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graphene pair to the rheological and mechanical properties of polyethylene matrix based nanocomposites
Applied Clay Science 150 (2017) 244–251
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Research paper
Contribution of the organo-montmorillonite/graphene pair to the rheological and mechanical properties of polyethylene matrix based nanocomposites
MARK
Fath Eddine Zakaria Rahmaouia, Pascal Medericb,⁎, Nourredine Aït Hocinec, Aïcha Aït Saadaa, Nathalie Poirotd, Idir Belaidia a
Department of Mechanical Engineering, University of M'hamed Bougara, Boumerdes, Algeria IRDL, CNRS-FRE 3744, Université de Bretagne Occidentale, 6 avenue Victor Le Gorgeu, CS 93837, 29238 Brest, Cedex 3, France LMR, INSA Centre Val de Loire, 3 rue de la chocolaterie, BP 3410, 41034 Blois, France d GREMAN, IUT de Blois, 15 rue de la chocolaterie, 41000 Blois, France b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Montmorillonite Graphene Nanocomposite Structure Mechanical properties Rheological properties
Since the last decade, graphene nanoplatelets with their exceptional physical properties are used as fillers in thermoplastic blends. In this work, the influence of commercial graphene nanoplatelets on properties of a high density polyethylene, in solid and melt states, was investigated, in comparison with the one of organically modified montmorillonite fillers. The use of a compatibilizer, a maleic anhydride grafted polyethylene, led to a clay based nanocomposite, with some improved mechanical and rheological properties, but with disappointing mechanical properties at break. On the other hand, by reducing the viscosity during mixing, the added compatibilizer slightly lowered the degree of dispersion of high aspect ratio graphene particles, weakening the material. More interestingly, the nanocomposite constituted with both clay nanoplatelets and lamellar graphene particles exhibited better reinforcing characteristics, in melt and solid states. This result can be partially explained by the high viscosity of the clay based nanocomposite which helps in the separation of graphene particles during mixing.
1. Introduction Applicative and fundamental research in the field of nanocomposites, which offer better end-use properties than those of microcomposites, at equivalent filler volume fraction, has significantly evolved. Since two decades, the relations between processing, structure and macroscopic behavior of this class of nanomaterials were thoroughly studied, as reviewed by Camargo et al. (2009). In particular, organically modified montmorillonite (OMt) is now successfully used as lamellar nanofiller for the elaboration of polar or apolar thermoplastic based nanocomposites (Ray and Okamoto, 2003), presenting improved mechanical (Alexandre and Dubois, 2000; Kato et al., 2003; Hotta and Paul, 2004; Aït Hocine et al., 2008), barrier (Alexandre and Dubois, 2000; Kato et al., 2003; Hotta and Paul, 2004; Alexandre et al., 2009) and flammability resistance (Gilman et al., 2000) properties. Moreover, melt-state rheological properties of OMt based nanocomposites have also attracted significant attention (Krishnamoorti and Yurekli, 2001), because of their relevance in the identification of the processing/ structure/rheological properties relations (Médéric et al., 2006). More ⁎
recently, graphene (G), which is the fewest layer limit of graphite, has emerged as a bidimensional material, with a high aspect ratio and exceptional properties (Zhu et al., 2010). Various routes leading down to the individual graphene layer, such as mechanical exfoliation (Novoselov et al., 2004), chemical conversion of graphite to graphene oxide (Stankovich et al., 2007), chemical vapor deposition (Kim et al., 2009), and total organic synthesis (Watson et al., 2001), have been largely reviewed by Allen et al. (2009). But some advances are still required (Singh et al., 2011), especially in the low cost production of large amounts of graphene nanoplatelets (Zhu et al., 2010). 2-D graphene is used as filler for polymer based nanocomposites (Kuilla et al., 2010; Dhand et al., 2013), because it presents a great potential for applications requiring improved electrical, thermal, mechanical and barrier properties (Potts et al., 2011). However, for an apolar polyolefin matrix, such as polyethylene, the incorporation of a compatibilizer, that is commonly a maleic anhydride grafted polyethylene, is required in order to obtain an organically modified montmorillonite based nanocomposite (Hasegawa et al., 1998) or a graphene based nanocomposite (Schniepp et al., 2006; Kim
Corresponding author. E-mail address: [email protected] (P. Mederic).
http://dx.doi.org/10.1016/j.clay.2017.09.037 Received 3 July 2017; Received in revised form 20 September 2017; Accepted 26 September 2017 Available online 05 October 2017 0169-1317/ © 2017 Elsevier B.V. All rights reserved.
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organically modified montmorillonite, namely Cloisite® C20A, commercialized by Southern Clay Products (Gonzales, Texas, TX). This organoclay is a dimethyl dihydrogenated-tallow ammonium exchanged montmorillonite. The individual C20A particles are nanoplatelets with ~1 nm thickness and ~500 nm length. The density of C20A organoclay is ~2 g/cm3. Polar graphene particles, referenced N006-P nano-graphene platelets and supplied by Angstron Materials (Ohio), present an oxygen percentage of 4%, average x–y dimensions less than 5 μm and a thickness ranging from 10 to 20 nm. The density of graphene is also close to 2 g/cm3.
et al., 2011; Seo et al., 2013; Vasileiou et al., 2014). In this case, graft ratio and average molar mass of the compatibilizer, as well as compatibilizer/filler ratio, play a key role in the final structure and resulting physical properties of the thermoplastic based nanocomposite. These parameters have thoroughly been studied for clay based nanocomposites (Hotta and Paul, 2004; Chrissopoulou et al., 2005; Wang et al., 2005; Durmus et al., 2007), but are still in discussion for graphene based nanocomposites (Seo et al., 2013; Mittal and Krauss, 2014). On the other hand, the melt intercalation technique is often used for the elaboration of polyolefin based nanocomposites because it requires no toxic solvent and it is practical for manufacturing in large scale (Zhang et al., 2010). OMt based nanocomposites are characterized by the existence of a three-dimensional percolated network mesostructure at low clay mass fractions, usually ~2% for polar matrix (Utracki and LyngaaeJørgensen, 2002) or polyolefin/compatibilizer matrix (Durmus et al., 2007). The low mass fraction threshold is explained by the fine dispersion of anisometric OMt nanoparticles within thermoplastic matrix. It was put in evidence from linear or nonlinear properties in melt state (Hoffmann et al., 2000; Ren et al., 2000; Utracki and LyngaaeJørgensen, 2002; Aubry et al., 2005; Durmus et al., 2007) and in solid state (Aït Hocine et al., 2008; Hassan et al., 2015), and, to a lesser degree, from gas barrier (Alexandre et al., 2009) and thermal (Hassan et al., 2015) properties. Similarly, the incorporation of a low percentage of graphene into a polyolefin matrix was shown to significantly improve mechanical, rheological and thermal properties (Song et al., 2011; El Achaby et al., 2012; El Achaby and Qaiss, 2013), because graphene nanosheets are well-dispersed and exhibit a high specific surface area, especially favoring the efficient transfer of stresses through the matrix/ filler interface (Srivastava et al., 2011). Recent studies have shown that it is conceivable to improve the dispersion of carbon black particles or carbon nanotubes by incorporating them with organically modified montmorillonite nanoplatelets into an elastomer (Pradhan et al., 2015; Annadurai et al., 2016) or a thermoplastic (Ma et al., 2007; Silva et al., 2014) matrix. The apparent synergy between the two fillers leads to an improvement of flame retardancy, mechanical, electrical and acoustic properties. Synergy between fillers in G OMt polylactic acid nanocomposites was also reported (Bouakaz et al., 2015). The paper focuses on the influence of composition on structural, thermal, mechanical and rheological properties of thermoplastic based nanocomposites constituted of a compatibilizer/polyethylene matrix filled with OMt, G, or OMt and G nanoplatelets, aiming at the study of the mechanisms of a possible synergy between the two kinds of fillers.
