Materials and Design 134 (2017) 103–110
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Graphene-based masterbatch obtained via modified polyvinyl alcohol liquid-shear exfoliation and its application in enhanced polymer composites Douglas A. Simon a,b, Eveline Bischoff a,b, Giovanna G. Buonocore c, Pierfrancesco Cerruti d,e, Maria G. Raucci c, Hesheng Xia f, Henri S. Schrekker a, Marino Lavorgna c,e,⁎, Luigi Ambrosio c, Raquel S. Mauler a,⁎ a
Institute of Chemistry, Universidade Federal do Rio Grande do Sul – UFRGS, Av. Bento Gonçalves 9500, Porto Alegre, RS 91501-970, Brazil Instituto Federal do Rio Grande do Sul – IFRS – Campus Farroupilha, Av. São Vicente 785, Farroupilha, RS 95180-000, Brazil c National Research Council, Institute for Polymers, Composites and Biomaterials, P.le E. Fermi 1, 80155 Portici, Naples, Italy d National Research Council, Institute for Polymers, Composites and Biomaterials, Via Campi Flegrei 34, 80078 Pozzuoli, Naples, Italy e National Research Council, Institute of Polymer, Composites and Biomaterials, Via Previati 1/C, 23900 Lecco, Italy f State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute, Sichuan University, Chengdu 610065, China b
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Production of scalable single step graphene-based masterbatch. • Application of graphene-based masterbatch directly into nanocomposites. • Exceptional improvement in properties for packaging with 0.1% of filler addition.
a r t i c l e
i n f o
Article history: Received 22 May 2017 Received in revised form 12 August 2017 Accepted 15 August 2017 Available online 15 August 2017 Keywords: Graphene production Graphene masterbatch Polymer composite Barrier and mechanical properties Packaging
a b s t r a c t A simple and inexpensive method for the production of graphene-based masterbatch via polymer-assisted shear exfoliation of graphite in water was comprehensively investigated. In detail, a modified polyvinyl alcohol (mPVOH) characterized by surface energy comparable with that of graphene was used as surfactant for the production of graphene-like particles. The proposed approach allowed a yield in graphene-like particles higher than that obtained by using common surfactants, along with a narrower size distribution. A mPVOH-masterbatch containing 4.38 wt% of graphene-like particles was produced by removing the aqueous solvent from a dispersion and directly used for production of polymer nanocomposites by melt processing. Films prepared by blending the masterbatch with polyvinyl alcohol in order to have a graphene-like particles content equal to 0.3 wt% showed a 78% reduction in water permeability and a 48% increase in storage modulus as compared with pristine polymers. Improved barrier properties were also observed for polylactic acid (PLA) and low-density polyethylene (LDPE)-based composite films, whereas an increment of about 520% in the storage modulus was observed for the composite obtained with PLA. The obtained results are very relevant and the proposed process will open up a new pathway for using graphene-based masterbatch in the packaging industry. © 2017 Elsevier Ltd. All rights reserved.
⁎ Corresponding authors. E-mail addresses:
[email protected] (M. Lavorgna),
[email protected] (R.S. Mauler).
http://dx.doi.org/10.1016/j.matdes.2017.08.032 0264-1275/© 2017 Elsevier Ltd. All rights reserved.
