Elsevier

Composites Part B: Engineering

Volume 148, 1 September 2018, Pages 272-280
Composites Part B: Engineering

Nanoscale friction of graphene oxide over glass-fibre and polystyrene

https://doi.org/10.1016/j.compositesb.2018.04.001Get rights and content

Abstract

Coatings of graphene oxide over two substrates of glass-fibre and polystyrene were obtained by electrophoretic deposition (EPD). A chemical reduction of graphene oxide by exposure to hydrazine hydrate at 100 °C significantly changes the interfacial interaction with the substrate as well as the tribology. Spectroscopic techniques like Fourier transform infrared, Raman spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction showed that the treatment with hydrazine replaces oxygen functional groups and also induces roughness, a structural disorder and decreases the interlayer separation in the transition from graphene oxide (GO) to reduced graphene oxide (rGO). Treatment with hydrazine reduces adhesion and friction force against diamond like carbon coated Si probe (DLC AFM) at the basal plain of the coatings. Investigation at the edges revealed that the presence of oxygenic functional groups leads to higher shear strength with glass-fibre and polystyrene which reduces after treatment with hydrazine.

Introduction

Graphene oxide (GO) is a layered material constituted by graphene sheets functionalized with epoxy and hydroxyl groups [1,2]. The presence of oxygen functional groups makes GO highly dispersible in polar media such as aqueous solutions [3]. This feature of GO is important for the preparation of nanocomposites and superior if compared with CNT, graphene and metallic oxide nanomaterials, since these have a tendency to agglomerate during the synthesis process [[3], [4], [5], [6], [7]]. GO has an amphiphilic character that gives rise to extensive interactions with the polymers. It has been stated that the edge polar groups especially carboxylic of GO might form a chemical bond with the polar polymers, such as hydrogen bonds, while the basal plain groups like phenol hydroxyl and peroxide groups consists of a network of hydrophobic polyaromatic island of unoxidized benzene rings [8] that may induce some physical interlinking such as C–H, π–π, etc [9].

Modification of functional groups can tune the surface interactions of GO, useful in a wide range of applications that include sensing and self-healing [[10], [11], [12]]. Numerous results have been reported which prove the possibility to tune the interfacial adhesion between GO and the substrates both with chemical and physical treatments. For instance, pre-treated polyethylene terephthalate (PET) showed prominent adherence to GO film through electrostatic adhesion [1]. Addition of multi-wall carbon nanotubes (MWCNT) and GO can reduce the wear rate by 40% which significantly enhance tribological performances as compared to the MWCNT/epoxy composites. It was observed that GO enhances the MWCNT-epoxy adhesion/interlocking and the glass transition temperature of the composite [13]. GO sheets decorated with nano diamond crystals effectively hindered the aggregation of GO and played a vital role to enhance fracture toughness through crack pinning mechanism in the epoxy polymer matrix composite [14]. Chen et al. [15] modified GO substrate through amino groups to produce covalent bonds between GO and glass-fibre (GF) which enhanced strength and toughness between GF and polymer matrix. Inclusion of GO as an interphase in epoxy/glass composites results in an improved load-transfer between the matrix and the fiber [16].

The investigation of GO-substrate interfacial interaction is advantageous to evaluate interfacial adhesion between graphene-based fillers, fibers, and polymer matrix [17]. In fact, the mechanical performances of structural composites markedly depend on the way the load is transferred from the matrix to the load-bearing reinforcements [16,18,19], especially with the involvement of shear stresses [20]. Several reports revealed that functionalized GO can provide a mechanical reinforcement in polymer composites higher than graphene [21,22]. Well dispersed GO sheets effectively modify the surface energy and can improve the wettability between fiber and matrix to inhibit crack propagation in the final composite [23]. Good interfacial interaction is essential to ensure efficient load transfer from polymer matrix to the fillers, which helps to reduce stress concentration and improve overall mechanical properties [24]. The GF/epoxy composite display strong hydrogen bonds between GO and GF/epoxy [24]. The polar groups in GO are helpful in enhancing the interfacial adhesion by establishing physical-chemical bonding [7]. Feng et al. [25] found that GO sheets functionalized with polystyrene (PS) chains are able to play a positive effect on the thermal and mechanical properties of the PS related composite. Similarly, the strong interfacial interaction between GO and poly (methyl methacrylate) (PMMA) yields ductile and tougher composites than the pristine PMMA [22].

Several studies indicate that the interaction between GO and substrate is a critical parameter to govern the mechanics of load transfer in polymer composites as well as for the stability of coatings [26]. In this scenario, shear strength (τ) measurement is one of the viable options for the assessment of interfacial adhesion between film and substrate. It is a measurement of the resistance against shear loading of the coating-substrate interface (adhesive strength) or the strength of the coating itself (cohesive strength) [27]. Despite its significance, experimental measurements of the shear strength for GO over polymer substrates have rarely been reported. One of the prime reasons for their scarcity is the interfacial behaviour of GO which is intricately associated with a variety of functional groups and the presence of topological defects [28]. The variation in functional groups diverges the shear response that leads to a wide range of friction characteristics [29], therefore τ depends on the material chemistry and functional structure which determines the physical properties.

In the present work, GO coatings were deposited over polystyrene and glass fibers substrates. After chemical reduction by hydrazine hydrate they were referred as reduced graphene oxide. The impact of oxygen functional groups and their modification after reduction was analysed through spectroscopic and crystallographic techniques. Adhesion forces and friction response between GO and rGO against AFM (atomic force microscopy) tip was investigated and the shear strength (τ) of GO or rGO coating over GF and PS were evaluated. This aim was reached through tribological studies by atomic force microscopy. The chemical modification significantly changes tribological characteristics of the coated sheets and allows to probe elastic/plastic response of thin films behaviour under compressive and shear stresses.

Section snippets

Synthesis and coating of graphene oxide over glass-fibre and polystyrene

Graphene oxide was synthesized by following the Hummer's method with slight modification [30]. Briefly, graphite powder (1 g) was added to H2SO4 (46 ml) in an ice-cooled bath. This was followed by adding NaNO3 (1 g) and stirring for 15 min. Then KMnO4 (6 g) was slowly added into the mixture to avoid a spontaneous exothermic reaction. The mixture was then stirred for at least 24 h at 35 °C. Finally, an excess of distilled water was added to the above mixture while the temperature was kept at

Morphology and chemical characterization

The steps involved in the preparation of GO/GF and GO/PS is showed in a schematic diagram in Fig. 1. The EPD procedure is implemented to coat cylindrically shaped GF of approximate diameter of 16 μm and over flat PS. Prepared coatings were exposed to hydrazine hydrate at 100 °C for 24 h. The comprehensive methodology is described in “Method” section. The morphology and distribution of produced GO sheets from EPD procedures are shown in Fig. 2 by FESEM (a, b) and AFM (c, d). GO sheets are

Conclusion

GO and rGO through hydrated hydrazine were investigated over GF and PS surfaces. The morphological and chemical characterization revealed a significant impact of chemical reduction. The roughness of the coating was found to increase after hydrazine treatment especially on GO/PS due to the involvement of additional hydrazine groups. A substantial amount of oxygenic functional groups of graphene oxide were replaced by hydrazine and resulted in a small quantity of O and N atoms. The interlayer

Acknowledgement

N.M.P. is supported by the European Commission H2020 under the Graphene Flagship Core 2 No. 785219 (WP14 ″Polymer composites") and under the Fet Proactive "Neurofibres" No. 732344.

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    Current affiliation: Department of Mathematics and Physical science, University of Sussex, Brighton, UK.

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