Reversibly Cross-linked High-performance Polyurethane Materials:Self-healing,Recycling,and Upcycling

Polymer materials are widely used in various fields of social production and life.However,it is undeniable that compared to other inorganic materials auch as metals,polymer materials are more prone to damage.Once damaged,waste polymers are difficult to be recycled or restored to their initial performance,leading to serious resource waste and environmental pollution issues.In order to achieve sustainable development for polymer science,endowing polymer materials with self-healing and recyclability properties becomes essential.The design concept of reversibly cross-linking polymers provides an effective solution on the above issues of environment and resources.By introducing reversible cross-linking sites into the polymer structure,the polymer segments can be depolymerized and rebuilt under specific stimuli,aiming for realizing the efficient healing and recycling of polymer materials.Polyurethanes are an important class of high-performance polymer materials.Due to their excellent physicochemical properties,the chemical structure of polyurethanes can be designed and adapted to meet specific needs.However,most of the reported polyurethanes do not exhibit healing and recycling capacities,leading to extremely serious environmental problems.In this paper,reversible cross-linked polyurethane materials were designed by introducing different kinds of reversible interactions into the polyurethane segments,and both the synthesis of polymer materials with"on-demand"mechanical properties and their healing/recycling properties can be realized.Herein,three high-performance polyurethane materials suitable for different scenarios are designed and synthesized,and new ideas of recycling waste polymers are proposed,including the conversion of"elastomers-to-plastics"and the realization of upcycling polymer composites.The specific research contents are described as follows:1.Healable,recyclable and scratch-resistant polyurethane elastomers based on strong and weak hydrogen bond cross-linkings.The PU-UPy_x-DPA_yelastomer was successfully fabricated by using prepolymers（fabricated with polytetrahydrofuran and4,4’-dicyclohexylmethane diisocyanate（HMDI））with different types of hydrogen bonds（derived from carbamate,carboxylic acid,and 2-urea-4-[1H]-pyrimidine（UPy）groups）.This elastomer has high elasticity,a high tensile strength of≈73.6 MPa and a high toughness of≈257.2 MJ m^-3,and exhibits good scratch and tear resistance,guaranteeing its service life.Due to the highly dynamic nature of hydrogen bonds,PU-UPy_x-DPA_ycan be efficiently healed and recycled at high temperature to restore its original mechanical properties.In addition,due to the large number of hydrogen bonding sites within,PU-UPy_x-DPA_ycan be uniformly dispersed with organic dyes,fluorescent dyes,and upconversion nanoparticles to achieve on-demand color and luminescence properties,greatly broadening its application range.This part of the work provides a new idea for the preparation of high-strength,healable and recyclable polyurethane elastomers.2.Polyurethanes cross-linked with coordination bonds for the conversion of elastomers and plastics.We fabricated the PU-HBA elastomer containing microphase separation structures using the reversible cross-linking of hydrogen bond array.The PU-HBA elastomer has a high breaking strength（≈80.3 MPa）,a high Young’s modulus（≈70.8 MPa）,and high toughness(about 198.8 MJ m^-3).PU-HBA exhibits excellent scratch resistance and can be fully healed and recycled at high temperature for multiple times.Subsequently,Mn²⁺were introduced into the material,and the as-prepared elastomer can be converted into the high-performance plastic PU-HBA-Mn²⁺based on the coordination interactions between Mn²⁺and the carbonyl groups within the polymer.The PU-HBA-Mn²⁺plastic has a high Young’s modulus（≈411.2 MPa）,high breaking strength（≈81.3 MPa）and high toughness(≈314.1 MJ m^-3).PU-HBA-Mn²⁺has excellent energy dissipation and impact resistance,and its breaking energy is up to 5.24J,which is much higher than most reported plastics.By using EDTA-2Na,Mn²⁺can be removed from the material,making it very easy to re-convert PU-HBA-Mn²⁺into the PU-HBA elastomer.Building upon the molecular design presented in previous chapter,we introduced a second reversible crosslinking interaction,with a strength between covalent bonds and supramolecular forces,into a single supramolecular crosslinked polymer system.This concept enables the cost-effective conversion of two high-performance materials with distinguish physiochemical properties,providing a new design strategy for the economic development of the polymer industry.3.Polyurethane plastics containing thermal-cleavable aromatic pinacol bonds for the fabrication of high-performance carbon fiber-reinforced composites.The molecular design concept of polymers continues the ideas developed in the previous chapter.The PU-AP thermoset was synthesized using prepolymers（fabricated with polytetrahydrofuran and HMDI）with four-arm aromatic pinacol cross-linker and butanediamine.A large number of hydrogen bond sites exist in PU-AP,which enables it with high-strength adhesion with carbon fiber cloth.As a result,multi-layer high-performance carbon fiber-reinforced polymer composite（denoted as CF/PU-AP）can be successfully fabricated with a high breaking strength of≈870 MPa and a Young’s modulus of≈22.31 GPa.Because aromatic pinacols are cleavable upon heating,PU-AP exhibits excellent environmental stability,and can be converted into a high-performance and soluble elastomer PU-DM under heating conditions,enabling the nondestructive recycle of carbon fiber cloth in solution,thus realizing the upcycling of CF/PU-AP.The recycled PU-DM elastomer has high mechanical strength,healability,recyclability and damage resistance.This work provides an effective approach for the recycling of high-performance fiber-reinforced composite materials and offers a highly innovative solution to the urgent problem of recycling fiber-reinforced composite materials wastes.