2.2. Preparation of nanocomposites All samples have been prepared by simultaneous melt mixing in a Haake Rheomix 600 internal mixer, at a temperature of 160 °C. The blade rotational speed was of 100 rpm during 6 min. The processing conditions, hence the resulting specific mixing mechanical energy (Médéric et al., 2009), were chosen to avoid the degradations of the matrix and compatibilizer and, more particularly, that of the organic modifier of clay (Xie et al., 2001). Polymers and fillers were manually pre-mixed before being thrown into the mixer. The samples were pelletized and processed by compression molding at 160 °C between 2 mm thick plates. Pressure was increased by steps from 0 to 25 MPa, in order to avoid the formation of air bubbles. The compositions of all samples are listed in Table 1, in filler mass fractions ϕm. In the study, the compatibilizer mass fraction was fixed at 40%, which was shown to be a relevant content in order to obtain exfoliated OMt polyolefin/compatibilizer nanocomposites (Lertwimolnun and Vergnes, 2005). Moreover, in the case of G polyolefin/compatibilizer, the (G:compatibilizer) ratio is usually chosen ranging from (1:3) to (1:10) (Seo et al., 2013). At last, for the G OMt PE/grPE nanocomposite, the ratio of (G:OMt) was chosen (1:1), which was shown to be the most interesting in terms of structural and mechanical properties for multiwalled carbon nanotubes/organically modified montmorillonite hybrids (Pradhan et al., 2015). 2.3. Characterizations 2.3.1. Structural characterization Samples were microtomed under a frozen atmosphere into liquid nitrogen and their nanostructures were observed by transmission electron microscopy (TEM), using a JEOL JEM-1230 microscope at 120 kV.
2. Experimental part 2.3.2. Differential scanning calorimetry The influence of graphene or/and clay particles on the PE crystallinity was studied from DSC measurements conducted on a PerkinElmer DTA-7, under a nitrogen atmosphere. The samples of about 10 mg were heated from room temperature up to 250 °C at a rate of 10 °C/min, held at 250 °C for 10 min, then were cooled from 250 °C to
2.1. Materials A high density polyethylene, noted PE in the text, referenced as HDPE 5502 and supplied by POLYMED, was used as matrix. Its density is 0.94 g/cm3 and its melting point is 139.9 °C. The number average molar mass is 17,300 g/mol, corresponding to a degree of polymerization of about 600, and the polydispersity index is 7.8 (Liu et al., 2010). A maleic anhydride grafted polyethylene, referenced as Orevac® 18,507, supplied by Arkema was used as compatibilizer, noted grPE in the text. The maleic anhydride grafting level is inferior to 1%. The density and the melting point of the compatibilizer are 0.95 g/cm3 and 128 °C, respectively. The Newtonian complex viscosity of both thermoplastics, measured at 160 °C, is: η0⁎ ~ 250,000 Pa·s and η0⁎ ~ 55,000 Pa·s for the PE matrix and the compatibilizer, respectively. The compatibilized blend obeys to a blending law, meaning that the two thermoplastics are miscible, hence a Newtonian viscosity of ~ 170,000 Pa.s for the blend constituted of 40% of compatibilizer used in this work. Clay and graphene particles were added to the matrix. The clay is an
Table 1 Compositions (wt%) of the studied materials. Samples
PE
OMt
G
grPE
PE grPE PE/grPE G PE
100 0 60 98 95 89 55 95 55 50
0 0 0 0 0 0 0 5 5 5
0 0 0 2 5 11 5 0 0 5
0 100 40 0 0 0 40 0 40 40
G PE/grPE OMt PE OMt PE/grPE G OMt PE/grPE
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specific mixing mechanical energy is sufficient to obtain a complete intercalation (Lertwimolnun and Vergnes, 2005; Beuguel et al., 2015), which favors the exfoliation of OMt particles. On the other hand, TEM micrographs of PE matrix filled with 5 wt% G indicate the presence of micrometric graphene stacks well-distributed within PE matrix (Fig. 1e). Most of the microparticles, with a nondescript shape and a crumpled appearance, are constituted of a random arrangement of long and folded graphene nanoplatelets (Fig. 1f), as already described by Mittal et al. (2016). Moreover, G microparticles are longer than OMt aggregates, reaching up to 5 μm length (Fig. 1e). However, a few shorter and thinner G particles are present within PE matrix (Fig. 1e). It is worth noticing that straight elements, of ~100 nm thick and with a micrometric length, are characteristic constituents of some G particles (Fig. 1e and g). At last, the presence of compatibilizer with graphene does not seem to significantly modify the structure of graphene based composite (Fig. 1h). Fig. 2 presents TEM micrographs of the G OMt PE/grPE nanocomposite. Simultaneous incorporation of 5 wt% of OMt and 5 wt% of G into the PE/grPE blend leads to a structure constituted of few micrometric G particles (Fig. 2a) and numerous OMt nanoplatelets (Fig. 2b), that is a superposition of G PE and OMt PE/grPE structures. This result confirms that the compatibilizer presents a better affinity with the clay. However, Fig. 2c could leave thinking that the separation of some straight elements of G particles would be favored by a synergism of clay nanoparticles with graphene particles (Bouakaz et al., 2015).
30 °C. In addition to determine the melting (Tm) and crystallization (Tc) temperatures, these experiments are also used to evaluate the melting enthalpy ΔHm of the materials. Then, the degree of crystallinity Xc was calculated using the following equation:
Xc =
∆Hm ∆Hm′ (1 − 0, 01 ∅m )
(1)
′
with Δ Hm the specific enthalpy at the melt state of 100% crystalline PE and equal to 293 J/g. 2.3.3. Micro-hardness Micro-hardness was measured on the larger surface of a molded material plate, using Micro-vickers hardness tester THV-501, equipped with a light microscope. The indenter was a square shaped diamond pyramid with top angle of 136°. A load P = 0,98 N was applied during 30 s and the mean diagonal length d of the indenter footprint was measured. Micro-hardness was calculated from the following equation (Pandis et al., 2009), according to ASTM E 384 – 99:
HV = 1.854 ×
P d2
(2)
At least, 5 measures were performed on each sample. The hardness measurements were performed so that the diagonal lengths of the indenter footprint size is of about 100 μm, which warranties that the local heterogeneity is averaged with regard to the particle size and interparticular distance of the studied materials (Pandis et al., 2009).
