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Enhancement of the thermal conductivity of polypropylene with low loadings of CuAg alloy nanoparticles and graphene nanoplatelets
Materials Today Communications 21 (2019) 100695
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Enhancement of the thermal conductivity of polypropylene with low loadings of CuAg alloy nanoparticles and graphene nanoplatelets
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Diana Iris Medellín-Bandaa, Dámaso Navarro-Rodrígueza,*, Salvador Fernández-Tavizóna, Carlos Alberto Ávila-Ortaa, Gregorio Cadenas-Pliegob,*, Victor Eduardo Comparán-Padillab a b
Departamento de Materiales Avanzados (DMA), Centro de Investigación en Química Aplicada, Blvd. Enrique Reyna 140, C.P. 25294, Saltillo Coahuila, Mexico Departamento de Síntesis de Polímeros (DSP), Centro de Investigación en Química Aplicada, Blvd. Enrique Reyna 140, C.P. 25294, Saltillo Coahuila, Mexico
A R T I C LE I N FO
A B S T R A C T
Keywords: Polypropylene Nanocomposite CuAg alloy Graphene Thermal conductivity
Isotactic polypropylene (iPP), silver-rich shell CuAg nanoalloys and graphene nanoplatelets (GNPs) were used to prepare thermally conductive binary (iPP/CuAg and iPP/GNPs) and ternary (iPP/CuAg/GNPs) nanocomposites with filler compositions from 0.1 to 5 wt.% through melt processing. All nanocomposites were thermally and electrically characterized. Results showed that both nanoparticles nucleate the crystallization of iPP, shifting the crystallization transition toward higher temperature as compared with that of iPP. The nanoparticles dispersion was not optimum but was enough to make the nanocomposites thermally conductive between 0.65 and 1.56 W m−1 K−1 and more thermally stable (443 °C) than neat iPP (347 °C). For the iPP/CuAg nanocomposites, the increase in the electrical conductivity from 10−15 to 10−3 S m−1 suggests the formation of a percolated network. In contrast, for iPP/GNPs nanocomposites, no improvement in this property was observed. Finally, for iPP/CuAg/GNPs nanocomposites, the measured electrical conductivity (10−15 S m−1) suggests that the polymerGNP interface is acting as a charge carrier trap. This effect is commonly reported in binary nanocomposites but little or no data exists for ternary NCs.
1. Introduction
The thermal conductivity of polymers can be enhanced by the inclusion of highly thermally conductive metallic fillers such as copper, silver and aluminium particles. Copper has been particularly explored due to its high thermal conductivity (∼400 W m−1 K−1 at room temperature) and relatively low cost [3], although it shows strong tendency to oxidation [7]. Silver is also a high thermally conductive metal (∼430 W m−1 K−1 at room temperature) but its high cost limits its use in high-volume commercial applications. The problems associated with the oxidation of copper and the high cost of silver could be solved by combining Cu and Ag into specific bimetallic configurations as for instance the core – shell CuAg nanoparticles [8]. Other drawback of Cu and Ag, and that all metals share, is their inherent electrical conductivity; although, this property could be counterbalanced with charge carrier traps [9]. On the other hand, carbon allotropes such as carbon nanotubes and graphene have emerged as an alternative to the metallic nanoparticles since they have the advantage of not being prone to oxidation at the nanocomposite processing conditions, and also, of conferring good mechanical reinforcement [10]. Single-layer graphene possesses exceptional properties among them a thermal conductivity of ∼5000 W m−1 K−1 [11] that largely surpasses that reported for highly
In simple terms, polymer nanocomposites (PNs) consist of polymer matrices and nanoscale fillers. They show substantial property enhancements at lower filler loadings than polymer composites (microscale fillers), however, exfoliation and homogeneous dispersion of nanofillers are often challenging to perform and, for practical purposes, they are critical as they determine the final properties and potential applications [1,2]. In recent years, considerable research efforts have been devoted to the obtention of thermally conductive thermoplastics by using highly conductive nanoparticles [3]. The large specific surface area, through which nanoparticles interact with the polymer matrix, makes possible to significantly enhance the thermal conductivity at low filler loadings [4]. Moreover, the surface of nanoparticles can be engineered to further improve their interaction with the polymer matrix, and so, reduce the interfacial heat transfer resistance [5]. In polymer nanocomposites the heat transfer occurs via phonons (crystal lattice vibrations) [4]. Heat flux through electrons is negligible, although it can become significant beyond the percolation threshold where interparticle contacts form electron conductive pathways [6].
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Corresponding authors. E-mail addresses: [email protected] (D. Navarro-Rodríguez), [email protected] (G. Cadenas-Pliego).
https://doi.org/10.1016/j.mtcomm.2019.100695 Received 14 September 2019; Accepted 11 October 2019 Available online 19 October 2019 2352-4928/ © 2019 Elsevier Ltd. All rights reserved.
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0.9 g cm−3). Grade 3 graphene nanoplatelets (GNPs) with average size/ thickness of 2.0 μm/10 nm, surface area of 600–700 m2 g−1; and purity of > 97%, were supplied by Cheap Tubes Company. GNPs were characterized by X-ray diffraction, showing the characteristic 002 peak around 26° in 2θ.
thermally conductive metals [12]. The thermal conductivity of graphene nanoplatelets ranges from 1000 to 5300 W m−1 K−1 depending on raw materials and fabrication method [13]. Defect-free graphene also shows a remarkable mobility of charge carriers (200,000 cm2 V−1 s−1) that makes it an ideal filler to prepare electrically conductive nanocomposites with low filler loadings [11]. The percolation threshold can be as low as 0.1 wt.% [14], although there are some research works reporting percolation thresholds higher than 5 wt. % [15]. Recent investigations have proved that the polymer-graphene interface can act as a charge carrier trap, and thererefore, it can suppress the electron transport across the nanocomposite [16]. The charge carrier trap effect of the polymer-graphene interface has been reported to occur at very low filler loadings (< 0.01 wt.%) where nanoparticles are far apart and no conduction paths of charge carriers exist in the nanocomposite. This effect appears to be useful in making polymer/ filler nanocomposites more insulating than the polymer matrix. For instance, Li et al. prerapred polyethylene/GNP nanocomposites with 0.005 wt.% of GNP that showed a lower electrical conductivity (1.98 × 10−16 S m−1) than the one measured for neat polyethylene (3.18 × 10−15 S m−1) [9]. This effect is observed at higher concentrations (1 wt.%) provided that no conduction paths exist (below the percolation threshold) in the nanocomposite [17]. This makes graphene nanoplatelets highly attractive as nanoscale fillers for the development of light-weight thermal conducting/electrical insulating nanocomposites. On the other hand, polypropylene (PP) is one of the five main thermoplastic resins used worldwide due to its low density, easy processing, good recyclability, and particularly for the exceptional combination of thermo-mechanical properties not found in almost any other thermoplastic [18]. However, its application in some important fields (e.g. flexible electronics, heat exchangers, etc.) is often limited by its own low thermal conductivity (0.22 W m−1 K−1) [19]. To overcome this limitation, while preserving most of its characteristic properties, PP has been loaded with small amounts of thermally conductive nanofillers, like carbon nanotubes (∼0.35 W m−1 K−1 with 4 vol.% of CNT) [1], graphene nanoplatelets (∼0.85 W m−1 K−1 with 16.7 wt.% of GNPs) [20], and others [21]. PP has also been loaded with two different nanoparticles (expanded graphite and multi-walled CNT) whose combination produced a double percolated network that resulted in a higher thermal conductivity (1.5 W m−1 K−1 at 20 wt.%) as compared with that of the binary PP/ MWCNT (∼0.75 W m−1 K−1 with 20% wt. %) and PP/EG (∼1.1 W m−1 K−1 with 20 wt.%) counterparts [15]. The nucleating effect of nanofillers has attracted great interest because it can produce high crystallization rates and degrees that could result in improved thermal conductivity as reported for polyethylene [22] and some other polymers [23]. However, polypropylene seems to be an exception due to its low chain stacking density (0.93 g cm−3) and phonon scattering created by the vibrational mode of the methyl group [6]. In the present work, the morphological characteristics, the thermal behavior, and the thermal/electrical conductivity property of binary and ternary nanocomposites, prepared with iPP, CuAg alloy nanoparticles and GNPs, were studied. The thermal conductivity of the resulting PNs was substantially enhanced at relatively low filler loadings (≤5.0 wt.%) suggesting that they are good candidates for applications requiring light-weight thermal conducting / electrical insulating materials.
