materials Review Polymer Nanocomposites—A Comparison between Carbon Nanotubes, Graphene, and Clay as Nanofillers Mrinal Bhattacharya Department of Bioproducts and Biosystems Engineering, University of Minnesota, St. Paul, MN 55108, USA; [email protected]; Tel.: +1-612-625-5234 Academic Editor: Biqiong Chen Received: 17 December 2015; Accepted: 18 March 2016; Published: 1 April 2016 Abstract: Nanofilled polymeric matrices have demonstrated remarkable mechanical, electrical, and thermal properties. In this article we review the processing of carbon nanotube, graphene, and clay montmorillonite platelet as potential nanofillers to form nanocomposites. The various functionalization techniques of modifying the nanofillers to enable interaction with polymers are summarized. The importance of filler dispersion in the polymeric matrix is highlighted. Finally, the challenges and future outlook for nanofilled polymeric composites are presented. Keywords: polymer nanocomposites; carbon nanotubes; graphene, clay; properties 1. Introduction The use of fillers for the enhancement of polymer properties has been well documented. Initially, fillers were used to reduce the cost of the polymeric products. However, with time, fillers became an integral part in many applications, particularly for reinforcing the mechanical properties of the polymer. “Reinforced” polymers consist of a polymeric matrix and a relatively stiff inorganic filler that undergoes dramatic change in modulus or stress at given strain over the pure polymer. Traditional fillers include talc, glass fibers, carbon black, and calcium carbonate particles in the micrometer range. However, most micron sized traditional fillers require high loading for modest property enhancement, causing problems in melt flow and processing due to the high viscosity of the filled materials. Furthermore, the high density of traditional fillers also leads to heavier composites. Finally, the lack of interfacial interaction between the filler and the polymeric matrix leads to weak interfacial adhesion and results in failure. A broad diversity of filler sizes has been used in reinforcing polymeric matrix. Edwards [1] in his review for filler reinforcement observed that “there is, nevertheless, good evidence that small particle size is a necessary requirement, and very likely the predominant requirement, for the reinforcement effect in rubber”. High degree of reinforcement is observed in the particle size range of 100 nm and below [1]. It has been reported that nylon 6 required three times more mass of glass fibers than clay montmorillonite (MMT) platelets to cause a doubling in the modulus [2]. Nanofillers in the range of 3%–5% by weight achieve the same reinforcement as 20%–30% of microsized fillers. Thus, nanocomposites have a weight advantage over conventional composites and, nanoscale materials have emerged as an attractive candidate as fillers as their increased specific interfacial area enables potentially higher interfacial interactions and hence, higher modulus. Nanofillers can be categorized on the basis of their dimensions—one dimensional which include nanotubes and nanowires [3,4], two dimensional such as nanoclays [5] and graphene [6], and three dimensional such as spherical [7] and cubical nanoparticles [8]. Carbonaceous nanofillers such as nanotubes and graphene display excellent properties due to their high mechanical strength and high aspect ratio. Graphene is a two-dimensional single atom thick sheet composed of sp2 carbon structure arranged in a honeycomb structure (Figure1). It can be considered as a fundamental building block for Materials 2016, 9, 262; doi:10.3390/ma9040262 www.mdpi.com/journal/materials Materials 2016, 9, 262 2 of 35 Materials 2016, 9, 262 2 of 34 all sp2 hybridized carbon allotropes. It can be wrapped up into 0D fullerenes, rolled into 1D nanotubes orfundamental 3D graphite building when a numberblock for of all graphene sp2 hybridized layers stack carbon up. allotropes. The 2D geometry It can be of graphenewrapped nanosheetsup into 0D isfullerenes, responsible rolled for into the maximum1D nanotubes value or 3D of itsgraphite surface when to volume a number ratio. of graphene These are lay favorableers stack for up. good The reinforcement2D geometry of of graphene polymers. nanosheets is responsible for the maximum value of its surface to volume ratio.Carbon These are nano favorable tubes have for good attracted reinforc attentionementbecause of polymers. of their unusual structures and properties. HighCarbon Young’s nano modulus tubes in have the directionattracted ofattention the nanotube’s because axis,of their electrical unusual conductivities structures and that properties. vary from insulatingHigh Young’s to metallic, modulus and in their the hollowdirectio structuresn