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D.A. Simon et al. / Materials and Design 134 (2017) 103–110
1. Introduction Graphene has a unique, single-atomic-layer 2D structure consisting of sp2 carbons bonded together in a honeycomb lattice [1]. The peculiar properties of graphene as single layers include mechanical strength (Young's modulus of ~1.0 TPa), electrical conductivity (intrinsic charge mobility of 200,000 cm2 V− 1 s−1), and thermal conductivity (~5300 W m−1 k−1) [2]. All these properties make the graphene a suitable candidate for potential use in a wide range of applications, including composites [3–5], electronics [6], gas barrier and membranes [7–9] and thin conductive films [10]. Graphene was first identified by means of micromechanical cleavage of graphite powder in 2004 [1,11]. Afterwards other methods have been developed to increase graphene production scalability [12]. Alongside with graphene as a single layer of C atoms, are also very interesting from industrial point of view both the few layers graphene (FLG) particles which consist of nanoparticles characterized by a number of layers between 3 and 10, and graphene nanoparticles (GNP) which have up to 100 graphene layers [13]. Unfortunately, the increase in the number of stacked layers erodes almost all physical properties that make the graphene an astonishing material. It has been found for instance that the Young's modulus decreases by about 40–60% moving from a single layer to a particle with 5 to 10 layers stacked [14]. However, the production of a few grams of single layer graphene by using any known processes is very difficult as well as not-sustainable. Thus, in order to produce sustainable and innovative polymer-based composites it is necessary to have a tradeoff among properties, costs and processability. In this respect, the use of FLGs and GNPs, characterized by a more accessible cost despite a lower intrinsic quality, could represent a more viable approach, alternative to the high quality single layer graphene [13] and FLG can outperform graphene for polymer-based composites due to better distribution [14]. Top-down approaches addressed to exfoliate graphite into graphene-like nanoparticles consisting of a mixture of single layer graphene, FLGs and GNPs particles are very promising for the largescale production due to their simplicity [15,16]. In this context, liquid shear exfoliation (LSE) is recently attracting huge scientific and industrial attention because it is a low-cost and scalable process exhibiting high-throughput potential, as well as the possibility of integration with other processes [17]. In these solution-based techniques, raw material, such as graphite powder, is dispersed in organic solvents or aqueous surfactant solutions. Shock waves and cavitation generated by sonication as well as shear produced by a rotor break apart the graphite flakes into small particles as single layer graphene and few layers of graphene [18]. After removing the larger particles by centrifugation, a homogeneous liquid dispersion, including separated monolayers, FLGs and GNPs particles is currently obtained [19]. To achieve a large production rate of defect-free and high aspect ratio graphene with LSE in water, the surfactant choice plays an important role. It has been stated that in order to increase yield, surface energy (δS) of exfoliating solutions should be similar to that of graphene at 46.7 mN/m [20,21]. The obtained graphene-like particles are commonly stabilized by the surfactant molecules which could affect the properties of graphene and limit their applications [22]. However, after the production of reliable graphene-like particles, the problem of how to produce graphene-based polymer nanocomposite still a challenging issue. Generally, after graphite exfoliation, the process consists of several steps including separation of large particles, removal of surfactants and solvent when it is economically and technologically possible, separation of graphene/FLG/GNP by centrifugation and refining of filler distribution by a stepwise approach consisting of redispersion, centrifugation, washings and final evaporation of solvent and finally dispersion in the polymeric matrix [17,22]. This process is very time-consuming and hinders the real industrial applications of graphene-based nanocomposites.
