Design and analysis of dual-curing systems

Dual-curing processing is a method to prepare thermoset materials through two polymerization reactions carried out simultaneously or sequentially. In these processes, a firm understanding of reaction mechanisms enables the design of catalytic systems to control reaction kinetics and to impart sequentiality to the curing reactions. Material properties at different curing stages are dictated by the choice of monomers. Furthermore, by employing click-based approaches, eco-friendly and efficient dual-curing processes can be designed. In this thesis, a number of sequential dual-curing systems were prepared and characterized. Sequentiality was either intrinsic due to the monomers used or it was achieved by employing latent catalysts. The dual-curing systems were designed with an objective of improved physical and mechanical properties of the fully cured materials. The majority of the monomers were processable through click reactions, although a few processes did not strictly fit click criteria. In terms of the characterized properties, this did not pose any shortcoming. Due to the limited number of related publications, the thiol-epoxy reaction was investigated in more detail. Accurate phenomenological and mechanistic models of reaction kinetics were developed to study reaction kinetics in and out of dual- curing context. For reactive latency, a new family of photobase generators (PBGs) were developed. As the name implies, these PBGs liberated base catalysts upon UV irradiation. The possibility of thermal initiation of some of these PBGs was also demonstrated. Storage stabilities of uncured and partially-cured (i.e. intermediate) materials were significantly improved since PBGs allowed temporal control over curing stages. In some dual-curing systems, step-wise click polymerizations such as Michael additions were combined with chain-wise homopolymerizations such as acrylate photopolymerizations. In these systems, the initial step-growth proces delivered intermediate materials with desirable properties such as polymer network homogeneity, high gel point conversion, and low polymerization shrinkage. The chain-wise process was carried out as a second curing stage, at the end of which final materials were obtained with increased crosslinking density, hardness and Tg. In all dual-curing systems presented here, final materials had significantly improved properties compared to intermediate materials, regardless of the nature of the curing processes. In one part of the project, a new set of catalyst comonomers were designed. These comonomers, which were also prepared using click-based procedures, had pendant allyl functionalities and wielded tertiary amine groups in their structure. The tertiary amines catalyzed a thiol-acrylate reaction carried out as a first curing stage in a dual-curing system. Later, as a second curing stage, the pendant allyl groups of the comonomers participated in thiol-ene polymerizations with the excess thiols initially present in the formulation, thereby getting incorporated into the final polymer network. The dual-curable materials developed here can be used in diverse applications ranging from high-performance adhesives, to rigid shape-memory materials. As a matter of fact, a preliminary demonstration of these two applications is provided. Prospectively, the materials presented here could benefit from a more detailed characterization in the context of specific applications. Without a doubt, such an effort would increase the possibility of successful commercialization of these formulations.El curat dual és una metodologia utilitzada per preparar materials termoestables mitjançant la combinació de dos processos de polimerització que poden tenir lloc de forma simultània o seqüencial. Conèixer en profunditat els mecanismes de reacció que regeixen aquests processos és l'eina clau per al disseny de sistemes catalítics que permetin controlar la cinètica de la reacció i aconseguir la seqüencialitat de les etapes de curat. Les propietats del material en les diferents etapes de curat estan determinades per l'elecció dels monòmers. A més, la utilització de reaccions de tipus click permet dissenyar processos de curat més ecològics i eficients. En aquesta Tesi, es van preparar i caracteritzar diferents sistemes de curat seqüencials. La seqüencialitat en el curat es va aconseguir amb la selecció adequada de monòmers o mitjançant l'ús de catalitzadors latents. Els sistemes de curat dual es van dissenyar també amb l'objectiu de millorar les propietats físiques i mecàniques dels materials completament curats. La majoria dels monòmers van ser curats mitjançant reaccions de tipus click. Encara que algunes de les etapes de curat no s'ajustaven estrictament als criteris acceptats per les reaccions click, això no va suposar un problema pel que fa a les propietats dels materials preparats. La cinètica de reacció tiol-epoxi es va investigar amb més detall, atès el nombre limitat de publicacions existents sobre aquest tema. Es van desenvolupar models fenomenològics i mecanístics avançats per estudiar la cinètica d'aquesta reacció dins i fora del context del curat dual. Per aconseguir sistemes latents, es va desenvolupar una nova família de fotobases latents. Com el seu nom indica, aquestes fotobases, després de ser irradiades mitjançant llum UV, alliberen una base que actua com a catalitzador del curat. També es va demostrar la possibilitat d'activar tèrmicament les fotobases. La utilització d'aquestes va permetre el control temporal de les etapes de curat, augmentant significativament l'estabilitat durant l'emmagatzematge dels materials no curats i parcialment curats, després de la primera etapa. En alguns sistemes duals, es van combinar polimeritzacions click per etapes com ara les addicions de Michael amb homopolimeritzacions en cadena tals com la fotopolimerització d'acrilats. En aquest tipus de sistemes, la polimerització per etapes en la primera etapa de curat permet obtenir materials intermedis amb propietats desitjables com ara alta homogeneïtat i conversió a la gelificació i baixa contracció durant el curat. La polimerització en cadena que té lloc durant la segona etapa de curat, va permetre obtenir materials finals amb major densitat d'entrecreuament, duresa i temperatura de transició vítria. Independentment de la naturalesa dels processos de curat utilitzats, tots els materials finals van presentar propietats tèrmiques i mecàniques significativament millorades en comparació amb els materials intermedis. En una part d'aquesta Tesi, es va dissenyar una nova família de comonòmers que actuaven simultàniament com a catalitzadors. Aquests comonòmers, que també es van preparar mitjançant procediments basats en la química click, tenien funcionalitats alíliques terminals i amines terciàries en la seva estructura generades durant la seva síntesi. Aquestes amines van actuar com a catalitzadors de la primera etapa de curat tiol-acrilat en un sistema de curat dual. En la segona etapa de curat, els grups al·lil terminals dels comonòmers van reaccionar amb grups tiols en excés presents en la formulació mitjançant una reacció tiol-è fotoinduïda. Els materials preparats mitjançant curat dual en aquest treball es poden utilitzar en una àmplia gamma d'aplicacions avançades que van des d'adhesius d'alt rendiment fins a materials amb memòria de forma