3.2. Thermal properties 2.3.4. Tensile tests Tensile tests were performed on an universal testing machine (MTS), using normalized specimens (ISO-527-2) of studied composites, at room temperature. A constant crosshead speed of 5 mm/min, corresponding to strain rate of 10− 3 s− 1, was used. The deformation was measured by video extensometer to avoid errors due to the sliding of the specimen relative to the grips. Young's modulus, E, strain at break, εb, and stress at break, σb, were determined from stress-strain curves for all composites. At least two specimens were tested for each material.
Table 2 presents thermal properties of all materials studied. It is noted that graphene and clay particles have little effect on the melting and crystallization temperatures of PE: Tm ~ 140 °C ± 2 °C and Tc ~ 117 °C ± 2 °C. Similarly, the crystallinity degrees of nanocomposites are close ( ± 3%) to that of their corresponding matrix, PE or PE/grPE, ~ 48% (Table 2), meaning the presence of clay has only a small effect on the amount of crystallites (Ogata et al., 1997) or graphene (Young et al., 2012) particles. This suggests that the fillers used in the study do not act as a nucleating agent for crystallization, in part due to the high molar mass of the high density polyethylene matrix used in this work (Mittal et al., 2016).
2.3.5. Rheology Linear shear oscillatory measurements were performed using a controlled stress rheometer (GEMINI from Malvern Instruments) equipped with parallel disks of 25 mm diameter and 2 mm spacing. All samples were characterized at 160 °C and all measurements were performed under nitrogen. Rheometrical data were shown to be reproducible within ± 5%.
3.3. Mechanical properties Fig. 3 presents the normalized hardness and normalized Young's modulus of G PE composites as a function of G content, relative to the values of hardness and Young's modulus measured for the PE matrix: 38.1 MPa and 1300 MPa, respectively. The values corresponding to the 5 wt% OMt PE composite are also displayed in Fig. 3, aiming to compare the influence on mechanical properties of the two high aspect ratio fillers, which exhibit similar dispersion levels at micrometric scale (Figs. 1a and c). Relative Young's modulus and micro-hardness of G PE composite follow the same trends versus G mass fraction. They are strongly increased by the incorporation of 2% of G particles, but the growth is weaker above ϕm = 2%. However, the improvement is more marked in terms of Young's modulus. Considering that the crystallinity degree of composites is not significantly affected by the presence of graphene (Table 2), it can be concluded that the G microparticles are directly responsible for the increase in the micro-hardness and rigidity of G PE composites. Fig. 3 also shows that the hardness and rigidity of the matrix filled at 5 wt% of G are clearly superior to those of the matrix filled at 5 wt% of OMt, for similar micrometric dispersions. This result shows the importance of the mechanical properties of fillers used as reinforcement particles and confirms the industrial potential of G particles. Let us remind that the Young's modulus was estimated ~1000 GPa for G particles (Young et al., 2012), against only ~180 GPa
3. Results and discussion 3.1. Structure The dispersion state and the structure of fillers were observed on TEM micrographs for the OMt PE, OMt PE/grPE, G PE and G PE/grPE materials (Fig. 1). TEM micrographs of PE matrix filled with 5 wt% OMt (Fig. 1a) clearly show that these materials can be considered as microcomposites. Indeed, numerous oval OMt aggregates, with a length of ~ 1 μm and a thickness of a few hundreds of nanometers, are welldispersed within PE matrix, without the interparticular distance exceeding ten micrometers (Fig. 1a). Moreover, some OMt thinner particles, with ~10 nm thickness are also evidenced (Fig. 1b). Although some OMt aggregates are observed (Fig. 1c), the addition of 40% of compatibilizer leads to a mainly exfoliated structure of the OMt PE nanocomposite (Fig. 1d), as reported in literature (Lertwimolnun and Vergnes, 2005). Indeed, the presence of compatibilizer facilitates the intercalation of PE matrix chains into OMt galleries, by improving the interactions between the PE matrix and OMt particles. Moreover, the 246
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Fig. 1. TEM micrographs of OMt PE (a, b), OMt PE/grPE (c, d), G PE (e–g), G PE/grPE (h) samples.
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Fig. 2. TEM micrographs of G OMt PE/grPE nanocomposite.
(Figs. 3 and 4a). On the contrary, properties at break, in particular stress at break, are weakened at a mass filler fraction of 5% (Fig. 4a). In presence of a compatibilizer, the relative Young's modulus of the OMt based nanocomposite (Fig. 4b) is increased, which is attributed to the fine dispersion of OMt nanoparticles (Fig. 1e and f). On the other hand, the properties at break are strongly affected, as reported in literature for highly filled nanocomposites, beyond percolation threshold (Aït Hocine et al., 2008). Although the structures of G PE and G PE/ grPE microcomposites are almost identical (Fig. 1c and g), a diminution of Young's modulus is stated, when the compatibilizer was added to G PE composites. This result suggests that the decrease of viscosity during mixing, due to the presence of compatibilizer, leads to a lower dispersion of G stacks. So, the strong decrease of relative mechanical properties at break of the G PE/grPE composite could be also explained by the presence of larger particles, which would represent stress concentration sites weakening the material properties, in particular stress and strain at break. At last, the high increase (~ 60%) of the Young's modulus of G OMt PE/grPE nanocomposite, compared to the limited improvement of the rigidity of all other studied materials (Fig. 4a and b), argues for a reinforcement of the network. This reinforcement is due to a greater amount of fillers (5 wt% of G particles and 5 wt% of OMt nanoplatelets), but also to the existence of a synergy between polar graphene microparticles and apolar clay nanoparticles, as suggested by TEM micrographs (Fig. 2) and already reported in the literature (Bouakaz et al., 2015). Such a structure makes the material fragile and, thus, leads to the collapse of the mechanical properties at break (Fig. 4b), as showed for highly filled OMt based nanocomposites (Aït Hocine et al., 2008).
Table 2 Thermal properties of materials. Materials
Tm (°C)
Tc (°C)
ΔHm (J/g)
Xc (%)
PE OMt PE G PE PE/grPE OMt PE/grPE G PE/grPE G OMt PE/grPE
139.9 141.5 139.7 140.0 138.7 139.3 138.1
116.7 115.9 117.9 117.2 117.7 119.3 118.1
143.1 143.1 127.1 138.6 138.1 140.3 123.5
48.9 51.4 45.6 47.3 49.6 50.4 46.8
Fig. 3. Relative mechanical properties of G PE composites as a function of mass fraction and of the 5 wt% OMt PE composite.
for OMt particles (Ogata et al., 1997). Young's modulus, as well as stress and strain at break, normalized relative to the corresponding matrix, are displayed as a function of the material composition, in Fig. 4a and b, for PE and PE/grPE based materials, respectively. In the case of the PE matrix, the Young's modulus was shown to be increased in presence of fillers, in a significant way for G particles
3.4. Rheological properties Fig. 5 presents complex viscosity, η⁎, as a function of frequency, f, for PE matrix and PE matrix filled with 5 wt% OMt, with or without compatibilizer. The complex viscosity of OMt PE composite exhibits a plateau at low frequencies, defining a Newtonian viscosity, slightly higher than Fig. 4. Linear and nonlinear mechanical properties of matrices filled with 5 wt% of G and/or 5 wt% of OMt, normalized relative to (a) the PE matrix and to (b) the PE/grPE matrix. E = 1300 MPa, σb = 148 MPa and εb = 188.5% for the PE matrix, and E = 1420 MPa, σb = 134 MPa and εb = 189% for the PE/grPE matrix.
