Search

Covalent adaptable networks (CANs) possess multiple functions including reprocessing (or recyclability), self‐healing, welding, shape shifting, 3D printing, etc., due to the network rearrangement from dynamic bonds, and favorable performance from their cross‐linked feature, and they are supposed to be as sustainable alternatives to thermosets. However, the thermal and mechanical properties, and stability of CANs are often sacrificed for rapid network rearrangement. In this paper, fast‐reprocessing CANs with high performance are facilely constructed by in situ polymerization and dynamic cross‐linking of styrene (St), maleic anhydride (MA), and acetal diol (BHAD). The rigid and hydrophobic polymer backbone endow the materials with high glass transition temperatures, mechanical performance, and water resistance. Besides, carboxylic group‐catalyzed dual dynamic ester and acetal‐based networks exhibit faster stress relaxation and realize extrusion reprocessing. This work provides an ingenious and simple strategy of construction of CANs combining rapid network rearrangement and excellent comprehensive performance, which is beneficial for the application of CANs. [ABSTRACT FROM AUTHOR]

Covalent adaptable networks (CANs) exhibit recyclability such as reprocessing, but it's a challenge to address the contradiction between reprocessing rate and performance. Here, pendent aliphatic chain anhydride monoesters are innovatively introduced into epoxy CANs based on transesterification, which efficiently accelerates the reprocessing without sacrificing thermal and mechanical properties. The transesterification rate is raised on account of the flexible aliphatic chain‐promoted segment movement and dynamic transfer auto‐catalysis. When the carbon number reflecting the length of the pendent chain is 12, the epoxy CAN exhibits the fastest stress relaxation or reprocessing. Computation via molecular dynamics simulation demonstrates that the increased segmental mobility from the pendent aliphatic chains contributes to enhanced reprocessability. Interestingly, the crystallization of the pendent aliphatic chains maintains or even improves the thermal and mechanical properties. Thus, introducing a flexible and crystallizable aliphatic side chain is an innovative and efficient approach to accelerate dynamic reactions and network arrangement while improving performance. [ABSTRACT FROM AUTHOR]

Covalent adaptable networks (CANs) are a new type of polymers, which possess excellent performance of thermosets and reprocessability of thermoplastics. Nevertheless, it is still a challenge to realize rapid reprocessing and postadjusting (adjust properties after preparation). Herein, for the first time, a method of combining Schiff base and Diels–Alder adduct in one network is developed to achieve rapid reprocessing and postadjusting. Through the dissociation of the Diels–Alder adduct at high temperatures, the cross‐link densities of the networks are reduced, thereby accelerating the rearrangement of the networks and realizing the rapid reprocessing and self‐healing. Moreover, the reconnecting degree of network after dissociation of Diels–Alder adduct can be easily controlled by annealing; as a result, the properties of the obtained CANs are postadjustable. This work provides a simple and promising approach of achieving excellent reprocessing and postadjusting for CANs via the synergism of an associative dynamic chemistry with a dissociative dynamic chemistry. [ABSTRACT FROM AUTHOR]

Polyurethane (PU), as a polymer material with versatile product forms and excellent performance, is used in coatings, elastomers, adhesives, and foams widely. However, the raw materials (polyols and isocyanates) of PU are usually made using petroleum‐derived chemicals. With the concern for depletion of petroleum resources and the associated negative impact on the environment, developing technologies that can use renewable raw materials as feedstock has become a research hotspot. Lignin, as an abundant, natural, and renewable organic carbon resource, has been explored as raw material for making polyurethanes because it possesses rich hydroxyl groups on its surface. Meanwhile, compared to vegetable oils, lignin does not compete with food supply and performance of the resulting products is superior. Lignin or modified lignin has been shown to impart the polyurethane material with additional functionalities, such as UV‐blocking ability, hydrophobicity, and flame retardancy. However, the utilization of lignin has encountered some challenges, such as product isolation, heterogeneity, aggregation, steric hindrance, and low activity. This paper summarizes recent research progress on utilizing lignin and modified lignin for bio‐based polyurethane synthesis with a focus on elastomers and foams. Opportunities and challenges for application of the lignin‐based polyurethanes in various fields are also discussed. [ABSTRACT FROM AUTHOR]