Latent curing of epoxy-thiol thermosets

Epoxy-thiol curing is a click reaction which allows quantitative yield of the end products. The base-catalyzed reaction is rapid at low temperatures so it is most often desirable to harness reactivity by using latent catalysts. In this work, we used triazabicyclodecene tetraphenylborate (TBD·HBPh4) as a photobase generator (PB). We activated the PB either thermally or by UV light and monitored reaction kinetics by DSC and FTIR methods. Depending on the catalytic system used, the rate of the thiol-epoxy reaction was ordered as follows: Neat base > UV activated PB > thermally activated PB > uncatalyzed system. A series of isothermal and non-isothermal DSC experiments were run on non-irradiated and irradiated samples in order to study the effect of PB content and UV irradiation duration on PB activation efficiency and latency/storage stability. The data from DSC were analyzed using model-free linear isoconversional methods to estimate kinetic parameters such as activation energies. In addition, the kinetics data for both activation methods were shown to be accurately represented by multi-term Kamal models. The storage stability of the systems were studied at room temperature and was shown to fit well to the predictions of the kinetic model.Postprint (author's final draft

Sequential curing of amine-acrylate-methacrylate mixtures based on selective aza-Michael addition followed by radical photopolymerization

Dual curing systems find various uses in industry with the process flexibility they provide which allows tailoring properties at different curing stages in accordance with application requirements. A safe and efficient dual curing scheme is proposed here for a set of mixtures containing different proportions of acrylates and methacrylates. The first curing stage is a stoichiometric aza-Michael addition between acrylates and an amine, followed by photo-initiated radical homopolymerization of methacrylates and remaining acrylates. An analysis of aza-Michael reaction kinetics confirmed that amines react selectively with acrylates, leaving methacrylates unreacted after the first curing stage. It was found that acrylate-rich mixtures achieve complete global conversion at the end of the scheme. However, the highest crosslinking density and thermal resistance was observed in a methacrylate-rich formulation. The resulting materials show a wide range of viscoelastic properties at both curing stages that can be tailored to a variety of industrial application needs.Postprint (author's final draft

