Fabrication of exfoliated graphene-based polypropylene nanocomposites with enhanced mechanical and thermal properties

Abstract

Despite the great potential of graphene as the nanofiller, to achieve homogeneous dispersion remains the key challenge for effectively reinforcing the polymer. Here, we report an eco-friendly strategy for fabricating the polymer nanocomposites with well-dispersed graphene sheets in the polymer matrix via first coating graphene using polypropylene (PP) latex and then melt-blending the coated graphene with PP matrix. A ∼75% increase in yield strength and a ∼74% increase in the Young’s modulus of PP are achieved by addition of only 0.42 vol% of graphene due to the effective external load transfer. The glass transition temperature of PP is enhanced by ∼2.5 °C by incorporating only 0.041 vol% graphene. The thermal oxidative stability of PP is also remarkably improved with the addition of graphene, for example, compared with neat PP, the initial degradation temperature is enhanced by 26 °C at only 0.42 vol% of graphene loading.

Introduction

Compared with traditional composites, polymer nanocomposites exhibit dramatic changes in some properties at very low loadings (generally ≤2 vol%) of nanofillers like exfoliated nano-silicate layers [1], [2], [3], carbon nanotubes [4], [5], and graphite nanoplatelets [6], [7], [8], [9]. However, the performance conferred by these nanofillers can be achieved only when homogeneous dispersion of nanofillers and strong interfacial adhesion between the nanofillers and the polymer matrix are realized. Graphene, a single-layered two-dimensional (2D) structure, is regarded as the strongest material to date by theoretical and experimental results [10], [11]. Chemically similar to carbon nanotubes and structurally analogous to silicate layers [12], graphene nanosheets are considered to be the most promising alternative to simultaneously improve the mechanical properties and barrier properties as well as thermal properties. Despite the potential of graphene as the nanofiller, to achieve homogeneous dispersion remains the key challenge for effectively reinforcing the polymer, especially for the non-polar polymer like polypropylene (PP) and polyethylene (PE) [13], two kinds of widely used general plastics in the world. To achieve the uniform dispersion of graphene in the polymer matrix, solution-mixing is the ideal strategy when these polymers can dissolve in common polar organic solvents. To date, it has been successfully achieved to disperse graphene sheets or graphene oxide in some polar polymers such as poly(methyl methacrylate) (PMMA), poly(acrylonitrile) (PAN), poly(acrylic acid) (PAA), polyester, epoxy resin, thermoplastic polyurethane (TPU), poly(vinyl alchol) (PVA) using solution-mixing technique [14], [15], [16], [17], [18], [19], [20], [21], [22]. Moreover, exfoliated polystyrene (PS)/graphene nanocomposites are also obtained using phenyl isocyanate-treated graphite oxide and using DMF as the solvent [23].

When it comes to the graphene nanocomposites based on PP and PE, solution-mixing technique becomes impossible since PP and PE are only soluble in solvents like xylene and trichlorobenzene above 120 °C. Galland [24] and Yu [13] have respectively employed the in situ Ziegler–Natta polymerization method to successfully create the exfoliated polyolefin/graphite nanocomposites, however, leaving the mechanical and thermal properties not investigated. Additionally, this method also has limited applicability and scalability. Recently, Song [25] prepared PE/graphene oxide nanocomposites using melt-blending method by adopting PE-grafted-graphene oxide. Despite relative good dispersion of graphene oxide in PE matrix, limited improvements in the mechanical properties were achieved (a maximum increase by ∼20% in the Young’s modulus, and ∼13% in the tensile strength). Kim et al. [26] have recently also fabricated exfoliated graphene/PE nanocomposites via employing PE functionalized analogs (amine, nitrile and isocyanate) with thermally reduced graphene, and the tensile modulus was greatly enhanced and higher for composites with functionalized PE when 1,2-dichlorobenzene was used as solvent for solvent mixing. Only until recently Torkelson [12], [27] successfully prepared the fully exfoliated PP/graphite nanocomposites by the solid-state shear pulverization (SSSP) technique, and incorporating 2.5 wt% graphite could lead to a ∼100% increase in the Young’s modulus and a ∼60% increase in the yield strength relative to neat PP. Moreover, the mechanical performance may still has room to improve at a relatively high loading of 2.5 wt% graphite or remains at a less addition of graphene nanosheets. In addition, latex technology has already been successfully applied for incorporating carbon nanotubes or graphene into a polymer matrix [28], [29], [30], [31]. Koning [28], [29] uses sodium dodecylsulfate (SDS) or polysaccharide (Gum Arabic, GA) as the surfactant and polystyrene (PS) latex as the polymer matrix to successfully fabricate well-dispersed PS/carbon nanotubes (CNTs) nanocomposites with low percolation thresholds of 0.3 wt%. Miltner [30] has incorporated CNTs into the PP latex with the help of sonication to prepare well-dispersed PP/CNTs composites, and investigated the induced nucleation effect of CNTs on the polymer matrix. Recently, Loos [31] has prepared exfoliated polystyrene (PS)/graphene nanocomposites using poly(styrene sulfonate) (PSS) as the stabilizer for graphene during the reduction of graphene oxide followed by mixing PSS coated graphene with PS latex. All of their work employed the surfactant as the stabilizer of nanofillers and the polymer latex as the polymer matrix.

Here, we report the creation of fully exfoliated PP/graphene nanocomposites with the graphene coated with PP latex, which overcomes the stacking together again of graphene sheets during melt-blending and provides superb polymer–particle interaction and effective load transfer. Consequently, A ∼75% increase in yield strength and a ∼74% increase in the Young’s modulus of PP are obtained by a graphene sheet addition of only 0.42 vol% (1.0 wt%). Compared with Torkelson’s work [12], [27], comparable improvements in the mechanical properties could be achieved at relatively lower graphene loading. Additionally, the thermal oxidative stability is also significantly improved with the addition of graphene sheets due to the barrier effects. This is very promising to fabricate high-performance polymer nanocomposites with graphene as the nanofiller.