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Preparation, microstructure and properties of polyethylene aluminum nanocomposite dielectrics
Composites Science and Technology 68 (2008) 2134–2140
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Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Preparation, microstructure and properties of polyethylene aluminum nanocomposite dielectrics Xingyi Huang a,b, Pingkai Jiang a,b,*, Chonung Kim a,b,c, Qingquan Ke a,b, Genlin Wang a,b a
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Shanghai Key Laboratory of Electric Insulation and Thermal Aging, Shanghai 200240, China c Electrical Engineering Faculty, Kim Chaek University of Technology, Pyongyang, Democratic People’s Republic of Korea b
a r t i c l e
i n f o
Article history: Received 8 April 2007 Received in revised form 7 March 2008 Accepted 18 March 2008 Available online 25 March 2008 Keywords: A. Polymer–matrix composites (PMCs) B. Electrical properties B. Mechanical properties B. Microstructure
a b s t r a c t Linear low density polyethylene (PE) aluminum (Al) nanocomposites were prepared and their morphology and properties were investigated. The particular attention was given to the structure/property relationship of the nanocomposites and an equivalent electric circuit model was proposed to interpret the dependences of the dielectric behaviors of the PE/Al nanocomposites on the nanoparticle concentration and the measuring frequency. The increase of the dielectric constant shows the effectiveness of Al nanoparticles in altering the intrinsic dielectric properties of PE. The microstructure transition revealed by morphological investigations has been also verified at the same composition as that from the rheological data. And also, it has been found that the nanocomposites at high filler concentrations still keep good mechanical properties and breakdown strength, which is very important for practical applications. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Polyethylene is known to be one of the most widely used polymeric insulating materials. The high dielectric strength, very low electrical conductivity and remarkable mechanical properties allow it to be used as an outstanding insulation in cable applications [1]. However, its low dielectric constant makes it impossible for PE to be used for electrical stress control in cable terminations. At cable terminations, the electric field is concentrated at the cutback point, which may be great enough to cause the dielectric breakdown of the air, resulting in the surface and internal discharge of the cable insulation, which will ultimately destroy the cable insulation, causing premature failure. The use of high-dielectric-constant materials has been strongly recommended in order to reduce the high voltage stress below the levels at which the dielectric breakdown or partial discharge of air would occur in the insulation. The common approach to increase the dielectric constant of a polymer is known to adopt microscale ceramics of high dielectric constant, carbon fiber or nanotubes as fillers [2], e.g., BaTiO3 [3] and muti-walled carbon nanotube (MWCT) [4]. For the polymer/ ceramic composites, high filler loadings are usually needed in order to realize the values of dielectric constant high enough for
* Corresponding author. Address: School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. Tel.: +86 21 54740787; fax: +86 21 54746520. E-mail address: [email protected] (P. Jiang). 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.03.009
the purposed industrial applications. However, high filler loading is sure to deteriorate the mechanical and other electrical properties. Furthermore, the dielectric constant of BaTiO3 is strongly dependent on grain size and temperature [5]. Carbon fiber/nanotubes with the larger aspect ratios and the high conductivities can produce very low percolation threshold and high dielectric constant [6], while a very low value of percolation threshold gives a big challenge to obtain reproducible products for practical applications. Some novel metal particles, e.g., silver particles can be used to increase the dielectric constant of polymers [7], whereas their high cost is a main disadvantage for practical applications. It should be noted that, although several polymer/metal nanocomposites with very high dielectric constant have been prepared, few paper have dealt with the mechanical property, processing performance and dielectric strength of the nanocomposites up to now, which are regarded to be of a great importance from the viewpoint of the practical applications. In this paper, we report the preparation of PE/Al nanocomposites by using the solution compounding method and the investigations on the microstructure of the nanocomposites. Some characterizations for the prepared materials and their processing, including mechanical properties, dielectric and rheological characteristics are also investigated. Aluminum (Al) is selected because of its self-passivation characteristic. The self-passivated oxide layer forms a thin insulating boundary around the surface of its metallic core, allowing the polyethylene/Al composites to have a high dielectric constant [8].
