Effect of grain boundaries on the interfacial behaviour of graphene-polyethylene nanocomposite

TL;DR: In this article, the effect of grain boundaries on the interfacial properties of bi-crystalline graphene/polyethylene based nanocomposites was investigated, where molecular dynamics based atomistic simulations were performed in conjunction with the reactive force field parameters to capture atomic interactions within graphene and polyethylene atoms.

About: This article is published in Applied Surface Science.The article was published on 2019-03-15 and is currently open access. It has received 69 citations till now. The article focuses on the topics: Graphene & Nanocomposite.

Summary

1. Introduction

• Graphene is a two-dimensional (2D) nanomaterial with honeycomb crystal lattice domain [1, 2] .
• Due to exceptional mechanical, thermal and electrical properties, graphene is emerging as a potential candidate for the reinforcement of nanocomposites [3] [4] [5] .
• They attributed higher interfacial strength for relatively stronger covalent bonds formed by the functional groups as compared to weak non-bonded van der Waals interactions.
• Several computational and experimental works have already been performed to characterize the mechanical properties of bi-crystalline graphene [38] [39] [40] [41] [42] .

2. Modelling details

• The classical mechanics based MD approach was used to perform all the simulations.
• In all these aforementioned configurations, the bi-crystals of graphene nanosheets were randomly oriented in the PE matrix for maintaining the realistic condition.
• Finally, the system was relaxed again under the influence of NPT ensemble at 100 K for a total time period of 25 ps.
• For avoiding the stress fluctuations during tensile strain analysis, Velocity-Verlet algorithm with a relatively smaller integration time step of 0.15 fs was opted.

3.1. Effect of defected graphene on the tensile strength of PE nanocomposites

• ReaxFF potential parameters have already been validated for simulating the mechanical properties of pristine and bi-crystalline graphene in their previous articles [41, 57] .
• Thus, for the same percentage of reinforcement in nanocomposite, a higher degree of interfacial strength is desirable; which can be achieved by either functionalising the interface or inducing geometrical defects in the nanofillers domain.
• Stress-strain responses for different types of bi-crystalline graphene reinforced PE nanocomposites are plotted in Fig. 3 and Fig. 4 for AC and ZZ configurations of graphene, respectively.
• These explanations can also be found and related to their earlier research articles [41, 42] in conjunction with the recommendation of Grantab et al. [40] work; hence, it complements their current efforts.
• In each of the simulations, uniaxial tensile loading was applied perpendicular as well as parallel to the GB.

3.2. Effect of defected graphene on the shear strength of PE nanocomposites

• After predicting tensile strength of nanocomposites, next set of simulations were performed to investigate the shear strength of the interface between graphene and PE matrix.
• In order to capture the shear strength at the interface, simulations were performed with periodic boundary conditions imposed only in two principal directions, whereas the third principal direction was used to pull the graphene out of polymer matrix as illustrated in Fig. 7 .
• In the graphene reinforced PE system, the pristine and bi-crystalline graphene nanosheets were pulled out of the PE matrix with a velocity of 0.0001 Å/fs along x-direction (non-periodic) and the resulting shear force on the graphene nanosheets in the pullout direction was plotted in Fig. 8 .
• The resulting maximum interfacial shear strength (τ xy-max ) was calculated with the help of surface area of graphene nanosheet as per equation 4.

3.3. Effect of defected graphene on the cohesive strength of PE nanocomposites

• In the last subsection, simulations were performed to estimate the cohesive strength of interface for different configurations of nanocomposites.
• It can be inferred from the trend plotted in Fig. 10 that similar to shear strength, normal interfacial stress of nanocomposite also improved significantly, while reinforced with bicrystalline graphene as compared to pristine graphene.
• Hence, these GB act as the path for load transfer to take place and helps in establishing a strong mechanical interlocking with high cohesive strength.
• Higher mis-orientation angle GB configurations possess higher normal interfacial strength and vice-versa.
• On a comparing note (Fig. 8 and Fig. 10 ), shear stress values at the interface are less than the cohesive/normal stress.

4.0 Conclusion

• In summary, simulations were performed to study the reinforcing capabilities of bi-crystalline graphene as compared to pristine graphene nanosheet.
• The authors also perceived that wrinkling with substantial out-of-plane deformation in bi-crystalline graphene containing higher mis-orientation angle GB resulted in more number of adhesion points and better non-bonding interaction at the interface; which were the main mechanisms causing an increment in the tensile strength.
• But emerges as a superior reinforcement for polymer based nanocomposites.

