Microstructure and properties of polypropylene composites filled with silver and carbon nanotube nanoparticles prepared by melt-compounding

Materials Science and Engineering B 142 (2007) 55–61

Microstructure and properties of polypropylene composites filled with silver and carbon nanotube nanoparticles prepared by melt-compounding G.D. Liang, S.P. Bao, S.C. Tjong ∗ Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China Received 23 April 2007; accepted 25 June 2007

Abstract Polypropylene reinforced with silver and multi-wall carbon nanotubes (MWNTs) particles were prepared by melt-compounding process. The morphology of PP nanocomposites was characterized by optical microscopy, scanning electron microscopy and transmission electron microscopy. Moreover, the electrical properties of binary PP/Ag and ternary PP/Ag/MWNT nanocomposites were examined. Microstructural examination revealed that the Ag nanoparticles acted as nucleation sites for PP, and induced the formation of ␤-form PP crystals. Therefore, PP transcrystals were formed adjacent to individual Ag particles and clusters. These insulating PP transcrytals blocked the conductive pathways in PP/Ag nanocomposites, thereby degrading their electrical properties. Annealing treatment of PP/Ag nanocomposites led to the formation of voids and microcracks in PP matrix as a result of structural transformation of ␤- to ␣-form PP. Such transformation also deteriorated the electrical properties of PP/Ag nanocomposites. The addition of a small amount of MWNT promoted significantly the electrical properties of PP/Ag/MWNT hybrid composites. © 2007 Elsevier B.V. All rights reserved. Keywords: Electrical measurements; Polymers; Silver; Carbon nanotube; Percolation phenomena

1. Introduction Conductive polymer composites are well recognized to exhibit unique characteristics such as high flexibility of polymer matrix, ease of fabrication, low cost and light weight. The composites are generally reinforced with fillers of micrometer sizes such as carbon blacks and metallic particles [1–3]. The electrical and mechanical properties of the microcomposites depend greatly on the shape, content and dispersion of filler. In addition, the structure of polymer matrix also plays an important role on the electrical and mechanical properties of the microcomposites. Electrical conductivity of these materials rises sharply when the filler content reaches a critical concentration, commonly known as percolation threshold. In this case, the average distance between the fillers becomes smaller, thereby favoring formation of conductive paths within the composites. Polymer microcomposites generally require a large filler content to achieve percolation threshold concentration. The additions of

∗

Corresponding author. E-mail address: [email protected] (S.C. Tjong).

0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.06.028

large filler content to polymers lead to poor processability and inferior mechanical performances. Recently, nanoparticle-filled polymers have attracted considerable interest because they exhibit enhanced electrical and mechanical properties by adding nanofillers at low loading levels [4–6]. Silver exhibits the largest electrical and thermal conductivities among all the metals [7]. Silver nanoparticles have found wide applications in catalysis, antimicrobials, conductive inks and electronic devices [8–10]. Therefore, functional nanocomposites with desired properties can be tailored by incorporating Ag nanoparticles into the polymers [11,12]. Nanoparticles with high aspect ratios such as carbon naotubes (CNTs) possess remarkable structural, electronic and thermal properties. They are excellent nanofillers for polymers to achieve these properties. Depending on the diameter dimensions, CNTs can be divided into single-walled and multi-walled. Owing to rapid advancement in materials synthesis, chemical vapor deposited multi-walled nanotubes (MWNTs) are available commercially in large quantities at relatively low cost. Generally, Ag nanoparticles and MWNTs tend to distribute non-uniformly in polymers as agglomerated clusters due to their large surface areas and/or aspect ratios.

