The use of reactive diluents is undeniably of paramount importance to develop epoxy resins which would meet more demanding and restrictive processes and applications in terms of viscosity and glass transition temperature. In the context of developing resins with low carbon impacts, 3 natural phenols namely carvacrol, guaiacol and thymol were selected and converted into monofunctional epoxies using a general glycidylation procedure. Without advanced purification, the developed liquid-state epoxies showed very low viscosities of 16 cPs to 55 cPs at 20°C, which could be further reduced to 12 cPs at 20 °C when purification by distillation is applied. The dilution effect of each reactive diluent on DGEBA's viscosity was also assessed for concentrations ranging from 5 to 20 wt% and compared to commercial and formulated DGEBA-based resin analogues. Interestingly, the use of these diluents reduced the initial viscosity of DGEBA by a factor of ten while maintaining glass transition temperatures above 90 °C. This article provides compelling evidence of the possibility of developing new sustainable epoxy resins with characteristics and properties that can be fine-tuned by only adjusting the reactive diluent concentration.
Epoxy vitrimers encompass many advantages compared to traditional epoxy materials such as recyclability, repairability, and reprocessability. These properties are induced by the incorporation of dynamic reversible covalent bonds. Recently, the incorporation of aromatic disulfide bridges that are dynamic has expanded the development of new eco-friendly epoxy materials. Herein, we studied a bio-based aliphatic disulfide based on cystamine as a hardener with a vanillin-derived bio-sourced epoxy to prepare fully bio-based epoxy vitrimers. This article provides a comparative study between cystamine and an aromatic disulfide benchmark hardener issued from petrol resources. This work demonstrated that the presence of this aliphatic hardener has a significant influence not only on the reactivity, but most importantly on the resulting dynamic properties. An interesting yet counterintuitive accelerating effect of the dynamic exchanges was clearly demonstrated with only 2 to 20% of molar fraction of cystamine added to the aromatic disulfide formulation. A similar glass transition was obtained compared to the purely aromatic analogue, but relaxation times were decreased by an order of magnitude.
The molecular simulation of interfacial systems is a matter of debate because of the choice of many input parameters that can affect significantly the performance of the force field of reproducing the surface tension and the coexisting densities. After developing a robust methodology for the calculation of the surface tension on a Lennard-Jones fluid, we apply it with different force fields to calculate the density and surface tension of pure constituents of epoxy resins. By using the model that best reproduces the experimental density and surface tension, we investigate the impact of composition in mass fraction on uncured epoxy resins and the effects of degree of cross-linking on cured resins.
Phase behavior modulation of liquid crystalline molecules can be addressed by structural modification at molecular level. Starting from a rigid rod-like core -, e.g., composed by aromatic rings - reduction of the symmetry or increase of the steric hindrance by different substituents generally reduces the clearing temperature. Similar approaches can be explored to modulate the properties of Liquid Crystalline Networks (LCNs)- shape-changing materials employed as actuators in many fields. Depending on the application, the polymeric properties have to be adjusted in terms of force developed under stimuli, kinetics of actuation, elasticity, and resistance to specific loads. In this paper, we explore the crosslinker modification at the molecular level and its effect on photoresponsive LCNs prepared by acrylate photopolymerization, towards the optimization of their properties, and the development of light-responsive artificial muscles. The synthesis and characterization of photopolymerizable crosslinkers, bearing different lateral groups on the aromatic core is reported. Such molecules were demonstrated able to strongly modulate the material mechanical properties, such as kinetics and maximum tension under light actuation, opening up to interesting materials for biomedical applications. This article is protected by copyright. All rights reserved.
In the nanomedicine field, there is a need to widen the availability of nanovectors to compensate for the increasingly reported side effects of poly(ethene glycol). Nanovectors enabling cross-linking can further optimize drug delivery. Cross-linkable polyoxazolines are therefore relevant candidates to address these two points. Here we present the synthesis of coumarin-functionalized poly(2-alkyl-2-oxazoline) block copolymers, namely, poly(2-methyl-2-oxazoline)-block-poly(2-phenyl-2-oxazoline) and poly(2-methyl-2-oxazoline)-block-poly(2-butyl-2-oxazoline). The hydrophilic ratio and molecular weights were varied in order to obtain a range of possible behaviors. Their self-assembly after nanoprecipitation or film rehydration was examined. The resulting nano-objects were fully characterized by transmission electron microscopy (TEM), cryo-TEM, multiple-angle dynamic and static light scattering. In most cases, the formation of polymer micelles was observed, as well as, in some cases, aggregates, which made characterization more difficult. Cross-linking was performed under UV illumination in the presence of a coumarin-bearing cross-linker based on polymethacrylate derivatives. Addition of the photo-cross-linker and cross-linking resulted in better-defined objects with improved stability in most cases.