2.2. Synthesis of CuAg alloy nanoparticles (NPs) An aqueous solution of copper hydroxide (5 g in 25 mL of H2O) was first introduced into a 500 mL two-neck round-bottom flask where it was heated to 60 °C and held under stirring and inert atmosphere (bubbled argon) for 1 h. Then, 2 mL of hydrazine hydrate and 25 mL of 4-aminobutyric acid solution (1.6 M in glycerol) were slowly added. After 1 h of reaction, the temperature was raised to 100 °C. Next, an aqueous silver nitrate solution (3 g in 50 mL of H2O) was added, keeping the temperature at 100 °C. After 2 h of reaction, the solution was allowed to cool down to room temperature to be filtered. The solid was sequentially and repeatedly washed with deionized water and methanol until obtaining a fine brown solid (CuAg alloy NPs), which was finally dried in a vacuum oven and kept under inert atmosphere. 2.3. Preparation of nanocomposites Nanocomposites were processed by melt extrusion, using a co-rotating twin-screw microextruder (Xplore), model IM15, operated at 175 °C. The iPP pellets and nanofillers were first mixed in a beaker and then fed into the extruder at a screw speed of 60 rpm (residence time of around 5 min). The melt was then transferred to an Xplore micro injector operating at a Tbarrel of 180 °C, a Tmold of 27 °C, and an injection pressure of 1.1 MPa. 3 mm thick sheets and disks were prepared by compression molding at 180 °C under a pressure of 10 MPa. Filler loadings were 0.25, 1.0, 2.5, and 5.0 wt.%. The nanocomposites were labelled as follows: polymer/fillercontent (for example iPP/CuAg1.0). Neat iPP (control) was processed under similar conditions. 2.4. Characterization of the CuAg nanoparticles and iPP/CuAg, iPP/GNPs and iPP/CuAg/GNPs nanocomposites The morphology as well as the particle size and particle size distribution of the CuAg alloy nanoparticles was determined by conventional transmission electron microscopy (TEM). The high-resolution electron microscopy (HRTEM) micrographs, the selected-area electron diffraction (SAED) patterns, and the energy dispersive X-ray spectroscopy (EDS) mapping (EDAX – EDS Genesis) were all performed in a FEI-TITAN 80–300 kV microscope operated at an accelerating voltage of 300 kV. The Image J processing program was used for nanoparticle size distribution analysis. Specimens were prepared by deposition of one drop of a colloidal methanol solution onto lacey carbon coated nickel grids. The morphological and elemental analysis (EDS mapping; EDAX Octane Plus) were also studied with a scanning electron microscope (SEM) from JEOL (JCM6000) using copper grids for sample deposition. X-ray diffraction (XRD) analysis was performed from 5 to 90° in 2θ (at 0.02°s−1) in a diffractometer from Bruker (eco D8 Advanced), equipped with a sealed copper tube (CuKα, λ = 0.154 nm) and Lynxeye detector. The thermal analysis was conducted in a differential scanning calorimeter (DSC) from TA Instruments (2500 Discovery series). All samples were heated to 200 °C for 5 min to erase any previous thermal history. They were then cooled (first cooling) and heated (second heating) at a temperature rate of 10 °C min−1. The thermal stability of neat iPP and nanocomposites was examined in a thermogravimetric analyzer from TA Instruments (TGA 5500, Discovery series) where samples were heated from room temperature to 800 °C at a heating rate of 10 °C min−1 under nitrogen atmosphere. The reported degradation temperature corresponds to the 2 wt.% loss temperature (T2%). The apparent (C) and specific (Cp) heat capacities were determined with disk-shaped samples (3.5 and 0.4 mm thick, respectively) according to
2. Experimental part 2.1. Materials Copper hydroxide, silver nitrate, hydrazine hydrate, 4-aminobutyric acid, glycerol, and methyl alcohol, all from Sigma-Aldrich (reagentgrade), were used as received. Isotactic polypropylene (iPP) was purchased from Indelpro Company (Grade: HG009, MFI 6.5 dg min−1, 2
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by DSC (Fig. 2). The first heating/cooling DSC trace showed a Tm/Tc of 774 °C (measured at the onset), which is downshifted only 6 °C with respect to the eutectic composition of the CuAg alloy. Such depression in Tm is to certain point expected for nanoscale bimetallic alloys whose surface/volume ratio is higher than that of their bulk counterparts. For instance, Delsante et al. measured a melting point depression of around 14 °C for side-segregated CuAg nanoparticles of 10 to 15 nm [30]. These authors suggested that the melting point depression may be due either to the effect of the large surface/volume ratio with respect to the bulk or to the presence of copper oxide (Cu2O) formed during the processing of nanoparticles. The second and third heating traces show a small endotherm around 840 °C that is typical of hyper or hypoeutectic CuAg alloys [29]. The absence of this small endotherm in the first heating trace confirms the eutectic composition of the synthesized CuAg nanoalloys before heating. So, the alloy composition changes from eutectic to non-eutectic when heated above its melting temperature.
the Standard Test Method for Thermal Conductivity and Thermal Diffusivity by Modulated Temperature Differential Scanning Calorimetry (ASTM E1952-11). Tests specimens were obtained by compression molding. Measurements were carried out in a modulated DSC (MDSC) from TA Instruments (2500 Discovery series). C and Cp measurements were preformed at 27 °C and recorded in mJ K−1 and J g−1 K−1, respectively. The amplitude of the temperature modulation was ± 0.5°K and the modulation period (P) was 60 s. The thermal conductivity (κ) was calculated from C and Cp according to:
κ=
8 L C2 Cp m d 2 P
where L, m and d are the specimen length (mm), thick specimen mass (mg), and thick specimen diameter (mm), respectively. Volume resistivity (VR) measurements were performed at room temperature with a Keithley 6517B electrometer / high resistance meter – Keithley 8009 resistivity test fixture according to the D-257 ASTM standard test. VR tests were done on circular plates of regular thickness prepared by compression molding. The electric conductivity (σ) was taken as 1/VR. The VR of neat iPP was measured to validate this method. The measured VR of 1015 Ω m (σ = 10−15 S m–1) for neat iPP is similar to the one reported in technical literature (1014–1015 Ω m) [24].
3.2. Morphology of the iPP/CuAg and iPP/CuAg/GNPs nanocomposites The dispersion of nanofillers in the iPP/CuAg and iPP/CuAg/GNPs nanocomposites was studied by SEM. An illustrative SEM micrograph (Fig. 3A) of an iPP/CuAg nanocomposite shows some agglomerates of around 1 μm in size. It can be seen that particles are embedded in the matrix but without interacting with it as expected for metallic particles dispersed in a non-polar matrix. The EDS mapping of both copper (red) and silver (green) shows a fairly good distribution within and around the micro scale particles (Fig. 3B). A close inspection permits the observation of small red and green dots in all the micrograph (Fig. 3C and D), indicating that the melt processing is suitable to disperse the nanoparticles in the matrix although, it should be said the particles dispersion can still be improved. The SEM analysis of the iPP/GNPs/CuAg nanocomposites also showed a good dispersion of nanoparticles, although, there is a considerable number of agglomerates of around 1 μm in size (Fig. 4). X-ray diffraction is a quite useful technique to determine the structural characteristics of nanocomposites, particularly those involving semicrystalline polymers. The XRD patterns of the iPP/CuAg and iPP/GNPs nanocomposites are shown in Figs. 5 and 6, respectively. In order to compare, the XRD patterns of the neat iPP and the corresponding nanoparticles (CuAg or GNPs) were also included. There are three main established crystal structures for iPP: α-monoclinic, β-trigonal and γ-orthorombic [31]. α (110, 040, and 130) and β (300 and 301) phases are clearly distinguished in the iPP/CuAg and iPP/GNPs nanocomposites; the α phase is by far the dominant crystalline structure in both nanocomposites. It can also be noticed that the amorphous broad band (between 10 and 25°) becomes less intense as the content of nanoparticles increases, suggesting that more crystals were formed with increasing filler loadings. This effect was later confirmed by the increasing ΔHm values measured by DSC. For the iPP/CuAg nanocomposites, the expected diffraction peaks of the copper and silver arrays appeared in the XRD patterns at 38 Ag(111), 44 Cu(111), 45 Ag(200), 50 Cu(200), and 65° Ag(220), although two additional reflections at 37 Cu2O(111) and 61 °Cu2O(220) seem to indicate that copper was partially oxidized during processing (Fig. 5). On the other hand, the characteristic peak of GNPs at 26.3° (002) is not perceived in any of the iPP/ GNPs nanocomposites. The intercalation of the polymer chains with the graphene sheets might produce an expansion of the graphitic material that may be reflected in a broadening and down-shifting of the diffraction peak. The full separation into individual or few graphene sheets (exfoliation) normally produces a complete vanishing of such a peak [14]. The use of small amounts of GNPs makes difficult to determine if the melt blending produced a broadening/down-shifting or a full vanishing of the 002 peak. The absence of this peak in the XRD pattern of the iPP/GNPs nanocomposites was interpreted as an intercalation of the polymer chains with the graphene sheets. Exfoliation was discarded as no improvement in electrical conductivity (as later
3. Results and discussion 3.1. Morphology and composition of the CuAg alloy nanoparticles The CuAg nanoalloy was first studied by TEM where it was observed that most nanoparticles are quasi spherical in shape (Fig. 1A). Icosahedral nanoparticles were also observed, but in a relatively small number. One example is shown in the HRTEM image (Fig. 1B) where lattice fringes of both fcc silver (0.233 nm) and fcc copper (0.205 nm) phases are distinguished [25]. The particle size, measured from TEM micrographs, ranged from 2 to 40 nm with an average diameter of around 18 nm (Fig. 1C). The diffraction rings in the SAED pattern (Fig. 1D) are associated with the crystallographic planes (111, 200, 220, and 311) of both copper and silver atom arrays [26]. The EDS analysis (Fig. 1E) gave us only a rough idea of the elemental composition because from dot to dot (nanoscale mapping) the Ag atom% varied from 20 to 60, and more frequently from 40 to 50. The reported structures of bimetallic nanoalloys are: i) the onephase or fully mixed configuration, where the two metals coexist either ordered or randomly distributed; (ii) the two-phase or phase separated configuration, where the two metals are segregated but coexisting at nanodomain level (case of the core-shell and the Janus-like configurations); and (iii) various possible intermediate configurations [27]. In the preparation of CuAg nanoalloys all these configurations are possible even for the same atom composition. The configuration of CuAg nanoalloys depends on the preparation method, oxidation state, particle size, and so on [28,29]. For instance, Radnóczi et al. prepared (cosputtered method) CuAg nanoparticles that below 5 nm (diameter) crystallized into a one-phase solid at all atom composition whereas above 5 nm the formation of one or two-phase solids depended on the atom composition [28]. The HRTEM image (Fig. 1B) and SAED pattern (Fig. 1D) seem indicate the existence of CuAg alloy nanoparticles of various configurations, although, according to the average size (18 nm) and in line with Radnóczi et al. observations, the CuAg alloy nanoparticles synthesized in this work are two-phase structured. This result was expected because the sequential addition of copper and silver salts may allow the obtention of CuAg nanoparticles with a silver-rich shell (or copper-rich core) [8]. The reported melting temperature (Tm) for a bulk CuAg alloy eutectic composition (around 60 atom% Ag) is 780 °C [29] whereas that for pure Cu and Ag is 1084.62 and 961.78 °C, respectively [12]. The eutectic temperature of the CuAg alloy nanoparticles was determined 3
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Fig. 1. TEM (A) and HRTEM (B) images of the CuAg alloy nanoparticles, particle diameter versus frequency plot (C), SAED pattern where diffraction peaks are indicated (D), and energy versus counts plot from EDS mapping (E).
