Design of sustainable polymer networks for advanced applications based on dynamic covalent bonds

Synthetic polymers are one of the most important inventions of mankind, which are often categorized into two basic categories named thermoplastics and thermosets. Thermoplastics are linear polymer chains with the capacity to be reprocessed at high temperatures. Thermoset polymers, on the other hand, are made up of chemically crosslinked networks. Owing to their different topological structure, thermosets usually outperform thermoplastics in shape stability and creep resistance, but have worse reprocessing and recyclability. With the development of covalent combinatorial chemistry, vitrimers, a group of dynamic polymer network, were first introduced by Leibler et al. in 2011. As a class of new covalent associative networks (CANs), vitrimers provide a solution to combine the best of both worlds. By including an appropriate transesterification catalyst to polyester-based epoxy resin networks, the permanent networks exhibit a gradual viscosity decrease upon heating, similar to vitreous silica, which is why the term “vitrimer” was coined.In the last decade, vitrimers have drawn significant attention due to their recyclability, but they shouldn't be solely constrained to recycling thermosets in traditional plastic applications, like packaging, casting, and construction. Therefore, we have developed a library of sustainable dynamic polymer networks, vitrimers, for advanced applications. Owing to the synergy of dynamic covalent bond exchange reactions and employed function groups, a series of novel vitrimers were created to fit various applications, including water remediation, motion monitoring, energy harvesting and healable 3D printing

Robust Superamphiphilic Membrane with a Closed-Loop Life Cycle

Oil-spill remediation is an international environmental challenge, and superamphiphilic membranes, as a promising solution, have recently drawn lots of attention. However, the robustness of the conventional membrane design is less satisfying under severe conditions during practical applications. Additionally, it is unavoidable for the membranes to face a series of foulants in their practical working environment, for example, algae and sand. These foulants will block the membrane, which leads to a new economic and environmental problem in terms of waste management at the end of their life. To address the aforementioned challenges, a new generation of superamphiphilic vitrimer epoxy resin membranes (SAVER) to separate oil and water efficiently is reported. Similar to classical epoxy resins, SAVER shows strong mechanical robustness and sustains exposure to aqua regia and sodium hydroxide solutions. Furthermore, the blocked membrane can be easily recovered when contaminated with mixed foulants by using dynamic transesterification reactions in the polymer network. The ease with which biobased SAVER can be manufactured, used, recycled, and re-used without losing value points to new directions in designing a closed-loop superamphiphilic membrane life cycle

Tunable wettability of polymer films by partial engulfment of nanoparticles

A series of poly(methyl methacrylate) (PMMA) surfaces decorated by Cu nanoparticles (NP) with gradually varied morphology were prepared by high-pressure CO2 treatment at various time spans. Combining the characterizations of transmission electron microscopy (TEM) and atomic force microscopy (AFM), an accurate three-dimensional view of the morphology of the surfaces was presented. Subsequently, the wettability of the surfaces decreases near linearly with the increase of the apparent height of the decorating NPs in both static (static contact angle) and dynamic (contact angle hysteresis) aspects. The observed tendency contradicts to the Wenzel or Cassie-Baxter model and is explained by the contribution of nanomeniscus formed between the decorating NP and the flat substrate. The capillary pressure from this meniscus is negative and results in the increase of the contact angle with the apparent height (H-N) of the Cu NPs decorating the PMMA surface. In addition, the effect of the coverage (C-N) by NPs on the wettability can be explained on the same basis. Our experiment demonstrates the important influence of the nanomeniscus on the wettability, which is usually not taken into account. The results in this work provide a comprehensive understanding of how nanostructure affects the wettability of the decorated surfaces and shed light on how to obtain certain wettability through nanostructuring of the surface morphology

Novel MXene sensors based on fast healing vitrimers

Soft matter containing Ti3C2Tx MXenes exhibits promising potential in electromechanical sensor development. Current systems suffer from a decrease in sensibility up to complete breakdown due to small structural defects that will be generated during their longtime practical service. Various non-covalent hydrogel systems, based on hydrogen bonding and ionic coupling, have been employed to improve their durability related to their repairability. However, Ti3C2Tx MXenes are not stable in those networks, since they will be irreversibly oxidized in high humidity environment during practical application. Here, we report the use of a novel dynamic covalent bond based network – a MXene acrylate vitrimer network (MAVIN) with a low glass transition temperature, which can not only be repaired fast with high efficiency but also protects the MXenes in sensor applications from oxidation under working conditions. In addition, owing to the strong microwave absorptivity of Ti3C2Tx and of the flexible dynamic covalent bond network, a damaged MAVIN sensor can be repaired by microwave radiation with a high healing efficiency of 92.4% within 1 minute, which is as good as the best healing efficiency reported in literature so far but 30 times faster. With stability at a voltage of 3 V and fast healing demonstrated, MAVIN promises potential usage in reliable and sustainable strain sensors

Photopolymer Resins with Biobased Methacrylates Based on Soybean Oil for Stereolithography

