Elsevier

Carbon

Volume 93, November 2015, Pages 830-842
Carbon

Influence of hydrogen functionalization on the fracture strength of graphene and the interfacial properties of graphene–polymer nanocomposite

https://doi.org/10.1016/j.carbon.2015.05.101Get rights and content

Abstract

Using molecular dynamics and classical continuum concepts, we investigated the effects of hydrogen functionalization on the fracture strength of graphene and also on the interfacial properties of graphene–polymer nanocomposite. Moreover, we developed an atomistic model to assess the temperature and strain rate dependent fracture strength of functionalized graphene along various chiral directions. Results indicate that hydrogen functionalization at elevated temperatures highly degrade the fracture strength of graphene. The functionalization also deteriorates the interfacial strength of graphene–polymer nanocomposite. Near-crack-tip stress distribution depicted by continuum mechanics can be successfully used to investigate the impact of hydrogen passivation of dangling carbon bonds on the strength of graphene. We further derived a continuum-based model to characterize the non-bonded interaction of graphene–polymer nanocomposite. These results indicate that classical continuum concepts are accurate even at a scale of several nanometers. Our work provides a remarkable insight into the fracture strength of graphene and graphene–polymer nanocomposites, which are critical in designing experimental and instrumental applications.

Introduction

The extraordinary electronic, magnetic, and mechanical properties of graphene, a single layer of graphite, have drawn remarkable attention from scientists and engineers. Graphene-based nanoelectromechanical systems (NEMS), such as resonators [1], have demonstrated intriguing applications in various engineering disciplines from telecommunication [2] to biomedicine [3]. Graphene reinforcements greatly improve the fracture toughness and fatigue crack propagation resistance of composite materials [4], and these reinforcements also enhance the electronic and magnetic properties of composite [3].

As in many crystalline materials, defects such as vacancies (missing atoms) and adatoms (presence of foreign atoms) are unavoidable during synthesis and fabrication of graphene-based systems [3], [5]. Vacancy defects drastically reduce the strength and stiffness of graphene [6], which critically influences the performance of NEMS. In many applications, however, the surface of graphene must be modified by introducing defects in order to achieve desired functionalities. As an example, a two-dimensional amorphous graphene membrane can be obtained by means of electron irradiation-induced vacancies, which opens new possibilities to engineer graphene-based NEMS [7]. Chemical functionalization, which is addition of foreign atoms or functional groups, could induce better interaction between graphene and the composite matrix, which is essential in fabricating nanocomposites with advanced electromechanical properties [8], [9].

Functionalization is widely used surface modification method to control the electronic and magnetic properties of graphene. Hydrogen functionalization creates new band gap openings in graphene [10], and carbon adatoms significantly modify the electronic and magnetic properties of graphene [11], [12], [13]. Functionalization also allows graphene to combine with various other polymer groups, which is critical when graphene is used as reinforcement in composite materials [14], [15], [16].

Most of the studies on functionalized graphene have focused on the electronic and magnetic properties [10], [11], [12], [13]. Understanding influence of adatoms and functional groups on the mechanical and interfacial properties of graphene-base systems is vitally important in many applications such as in graphene-based composite materials, where the adsorption of adatoms and functional groups is unavoidable [8], [9], [14], [15], [16]. In graphene-based composites, interaction between graphene and composite matrix is governed by non-bonded interactions, which is mainly van der Waals force [14], [15], [16]. The adsorption of adatoms could have a significant impact on the non-bonded interactions between graphene and composite matrix. Molecular dynamics simulation studies reveal that hydrogen adsorption can have a significant impact on the strength of graphene and its allotropes [17], [18]. On the other hand, dangling bonds in defective graphene, which are created by defects (e.g., vacancies), can be passivated by hydrogen adatom thereby stabilizing defective graphene. Therefore, an in-depth understanding of the effects of hydrogen adatoms on the fracture strength and interfacial properties of graphene is critically important.

Graphene is subjected to high temperatures (∼1300 K) during the fabrication of graphene-based composite materials [4]. Several molecular dynamics simulation studies have been conducted on the temperature dependent mechanical properties of pristine graphene [19], [20] and graphene with vacancies [21]. However, the effects of temperature on functionalized graphene has not been well understood. The behavior of functionalized graphene sheets could be significantly defer from that of pristine sheets since functional groups (or adatoms) transform the hybridization of carbon in graphene from sp2 to sp3. On the other hand, most of the recent studies focus on the mechanical properties graphene along two principle directions, i.e., armchair and zigzag. Graphene can be subjected to mechanical loading along any direction during practical applications such as in NEM and composites. Therefore, understanding the behavior of graphene along various chiral directions is important in designing graphene-based systems.

This paper presents a comprehensive molecular dynamics simulation study on the effects of hydrogen functionalization (i.e., hydrogen adatoms) on the fracture strength of graphene. Also the influence of temperature on the fracture strength along various chiral directions has been studied. Further, fracture strength of hydrogen-passivated graphene was investigated using the concept of near-crack-tip stress distribution. A continuum-based model to describe the macroscopic properties arising from the non-bonded interaction of graphene–polymer composite was also developed. Finally, the influence of hydrogen adatoms on the interfacial properties of graphene–polymer composite was investigated.

The paper is organized as follows: Section 2 describes molecular dynamics simulations of hydrogen functionalized graphene. Section 3 presents the influence of adatoms and temperature on the fracture strength of graphene. Further, in this section, an atomistic model is developed to assess the temperature and strain rate dependent fracture strength of functionalized graphene along various chiral directions. In Section 4, we derive a continuum-based model to characterize the non-bonded interaction of graphene–polymer nanocomposite. The influence of hydrogen functionalization on the interfacial properties of the composite is also investigated in this section. Conclusions are drawn in the final section.

Section snippets

Modeling of graphene

Experimental studies on the mechanical behavior of graphene are limited due to practical problems in designing experiments at the nanoscale [1], [2], [3], [4]. Therefore, atomistic simulations such as quantum mechanics (QM) and molecular dynamics (MD) play a vital role in investigation of the mechanical behavior of graphene. MD is computationally very efficient compared to QM models, and empirical data are incorporated in evaluating certain parameters of MD potential fields [22], [23]. QM

Effects of adatom on stress–strain relation

For a fixed adatom concentration, we performed five MD simulations with different randomly distributed adatoms. In these five simulations, the average strength and stiffness of graphene are less sensitive (<5%) to the distribution of adatoms in the sheet. Therefore, the average strength and stiffness were used for the analysis.

Fig. 2 compares the stress–strain curves of an armchair graphene sheet with various concentrations of hydrogen adatoms. The figure shows that the adatoms highly degrade

Molecular dynamics simulations

We conducted a comprehensive MD simulation study on the interfacial fracture of a graphene–polymer (polyethylene) system. The size of the polymer matrix is 30 × 30 × 11 Å, with 448 monomers (–CH2–). The edges of a polymer chain were connected to an additional hydrogen atom, which makes the terminal group stable (i.e., –CH3). Influence of free lateral surfaces were eliminated by applying periodic boundary conditions along the in plane directions (x and y) of the simulation box, and a surface along z

Conclusions

We used molecular dynamics simulations to investigate the influence of hydrogen functionalization on the fracture strength graphene and also on the non-bonded interaction of graphene–polymer nanocomposite. The results indicate that hydrogen functionalization of graphene, which is quite beneficial in some advanced nanomechanical applications, could results a weak and unstable graphene and even degrade the interfacial properties of graphene–polymer composites. The fracture strength of graphene

Acknowledgments

This work was financially supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada. Computing resources were provided by WestGrid and Compute/Calcul Canada.

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