of the nanotube are attractive’s axis, features. electrical Carbon conductivities nanotubes arethat half vary as densefrom insulating as aluminum to metallic, and have and tensile their strengths hollow structures 20 times that are ofattractive high strength features. alloys Carbon [9]. Studiesnanotubes [10, 11are] havehalf as shown dense that as nanotubesaluminum display and have extraordinary tensile strengths mechanical 20 times properties—tensile that of high strength modulus alloys of 1 TPa,[9]. tensileStudies strength [10,11] inhave the shown range of that 50–150 nanotubes GPa and display a failure extraordinary strain in excess mechanical of 5%. properties—tensile modulusA graphene of 1 TPa, nanosheet tensile strength with ain Young’s the range modulus of 50–150 of GPa 1 TPa and and a failure ultimate strain strength in excess of of 130 5%. GPa is oneA ofgraphene the strongest nanosheet materials with knowna Young’s [12 ].modulus It has aof high 1 TPa specific and ultimate area of 2600strength m2/g, of 130 very GPa high is electricone of the conductivity strongest material (6000 S/cm)s known [13 [12]], thermal. It has a conductivityhigh specific (~5000area of 2600 W/mK) m2/g, [14 very] and high high electric gas impermeabilityconductivity (6000 [15 ].S/cm) [13], thermal conductivity (~5000 W/mK) [14] and high gas impermeability [15]. FigureFigure 1.1. GrapheneGraphene isis aa 2D2D building building material material for for carbon carbon materials materials of suchof such as 0Das buckyballs,0D buckyballs, 1D nanotubes1D nanotubes or 3D or graphite.3D graphite. Reproduced Reproduced from from Reference Reference [13] [13] with with permission. permission. Clays are naturally found as platelets, stacked from a few to as many as one thousand sheets. A Clays are naturally found as platelets, stacked from a few to as many as one thousand sheets. single sheet of MMT was reported to have an in plane Young’s modulus ranging between 178 and A single sheet of MMT was reported to have an in plane Young’s modulus ranging between 178 and 265 GPa [16,17]. These excellent properties of nanofillers make them suitable candidates for 265 GPa [16,17]. These excellent properties of nanofillers make them suitable candidates for reinforcing reinforcing polymer matrix. Montmorillonite (MMT), is the most widely used clay nanofiller, polymer matrix. Montmorillonite (MMT), is the most widely used clay nanofiller, sandwiched between sandwiched between two silicate layers of an octahedral sheet of alumina. The nanometer-scale two silicate layers of an octahedral sheet of alumina. The nanometer-scale sheets of aluminosilicates sheets of aluminosilicates have dimensions of 1–5 nm thickness and 100–500 nm in diameter have dimensions of 1–5 nm thickness and 100–500 nm in diameter (Figure2). These dimensions lead (Figure 2). These dimensions lead to platelets of high (>50) aspect ratio. Hence, when blended with to platelets of high (>50) aspect ratio. Hence, when blended with polymer, it enables stress transfer polymer, it enables stress transfer from the polymer to the mineral. The stiffness of the clay minerals from the polymer to the mineral. The stiffness of the clay minerals results in increased mechanical results in increased mechanical properties of the blend. properties of the blend. This article reviews the processing techniques for developing nanocomposites and summarizes This article reviews the processing techniques for developing nanocomposites and summarizes their properties. The focus will be between 1D versus 2D carbonaceous nanofillers (nanotubes versus their properties. The focus will be between 1D versus 2D carbonaceous nanofillers (nanotubes versus graphene), and between two different 2D nanofillers (graphene versus clay). We limit the discussion graphene), and between two different 2D nanofillers (graphene versus clay). We limit the discussion to to 1D and 2D fillers as it has been reported that 2D fillers provide higher degree of reinforcement 1D and 2D fillers as it has been reported that 2D fillers provide higher degree of reinforcement than than spherical shaped fillers [18]. spherical shaped fillers [18]. Materials 2016, 9, 262 3 of 35 Materials 2016, 9, 262 3 of 34 Figure 2. Structure of montmorill montmorillonite.onite. Reproduced from referencereference [5]] withwith permission.permission. 2. Considerations Considerations for for Developing Developing Nanocomposites Nanocomposites Because ofof the the contrast contrast in composition, in composition, interaction, interaction and properties, and properties between dissimilar between components dissimilar incomponents nanocomposites, in nanocomposites, several key factors several affect key thefactors role thataffect nanoparticles the role that play nanoparticles as reinforcing