The accessibility of renewable materials that are both sustainable and competitive is essential to accommodate the rapid growth in consumption of 3D printing materials. We have developed biobased photopolymer resins based on modified soybean oil for application in commercial stereolithography printers. First, soybean oil methacrylates with various functionalities were successfully synthesized from epoxidized soybean oil as an alternative to commercially available soybean oil acrylate. A library of photoresins was created by mixing up to 80% of the biobased (meth)acrylate oligomers with biobased diluents and a photoinitiator. The resin composition was optimized to achieve a maximum biobased content and a low viscosity. The manufactured parts demonstrated complete layer fusion and accurate print quality. Stiffness and toughness can be tuned by altering the chemical composition or the number of functional groups per oligomer. These biobased materials can be employed to reduce the environmental impact of additive manufacturing while being competitive with current fossil-based resins from commercial manufacturers

Biobased Photopolymer Resin for 3D Printing Containing Dynamic Imine Bonds for Fast Reprocessability

Acrylic photopolymer resins are widely used in stereolithographic 3D printing. However, the growing demand for such thermosetting resins is weighing on global issues such as waste management and fossil fuel consumption. Therefore, there is an increasing demand for reactive components that are biobased and enable recyclability of the resulting thermoset products. In this work, the synthesis of a photo-cross-linkable molecule containing dynamic imine bonds based on biobased vanillin and dimer fatty diamine is described. Using the biobased building blocks, formulations containing reactive diluent and a photoinitiator were prepared. The mixtures could be rapidly cross-linked under UV light, yielding vitrimers. Using digital light processing, 3D-printed parts were prepared, which were rigid, thermally stable, and reprocessed within 5 min at elevated temperature and pressure. The addition of a building block containing a higher concentration of imine bonds accelerated the stress relaxation and improved the mechanical rigidity of the vitrimers. This work will contribute to the development of biobased and recyclable 3D-printed resins to facilitate the transition to a circular economy.</p

Lipoic acid-based vitrimer-like elastomer

Dynamic covalent networks (DCNs) are materials that feature reversible bond formation and breaking, allowing for self-healing and recyclability. To speed up the bond exchange, significant amounts of catalyst are used, which creates safety concerns. To tackle this issue, we report the synthesis of a lipoic acid-based vitrimer-like elastomer (LAVE) by combining (i) ring-opening polymerization (ROP) of lactones, (ii) lipoic acid modification of polylactones, and (iii) UV crosslinking. The melting temperature (Tm) of LAVE is below room temperature, which ensures the elastic properties of LAVE at service temperature. By carefully altering the network, it is possible to tune the Tm, as well as the mechanical strength and stretchability of the material. An increase in polylactone chain length in LAVE was found to increase strain at break from 25% to 180% and stress at break from 0.34 to 1.41 MPa. The material showed excellent network stability under cyclic strain loading, with no apparent hysteresis. The introduction of disulfide bonds allows the material to self-heal under UV exposure, extending its shelf life. Overall, this work presents an environmentally friendly approach for producing a sustainable elastomer that has potential for use in applications such as intelligent robots, smart wearable technology, and human-machine interfaces.</p

Lipoic acid-based vitrimer-like elastomer

Dynamic covalent networks (DCNs) are materials that feature reversible bond formation and breaking, allowing for self-healing and recyclability. To speed up the bond exchange, significant amounts of catalyst are used, which creates safety concerns. To tackle this issue, we report the synthesis of a lipoic acid-based vitrimer-like elastomer (LAVE) by combining (i) ring-opening polymerization (ROP) of lactones, (ii) lipoic acid modification of polylactones, and (iii) UV crosslinking. The melting temperature (Tm) of LAVE is below room temperature, which ensures the elastic properties of LAVE at service temperature. By carefully altering the network, it is possible to tune the Tm, as well as the mechanical strength and stretchability of the material. An increase in polylactone chain length in LAVE was found to increase strain at break from 25% to 180% and stress at break from 0.34 to 1.41 MPa. The material showed excellent network stability under cyclic strain loading, with no apparent hysteresis. The introduction of disulfide bonds allows the material to self-heal under UV exposure, extending its shelf life. Overall, this work presents an environmentally friendly approach for producing a sustainable elastomer that has potential for use in applications such as intelligent robots, smart wearable technology, and human-machine interfaces.</p

Flexible Vitrimers for Self-healable Triboelectric Nanogenerators

Despite rapid advancements in the development of triboelectric nanogenerators (TENG), mechanical defects generated during their working condition limit the service lifetime of these devices. Polyester-based vitrimers can offer a solution to solve this issue via their dynamic bond exchange reactions. However, they often require high temperatures and a long time to crosslink, rendering them not useful for large-scale TENG fabrication. To solve these issues, a self-healable vitrimer-based triboelectric nanogenerator (VITENG) is developed based on diacrylate poly(dimethylsiloxane) (AA-PDMS) via a fast thiol-Michael reaction. By including 2,3-dihydroxypropyl methacrylate (DHPMA), hydroxy end dangling side chains are generated in the matrix, which increased the PDMS network's flexibility and transesterification efficiency. Efficient transesterification occurs on the abundant dynamic β-hydroxy ester bonds in the PDMS network, resulting in 100% efficient mechanical damage repair. The VITENG demonstrates a good performance with an output voltage of 135 V under a load of 10 N, demonstrating the potential for mechanical energy harvesters with long lifetimes and high design flexibility