Design,Synthesis And Application Of Biobased Epoxy Vitrimer

Epoxy thermosets from fossil resources have permanent crosslinked structures,which can not be reprocessed,non-biodegradable and difficult to be recycled.Its large-scale application leads to problems such as resource crisis,environmental pollution and resource waste.Therefore,increasing attentions have been paid to designing sustainable epoxy thermosets from renewable resources.Among them,vegetable oils with many advantages including multiple functional groups,easy availability and low cost are considered to be an ideal substitute for fossil raw materials for the synthesis of a variety of sustainable polymers.However,common vegetable oil-based polymers have flexible aliphatic long chains,resulting in poor mechanical properties,and the permanently crosslinked network structure also makes them unable to be reprocessed,which both greatly limit the practical application of materials.The introduction of dynamic covalent bonds into the cross-linked polymer network provides an effective method for repeated processing of thermoset materials.Crosslinked polymers with dynamic covalent bonds are called covalent adaptive networks(CANs),which behave like thermosets at service temperature and can be reprocessed at elevated temperature,due to the rearrangement of network topology via exchange reaction of dynamic covalent bonds,thus resulting in mobility.As a special type of CANs,vitrimers have drawn considerable interests as they retain network integrity during exchange reaction due to the associative exchange mechanism.Based on the design concept of vitrimer,this paper introduced dynamic covalent bonds into vegetable oil-based polymers,successfully prepared a variety of biobased epoxy vitrimers,and discussed the feasibility of using them as composite matrixes.The specific research results obtained are as follows:Firstly,we reported a series of reprocessable and weldable epoxidized soybean oilbased vitrimer(ESOV)with adjustable mechanical properties,which were synthesized by catalyst-free curing of epoxidized soybean oil(ESO)with 4,4?-dithiodiphenylamine(APD)as a curing agent.The introduction of APD,on the one hand,enables the reprocessing of the ESO-based polymer,on the other hand,it introduces rigid structures,which improves the mechanical properties.By adjusting the curing time,the mechanical properties of ESOV can be adjusted.With the extension of curing time,the crosslinking density of ESOV gradually increased,resulting in the gradual increase of Young's modulus and tensile strength of ESOV,and the gradual decrease of elongation at break.Due to the dynamic exchange of disulfide bonds in the APD at high temperatures,the network topology was rearranged,giving ESOV excellent reproducibility.After repeatedly cutting and compression molding for several times,the mechanical properties,gel fraction and chemical structure of ESOV remained almost unchanged.Finally,the welding test was carried out on ESOV,and the mechanical properties of the samples were completely restored after welding,proving that ESOV has good weldability.At the same time,in order to further improve the mechanical properties of epoxy vitrimer,aromatic imine bonds with ?-? conjugation effect were selected to replace disulfide bonds for modification.Herein,vanillin and 4-aminophenol were used as raw materials to synthesize a bisphenol curing agent(VA)with dynamic imine bond,and then we used it to cure ESO to synthesize an ESO-based epoxy vitrimer(ESO-VA)based on imine bonds.Due to the rigid benzene ring structure in the aromatic imine bond and the?-? conjugation effect,the modulus of ESO-VA was increased to 41.4 MPa,and the strength was increased to 7.7 MPa.The introduction of imine bonds makes the topological rearrangement of the network occur at high temperatures,giving ESO-VA reprocessability.After reprocessing for several times,its mechanical properties,gel fraction and chemical structure remained unchanged.In addition,ESO-VA could also be used as a matrix to prepare carbon fiber reinforced composite,and the composite showed better mechanical strength and modulus than ESO-VA.The dynamic properties of the imine bonds in ESOVA enabled the composite to be recycled in different ways: by deconstructing the ESOVA network under acidic conditions,valuable carbon fiber fabrics can be recycled without changing its structure and performance;after the composite is pulverized,it can be recycled into short carbon fiber filled composite through traditional thermal processing technology,which has potential application value in non-structural aspects.Finally,we used glycerol-derived glycerol triglycidyl ether(Gte)to replace the flexible long-chain epoxidezed soybean oil(ESO),and reacted with VA containing dynamic imine bonds to obtain a The high-strength and biobased epoxy vitrimer(Gte-VA)based on imine bonds.Compared with ESO,because of its shorter chain structure,Gte can obtain higher crosslinked density on the one hand,and on the other hand,it can introduce more aromatic imine bond structures under the same quality,thus greatly improving the mechanical properties.The biobased epoxy vitrimer showed a Young's modulus of 1.6 GPa and a tensile strength of 62 MPa,which was close to the mechanical properties of amine-cured bisphenol A diglycidyl ether.In addition,Gte-VA showed excellent reprocessability,recyclability and UV shielding performance: due to the dynamic exchange of imine bonds at high temperatures,Gte-VA can be reprocessed,and its mechanical properties,chemical structure and thermodynamic properties are maintained;due to the imine-amine reversible exchange reaction of the imine bonds,Gte-VA can be degraded in the amine solution,and a complete recycled sample with unchanged strength and chemical structure can be obtained after removing the amine solution;because the imine bonds have the absorbing effect on UV light,Gte-VA shows a shielding effect in the full UV light range.In addition,Gte-VA can also be used as a matrix to prepare carbon fiber reinforced composite.Based on the imine-amine reversible exchange reaction of the imine bonds,after the resin was degraded in the amine solution,the carbon fiber fabrics can be recycled without damaging its structure and performance.After recombining the degraded resin with the recycled carbon fiber fabric,a novel carbon fiber reinforced composite with mechanical properties similar to the original sample can be obtained,achieving the complete recycling of the carbon fiber reinforced composite.In this paper,a simple and green preparation process was used to directly cure commercial biobased epoxy resins with curing agents containing dynamic covalent bonds to prepare a series of biobased epoxy vitrimers based on dynamic covalent bonds.By adjusting the crosslinking density,the type of dynamic covalent bonds and the structure of epoxy resin,the mechanical properties of the biobased epoxy vitrimer were adjusted,and a biobased epoxy vitrimer with mechanical properties close to the amine-cured bisphenol A diglycidyl ether was finally prepared.The introduction of dynamic covalent bonds not only realizes the high performance of biobased epoxy thermosets,but also gives them reprocessability and recyclability,and successfully realizes the application in the preparation of high-performance,recyclable carbon fiber reinforced composites.This research has successfully solved the problems of poor mechanical properties,unreprocessable,difficult to recycle,and cumbersome preparation process of biobased epoxy thermosets,which opens up a new way for the preparation and application of biobased epoxy vitrimer,and also contributes to the vigorous development of the vegetable oil industry.