Dynamically Cross-Linked Recyclable Polymeric Materials

In recent years,a variety of polymeric materials play an important role in industry and daily life.But at the same time,it is undeniable that many polymeric materials are discarded after use,which not only makes us continue to synthesize various polymeric materials and exacerbates the shortage of resources,but also causes a large amount of environmental pollution.Considering most polymers are difficult to degrade in natural conditions,it is very necessary to extend their service life and realize their sustainability.However,because the thermoplastic polymers will inevitably decompose and reduce the value of the material during repeated mechanical processing,and thermosetting polymers are insoluble,non-melting and difficult to be reprocessed,it still remains full of challenges to achieve multiple processing of polymer materials and maintain their use value.Supramolecular interactions,which are mainly composed of dynamic covalent bonds and noncovalent interactions,will be the effective methods to solve the abovementioned problems.Introducing supramolecular interactions into polymeric materials can endow them with not only certain cross-linking density and excellent mechanical properties,but also dynamic properties between chain segments under specific stimuli.As a result,processing and recycling of polymeric materials under mild conditions can be realized.Herein,we designed and synthesized three soluable aromatic engineering polymers and they were the main body of the corresponding materials.By modifying different kinds of supramolecular interactions,solution-processable and recyclable polymer materials were successfully prepared.Furthermore,their mechanical properties,thermal stability and chemical recycling capacities were studied in detail.The research contents are as follows:1.Polymeric complex nanoparticles enable the fabrication of mechanically super-strong and recyclable poly(aryl ether sulfone)-based polymer composites.In this study,we successfully synthesized carboxylic acid-functionalized poly(aryl ether sulfone)(PAES-COOH)and it was complexed with commercialized polyvinylpyrrolidone(PVPON)in N,N-dimethylformamide(DMF)followed by the evaporation of the solvent to form PAES-COOH/PVPON composite films.Due to the difference of the structure phases of the two polymers in DMF,complex nanoparticles were in-situ generated to effectively strengthen the polymer composites.The PAES-COOH/PVPON_10%composite,which contains 10 wt%PVPON,exhibits a tensile strength of 104.8 MPa,which is?2.0 times higher than the initial PAES-COOH and even higher than that of typical PAES.The PAES-COOH/PVPON composites possess excellent solvent-assisted healability and recyclability to recover their original mechanical strength,thus prolonging its service life.In this work,we propose that in-situ generation of nanoparticles is an effective means of reinforcing materials.The results of this work will be helpful for the design of high strength polymer composites in the future.2.Solution-processable poly(aryl ether ketone)supramolecular thermosets based on dynamic boroxines.In this work,a series of low-molecular-weight bisphenol A type poly(aryl ether ketone)s(PAEKs)were synthesized,and phenylboronic acid was further modified at the end of the PAEK.Boroxines were generated by the dehydrating and trimerizing the phenylboronic acids at high temperature,and the PAEK thermosets cross-linked with boroxines(PAEK-B₃O₃)were successfully fabricated.Among all PAEK samples,PAEK-B₃O₃ prepared with the PAEK with the average molecular weight of 9400 has a tensile strength up to 97.8 MPa,which is even higher than that of bisphenol A type PAEK-B₃O₃ with high molecular weight.Boroxines have a high thermal decomposition temperature,which helps PAEK-B₃O₃retain excellent thermal stability.Based on the dynamic properties of boroxines in alcohols,PAEK-B₃O₃ can realize its chemical recycling with the assistance of solvent,thus realizing the resource saving.With the excellent solution processability of PAEK-B₃O₃,it can be not only processed into different shapes in the mold and given practical applications,but also can be complexed with inorganic fillers in the solvent to achieve further reinforcement.This work will provide a new strategy for the solution-processing,chemical recycling and further enhancement of high-performance engineering polymers.3.Mechanically robust supramolecular plastics with energy-saving and highly efficient closed-loop recyclability.We synthesized low-molecular-weight poly(aryl ether ketone)PAEK containing phenylboronic acids of as well as the molecule CB containing both phenylboronic acid and carboxylic acid.The two monomers were complexed in solution followed by evaporating the solvent to prepare the PAEK-CB plastics.Owning to the covalent cross-linking from boroxines and the hydrogen-bonding interactions between carboxylic acids,the tensile strength of the PAEK-CB plastics can be up to 46.7 MPa.The PAEK-CB plastics can be dissolved with the assistance of solvent.By adding excessive ethanol,the two monomers can be separated easily through the precipitation method,and the recovery rates of the two monomers are both higher than 90%,thus realizing the closed-loop recovery of thermosets.The recovered monomoers can be repeatedly manufactured into the original PAEK-CB plastics for five cycles,which can surely extend its service life.The recovered PAEK also enables the upcycling by complexing with PVA or EVOH to fabricate boronic ester-cross-linked supramolecular thermosets.This work will provide an effective way for the closed-loop recovery of thermosets and the efficient separation of initial monomers.