All-atom molecular dynamics (MD) simulations were performed with the CHARMM force field to characterize various epoxy resins, such as aliphatic and bisphenol-based resins. A multistep cross-linking algorithm was established, and key properties such as density, glass temperature, and elastic modulus were calculated. A quantitative comparison was made and was proven to be in good agreement with experimental data, with average absolute deviations between experiments and molecular simulation comprised between 2% and 12%. Additional findings on structure-property relationships were highlighted such as the effect of the cross-linking rate and oligomerization of the resin.
We reported molecular simulations of the interactions between water, an epoxy prepolymer (DGEBA) and an hardener (IPDA) on an aluminum surface. This work proposes a comprehensive thermodynamic characterization of the adhesion process from the calculation of the different interfacial tensions. The cross-interactions between the atoms of the metal surface and the different molecules are adjusted so as to reproduce the experimental work of adhesion. Water nanodroplets on the metal surface are then simulated to predict its contact angle. Liquid-vapor surface tensions of the epoxy prepolymer (DGEBA) and hardener (IPDA) and the solid-vapor surface tension of the aluminum surface are also calculated to provide the solid-liquid interfacial tension that remains very difficult to obtain from the mechanical definition.
Unidirectional bamboo reinforced cardanol-based epoxy composites were prepared by a close mold method. Two morphologies of reinforcements were used in this research: bamboo fibres and bamboo strips. The present article investigates the influence of bamboo reinforcements on the thermal and mechanical properties of the biobased matrix. Differential Scanning Calorimetry analyses showed that the introduction of bamboo does not modify the physical properties of the matrix. DMA analyses in shear mode showed an improvement of the shear conservative modulus that reaches 1.7 ± 0.1 GPa. This value that is independent from the morphology of reinforcements, indicates the existence of physical interactions. The continuity of matters between bamboo strips or bamboo fibres and the matrix observed by SEM confirms this result. Nevertheless, in tensile mode, the improvement of the tensile conservative modulus is specific to the morphology used. Indeed, bamboo strips composites is 7.7 ± 0.8 GPa, while for bamboo fibres composites, it reaches 9.6 ± 0.8 GPa. This result is explained by the optimisation of stress transfer thanks to the specific morphology of bamboo fibres. A significant increase is also observed for the rubbery modulus due to entanglements specific of bamboo reinforcement
The radical-induced cationic frontal photopolymerization (RICFP) of fully biobased epoxy composites is successfully demonstrated. This curing strategy considerably reducess the curing time and improves the efficiency of the composite fabrication. Two different natural fibre fabrics made of cellulose and flax fibres are embedded in two epoxy matrices, one derived from vanillin (diglycidylether of vanillyl alcohol-DGEVA) and the other from petroleum (3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate-CE). After RICFP the composites are characterized by means of dynamic mechanical thermal analysis and tensile tests. The mechanical properties improved with increasing fibre content, confirming a strong adhesion between the matrix and the reinforcing fibre fabrics, which is further evidenced by scanning electron microscopy analyses of the fracture surfaces. Furthermore, these fully bio-based composites possess comparable or even higher mechanical strength compared with the corresponding epoxy composites fabricated with conventional CE resin. A promising facile route to high-performing natural fibre-biobased epoxy resin composites is presented. This article is protected by copyright. All rights reserved
The importance of electrically functional biomaterials is increasing as researchers explore ways to utilise them in novel sensing capacities. It has been recognised that for many of these materials the state of hydration is a key parameter that can heavily affect the conductivity, particularly those that rely upon ionic or proton transport as a key mechanism. However, thus far little attention has been paid to the nature of the water morphology in the hydrated state and the concomitant ionic conductivity. Presented here is an inelastic neutron scattering (INS) experiment on hydrated eumelanin, a model bioelectronic material, in order to investigate its 'water morphology'. We develop a rigorous new methodology for performing hydration dependent INS experiments. We also model the eumelanin dry spectra with a minimalist approach whereas for higher hydration levels we are able to obtain difference spectra to extract out the water scattering signal. A key result is that the physi-sorbed water structure within eumelanin is dominated by interfacial water with the number of water layers between 3-5, and no bulk water. We also detect for the first time, the potential signatures for proton cations, most likely the Zundel ion, within a biopolymer/water system. These new signatures may be general for soft proton ionomer systems, if the systems are comprised of only interfacial water within their structure. The nature of the water morphology opens up new questions about the potential ionic charge transport mechanisms within hydrated bioelectronics materials.
The feasibility of a solar-driven photoelectrochemical process to generate hydrogen fuel from metal mine polluted water while simultaneously recovering heavy metals has been explored. Electron transport from the photoanode to the cathode plays a key role in generating hydrogen (37.6 µmol h–1 cm–2 at 0.2 V RHE, 1 sun illumination), and scavenging Zn2+ ions in the form of ZnO.