Recent reports have described the possibility of producing nanocomposites via masterbatch dilution process [23–26]. However, as for the nanocomposites based on graphene and its derivatives, this approach has been applied only for reduced graphene oxide [27,28] and expanded graphene oxide [29]. Table S1 (in Supplementary information) summarizes the main papers which deal with the graphenebased masterbatch production and applications and it is clearly shown that there is a lack of papers focused on graphene-like particles. In this paper, we demonstrate that a modified polyvinyl alcohol (mPVOH) which is currently known as a high amorphous polyvinyl alcohol is very efficient for the production of graphene-like particles, i.e. a mixture of graphene single layer, FLG and GNP particles by using liquid-shear exfoliation in aqueous solutions. The exfoliating effectiveness of the mPVOH as surfactant was compared with those of common surfactants such as Polysorbate 80, Triton X-100 and sodium dodecylbenzene sulfonate. The obtained dispersions, after the exfoliation process, exhibited a high concentration of graphene-like particles which are characterized by narrow particle size distribution. The aqueous dispersion was freeze-dried to remove the water solvent and the resulting solid was used directly as a masterbatch for incorporating graphene-like particles into selected polymer matrices. To this aim, commercial extrusion grade mPVOH, low density polyethylene (LDPE) and polylactic acid (PLA) were mixed with the graphene-based masterbatch by melt compounding, producing innovative graphenebased composites through a versatile and ease approach. The composites were characterized in terms of barrier and mechanical properties and the results show that the approach proposed is suitable to open new opportunities to the use of graphene-based composites in the field of packaging applications. 2. Experimental 2.1. Materials Natural graphite flakes, Polysorbate 80, Triton X-100 and Sodium dodecylbenzenesulfonate (SDBS) were purchased from Sigma-Aldrich. Water soluble mPVOH and extrusion grade EGmPVOH (MFR 4.0 g/10 min, 190 °C, 2.16 kg) were purchased from Nippon Gohsei. LDPE (PB608) was supplied from Braskem S.A. and PLA (Purapol L100IXS) was obtained from Purac company. All materials were used as received. 2.2. Polymer-assisted liquid-exfoliation protocol Dispersions consisting of 25 g of graphite and 8 g mPVOH in 1 L of deionized water were mixed at high-shear by using two protocols (see below for more details). In both cases the mixing duration lasted for 100 min, and dispersion aliquots were collected at different times (i.e. 3, 7, 15, 25, 40, 55, 70, 85 and 100 min). Similar dispersions were prepared by using SDBS, Triton X-100 and Polysorbate 80 to compare their graphite exfoliation effectiveness with that of mPVOH. The two high-shear mixing approaches indicated as Protocol I and Protocol II, were as follows: – Protocol I: the shear and hydrodynamic pressure were generated by using a high-speed steel blender with 28 mm diameter blades which operated at 24.000 rpm. – Protocol II: a rotor-stator system (Ultra-Turrax T25 – IKA) equipped with a S25N 25GST dispersing element was used to produce shear forces at 12.000 rpm.
After the exfoliation step, the dispersions and aliquots were centrifugated at 4000 rpm for 60 min to remove large particles. The supernatants were collected and analyzed in order to evaluate the quality and size of obtained graphene-like particles. Finally, the resulting
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supernatants were freeze-dried overnight to remove water. The graphene-based masterbatch (GrM) consisting of graphene-like particles dispersed in the mPVOH was then stored at room temperature until being further used for the production of polymer composites. 2.3. Characterization of graphene dispersions and graphene-based masterbatch In order to evaluate the content of graphene-like particles in the dispersions (obtained after 100 min of high-shear mixing), two different methods, i.e. filtration and TGA measurements were carried out. In the first approach, 30 mL of the dispersion was vacuum filtrated by using alumina membranes with pore size of 0.2 μm. The obtained graphene film was subsequently rinsed with deionized water several times to remove the surfactant. The films were dried at 70 °C under vacuum for 48 h and then they were weighed. Alternatively, thermogravimetric analysis (TGA) was used to determine the residues of the graphene dispersions heated at 750 °C under nitrogen flow which were then compared with the TGA residue of mPVOH aqueous solution. By using the effective content of graphene-like particle in the dispersion it was possible to determine the extinction coefficient value (α) for the Lambert-Beer equation and apply UV/Visible spectroscopy to evaluate graphene-like particle concentration in all aliquots, during the exfoliation process. UV–Vis absorption spectra of graphene dispersions were collected at λ equal to 660 nm with an Agilent Cary 60 UV/Vis spectrophotometer, using 10 mm plastic cuvettes. Dispersion samples were diluted in deionized water (1:8 v/v) to avoid absorption saturation. The surfactant absorption at 660 nm is negligible as evident from a reference spectrum of a surfactant solution at high concentration (equal to 5 wt%) (spectrum not reported for sake of brevity). Raman spectra of the graphene-like particles were measured by Jobin-Yvon HR LabrRam micro-Raman by using a 533 nm HeNe laser emission on glass slide. The samples were dried out on the slide from a 1 mL drop before the measurement. Bright field transmission electron microscopy (TEM) experiments were performed on a FEI TECNAI G12 Spirit-Twin (120 