Elsevier

Journal of Alloys and Compounds

Volume 684, 5 November 2016, Pages 254-260
Journal of Alloys and Compounds

Strong and ductile graphene oxide reinforced PVA nanocomposites

https://doi.org/10.1016/j.jallcom.2016.05.162Get rights and content

Highlights

  • PVA-GO (0.3 wt%) exhibited 150% increment in tensile strength over PVA.

  • Composite with in-situ reduced fillers showed excellent ductility (220% strain).

  • PVA-GO films had randomly oriented fillers in the matrix with strong interfacial.

  • PVA-rGO composites had preferred orientation of the fillers in the plane of the film.

  • Such interesting mechanical properties with only 0.3 wt% fillers are new information.

Abstract

Graphene oxide (GO) and its derivatives have been widely used as reinforcement in polymer matrices for enhanced mechanical and electrical properties. While graphene has higher elastic modulus and tensile strength than that of graphene oxide, it is the latter’s hydrophilic nature that provides a distinct advantage as a filler in the aqueous processing of polymer nanocomposites. In this work, we have tried to disseminate the effects of GO and reduced graphene oxide (rGO) as fillers in polyvinyl alcohol (PVA) matrix. At a loading of only 0.3 wt% GO in PVA, we observed ∼150% increment both in elastic modulus and tensile strength, which is unprecedented. The property enhancement was attributed to the homogeneous distribution of fillers, and strong interfacial interactions (hydrogen bonds) between the fillers and the matrix. Composites fabricated after in-situ reduction of GO fillers had elastic modulus comparable to that of pure PVA, but had considerably improved tensile strength. The failure strain of rGO based composites was much higher than that of both pure PVA and GO reinforced PVA composites, thus showing enhanced ductility. The microstructure of the PVA-rGO composites exhibited alignment of the fillers in the plane of the polymer film, with interfacial bonds between the fillers and matrix only at the rGO sheet edges. The distribution of GO fillers in the PVA-GO composites, however, was random with no preferred orientation. A higher degree of interfacial interactions and the homogeneous distribution of the fillers led to enhanced elastic modulus and strength, along with considerable ductility for the PVA-GO composites.

Introduction

Graphene exhibits exceptional thermal, mechanical and electrical properties [1] for which it has gained tremendous scientific attention in the recent years. Its thermal conductivity (∼5300 Wm−1 K−1) and tensile elastic modulus (∼1060 GPa) are amongst the best, and its tensile strength is comparable to that of carbon nanotubes [2], [3]. Besides, its large theoretical specific surface area (2630 m2 g−1) [4], high intrinsic mobility (200,000 cm2 v−1 s−1) [5], [6], and optical transmittance (∼97.7%) merit attention for exploitation in many functional applications [7], [8]. One of the most promising areas of this material is in polymer nanocomposites, where the exceptionally high elastic modulus and tensile strength of graphene or graphene derivatives [9] manifest in significant improvement in the mechanical properties of the composites. It is logical to expect that the interface-controlled properties are functions of the structure of graphene (or its derivatives) in a polymer nanocomposite. Exfoliation of graphene oxide produced by thermal or sonication methods provide morphology with varying lateral dimensions [10], [11], which influences the degree of interfacial interactions and mechanical interlocking. Additionally, interfacial chemistry of the fillers also plays an important role in the mechanical and electrical properties of reinforced polymer composites. According to the Lerf-Klinowski model [12], graphene oxide (GO) produced by Hummers or modified Hummers method creates graphene sheets with hydroxyl and epoxy groups present in the basal plane, and carboxylic acid groups at the edges of the sheets [13]. The presence of these functional groups makes GO hydrophilic, enabling its easy dispersion in polar solvents to form a stable colloid. This property of GO provides excellent opportunity for exploitation as reinforcement in aqueous based polymer composite fabrication.

Polyvinyl alcohol (PVA) is a hydroxyl rich, non-toxic, biocompatible, and water-soluble polymer system, which can be processed by aqueous methods. It is an important engineering polymer with various applications in membrane technology [14], fuel cells [15], drug delivery [16], and shape memory effects [17]. It has often been considered as a model polymer matrix to study the reinforcement behaviour of GO based fillers. Several authors have reported the strong influence of filler loading and the synthesis routes on the mechanical behaviour with moderate to more than 100% increase in tensile strength with filler loadings in PVA composites having of 2–3% fillers [18], [19], [20], [21], [22], [23]. Mo et al. fabricated GO based PVA composites with tensile strength of 280 MPa and elastic modulus of 13.5 GPa, albeit with 50 wt% loading of fillers [24]. These reports have shown mechanical property improvement with filler content that remained in the levels of a few percent or more. Progressively, however, comparable levels of mechanical property enhancements have reported for matrices with lower levels of filler loading, eg. 0.5–0.7 wt%, with approaches, such as in-situ reduction of GO, and covalent functionalization of GO filler [25], [26], [27], [28]. Additionally, synergistic effect of two types of fillers (multiwalled carbon nanotubes and graphene oxide nanosheets) has recently exhibited enhanced tensile strength with 1.25 wt% fillers [29].

A few general trends can be observed despite the scatter reported in the literature on the mechanical properties of PVA based composites. Increase in the tensile elastic modulus has generally resulted in the lowering of failure strain [24], [30], [31], [32], [33]. The ductile to brittle transition has recently been related to the dehydration of water in the composite and its interfacial bonding [34]. More importantly, the difference in the effect of graphene oxide and graphene on the mechanical properties in PVA (or any polymer system) has not been clearly spelt out. Additionally, since the word graphene is often used to describe functionalised or partially reduced graphene oxide (rGO), careful reading of the original papers is often necessary to determine the exact type of filler employed [35]. The purpose of this study is to explore the possibility of fabricating composites that possess high elastic modulus along with considerable failure strain (which is difficult to synthesize, but desirable for engineering applications). We have also addressed the subtle difference in the effect of GO and rGO on the properties of the composites by fabricating composites with only 0.3 wt% of fillers. GO was used via solution blending technique with water as the processing solvent. Subsequently, the GO loaded polymer blend was reduced in-situ to prepare rGO loaded composite films. Tensile properties of the films have been measured followed by a series of chemical and structural characterization techniques.

Section snippets

Experimental

Graphite fine powder (Loba Chemie, 99% pure, particle size 50 μm), potassium permanganate (Loba Chemie), sulphuric acid (98%, Merck), hydrochloric acid (98%, Merck), nitric acid (Merck), PVA with molecular weight 1,15,000 and degree of polymerization 1700–1800, were procured from commercial sources and used without any further purification. Composite fabrication and characterization methods are outlined as follows. Synthesis of graphene oxide was carried out by the modified Hummers method with

Results and discussions

The X-ray diffractogram of graphite flakes exhibited a sharp diffraction peak at 26.6°, corresponding to the (002) reflections, and d-spacing value of 0.336 nm (Fig. 1). The diffractogram of GO powders exhibited a peak at a 2θ value of 10.9°, which corresponds to d-spacing of 0.78 nm. Such a change in the position of the diffraction peak and interlayer spacing is well reported for the formation of graphene oxide, which arises due to the existence of oxygen containing functional groups that

Summary

PVA composites with minimal GO loading of 0.3 wt% exhibited ∼150% increment in tensile elastic modulus as well as strength, and higher failure strain than those of pure PVA films. The improvement in the mechanical properties was ascribed to stronger interfacial bonding between the fillers and the matrix, and the wrinkled morphology of the fillers. The comparatively higher failure strain of the PVA-GO composites along with a higher elastic modulus resulted from the randomly oriented morphology

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