Acetoacetate based thermosets prepared by dual-Michael addition reactions

A novel set of dual-curable multiacetoacetate-multiacrylate-divinyl sulfone ternary materials with versatile and manipulable properties are presented. In contrast to common dual-curing systems, the first stage polymer herein consists of a densely crosslinked, high Tg network as a result of base-catalyzed multiacetoacetate-divinyl sulfone Michael addition. A more flexible secondary network forms after base-catalyzed Michael addition of remaining multiacetoacetate to multiacrylate. Curing is truly sequential as the rates of the two Michael additions are significantly different. Curing kinetics were analyzed using differential scanning calorimetry (DSC) and Fourier-transform infrared (FTIR). The materials at each curing stage were characterized using dynamic mechanical analysis (DMA) and SEM. Although some phase separation was observed in certain formulations, the incompatibilities were minimized when the molar percentage of the acetoacetate-divinyl sulfone polymer network was above 75%. Furthermore, the environmental scanning electron microscopy (ESEM) images of these materials show that the more flexible acetoacetate-acrylate phase is dispersed in the form of polymeric spheres within the rigid acetoacetate-divinyl sulfone matrix. This unique dual microstructure can potentially render these materials highly resilient in applications requiring densely crosslinked polymer architectures with enhanced toughnesPostprint (published version

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.Postprint (published version

Structural design of CANs with fine-tunable relaxation properties: a theoretical framework based on network structure and kinetics modeling

In this work, we present a model capable of reproducing the stress relaxation dynamics of a wide range of relaxation processes in covalent adaptable networks (CANs) produced by stepwise polymerization. The proposed model captures the effective elastic response of the material subject to an initial stress by analogy with a network decrosslinking process. The combination of a recursive structural model and a kinetic model for the bond exchange reaction makes it possible to predict the expected stress relaxation profile in simple and complex systems depending on the structure of the network, the rate of bond exchange of the different components, and the presence of permanent bonds. After the basic features of the model are analyzed, its prediction capabilities are validated by simulating a number of scenarios taken from the literature. The results show that tailoring of the network architecture enables unprecedented flexibility in the design of CAN-based materials.This work was funded by the Spanish Ministry of Science and Innovation (MCIN/AEI/10.13039/501100011033) through R&D project PID2020-115102RB-C22 and also by Generalitat de Catalunya (2017-SGR-77). X. Fernández-Francos and O. Konuray acknowledge the Serra-Hunter ́ programme (Generalitat de Catalunya).Postprint (published version

Epoxy Doped, Nano-scale Phase-separated Poly-Acrylates with Potential in 3D Printing

An efficient method to improve the mechanical performance of a commercially available photocure resin is described wherein the resin is modified with a mixture of a cycloaliphatic epoxy and an anhydride curing agent. Photocured samples are thermally treated in a subsequent step to cure the epoxy to obtain an interpenetrated polymer network (IPN) and also complete reaction of the acrylate monomers remaining from the photocure. The latter is accomplished by a thermal radical initiator added earlier into the formulation together with the epoxy-anhydride. The thermal properties and microstructure of the resulting IPN are analyzed. Uniform and quantitative conversions are obtained, with glass transition temperatures comparable to conventional epoxies. The liquid, uncured samples containing different amounts of epoxy are stable at 30 °C for several weeks. In the fully cured epoxy-rich materials, nano-scale phase separation is observed by atomic force microscopy. This is corroborated by the existence of multiple relaxations determined by dynamic mechanical analysis analysis. Specimens from a formulation containing 50% by weight of epoxy-anhydride are 3D printed in a customized masked image processing stereolithography, thermally treated, and are subjected to compression tests. Results show that Young's modulus increases by 900% over the neat resinPeer ReviewedPostprint (published version