kV, LaB6) microscope equipped with a FEI Eagle 4 k CCD camera. Dry samples were prepared on holey carbon-coated copper grids (300 mesh, lacey carbon, Ted Pella) by placing a drop of a diluted dispersion on a grid and allowing it to dry at ambient conditions before storage. Also, ultrathin specimens (~ 70 nm) were cut from the middle section of injectionmolded specimens, in the perpendicular direction to the melt flow and under cryogenic conditions, by using a RMC CRX microtome equipped with a diamond knife. The cutting was performed at − 120 °C, and the film was retrieved onto 300 mesh Cu grids. Dynamic Light Scattering (DLS) measurements were made with a Malvern Zetasizer Nano ZS, using a 633 nm HeNe laser. Dispersion samples were tested in plastic cuvettes having 10 mm path length. The machine was operated in backscatter mode at an angle of 173°. Samples were equilibrated to 25 °C for 120 s prior to measurement. Values for solvent parameters at 25 °C were used from software library. An automatic measurement duration setting was used, with automatic measurement positioning and automatic attenuation. The samples were analyzed as prepared without further dilution. After the production of graphene-based masterbatch by high shear exfoliation, centrifugation and freeze-drying, the graphene masterbatch was further analyzed to assess the final concentration of graphene. That was determined by subtracting the residue of a mPVOH solution at 750 °C under nitrogen flow from the TGA residue of the graphene-based masterbatch (GrM). The thickness of graphene-like particles was measured by atomic force microscopy (AFM), using an SPM 5500 AFM instrument from Agilent Technologies operating in acoustic mode. The experiments were performed on a dilute sample of the dispersion applied on a silicon surface. A conventional silicon tip with resonance frequencies of 240– 440 kHz and spring constant of the tip ranging from 20 to 75 N/m was
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utilized. The free oscillating amplitude was 3.0 V and the set point and the gains were adjusted to improve the image resolution. 2.4. Preparation and characterization of polymer composites with graphene-based masterbatch The most important application of graphene-like particles produced by LSE is in the field of multifunctional polymer composites, as it seems to be the only method capable to produce the large amounts as required at industrial scale. In order to validate the potentials of GrM in the production of films for packaging three different composites were prepared by blending the masterbatch with commercial EGmPVOH, LDPE and PLA as matrices. The addition of GrM to the polymer matrix was realized by using a Haake Rheomix mixer equipped with two co-rotating rotors, operating at 190 °C and with rotor speed fixed at 100 rpm. The final graphene-like particle contents were 0.1 and 0.3 wt% for EGmPVOH and 0.3 wt% for LDPE and PLA-based composites. After mixing, film samples were obtained by compression molding at 190 °C and 10 MPa pressure, resulting in a regular thickness of about 200 μm. Differential scanning calorimetry (DSC) analysis was performed by using a Perkin Elmer DSC 6000, operating under nitrogen flow. The samples were heated from 30 to 200 °C at a heating and cooling rate of 10 °C/min. Dynamic mechanical analysis (DMA) was performed by using a Perkin Elmer DMA 8000 instrument operating in tension mode at a fixed frequency of 1 Hz and 0.005 mm strain. The films were heated from 30 to 100 °C at a rate of 3 °C/min. Water permeability tests were performed by using a Permatran W3/ 31 device (Mocon, Germany). Samples with a surface area of 5 cm2 were tested at 25 °C. Permeation tests were performed by setting the relative humidity at the downstream and upstream sides of the film to 0% and 50% respectively. A flow rate of 100 sccm of nitrogen was used. Each test was carried out in triplicate in order to determine the mean and the standard deviation. 3. Results and discussion The surface tension of water at 25 °C is about 72.8 mN/m [21] while for graphene it has been estimated to be equal to 46.7 mN/m [30]. It is well known that the best graphite exfoliation (i.e. maximum process yield of graphene-like particles with satisfactory aspect ratio) is realized by minimizing the surface energy difference between graphene and solvent, which in this case is a solution of mPVOH in water. The surface tension of the mPVOH/water solution was measured at 23 °C by using a contact angle apparatus, showing values slightly lower than 50 mN/m. This value agrees with some results reported in the literature. In fact, it has been found that for solutions with graphene concentrations higher than 2 wt% the value reduces to 48 mN/m [31]. In the case at hand, it is likely that during high-shear mixing due to heat generation, the surface tension reduces and approximates to the values of 45 mN/m, which results to be within the optimal range for graphene exfoliation of 40–50 mN/m according to Pugno et al. [13]. 