Xavier Fernández-Francos

Environmentally-friendly processing of thermosets by two-stage sequential aza-Michael addition and free-radical polymerization of amine-acrylate mixtures

A new dual-curing, solvent-free process is described for the preparation of tailor-made materials from off-stoichiometric amine–acrylate formulations. The first stage reaction is a self-limiting click aza-Michael addition between multifunctional amine and acrylate monomers with an excess of acrylate groups. The second stage reaction is a photoinduced radical polymerization of the unreacted acrylate groups. By selecting the structure of the monomers and the stoichiometry of the formulations, mechanical and thermal characteristics of the intermediate and final materials can be tuned. The materials obtained after the first curing stage can be gelled or ungelled and loosely or tightly crosslinked at the end of the second curing stage. The methodology used allows to obtain storable and processable intermediate polymers and final networks with optimum properties for different applications. The presence of amines in the reaction medium overcomes the intrinsic oxygen inhibition of acrylate free-radical polymerizations, resulting in a quasi complete cure.

Sequential curing of off-stoichiometric thiol-epoxy thermosets with a custom-tailored structure†

A new dual-curing system based on sequential thiol–epoxy click polycondensation and epoxy anionic homopolymerization was studied. Formulations of diglycidyl ether of bisphenol A and trimethylolpropane tris(3-mercaptopropionate) with 1-methylimidazole as a base catalyst and excess of epoxy groups were prepared and characterized. The curing process is sequential: fast thiol–epoxy polycondensation takes place first, followed by slower homopolymerization of excess epoxy groups. This makes it possible to define curing sequences with easy time–temperature control for both curing stages. The network build-up process during the first curing stage can be easily modelled assuming ideal polycondensation, which allows tailoring the structure and properties of the intermediate materials. The homopolymerization of the excess epoxy groups in the second curing stage results in a higher glass transition temperature (Tg) in comparison with the stoichiometric thiol–epoxy material, thus extending the application of thiol–epoxy thermosets to wider temperature ranges.

Preparation of click thiol-ene/thiol-epoxy thermosets by controlled photo/thermal dual curing sequence

A new sequential two steps photo and thermal process for the preparation of click thiol-ene/thiol-epoxy thermosets is described. Commercially available diglycidyl ether of bisphenol A (DGEBA), triallylisocyanurate (TAIC) and pentaerythritol tetrakis (3-mercaptopropionate) (PETMP) were combined to produce tailored materials with a 75, 50 and 25% of thiol-ene/thiol-epoxy networks. A photoinitiator was used to trigger the radical thiol-ene polymerization and a latent amine precursor was used to start the base-catalyzed thiol-epoxy click reaction. Neat thiol-ene and thiol-epoxy materials were prepared and taken as the references. The use of a latent amine precursor in adequate proportion and under suitable reaction conditions allowed us to reach a dual system with two well-defined steps, stable intermediate materials and well-controlled structure after the first curing stage and at the end of the curing process. This process overcomes some limitations observed in analogous curing systems reported previously such as the absence of latency for the second curing stage leading to unstable materials in the intermediate stage. Both chemical reactions were studied by FTIR and calorimetry. The latency of the different formulations was studied by DSC and rheometry. The materials prepared were characterized by thermal mechanical analysis and thermogravimetry.

Epoxy-Based Shape-Memory Actuators Obtained via Dual-Curing of Off-Stoichiometric “Thiol–Epoxy” Mixtures

In this work, epoxy-based shape-memory actuators have been developed by taking advantage of the sequential dual-curing of off-stoichiometric “thiol–epoxy” systems. Bent-shaped designs for flexural actuation were obtained thanks to the easy processing of these materials in the intermediate stage (after the first curing process), and successfully fixed through the second curing process. The samples were programmed into a flat temporary-shape and the recovery-process was analyzed in unconstrained, partially-constrained and fully-constrained conditions using a dynamic mechanical analyzer (DMA). Different “thiol–epoxy” systems and off-stoichiometric ratios were used to analyze the effect of the network structure on the actuation performance. The results evidenced the possibility to take advantage of the flexural recovery as a potential actuator, the operation of which can be modulated by changing the network structure and properties of the material. Under unconstrained-recovery conditions, faster and narrower recovery-processes (an average speed up to 80%/min) are attained by using materials with homogeneous network structure, while in partially- or fully-constrained conditions, a higher crosslinking density and the presence of crosslinks of higher functionality lead to a higher amount of energy released during the recovery-process, thus, increasing the work or the force released. Finally, an easy approach for the prediction of the work released by the shape-memory actuator has been proposed.

Thermal curing and photocuring of a DGEBA modified with multiarm star poly(glycidol)-b-poly(ε-caprolactone) polymers of different arm lengths

The influence of two multiarm star polymers, hyperbranched poly(glycidol)-b-poly(ε-caprolactone) of different arm lengths, on the thermal curing and the photocuring of a diglycidyl ether of bisphenol A epoxy resin (DGEBA) is studied. Star polymer with short arms PCL-10 decelerates more the thermal curing than the polymer with long arms PCL-30 because the latter is less solubilized in the epoxy matrix and its effect on the polymerization of the resin and the thermal–mechanical properties is less important. The kinetic triplet corresponding to the thermal curing of the different formulations has been determined. In the analysis of the photocuring process, we have also found that short-arm star PCL-10 is better solubilized in the epoxy matrix and its effect on the photocuring kinetics is more significant than that of the long-arm star. The effect of both polymers on the thermal–mechanical properties of the cured thermosets is less important due to the lower solubility at the relatively low photocuring temperatures.

