Materials Today Communications
Facile fabrication of graphene oxide/poly(styrene-co-methyl methacrylate) nanocomposite with 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.
Section snippets
Materials
Styrene (St, 99%) and methyl methacrylate (MMA, 99%) liquid monomers, graphite, hydrogen peroxide (H2O2, 30%), sulfuric acid (H2SO4, 95.098%), azobisisobutyronitrile (AIBN, 98%) were purchased from Sigma-Aldrich. Potassium permanganate (KMnO4, 99%) was bought from Caledon. Phosphoric acid (H3PO4, 85%), methanol, and tetrahydrofuran (THF, 99%) were purchased from Fisher Scientific. St and MMA liquid monomers were passed through a column packed with Al2O3 powder, and AIBN was recrystallized by
Characterization of GO
FTIR-ATR spectra of pristine graphite and GO are shown in Fig. 2a. The GO exhibits characteristic broad absorption band in the range of ∼3000 to 3500 cm-1, which is ascribed to the stretching vibration of OH groups. The absorption peak observed at 1735 cm-1 corresponds to the stretching vibration of CO bonds in the carboxyl group, while the sharp absorption band at 1614 cm-1 is related to the stretching of sp2 alkene (CC) bonds in unoxidized graphitic domain [21]. The peak at 1221 cm-1 can be
Conclusion
In this article, we optimized the monomer ratio of styrene and methyl methacrylate to synthesize poly(styrene-co-methyl methacrylate) copolymer and reported the effect of GO on the microstructural, mechanical, and thermal behavior of the optimized copolymer, and further elucidated the reinforcement effect by varying GO content from 0.075 to 1 wt.%. The injection-molded GO (0.1) nanocomposite exhibited a ∼14.6% improvement in strength and a ∼15% increase in ductility, leading to a notable
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgment
The authors acknowledge the partial financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and Alberta Innovates.
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