X. Huang et al. / Composites Science and Technology 68 (2008) 2134–2140
2. Experimental part 2.1. Materials The polyethylene used is linear low density polyethylene from ExxonMobil in Saudi Arabia. It has a Melt Flow Index (MFI) of 3.6 g/min by ASTM D1238 and a density of 0.924 g/cm3 by ASTM D4703/D1505. The Al nanoparticles (purchased from Hongwu nanomaterials Co. China) with an average diameter of 102 nm according to ISO/TS 13762 were used as received.
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than 5% was used, which was in the linear viscoelastic regime for all samples. The tensile tests were performed according to ASTM D 638-2003 by using an Instron series IX 4465 materials tester, with a grip separation rate of 250 mm/min, with 1 mm-thick dumbbell type specimens. DMA experiments were performed using the Netzsch 242C instrument. The deformation applied was the cantilever mode, and the mean dimensions of sample plaques were 40 mm 10 mm 0.92 mm. The temperature varied from 150 °C up to the melting temperature (about 120 °C). The frequency was 10 Hz and the heating rate 5 °C/min. The amplitude of deformation is 0.03%.
2.2. Preparation of PE/Al nanocomposites 3. Results and discussion Al nanoparticles were added to analytically pure trichloromethane (CHCl3) to give a concentration of 5 mg of Al/ml of CHCl3, and dispersed in a sonication bath (40 kHz, 250 W) for 2 h on the basis of the desired weight fraction of Al in the final composites at room temperature. At the same time, an appropriated quantity of PE was dissolved in chemically pure para-xylene (pX) to give a concentration of 10 mg of PE/ml of pX at 368 K. Then, the suspension of Al/CHCl3 was added to a solution of PE/pX, and the resulting mixture was stirred with an electric mixer (agitating speed is about 2500 r/min) at 368 K for 2 h. Afterward, the mixture were heated at 368 K for 6 h so as to evaporate the solvent. The resulting nanocomposites were dried in a vacuum oven at about 393 k for 24 h to evaporate the solvent completely. In order to make the results of the neat and filled polymer samples comparable with each other, the neat PE was treated with the same process as the filled polymer samples were done. Thin films with thickness of around 250 ± 10 lm were used for dielectric breakdown strength measurements. For the sake of convenience, the composites were denoted using the following notation: PE-particle weight, thus PE-2.0 indicates the composites with 2.0 wt.% Al nanoparticles. 2.3. Characterization The morphology and dispersion of the Al nanoparticles in the PE were investigated using JEOL JEM-4701 field emission scanning electron microscopy (FE-SEM). Cylindrical samples were broken in the liquid nitrogen and sputtered with thin layers of gold to avoid accumulation of charges. The high resolution transmission electron microscopy (HRTEM) investigations were performed on a JEOL 2100F transmission electron microscope, which was operated at an accelerating voltage of 200 kV. The sample was prepared by placing a drop of a dilute dimethylformamide-dispersion of Al nanoparticles on the holey-carbon coated copper TEM grid. Dielectric spectroscopy measurements were performed by using a broadband dielectric spectrometer (Novocontrol GmbH, Germany). Sample thickness is about 1 mm in all cases. Dielectric constant (e0 ) was recorded in the discrete frequencies ranged from 0.1 Hz to 1 MHz. Dielectric breakdown testing was performed on an AHDZ-10/100 alternating-current dielectric strength tester (Shanghai Lanpotronics Corp., Shanghai, China) according to ASTM D 1492004. The specimens were immersed in the pure silicon oil, between two 10-mm-diameter copper ball electrodes opposing to each other. The lower electrode was connected to earth and an increasing AC voltage with a rate of 2 KV/s was applied to the upper electrode until the sample failed. Twenty breakdown tests were performed on each specimen. And 63.2% electrical breakdown characteristics were calculated from the experimentally obtained data by using Weibull statistical method [9]. A rotational rheometer, Gemini 200HR (Bohlin instruments, UK) with a parallel-plate geometry (diameter of 25 mm), was used for rheological measurements. Small amplitude oscillatory shear was performed in the frequency range 0.01–100 rad/s at 160 °C. A strain of less
3.1. Morphology In the case of nanoparticle-filled polymer composites, the nanofillers inside the polymer matrix generally exist in the two states: single particles and particle agglomerates (clusters). Metal-filled insulation systems can be simulated by means of a large number of equivalent elementary capacitors where the single particles or agglomerates (clusters) can act as electrodes when an external electric field is applied on the composites [10]. Since the above-mentioned elementary capacitors are considered be connected both in parallel and in series with one another, the dielectric constant of PE/Al nanocomposites can be affected by the following factors: the number of the elementary capacitors and the thickness of the intercapacitor layer. Furthermore, all of these two factors are directly related to the dispersion state of the fillers. Take into account this consideration, it is necessary to investigate the dispersion of the nanoparticles within the polyethylene matrix. Fig. 1a–f shows the FE-SEM micrographs of the cross section of the nanocomposites prepared by solution compounding. From the FE-SEM images, it can be seen that the dispersion in nanoscale is obtained when the content of Al nanoparticles is lower than 4 wt.