Bio‐based epoxy resins are attracting widespread interest in the field of polymer thermosets as environmentally friendly building block. In the present study, the feasibility of applying UV‐curable epoxidized vegetable oils (EVOs) as anti‐corrosion coating is investigated. Rheological characterization of EVOs is carried out, and their viscosity‐shear relationship is evaluated. The cationic UV‐curing of EVOs successfully gives rise to crosslinked materials with a wide range of thermo‐mechanical properties, evaluated by differential scanning calorimetric analysis and dynamic thermal mechanical analysis. A high epoxy‐group conversion, ranging from 93% to 99%, is always obtained. The thermal stability and surface properties of the bio‐based coatings, such as, pencil hardness, adhesion, solvent resistance, and contact angle, are analyzed. Moreover, the corrosion protection effectiveness of the coatings is characterized by potentiodynamic polarization and electrochemical impedance spectroscopy measurements. In addition, field emission scanning electron microscopy is used to assess the samples morphology after corrosion tests. This paper presents a comprehensive characterization of the bio‐based thermosets obtained from three different epoxidized triglycerides; the specific application for anticorrosion coatings is investigated using electrochemical measurements. Results highlight the good corrosion protection behavior of the coatings, specifically for the formulation having the highest epoxy content; this can be correlated to the high cross‐linking density and higher glass transition temperature.
Beyond the need to find a non-toxic alternative to DiGlycidyl Ether of Bisphenol-A (DGEBA), the serious subject of non-epichlorohydrin epoxy resins production remains a crucial challenge that must be solved for the next epoxy resin generations. In this context, this study focuses on the valorization of vegetable oils (VOs) into thermoset materials by using (i) epoxidation of the VOs through the “double bonds to epoxy” synthetic route and (ii) synthesis of crosslinked homopolymers by UV or hardener-free thermal curing processes. A thorough identification, selection and physico-chemical characterization of non-edible or non-valuated natural vegetable oils were performed. Selected VOs, characterized by a large range of double bond contents, were then chemically modified into epoxides thanks to an optimized, robust and sustainable method based on the use of acetic acid, hydrogen peroxide and Amberlite® IR-120 at 55 °C in toluene or cyclopentyl methyl ether (CMPE) as a non-hazardous and green alternative solvent. The developed environmentally friendly epoxidation process allows reaching almost complete double bond conversion with an epoxy selectivity above 94% for the 12 studied VOs. Finally, obtained epoxidized vegetable oils (EVOs), characterized by an epoxy index from 2.77 to 6.77 meq. g⁻¹ were cured using either UV or hardener-free thermal curing. Both methods enable the synthesis of 100% biobased EVO thermoset materials whose thermomechanical performances were proved to linearly increase with the EVOs' epoxy content. This paper highlights that tunable thermomechanical performances (Tα from −19 to 50 °C and Tg from −34 to 36 °C) of EVO based thermoset materials can be reached by well selecting the starting VO raw materials.
The development of solution-processed photovoltaic (PV) devices for indoor applications has recently attracted widespread attention owing to their outstanding potential in harvesting energy efficiently for low-power-consumption electronic devices, such as wireless sensors and internet of things (IoTs). In particular, organic PV (OPV), perovskite PV (PPV) and quantum dot PV (QDPV) are among the most promising emerging photovoltaic technologies that have already demonstrated strong commercialisation potential for this new market, owing to their excellent yet highly tuneable optoelectronic properties to meet the demands for specific applications. In this review, we summarise the recent progress in the development of OPV, PPV and QDPV for indoor applications, showing the rapid advance in their device performance in conjunction with highly diverse materials and device designs, including semi-transparent, flexible and large-area devices. The remaining challenges of these emerging indoor PV technologies that need to be urgently addressed toward their commercialisation, including in particular their limited stability and high ecotoxicity, will be discussed in detail. Potential strategies to address these challenges will also be proposed.
For the first time, the effect of reactant structure, stoichiometry and heating rate on the reactivity of epoxidized perilla oil (EPLO) and epoxidized safflower oil (ESFO) with dicarboxylic acids (DCAs) was studied using in situ FT-IR. The epoxy content in the monomer structure was found to affect the copolymerization system’s reactivity, with epoxidized linseed oil (ELO) considered as a reference. In this study we discuss also the influence of the DCA structure on the copolymerization reactivity. Two aromatic diacids, dithiodibenzoic acid (DTBA) and diphenic acid (DPA), were studied and compared in the copolymerization of the 3 EVOs, in the presence of imidazole (IM) initiator. The kinetics of these reactions were followed by in situ FT-IR. The corresponding activation energies were calculated via different kinetic models. These data highlight the higher reactivity of the EPLO monomer and the DTBA hardener.