3.1. Characterization of graphene dispersions In order to evaluate the capability of the various surfactants to exfoliate graphite, UV/Vis spectra of the dispersions collected after centrifugation were recorded, as shown in Fig. 1. Samples were prepared according to Protocol I and diluted 1:8 with deionized water to avoid saturation. SDBS has the lowest absorbance at 660 nm equal to 0.30 a.u. while Polysorbate 80 and Triton-×100 performed similarly at 0.83 and 0.90 a.u., respectively. Under the same conditions, mPVOH has the highest absorbance equal to 1.69 a.u., as a confirmation of the great potential to exfoliate graphite into graphene-like particles. The concentration of graphene-like particles in the dispersion (i.e. after 100 min of high-shear mixing) was measured by weighing the
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Fig. 1. UV/Vis spectra for graphene dispersions (1:8 dilution) prepared with several surfactants by Protocol I.
Fig. 2. Raman spectra of graphite flakes (black) and graphene-like particle dispersion in mPVOH (blue curve). The inset shows 2D peak shift in the range between 2500 and 2800 cm−1. The arrows point to the main peak. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
amount of solid in 30 mL of dispersion by both vacuum filtration and drying and TGA analysis. By taking into account these concentrations value, the extinction coefficient for the graphene-like particles at 660 nm (α660) was estimated from the UV–Vis absorbance by using the Lambert-Beer eq. [22]. The value obtained is α 660 = 1306 mL mg− 1 m − 1 in agreement with previous reports for
surfactant-stabilized aqueous graphene dispersions [21]. The concentration of graphene-like particles present in the dispersion of mPVOH after 100 min of high-shear mixing is 1.04 mg mL − 1. The dispersion of graphene-like particles in mPVOH remains stable for several months (see Fig. S1 in the Supplementary information). Fig. 2 compares Raman spectra of both starting graphite and graphene-like particles produced using mPVOH as surfactant. Graphite exhibits its characteristic spectrum with a clear G (~ 1581 cm−1) and 2D (~2715 cm−1) bands [32]. A downshift of the 2D band to lower frequencies (about 23 cm−1) was noted in the spectrum of graphene-like particle samples together with an intensity enhancement compared to that of graphite, which is indicative of the few-layer nature of mPVOH-stabilized graphene in the dispersions, in agreement with previous reports [33]. A notable increase of the D band intensity is also observed for the graphene-like particle samples, indicating that the number of defects or particle edges is increased during exfoliation. Such defects can be divided into two main types: basal-plane defects and edge defects [17]. Basal plane defects bring about a G bands broadening, as found in the chemically reduced graphene, while edge defects are introduced by fragmentation effect and are unavoidable [33]. In this case, since the narrow G band has not changed, the modification of the D band is mainly ascribed to edge contributions, in agreement with the findings of Varrla et al. [19] confirming that no basal-plane defects are introduced during the liquid shear exfoliation. Fig. 3 shows a bright field TEM image of typical graphene-like particles, both nanosheets and FLGs, prepared by LSE by using mPVOH as surfactant. In particular, it is possible to observe a cluster composed by several monolayers and FLGs spread across a 120 μm2 area (Fig. 3a and inset) and a single FLG particle composed of 5–10 layers with a length of about 4.9 μm and a width of about 2 μm (Fig. 3b and Fig. S4 in the Supplementary information). In both cases the carbon grid is seen underneath indicating the presence of only a limited number of stacked layers. This is confirmed by AFM topography of a spin coated dispersion deposited over a silicon wafer from which it is possible to detect the presence of FLG particles with 3–5 graphene layers (see Fig. S5 in the Supporting information). Ferrari et al. [15] pointed out that typical values for particle sizes produced by LSE are in the range of 0.01 to 1 μm. In this case, the FLG is exceptionally large which is very important to have effectively enhanced polymer composites. The particle size distribution (PSD) of graphene-like particles can be accurately measured by Dynamic Light Scattering (DLS) as reported by Lotya et al. [34]. PSD of dispersions aliquots obtained by using Protocol I are shown in Fig. 4. The primary peak of the distributions related to the collected aliquots, which result very narrow, is centered at about 400 nm. Despite the size of graphene-like particles is comparable with those shown in other reports [17,35], it appears very interesting the regularity of particle size which does not change significantly with the exfoliation time. This result which may be tentatively ascribed to a direct effect of
Fig. 3. TEM images of graphene-like particles deposited from the dispersions. (A) A cluster of graphene nanoparticles and (B) a large FLG particle.
