Functionalization enhancement on interfacial shear strength between graphene and polyethylene
Graphical abstract
Introduction
Graphene, a two dimensional and single-atom-thick sheet of sp2- hybridized carbon atoms [1], has been widely investigated as nano-fillers to enhance the mechanical, electrical and thermal properties of graphene/polymer composites [2], [3], [4], [5], [6], [7]. Some previous studies on graphene/polymer composites showed that mechanical properties of polymer matrix can be remarkably enhanced by rfilling smaller amounts of graphene sheets than other nano-fillers [3], [4], [5], [6]. For example, Ramanathan et al. [4] reported that by adding only 1 wt% of functionalized graphene sheets (FGS) into poly(methyl methacrylate) (PMMA) matrix, the elastic modulus was increased by approximately 80% and ultimate strength was improved by nearly 20% over that of the pure PMMA.
Interfacial load transfer from the polymer matrix to the graphene, plays a critical role in determining the overall mechanical performance of graphene/polymer composites [3], [8], [9]. It has been confirmed that introducing functional groups bonded to graphene may lead to a strong graphene/polymer interface and excellent mechanical performance of graphene/polymer composites. Several possible reasons were put forward to explain the mechanisms of mechanical reinforcement. Graphene has high specific surface area and wrinkled morphology at the nanoscale. This wrinkled morphology could be further enhanced by structural defects and functional groups on graphene sheet [10], [11], [12], [13], [14], and lead to mechanical interlocking between the FGS and polymer matrix [4]. Besides, the functional groups on FGS may form hydrogen bonds or covalent bonds across the interface with some polar polymers, such as PMMA or epoxy, which would result in stronger interfacial interactions and consequently enhanced mechanical properties [15], [16], [17].
Molecular dynamics (MD) simulations is a powerful method to investigate the interfacial characteristics of graphene/polymer composites [18], [19], [20], [21], which could not be accessible by the experimental tools. A few simulations investigated the change of structural and dynamical properties of polymer matrix near the graphene surface [22], [23], [24]. Their results showed that a dense polymer layer is formed in the vicinity of graphene due to intermolecular interaction across the interface. And the polymer chains in the dense layer show a tendency to reorient to almost parallel to graphene surface. This more ordered structure, resulted from contact with graphene surface, may lead to mechanical properties exceeding those of unaffected amorphous polymer [25]. Nikkhah et al. [26] studied the effect of functional groups, such as NH2, COOH, OH and CH3 groups, on the layering structure of polymer. The functional groups with various surface density might change the peak density and location of the first polymer layer near the graphene, because the insertion of functional groups might significantly change of interfacial interaction between the FGS and polymer matrix. Because of the increased electrostatic interaction, functional groups with high electronegativity lead to high interfacial interaction between FGS and polymer. However, the surface roughness of FGS, which could be enhanced by larger groups, might sufficient to lead to decrease of van der Waals (vdW) interaction and negative effect on interfacial interaction [26], [27], [28].
Pull-out simulation is always used to study interfacial mechanical properties and load transfer mechanism along the interface of nanocomposites. Lv et al. [18] using pull-out simulation investigated the influence of functional groups on the interfacial mechanical characteristics of FGS/polymer composites, including OH, NH2, COOH, F and CH3 groups. Some functional groups, like NH2 and OH groups for FGS/polyethylene (PE) systems, could significantly enhance the interfacial interaction energy and interfacial shear stress of the hybrid systems. Their results showed that interfacial shear stress is basically dominated by the interfacial interaction between the FGS and polymer. However, mechanical interlocking along the interface enhanced by larger functional groups, might also have a significant effect on the interfacial shear stress. This effect of mechanical interlocking could be obviously seen in the research with pristine graphene by Liu et al. [29]. They found that the wrinkled graphene/PE system lead to larger interfacial interaction energy and much higher maximum interfacial shear force compared to the ideal flat graphene/PE system. Ding et al. [30] studied the effect of oxidation degree on the interfacial and overall mechanical properties of graphene oxide (GO)/poly(vinyl alcohol) (PVA) system. The high oxidation degree of GO sheet would enhance the interfacial binding energy, interfacial shear stress and overall mechanical properties, such as Young’s modulus, shear modulus and bulk modulus. Their results suggested that the larger amount of hydrogen bonds formed at the interface caused by higher oxidation degree of GO, lead to the enhancement of mechanical properties. Wang et al. [20] also studied the influence of surface functionalization on the interfacial load transfer in FGS/PE nanocomposites. Their results showed that O functionalized graphene leads to larger interfacial shear force than H functionalized and pristine ones during pull-out process. A mechanism of interfacial load transfer was suggested by the authors, which consists of two contributing parts: the formation of new surface and relative sliding along the interface.
However, the reinforcing mechanism of functional groups on interfacial load transfer of graphene/polymer composites has not been well understood. Although mechanical interlocking is considered to be an important reinforcing mechanism of interfacial shear strength, no mechanical interlocking phenomenon has been reported in previous MD simulations. Moreover, the PCFF and COMPASS force field, which were used in previous MD simulation, have fixed partial charge on individual atom during the simulation. This defect of force field may lead to serious error in calculation of interaction energy involving the polar functional groups, which may cause a misunderstanding of the reinforcing mechanism.
In this paper, pull-out processes were simulated to investigate the interfacial mechanical properties between the FGS and PE matrix by MD simulation with ReaxFF reactive force field. Moreover, the interfacial structure of polymer and the interfacial interaction in the equilibrium FGS/PE systems were also analyzed. The purpose of this paper is to elucidate the enhancement mechanism of interfacial shear strength in the FGS/PE systems.
Section snippets
Computational methods
To set up the initial atomistic structures, the full atom model of amorphous PE layer was established with the dimensions of 80 Å × 70 Å × 30 Å and the density of 0.8 g/cm3 using Material Studio developed by Accelrys Inc. The PE layer consists of 286 molecule chains, and each molecule (CH3(CH2)18CH3) is composed of 10 monomers (C2H4). The monolayer graphene sheet with a size of 60.5 Å × 40.3 Å was also constructed using Material Studio. The effect of unsaturated carbon atom at the edge of graphene sheet
Interfacial structure of equilibrium systems
To study the structure of polymer in the vicinity of FGS, local mass density of PE were calculated for the equilibrated FGS/PE systems. We partitioned the PE along the z direction into several small bins (0.6 Å in thickness) parallel to the interface. In each small bin, the mass density was calculated as the total atomics mass of PE divided by the volume of bin. The final local mass density was the average over the configurations in the production MD run. Fig. 2 shows the variation of mass
Conclusions
In this paper, pull-out processes were simulated to investigate the interfacial mechanical properties between the FGS and PE matrix by using MD simulation with ReaxFF reactive force field. The interfacial structure of polymer and interfacial interaction in the equilibrium FGS/PE systems were also analyzed to reveal the enhancement mechanism of interfacial shear strength.
The layer structure of polymer in the vicinity of FGS was observed in the equilibrium FGS/PE systems. We found that the
Acknowledgment
The project is supported by the Fundamental Research Funds for the Central Universities of China (Grant No. CDJZR12248801).
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