Facile fabrication of graphene oxide/poly(styrene-co-methyl methacrylate) nanocomposite with high toughness and thermal stability

Highlights

• Optimization of monomer ratios of poly(styrene-co-methyl methacrylate) copolymer based on mechanical properties

• Mechanical, thermal, and structural analysis of the effect of graphene oxide concentration in the copolymer matrix

• Improvement of the mechanical and thermal properties of the copolymer by adding a small amount of graphene oxide via solvent-free in situ bulk polymerization

• Thermal stability of the nanocomposite was found to be much better than the neat copolymer

Abstract

Graphene and related nanomaterial-based polymer composites have shown the potential to resolve the long-standing conflict between strength and toughness, the two vital mutually exclusive mechanical properties. The uniform dispersion of the nanofillers in polymer matrices to attain strong matrix-filler interfacial bonding, which is essential for effective load transfer between the polymer matrix and fillers, is the least investigated aspect and a major challenge in composite engineering. Copolymeric materials can be exploited to enhance the distribution of nanofillers. Herein the optimization of monomer ratios of the poly (styrene-co-methyl methacrylate) copolymer and a facile method to fabricate graphene oxide (GO) reinforced nanocomposites using in situ bulk copolymerization are reported. The ultimate tensile strength, failure strain, and storage modulus of the injection molded copolymer were increased by 14.6, 15, and 43%, respectively, by adding only 0.1 wt.% GO. Also, the thermogravimetric analysis revealed that the thermal stability of the nanocomposite is much better than the neat copolymer. Crack arresting mechanism and dispersion state of GO sheets in the copolymer matrix were also investigated using scanning and transmission electron microscopes. Thus, this paper provides a methodology for uniform dispersion of GO in copolymeric materials to attain high toughness and thermal stability.

Introduction

Graphene is a flat monolayer of carbon atoms tightly packed in a two-dimensional honeycomb lattice [1]. A great deal of effort has been made to develop lightweight, strong, and tough graphene-reinforced composite materials owing to the exceptional physical properties of graphene. For instance, Young's modulus near 1 TPa and almost 100 times greater tensile strength (130 GPa) than steel [2], thermal conductivity above 3000 Wm^-1 K^-1 [3], complete impermeability to gases [4], and extremely high specific surface area [5]. However, due to the large aspect ratio, van der Waals, and π-π interactions, exfoliated graphene layers have a strong tendency to aggregate and phase separate, resulting in poorly dispersed bundles and agglomerates in polymer-matrix [6]. Nevertheless, one fundamental property of graphene is that it can be chemically functionalized by oxygen-containing groups such as epoxy, hydroxyl, carbonyl, and carboxylic acid groups—this oxidized form of graphene is known as graphene oxide (GO). The hydrophilicity-hydrophobicity balance of GO, ability to dissolve and form stable colloid solutions in water and a wide range of organic solvents, and better interfacial-bonding capabilities of its functional groups—attached to the basal plane and edges—with polymer-matrix, made GO a promising candidate for filler-reinforced polymer composites.

As reported in recent works of literature, the addition of GO in polymer matrices leads to considerable improvements of not only the mechanical properties including strength, toughness, stiffness, and hardness but also the viscoelastic and thermal properties of the virgin polymers [[7], [8], [9], [10], [11], [12], [13]]. Solution blending, melt blending, and in situ polymerization are the three main methods to prepare GO-based polymer nanocomposites [14]. The homogeneous dispersion and exfoliation of GO sheets in polymer matrices are difficult to achieve in the case of melt blending due to the high viscosities of polymer melts. Besides, GO sheets undergo in situ thermal reduction and lose oxygen-containing functional groups, which leads to a weak interaction between GO sheets and polymer chains, and as a consequence, inferior mechanical properties are observed [15]. Also, melt blending of two immiscible polymers in the presence of nanofillers results in selective localization of nanofillers in only one phase or at the interface of the two phases [16]. This inhomogeneous distribution decreases the reinforcement effect of the nanofillers. Though it is much easier to get uniform dispersion and full exfoliation of GO nanosheets via solution blending, this processing technique is not environmentally friendly, and solvent removal is a critical issue [17]. On the other hand, in situ bulk polymerization is a facile solvent-free method through which graphene can be dispersed homogeneously in polymer matrices [14].

Copolymerization is an effective way to tune the properties of virgin polymers. For instance, the copolymer of styrene (St) and methyl methacrylate (MMA) combines the excellent ease of processing of polystyrene (PS) with the high-strength and crystal clarity of poly(methyl methacrylate) (PMMA). This copolymer has ubiquitous use in the homeware and toy industries. The monomer ratio and dispersion state of nanofillers have a strong influence on the polymer nanocomposite properties. Therefore, optimization of the monomer ratios of a copolymer, and homogeneous dispersion of the fillers can enhance the mechanical as well as thermal properties and, in turn, will significantly broaden the scope of the material.

Although organoclay, montmorillonite, graphite, carbon nanotube, and graphene reinforced PS and PMMA immiscible blends prepared by melt mixing or solution blending have been studied before [15,16,[18], [19], [20]], no research article, to date, reported the reinforcement of poly(St-co-MMA) with GO or even any graphene-related nanomaterials via in situ bulk copolymerization, and evaluated the effect of GO during and after the copolymerization. Moreover, although poly(St-co-MMA) was synthesized before, to the best of our knowledge, no research has been done to optimize the monomer ratio in terms of mechanical properties. Herein, initially, we optimized the monomer ratio to synthesize poly(St-co-MMA). Later, we tried to improve the mechanical and thermal properties of the material by adding GO at a minimal level. We also performed microstructural and chemical characterizations and analyzed the effect of GO wt.% on the properties of the copolymer. GO is expected to provide better interface bonding and filler loading effect at extraordinarily low filler content predominantly due to functional groups containing many reactive zones. Also, PS is non-polar, whereas PMMA consists of a non-polar hydrocarbon backbone and somewhat polar pendant ester groups. The difference in polarity of the repeating units in the copolymer chain is also anticipated to affect the dispersion morphology of the GO sheets. Furthermore, we devised an ingenious experimental procedure to initiate in-situ copolymerization under ultrasonication at 60 °C to prevent aggregation and achieve high dispersion, a process of prime industrial importance for the preparation of well-dispersed nanocomposites. The prepared strong and thermally stable copolymer nanocomposite should find applications such as in sports, toy, coating, automobile industries, etc.