the nucleating effect of both CuAg nanoalloys and graphene nanoplatelets, and second, higher enthalpy values (ΔH), indicating a higher degree of crystallinity for the nanocomposites [32]. The dominant phase corresponds to the α-crystal (monoclinic) as already mentioned. The DSC traces of the ternary (iPP/CuAg/GNPs) nanocomposites also show that Tc rises with increasing contents of nanoparticles (Fig. 10). Their Tm, Tc, ΔHm, and ΔHc values are slightly smaller than those of the binary nanocomposites with similar filler loadings (Table 1). The crystalline degree (Xc) of neat iPP and nanocomposites was obtained from the following expression: Xc = (ΔHm/(1 – x) ΔH0) × 100, where ΔH0 is the melting enthalpy (165 J g−1) for a 100% crystalline iPP [33], and x is the filler weight fraction. Xc for neat iPP is 52.1% whereas that for binary composites is higher than 56%. The highest Xc value (63.2%) was registered for the binary nanocomposites with 2.5 wt.% of nanofiller. The TGA traces showed marked differences in thermal stability
dscussed in ths paper) was observed. The XRD patterns of the ternary iPP/CuAg/GNPs nanocomposites (Fig. 7) showed a similar feature to those of the nanocomposites prepared with only one type of nanoparticle. 3.3. Thermal behavior of iPP/CuAg, iPP/GNPs and iPP/CuAg/GNPs nanocomposites The effect of CuAg and GNPs nanoparticles on the thermal behavior of the host iPP matrix was examined by DSC and TGA. The second heating and first cooling DSC traces of the iPP/CuAg and iPP/GNPs nanocomposites are depicted in Figs. 8 and 9, respectively. In order to compare, the heating and cooling traces of neat iPP (control) were also included. Two important differences between the thermograms of nanocomposites and neat iPP are readily observed. First, an increase of the crystallization temperature (Tc) that can be simply associated with 4
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Fig. 4. SEM micrograph of the iPP/CuAg2.5/GNPs2.5 nanocomposite.
iPP/GNPs nanocomposites is higher than that reached by the iPP/CuAg nanocomposites. It is not so simple to determine why one filler is better than the other when there are many filler factors (nature, shape, size, adhesion to the matrix, dispersion, orientation, etc.) affecting the heat transfer across the nanocomposites [34]. Copper and silver nanoparticles as well as graphene nanoplatelets have been reported to be good thermal conductive enhancers [3,20,35], and values here obtained are comparable to those reported in literature for iPP-based nanocomposites, where a homogeneous distribution of particles was presumed [15]. The combination of two different conducting nanoparticles has been already explored to enhance the thermal conductivity of nanocomposites by the so-called “synergistic effect”, although this effect not always occurs [36]. The ternary iPP/CuAg/GNPs nanocomposites showed similar thermal conductivities as compared with that of the binary iPP/CuAg nanocomposites of the same composition (2.5 and 5 wt.%). The thermal conductivity of the ternary iPP/ CuAg1.25/GNPs1.25 nanocomposite was also similar to that of the binary
Fig. 2. Heating and cooling DSC thermograms of CuAg alloy nanoparticles.
between neat iPP and the iPP/CuAg (Fig. 11A), iPP/GNPs (Fig. 11B), and iPP/CuAg/GNPs (Fig. 11C) nanocomposites. The iPP/CuAg nanocomposites showed a higher T2% (∼376 °C) as compared with that of neat iPP (T2% = 345 °C). For iPP/GNPs nanocomposites only iPP/ GnPs5.0 showed a higher T2% (∼375 °C) than that of the pure polymer. Finally, the very high T2% (443 °C) for the two ternary nanocomposites indicates that the combination of the two distinct nanoparticles creates synergy in thermal stability.
3.4. Thermal and electrical conductivities of nanocomposites The thermal conductivity data of the binary (iPP/CuAg and iPP/ GNPs) and ternary (iPP/CuAg/GnPs) nanocomposites are listed in Table 1. It can be readily noticed that the thermal conductivity of the
Fig. 3. SEM micrograph (A) and EDS mapping of copper (red)/silver (green) (B), silver (C) and copper (D) for the iPP/CuAg2.5 nanocomposite. 5
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Fig. 8. First cooling and second heating DSC traces of neat iPP and iPP/CuAg nanocomposites.
Fig. 5. XRD patterns of the CuAg alloy nanoparticles, pure iPP and iPP/CuAg nanocomposites.
Fig. 9. First cooling and second heating DSC traces of neat iPP and iPP/GNPs nanocomposites.
Fig. 6. XRD patterns of GNPs, pure iPP and iPP/GNPs nanocomposites.
Fig. 7. X-ray diffractograms of iPP, iPP/CuAg1.25/GNPs1.25 and iPP/CuAg2.5/ GnPs2.5. Fig. 10. First cooling and second heating DSC traces of neat iPP and iPP/CuAg/ GNPs nanocomposites.
iPP/GNPs2.5 nanocomposite. Only iPP/CuAg2.5/GNPs2.5 showed a much lower thermal conductivity (1.21 W m−1 K−1) as compared with its iPP/GNP5.0 counterpart (1.56 W m−1 K−1). In literature reports it was proposed that, in ternary nanocomposites, a double percolated network is formed creating synergy in heat conduction [15]. Such synergy was not observed in our nanocomposites. In polymer nanocomposites the thermal conductivity is affected by the scattering of energy carriers at the matrix-filler interface [34]. The specific surface area of particles drastically increases when the particle size is scaled down from micro- to nano-scale, meaning that the thermal
conductivity of nanocomposites is largely determined by the interfacial effects. Our results show that the thermal conductivity of polymers was improved by the inclusion of highly conductive CuAg and GNPs nanoparticles, nevertheless, the resulting thermal conductivity is lower than expected due mainly to the thermal resistance of the matrix-filler (M-F) interface. Different highly thermally conductive fillers (metals, ceramics, carbon derivatives, etc.) have been studied over the years, leading to more or less comparable results because all of them follow 6
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Table 1 Thermal characteristics of iPP, iPP/CuAg, iPP/GNPs, and iPP/CuAg/GNPs. The crystalline degree and electrical conductivity are included. Nanocomposite
Tc (°C)
ΔHc (J g−1)
Tm (°C)
ΔHm (J g−1)
Xc (%)
κ (W m−1 K−1)
σ (S m−1)
iPP iPP/CuAg0.25 iPP/CuAg1.0 iPP/CuAg2.5 iPP/CuAg5.0 iPP/GNP0.25 iPP/GNP1.0 iPP/GNP2.5 iPP/GNP5.0 iPP/CuAg1.25/GNP1.25 iPP/CuAg2.5/GNP2.5
112.5 126.2 125.7 127.4 128.4 126.4 127.4 128.4 129.6 127.0 128.5
90.9 94.9 92.4 99.1 93.4 94.7 92.3 98.6 92.6 88.9 91.7
164.3 165.8 165.7 165.5 166.0 165.0 165.5 165.5 166.0 165.1 165.4
85.9 95.3 92.5 101.6 94.8 94.2 91.4 101.6 94.4 92.1 93.7
52.1 57.9 56.6 63.2 60.5 57.2 56.0 63.2 60.2 57.2 59.8
0.21 0.65 0.89 0.99 1.28 0.81 0.99 1.01 1.56 1.08 1.21
1 × 10−15 1 × 10−13 3 × 10−6 3 × 10−4 5 × 10−3 1 × 10−15 1 × 10−13 3 × 10−15 6 × 10−13 5 × 10−15 3 × 10−14
Fig. 11. TGA thermograms of neat iPP and its nanocomposites with CuAg (A), GNPs (B) and CuAg/GNPs (C).
hybrid nanocomposites as compared with the single-particle ones [15], however, there are some reports where the electrical conductivity was lower in ternary than in binary nanocomposites. For instance, Paszkiewicz et al. looking for synergistic effects prepared ternary nanocomposites with low-density polyethylene (LDPE) and two highly thermally and electrically conductive nanoparticles (CNT and GNPs) [36]. Contrary to what was expected, they found a lower electrical conductivity for the ternary LDPE/CNT/GNPs nanocomposites than for binary LDPE/CNT ones despite the fact that CNT and GNP interlocate and form local percolated networks. They reported a σ value of around 100 and 10−13 S m−1 for LDPE/CNT and LDPE/CNT/GNP nanocomposites (both with 3 wt.% of nanoparticles), respectively. Authors suggested that a thin polymer film prevents the direct contact between CNT and GNP, introducing an insulating layer in the tunneling barrier for electrical transport. The thin film is in effect an insulating layer but taking into account the large difference between these two values, one can suppose that the LDPE-GNP interface is acting as a charge carrier trap. It should be emphasized that this effect has been reported to occur mainly at low filler contents (< 1 wt.%) where nanoparticles are far apart and no conduction paths of charge carriers exist in the nanocomposite [9,17]. There are few research works where much higher contents of graphene nanoplatelets were used to make the polymer nanocomposites less conductive than the neat polymer. For instance, Gaska et al. reported a polyethylene/GNPs nanocompostite with 5 wt.% of GNPs that, at low fields (< 20 kV mm−1), showed a σ value of ∼ 7 × 10−17 S m−1 whereas the measured one for neat polyethylene was ∼1 × 10−16 S m−1; both processed by extrusion (using a compression screw) under similar conditions [39]. These authors suggested that the graphene nanoplatelets, all aligned perpendicular to the electric field, act as charge trapping sites, limiting the charge transport through the material. In our ternary iPP/CuAg/GNPs nanocomposites, the GNPs nanoparticles (non-exfoliated) might be far apart and they are probably acting as charge carrier traps rather than charge conduction links of a conducting network. So, our results confirm that the charge carrier trapping effect can occur at relatively high conents of GNPs (up to 5 wt.
