Associate Professor Jing Yu (Tsinghua University), Professor Huajian Gao (Tsinghua University), and Dr. Quan Chen (Changchun Institute of Applied Chemistry, Chinese Academy of Sciences) recently developed a class of supramolecular elastomerswith high mechanical properties and efficient chemical recovery, called BNOSE, that are based on boron-nitrogen (B–N) and boron-oxygen (B–O) dynamic bonds. The dynamic B–N and B–O bonds in BNOSE provide robust interchain forces and degradation in mild ethanol solvents, resulting in a material with excellent mechanical properties and chemical recovery. Having a tensile strength of over 43 MPa and a toughness above 121 MJ/m³, BNOSE outperforms the vast majority of commercial elastomers and existing chemically recovered thermoplastic elastomers. BNOSE offers a sustainable solution without sacrificing mechanical performance, demonstrating potential in a variety of fields, such as soft robotics and flexible electronics. In addition, its scalable design approach can be extended to other polymer systems to meet the growing demand for recyclable high-performance materials. This work was published in CCS Chemistry.
Background:
Elastomers, or thermoplastics, are important basic materials for advancing various technologies in society. However, post-consumer waste continues to be largely disposed of through incineration or dumped in landfills, which not only wastes resources but also places a significant strain on the environment. Traditional thermoset elastomers, while strong and durable, are difficult to recycle due to their stable cross-linking network. Recyclable elastomers are often limited in their mechanical properties because of the instability of their chemical bonds. How to break through this bottleneck to achieve high mechanical properties and chemical recovery is the key to promoting the development of sustainable materials and related fields.
Highlights of this article:
In response to the aforementioned challenges, Profs. Yu Jing and Gao Huajian and Dr. Chen Quan developed BNOSE, a class of supramolecular elastomers based on boron-nitrogen (B-N) coordination and boron oxygen (B-O) dynamic bonding that exhibits excellent mechanical properties along with efficient chemical recovery. The polymer chains form reversible B-N bonds, enhancing the strength and toughness of the material, while the B-O bonds within the polymer chains can be depolymerized by adding ethanol to recover pyridinboric acid and polytetrahydrofuran (PTMEG) monomers (Figure 1). The method offers a simple, environmentally friendly, and more economical alternative to traditional recycling methods, such as metal catalysts or high-temperature degradation. With a tensile strength of over 43 MPa, an elongation of over 800%, and a toughness of 121.76 MJ/m³, BNOSE-Py4 materials successfully balance high mechanical properties and chemical recyclability, providing a novel approach for designing next generation sustainable polymer materials.
DFT calculations show that the boron-nitrogen coordination bond energy formed by 4-pyridine boric acid was greater than that of the structure formed by 3-pyridine boric acid coordination in the control group, BNOSE-Py3 and BNOSE-Ph systems, which provided theoretical support for the energy difference at the molecular level (Figure 2a) and was further verified experimentally upon testing the mechanical properties. Compared with the control BNOSE-Py3 and BNOSE-Ph systems, BNOSE-Py4 has a higher elastic modulus and tensile strength, while the former has an advantage in tensile rate due to weak coordination bond energy (Figure 2b).
Due to the low bonding energy of B-O bonds, hydrolysis or alcohol hydrolysis occurs with ease in the presence of nucleophiles. Submerging the material in ethanol overnight causes polymer degradation, which results from the reaction of the ethanol's hydroxyl OH with the B-O bonds. By lowering the pH of the system, the aqueous phase was separated to obtain pyridinborate monomer (Py4), and the oligomers recovered from the organic phase could further hydrolyze the carbamate group under alkaline conditions to recover polytetrahydrofuran monomers, with recoveries of 82% and 91%, respectively (Figure 3).
As as result of the characteristics of linear polymers, the BNOSE material system can be solvent recovered and remodeled. The multiple recycling methods are suitable for multi-platform application scenarios, such as magnetic response robots, temperature sensors, flexible electronic skins, etc., which is particularly important in gentle recycling required for valuable and sensitive components (Figure 4).
Summary and outlook:
The supramolecular elastomer, BNOSE, was successfully developed and combines high mechanical properties with gentle and efficient chemical recovery, breaking through current bottlenecks in the development of sustainable elastomers. The small difference in the chemical structure of BNOSE-Py4 and BNOSE-Py3 has a significant impact on the mechanical properties of the prepared materials, reflecting the importance of precise design. In the future, by expanding functional boric acid polymeric monomers, material performance is expected to be further optimized for additional function and value. More wide-spread use in high-performance sustainable materials is highly anticipated.
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About the journal: CCS Chemistry is the Chinese Chemical Society's flagship publication, established to serve as the preeminent international chemistry journal published in China. It is an English language journal that covers all areas of chemistry and the chemical sciences, including groundbreaking concepts, mechanisms, methods, materials, reactions, and applications. All articles are diamond open access, with no fees for authors or readers. More information can be found at https://www.chinesechemsoc.org/journal/ccschem .
About the Chinese Chemical Society: The Chinese Chemical Society (CCS) is an academic organization formed by Chinese chemists of their own accord with the purpose of uniting Chinese chemists at home and abroad to promote the development of chemistry in China. The CCS was founded during a meeting of preeminent chemists in Nanjing on August 4, 1932. It currently has more than 120,000 individual members and 184 organizational members. There are 7 Divisions covering the major areas of chemistry: physical, inorganic, organic, polymer, analytical, applied and chemical education, as well as 31 Commissions, including catalysis, computational chemistry, photochemistry, electrochemistry, organic solid chemistry, environmental chemistry, and many other sub-fields of the chemical sciences. The CCS also has 10 committees, including the Woman's Chemists Committee and Young Chemists Committee. More information can be found at https://www.chinesechemsoc.org/ .
