| Epoxy resins are widely used in various fields because of their excellent properties.However,due to the poor toughness and short service life of some common used epoxy resins,the toughening and modification of epoxy resins and the design of repairable epoxy resins have become a topic of concern for scientists all over the world.In recent years,more and more studies have been carried out on the design and construction of reversible materials such as epoxy resin,polyurethane.Moreover,many feasible methods and simple healing effects have emerged.However,further research is needed on the formation of epoxy resins with recycling ability.At present,there are many experimental methods to construct self-healing or reusable materials,such as physical association and association,weak hydrogen bond interaction between molecules,reversible covalent bond and so on.Diels-Alder（DA）reaction is a reversible chemical reaction,which is widely used in the design of self-repairing materials.Many works have been done on the basic research of this reaction and its application for the design of repairable materials.Thus,it is necessary to establish a complete system in the design and characterization of materials formed by Diels-Alder reaction,so as to realize the reversibility of the synthesized materials from both macro and micro perspectives.In this paper,polyethylene glycol diglycidyl ether（PEGDGE）was extended by furan methylamine to prepare long linear polymer chains with furan structure,which were cured with different proportions of diphenylmethane bismaleimide（BMI）.A series of polymer crosslinking networks were obtained.Because the main chain of the selected polymer is hydrophilic and the chain is flexible,the reaction can be carried out more adequate during the curing process.This will help us to study the basic characteristics of Diels-Alder and its inverse reaction on the basis of Diels-Alder.First,the conversion and reaction rates of Diels-Alder reaction at different temperatures were studied by infrared spectroscopy.Compared with some studies on Diels-Alder reaction,the materials cured with PEGDGE structure as the main body could achieve higher conversion at room temperature,and the activation energy of the reaction was similar to most reports.The temperature range of the reaction was obtained by differential scanning calorimetry（DSC）.Dynamic mechanical analysis（DMA）was used to characterize the reduction process of modulus caused by retro-Diels-Alder（rDA）reaction.It is important to understand the reverse decomposition of materials caused by rDA reaction.In addition,we have studied the transition of materials from gel to solution at rDA reaction temperature and the fracture recovery ability of the materials was characterized.The mechanical properties of the material before and after recovery were characterized,and good recovery effect was obtained.This enables us to develop a complete experimental route and characterization method for the design of reusable materials by Diels-Alder reaction,and achieves an ideal application effect.Next,we studied the long linear flexible polymer chains containing furan structure with side groups obtained from polypropylene glycol diglycidyl ether（PPGDGE）extended by furan methylamine.A series of polymer cross-linking networks were obtained by curing with different proportions of BMI.PPGDGE was chosen for the study because of its hydrophobic property,which is more suitable for the practical application than PEGDGE.Due to its good stretchability,it is more suitable as a toughening agent.Therefore,it is necessary to study the properties of reversible materials with PPGDGE as the main structure before modifying epoxy resin.In this work,we follow the previous part of the research methods and continue to study its conversion.The thermal cycle curves of thermal decomposition of materials were measured by differential scanning calorimetry（DSC）.The endothermic peaks of rDA reaction appeared at the first heating,however there were no exothermic peaks and endothermic peaks at cooling and reheating process,which indicated that the Diels-Alder reaction did not occur in a short time.In addition,we added the TGA test to determine the thermogravimetric curves of raw materials and syntheses.This work ensures the complete morphology of materials under the condition of rDA reaction and enriches the experimental methods.Tensile tests results show that the elongation of PPGDGE cured materials can reach 230%at high strength.In addition,the cured materials were placed at 40℃,60℃,80℃,100℃and 120℃for 30minutes,respectively,and characterized by infrared spectroscopy.The dynamic equilibrium state of DA reaction and rDA was analyzed in detail.The experimental results showed the reversible reaction characteristics of DA reaction more clearly.Finally,we designed and realized the synthesis of reusable E51 epoxy resin in two ways.The first way is mixing E51 epoxy resin and PPDGE according to different proportion of epoxy functional groups.Then,the linear long chain E51-FA-PPGDEG with different proportion of E51 structure in the chain segment was synthesized by adding furan methylamine（FA）with equal molar number of functional groups.The polymer networks of different E51 ratios were obtained by curing with different proportion of curing agent BMI.The cured materials were named M（0-100）,M（25-75）,M（50-50）,M（75-25）and M（100-0）in five proportions of E51 content from 0%to 100%.The other way is to obtain E51-FA and PPDGE-FA by chain-extending E51 epoxy resin and PPGDGE with furan methylamine（FA）,respectively.To make E51 account for 25%,50%and 75%,the furan functional groups molar ratios of E51-FA and PPGFA was calculate before mixing them together and then curing with different BMI ratios.The materials were named D（25-75）,D（50-50）,D（75-25）.Each material also contains cured products of different BMI ratios.Next,we studied the reaction kinetics of DA reaction.The results show that the DA reaction follows the second-order reaction kinetics.In the synthesis of several materials,the reaction rate of M（0-100）,M（25-75）,M（50-50）increases with the increase of E51 content.It is noteworthy that the reaction rate of M（50-50）is 0.01749 M-1·L·min-1,which is even higher than that of PEGFA/BMI material at 60℃.However,M（75-25）and M（100-0）with higher E51 content are difficult to react completely at room temperature due to the difficulty of chain movement.Through thermal analysis of these materials,their rapid rDA reaction and low-speed positive DA reaction were verified again.With the increase of E51 content,the glass transition temperature（Tg）increases and the thermal stability increases slightly.The analysis of tensile stress-strain curves of different materials in this system show that with the increase of E51content,the material structure gradually changes from high elastic rubber system to glass plastic system at room temperature.Starting from the M（50-50）system with 50%E51content,the tensile curve shows obvious yield phenomenon.Compared with the M（100-0）system with 100%E51 content,the toughness of the material was improved by adding PPGDGE,which changed from brittle fracture to ductile fracture.For the effect of different curing agent content on the properties of cured materials,we discussed the causes of the change of properties of M（m-n）series and D（m-n）series from the aspects of reaction selectivity caused by structure and reaction rate.In the end,we studied the reusability of materials.On the one hand,the scratch repair ability of materials under heat treatment was characterized.The results of scanning electron microscopy（SEM）showed that the introduction of flexible PPDGE made the materials fluidity after rDA reaction,which was conducive to the self-repair of materials.On the other hand,the fracture spline remodeling ability and the properties after remodeling were analyzed,and the tensile test showed that the strength and modulus of the material will be restored to the original state.Therefore,a series of epoxy resins with different toughness and reusability were synthesized.In a word,we have designed a complete route to develop reusable materials and obtained epoxy resin materials which can be reused in limited conditions.It can be used for reference in the design and development of new materials. |
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