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process of Protocol I as a sliding of layers and ascribed the regularity in the particle size to the sliding taking place in groups of 5 to 10 layers at a once. However, it has also been reported that the exfoliation may take place by erosion [37] or fragmentation mechanism [38]. As for the broad PSD obtained through the Protocol II it is worth noting that the rotor-stator induces a very high localized shear which likely brings about a random breakage of the particle layers, as found by Patton et al. [17]. 3.2. Characterization of graphene-based masterbatch
Fig. 4. Particle size distribution of graphene-like particles stabilized with mPVOH at different exfoliation times by using Protocol I (in the legend are indicated the times of high-shear application).
the mPVOH on the exfoliation mechanism was not previously reported in the literature, wherein the particle size seems to be dependent on both time and surfactant [35]. A small peak is observed at around 5 μm which is assigned to bigger particles (as the one shown in Fig. 3) despite it may also be ascribed to small dust particles or air bubbles in the dispersion [34]. However, it appears evident that the particle size distribution clearly depends on the method adopted for applying the high shear, i.e. Protocol I and Protocol II. Fig. 5 shows the PSD for graphene-like particles (i.e. mixture of graphene single layer, FLG and GNP particles) stabilized with mPVOH, Polysorbate 80 and Triton X-100 and produced after 100 min of high shear-mixing. The results show that the larger size is obtained when the liquid exfoliation is assisted by mPVOH as surfactant, confirming that the mPVOH has a positive effect on the exfoliation mechanism to preserve the high aspect ratio of the particles. In detail, it results that the mean particle size is almost double as compared to those obtained by using conventional surfactants. The graphene-like particles produced by Protocol II present a broad size distribution (i.e. resulting in size from 70 nm to 3 μm) and a larger mean size. This result could be ascribed to the different exfoliation mechanisms which occur when both different surfactants and methods are adopted for the high shear-liquid exfoliation. Alaferdov et al. [35] and Coleman et al. [36] described the exfoliation of graphite in a similar
Fig. 5. PSD of graphene-like particles obtained by using both different surfactants with Protocol I and mPVOH with Protocol II. Mean size values are indicated over the curves in nm.