similar heat transfer mechanisms [37]. At low loadings, where nanoparticles are far apart, the interfacial resistance has a strong effect on the heat conduction, although the large mean inter-particle distances seems to be more detrimental than the interfacial resistance. At high filler loadings, the filler-filler (F-F) contact gives rise to the percolation threshold, which is a critical property for the electrical conduction but much less relevant for the thermal conduction because the F-F interface can also give rise to thermal resistance [38]. On the other hand, the electrical conductivity of the iPP/GNPs nanocomposites is markedly different to that of the iPP/CuAg at similar filler composition (Table 1). The electric conductivity of the iPP/GNPs nanocomposites (10−15–10−13 S m−1)) showed no real improvement as compared with that of neat iPP (10−15 S m−1). This means that the graphene nanoplatelets are far apart and therefore they do not provide interparticle contacts for the electric conduction. A similar conductivity (σ ∼ 10−14 S m−1) was measured by Imran et al. for binary iPP/GNPs nanocomposites with GNP loadings of 0.2–4.8 wt.% prepared by melt processing (extrusion) [20]. These authors suggested that the percolation threshold was not reached in their nanocomposites below 10 wt.%. We also arrived to the same conclusion because the extrusion process did not provide iPP/GNPs nanocomposites with exfoliated nanoparticles. In contrast, the iPP/CuAg nanocomposites showed increasing σ values with increasing filler contents reaching a σ of 10−6, 10−4, and 10−3 S m−1 for iPP/CuAg1.0, iPP/CuAg2.5, and iPP/CuAg5.0, respectively. From these results we would expect comparable or higher σ values for the ternary iPP/CuAg/GNPs nanocomposites with similar total filler content, however, for such nanocomposites no improvement in this property was observed. The measured low values (5 × 10−15 S m−1 for iPP/CuAg1.25/GNPs1.25 and 3 × 10−14 S m−1 for iPP/CuAg2.5/ GNPs2.5) suggest that no electron conduction paths (or networks) were formed in the ternary nanocomposites despite the relatively high content of both CuAg and GNPs (2.5 and 5 wt.% of total filler). The combination of two nanoparticles for improving the electrical conductivity of polymers has been already explored by other groups. Intermediate or improved electrical conductivity is usually found in two-particle or 7
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%) provided that they do not form conduction paths. In this way, the ternary iPP/CuAg/GNPs nanocomposites became more insulating than the binary iPP/CuAg ones. The charge carrier trap effect is of course undesirable for the development of electric conducting nanocomposites, but it results advantageous for the preparation of high thermal/ low electrically conductive materials, which are suitable for modern electronic devices for which heat releasing is a key factor.
Polym. Sci. 59 (2016) 41–85. [7] D.A. Pomogailo, L.A. Petrova, D.A. Pomogailo, E.I. Knerelman, A.M. Kolesnikova, S.V. Barinov, K.A. Kydralieva, G.I. Dzhardimalieva, Nanosci. Technol. 8 (2017) 27–40. [8] N.R. Kim, K. Shin, I. Jung, M. Shim, H.M. Lee, J. Phys. Chem. C 118 (2014) 264324–264331. [9] Z. Li, B. Du, C. Han, H. Xu, Sci. Rep. 7 (2017) 4015, https://doi.org/10.1038/ s41598-017-04196-5. [10] R.J. Young, M. Liu, I.A. Kinloch, S. Li, X. Zhao, C. Valles, D.G. Papageorgiou, Compos. Sci. Technol. 154 (2018) 110–116. [11] S. Park, R.S. Ruoff, Nat. Nanotechnol. 4 (2009) 217–224. [12] D.R. Lide, Handbook of Chemistry and Physics, 84th ed., CRC Press, New York, 2003. [13] S. Chinkanjanarot, J.M. Tomasi, J.A. King, J.M. Odegard, Carbon 140 (2018) 653–663. [14] A. Cruz-Aguilar, D. Navarro-Rodríguez, O. Pérez-Camacho, S. Fernández-Tavizón, C.A. Gallardo-Vega, M. García-Zamora, E. Díaz Barriga-Castro, Mater. Today Commun. 16 (2018) 232–241. [15] K. Wu, Y. Xue, W. Yang, S. Chai, F. Chen, Q. Fu, Compos. Sci. Technol. 130 (2016) 28–35. [16] Z. Li, J. Su, B. Du, Z. Hou, C. Han, Nanomaterials 8 (2018) 956. [17] Z. Jing, C. Li, H. Zhao, G. Zhang, B. Han, Materials 9 (2016) 680. [18] H.A. Maddah, Am. J. Polym. Sci. 6 (2016) 1–11. [19] M.H. Vakili, H. Ebadi-Dehaghani, M. Haghshenas-Fard, J. Macromol. Sci. Part B: Phys. 50 (2011) 1637–1645. [20] K.A. Imran, J. Lou, K.N. Shivakumar, J. Appl. Polym. Sci. 135 (9) (2018) 11. Article ID 45833. [21] L. Chen, H.-F. Hu, S.-J. He, Y.-H. Du, N.-J. Yu, X.-Z. Du, J. Lin, S. Nazarenko, PLoS One 12 (2017) e0170523. [22] R. Haggenmueller, C. Guthy, J.R. Lukes, J.E. Fisher, K.I. Winey, Macromolecules 40 (2007) 2417–4121. [23] T. Weime, L. Duan, N. Mys, L. Cardon, D.R. D’hooge, Polymers 11 (2019) 87, https://doi.org/10.3390/polym11010087. [24] J. Brandrup, E.H. Immergut, E.A. Grulke, fourth ed., Polymer Handbook 1 WileyInterscience, New Jersey, 1999. [25] M.K. Sharma, R.D. Buchner, W.J. Scharmach, V. Papavassiliou, M.T. Swihart, Aerosol Sci. Technol. 47 (2013) 858–866. [26] A. Sakthisabarimoorthi, M. Jose, S.A. Martin Briti Dhas, J. Mater. Sci.: Mater. Electron. 28 (2017) 4545–4552. [27] F. Calvo, Phys. Chem. Chem. Phys. 17 (2015) 27922–27939. [28] G. Radnóczi, E. Bokányi, Z. Erdélyi, F. Misják, Acta Mater. 123 (2017) 82–89. [29] J. Sopousek, J. Pinkas, P. Broz, J. Bursik, V. Vykoukal, D. Skoda, A. Styskalik, O. Zobac, J. Vrest’al, A. Hrdlicka, J. Simbera, J. Nanomater. 2014 (2014) 13. Article ID 638964. [30] S. Delsante, G. Borzone, R. Novakovic, D. Piazza, G. Pigozzi, J. Janczak-Rusch, M. Pillonid, G. Ennas, Phys. Chem. Chem. Phys. 17 (2015) 28387–28393. [31] R. Krache, R. Benavente, J.M. López-Majada, J.M. Pereña, M.L. Cerrada, E. Pérez, Macromolecules 40 (2007) 6871–6878. [32] M. El Achaby, F.E. Arrakhiz, S. Vaudreuil, A. el Kacem Qaiss, M. Bousmina, O. FassiFehri, Polym. Compos. 33 (2012) 733–744. [33] J.E. Mark, Polymer Data Handbook, Oxford University Press, 1999. [34] J. Ordoñez-Miranda, R. Yang, J.J. Alvarado-Gil, Thermal conductivity of particulate nanocomposites, in: X. Wang, Z.M. Wang (Eds.), Nanoscale Thermoelectrics, Springer, New York, 2014, pp. 93–139. [35] L. Rivière, A. Lonjon, E. Dantras, C. Lacabanne, P. Olivier, N. Rocher Gleizes, Eur. Polym. J. 85 (2016) 115–125. [36] S. Paszkiewicz, A. Szymczyk, D. Pawlikowska, J. Subocz, M. Zenker, R. Masztak, Nanomaterials 8 (2018) 264, https://doi.org/10.3390/nano8040264. [37] N. Burger, A. Laachachi, M. Ferriol, M. Lutz, V. Toniazzo, D. Ruch, Prog. Polym. Sci. 61 (2016) 1–28. [38] Y. Su, J.J. Li, G.J. Weng, Carbon 137 (2018) 222–233. [39] K. Gaska, X. Xu, S. Gubanski, R. Kádár, Polymers 9 (2017) 11, https://doi.org/10. 3390/polym9010011.
4. Conclusion Binary and ternary nanocomposites based on iPP, silver-rich shell CuAg alloy nanoparticles and/or graphene nanoplatelets were prepared by melt processing, and their morphological and thermal/electrical conductivity properties were determined. SEM and TEM analyses showed a good dispersion of nanoparticles despite the observation of microscale CuAg agglomerates. In both binary and ternary nanocomposites, the Tc and crystalline degree increased as the content of nanoparticles increased, suggesting a nucleating effect of the nanoparticles. The thermal stability (T2% = 443 °C) of the ternary nanocomposites was superior to that of the binary counterparts (T2% ∼ 375 °C) of similar filler loading. The attained thermal conductivity of 1.56 W m−1 K−1 is comparable to that reported for iPP/ filler nanocomposites, although the ternary nanocomposites, herein reported, are electrical insulators (10−15–10−14 S m−1); the polymerGNP interface is likely to act as a charge carrier trap. The charge carrier trapping effect appears not yet to have been reported in ternary nanocomposites. Theoretical and further experimental analysis of this effect may perhaps result a new strategy to prepare light-weight thermal conducting / electrical insulating materials. Acknowledgements Financial support from CONACYT Mexico(Project: 281164: Consolidation of the National Laboratory on Graphene Materials) is gratefully appreciated. Authors also appreciate the technical assistance of Enrique Díaz Barriga-Castro (TEM), Jesús Cepeda (SEM), Guadalupe Méndez (DSC), Gilberto Hurtado (electric conductivity measurements), Joelis Rodríguez (XRD), Uriel Sierra (graphene analysis) and Alfonso Mercado (Lab. facilities) from CIQA. References [1] M.S. Nurul, M. Mariatti, J. Thermoplast. Compos. Mater. 26 (2011) 627–639. [2] S. Mahdi Hamidinejad, R.K.M. Chu, B. Zhao, C.B. Park, T. Filleter, ACS Appl. Mater. Interfaces 10 (2018) 1225–1236. [3] D. Zhu, W. Yu, H. Du, L. Chen, Y. Li, H. Xie, J. Nanomater. 2016 (2016) 6. Article ID 3089716. [4] A. Li, C. Zhang, Y.F. Zhang, Polymers 9 (2017) 437. [5] K. Yang, M. Gu, Y. Guo, X. Pan, G. Mu, Carbon 47 (2009) 1723–1737. [6] H. Chen, V.V. Ginzburg, J. Yang, Y. Yang, W. Liu, Y. Huang, L. Du, B. Chen, Prog.