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Isotactic polypropylene (PP) is important commercial plastics widely used to produce household goods and automotive parts due to its versatile properties such as high chemical resistance, low density, shape and dimensional stability. The application of PP in industrial sectors can be further expanded once its electrical and mechanical performances have been highly upgraded. These can be achieved by reinforcing PP with inorganic nanoparticles. PP can crystallize in several structures such as monoclinic ␣-form, trigonal ␤-form and triclinic ␥form [13–15]. Among these crystalline structures, ␣-form is dominant, but the ␤-form can be induced under appropriate conditions of heating [16], applied strain [17] and presence of special nucleators [14,15]. The ␤-form spherulites are composed of radial arrays of parallel-stacked lamellae with preferred radial growth direction of lamellae along certain crystallographic direction [13–15]. Numerous studies have reported that this special arrangement of macromolecules favors an enhancement of the impact strength for PP [14,15]. However, the electrical properties of the ␤-form PP are rarely reported in literature. In a previous work, we have reported that the Ag nanoparticle additions promote formation of the ␤-PP, thereby enhancing both tensile stress and impact strength of PP considerably [18]. The work described herein concerns the effect of Ag nanoparticle and MWNT additions on electrical conductivity and dielectric behavior of PP nanocomposites. 2. Experimental 2.1. Sample preparation The PP/Ag and ternary PP/Ag/MWNT nanocomposites were prepared by melt-blending PP pellets with Ag nanoparticles or PP with mixed Ag and MWNT powders in a Brabender mixer, respectively. Isotactic polypropylene with a molecular weight Mw = 250, 000 and a melt flow index of 12 g/10 min was obtained commercially from HMC Polymers Co. Ltd., Silver and MWNT powders were supplied by Nanostructured and Amorphous Materials Inc. Silver particles of nanometer sizes were used to form PP nanocomposites. The average diameter of Ag nanoparticles was determined to be ∼60 nm using TEM micrograph and SemAfore software (Fig. 1(a) and (b)). Fig. 2 is a TEM micrograph showing the morphology of as-received MWNTs. Apparently, MWNTs with length up to micrometer scale tended to entangle each other. To disperse the fillers into the polymer matrix uniformly and to avoid thermal degradation of polymer matrices, the mixing time was set at 200 ◦ C for 15 min. The blended mixtures were then hot pressed at 230 ◦ C under 10 MPa into plates of 1 mm thickness. Disk samples of 12 mm diameter were punched from these plates. 2.2. Microstructural examination The morphology of PP/MWNT nanocomposites was observed using an optical microscope (Olympus BH2-UMA) equipped with a camera (Olympus DP 11). The samples were sandwiched between two microscopic glass slides and then

Fig. 1. (a) TEM micrograph and (b) size distribution of Ag nanoparticles.

mounted on a hot stage. They were held at 230 ◦ C for 5 min to eliminate thermal history and pressed into thin film of ∼0.1 mm thickness and quickly cooled to room temperature. The morphology of the samples was also examined with scanning electron microscopy (SEM, JEOL JSM model 820). The samples for SEM examination were fractured in liquid nitrogen. They were coated with a thin layer of gold prior to SEM observation. The dispersion of Ag particle in PP matrix was also investigated using a transmission electron microscope (TEM; Philips CM20). Ultrathin specimens (∼70 nm) were cut from the middle section of compression-molded plaque using a Reichert Ultracut microtome under cryogenic conditions. The films were then retrieved onto the Cu grids. The structure of the PP/Ag nanocomposites was investigated using the X-ray diffraction technique. XRD measurements were performed with a Philip X’Pert diffractometer equipped with Ni-filtered Cu K␣ radiation, having a wavelength of 0.154 nm.

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Fig. 3. Optical micrograph of as-prepared PP/3.0 wt% Ag nanocomposite.

Fig. 2. TEM micrograph of MWNTs.

The diffractometer was scanned in 2θ range of 5–40◦ and the scanning rate used was 1.2◦ min−1 . The melting behavior of PP/Ag nanocomposites was investigated with a DSC Instrument (model TA 2910) under a nitrogen atmosphere. The PP/Ag nanocomposites (about 5 mg) were sealed in aluminum pans. The samples were heated to 230 ◦ C at a rate of 10 ◦ C min−1 . The change of heat flow versus time was recorded. 2.3. Electrical properties Samples for the dielectric and resistivity measurements were coated with silver paint prior to the tests. Two metallic electrodes were then connected to the samples via silver wires. The dielectric constant and resistivity of samples were measured by employing an impedance analyzer (Agilent model 4294) in the frequency range of 102 to 107 Hz at room temperature. 3. Results and discussion