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et al., 2015). This result suggests that G stacks observed in TEM micrographs (Fig. 1g) are micrometric particles, with a high character of lamellarity, constituted of a few tens of graphene layers. Fig. 7 shows complex viscosity, η⁎, and elastic modulus, G′, as a function of frequency in the linear viscoelastic domain, for PE, G PE/ grPE, OMt PE/grPE and G OMt PE/grPE samples. Mass fractions of graphene and clay are 5%. The results show that, like the PE/grPE blend filled with 5 wt% of OMt, the PE/grPE blend filled with 5 wt% of OMt and 5 wt% of G exhibits no terminal regime (η⁎∝η0⁎ and G′ ∝ f2) at the lowest frequencies studied. Indeed, for both samples, the viscosity curve (Fig. 7a) and the elastic modulus curve (Fig. 7b) show, respectively, the existence of a yield behavior and the frequency independent elastic modulus on long time scales (low frequencies). In the case of OMt based nanocomposites, the pseudosolid behavior is usually attributed to the formation of a percolated network (Krishnamoorti and Yurekli, 2001) of anisometric OMt nanoparticles, observed by TEM (Fig. 1f). Fig. 7a also shows that the pseudosolid behavior of OMt PE/grPE nanocomposite is significantly enhanced by adding 5 wt% of G. Indeed, at f = 10− 4 Hz, the presence of 5 wt% of G increases by ~1.000,000 Pa.s the OMt PE/grPE nanocomposite viscosity, whereas it only increases by 100,000 Pa.s the PE matrix viscosity (Fig. 6), i.e. ten time less. Fig. 8 presents the reinforcing factor of fillers, defined as the ratio between the elastic modulus, G'c, of the filled matrix and the elastic modulus, G'm, of the matrix (PE or PE/grPE) (Bouakaz et al., 2015), as a function of frequency for composites with 5 wt% G or/and 5 wt% OMt. The reinforcing effect of G microparticles dispersed within PE matrix, although it is weak, is higher than that of OMt microparticles, which is due to the differences between geometrical and mechanical characteristics of the two fillers, previously evoked in the study. Moreover, the reinforcing factors of G particles on the PE and PE/grPE matrices (Fig. 7) are quite similar, meaning that the compatibilizer does not play a key role in the dispersion of G particles, as observed on TEM micrographs (Fig. 1c and g). However, the weak reduction of reinforcing effect of G particles in presence of compatibilizer also suggests that the compatibilizer, by reducing the viscous effects during mixing, would have trend to slightly lower the degree of graphene dispersion. On the other hand, the significant improvement in the reinforcing factor values of OMt PE/grPE nanocomposite can be explained by the high dispersion level of OMt nanoparticles (Fig. 1f). At last, the highest values of reinforcing factor are obtained for the G OMt PE/grPE sample, over the whole frequency range. Indeed, G particles strengthen the percolation network of OMt nanoplatelets. But, the increase of complex viscosity (Fig. 7a), elastic modulus (Fig. 7b) and reinforcing effect (Fig. 8) of the G OMt PE/grPE sample relative to the OMt PE/grPE one, even at high frequencies (Lim and Park, 2001), could be also the signature of a thinner G structure than the expected structure with adding 5% of G microparticles to the PE/grPE blend. This result could be explained by the high viscosity of the PE/grPE matrix filled with OMt nanoplatelets which would favor the separation of G particles by strongly improving the shear effect during mixing. Moreover, the presence of OMt nanoplatelets within the initiated stacking defects, when they are sufficiently large, could facilitate the separation of G particles (Fig. 2c). We cannot exclude that the G particles could play a role in the OMt exfoliation degree, improving it by collisions between the two fillers during mixing.
Fig. 5. Complex viscosity of 5 wt% OMt based materials as a function of frequency.
that of PE matrix. This small viscosity increase is the signature of the presence of OMt microparticles observed on TEM micrographs (Fig. 1a). On the contrary, compatibilized nanocomposite is a structured medium exhibiting a pseudosolid behavior characterized by an apparent yield stress. Indeed, the use of a compatibilizer favors the intercalation of PE chains into OMt galleries, leading to a mainly exfoliated structure (Fig. 1f), as reported for compatibilizer/polyolefin blends filled with OMt particles (Lertwimolnun and Vergnes, 2005). Therefore, at ϕm = 5%, the viscoelastic behavior of OMt based nanocomposite is governed by a percolation network of nanoparticles, at low frequencies, and dominated by the PE matrix, at high frequencies. Fig. 6 presents the complex viscosity of G PE composites as a function of frequency, at different G mass fractions. The complex viscosity increases with increasing G mass fraction over the whole range of frequencies investigated. At ϕm = 5%, without compatibilizer, the viscosity increase due to rigid G fillers is higher than that attributed to OMt particles (Fig. 5). In spite of uncertainty in the determination of the Newtonian viscosity of materials, the relative viscosity, ηr, defined as the ratio of the Newtonian viscosity of the G based composite to that of PE matrix, is displayed as a function of G volume fraction in the insert of Fig. 6. Following the method proposed by Utracki and Lyngaae-Jørgensen (2002), based on the fit of relative viscosity versus volume fraction curve to the second-order Einstein type equation, the intrinsic viscosity [η] and interaction constant k were determined: k ~ 0.8 and [η] ~ 26. This value of [η], leading to an aspect ratio of ~100 (Utracki and Lyngaae-Jørgensen, 2002) close to this given by the supplier, is superior to [η] ~ 3 corresponding to the micrometric C20A particles dispersed in PE matrix (Médéric et al., 2013), but strongly inferior to [η] ~ 60, reported for the exfoliated OMt nanoplatelets with an aspect ratio of ~200 (Aubry et al., 2005; Beuguel
4. Conclusion The influence of graphene or/and organically modified montmorillonite fillers on mechanical properties, in solid or melt state, of a high density polyethylene matrix or a maleic anhydride grafted polyethylene/polyethylene blend was investigated. It was established that the presence of fillers does not significantly modify the crystallization degree of materials studied, meaning that the different effects observed in mechanical properties can only be attributed to the structure of
Fig. 6. Complex viscosities of G PE composites as a function of frequency, at different G mass fractions. Insert: Relative viscosity, ηr, as a function of G volume fraction.
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Fig. 7. Complex viscosity (a) and elastic modulus (b) as a function of frequency for PE, G PE/grPE, OMt PE/grPE and G OMt PE/grPE samples.