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Enhancement of the thermal conductivity of polypropylene with low loadings of CuAg alloy nanoparticles and graphene nanoplatelets
T
Diana Iris Medellín-Bandaa, Dámaso Navarro-Rodrígueza,*, Salvador Fernández-Tavizóna, Carlos Alberto Ávila-Ortaa, Gregorio Cadenas-Pliegob,*, Victor Eduardo Comparán-Padillab a b
Departamento de Materiales Avanzados (DMA), Centro de Investigación en Química Aplicada, Blvd. Enrique Reyna 140, C.P. 25294, Saltillo Coahuila, Mexico Departamento de Síntesis de Polímeros (DSP), Centro de Investigación en Química Aplicada, Blvd. Enrique Reyna 140, C.P. 25294, Saltillo Coahuila, Mexico
A R T I C LE I N FO
A B S T R A C T
Keywords: Polypropylene Nanocomposite CuAg alloy Graphene Thermal conductivity
Isotactic polypropylene (iPP), silver-rich shell CuAg nanoalloys and graphene nanoplatelets (GNPs) were used to prepare thermally conductive binary (iPP/CuAg and iPP/GNPs) and ternary (iPP/CuAg/GNPs) nanocomposites with filler compositions from 0.1 to 5 wt.% through melt processing. All nanocomposites were thermally and electrically characterized. Results showed that both nanoparticles nucleate the crystallization of iPP, shifting the crystallization transition toward higher temperature as compared with that of iPP. The nanoparticles dispersion was not optimum but was enough to make the nanocomposites thermally conductive between 0.65 and 1.56 W m−1 K−1 and more thermally stable (443 °C) than neat iPP (347 °C). For the iPP/CuAg nanocomposites, the increase in the electrical conductivity from 10−15 to 10−3 S m−1 suggests the formation of a percolated network. In contrast, for iPP/GNPs nanocomposites, no improvement in this property was observed. Finally, for iPP/CuAg/GNPs nanocomposites, the measured electrical conductivity (10−15 S m−1) suggests that the polymerGNP interface is acting as a charge carrier trap. This effect is commonly reported in binary nanocomposites but little or no data exists for ternary NCs.
1. Introduction
The thermal conductivity of polymers can be enhanced by the inclusion of highly thermally conductive metallic fillers such as copper, silver and aluminium particles. Copper has been particularly explored due to its high thermal conductivity (∼400 W m−1 K−1 at room temperature) and relatively low cost [3], although it shows strong tendency to oxidation [7]. Silver is also a high thermally conductive metal (∼430 W m−1 K−1 at room temperature) but its high cost limits its use in high-volume commercial applications. The problems associated with the oxidation of copper and the high cost of silver could be solved by combining Cu and Ag into specific bimetallic configurations as for instance the core – shell CuAg nanoparticles [8]. Other drawback of Cu and Ag, and that all metals share, is their inherent electrical conductivity; although, this property could be counterbalanced with charge carrier traps [9]. On the other hand, carbon allotropes such as carbon nanotubes and graphene have emerged as an alternative to the metallic nanoparticles since they have the advantage of not being prone to oxidation at the nanocomposite processing conditions, and also, of conferring good mechanical reinforcement [10]. Single-layer graphene possesses exceptional properties among them a thermal conductivity of ∼5000 W m−1 K−1 [11] that largely surpasses that reported for highly
In simple terms, polymer nanocomposites (PNs) consist of polymer matrices and nanoscale fillers. They show substantial property enhancements at lower filler loadings than polymer composites (microscale fillers), however, exfoliation and homogeneous dispersion of nanofillers are often challenging to perform and, for practical purposes, they are critical as they determine the final properties and potential applications [1,2]. In recent years, considerable research efforts have been devoted to the obtention of thermally conductive thermoplastics by using highly conductive nanoparticles [3]. The large specific surface area, through which nanoparticles interact with the polymer matrix, makes possible to significantly enhance the thermal conductivity at low filler loadings [4]. Moreover, the surface of nanoparticles can be engineered to further improve their interaction with the polymer matrix, and so, reduce the interfacial heat transfer resistance [5]. In polymer nanocomposites the heat transfer occurs via phonons (crystal lattice vibrations) [4]. Heat flux through electrons is negligible, although it can become significant beyond the percolation threshold where interparticle contacts form electron conductive pathways [6].
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Corresponding authors. E-mail addresses: [email protected] (D. Navarro-Rodríguez), [email protected] (G. Cadenas-Pliego).
https://doi.org/10.1016/j.mtcomm.2019.100695 Received 14 September 2019; Accepted 11 October 2019 Available online 19 October 2019 2352-4928/ © 2019 Elsevier Ltd. All rights reserved.
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0.9 g cm−3). Grade 3 graphene nanoplatelets (GNPs) with average size/ thickness of 2.0 μm/10 nm, surface area of 600–700 m2 g−1; and purity of > 97%, were supplied by Cheap Tubes Company. GNPs were characterized by X-ray diffraction, showing the characteristic 002 peak around 26° in 2θ.
thermally conductive metals [12]. The thermal conductivity of graphene nanoplatelets ranges from 1000 to 5300 W m−1 K−1 depending on raw materials and fabrication method [13]. Defect-free graphene also shows a remarkable mobility of charge carriers (200,000 cm2 V−1 s−1) that makes it an ideal filler to prepare electrically conductive nanocomposites with low filler loadings [11]. The percolation threshold can be as low as 0.1 wt.% [14], although there are some research works reporting percolation thresholds higher than 5 wt. % [15]. Recent investigations have proved that the polymer-graphene interface can act as a charge carrier trap, and thererefore, it can suppress the electron transport across the nanocomposite [16]. The charge carrier trap effect of the polymer-graphene interface has been reported to occur at very low filler loadings (< 0.01 wt.%) where nanoparticles are far apart and no conduction paths of charge carriers exist in the nanocomposite. This effect appears to be useful in making polymer/ filler nanocomposites more insulating than the polymer matrix. For instance, Li et al. prerapred polyethylene/GNP nanocomposites with 0.005 wt.% of GNP that showed a lower electrical conductivity (1.98 × 10−16 S m−1) than the one measured for neat polyethylene (3.18 × 10−15 S m−1) [9]. This effect is observed at higher concentrations (1 wt.%) provided that no conduction paths exist (below the percolation threshold) in the nanocomposite [17]. This makes graphene nanoplatelets highly attractive as nanoscale fillers for the development of light-weight thermal conducting/electrical insulating nanocomposites. On the other hand, polypropylene (PP) is one of the five main thermoplastic resins used worldwide due to its low density, easy processing, good recyclability, and particularly for the exceptional combination of thermo-mechanical properties not found in almost any other thermoplastic [18]. However, its application in some important fields (e.g. flexible electronics, heat exchangers, etc.) is often limited by its own low thermal conductivity (0.22 W m−1 K−1) [19]. To overcome this limitation, while preserving most of its characteristic properties, PP has been loaded with small amounts of thermally conductive nanofillers, like carbon nanotubes (∼0.35 W m−1 K−1 with 4 vol.% of CNT) [1], graphene nanoplatelets (∼0.85 W m−1 K−1 with 16.7 wt.% of GNPs) [20], and others [21]. PP has also been loaded with two different nanoparticles (expanded graphite and multi-walled CNT) whose combination produced a double percolated network that resulted in a higher thermal conductivity (1.5 W m−1 K−1 at 20 wt.%) as compared with that of the binary PP/ MWCNT (∼0.75 W m−1 K−1 with 20% wt. %) and PP/EG (∼1.1 W m−1 K−1 with 20 wt.%) counterparts [15]. The nucleating effect of nanofillers has attracted great interest because it can produce high crystallization rates and degrees that could result in improved thermal conductivity as reported for polyethylene [22] and some other polymers [23]. However, polypropylene seems to be an exception due to its low chain stacking density (0.93 g cm−3) and phonon scattering created by the vibrational mode of the methyl group [6]. In the present work, the morphological characteristics, the thermal behavior, and the thermal/electrical conductivity property of binary and ternary nanocomposites, prepared with iPP, CuAg alloy nanoparticles and GNPs, were studied. The thermal conductivity of the resulting PNs was substantially enhanced at relatively low filler loadings (≤5.0 wt.%) suggesting that they are good candidates for applications requiring light-weight thermal conducting / electrical insulating materials.