note that pronounced transcrystalline morphologies are formed adjacent to the Ag nanoparticles/clusters. This implies that the Ag nanoparticles act as nucleation sites for PP molecules. Thus, PP molecular segments are anchored to the Ag nanoparticles having large surface areas during crystallization. Fig. 6 shows the typical XRD profiles of pristine PP, as-prepared and annealed PP/0.1 wt% Ag nanocomposites. Annealing was performed at 90 ◦ C for 80 h. The XRD profile of pristine PP shows five distinct reflection peaks located at 2θ = 14.1◦ , 16.9◦ , 18.5◦ , 21.3◦ and 21.8◦ , associated with the diffraction of ␣-form PP. A strong reflection peak appears at 2θ = 16.1◦ , ascribed to the (3 0 0) crystalline plane of ␤-form PP [14,15]. Thus the Ag nanoparticle additions promote the formation of ␤-form PP crystals. This ␤-form PP diffraction peak disappears in the pattern of annealed specimen, indicating that the ␤-form PP is unstable and transforms into ␣-form. In general, the ␤–␣ form transition occurs more readily in the ␤form PP specimens during tensile or compressive deformation [19]. Fig. 7 shows the SEM micrograph of cryogenic fractured surface of annealed PP/3.0 wt% Ag nanocomposite. Some voids and cracks are observed in PP matrix. This is attributed to the trans-

3.1. Microstructure of PP/Ag composites Fig. 3 is an optical micrograph showing the microstructure of PP/3.0 wt% Ag nanocomposite. It can be seen that Ag nanoparticles and their agglomerated clusters are uniformly dispersed in PP matrix. Agglomerated Ag clusters are resulted from the high surface areas of Ag nanoparticles. Fig. 4 shows the SEM micrograph of cryogenic fractured surface of the as-prepared PP/3.0 wt% Ag nanocomposite. The Ag nanoparticles are recognized as bright spots in the micrograph. Moreover, clusters (indicated by the arrows) resulted from agglomeration of Ag nanoparticles can be readily seen. Fig. 5 shows the polarizing optical micrograph of PP/3.0 wt% Ag nanocomposite. Sheaf-like spherulites associated with the formation of ␤-form PP crystals are observed. It is interesting to

Fig. 4. SEM micrograph of as-prepared PP/3.0 wt% Ag nanocomposite.

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Fig. 7. SEM micrograph of annealed PP/3.0 wt% Ag nanocomposite.

Fig. 5. (a) Polarizing and (b) conventional optical micrographs of PP/3.0 wt% Ag nanocomposite taken from the same location of specimen. The arrows denote the Ag nanoparticle clusters.

Fig. 8 shows the heating traces of the as-prepared and annealed PP/0.1 wt% Ag nanocomposite specimens at a rate of 10 ◦ C min−1 . The heating trace of the as-prepared PP/0.1 wt% Ag nanocomposite shows the presence of three strong endothermic peaks at 146.2, 153.2 and 164.2 ◦ C. The first two peaks are associated with the melting of the original ␤1 -phase and subsequent re-crystallization to produce a more stable structure (␤2 -phase) during heating. The last peak at 164.2 ◦ C is related to the melting of the ␣-form PP crystals [14,17]. In contrast, only single endothermic peak located at 164 ◦ C is observed for the annealed specimen. This further confirms that the ␤-form PP crystals transform into ␣-form during annealing. 3.2. Electrical properties of PP/Ag composites

formation of ␤-form PP crystals into ␣-form via re-arrangement of macromolecules during annealing. Since the density of the ␤form PP crystals is lower than that of ␣-form, voids and cracks are formed during annealing treatment.

Fig. 9(a) and (b) shows the variations of dielectric constant and conductivity with filler content for the as-prepared and annealed PP/Ag nanocomposites, respectively. For the

Fig. 6. XRD profiles of neat PP, as-prepared and annealed PP/0.1 wt% Ag nanocomposite specimens. Annealing was performed at 90 ◦ C for 80 h.

Fig. 8. DSC heating traces of the as-prepared and annealed PP/0.1 wt% Ag nanocomposites.

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Fig. 10. TEM micrograph of PP/60 wt% Ag nanocomposite.

conductivity. In crystalline polymers composites, annealing treatment usually produces more perfect crystalline structure of polymer matrix. As a result, the dielectric constant and conductivity of crystalline polymer composites are usually enhanced due to the increase of filler concentration in the amorphous phase of polymer after annealing [20]. The reduced dielectric constant and conductivity for the annealed samples may result from the holes and microcracks formed during annealing as a result of transformation of ␤-form into ␣-form PP crystals. Fig. 9. Variations of (a) dielectric constant and (b) conductivity with filler content for as-prepared and annealed PP/Ag nanocomposites.

