Effects of different parameters on thermal and mechanical properties of aminated graphene/epoxy nanocomposites connected by covalent: A molecular dynamics study
Introduction
Epoxy and epoxy-based composite have been used in electronics as adhesives, underfills and moulding compounds [1,2]. The source of diglycidyl ether of bisphenol A (DGEBA) is easily accessible and it is also widely used in industry. It has been found that the thermal and mechanical properties of epoxy nanocomposites can be effectively improved by controlling nano-filler materials.
Graphene (Gr) is viewed as a kind of two-dimensional nanomaterial with sp2 hybridization orbitals [3], which has become one of the strongest materials due to its exceptional properties [4] (e.g. high elasticity, high surface area and fracture strength [5]). Only a small amount of exfoliated Gr can significantly enhance the thermodynamic and mechanical properties of epoxy nanocomposites [4]. To study the properties of Gr/epoxy nanocomposites, a large number of works have been carried out. Sun et al. [2] performed the MD simulation to investigate the interfacial energy between Gr and epoxy using two force fields (PCFF and Dreiding). Rahman et al. [6] inserted the Gr sheet into epoxy resin in order to improve the mechanical tensile properties of crosslinked Gr/epoxy nanocomposites. It was discovered that the mass fraction of Gr between 1% and 3% can effectively enhance the elasticity modulus of epoxy composites. Furthermore, the fact that the dispersed Gr system has a better elastic modulus in the plane compared with that of the agglomerated Gr system has also been proved. Li et al. [7] investigated that Stone-Wales and Multi-vacancy of Gr can significantly improve the interfacial thermal conductivity between Gr and epoxy. Different forms of Gr/epoxy nanocomposites have been studied by Shiu et al. [8] and they found that the thermal and mechanical properties of the composites can be improved by dispersed Gr sheets.
However, the interfacial energy between Gr and epoxy resin in Gr/epoxy nanocomposites is very low. Numerous studies have shown that the interfacial energy can be improved effectively by surface modification of Gr (i.e. Functional groups are grafted on the surface of Gr). The effects of surface modification of Gr (especially AG) on properties of epoxy composites was studied in several research works [3,[8], [9], [10], [11], [12], [13], [14]]. Melro et al. [3] investigated the interfacial shear strength and drawing force of FG layers (carboxyl, carbonyl, hydroxyl) and epoxy composites. Wang et al. [9] proved that FG (Gr with amino or carboxyl)/epoxy nanocomposites have higher thermal conductivity than the original Gr/epoxy nanocomposites. Yadav et al. [10] found that the surface modification of Gr with amino and carboxyl groups greatly improves Tg of Gr/epoxy nanocomposites. Nikkhah et al. [[11], [12], [13], [14]] applied MD method to explore various functional groups (amino (NH2), carboxyl (COOH) and hydroxyl (OH), etc.) covered with polyethylene at different densities. It was proved that the interfacial energy between AG and polyethylene is higher than other FG and polyethylene. In addition, the increased density of superficial NH2 groups will increase the adhesion force, because the increase of interfacial atomic density leads to the reduction of surface atomic roughness. There are some experimental researches on improving thermal and mechanical properties [[15], [16], [17], [18], [19]]. Lee et al. [17] found that Gr with NH2 and COOH display better mechanical and thermal properties of composites than pristine Gr and Gr with only COOH. In particular, Rafiee et al. [18,19] proved that a better interfacial interaction between nanoparticles and epoxy is essential in enhancing the thermal and mechanical properties of nanocomposite materials. It's well known that covalent bonding is much stronger than van der Waals Force. However, it was reported that the non-covalent bonding (van der Waals Force) is the main connection between FG and epoxy resin in most paper and the influence of covalent bonding has rarely been studied in the available literature.
This paper is dedicated to using MD simulation to study the effect of C–N covalent bonds between AG and epoxy on the thermal and mechanical properties of AG/epoxy nanocomposites. Not all FGs can improve the performance of Gr/epoxy nanocomposites. AG not only retains some excellent properties of Gr sheet but also enhances the interfacial interaction between Gr and epoxy, thus it can improve the thermal and mechanical properties of epoxy composites. The main research parameters (including E, Tg, CTE and Eint) are characterized. Besides, the effects of crosslinking degree, mass fraction and FG types (COOH and OH) are also considered. The results of MD simulation have a good agreement with the experimental research and the underlying mechanism resulted in the improvement of the thermal and mechanical properties of AG/epoxy nanocomposites can be explained from the microscopic perspective.
Section snippets
Construction of crosslinking AG/Epoxy nanocomposites
To study the thermal and mechanical properties of AG/epoxy nanocomposites, the atomistic model of the nanocomposites should be first constructed. The molecular structure consists of DGEBA resin, 1, 3-benzenediamine curing agent in the ratio of 198:172 and the AG (amino groups were randomly attached on the surface of Gr by covalent bonds and the surface density was 4.77 groups nm−2) with a mass fraction of 1.2%. The initial density and size of the model were 1.1 gcm−3 and ,
Radial distribution function (RDF)
The total RDF of different epoxy systems (epoxy, G/epoxy nanocomposites and AG/epoxy nanocomposites) at 300 K is shown in Fig. 5. The crosslinking degree, model size and proportion of the three crosslinking systems are consistent (AG and 1, 3-benzenediamine have the same effect on DGEBA.), thus the trend of these three RDF curves is very close. Common features are demonstrated for the three different epoxy systems. The absence of any RDF at the distance less than 0.9 Å is due to the excluding
Conclusion
In this paper, the AG/epoxy nanocomposites molecular system was firstly constructed by C–N chemical bonds and its thermal and mechanical properties (Tg, CTE, E and Eint) were investigated in detail. The influence of crosslinking degree, mass fraction and FG type was considered. The main conclusions are as follows.
- 1.
With the increase of crosslinking degree, the Tg and CTE of AG/epoxy nanocomposites tended to be improved, because the amino groups were fully reacted at 90% crosslinking degree, the
Author contributions
Hui-Ping Yu performed the simulation and edited the paper. Zhi-Hao Tong analyzed the data and wrote the original draft. Pei Chen, An-Wen Cai and Fei Qin provided suggestions on the concept and research methodology, and participated in data analysis.
Declaration of competing interest
The authors declare that they have no known competing financialinterestsor personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 11272020), which is gratefully acknowledged.
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