Diffusion- and Mobility-Controlled Self-Healing Polymer Networks with Dynamic Covalent Bonding

Remolding and Deconstruction of Industrial Thermosets via Carboxylic Acid-Catalyzed Bifunctional Silyl Ether Exchange

Convenient strategies for the deconstruction and reprocessing of thermosets could improve the circularity of these materials, but most approaches developed to date do not involve established, high-performance engineering materials. Here, we show that bifunctional silyl ether, i.e., R'O-SiR₂-OR'', (BSE)-based comonomers generate covalent adaptable network analogues of the industrial thermoset polydicyclopentadiene (pDCPD) through a novel BSE exchange process facilitated by the low-cost food-safe catalyst octanoic acid. Experimental studies and density functional theory calculations suggest an exchange mechanism involving silyl ester intermediates with formation rates that strongly depend on the Si-R₂ substituents. As a result, pDCPD thermosets manufactured with BSE comonomers display temperature- and time-dependent stress relaxation as a function of their substituents. Moreover, bulk remolding of pDCPD thermosets is enabled for the first time. Altogether, this work presents a new approach toward the installation of exchangeable bonds into commercial thermosets and establishes acid-catalyzed BSE exchange as a versatile addition to the toolbox of dynamic covalent chemistry.

Hyaluronic Acid-PEG-Based Diels-Alder In Situ Forming Hydrogels for Sustained Intraocular Delivery of Bevacizumab

Retinal diseases are the leading cause of visual impairment worldwide. The effectiveness of antibodies for the treatment of retinal diseases has been demonstrated. Despite the clinical success, achieving sufficiently high concentrations of these protein therapeutics at the target tissue for an extended period is challenging. Patients suffering from macular degeneration often receive injections once per month. Therefore, there is a growing need for suitable systems that can help reduce the number of injections and adverse effects while improving patient complacency. This study systematically characterized degradable "in situ" forming hydrogels that can be easily injected into the vitreous cavity using a small needle (29G). After intravitreal injection, the formulation is designed to undergo a sol-gel phase transition at the administration site to obtain an intraocular depot system for long-term sustained release of bioactives. A Diels-Alder reaction was exploited to crosslink hyaluronic acid-bearing furan groups (HAFU) with 4 arm-PEG10K-maleimide (4APM), yielding stable hydrogels. Here, a systematic investigation of the effects of polymer composition and the ratio between functional groups on the physicochemical properties of hydrogels was performed to select the most suitable formulation for protein delivery. Rheological analysis showed rapid hydrogel formation, with the fastest gel formation within 5 min after mixing the hydrogel precursors. In this study, the mechanical properties of an ex vivo intravitreally formed hydrogel were investigated and compared to the in vitro fabricated samples. Swelling and degradation studies showed that the hydrogels are biodegradable by the retro-Diels-Alder reaction under physiological conditions. The 4APM-HAFU (ratio 1:5) hydrogel formulation showed sustained release of bevacizumab > 400 days by a combination of diffusion, swelling, and degradation. A bioassay showed that the released bevacizumab remained bioactive. The hydrogel platform described in this study offers high potential for the sustained release of therapeutic antibodies to treat ocular diseases.

Humins Blending in Thermoreversible Diels-Alder Networks for Stiffness Tuning and Enhanced Healing Performance for Soft Robotics

Humins waste valorization is considered to be an essential pathway to improve the economic viability of many biorefinery processes and further promote their circularity by avoiding waste formation. In this research, the incorporation of humins in a Diels-Alder (DA) polymer network based on furan-maleimide thermoreversible crosslinks was studied. A considerable enhancement of the healing efficiency was observed by just healing for 1 h at 60 °C at the expense of a reduction of the material mechanical properties, while the unfilled material showed no healing under the same conditions. Nevertheless, the thermal healing step favored the irreversible humins polycondensation, thus strengthening the material while keeping the enhanced healing performance. Our hypothesis states a synergistic healing mechanism based on humins flowing throughout the damage, followed by thermal humins crosslinking during the healing trigger, together with DA thermoreversible bonds recombination. A multi-material soft robotic gripper was manufactured out of the proposed material, showing not only improved recovery of the functional performance upon healing but also stiffness-tunable features by means of humins thermal crosslinking. For the first time, both damage healing and zone reinforcement for further damage prevention are achieved in a single intrinsic self-healing system.