Polymer/graphene composites are currently manufactured by using graphene and its derivatives (graphene oxide, reduced graphene oxide and functionalized graphene) via in situ polymerization, solution blending or melt compounding [7,39]. However, the most common industrial process for the dispersion of particles into a polymeric matrix is by using a filler concentrated masterbatch. Due to the narrower PSD, the graphene dispersion prepared by using the Protocol I procedure (i.e. after 100 min of exfoliation) was selected for the production of graphene-based masterbatch through a preliminary step finalized to the removal of water solvent by freeze-drying approach. TGA thermograms for pristine mPVOH and graphene-based masterbatch (GrM) are shown in Fig. 6. At 750 °C the residue for the GrM is equal to 4.38 wt% which is significantly higher than that of mPVOH (around 0.001 wt%). This residue is ascribed to the presence of graphene-like particles dispersed in the mPVOH phase constituting the masterbatch. Moreover, the presence of graphene-like particles dispersed within the polymeric matrix reduces its thermal stability, and despite some papers have reported the opposite [40] for several thermoplastics, the same effect has already been identified for PVOH filled with graphene oxide [41]. 3.3. Characterization of polymer composites obtained by using graphenebased masterbatch Polymer composites have been extensively studied with the premise of improving mechanical, electrical and thermal properties of polymers [42–44]. In general, the key factors which need to be optimized for obtaining materials with enhanced structural and functional properties are both appropriate dispersion and interfacial interactions between the polymeric matrix and filler [45]. As for graphene filler obtained through LSE procedure, it has been shown that the chemical affinity between graphene covered with the common surfactants and polymers is often very low [39]. The graphene-based masterbatch (GrM) produced by using the mPVOH has been tested in three different polymers commonly applied
Fig. 6. TGA curves of pristine mPVOH (black) and graphene-based masterbatch (red curve). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 7. (A) Water vapor permeability for EGmPVOH and its composites with graphene-based masterbatch (the numerical values on the bar columns represent the reduction of water permeability with respect to the pristine polymer) and (B) storage modulus for the composite containing 0.3 wt% graphene compared to the pristine polymer.
in the packaging industry, i.e. EGmPVOH, LDPE and PLA. EGmPVOH is highly compatible with GrM, as it consists of a different grade of mPVOH (always provided by Nippon Gonsei) which has been modified with additives for enabling processability and minimizing the thermal degradation of polymer during the extrusion process. Detailed information on composites compositions are reported in Table S2 (in Supplementary information). Fig. 7A) and B) show the barrier properties in term of water vapor permeability and thermo-mechanical properties of composites obtained by blending GrM with EGmPVOH. It was found that water permeability decreased by about 53% with 0.1 wt% of graphene-like particles and about 78% with 0.3 wt%. This effect is ascribed to the formation of a very tortuous morphology which hinders the diffusion of water molecules throughout the film samples [46]. However, since the graphene-based particles do not have a good affinity for water, it is also evident that the composites present a low water vapor solubility which contributes to reduce the overall water vapor permeability. An increase in the storage modulus was also observed all over the investigated temperature range with an increase of about 48% at room temperature ascribed to a strong interaction which take place between the graphene-like particles and the matrix. This interaction, which will be further investigated in the future, is likely responsible for the long-lasting stability of the graphene-like dispersion stabilized with mPVOH (as shown in Fig. S1). These results are much higher than those found by Shao et al. [47] using graphene oxide, and could be similar to those described by Wang et al. [9] using a time consuming layer-by-layer procedure for graphene oxide in poly (ethyleneco-vinyl alcohol). When pristine mPVOH, which is hugely hydrophilic, is mixed with LDPE, a non-polar polymer, it does not affect the water permeability. However, a reduction of about 21% is obtained in presence of 0.3 wt% of graphene-like particles added via graphene-based masterbatch (Fig. 8A). The amount of crystalline phase of the blend between LDPE
and GrM remains similar to that of pristine LDPE (Fig. S2 and Table S3 in Supplementary information) with no change in the mechanical properties (Fig. 8B). Checchetto et al. also reported that GNPs inclusion does not change the crystalline fraction nor the size of the LDPE crystallites [48].This result was expected since it is probable that the graphenelike particles remain encapsulated in the mPVOH matrix, which is not miscible with polyethylene. The graphene-like particles were homogeneously dispersed in the hosting polymeric matrix (see Fig. S6 in Supplementary information) by creating a tortuous pathway which contributes to the decrement of the water vapor permeability. In PLA, a polar polymer like mPVOH, a small decrease (about 10%) on its barrier property was found when 0.3 wt% graphene-like particles were added by using the masterbatch (Fig. 9A). This reduction is partially due to the presence of mPVOH since water permeability decreases by 3.8% when only mPVOH was added. However, even in this case, it is inferred that the largest contribution to the reduction of water permeability is due to the induced tortuous path due to the presence of well dispersed 2D particles (see Fig. S6 in Supporting information), with some contribution from an increase in Tg [49]. Similar results were obtained by Ambrosio-Martín et al. using reduced graphene oxide [23]. They obtained a reduction of water vapor permeability of about 4% and 13% by using respectively 0.1 wt% and 0.5 wt% of graphene as filler in PLA. PVOH and PLA have high compatibility in the range of composition herein investigated (the weight ration of PLA to mPVOH is about to 94/6) [50] and the addition of GrM improved the storage modulus from less than 6 × 109 Pa for pristine PLA to about 4.1010 Pa for the composite which corresponds to an increment of about 520% for a content of about 0.3 wt% of graphene-like particles. At the same time, an increase in Tg was found in DMA (Fig. 9B) and DSC analysis (shown in Fig. S3 in the Supplementary information). DSC shows a change in Tg from 55 °C (for pristine PLA) to approximately 61 °C (closer to mPVOH Tg at 69 °C).