Recyclable dual-curing thiol-isocyanate-epoxy vitrimers with sequential relaxation profiles

Two series of poly(thiourethane)s-poly(thiol-epoxy) hybrids were prepared from dual-curing, stoichiometric thiol:isocyanate:epoxy mixtures. 1-methylimidazole (MI) was used as initiator for the thiol-isocyanate and thiol-epoxy curing reactions. A salt of tetraphenylborate derived from highly basic amine 1,5,7 triazabicyclo [4,4,0]dec-5-ene (TBD) was used as catalyst for the activation of bond exchange reactions. The curing kinetics and the glass transition temperature (Tg) of the samples were studied by differential scanning calorimetry (DSC). Thermal stability was characterized by thermogravimetric analysis (TGA). Stress relaxation dynamics related to the bond exchange reactions were studied by dynamic mechanical analysis (DMA). Recyclability of the fully cured samples according to the DMA results was carried out by hot-pressing at elevated temperatures. The results show that the curing process takes place in a controlled and sequential way: the thiol-isocyanate reaction takes place first, at moderate temperatures, followed by the thiol-epoxy reaction, which is completed at higher temperatures. The analysis of stress relaxation evidences a complex two-step relaxation behavior depending on the contribution of isocyanate and epoxy groups to the mixture. Isocyanate-rich materials show a simple relaxation process corresponding to the trans-thiocarbamoylation exchange reactions. Epoxy-rich materials show, in contrast, a two-step relaxation processes evidencing that trans-thiocarbamoylation alone may not be sufficient to relax the stress completely, due to an apparently permanent network structure. However, complete stress relaxation is possible for epoxy-rich materials through additional bond exchange reactions such as transesterification or dynamic thiol-Michael, but at a slower rate. Depending on the composition and the temperature, full recycling of the material can be achieved at moderate temperatures in a timescale of minutes to hours.This work was funded by the Spanish Ministry of Science and Innovation (MCIN/AEI/10.13039/501100011033) through R&D project PID2020-115102RB-C22, and also by Generalitat de Catalunya (2021- SGR-00154), Spain. X. Fernández-Francos and O. Konuray acknowledge the Serra-Húnter programme (Generalitat de Catalunya).Peer ReviewedPostprint (published version

Kinetics analysis and simulation of sequential epoxy dual-curing systems with independent thermal activation

The curing kinetics of a sequential dual-curing system based on an off-stoichiometric amine-epoxy formulation with intermediate latent reactivity has been analyzed. The first curing stage is an epoxy-amine polycondensation taking place at low temperatures, while the second curing stage is an anionic homopolymerization of the excess epoxy groups, taking place at high temperatures and catalyzed by a latent base. The different reactivity of both polymerization processes allows an excellent separation into well-defined curing stages each of which can be analyzed individually. The kinetics of the two curing stages have been analyzed by integral isoconversional procedures and model-fitting methods. Both methodologies successfully simulated each curing stage and also the global curing process, showing that it is possible to control the activation of both curing stages. Isoconversional integral analysis is a simple yet powerful method that can be used for the simulation of temperature-controlled curing programmes. Model-fitting analysis is more suitable for the flexible simulation of processing scenarios such as the curing of composites.Postprint (author's final draft

Sequential curing of thiol-acetoacetate-acrylate thermosets by latent Michael addition reactions

Thiol-acetoacetate-acrylate ternary dual-curing thermosets were prepared by a sequential process consisting of thiol-Michael addition to acrylates at room temperature followed by Michael addition of acetoacetates to acrylates at moderately elevated temperature. The curing sequence can be controlled with the help of the different acidities of the protons on thiol and acetoacetate groups, the favorable pKa of the base used as catalyst and the self-limiting character of Michael additions. The latency of the curing steps can be regulated by selection of the right catalysts, temperature and curing conditions. The properties of the intermediate and final materials can be tuned by changing the structure of the monomers and the contribution of both Michael addition reactions.Postprint (author's final draft