In this study, we investigate the underlying origin of the high performance of PM6:Y6 organic solar cells. Employing transient optoelectronic and photoemission spectroscopies, we find that this blend exhibits greatly suppressed charge trapping into electronic intra-bandgap tail states compared to other polymer/non-fullerene acceptor solar cells, attributed to lower energetic disorder. The presence of tail states is a key source of energetic loss in most organic solar cells, as charge carriers relax into these states, reducing the quasi-Fermi level splitting and therefore device VOC. DFT and Raman analyses indicate this suppression of tail state energetics disorder could be associated with a higher degree of conformational rigidity and uniformity for the Y6 acceptor. We attribute the origin of such conformational rigidity and uniformity of Y6 to the presence of the two alkyl side chains on the outer core that restricts end-group rotation by acting as a conformation locker. The resultant enhanced carrier dynamics and suppressed charge carrier trapping are proposed to be a key factor behind the high performance of this blend. Low energetic disorder is suggested to be a key factor enabling reasonably efficient charge generation in this low energy offset system. In the absence of either energetic disorder or a significant electronic energy offset, it is argued that charge separation in this system is primarily entropy driven. Nevertheless, photocurrent generation is still limited by slow hole transfer from Y6 to PM6, suggesting pathways for further efficiency improvement.
Roll-to-roll coating of all active layers is demonstrated for a P-I-N perovskite solar cell stack, using a single step perovskite ink with an acetonitrile solvent system and flexible plastic substrate. A slot-die coating roll-to-roll process with a common coating speed for all layers is developed by using appropriate length meniscus guides as part of the coating head for each ink rheology. High performance devices are demonstrated with four roll-to-roll slot-die coated layers and evaporated top electrode, the drying conditions of the perovskite layer are optimized and found to be critical to achieving good performance. Multi solvent blend systems for the electron collection layer are developed that are more industrially compatible than the commonly used chlorobenzene solvent system and make use of a gradient of solvent volatilities to give both good macro film formation and rapid drying. A stabilised power conversion efficiency of 12.2% is demonstrated, that is the highest reported to date for devices with all layers other than the top electrode deposited roll-toroll. This work demonstrates the feasibility of a roll-to-roll fabrication process for perovskite solar cells that could be transferred to a fully inline roll-to-roll process with all coating and drying stages made sequentially on one line running at a common coating speed and further demonstrates the potential to produce high efficiency photovoltaics using roll-to-roll fabrication methods.
This paper demonstrates the high reactivity and potentiality of bio‐renewably obtained cardanol‐based epoxy monomers as well as the possibility of tuning the final thermo‐mechanical properties, by changing either the epoxy content or the chemical structure of the starting photocurable resin. The reactivity of the cardanol monomers towards the cationic UV‐curing process was investigated by FT‐IR analysis and it was shown that a high epoxy group conversion was achieved. Furthermore, thermomechanical properties were investigated on crosslinked films by means of DSC and DMTA analysis. This article is protected by copyright. All rights reserved.
Perovskite solar cells with high power-per-weight have great potential to be used for aerospace applications such as satellites or high-altitude pseudo-satellites. These latter are unmanned aircrafts exclusively powered by solar energy, typically flying in the stratosphere where conditions of pressure, temperature and illumination are critically different than on earth surface. In this work, we evaluate the performance and stability of high efficiency perovskite solar cells under mimic stratospheric environment. In-situ measurements at controlled conditions of pressure, temperature and illumination were developed. We show that the cells can operate efficiently in a large range of temperatures from -50°C to +20°C, with a maximum power conversion efficiency at -20°C, which is ideal for use in the stratosphere. Besides, performances are maintained after a number of temperature cycles down to -85°C, representative of temperature variations due to diurnal cycles. An efficient encapsulation is developed, which could be critical to avoid the accelerated degradation of the cells under vacuum. Finally, a promising stability for 25 days of day-night cycles was demonstrated, which suggests that perovskite solar cells could be used to power high altitude pseudo-satellites.
The demonstration of photovoltaic devices with high power conversion efficiencies using low cost perovskite materials hints at the possibility of dramatically lowering the cost of solar energy. Key to further exploiting the potential of these materials is developing rapid processing techniques that can be used to deliver lower cost high throughput manufacture. This work details the development of low viscosity rapid drying perovskite formulations designed to give high quality solar films when slot-die coated on flexible roll-to-roll compatible substrates. A single step slot-die compatible perovskite ink based on an acetonitrile/methylamine solvent system utilizing a chloride additive is developed, resulting in large area perovskite films from slot-die coating under ambient conditions. The drying conditions for the perovskite film are optimized and fast (<10 min), low temperature (<120 °C) drying of slot-die coated films on flexible substrates are demonstrated and result in high performance devices.
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