%: the individually dispersed particles, even very small nanoparticles can be found in Fig. 1b. With further loading with Al nanoparticles, particle clusters begin to appear from place to place and their magnitudes are found to further increase. Starting at 12 wt.%, nanoparticle agglomerates larger than microscale can be seen (Fig. 1d), and the microstructure transition is observed to occur up to 16 wt.% (Fig. 1e). The transition mentioned here is defined as such a process that the unconnected topological agglomerates continue to grow until the polymer–nanoparticle agglomerates networks (a state that the nanoparticle clusters have formed and contact between one another) are formed (Fig. 1f). The dispersion of nanofillers into a polymer matrix is determined by the kinetics of composite preparation. The attractive van der Waals forces between nanoparticles are relatively weak because of the long particle-to-particle distance at low nanoparticle loading, the agglomerated nanoparticles are thought to be easily broken up by the shear force provided by the mixing device, which may be the main reason why the nanoscale dispersion is obtained for the nanocomposites with nanoparticle loading lower than 4 wt.%. With further increasing the Al nanoparticle loading, the particle-to-particle distance decreases exponentially and the more attractive van der Waals forces begin to create agglomeration of the particles from place to place. 3.2. Dielectric characteristics Fig. 2 presents the behaviors of the frequency dependences of the real part of the dielectric constant (e0 ) for different Al nanoparticle contents. It can be seen that the nanocomposites with the Al nanoparticle content lower than 12.0 wt.% are observed to have lit-
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Fig. 1. FE-SEM micrographs of (a) PE, (b) PE-4.0, (c) PE-8.0, (d) PE-12.0, (e) PE-16.0 and (f) PE-24.0.
Fig. 2. Frequency dependences of e0 for the PE nanocomposites.
tle dependences on frequency, resulting in nearly the same value of e0 in the total range of the measuring frequencies. Starting at 16 wt.%, the e0 values of the nanocomposites have significantly different dependences on frequency: a much significant decrease of
the dielectric constant can be observed when the frequency increases in the extra low frequency range, and the addition of Al nanoparticles results in much higher e0 value. For example, at 0.1 Hz, the e0 value increases from 2.35 for pure PE to 24.14 for PE-16.0. Such an increase of the e0 value shows the effectiveness of Al nanoparticles in altering the intrinsic dielectric properties of the base polymers, giving us a hint to overcome one of the main disadvantages of PE from the viewpoint of its application, e.g., low capability of charge storage. This increase of dielectric constant observed in the composites with higher loading levels is caused by the interfacial polarization, which is understood to occur in the dielectrics composed of two or more materials with significantly different electrical properties. The above-mentioned interfacial polarization can cause the formation of electric charge at the interfaces between different materials when voltage is applied to the materials [11]. At 32 wt.%, however, the dielectric constant reaches the maximum point at the low frequency range and begins to decrease as the filler fraction increases. It is also interesting to pay attention to that the dielectric constant transition at 24 wt.% only takes place at extra-low frequency range but not at the higher frequency range. As shown in Fig. 2, a moderate increase of the dielectric constant in the frequency range of 100 Hz to 1 MHz is observed as the Al weight fraction increases up to 24 wt.%, and then any significant changes of the dielectric permittivity can not be found over 24 wt.%. These features might be attributed to that
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there are different mechanisms for dielectric polarization of the PE/ Al nanocomposites considered here. A detailed description of the mechanisms was presented elsewhere [12]. The above-mentioned dependences of the dielectric behaviors of the PE/Al nanocomposites on the nanoparticle concentration and the measuring frequency can be interpreted by means of the equivalent electric circuit model in this study. As well known, a dielectric material can be generally modeled with the parallel or serial combination of the capacitor and the resistor: in the equivalent electric circuit, the pure polarization and the electric power loss in the dielectric material components can be characterized in terms of the equivalent parallel or serial capacitance (or dielectric permittivity) and resistance (or ac resistivity). In the case of the PE/Al nanocomposites considered here, the general equivalent electric circuit model can be supposed as shown in Fig. 3. In Fig. 3, CPE represents the average capacitance of PE domain between two electrodes; CAl, RAl are the equivalent average capacitances and ac resistances between the Al core and the internal interface of the oxide layer, respectively; C0, R0 are the equivalent average capacitance and ac resistance of the oxide layer, respectively; Ci, Ri are the equivalent average capacitance and ac resistance between nanoparticles (or between nanoparticles and the electrodes), respectively. When the nanofiller concentration is very low (below about 4 wt.