Hydrogen-Bond-Assisted Diels–Alder Kinetics or Self-Healing in Reversible Polymer Networks? A Combined Experimental and Theoretical Study

Diels–Alder (DA) cycloadditions in reversible polymer networks are important for designing sustainable materials with self-healing properties. In this study, the DA kinetics of hydroxyl-substituted bis- and tetrafunctional furans with bis- and tris-functional maleimides, both containing ether-functionalized spacers, is investigated by modelling two equilibria representing the endo and exo cycloadduct formation. Concretely, the potential catalysis of the DA reaction through hydrogen bonding between hydroxyl of the furans and carbonyl of the maleimides or ether of the spacers is experimentally and theoretically scrutinized. Initial reaction rates and forward DA rate constants are determined by microcalorimetry at 20 °C for a model series of reversible networks, extended with (i) a hydroxyl-free network and hydroxyl-free linear or branched systems, and (ii) polypropylene glycol additives, increasing the hydroxyl concentration. A computational density-functional theory study is carried out on the endo and exo cycloadditions of furan and maleimide derivatives, representative for the experimental ones, in the absence and presence of ethylene glycol as additive. Additionally, an ester-substituted furan was investigated as a hydroxyl-free system for comparison. Experiment and theory indicate that the catalytic effect of H-bonding is absent or very limited. While increased concentration of H-bonding could in theory catalyze the DA reaction, the experimental results rule out this supposition.

Designing End-of-life Recyclable Polymers via Diels-Alder Chemistry: A Review on the Kinetics of Reversible Reactions

The purpose of this review is to critically assess the kinetic behaviour of the furan/maleimide Diels-Alder click reaction. The popularity of this reaction is evident and still continues to grow, which is likely attributed to its reversibility at temperatures above 100°C, and due to its bio-based "roots" in terms of raw materials. This chemistry has been used to form thermo-reversible crosslinks in polymer networks, and thus allows the polymer field to design strong, but also end-of-life recyclable thermosets and rubbers. In this context, the rate at which the forward reaction (Diels-Alder for crosslinking) and its reverse (retro Diels-Alder for de-crosslinking) proceed as function of temperature is of crucial importance in assessing the feasibility of the design in real-life products. Differences in kinetics based from various studies are not well understood, but are potentially caused by chemical side groups, mass transfer limitations, and on the analysis methods being employed. In this work we attempt to place all the relevant studies in perspective with respect to each other, and thereby offer a general guide on how to assess their recycling kinetics. This review sheds light on the kinetics on the furan/maleimide Diels-Alder reaction. This popular reaction opens up a path to develop end-of-life recyclable polymer networks with self-healing properties. The factors affecting reaction kinetics are discussed, and the importance of accurate reaction kinetics in the context of polymer reprocessing is highlighted. This article is protected by copyright. All rights reserved.

Reversible hetero-Diels–Alder amine hardener as drop-in replacement for healable epoxy coatings

Replacing commercial hardeners with bio-sourced fatty acids linked by hetero Diels–Alder (HDA) motifs enabled epoxy-amine coatings with intrinsic self-healing properties. The HDA-based coatings demonstrate scratch healing at 95 °C within 15 min.

The Influence of the Furan and Maleimide Stoichiometry on the Thermoreversible Diels-Alder Network Polymerization

In recent work, the thermoreversible Diels–Alder reaction between furan and maleimide functional groups has been studied extensively in the context of self-healing elastomers and thermosets. To elaborate the influence of the stoichiometric ratio between the maleimide and furan reactive groups on the thermomechanical properties and viscoelastic behavior of formed reversible covalent polymer networks, a series of Diels–Alder-based networks with different stoichiometric ratios was synthesized. Differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and dynamic rheology measurements were performed on the reversible polymer networks, to relate the reversible network structure to the material properties and reactivity. Such knowledge allows the design and optimization of the thermomechanical behavior of the reversible networks for intended applications. Lowering the maleimide-to-furan ratio creates a deficit of maleimide functional groups, resulting in a decrease in the crosslink density of the system, and a consequent decrease in the glass transition temperature, Young’s modulus, and gel transition temperature. The excess of unreacted furan in the system results in faster reaction and healing kinetics and a shift of the reaction equilibrium.