as-prepared PP/Ag nanocomposites, the dielectric constant is quite small and changes little with increasing filler content up to 20 wt%. When the Ag content exceeds 30 wt%, the dielectric constant increases with increasing filler content. A similar dielectric behavior is observed for annealed nanocomposites. From Fig. 9(b), the electrical conductivity of all samples increases gradually with increasing filler content. No abrupt increase in either dielectric constant or electrical conductivity, known as percolation phenomenon, is observed for the specimens investigated, indicating that conductive Ag pathway is not formed in these specimens. Fig. 10 shows the TEM micrograph of PP/60 wt% Ag nanocomposite. The dark spots are recognized as Ag particles. It is apparent that the Ag nanoparticles are still distributed independently in PP matrix even at high Ag loading of 60 wt%. Accordingly, there exists no formation of the nanofiller conductive network. These Ag nanoparticles dispersed independently in PP matrix are separated by insulating PP transcrystals as shown in Fig. 5. As a result, the PP/Ag nanocomposites exhibit poor electrical properties even at high Ag loading. Comparing to the as-prepared PP/Ag nanocomposites, annealed specimens show smaller dielectric constant and

3.3. Electrical properties of PP/Ag/MWNT composites Owing to the lack of conductive paths, PP/Ag nanocomposites exhibit poor electrical properties. In order to improve the electrical properties of PP/Ag nanocomposites, MWNTs having large aspect ratio (shown in Fig. 2) are compounded with PP and nano-Ag to form hybrid nanocomposites. Fig. 11(a) and (b) shows the variations of dielectric constant and conductivity of PP/Ag/MWNT composites with MWNT content, respectively. The dielectric constant and conductivity of PP/Ag/MWNT composites increase gradually with MWNT content at low filler loading. As the MWNT content reaches 2 wt%, the dielectric constant increases sharply by two orders of magnitude, and the conductivity by more than four orders of magnitude, exhibiting the percolation phenomenon. This reveals that MWNTs with large aspect ratio favor the formation of conductive pathways in the PP/Ag/MWNT hybrid nanocomposites. Fig. 12 shows TEM micrograph of the PP/10 wt% Ag/2.0 wt% MWNT nanocomposite. It is apparent that the Ag particles individually dispersed are linked together with long MWNTs. The nanotubes act as bridges for individual Ag particles, yielding high electrical conductivity for the ultimate composites [21]. Fig. 13 depicts the possible structure of the PP/Ag/MWNT hybrid composites. For the purpose of comparison, the variations of dielectric constant and conductivity of binary PP/MWNT composites are

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Fig. 13. Schematic illustration of PP/Ag/MWNT composites. The gray spheres denote Ag particles and solid lines represent MWNTs.

also presented in Fig. 11. Comparing to ternary PP/Ag/MWNT hybrid composites, PP/MWNT nanocomposites exhibit larger percolation threshold (∼3.0 wt%). This clearly indicates that the Ag particles facilitate the formation of conductive pathways in the presence of MWNT. It is likely that Ag particles act as anchors among MWNTs clusters and facilitate the electron transfer among MWNT, as shown in Fig. 13. 4. Conclusions Fig. 11. Variations of (a) dielectric constant and (b) conductivity with MWNT content for PP/MWNT and PP/Ag/MWNT nanocomposites. The nano-Ag content is fixed at 10 wt% for PP/Ag/MWNT hybrid nanocomposites.

1. Microstructural examinations show that silver nanoparticles act as nucleation sites for crystallization of PP molecules and induce the formation of ␤-form PP. As a result, PP transcrystals are formed adjacent to Ag particle clusters. These insulating PP transcrytals inhibit the formation of conductive pathway in PP/Ag nanocomposites, thereby degrading their electrical properties. 2. Voids and microcracks are formed within the PP matrix of PP/Ag nanocomposites during annealing due to the transformation of ␤-form to ␣-form PP. Such structural transformation also weakens the electrical properties of PP/Ag nanocomposites. 3. The addition of a small amount of MWNTs promotes significantly the electrical properties of ternary PP/Ag/MWNT nanocomposites. The nanotubes act as bridges among Ag nanoparticles, thus facilitating the formation of conductive paths in the PP/Ag/MWNT hybrid composites. Acknowledgement

Fig. 12. TEM micrograph of PP/10 wt% Ag/2.0 wt% MWNT composite.

The work described in this paper was fully supported by a CERG grant from Research Grants Council, Hong Kong Special Administrative Region, China (project no. CityU 1123/04E).

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