% according to our experiment results), CAl and C0 can be neglected because the separate nanoparticles are so small and the nanoparticle-to-nanoparticle distance is so long that it could be impossible for the interfacial polarization to be formed, thus Ci being less different in magnitude from CPE, which leads to the conclusion that in the case of the relatively low Al nanofiller concentrations below 4 wt.% the dielectric behaviors of the PE/Al nanocomposites are very similar with those of the well-known nonpolar dielectrics (e.g., polyethylene, polypropylene, etc.), that is, dielectric permittivity of a nanocomposite keeps a defined low value in the total frequency range of 0.1 Hz to 1 MHz. The further increase of the nanofiller concentration causes both the increase of the effective average surface area of the nanoparticle and the decrease of the nanofiller-to-nanofiller space. It is well understood that the larger the surface of a dielectric specimen and the shorter the electrode-to-electrode space (the thickness of the dielectric material), the higher the capacitance of the system.
Therefore, CAl, C0 and Ci tend to further and further increase as the nanofiller loading becomes higher in the total frequency range under consideration, especially in the extra-low frequency range. It can be expected from the investigation of the dielectric behaviors that the increase of CAl and C0 could be predominant over that of Ci in the extra-low frequency range because they are closely related to the electrical conduction phenomena happened mainly in such low frequencies. In other words, the dielectric phenomena observed in the extra-low frequencies may be ascribed to the interfacial polarization due to the motion of the free electrical charge carriers both in the Al core and the oxide layer. In other hands, the higher the frequency becomes, the more difficult the motion of the free charge carriers, which may result in that the increase of Ci with the nanofiller loading level increasing becomes much more significant than those of CAl and C0 in the relatively high frequencies over 1 kHz. However, the increase of the nanofiller loading over about 24 wt.% begins to cause the significant agglomeration of the nanoparticles as mentioned above, resulting in the significant decrease of the effective surface area of the nanofiller and the increase of the equivalent average thickness of the oxide layer, which hints at the fact that CAl and C0 in the extra-low frequencies begin to decrease again beyond a relatively high loading level (RAl and R0 conversely begin to increase, causing the decrease of the dielectric loss tangent). This may be the main reason why in the extra-low frequency range the dielectric permittivity of the PE/Al nanocomposite shows the decreasing tendency again beyond the nanofiller loading level of about 24 wt.%. And also, little changes of the permittivity value in the comparatively high frequencies over 1 kHz can be seen beyond the nanofiller loading level of 24 wt.%, as can shown in Fig. 2, which suggests that although the nanoparticles becomes closer and closer with neighboring ones so as to contact with one another as the nanofiller concentration increases further and further, the complete contacts between nanoparticles cannot be realized and it is quite possible to suppose the existence of the apparent contact resistance between nanoparticles even in the high loading levels. Then, the physical meanings of Ci and Ri represented in the equivalent electric circuit shown in Fig. 3 are changed into the equivalent average capacitance and ac resistance in a nanofillerto-nanofiller contact point. Beyond 24 wt.%, further increase of the nanoparticle concentration no longer change the nanofiller-to-nanofiller contact spaces, allowing the permittivity of the PE/Al nanocomposite in the higher frequencies over 1 kHz to keep nearly unchanged. This mechanism can be supported by the previous research [13]. For dielectric application, the dielectric strength should be considered so as to check the ability to withstand high electric field. Fig. 4 presents the dependences of the breakdown strength on the content of Al nanoparticles. It can be seen that the breakdown strength tends to sharply decrease as Al nanoparticle loading level increases in the range of the comparatively low contents of nanofillers, whereas in the range of nanofiller contents above the microstructure transition concentration (16 wt.%), it is found to be kept approximately constant. According to Beale and Duxbury’s model used to describe dielectric breakdown in metal-loaded dielectric composites [14], the breakdown strength Ebre decreases logarithmically with the linear dimension L of the composites when the volume fraction of the metal pmetal is below the percolation threshold pc and behaves as Ebre ¼
Fig. 3. Schematic diagram (a) and equivalent electric circuit model (b) for a PE/Al nanocomposites.