Diels-Alder Cross-Linked, Washing-Free Hydrogel Films with Ordered Wrinkling Patterns for Multicellular Spheroid Generation

Three-dimensional (3D) multicellular spheroids are a new generation in vitro cell model, however, their applications are severely limited by difficulties in their generation. Here patterned poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogel films were synthesized for their generation. Instead of polymerization of HEMA monomers in the presence of a cross-linker, here the PHEMA films were synthesized by cross-linking furan-functionalized linear PHEMA, PHEMA-furan, and maleimide-functionalized linear PHEMA, PHEMA-mal, via Diels-Alder (DA) reaction between furan and maleimide groups. A thermal treatment temperature of 75 °C was chosen for the cross-linking reaction. The occurrence of DA reaction was confirmed by IR spectra. Using this method, cross-linked PHEMA films with smooth surface were successfully synthesized in situ in the well of cell culture plates. The films were then patterned by simply adding water to swell them. Highly ordered, honeycomb-like wrinkling patterns were successfully obtained by adjusting the furan and maleimide contents in the precursor linear polymers. The patterned hydrogel films were used to generate multicellular spheroids. Guided by the patterns, 3D spheroids with narrow size distribution, tunable size, and high cell viability were successfully obtained. The patterned PHEMA films reported here exhibited a lot of advantages. The patterning method was quite simple and required no template or special equipment. They were synthesized in situ in commercial cell culture plates. Particularly, thanks to the clean nature of the DA reaction, no low molecular weight monomer, cross-linker, initiator, or catalyst, which were potentially cytotoxic, was involved in the film synthesis, and no byproduct was produced and left in the film. The resulting films presented a high biocompatibility, allowing the avoidance of the tedious washing step. The films synthesized here were expected to have high potential for massive production of well-defined multicellular spheroids.

Biomimetic Boroxine-Based Multifunctional Thermosets via One-Pot Synthesis

Boroxine-based thermosets with remarkable mechanical tunability, self-healing ability, recyclability, and adhesive strength are of significant importance in various applications. However, complex multistep reactions are often required to prepare such thermosets. Herein, a facile one-pot approach to synthesize boroxine-based malleable thermosets is proposed. Random copolymers with pendant boronic acid groups were synthesized from alkenyl monomers containing boronic acids [4-vinylphenylboronic acid (4-VPBA), 3-vinylphenylboronic acid, or 3-acrylamidophenylboronic acid] and octadecanoxy polyethylene glycol methacrylate. Then, the as-prepared copolymers were cured to form thermosets with boroxine bonds. The tensile strengths of the thermosets were tailored to range from 9.3 to 27.5 MPa by increasing the concentration of 4-VPBA. Moreover, because of the reversible nature of dynamic boroxine bonds (transformation between boroxines and boronic acids) induced by water, the thermosets exhibit remarkable self-healing efficiency (up to 99%), tunable mechanical properties, and excellent recyclability. Additionally, the thermosets also demonstrate superior adhesive strength (as high as 73.9 MPa) on different substrates.

Self-Healing in Mobility-Restricted Conditions Maintaining Mechanical Robustness: Furan-Maleimide Diels-Alder Cycloadditions in Polymer Networks for Ambient Applications

Two reversible polymer networks, based on Diels–Alder cycloadditions, are selected to discuss the opportunities of mobility-controlled self-healing in ambient conditions for which information is lacking in literature. The main methods for this study are (modulated temperature) differential scanning calorimetry, microcalorimetry, dynamic rheometry, dynamic mechanical analysis, and kinetic simulations. The reversible network 3M-3F630 is chosen to study the conceptual aspects of diffusion-controlled Diels–Alder reactions from 20 to 65 °C. Network formation by gelation is proven and above 30 °C gelled glasses are formed, while cure below 30 °C gives ungelled glasses. The slow progress of Diels–Alder reactions in mobility-restricted conditions is proven by the further increase of the system’s glass transition temperature by 24 °C beyond the cure temperature of 20 °C. These findings are employed in the reversible network 3M-F375PMA, which is UV-polymerized, starting from a Diels–Alder methacrylate pre-polymer. Self-healing of microcracks in diffusion-controlled conditions is demonstrated at 20 °C. De-gelation measurements show the structural integrity of both networks up to at least 150 °C. Moreover, mechanical robustness in 3M-F375PMA is maintained by the poly(methacrylate) chains to at least 120 °C. The self-healing capacity is simulated in an ambient temperature window between −40 and 85 °C, supporting its applicability as self-healing encapsulant in photovoltaics.