Simultaneous Monitoring of Curing Shrinkage and Degree of Cure of Thermosets by Attenuated Total Reflection Fourier Transform Infrared (ATR FT-IR) Spectroscopy

We present a novel methodology to simultaneously monitor of the degree of cure and curing shrinkage of thermosetting formulations. This methodology is based on the observation of changes in the infrared absorption of reactive functional groups and the groups used as a standard reference for normalization. While the optical path length is exact and controlled in transmission infrared spectroscopy, in attenuated total reflection Fourier transform infrared (ATR FT-IR), the exact determination of volume changes requires the measurement of the refractive indices of the studied system throughout the curing process or at least an indirect parallel measurement of this property. The methodology presented here allows one to achieve quantitative measurements of the degree of cure and shrinkage for thermosets using in situ ATR FT-IR spectroscopy.

Analysis of the reaction mechanism of the thiol-epoxy addition initiated by nucleophilic tertiary amines

A kinetic model for thiol–epoxy crosslinking initiated by tertiary amines has been proposed. The kinetic model is based on mechanistic considerations and it features the effect of the initiator, hydroxyl content, and thiol–epoxy ratios. The results of the kinetic model have been compared with data from the curing of off-stoichiometric formulations of diglycidyl ether of bisphenol A (DGEBA) crosslinked with trimethylolpropane tris(3-mercaptopropionate) (S3) using 1-methylimidazole (1MI) as the initiator. The model has been validated by fitting the kinetic parameters to the experimental data under a variety of reaction conditions. In spite of the experimental uncertainty and model assumptions, the main features of the curing kinetics are correctly described and the reaction rates are quantitatively reproduced.

New Epoxy-Anhydride Thermosets Modified with Multiarm Stars with Hyperbranched Polyester Cores and Poly(ϵ-caprolactone) Arms

Multiarm star polymers with hyperbranched aromatic or aromatic-aliphatic cores and poly(ϵ-caprolactone) arms have been used as toughness modifiers in epoxy-anhydride formulations catalyzed by benzyldimethylamine. The curing process has been studied by dynamic scanning calorimetry, demonstrating little influence of the mobility of the reactive species and the hydroxyl content on the kinetics of this process. The obtained materials were characterized by thermal and mechanical tests and the microstructure by electron microscopy. Homogeneous thermosets have been obtained with a remarkable increase in impact strength without compromising glass transition temperature, thermal stability or hardness.

Thermal curing of an epoxy-anhydride system modified with hyperbranched poly(ethylene imine)s with different terminal groups

New hyperbranched polymers (HBP) have been synthesized by reaction of a poly(ethylene imine) with phenyl and t-butyl isocyanates. These HBPs have been characterized by 1H-NMR (nuclear magnetic resonance of hydrogen) and Fourier transform infrared spectroscopy. Their influence on the curing and properties of epoxy-anhydride thermosets has been studied by different techniques: differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and thermogravimetry (TG). The curing kinetics has been studied with DSC. Integral isoconversional method and the Šesták–Berggren model have been used to determine the activation energy and the frequency factor. The kinetic parameters are very similar for all the studied systems at the middle stage of the process, but changes are observed at the beginning and at the end of the process when these modifiers are used. The HBPs reduce the glass transition temperature of the cured materials. In addition, from the DMA analysis it can be seen that the HBP modifier obtained from phenyl isocyanate hardly changes the storage modulus, but the obtained ones from t-butyl isocyanate decrease it. TG analysis reveals a decrease in the onset temperature of the degradation process upon addition of the HBPs.

New understanding of the shape-memory response in thiol-epoxy click systems: towards controlling the recovery process

Our research group has recently found excellent shape-memory response in “thiol-epoxy” thermosets obtained with click-chemistry. In this study, we use their well-designed, homogeneous and tailorable network structures to investigate parameters for better control of the shape-recovery process. We present a new methodology to analyse the shape-recovery process, enabling easy and efficient comparison of shape-memory experiments on the programming conditions. Shape-memory experiments at different programming conditions have been carried out to that end. Additionally, the programming process has been extensively analysed in uniaxial tensile experiments at different shape-memory testing temperatures. The results showed that the shape-memory response for a specific operational design can be optimized by choosing the correct programming conditions and accurately designing the network structure. When programming at a high temperature (Txa0≫xa0Tg), under high network mobility conditions, high shape-recovery ratios and homogeneous shape-recovery processes are obtained for the network structure and the programmed strain level (εD). However, considerably lower stress and strain levels can be achieved. Meanwhile, when programming at temperatures lower than Tg, considerably higher stress and strain levels are attained but under low network mobility conditions. The shape-recovery process heavily depends on both the network structure and εD. Network relaxation occurs during the loading stage, resulting in a noticeable decrease in the shape-recovery rate as εD increases. Moreover, at a certain level of strain, permanent and non-recoverable deformations may occur, impeding the completion and modifying the whole path of the shape-recovery process.