| Epoxy resins have the advantages of good wettability, low curing shrinkage, strong adhesion, good mechanical properties and thermal and chemical stability. Therefore, as microelectronic packaging materials,adhesives, coatings and composite matrix resins, they are widely used in electronic appliances, aerospace and other fields. With the human exploration of the unknown world’s footsteps continue to move toward a harsher environment, it will certainly make a higher request for the epoxy resins. So, it is necessary to design and prepare the cross-linked epoxy network structure based on the relationship between the structure and properties to meet the required performance. However, a variety of reasons makes the study of the relationship between structure and performance by experimental methods alone to be a difficult research: 1) cross-linked network structure insoluble and infusible, resulting in many analysis and testing methods become difficult or even failure, 2) the intricate correlation between the factors that affect the structure of the cross-linked network makes it difficult to control a single variable, 3) physical aging and absorption of water and other factors that make it difficult to control the variables, 4) increasingly harsh experimental conditions result in high experimental costs. As a new type of research technology, molecular dynamics simulation can accurately build the molecular structure model and study its performance, so it has a unique advantage in the study of structural-performance relationship. With the improvement of computer computing power and development of molecular dynamics simulation technology, it is possible to solve the relationship between the performance and the structure of epoxy resin.In this paper, the effects of main chain structure on the thermomechanical properties of crosslinked epoxy resins are studied by molecular dynamics simulations. On the one hand, BOB—PACM and BOF—PACM cross-linked epoxy resin models with conversion rates of 90% were constructed by"stepwise" crosslinking algorithm, respectively. The glass transition temperature, thermal expansion coefficient, poisson’s ratio and glass modulus of two cross-linked epoxy systems were studied. These simulation results are consistent with the reported results, which proves the effectiveness of the model construction. Subsequent studies have found that, compared with the BOB—PACM system, the BOF—PACM system has lower mobility at the same temperature, with less free volume and higher van der Waals. So it is concluded that higher polarity arising from furan ring retards the mobility of chains in BOF-PACM, making its glassy modulus higher. One the other hand,the DGEBA-D400 and DGEBA-DETDA are studied. The changes of axial stress and energy during the stretching process at different temperatures are recorded. It shows that, as the temperature decreases, the modulus of the system increases gradually, meanwhile, the change rate of potential energy is gradually increased and the changes mainly come from the change of van der Waals force and electrostatic force. Therefore, it can be concluded that the mechanical properties of the epoxy resin, especially the modulus, are mainly affected by the non-bonding effect in the glassy state.Secondly, based on molecular dynamics simulations, models of supported epoxy film on silica substrate with epoxy in different curing degrees are constructed. By performing density profile analysis, the interphase region proves exist according to the difference in the density of the polymer in the adjacent silica substrate and the bulk region. Although the change of the interphase thickness is not observed, it is found that the polymer chains stick to the substrate more closely with the increasing conversion. By performing the radial distribution functions and the mean square displacements, the structure and motion ability of the interphase of supported epoxy resin films are studied. The results show that there is more hydrogen bond formed between the interfaces in the highly crosslinked system. Compared with the bulk region, the movement ability of the interfacial layer network chain is relatively weak. At the same time, with the increase of the crosslinking density,the mobility is gradually reduced. The stress—strain curves obtained from the tensile test show that the modulus of the interphase and elongation at break increase with the increasing of conversions. This is due to the formation of more hydrogen bonds, creating higher interaction force between the polymer and the substrate, retarding the movement of the polymer chains, increasing the rigid of the interphase.Thirdly, the mechanism of water absorption of epoxy resin systems with different curing degrees is studied by experimental and molecular dynamics simulations. In the experiment, the epoxy resin film with different curing degree of 200 μm was prepared by changing the curing process. The presence and diffusion behavior of water molecules in the process of water absorption of epoxy resin were characterized by on line infrared spectrum, difference spectrum and two-dimensional correlation infrared analysis. The results show that water in the epoxy resin system exist in the form of "free" and "combine",and the combined water are first formed, proves that the formation of hydrogen bonds in the epoxy resin plays an important role in water absorption.The equilibrium water absorption and diffusion coefficient of the epoxy resin system were studied by gravimetric method. The results show that with the increase of curing degree, both the equilibrium water absorption and diffusion coefficient show an increasing trend. The crosslinking epoxy resin system with 1% water molecules was established by molecular simulation method.The hydrogen bond interaction, the diffusion properties of water molecules and the distribution of free volume were studied. The simulation results were in good agreement with the experimental results. At the same time, from the simulation results, although the total free volume of epoxy resin decreases with the increase of curing degree, while the proportion of free size of larger size increases with the increase of curing degree, The large size of the free volume in the epoxy resin system in the diffusion of water molecules play a decisive factor.Finally, according to the previous research results, the modulus of epoxy resin is mainly affected by van der Waals force and electrostatic force, and the interfacial interaction between epoxy resin nanocomposites has an important effect on the performance of interphase and composite materials. It was decided to synthesize heptidene POSS particles which have strong interaction with DGEBA. Pre-reaction was used to increase the dispersibility of POSS in the epoxy system. The effects of the addition of this kind of POSS on the thermo-mechanical properties of epoxy resin were studied. The pretreatment of PETMP with the curing agent PETMP was carried out according to the click chemical reaction between mercapto and olefins, followed by the reaction between the mercapto and epoxy group to obtain the epoxy nanocomposites with different content of POSS. The structure and morphology of the epoxies are characterized by XRD and SEM. Although POSS are uniformly dispersed in the whole resin, it tends to form a crystalline region, and as the POSS content increased aggregation phenomenon is more obvious. DSC, TGAand DMA are used to measure the glass transition temperature, heat resistance and thermomechanical properties of these cured epoxies. The results show that the effect of pendant POSS on the glass transition temperature of epoxy is small. The addition of a small amount of POSS pendant groups greatly increases the glassy modulus of the epoxy resin,which we believe is due to the strong interaction between the well-dispersed hephenyl-based POSS particles and the epoxy resin. With the increasing content of POSS, the modulus of epoxy resin composites will decrease, and this is due to the sharp reduction of continuous phase epoxy resin matrix,which leads to the increase of defects in the systems. |
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