2.2. Synthesis of CuAg alloy nanoparticles (NPs) An aqueous solution of copper hydroxide (5 g in 25 mL of H2O) was first introduced into a 500 mL two-neck round-bottom flask where it was heated to 60 °C and held under stirring and inert atmosphere (bubbled argon) for 1 h. Then, 2 mL of hydrazine hydrate and 25 mL of 4-aminobutyric acid solution (1.6 M in glycerol) were slowly added. After 1 h of reaction, the temperature was raised to 100 °C. Next, an aqueous silver nitrate solution (3 g in 50 mL of H2O) was added, keeping the temperature at 100 °C. After 2 h of reaction, the solution was allowed to cool down to room temperature to be filtered. The solid was sequentially and repeatedly washed with deionized water and methanol until obtaining a fine brown solid (CuAg alloy NPs), which was finally dried in a vacuum oven and kept under inert atmosphere. 2.3. Preparation of nanocomposites Nanocomposites were processed by melt extrusion, using a co-rotating twin-screw microextruder (Xplore), model IM15, operated at 175 °C. The iPP pellets and nanofillers were first mixed in a beaker and then fed into the extruder at a screw speed of 60 rpm (residence time of around 5 min). The melt was then transferred to an Xplore micro injector operating at a Tbarrel of 180 °C, a Tmold of 27 °C, and an injection pressure of 1.1 MPa. 3 mm thick sheets and disks were prepared by compression molding at 180 °C under a pressure of 10 MPa. Filler loadings were 0.25, 1.0, 2.5, and 5.0 wt.%. The nanocomposites were labelled as follows: polymer/fillercontent (for example iPP/CuAg1.0). Neat iPP (control) was processed under similar conditions. 2.4. Characterization of the CuAg nanoparticles and iPP/CuAg, iPP/GNPs and iPP/CuAg/GNPs nanocomposites The morphology as well as the particle size and particle size distribution of the CuAg alloy nanoparticles was determined by conventional transmission electron microscopy (TEM). The high-resolution electron microscopy (HRTEM) micrographs, the selected-area electron diffraction (SAED) patterns, and the energy dispersive X-ray spectroscopy (EDS) mapping (EDAX – EDS Genesis) were all performed in a FEI-TITAN 80–300 kV microscope operated at an accelerating voltage of 300 kV. The Image J processing program was used for nanoparticle size distribution analysis. Specimens were prepared by deposition of one drop of a colloidal methanol solution onto lacey carbon coated nickel grids. The morphological and elemental analysis (EDS mapping; EDAX Octane Plus) were also studied with a scanning electron microscope (SEM) from JEOL (JCM6000) using copper grids for sample deposition. X-ray diffraction (XRD) analysis was performed from 5 to 90° in 2θ (at 0.02°s−1) in a diffractometer from Bruker (eco D8 Advanced), equipped with a sealed copper tube (CuKα, λ = 0.154 nm) and Lynxeye detector. The thermal analysis was conducted in a differential scanning calorimeter (DSC) from TA Instruments (2500 Discovery series). All samples were heated to 200 °C for 5 min to erase any previous thermal history. They were then cooled (first cooling) and heated (second heating) at a temperature rate of 10 °C min−1. The thermal stability of neat iPP and nanocomposites was examined in a thermogravimetric analyzer from TA Instruments (TGA 5500, Discovery series) where samples were heated from room temperature to 800 °C at a heating rate of 10 °C min−1 under nitrogen atmosphere. The reported degradation temperature corresponds to the 2 wt.% loss temperature (T2%). The apparent (C) and specific (Cp) heat capacities were determined with disk-shaped samples (3.5 and 0.4 mm thick, respectively) according to
2. Experimental part 2.1. Materials Copper hydroxide, silver nitrate, hydrazine hydrate, 4-aminobutyric acid, glycerol, and methyl alcohol, all from Sigma-Aldrich (reagentgrade), were used as received. Isotactic polypropylene (iPP) was purchased from Indelpro Company (Grade: HG009, MFI 6.5 dg min−1, 2
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by DSC (Fig. 2). The first heating/cooling DSC trace showed a Tm/Tc of 774 °C (measured at the onset), which is downshifted only 6 °C with respect to the eutectic composition of the CuAg alloy. Such depression in Tm is to certain point expected for nanoscale bimetallic alloys whose surface/volume ratio is higher than that of their bulk counterparts. For instance, Delsante et al. measured a melting point depression of around 14 °C for side-segregated CuAg nanoparticles of 10 to 15 nm [30]. These authors suggested that the melting point depression may be due either to the effect of the large surface/volume ratio with respect to the bulk or to the presence of copper oxide (Cu2O) formed during the processing of nanoparticles. The second and third heating traces show a small endotherm around 840 °C that is typical of hyper or hypoeutectic CuAg alloys [29]. The absence of this small endotherm in the first heating trace confirms the eutectic composition of the synthesized CuAg nanoalloys before heating. So, the alloy composition changes from eutectic to non-eutectic when heated above its melting temperature.
the Standard Test Method for Thermal Conductivity and Thermal Diffusivity by Modulated Temperature Differential Scanning Calorimetry (ASTM E1952-11). Tests specimens were obtained by compression molding. Measurements were carried out in a modulated DSC (MDSC) from TA Instruments (2500 Discovery series). C and Cp measurements were preformed at 27 °C and recorded in mJ K−1 and J g−1 K−1, respectively. The amplitude of the temperature modulation was ± 0.5°K and the modulation period (P) was 60 s. The thermal conductivity (κ) was calculated from C and Cp according to:
κ=
8 L C2 Cp m d 2 P
where L, m and d are the specimen length (mm), thick specimen mass (mg), and thick specimen diameter (mm), respectively. Volume resistivity (VR) measurements were performed at room temperature with a Keithley 6517B electrometer / high resistance meter – Keithley 8009 resistivity test fixture according to the D-257 ASTM standard test. VR tests were done on circular plates of regular thickness prepared by compression molding. The electric conductivity (σ) was taken as 1/VR. The VR of neat iPP was measured to validate this method. The measured VR of 1015 Ω m (σ = 10−15 S m–1) for neat iPP is similar to the one reported in technical literature (1014–1015 Ω m) [24].
3.2. Morphology of the iPP/CuAg and iPP/CuAg/GNPs nanocomposites The dispersion of nanofillers in the iPP/CuAg and iPP/CuAg/GNPs nanocomposites was studied by SEM. An illustrative SEM micrograph (Fig. 3A) of an iPP/CuAg nanocomposite shows some agglomerates of around 1 μm in size. It can be seen that particles are embedded in the matrix but without interacting with it as expected for metallic particles dispersed in a non-polar matrix. The EDS mapping of both copper (red) and silver (green) shows a fairly good distribution within and around the micro scale particles (Fig. 3B). A close inspection permits the observation of small red and green dots in all the micrograph (Fig. 3C and D), indicating that the melt processing is suitable to disperse the nanoparticles in the matrix although, it should be said the particles dispersion can still be improved. The SEM analysis of the iPP/GNPs/CuAg nanocomposites also showed a good dispersion of nanoparticles, although, there is a considerable number of agglomerates of around 1 μm in size (Fig. 4). X-ray diffraction is a quite useful technique to determine the structural characteristics of nanocomposites, particularly those involving semicrystalline polymers. The XRD patterns of the iPP/CuAg and iPP/GNPs nanocomposites are shown in Figs. 5 and 6, respectively. In order to compare, the XRD patterns of the neat iPP and the corresponding nanoparticles (CuAg or GNPs) were also included. There are three main established crystal structures for iPP: α-monoclinic, β-trigonal and γ-orthorombic [31]. α (110, 040, and 130) and β (300 and 301) phases are clearly distinguished in the iPP/CuAg and iPP/GNPs nanocomposites; the α phase is by far the dominant crystalline structure in both nanocomposites. It can also be noticed that the amorphous broad band (between 10 and 25°) becomes less intense as the content of nanoparticles increases, suggesting that more crystals were formed with increasing filler loadings. This effect was later confirmed by the increasing ΔHm values measured by DSC. For the iPP/CuAg nanocomposites, the expected diffraction peaks of the copper and silver arrays appeared in the XRD patterns at 38 Ag(111), 44 Cu(111), 45 Ag(200), 50 Cu(200), and 65° Ag(220), although two additional reflections at 37 Cu2O(111) and 61 °Cu2O(220) seem to indicate that copper was partially oxidized during processing (Fig. 5). On the other hand, the characteristic peak of GNPs at 26.3° (002) is not perceived in any of the iPP/ GNPs nanocomposites. The intercalation of the polymer chains with the graphene sheets might produce an expansion of the graphitic material that may be reflected in a broadening and down-shifting of the diffraction peak. The full separation into individual or few graphene sheets (exfoliation) normally produces a complete vanishing of such a peak [14]. The use of small amounts of GNPs makes difficult to determine if the melt blending produced a broadening/down-shifting or a full vanishing of the 002 peak. The absence of this peak in the XRD pattern of the iPP/GNPs nanocomposites was interpreted as an intercalation of the polymer chains with the graphene sheets. Exfoliation was discarded as no improvement in electrical conductivity (as later
3. Results and discussion 3.1. Morphology and composition of the CuAg alloy nanoparticles The CuAg nanoalloy was first studied by TEM where it was observed that most nanoparticles are quasi spherical in shape (Fig. 1A). Icosahedral nanoparticles were also observed, but in a relatively small number. One example is shown in the HRTEM image (Fig. 1B) where lattice fringes of both fcc silver (0.233 nm) and fcc copper (0.205 nm) phases are distinguished [25]. The particle size, measured from TEM micrographs, ranged from 2 to 40 nm with an average diameter of around 18 nm (Fig. 1C). The diffraction rings in the SAED pattern (Fig. 1D) are associated with the crystallographic planes (111, 200, 220, and 311) of both copper and silver atom arrays [26]. The EDS analysis (Fig. 1E) gave us only a rough idea of the elemental composition because from dot to dot (nanoscale mapping) the Ag atom% varied from 20 to 60, and more frequently from 40 to 50. The reported structures of bimetallic nanoalloys are: i) the onephase or fully mixed configuration, where the two metals coexist either ordered or randomly distributed; (ii) the two-phase or phase separated configuration, where the two metals are segregated but coexisting at nanodomain level (case of the core-shell and the Janus-like configurations); and (iii) various possible intermediate configurations [27]. In the preparation of CuAg nanoalloys all these configurations are possible even for the same atom composition. The configuration of CuAg nanoalloys depends on the preparation method, oxidation state, particle size, and so on [28,29]. For instance, Radnóczi et al. prepared (cosputtered method) CuAg nanoparticles that below 5 nm (diameter) crystallized into a one-phase solid at all atom composition whereas above 5 nm the formation of one or two-phase solids depended on the atom composition [28]. The HRTEM image (Fig. 1B) and SAED pattern (Fig. 1D) seem indicate the existence of CuAg alloy nanoparticles of various configurations, although, according to the average size (18 nm) and in line with Radnóczi et al. observations, the CuAg alloy nanoparticles synthesized in this work are two-phase structured. This result was expected because the sequential addition of copper and silver salts may allow the obtention of CuAg nanoparticles with a silver-rich shell (or copper-rich core) [8]. The reported melting temperature (Tm) for a bulk CuAg alloy eutectic composition (around 60 atom% Ag) is 780 °C [29] whereas that for pure Cu and Ag is 1084.62 and 961.78 °C, respectively [12]. The eutectic temperature of the CuAg alloy nanoparticles was determined 3
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Fig. 1. TEM (A) and HRTEM (B) images of the CuAg alloy nanoparticles, particle diameter versus frequency plot (C), SAED pattern where diffraction peaks are indicated (D), and energy versus counts plot from EDS mapping (E).
