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 (T » Tg), 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 (eD). 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 eD. Network relaxation occurs during the loading stage, resulting in a noticeable decrease in the shape-recovery rate as eD 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.Postprint (author's final draft

Laboratory 3.0: manufacturing technologies laboratory virtualization with a student-centred methodology

This paper presents a blended-learning strategy for improving the teaching method applied in the laboratory subject Manufacturing Technologies. The teaching method has been changed from a predominantly teacher-centred to an active learning system with a student-centred focus and e-learning activities. In face-to-face classes, a game-based learning platform has been used. This methodology ensured engaging classes at the same time that provided a useful live feedback for students and teachers. The virtualization of the laboratory was achieved by two different e-learning activities, self-assessment tasks and video clips. These e-learning tools have been used not only to improve the students’ learning but also to enhance their motivation. The results from academic outputs show a significant improvement after the new blended learning method is applied. Moreover, a student satisfaction survey shows the positive impact of the methodology on the students’ engagement and motivationPeer Reviewe

Laboratori 3.0: virtualització del laboratori amb una metodologia centrada en l'estudiant

This paper presents an improvement introduced to a laboratory subject by means of a student - centered blended - learning teaching strategy. The implemented virtual tools (videos and questionnaires) help to prepare the practical sessions and allow the sel f - assessment before and after each practical session. Students have shown a great satisfaction with the method. The analysis of the qualifications obtained has allowed an assessment of the degree of correlation between the different techniques used.Postprint (published version

Novel hybrid organic/inorganic poly(thiourethane) covalent adaptable networks

Organic-inorganic hybrid materials combine the advantages of both phases: hardness and strength of inorganic phase and elasticity and toughness of the organic matrix. In the present study, we have prepared nanocomposites with a poly(thiourethane) polymeric matrix and silsesquioxane-type structures, with thiols as reactive groups (POSS-A or POSS-B, synthesized in different pressure conditions), looking for a covalent interaction between both phases, and good dispersion. Due to the click behavior of the reaction between the isocyanate and the thiol groups, highly homogeneous materials are obtained. Both monomers, catalyst (dibutyltin dilaurate, DBTDL), and the POSS precursor (3-mercaptopropyl trimethoxysilane, MPTMS), are commercially available, which present the advantage of being industrially scalable. The incorporation of POSS leads to an increase in glassy and rubbery storage moduli and the temperature of the maximum of tan delta curve. The vitrimeric behavior of the poly(thiourethanes) improved with the POSS incorporation, getting lower relaxation times. With a higher proportion of closed cages, POSS-B leads to the most significant improvements. All the materials prepared showed high transparency and the fracture of POSS modified materials indicates an improved toughness.This work is part of the R & D projects PID2020-115102RB-C21 and PID2020-115102RB-C22 funded by MCINAEI/10.13039/501100011033. We acknowledge these grants and the Generalitat de Catalunya (2017-SGR-77 and BASE3D) . The authors declare the following financial interests/personal re-lationships which may be considered as potential competing interests: Angels Serra reports financial support was provided by Spain Ministry of Science and Innovation

Preparation and Characterization of a Series of Self-Healable Bio-Based Poly(thiourethane) Vitrimer-like Materials

A series of poly(thiourethanes) (PTUs) from biobased monomers have been synthesized. Limonene and squalene were transformed into polyfunctional thiols by thiol-ene reaction with thioacetic acid and further saponification. They were then reacted in different proportions with hexamethylene diisocyanate (HDI) in the presence of a catalyst to prepare bio-based poly(thiourethane) vitrimer-like materials. The different functionalities of squalene and limonene thiols (six and two, respectively) allow for changing the characteristics of the final material by only varying their relative proportions in the reactive mixture. The proportions of thiol and isocyanate groups were stoichiometric in all the formulations tested. An acidic and a basic catalyst were tested in the preparation of the networked polymers. As the acidic catalyst, we selected dibutyltin dilaurate (DBTDL), and as the basic catalyst, a tetraphenylborate salt of 1,8-diazabicyclo(5.4.0)undec-7-ene (BGDBU), which has the advantage of only releasing the base at high temperatures. The materials obtained were characterized by thermogravimetry and thermomechanical analysis. The vitrimeric-like behavior was evaluated, and we could see that higher proportions of the limonene derivative in the formulations led to faster stress relaxation of the material. The use of the base catalyst led to a much shorter relaxation time. The materials obtained demonstrated good self-healing efficiencyPeer ReviewedPostprint (published version

New bio-based materials obtained by thiol-ene/thiol-epoxy dual curing click procedures from eugenol derivates

