carbon nanotubes (CNTs) with exceptional physical and chemical properties are attracting significant interest in the field of tissue engineering. Several reports investigated CNTs biocompatibility and their impact in terms of cell attachment, proliferation and differentiation mainly using polymer/CNTs membranes. However, these 2D membranes are not able to emulate the complex in vivo environment. In this paper, additive manufacturing (3D printing) is used to create composite 3D porous scaffolds containing different loadings of multi-walled carbon nanotubes (MWCNT) (0.25, 0.75 and 3 wt%) for bone tissue regeneration. Pre-processed and processed materials were extensively characterised in terms of printability, morphological and topographic characteristics and thermal, mechanical and biological properties. Scaffolds with pore sizes ranging between 366 µm and 397 µm were successfully produced and able to sustain early-stage human adipose-derived mesenchymal stem cells attachment and proliferation. Results show that MWCNTs enhances protein adsorption, mechanical and biological properties. Composite scaffolds, particularly the 3 wt% loading of MWCNTs, seem to be good candidates for bone tissue regeneration.

Thermal Diffusivity Mapping of Graphene Based Polymer Nanocomposites.

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.