the nucleating effect of both CuAg nanoalloys and graphene nanoplatelets, and second, higher enthalpy values (ΔH), indicating a higher degree of crystallinity for the nanocomposites [32]. The dominant phase corresponds to the α-crystal (monoclinic) as already mentioned. The DSC traces of the ternary (iPP/CuAg/GNPs) nanocomposites also show that Tc rises with increasing contents of nanoparticles (Fig. 10). Their Tm, Tc, ΔHm, and ΔHc values are slightly smaller than those of the binary nanocomposites with similar filler loadings (Table 1). The crystalline degree (Xc) of neat iPP and nanocomposites was obtained from the following expression: Xc = (ΔHm/(1 – x) ΔH0) × 100, where ΔH0 is the melting enthalpy (165 J g−1) for a 100% crystalline iPP [33], and x is the filler weight fraction. Xc for neat iPP is 52.1% whereas that for binary composites is higher than 56%. The highest Xc value (63.2%) was registered for the binary nanocomposites with 2.5 wt.% of nanofiller. The TGA traces showed marked differences in thermal stability
dscussed in ths paper) was observed. The XRD patterns of the ternary iPP/CuAg/GNPs nanocomposites (Fig. 7) showed a similar feature to those of the nanocomposites prepared with only one type of nanoparticle. 3.3. Thermal behavior of iPP/CuAg, iPP/GNPs and iPP/CuAg/GNPs nanocomposites The effect of CuAg and GNPs nanoparticles on the thermal behavior of the host iPP matrix was examined by DSC and TGA. The second heating and first cooling DSC traces of the iPP/CuAg and iPP/GNPs nanocomposites are depicted in Figs. 8 and 9, respectively. In order to compare, the heating and cooling traces of neat iPP (control) were also included. Two important differences between the thermograms of nanocomposites and neat iPP are readily observed. First, an increase of the crystallization temperature (Tc) that can be simply associated with 4
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Fig. 4. SEM micrograph of the iPP/CuAg2.5/GNPs2.5 nanocomposite.
iPP/GNPs nanocomposites is higher than that reached by the iPP/CuAg nanocomposites. It is not so simple to determine why one filler is better than the other when there are many filler factors (nature, shape, size, adhesion to the matrix, dispersion, orientation, etc.) affecting the heat transfer across the nanocomposites [34]. Copper and silver nanoparticles as well as graphene nanoplatelets have been reported to be good thermal conductive enhancers [3,20,35], and values here obtained are comparable to those reported in literature for iPP-based nanocomposites, where a homogeneous distribution of particles was presumed [15]. The combination of two different conducting nanoparticles has been already explored to enhance the thermal conductivity of nanocomposites by the so-called “synergistic effect”, although this effect not always occurs [36]. The ternary iPP/CuAg/GNPs nanocomposites showed similar thermal conductivities as compared with that of the binary iPP/CuAg nanocomposites of the same composition (2.5 and 5 wt.%). The thermal conductivity of the ternary iPP/ CuAg1.25/GNPs1.25 nanocomposite was also similar to that of the binary
Fig. 2. Heating and cooling DSC thermograms of CuAg alloy nanoparticles.
between neat iPP and the iPP/CuAg (Fig. 11A), iPP/GNPs (Fig. 11B), and iPP/CuAg/GNPs (Fig. 11C) nanocomposites. The iPP/CuAg nanocomposites showed a higher T2% (∼376 °C) as compared with that of neat iPP (T2% = 345 °C). For iPP/GNPs nanocomposites only iPP/ GnPs5.0 showed a higher T2% (∼375 °C) than that of the pure polymer. Finally, the very high T2% (443 °C) for the two ternary nanocomposites indicates that the combination of the two distinct nanoparticles creates synergy in thermal stability.
3.4. Thermal and electrical conductivities of nanocomposites The thermal conductivity data of the binary (iPP/CuAg and iPP/ GNPs) and ternary (iPP/CuAg/GnPs) nanocomposites are listed in Table 1. It can be readily noticed that the thermal conductivity of the
Fig. 3. SEM micrograph (A) and EDS mapping of copper (red)/silver (green) (B), silver (C) and copper (D) for the iPP/CuAg2.5 nanocomposite. 5
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Fig. 8. First cooling and second heating DSC traces of neat iPP and iPP/CuAg nanocomposites.
Fig. 5. XRD patterns of the CuAg alloy nanoparticles, pure iPP and iPP/CuAg nanocomposites.
Fig. 9. First cooling and second heating DSC traces of neat iPP and iPP/GNPs nanocomposites.
Fig. 6. XRD patterns of GNPs, pure iPP and iPP/GNPs nanocomposites.
Fig. 7. X-ray diffractograms of iPP, iPP/CuAg1.25/GNPs1.25 and iPP/CuAg2.5/ GnPs2.5. Fig. 10. First cooling and second heating DSC traces of neat iPP and iPP/CuAg/ GNPs nanocomposites.
iPP/GNPs2.5 nanocomposite. Only iPP/CuAg2.5/GNPs2.5 showed a much lower thermal conductivity (1.21 W m−1 K−1) as compared with its iPP/GNP5.0 counterpart (1.56 W m−1 K−1). In literature reports it was proposed that, in ternary nanocomposites, a double percolated network is formed creating synergy in heat conduction [15]. Such synergy was not observed in our nanocomposites. In polymer nanocomposites the thermal conductivity is affected by the scattering of energy carriers at the matrix-filler interface [34]. The specific surface area of particles drastically increases when the particle size is scaled down from micro- to nano-scale, meaning that the thermal
conductivity of nanocomposites is largely determined by the interfacial effects. Our results show that the thermal conductivity of polymers was improved by the inclusion of highly conductive CuAg and GNPs nanoparticles, nevertheless, the resulting thermal conductivity is lower than expected due mainly to the thermal resistance of the matrix-filler (M-F) interface. Different highly thermally conductive fillers (metals, ceramics, carbon derivatives, etc.) have been studied over the years, leading to more or less comparable results because all of them follow 6
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Table 1 Thermal characteristics of iPP, iPP/CuAg, iPP/GNPs, and iPP/CuAg/GNPs. The crystalline degree and electrical conductivity are included. Nanocomposite
Tc (°C)
ΔHc (J g−1)
Tm (°C)
ΔHm (J g−1)
Xc (%)
κ (W m−1 K−1)
σ (S m−1)
iPP iPP/CuAg0.25 iPP/CuAg1.0 iPP/CuAg2.5 iPP/CuAg5.0 iPP/GNP0.25 iPP/GNP1.0 iPP/GNP2.5 iPP/GNP5.0 iPP/CuAg1.25/GNP1.25 iPP/CuAg2.5/GNP2.5
112.5 126.2 125.7 127.4 128.4 126.4 127.4 128.4 129.6 127.0 128.5
90.9 94.9 92.4 99.1 93.4 94.7 92.3 98.6 92.6 88.9 91.7
164.3 165.8 165.7 165.5 166.0 165.0 165.5 165.5 166.0 165.1 165.4
85.9 95.3 92.5 101.6 94.8 94.2 91.4 101.6 94.4 92.1 93.7
52.1 57.9 56.6 63.2 60.5 57.2 56.0 63.2 60.2 57.2 59.8
0.21 0.65 0.89 0.99 1.28 0.81 0.99 1.01 1.56 1.08 1.21
1 × 10−15 1 × 10−13 3 × 10−6 3 × 10−4 5 × 10−3 1 × 10−15 1 × 10−13 3 × 10−15 6 × 10−13 5 × 10−15 3 × 10−14
Fig. 11. TGA thermograms of neat iPP and its nanocomposites with CuAg (A), GNPs (B) and CuAg/GNPs (C).
hybrid nanocomposites as compared with the single-particle ones [15], however, there are some reports where the electrical conductivity was lower in ternary than in binary nanocomposites. For instance, Paszkiewicz et al. looking for synergistic effects prepared ternary nanocomposites with low-density polyethylene (LDPE) and two highly thermally and electrically conductive nanoparticles (CNT and GNPs) [36]. Contrary to what was expected, they found a lower electrical conductivity for the ternary LDPE/CNT/GNPs nanocomposites than for binary LDPE/CNT ones despite the fact that CNT and GNP interlocate and form local percolated networks. They reported a σ value of around 100 and 10−13 S m−1 for LDPE/CNT and LDPE/CNT/GNP nanocomposites (both with 3 wt.% of nanoparticles), respectively. Authors suggested that a thin polymer film prevents the direct contact between CNT and GNP, introducing an insulating layer in the tunneling barrier for electrical transport. The thin film is in effect an insulating layer but taking into account the large difference between these two values, one can suppose that the LDPE-GNP interface is acting as a charge carrier trap. It should be emphasized that this effect has been reported to occur mainly at low filler contents (< 1 wt.%) where nanoparticles are far apart and no conduction paths of charge carriers exist in the nanocomposite [9,17]. There are few research works where much higher contents of graphene nanoplatelets were used to make the polymer nanocomposites less conductive than the neat polymer. For instance, Gaska et al. reported a polyethylene/GNPs nanocompostite with 5 wt.% of GNPs that, at low fields (< 20 kV mm−1), showed a σ value of ∼ 7 × 10−17 S m−1 whereas the measured one for neat polyethylene was ∼1 × 10−16 S m−1; both processed by extrusion (using a compression screw) under similar conditions [39]. These authors suggested that the graphene nanoplatelets, all aligned perpendicular to the electric field, act as charge trapping sites, limiting the charge transport through the material. In our ternary iPP/CuAg/GNPs nanocomposites, the GNPs nanoparticles (non-exfoliated) might be far apart and they are probably acting as charge carrier traps rather than charge conduction links of a conducting network. So, our results confirm that the charge carrier trapping effect can occur at relatively high conents of GNPs (up to 5 wt.