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Fig. 8. Reinforcing factor as a function of frequency for all composites.
composites. Adding the compatibilizer to clay based composites makes it an exfoliated nanocomposite, as commonly reported in literature. This nanocomposite, with a filler percolation network, exhibits improved linear mechanical properties, for melt state and solid state, but disappointing mechanical properties at break. Mixed with the polyethylene matrix, commercial graphene platelets have a relatively high aspect ratio. However, the added compatibilizer decreases the matrix viscosity during mixing, leading to a lower dispersion of graphene particles. The resulting larger graphene particles weaken the material, giving it low mechanical properties at break. The composite constituted with both clay nanoparticles and lamellar graphene particles presents the most pronounced reinforcing effect, in linear regime, as well as the poorest mechanical properties at break. These results can be explained by the high viscosity of the clay based nanocomposite which increases the shear applied on graphene particles during mixing, hence favoring their separation. This separation mechanism could be amplified by the penetration of clay nanoplatelets into stacking defects of graphene particles, initiated by viscosity effect. As far as the hybrid materials constituted of two fillers are concerned, the influence of matrix, material composition, filler nature on apparent synergy between the two fillers should be thoroughly studied in future works. Acknowledgments The authors are grateful to Arkema (Serquigny, France) for the compatibilizer supply. References Aït Hocine, N., Médéric, P., Aubry, T., 2008. Mechanical properties of polyamide-12 layered silicate nanocomposites and their relations with structure. Polym. Test. 27,
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Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
Contribution of the organo-montmorillonite/graphene pair to the rheological and mechanical properties of polyethylene matrix based nanocomposites
MARK
Fath Eddine Zakaria Rahmaouia, Pascal Medericb,⁎, Nourredine Aït Hocinec, Aïcha Aït Saadaa, Nathalie Poirotd, Idir Belaidia a
Department of Mechanical Engineering, University of M'hamed Bougara, Boumerdes, Algeria IRDL, CNRS-FRE 3744, Université de Bretagne Occidentale, 6 avenue Victor Le Gorgeu, CS 93837, 29238 Brest, Cedex 3, France LMR, INSA Centre Val de Loire, 3 rue de la chocolaterie, BP 3410, 41034 Blois, France d GREMAN, IUT de Blois, 15 rue de la chocolaterie, 41000 Blois, France b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Montmorillonite Graphene Nanocomposite Structure Mechanical properties Rheological properties
Since the last decade, graphene nanoplatelets with their exceptional physical properties are used as fillers in thermoplastic blends. In this work, the influence of commercial graphene nanoplatelets on properties of a high density polyethylene, in solid and melt states, was investigated, in comparison with the one of organically modified montmorillonite fillers. The use of a compatibilizer, a maleic anhydride grafted polyethylene, led to a clay based nanocomposite, with some improved mechanical and rheological properties, but with disappointing mechanical properties at break. On the other hand, by reducing the viscosity during mixing, the added compatibilizer slightly lowered the degree of dispersion of high aspect ratio graphene particles, weakening the material. More interestingly, the nanocomposite constituted with both clay nanoplatelets and lamellar graphene particles exhibited better reinforcing characteristics, in melt and solid states. This result can be partially explained by the high viscosity of the clay based nanocomposite which helps in the separation of graphene particles during mixing.
1. Introduction Applicative and fundamental research in the field of nanocomposites, which offer better end-use properties than those of microcomposites, at equivalent filler volume fraction, has significantly evolved. Since two decades, the relations between processing, structure and macroscopic behavior of this class of nanomaterials were thoroughly studied, as reviewed by Camargo et al. (2009). In particular, organically modified montmorillonite (OMt) is now successfully used as lamellar nanofiller for the elaboration of polar or apolar thermoplastic based nanocomposites (Ray and Okamoto, 2003), presenting improved mechanical (Alexandre and Dubois, 2000; Kato et al., 2003; Hotta and Paul, 2004; Aït Hocine et al., 2008), barrier (Alexandre and Dubois, 2000; Kato et al., 2003; Hotta and Paul, 2004; Alexandre et al., 2009) and flammability resistance (Gilman et al., 2000) properties. Moreover, melt-state rheological properties of OMt based nanocomposites have also attracted significant attention (Krishnamoorti and Yurekli, 2001), because of their relevance in the identification of the processing/ structure/rheological properties relations (Médéric et al., 2006). More ⁎
recently, graphene (G), which is the fewest layer limit of graphite, has emerged as a bidimensional material, with a high aspect ratio and exceptional properties (Zhu et al., 2010). Various routes leading down to the individual graphene layer, such as mechanical exfoliation (Novoselov et al., 2004), chemical conversion of graphite to graphene oxide (Stankovich et al., 2007), chemical vapor deposition (Kim et al., 2009), and total organic synthesis (Watson et al., 2001), have been largely reviewed by Allen et al. (2009). But some advances are still required (Singh et al., 2011), especially in the low cost production of large amounts of graphene nanoplatelets (Zhu et al., 2010). 2-D graphene is used as filler for polymer based nanocomposites (Kuilla et al., 2010; Dhand et al., 2013), because it presents a great potential for applications requiring improved electrical, thermal, mechanical and barrier properties (Potts et al., 2011). However, for an apolar polyolefin matrix, such as polyethylene, the incorporation of a compatibilizer, that is commonly a maleic anhydride grafted polyethylene, is required in order to obtain an organically modified montmorillonite based nanocomposite (Hasegawa et al., 1998) or a graphene based nanocomposite (Schniepp et al., 2006; Kim
Corresponding author. E-mail address: [email protected] (P. Mederic).
http://dx.doi.org/10.1016/j.clay.2017.09.037 Received 3 July 2017; Received in revised form 20 September 2017; Accepted 26 September 2017 Available online 05 October 2017 0169-1317/ © 2017 Elsevier B.V. All rights reserved.
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organically modified montmorillonite, namely Cloisite® C20A, commercialized by Southern Clay Products (Gonzales, Texas, TX). This organoclay is a dimethyl dihydrogenated-tallow ammonium exchanged montmorillonite. The individual C20A particles are nanoplatelets with ~1 nm thickness and ~500 nm length. The density of C20A organoclay is ~2 g/cm3. Polar graphene particles, referenced N006-P nano-graphene platelets and supplied by Angstron Materials (Ohio), present an oxygen percentage of 4%, average x–y dimensions less than 5 μm and a thickness ranging from 10 to 20 nm. The density of graphene is also close to 2 g/cm3.
et al., 2011; Seo et al., 2013; Vasileiou et al., 2014). In this case, graft ratio and average molar mass of the compatibilizer, as well as compatibilizer/filler ratio, play a key role in the final structure and resulting physical properties of the thermoplastic based nanocomposite. These parameters have thoroughly been studied for clay based nanocomposites (Hotta and Paul, 2004; Chrissopoulou et al., 2005; Wang et al., 2005; Durmus et al., 2007), but are still in discussion for graphene based nanocomposites (Seo et al., 2013; Mittal and Krauss, 2014). On the other hand, the melt intercalation technique is often used for the elaboration of polyolefin based nanocomposites because it requires no toxic solvent and it is practical for manufacturing in large scale (Zhang et al., 2010). OMt based nanocomposites are characterized by the existence of a three-dimensional percolated network mesostructure at low clay mass fractions, usually ~2% for polar matrix (Utracki and LyngaaeJørgensen, 2002) or polyolefin/compatibilizer matrix (Durmus et al., 2007). The low mass fraction threshold is explained by the fine dispersion of anisometric OMt nanoparticles within thermoplastic matrix. It was put in evidence from linear or nonlinear properties in melt state (Hoffmann et al., 2000; Ren et al., 2000; Utracki and LyngaaeJørgensen, 2002; Aubry et al., 2005; Durmus et al., 2007) and in solid state (Aït Hocine et al., 2008; Hassan et al., 2015), and, to a lesser degree, from gas barrier (Alexandre et al., 2009) and thermal (Hassan et al., 2015) properties. Similarly, the incorporation of a low percentage of graphene into a polyolefin matrix was shown to significantly improve mechanical, rheological and thermal properties (Song et al., 2011; El Achaby et al., 2012; El Achaby and Qaiss, 2013), because graphene nanosheets are well-dispersed and exhibit a high specific surface area, especially favoring the efficient transfer of stresses through the matrix/ filler interface (Srivastava et al., 2011). Recent studies have shown that it is conceivable to improve the dispersion of carbon black particles or carbon nanotubes by incorporating them with organically modified montmorillonite nanoplatelets into an elastomer (Pradhan et al., 2015; Annadurai et al., 2016) or a thermoplastic (Ma et al., 2007; Silva et al., 2014) matrix. The apparent synergy between the two fillers leads to an improvement of flame retardancy, mechanical, electrical and acoustic properties. Synergy between fillers in G OMt polylactic acid nanocomposites was also reported (Bouakaz et al., 2015). The paper focuses on the influence of composition on structural, thermal, mechanical and rheological properties of thermoplastic based nanocomposites constituted of a compatibilizer/polyethylene matrix filled with OMt, G, or OMt and G nanoplatelets, aiming at the study of the mechanisms of a possible synergy between the two kinds of fillers.
