Covalent adaptable networks (CANs), which can reconfigure on-demand under photo- or thermal stimuli, have recently been pursued as an alternative to the traditional thermosetting polymers. While these materials have demonstrated excellent recyclability and reprocessability, the majority of them reported to date are based on non-renewable resources. Meanwhile, material recycling highly counts on the collection system, and any materials that inevitably escape from the collection system will eventually go to the environment, challenging nature’s ability to break down these materials. Therefore, CAN materials that possess both recyclability and degradability are highly desirable. In this work, we seek to simultaneously address the recyclability, renewability, and degradability of CAN materials. Spiro diacetal building blocks are derived from bio-based benzaldehyde and erythritol and then subjected to the curing process using bio-based epoxy soybean oil as crosslinkers, yielding fully biobased CAN materials. Owing to the dynamic and degradable features of acetal motifs, our CAN materials exhibit both good recyclability and acid degradability, and the degraded products are reusable for preparation of new CANs. In addition, by tuning the steric hindrance adjacent to the reactive phenol site, we are able to control the mechanical properties of CANs using different bio-based benzaldehydes (vanillin, ethyl vanillin, and syringaldehyde). The outcome of the current research provides a strategy for the design of recyclable and degradable bio-based CANs, which will extend the development of CANs. 

Recyclable, Degradable, and Fully Bio-Based Covalent Adaptable Polymer Networks Enabled by a Dynamic Diacetal Motif

End-of-Life wind turbine blades: Review on recycling strategies

Vitrimers are being actively studied as sustainable plastics. To that end, robust vitrimer films fabricated from a ternary aqueous solution of tannic acid (TA), boric acid (BA), and polyvinyl alcohol (PVA) are reported herein. The trimeric vitrimer films include B–O–C dynamic covalent bonds. The optimal vitrimer film exhibits excellent crosslinking density (0.0471 mol/cm3), tensile strength (77 MPa), and modulus (3.2 GPa). A carbon–fiber-reinforced vitrimer (CFRV) was prepared with a vitrimer film and carbon fiber at temperatures above Tv. The CFRV (tensile strength, 570 MPa; modulus, 21 GPa) can be used as a structural composite material. Additionally, it retains the unique recyclability and reprocessability of the vitrimer. The solution-cast film can be reproduced from the vitrimer solution prepared during recycling, and a CFRV similar to the original can be recreated by remolding the recovered carbon fiber with the reproduced vitrimer film. The synthesized biobased high-strength vitrimer films can facilitate the development of sustainable carbon–fiber-composite materials. 

Robust Biobased Vitrimers and Its Application to Closed-Loop Recyclable Carbon Fiber-Reinforced Composites

Self-healing behavior of binder has important potential for promoting low-carbon service and extending service life of asphalt pavement. A novel self-healing polyurethane modified asphalt with dynamic covalent bonds was prepared in this work. The chemical structure, thermal-oxygen stability, rheological properties, fatigue properties and self-healing capability of polyurethane modified asphalt with dynamic covalent bonds were systemically investigated through Fourier infrared spectrum test, thermal gravimetric test, dynamic shear rheology test, linear amplitude sweep(LAS) test and multiple fatigue-healing-fatigue test. The result indicates disulfide dynamic covalent bond and oxime urethane dynamic covalent bond were successfully introduced into polyurethane cross-linking structure in asphalt, and the thermal-oxygen stability of asphalt has a significant increase after modification under the environmental conditions of construction and service. Rheological properties analysis suggests the introduction of dynamic covalent bonds is beneficial to improve the low-temperature toughness of polyurethane modified asphalt, and enhances its low-temperature cracking resistance. LAS analysis indicates the fatigue life of polyurethane modified asphalt can also be lengthened by over 70% at different strain levels(1.5%, 2.5% and 5%) with the introduction of dynamic covalent bonds, which is attributed to the excellent conformation adjustment ability and stress dissipation ability of dynamic cross-linking structure in asphalt. The multiple fatigue-healing-fatigue test shows the self-healing capability of polyurethane modified asphalt has an improvement with the introduction of dynamic covalent bonds, especially at the later stage with serious fatigue damage, and oxime urethane dynamic covalent bond shows the better contribution effect as compared with disulfide bond. This is due to the reversible fracture-rearrangement characteristics of dynamic covalent bonds. This work provides a new idea for improving the self-healing capacity and rheological properties of binder, reducing the frequency of pavement maintenance and extending the pavement service life. 

Preparation and characterization of polyurethane-modified asphalt containing dynamic covalent bonds

Polymeric hydrogel materials with multiple functions are in great demand in practical biomedical scenarios. In this work, a self-healing hydrogel with both antimicrobial properties was prepared using a strategy that combines dynamic imine and borate ester bonds. In this hydrogel, polyvinyl alcohol (PVA) is used as the base network, and borax solution as the cross-linking agent, and borate ester bonds can be formed between these two. Dialdehyde carboxymethyl cellulose (DCMC) was selected to cross-link with the amino groups in carboxymethyl chitosan (CMCS) and polyethyleneimine (PEI) to form dynamic imine bonds. The PVA/PEI/DCMC/CMCS hydrogels prepared by double chemical cross-linking have good mechanical properties (maximum tensile strength up to 289 KPa and strain at the break up to 1025%). Due to the uniqueness of the two chemical bonds, the hydrogel material is self-healing at room temperature without additional stimulation. In addition, the inherent antibacterial properties of the raw materials in this hydrogel confer antibacterial properties, with a kill rate of up to 99% against E. coli and S. aureus. The multifunctional hydrogels developed in this study provide more ideas and references for the future application of hydrogel materials in practical scenarios. 

Dual-network hydrogels based on dynamic imine and borate ester bonds with antibacterial and self-healing properties

Introducing dynamic covalent bonding into thermoset polymers has received considerable attention because they can repair or recover when damaged, thereby minimizing waste and extending the service life of thermoset polymers. However, most of the yielded dynamic covalent bonds require an extra catalyst, high temperature and high-pressure conditions to trigger their self-healing properties. Herein, we report on a catalyst-free bis-dynamic covalent polymer network containing vinylogous urethane and disulfide bonds. It is revealed that the introduction of disulfide bonds significantly reduces the activation energy (reduced from 94 kJ/mol to 51 kJ/mol) of the polymer system for exchanging and promotes the self-healing efficiency (with a high efficiency of 86.92% after being heated at 100 °C for 20 h) of the material. More importantly, the mechanical properties of the healed materials are comparable to those of the initial ones due to the special bis-dynamic covalent polymer network. These results suggest that the bis-dynamic covalent polymer network made of disulfide and inter-vinyl ester bonds opens a new strategy for developing high-performance vitrimer polymers.

/pdf/self-healable-covalently-adaptable-networks-based-on-6dvj61ll.pdf

Self-Healable Covalently Adaptable Networks Based on Disulfide Exchange

Room temperature-curable, easily degradable, and highly malleable and recyclable vanillin-based vitrimers with catalyst-free bond exchange

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Self-Healable Covalently Adaptable Networks Based on Disulfide Exchange

Room temperature-curable, easily degradable, and highly malleable and recyclable vanillin-based vitrimers with catalyst-free bond exchange