Fig. 8. (A) Water vapor permeability for LDPE and LDPE containing 0.3 wt% graphene obtained by blending LDPE with graphene-based masterbatch (the numerical values on the bar columns represent the reduction of water permeability with respect to the pristine polymer) and (B) storage and loss moduli for LDPE and LDPE containing 0.3 wt% graphene composite.
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Fig. 9. (A) Water vapor permeability for pristine PLA and PLA composite with 0.3 wt% Graphene obtained by graphene-based masterbatch (the numerical values on the bar columns represent the reduction of water permeability with respect to the pristine polymer) and (B) storage and loss moduli for pristine PLA and PLA composite with 0.3 wt% graphene.
4. Conclusions In this paper, it has been identified a successful low-cost and scalable process to prepare a graphene-like particles masterbatch directly from a liquid shear exfoliation method by using a commercial modified polyvinyl alcohol as suitable surfactant. In particular, the mPVOH has shown better results for exfoliating (by liquid shear exfoliation) and stabilizing graphene-like particles (i.e. single layer graphene platelets, FLG and GNP particles) than other surfactants tested. Masterbatch produced by freeze drying the aqueous dispersion exhibits a very high graphene content (around 4.38 wt%) and can be directly applied on industrial process to produce improved films for packaging applications based on EGmPVOH, PLA and LDPE, with lower water permeability and enhanced mechanical properties. Therefore, the proposed methodology extends the scope for scalable high throughput liquid-phase production graphene, which can promote the applications of graphene via a simple masterbatch addition to polymeric matrices. The obtained results are very relevant and the proposed process will open up a new pathway for using graphene-based masterbatch for innovative and multifunctional composite materials addressed to the packaging industry.
Acknowledgements The authors are grateful to CAPES (Grant 310290/2015-5) and CNPq (Grant 400531/2013-5) for financial support. R. S. Mauler and H. S. Schrekker are grateful to CNPq for the PQ fellowships. This work was also supported by the Joint Laboratory for Graphene based Multifunctional Polymer Nanocomposites funded by CNR (Joint Lab Call (2015–2018) and established between the IPCB-CNR and the Polymer Research Institute of Sichuan University. Authors also are grateful to Dr. M. Pannico and Dr. N. M. Balzaretti for Raman analysis and Dr. A. R. Pohlmann for UV spectroscopy. Appendix A. Supplementary data The Supporting information shows a dispersion consisting of graphene-like particles in mPVOH after 8 months from the production, prepared by Protocol I, a summary of recent reports in the graphene masterbatch field and a table describe the detailed composition of polymer nanocomposites. DSC thermograms for LDPE and LDPE with 0.3% of graphene-like particles and PLA and PLA filled with 0.3% of graphenelike particles obtained by blending the polymeric matrix and the graphene-based masterbatch. Thermal properties and crystallinity are presented in a detailed table for LDPE and PLA based composites. Micrograph images obtained by TEM for the edge of a graphene-like particle and the its dispersed in LDPE and PLA matrix. Also, AFM topography for the graphene-like particles deposited over a silicon wafer with height measurements.
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