Figures

Table 1. LJ potential parameters for carbon (C) and hydrogen (H) atoms.

Table 3. Interaction energies between nanofiller and PE matrix

Table 2. Improvement in tensile strength for various configurations with respect to neat PE.

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Effect of grain boundaries on the interfacial behaviour of graphene-polyethylene nanocomposite" ?

Aim of this article was to investigate the effect of grain boundaries on the interfacial properties of bi-crystalline graphene/polyethylene based nanocomposites. 

Q2. What is the common technique for synthesising larger size graphene?

Chemical vapour deposition is the most commonly used technique for synthesising larger size graphene, but it results in polycrystalline structure. 

Q3. Why is graphene emerging as a potential candidate for the reinforcement of nanocomposites?

Due to exceptional mechanical, thermal and electrical properties, graphene is emerging as a potential candidate for the reinforcement of nanocomposites [3-5]. 

Q4. What is the effect of mis-orientation angle on graphene?

Higher mis-orientation angle configurations lead to redistribution of stress uniformly throughout the bi-crystalline graphene sheet that maximizes the load transfer phenomenon and helps in improving the tensile strength. 

Q5. What is the reason for the high tensile strength of graphene?

It is predicted from the post processing of dump files that higher mis-orientation angle configurations contain more energetic sites (due to high density of dislocations) relative to lower mis-orientation angles for a given weight percentage of graphene in PE; therefore, there would be more wrinkling in higher mis-orientation angles and thus high tensile strength. 

Q6. What are the main mechanisms causing an increase in the tensile strength of bi-?

The authors also perceived that wrinkling with substantial out-of-plane deformation in bi-crystalline graphene containing higher mis-orientation angle GB resulted in more number of adhesion points and better non-bonding interaction at the interface; which were the main mechanisms causing an increment in the tensile strength. 

Q7. What are the limitations of graphene synthesis?

Due to limitations associated with the synthesising techniques, nanomaterials e.g. large size graphene nanosheets are synthesised with geometrical defects such as vacancies, dislocations and grain boundaries (GB) [34, 35]. 

Q8. What is the effect of graphene on the mechanical properties of polymer composites?

Liu et al. [33] concluded in their work that grafting of graphene with polymer chains helps in improving the shear strength as well as graphene’s dispersion in the polymer matrix. 

Q9. What is the effect of the tensile strength of graphene on the GB?

Snapshots showing crazing and voids formation in PE when subjected to tensile loadAfter predicting tensile strength of nanocomposites, next set of simulations were performed to investigate the shear strength of the interface between graphene and PE matrix. 

Q10. What is the effect of the curved graphene on the tensile strength?

better interfacial properties have been predicted from the interaction energy trend for bi-crystalline graphene nanocomposites as compared to pristine graphene. 

Q11. How was the graphene pulled out of the PE matrix?

In order to capture the shear strength at the interface, simulations were performed with periodic boundary conditions imposed only in two principal directions, whereas the third principal direction was used to pull the graphene out of polymer matrix as illustrated in Fig.7. 

Q12. What is the effect of a curved graphene matrix on the tensile?

It was also predicted from the tensile deformation of above designed nanocomposites that after achieving the maximum tensile strength, permanent deformation in the form of voids and crazing starts generating in PE matrix as shown in Fig.6. 

Q13. What is the resulting shear force on graphene nanosheets?

In the graphene reinforced PE system, the pristine and bi-crystalline graphene nanosheets were pulled out of the PE matrix with a velocity of 0.0001 Å/fs along x-direction (non-periodic) and the resulting shear force on the graphene nanosheets in the pullout direction was plotted in Fig.8. 

Q14. What is the effect of the stress-strain response of graphene reinforced nanocompo?

Due to increased interaction, atoms configuring GB atoms were actually pulled by the PE chains that results in inducing wrinkles (crests and troughs) in the 2D bi-crystalline graphene; in contrast, the pristine graphene structure in PE/GRP nanocomposite relatively remained flattened (minimal out of plane displacement) during tensile deformation as captured in Fig.5. 

Q15. What is the effect of the stress-strain responses on graphene?

It can also be inferred from the stress-strain responses plotted in Fig.3 and Fig.4 that increment in tensile strength of nanocomposites is more prominent in bi-crystalline graphene containing higher mis-orientation angles. 

Q16. What is the way to prove that bi-crystalline graphene is a superior reinforcement?

All the simulations help in concluding that bi-crystalline graphene is a superior reinforcement for developing the future nanocomposites as compared to pristine PE nanocomposites.