Novel bio-based and dual-curable thermosets were prepared from eugenol derivatives. The curing sequence combined two click reactions, a photoinduced radical thiol-ene reaction followed by a thermally activated thiol-epoxy reaction. Eugenol was transformed into a triallyl (3A-EU) and a diallyl glycidyl derivative (2AG-EU) with high yields, and they were used as starting monomers in order to study the thiol-ene reaction and the dual-curing process, respectively. Three different thiol crosslinkers were tested, one commercially available tetrathiol derived from pentaerythritol (PETMP) and two other that were also synthesized: a trithiol derived from eugenol (3SH-EU) and a hexathiol derived from squalene (6SH-SQ). FTIR and DSC were used to monitor both curing stages and analyze the obtained materials. The results evidenced the occurrence of side reactions that led to incomplete thiol-ene reaction. The dual-curable materials showed higher Tgs than the materials obtained by a simple thiol-ene process and presented higher mechanical and thermomechanical performance.Postprint (author's final draft

Motion control in free-standing shape-memory actuators

In this work, free-standing shape-memory thermally triggered actuators are developed by laminating 'thiol-epoxy'-based glassy thermoset (GT) and stretched liquid-crystalline network (LCN) films. A sequential curing process was used to obtain GTs with tailored thermomechanical properties and network relaxation dynamics, and also to assemble the final actuator. The actuation extent, rate and time were studied by varying the GT and the heating rate in thermo-actuation with an experimental approach. The results demonstrate that it is possible to tailor the actuation rate and time by designing GT materials with a glass transition temperature close to that of the liquid-crystalline-to-isotropic phase transition of the LCN, thus making it possible to couple the two processes. Such coupling is also possible in rapid heating processes even when the glass transition temperature of the GT is clearly lower than the isotropization temperature of the LCN, depending on the network relaxation dynamics of the GT and the presence of thermal gradients within the actuators. Interestingly, varying the GT network relaxation dynamics does not affect the actuation extent. As predicted by the analytical model developed in our previous work, the modulus of the GT layer is mainly responsible for the actuation extent. Finally, to demonstrate the enhanced control of the actuation, specifically designed actuators were assembled in a three-dimensional actuating device able to make complex motions (including 'S-type' bending). This approach makes it possible to engineer advanced functional materials for application in self-adaptable structures and soft robotics.Postprint (author's final draft

Vitrimeric Epoxy-Amine Polyimine Networks Based on a Renewable Vanillin Derivative

A series of bio-based polyimine vitrimers was obtained and characterized. A diimine-diglycidyl monomer (DIDG) was synthesized by condensing vanillin with 4,4'-oxydianiline (ODA) and further glycidylation with epichlorohydrin. This compound was cross-linked with three different Jeffamines (Jeff230, JeffD400, and JeffT403) with a different number of poly(propylene glycol) units in their structure and different functionalities. Trimethylolpropane triglycidyl ether (TMPTE) was added to the formulation to improve the thermal, mechanical, and thermomechanical properties. All of the materials prepared showed Tg’s above 66 °C, good vitrimeric behavior being the maximum relaxation rate reached by the material prepared from JeffD400, which also allows the most extensive degradation when treated with an acidic aqueous solution. These polyimine vitrimers can entirely relax the stress in less than 10.5 min at 150 °C without any added catalyst. All of the materials prepared could be satisfactorily recycled up to 200 °C also presenting an excellent self-welding ability.Peer ReviewedPostprint (published version

The use of lanthanide triflates in the preparation of poly(thiourethane) covalent adaptable networks

Covalent adaptable networks (CANs) are new polymeric materials with the mechanical properties of thermosets and the possibility of being recycled like thermoplastics. Poly(thiourethane) networks have demonstrated vitrimeric-like behavior at high temperatures due to the trans-thiocarbamoylation process, which Lewis acids and bases can accelerate. In this study, we report the use of lanthanide triflates (La, Sm, Dy, Er, and Yb) as Lewis acid catalysts, a greener alternative to other metallic catalysts as dibutyltin dilaurate (DBTDL) widely used in poly (urethane) and poly(thiourethane) networks. Moreover, they are not as reactive as DBTDL, and the curing mixture can be manipulated for a longer time at room temperature. As monomers, trimethylolpropane tris(3- mercapto propionate) (S3), hexamethylene diisocyanate (HDI), and isophorone diisocyanate (IPDI) have been used. We have demonstrated that the materials prepared with lanthanum triflate present the lowest relaxation times than those prepared with other lanthanide triflates or DBTDL. Calorimetry (DSC) and infrared spectroscopy (FTIR) were applied to study the curing process. The materials obtained were fully characterized by thermog- ravimetric analysis (TGA) and thermomechanical tests (DMA).This work is part of the R&D projects PID2020-115102RB-C21 and PID2020-115102RB-C22 funded by MCNI/AEI/10.13039/ 501100011033. We acknowledge these grants and to the Generalitat de Catalunya (2021-SGR-00154 and BASE3D). F.G. thanks to MCNI/AEI for the grant PRE2018-084192.Postprint (published version

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 buildup 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 (T-g) in comparison with the stoichiometric thiol-epoxy material, thus extending the application of thiol-epoxy thermosets to wider temperature ranges.Postprint (published version