2.2. Preparation of nanocomposites All samples have been prepared by simultaneous melt mixing in a Haake Rheomix 600 internal mixer, at a temperature of 160 °C. The blade rotational speed was of 100 rpm during 6 min. The processing conditions, hence the resulting specific mixing mechanical energy (Médéric et al., 2009), were chosen to avoid the degradations of the matrix and compatibilizer and, more particularly, that of the organic modifier of clay (Xie et al., 2001). Polymers and fillers were manually pre-mixed before being thrown into the mixer. The samples were pelletized and processed by compression molding at 160 °C between 2 mm thick plates. Pressure was increased by steps from 0 to 25 MPa, in order to avoid the formation of air bubbles. The compositions of all samples are listed in Table 1, in filler mass fractions ϕm. In the study, the compatibilizer mass fraction was fixed at 40%, which was shown to be a relevant content in order to obtain exfoliated OMt polyolefin/compatibilizer nanocomposites (Lertwimolnun and Vergnes, 2005). Moreover, in the case of G polyolefin/compatibilizer, the (G:compatibilizer) ratio is usually chosen ranging from (1:3) to (1:10) (Seo et al., 2013). At last, for the G OMt PE/grPE nanocomposite, the ratio of (G:OMt) was chosen (1:1), which was shown to be the most interesting in terms of structural and mechanical properties for multiwalled carbon nanotubes/organically modified montmorillonite hybrids (Pradhan et al., 2015). 2.3. Characterizations 2.3.1. Structural characterization Samples were microtomed under a frozen atmosphere into liquid nitrogen and their nanostructures were observed by transmission electron microscopy (TEM), using a JEOL JEM-1230 microscope at 120 kV.
2. Experimental part 2.3.2. Differential scanning calorimetry The influence of graphene or/and clay particles on the PE crystallinity was studied from DSC measurements conducted on a PerkinElmer DTA-7, under a nitrogen atmosphere. The samples of about 10 mg were heated from room temperature up to 250 °C at a rate of 10 °C/min, held at 250 °C for 10 min, then were cooled from 250 °C to
2.1. Materials A high density polyethylene, noted PE in the text, referenced as HDPE 5502 and supplied by POLYMED, was used as matrix. Its density is 0.94 g/cm3 and its melting point is 139.9 °C. The number average molar mass is 17,300 g/mol, corresponding to a degree of polymerization of about 600, and the polydispersity index is 7.8 (Liu et al., 2010). A maleic anhydride grafted polyethylene, referenced as Orevac® 18,507, supplied by Arkema was used as compatibilizer, noted grPE in the text. The maleic anhydride grafting level is inferior to 1%. The density and the melting point of the compatibilizer are 0.95 g/cm3 and 128 °C, respectively. The Newtonian complex viscosity of both thermoplastics, measured at 160 °C, is: η0⁎ ~ 250,000 Pa·s and η0⁎ ~ 55,000 Pa·s for the PE matrix and the compatibilizer, respectively. The compatibilized blend obeys to a blending law, meaning that the two thermoplastics are miscible, hence a Newtonian viscosity of ~ 170,000 Pa.s for the blend constituted of 40% of compatibilizer used in this work. Clay and graphene particles were added to the matrix. The clay is an
Table 1 Compositions (wt%) of the studied materials. Samples
PE
OMt
G
grPE
PE grPE PE/grPE G PE
100 0 60 98 95 89 55 95 55 50
0 0 0 0 0 0 0 5 5 5
0 0 0 2 5 11 5 0 0 5
0 100 40 0 0 0 40 0 40 40
G PE/grPE OMt PE OMt PE/grPE G OMt PE/grPE
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specific mixing mechanical energy is sufficient to obtain a complete intercalation (Lertwimolnun and Vergnes, 2005; Beuguel et al., 2015), which favors the exfoliation of OMt particles. On the other hand, TEM micrographs of PE matrix filled with 5 wt% G indicate the presence of micrometric graphene stacks well-distributed within PE matrix (Fig. 1e). Most of the microparticles, with a nondescript shape and a crumpled appearance, are constituted of a random arrangement of long and folded graphene nanoplatelets (Fig. 1f), as already described by Mittal et al. (2016). Moreover, G microparticles are longer than OMt aggregates, reaching up to 5 μm length (Fig. 1e). However, a few shorter and thinner G particles are present within PE matrix (Fig. 1e). It is worth noticing that straight elements, of ~100 nm thick and with a micrometric length, are characteristic constituents of some G particles (Fig. 1e and g). At last, the presence of compatibilizer with graphene does not seem to significantly modify the structure of graphene based composite (Fig. 1h). Fig. 2 presents TEM micrographs of the G OMt PE/grPE nanocomposite. Simultaneous incorporation of 5 wt% of OMt and 5 wt% of G into the PE/grPE blend leads to a structure constituted of few micrometric G particles (Fig. 2a) and numerous OMt nanoplatelets (Fig. 2b), that is a superposition of G PE and OMt PE/grPE structures. This result confirms that the compatibilizer presents a better affinity with the clay. However, Fig. 2c could leave thinking that the separation of some straight elements of G particles would be favored by a synergism of clay nanoparticles with graphene particles (Bouakaz et al., 2015).
30 °C. In addition to determine the melting (Tm) and crystallization (Tc) temperatures, these experiments are also used to evaluate the melting enthalpy ΔHm of the materials. Then, the degree of crystallinity Xc was calculated using the following equation:
Xc =
∆Hm ∆Hm′ (1 − 0, 01 ∅m )
(1)
′
with Δ Hm the specific enthalpy at the melt state of 100% crystalline PE and equal to 293 J/g. 2.3.3. Micro-hardness Micro-hardness was measured on the larger surface of a molded material plate, using Micro-vickers hardness tester THV-501, equipped with a light microscope. The indenter was a square shaped diamond pyramid with top angle of 136°. A load P = 0,98 N was applied during 30 s and the mean diagonal length d of the indenter footprint was measured. Micro-hardness was calculated from the following equation (Pandis et al., 2009), according to ASTM E 384 – 99:
HV = 1.854 ×
P d2
(2)
At least, 5 measures were performed on each sample. The hardness measurements were performed so that the diagonal lengths of the indenter footprint size is of about 100 μm, which warranties that the local heterogeneity is averaged with regard to the particle size and interparticular distance of the studied materials (Pandis et al., 2009).
