| The oxygen atom in furan ring tends to form more hydrogen bonds that can be used as physical crosslinking points to reduce the movement of the chains segment,so that the glass transition temperature(Tg)and modulus of the furan epoxy resin are higher than that of the phenyl epoxy resin.However,it has also been reported that the Tg of furan epoxy resin is not higher than that of phenyl epoxy resin,although the furan ring forms more hydrogen bonds.Therefore,there is still an argument whether the furan ring can enhance Tg of epoxy resins.Hence,a clearer understanding of the influence of hydrogen bonds formed by furan rings on the thermodynamic properties of epoxy networks is required.Simultaneously,the preparation of nano-inorganic fillers/epoxy resin composites can satisfy requirements of the higher performance for epoxy resin,and the interface layer between the fillers and epoxy resin plays a very important role in the performance of the composites.However,many experimental methods cannot provide atomic information about the molecular accumulation,density distribution and molecular diffusion at the interface during the crosslinking process of epoxy resin,and the dynamic factors of the interface formation are still not clear.So,it is necessary to study the formation process of the interface layer and the local network structure in the nano-inorganic filler/epoxy resin composites.Due to the limitations of experimental methods,it is difficult to quantitatively characterize the strength of hydrogen bonds,and represent the stacking and diffusion information of atoms in the interface layer,so that molecular dynamics simulation method is employed in this study bexause it has unique advantages in in-depth understanding of the microstructure and physical properties of the network.The main achievements of this paper are as follows:Firstly,three epoxy molecules with similar structure of 2,5-furandimethanol-glycidyl ether(BOF),p-phenyldimethanol-glycidyl ether(pBOB)and 2,5-pentadienedimethanol glycidyl ether(BOC)were constructed,and 4,4'-diaminodiphenylmethane(DDM)was used as curing agent.A cross-linked epoxy resin system with a conversion of 90%was constructed by "step-by-step cross-linking" method.The changes in the microstructure and morphology of the network caused by hydrogen bonds and the effects of hydrogen bonds on the Tg and modulus of the epoxy resin system were investigated.Compared with pBOB-DDM and BOC-DDM systems,the BOF-DDM system contains the largest number of hydrogen bonds because the oxygen atoms in the furan ring are easy to form hydrogen bonds.The more hydrogen bonds in the BOF-DDM system decrease the average distance between the molecular chains of the network,which reduce the free volume fraction and narrow the free volume distribution,finally lead to an increase in Van der Waals energy.In order to overcome more van der Waals energy during the deformation process,the modulus of BOF-DDM eventually are increased.However,for DDM crosslinked epoxy resin network,the Tg of BOF-DDM system is lower than that of pBOB-DDM and BOC-DDM systems,more hydrogen bonds in BOF-DDM did not increase Tg which was consistent with the experimental results,the lower molecular weight and smaller steric hindrance of BOF-DDM molecule is more decisive factors for the reduction of Tg,which is different from most relevant reports on furanyl epoxy.Secondly,a model of epoxy resin/SiO2 interface was constructed by molecular dynamics simulation.The local concentration changes in the interface layer of furan epoxy resin and amine curing agent with the process of cross-linking reaction was investigated and the kinetic factors for the formation of interface layer was studied.The simulation results showed that the concentration of the resin and its cured material increases first,reaches the peak value,then gradually decreases,and finally tends to be stable along the direction away from the surface of SiO2.The concentration of amine group in the interface layer was higher than that of epoxy group because of a strong interfacial interaction energy between the curing agent molecule and the surface of SiO2 before the cross-linking reaction.Uneven distribution of the unreacted amine and epoxy groups was limited because the movement ability of the amine hydrogen and epoxy groups were further restricted when the cross-linking network of epoxy resin gradually formed with the increase of conversion.The concentration of residual amine group in the interface layer was still higher than that of epoxy groups when the conversion reaches 85%.Moreover,the enrichment of amine groups in the interface layer resulted in a lower cross-linking density. |
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