similar heat transfer mechanisms [37]. At low loadings, where nanoparticles are far apart, the interfacial resistance has a strong effect on the heat conduction, although the large mean inter-particle distances seems to be more detrimental than the interfacial resistance. At high filler loadings, the filler-filler (F-F) contact gives rise to the percolation threshold, which is a critical property for the electrical conduction but much less relevant for the thermal conduction because the F-F interface can also give rise to thermal resistance [38]. On the other hand, the electrical conductivity of the iPP/GNPs nanocomposites is markedly different to that of the iPP/CuAg at similar filler composition (Table 1). The electric conductivity of the iPP/GNPs nanocomposites (10−15–10−13 S m−1)) showed no real improvement as compared with that of neat iPP (10−15 S m−1). This means that the graphene nanoplatelets are far apart and therefore they do not provide interparticle contacts for the electric conduction. A similar conductivity (σ ∼ 10−14 S m−1) was measured by Imran et al. for binary iPP/GNPs nanocomposites with GNP loadings of 0.2–4.8 wt.% prepared by melt processing (extrusion) [20]. These authors suggested that the percolation threshold was not reached in their nanocomposites below 10 wt.%. We also arrived to the same conclusion because the extrusion process did not provide iPP/GNPs nanocomposites with exfoliated nanoparticles. In contrast, the iPP/CuAg nanocomposites showed increasing σ values with increasing filler contents reaching a σ of 10−6, 10−4, and 10−3 S m−1 for iPP/CuAg1.0, iPP/CuAg2.5, and iPP/CuAg5.0, respectively. From these results we would expect comparable or higher σ values for the ternary iPP/CuAg/GNPs nanocomposites with similar total filler content, however, for such nanocomposites no improvement in this property was observed. The measured low values (5 × 10−15 S m−1 for iPP/CuAg1.25/GNPs1.25 and 3 × 10−14 S m−1 for iPP/CuAg2.5/ GNPs2.5) suggest that no electron conduction paths (or networks) were formed in the ternary nanocomposites despite the relatively high content of both CuAg and GNPs (2.5 and 5 wt.% of total filler). The combination of two nanoparticles for improving the electrical conductivity of polymers has been already explored by other groups. Intermediate or improved electrical conductivity is usually found in two-particle or 7
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%) provided that they do not form conduction paths. In this way, the ternary iPP/CuAg/GNPs nanocomposites became more insulating than the binary iPP/CuAg ones. The charge carrier trap effect is of course undesirable for the development of electric conducting nanocomposites, but it results advantageous for the preparation of high thermal/ low electrically conductive materials, which are suitable for modern electronic devices for which heat releasing is a key factor.
Polym. Sci. 59 (2016) 41–85. [7] D.A. Pomogailo, L.A. Petrova, D.A. Pomogailo, E.I. Knerelman, A.M. Kolesnikova, S.V. Barinov, K.A. Kydralieva, G.I. Dzhardimalieva, Nanosci. Technol. 8 (2017) 27–40. [8] N.R. Kim, K. Shin, I. Jung, M. Shim, H.M. Lee, J. Phys. Chem. C 118 (2014) 264324–264331. [9] Z. Li, B. Du, C. Han, H. Xu, Sci. Rep. 7 (2017) 4015, https://doi.org/10.1038/ s41598-017-04196-5. [10] R.J. Young, M. Liu, I.A. Kinloch, S. Li, X. Zhao, C. Valles, D.G. Papageorgiou, Compos. Sci. Technol. 154 (2018) 110–116. [11] S. Park, R.S. Ruoff, Nat. Nanotechnol. 4 (2009) 217–224. [12] D.R. Lide, Handbook of Chemistry and Physics, 84th ed., CRC Press, New York, 2003. [13] S. Chinkanjanarot, J.M. Tomasi, J.A. King, J.M. Odegard, Carbon 140 (2018) 653–663. [14] A. Cruz-Aguilar, D. Navarro-Rodríguez, O. Pérez-Camacho, S. Fernández-Tavizón, C.A. Gallardo-Vega, M. García-Zamora, E. Díaz Barriga-Castro, Mater. Today Commun. 16 (2018) 232–241. [15] K. Wu, Y. Xue, W. Yang, S. Chai, F. Chen, Q. Fu, Compos. Sci. Technol. 130 (2016) 28–35. [16] Z. Li, J. Su, B. Du, Z. Hou, C. Han, Nanomaterials 8 (2018) 956. [17] Z. Jing, C. Li, H. Zhao, G. Zhang, B. Han, Materials 9 (2016) 680. [18] H.A. Maddah, Am. J. Polym. Sci. 6 (2016) 1–11. [19] M.H. Vakili, H. Ebadi-Dehaghani, M. Haghshenas-Fard, J. Macromol. Sci. Part B: Phys. 50 (2011) 1637–1645. [20] K.A. Imran, J. Lou, K.N. Shivakumar, J. Appl. Polym. Sci. 135 (9) (2018) 11. Article ID 45833. [21] L. Chen, H.-F. Hu, S.-J. He, Y.-H. Du, N.-J. Yu, X.-Z. Du, J. Lin, S. Nazarenko, PLoS One 12 (2017) e0170523. [22] R. Haggenmueller, C. Guthy, J.R. Lukes, J.E. Fisher, K.I. Winey, Macromolecules 40 (2007) 2417–4121. [23] T. Weime, L. Duan, N. Mys, L. Cardon, D.R. D’hooge, Polymers 11 (2019) 87, https://doi.org/10.3390/polym11010087. [24] J. Brandrup, E.H. Immergut, E.A. Grulke, fourth ed., Polymer Handbook 1 WileyInterscience, New Jersey, 1999. [25] M.K. Sharma, R.D. Buchner, W.J. Scharmach, V. Papavassiliou, M.T. Swihart, Aerosol Sci. Technol. 47 (2013) 858–866. [26] A. Sakthisabarimoorthi, M. Jose, S.A. Martin Briti Dhas, J. Mater. Sci.: Mater. Electron. 28 (2017) 4545–4552. [27] F. Calvo, Phys. Chem. Chem. Phys. 17 (2015) 27922–27939. [28] G. Radnóczi, E. Bokányi, Z. Erdélyi, F. Misják, Acta Mater. 123 (2017) 82–89. [29] J. Sopousek, J. Pinkas, P. Broz, J. Bursik, V. Vykoukal, D. Skoda, A. Styskalik, O. Zobac, J. Vrest’al, A. Hrdlicka, J. Simbera, J. Nanomater. 2014 (2014) 13. Article ID 638964. [30] S. Delsante, G. Borzone, R. Novakovic, D. Piazza, G. Pigozzi, J. Janczak-Rusch, M. Pillonid, G. Ennas, Phys. Chem. Chem. Phys. 17 (2015) 28387–28393. [31] R. Krache, R. Benavente, J.M. López-Majada, J.M. Pereña, M.L. Cerrada, E. Pérez, Macromolecules 40 (2007) 6871–6878. [32] M. El Achaby, F.E. Arrakhiz, S. Vaudreuil, A. el Kacem Qaiss, M. Bousmina, O. FassiFehri, Polym. Compos. 33 (2012) 733–744. [33] J.E. Mark, Polymer Data Handbook, Oxford University Press, 1999. [34] J. Ordoñez-Miranda, R. Yang, J.J. Alvarado-Gil, Thermal conductivity of particulate nanocomposites, in: X. Wang, Z.M. Wang (Eds.), Nanoscale Thermoelectrics, Springer, New York, 2014, pp. 93–139. [35] L. Rivière, A. Lonjon, E. Dantras, C. Lacabanne, P. Olivier, N. Rocher Gleizes, Eur. Polym. J. 85 (2016) 115–125. [36] S. Paszkiewicz, A. Szymczyk, D. Pawlikowska, J. Subocz, M. Zenker, R. Masztak, Nanomaterials 8 (2018) 264, https://doi.org/10.3390/nano8040264. [37] N. Burger, A. Laachachi, M. Ferriol, M. Lutz, V. Toniazzo, D. Ruch, Prog. Polym. Sci. 61 (2016) 1–28. [38] Y. Su, J.J. Li, G.J. Weng, Carbon 137 (2018) 222–233. [39] K. Gaska, X. Xu, S. Gubanski, R. Kádár, Polymers 9 (2017) 11, https://doi.org/10. 3390/polym9010011.
4. Conclusion Binary and ternary nanocomposites based on iPP, silver-rich shell CuAg alloy nanoparticles and/or graphene nanoplatelets were prepared by melt processing, and their morphological and thermal/electrical conductivity properties were determined. SEM and TEM analyses showed a good dispersion of nanoparticles despite the observation of microscale CuAg agglomerates. In both binary and ternary nanocomposites, the Tc and crystalline degree increased as the content of nanoparticles increased, suggesting a nucleating effect of the nanoparticles. The thermal stability (T2% = 443 °C) of the ternary nanocomposites was superior to that of the binary counterparts (T2% ∼ 375 °C) of similar filler loading. The attained thermal conductivity of 1.56 W m−1 K−1 is comparable to that reported for iPP/ filler nanocomposites, although the ternary nanocomposites, herein reported, are electrical insulators (10−15–10−14 S m−1); the polymerGNP interface is likely to act as a charge carrier trap. The charge carrier trapping effect appears not yet to have been reported in ternary nanocomposites. Theoretical and further experimental analysis of this effect may perhaps result a new strategy to prepare light-weight thermal conducting / electrical insulating materials. Acknowledgements Financial support from CONACYT Mexico(Project: 281164: Consolidation of the National Laboratory on Graphene Materials) is gratefully appreciated. Authors also appreciate the technical assistance of Enrique Díaz Barriga-Castro (TEM), Jesús Cepeda (SEM), Guadalupe Méndez (DSC), Gilberto Hurtado (electric conductivity measurements), Joelis Rodríguez (XRD), Uriel Sierra (graphene analysis) and Alfonso Mercado (Lab. facilities) from CIQA. References [1] M.S. Nurul, M. Mariatti, J. Thermoplast. Compos. Mater. 26 (2011) 627–639. [2] S. Mahdi Hamidinejad, R.K.M. Chu, B. Zhao, C.B. Park, T. Filleter, ACS Appl. Mater. Interfaces 10 (2018) 1225–1236. [3] D. Zhu, W. Yu, H. Du, L. Chen, Y. Li, H. Xie, J. Nanomater. 2016 (2016) 6. Article ID 3089716. [4] A. Li, C. Zhang, Y.F. Zhang, Polymers 9 (2017) 437. [5] K. Yang, M. Gu, Y. Guo, X. Pan, G. Mu, Carbon 47 (2009) 1723–1737. [6] H. Chen, V.V. Ginzburg, J. Yang, Y. Yang, W. Liu, Y. Huang, L. Du, B. Chen, Prog.
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