3.2. Thermal properties 2.3.4. Tensile tests Tensile tests were performed on an universal testing machine (MTS), using normalized specimens (ISO-527-2) of studied composites, at room temperature. A constant crosshead speed of 5 mm/min, corresponding to strain rate of 10− 3 s− 1, was used. The deformation was measured by video extensometer to avoid errors due to the sliding of the specimen relative to the grips. Young's modulus, E, strain at break, εb, and stress at break, σb, were determined from stress-strain curves for all composites. At least two specimens were tested for each material.
Table 2 presents thermal properties of all materials studied. It is noted that graphene and clay particles have little effect on the melting and crystallization temperatures of PE: Tm ~ 140 °C ± 2 °C and Tc ~ 117 °C ± 2 °C. Similarly, the crystallinity degrees of nanocomposites are close ( ± 3%) to that of their corresponding matrix, PE or PE/grPE, ~ 48% (Table 2), meaning the presence of clay has only a small effect on the amount of crystallites (Ogata et al., 1997) or graphene (Young et al., 2012) particles. This suggests that the fillers used in the study do not act as a nucleating agent for crystallization, in part due to the high molar mass of the high density polyethylene matrix used in this work (Mittal et al., 2016).
2.3.5. Rheology Linear shear oscillatory measurements were performed using a controlled stress rheometer (GEMINI from Malvern Instruments) equipped with parallel disks of 25 mm diameter and 2 mm spacing. All samples were characterized at 160 °C and all measurements were performed under nitrogen. Rheometrical data were shown to be reproducible within ± 5%.
3.3. Mechanical properties Fig. 3 presents the normalized hardness and normalized Young's modulus of G PE composites as a function of G content, relative to the values of hardness and Young's modulus measured for the PE matrix: 38.1 MPa and 1300 MPa, respectively. The values corresponding to the 5 wt% OMt PE composite are also displayed in Fig. 3, aiming to compare the influence on mechanical properties of the two high aspect ratio fillers, which exhibit similar dispersion levels at micrometric scale (Figs. 1a and c). Relative Young's modulus and micro-hardness of G PE composite follow the same trends versus G mass fraction. They are strongly increased by the incorporation of 2% of G particles, but the growth is weaker above ϕm = 2%. However, the improvement is more marked in terms of Young's modulus. Considering that the crystallinity degree of composites is not significantly affected by the presence of graphene (Table 2), it can be concluded that the G microparticles are directly responsible for the increase in the micro-hardness and rigidity of G PE composites. Fig. 3 also shows that the hardness and rigidity of the matrix filled at 5 wt% of G are clearly superior to those of the matrix filled at 5 wt% of OMt, for similar micrometric dispersions. This result shows the importance of the mechanical properties of fillers used as reinforcement particles and confirms the industrial potential of G particles. Let us remind that the Young's modulus was estimated ~1000 GPa for G particles (Young et al., 2012), against only ~180 GPa
3. Results and discussion 3.1. Structure The dispersion state and the structure of fillers were observed on TEM micrographs for the OMt PE, OMt PE/grPE, G PE and G PE/grPE materials (Fig. 1). TEM micrographs of PE matrix filled with 5 wt% OMt (Fig. 1a) clearly show that these materials can be considered as microcomposites. Indeed, numerous oval OMt aggregates, with a length of ~ 1 μm and a thickness of a few hundreds of nanometers, are welldispersed within PE matrix, without the interparticular distance exceeding ten micrometers (Fig. 1a). Moreover, some OMt thinner particles, with ~10 nm thickness are also evidenced (Fig. 1b). Although some OMt aggregates are observed (Fig. 1c), the addition of 40% of compatibilizer leads to a mainly exfoliated structure of the OMt PE nanocomposite (Fig. 1d), as reported in literature (Lertwimolnun and Vergnes, 2005). Indeed, the presence of compatibilizer facilitates the intercalation of PE matrix chains into OMt galleries, by improving the interactions between the PE matrix and OMt particles. Moreover, the 246
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Fig. 1. TEM micrographs of OMt PE (a, b), OMt PE/grPE (c, d), G PE (e–g), G PE/grPE (h) samples.
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Fig. 2. TEM micrographs of G OMt PE/grPE nanocomposite.
(Figs. 3 and 4a). On the contrary, properties at break, in particular stress at break, are weakened at a mass filler fraction of 5% (Fig. 4a). In presence of a compatibilizer, the relative Young's modulus of the OMt based nanocomposite (Fig. 4b) is increased, which is attributed to the fine dispersion of OMt nanoparticles (Fig. 1e and f). On the other hand, the properties at break are strongly affected, as reported in literature for highly filled nanocomposites, beyond percolation threshold (Aït Hocine et al., 2008). Although the structures of G PE and G PE/ grPE microcomposites are almost identical (Fig. 1c and g), a diminution of Young's modulus is stated, when the compatibilizer was added to G PE composites. This result suggests that the decrease of viscosity during mixing, due to the presence of compatibilizer, leads to a lower dispersion of G stacks. So, the strong decrease of relative mechanical properties at break of the G PE/grPE composite could be also explained by the presence of larger particles, which would represent stress concentration sites weakening the material properties, in particular stress and strain at break. At last, the high increase (~ 60%) of the Young's modulus of G OMt PE/grPE nanocomposite, compared to the limited improvement of the rigidity of all other studied materials (Fig. 4a and b), argues for a reinforcement of the network. This reinforcement is due to a greater amount of fillers (5 wt% of G particles and 5 wt% of OMt nanoplatelets), but also to the existence of a synergy between polar graphene microparticles and apolar clay nanoparticles, as suggested by TEM micrographs (Fig. 2) and already reported in the literature (Bouakaz et al., 2015). Such a structure makes the material fragile and, thus, leads to the collapse of the mechanical properties at break (Fig. 4b), as showed for highly filled OMt based nanocomposites (Aït Hocine et al., 2008).
Table 2 Thermal properties of materials. Materials
Tm (°C)
Tc (°C)
ΔHm (J/g)
Xc (%)
PE OMt PE G PE PE/grPE OMt PE/grPE G PE/grPE G OMt PE/grPE
139.9 141.5 139.7 140.0 138.7 139.3 138.1
116.7 115.9 117.9 117.2 117.7 119.3 118.1
143.1 143.1 127.1 138.6 138.1 140.3 123.5
48.9 51.4 45.6 47.3 49.6 50.4 46.8
Fig. 3. Relative mechanical properties of G PE composites as a function of mass fraction and of the 5 wt% OMt PE composite.
for OMt particles (Ogata et al., 1997). Young's modulus, as well as stress and strain at break, normalized relative to the corresponding matrix, are displayed as a function of the material composition, in Fig. 4a and b, for PE and PE/grPE based materials, respectively. In the case of the PE matrix, the Young's modulus was shown to be increased in presence of fillers, in a significant way for G particles
3.4. Rheological properties Fig. 5 presents complex viscosity, η⁎, as a function of frequency, f, for PE matrix and PE matrix filled with 5 wt% OMt, with or without compatibilizer. The complex viscosity of OMt PE composite exhibits a plateau at low frequencies, defining a Newtonian viscosity, slightly higher than Fig. 4. Linear and nonlinear mechanical properties of matrices filled with 5 wt% of G and/or 5 wt% of OMt, normalized relative to (a) the PE matrix and to (b) the PE/grPE matrix. E = 1300 MPa, σb = 148 MPa and εb = 188.5% for the PE matrix, and E = 1420 MPa, σb = 134 MPa and εb = 189% for the PE/grPE matrix.
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et al., 2015). This result suggests that G stacks observed in TEM micrographs (Fig. 1g) are micrometric particles, with a high character of lamellarity, constituted of a few tens of graphene layers. Fig. 7 shows complex viscosity, η⁎, and elastic modulus, G′, as a function of frequency in the linear viscoelastic domain, for PE, G PE/ grPE, OMt PE/grPE and G OMt PE/grPE samples. Mass fractions of graphene and clay are 5%. The results show that, like the PE/grPE blend filled with 5 wt% of OMt, the PE/grPE blend filled with 5 wt% of OMt and 5 wt% of G exhibits no terminal regime (η⁎∝η0⁎ and G′ ∝ f2) at the lowest frequencies studied. Indeed, for both samples, the viscosity curve (Fig. 7a) and the elastic modulus curve (Fig. 7b) show, respectively, the existence of a yield behavior and the frequency independent elastic modulus on long time scales (low frequencies). In the case of OMt based nanocomposites, the pseudosolid behavior is usually attributed to the formation of a percolated network (Krishnamoorti and Yurekli, 2001) of anisometric OMt nanoparticles, observed by TEM (Fig. 1f). Fig. 7a also shows that the pseudosolid behavior of OMt PE/grPE nanocomposite is significantly enhanced by adding 5 wt% of G. Indeed, at f = 10− 4 Hz, the presence of 5 wt% of G increases by ~1.000,000 Pa.s the OMt PE/grPE nanocomposite viscosity, whereas it only increases by 100,000 Pa.s the PE matrix viscosity (Fig. 6), i.e. ten time less. Fig. 8 presents the reinforcing factor of fillers, defined as the ratio between the elastic modulus, G'c, of the filled matrix and the elastic modulus, G'm, of the matrix (PE or PE/grPE) (Bouakaz et al., 2015), as a function of frequency for composites with 5 wt% G or/and 5 wt% OMt. The reinforcing effect of G microparticles dispersed within PE matrix, although it is weak, is higher than that of OMt microparticles, which is due to the differences between geometrical and mechanical characteristics of the two fillers, previously evoked in the study. Moreover, the reinforcing factors of G particles on the PE and PE/grPE matrices (Fig. 7) are quite similar, meaning that the compatibilizer does not play a key role in the dispersion of G particles, as observed on TEM micrographs (Fig. 1c and g). However, the weak reduction of reinforcing effect of G particles in presence of compatibilizer also suggests that the compatibilizer, by reducing the viscous effects during mixing, would have trend to slightly lower the degree of graphene dispersion. On the other hand, the significant improvement in the reinforcing factor values of OMt PE/grPE nanocomposite can be explained by the high dispersion level of OMt nanoparticles (Fig. 1f). At last, the highest values of reinforcing factor are obtained for the G OMt PE/grPE sample, over the whole frequency range. Indeed, G particles strengthen the percolation network of OMt nanoplatelets. But, the increase of complex viscosity (Fig. 7a), elastic modulus (Fig. 7b) and reinforcing effect (Fig. 8) of the G OMt PE/grPE sample relative to the OMt PE/grPE one, even at high frequencies (Lim and Park, 2001), could be also the signature of a thinner G structure than the expected structure with adding 5% of G microparticles to the PE/grPE blend. This result could be explained by the high viscosity of the PE/grPE matrix filled with OMt nanoplatelets which would favor the separation of G particles by strongly improving the shear effect during mixing. Moreover, the presence of OMt nanoplatelets within the initiated stacking defects, when they are sufficiently large, could facilitate the separation of G particles (Fig. 2c). We cannot exclude that the G particles could play a role in the OMt exfoliation degree, improving it by collisions between the two fillers during mixing.
Fig. 5. Complex viscosity of 5 wt% OMt based materials as a function of frequency.
that of PE matrix. This small viscosity increase is the signature of the presence of OMt microparticles observed on TEM micrographs (Fig. 1a). On the contrary, compatibilized nanocomposite is a structured medium exhibiting a pseudosolid behavior characterized by an apparent yield stress. Indeed, the use of a compatibilizer favors the intercalation of PE chains into OMt galleries, leading to a mainly exfoliated structure (Fig. 1f), as reported for compatibilizer/polyolefin blends filled with OMt particles (Lertwimolnun and Vergnes, 2005). Therefore, at ϕm = 5%, the viscoelastic behavior of OMt based nanocomposite is governed by a percolation network of nanoparticles, at low frequencies, and dominated by the PE matrix, at high frequencies. Fig. 6 presents the complex viscosity of G PE composites as a function of frequency, at different G mass fractions. The complex viscosity increases with increasing G mass fraction over the whole range of frequencies investigated. At ϕm = 5%, without compatibilizer, the viscosity increase due to rigid G fillers is higher than that attributed to OMt particles (Fig. 5). In spite of uncertainty in the determination of the Newtonian viscosity of materials, the relative viscosity, ηr, defined as the ratio of the Newtonian viscosity of the G based composite to that of PE matrix, is displayed as a function of G volume fraction in the insert of Fig. 6. Following the method proposed by Utracki and Lyngaae-Jørgensen (2002), based on the fit of relative viscosity versus volume fraction curve to the second-order Einstein type equation, the intrinsic viscosity [η] and interaction constant k were determined: k ~ 0.8 and [η] ~ 26. This value of [η], leading to an aspect ratio of ~100 (Utracki and Lyngaae-Jørgensen, 2002) close to this given by the supplier, is superior to [η] ~ 3 corresponding to the micrometric C20A particles dispersed in PE matrix (Médéric et al., 2013), but strongly inferior to [η] ~ 60, reported for the exfoliated OMt nanoplatelets with an aspect ratio of ~200 (Aubry et al., 2005; Beuguel
4. Conclusion The influence of graphene or/and organically modified montmorillonite fillers on mechanical properties, in solid or melt state, of a high density polyethylene matrix or a maleic anhydride grafted polyethylene/polyethylene blend was investigated. It was established that the presence of fillers does not significantly modify the crystallization degree of materials studied, meaning that the different effects observed in mechanical properties can only be attributed to the structure of
Fig. 6. Complex viscosities of G PE composites as a function of frequency, at different G mass fractions. Insert: Relative viscosity, ηr, as a function of G volume fraction.
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Fig. 7. Complex viscosity (a) and elastic modulus (b) as a function of frequency for PE, G PE/grPE, OMt PE/grPE and G OMt PE/grPE samples.
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Fig. 8. Reinforcing factor as a function of frequency for all composites.
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