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ðpc pmetal Þt ln L
ð1Þ
According to Eq. (1), the breakdown strength tends to zero if the metal fraction in the composites approaches the classical percola-
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Fig. 4. The breakdown strength corresponding to the cumulative failure probability of 63.2% for the nanocomposites at room temperature and 50 Hz.
tion threshold, whereas the nanocomposites with the nanoparticle contents above the microstructure transition concentration are observed to still have 40% breakdown strength of the neat PE, which may be attributed to that (i) Al nanoparticle has the passivated oxide layer (a kind of insulating layer) around its core surface, as shown in Fig. 5, which can act as the electrical barrier governing the tunneling current between the neighboring Al cores and (ii) the apparent contact resistance may be found between nanoparticles in the high nanoparticle concentrations because the existence of the undesirable voids and the solvent residue make it impossible for the complete contacts between the nanoparticles to be realized. It is worth noting that the nanocomposites with high particle loadings still have good dielectric breakdown characteristics, this being of a great significance for practical applications. 3.3. Melt rheological characteristics The rheological characteristics of polymer nanocomposites have both a practical significance related to the processing of compos-
ites and a scientific importance as a probe for dynamics and microstructures of the composites [15–17], and thus the nanocomposites have been further investigated from the viewpoint of their dynamic rheological properties. Fig. 6 is the experimental results for the various rheological parameters. It is clearly seen in Fig. 6a that the nanocomposites, especially those with high contents of Al nanoparticles have much greater viscosities than the neat PE. A Newtonian plateau at low frequencies is quite apparent for the nanocomposites with Al nanoparticle contents up to 12 wt.%. When the content of Al nanoparticle is larger than 16 wt.%, the plateau disappears and the nanocomposites show the strong shear thinning effect (the complex viscosity almost decreases linearly with the increasing frequency), and the reduction gradient of complex viscosity becomes greater as the nanoparticle content increases. Fig. 6b shows that the storage module G0 of the nanocomposite increases monotonously at all the measurement frequencies as the concentration of Al nanoparticle increases, and such an increase in the storage module is consistent with an increase in complex viscosity with the content of Al nanoparticle increasing. Much more significant increases in the complex viscosity, storage and loss module are observed at low frequencies: in our case, when the nanoparticle contents are ranged from 12 to 16 wt.% and from 16 to 24 wt.%, respectively, the differences of complex viscosities, storage and loss modules of the composites are found to reach one or two orders of magnitude at 0.01 rad/s, whereas they are found to be nearly unchanged or insignificantly changed both below 12 wt.% and above 24 wt.%, leading to the conclusion that 16 wt.% may be the critical point for the microstructure transition in the nanocomposites, as discussed above in more detail. A transition of the curve slopes can be found in Fig. 6b–c; the higher the content of nanofillers in nanocomposites, the further the curves of storage modulus G0 versus frequency or loss modulus G0 versus frequency are flattened, respectively; and a plateau would appear at low frequencies in the composites with Al nanoparticle loading higher than 24 wt.%. The low-frequency plateau is indicative of a ‘pseudo-solid-like’ behavior) [18]. As seen from Fig. 1f, the ‘pseudo-solid-like’ behavior may be derived from the polymer–nanoparticle agglomerates network structure formed in the nanocomposites with high particle loading. Pötschke et al. [16] have reported that the plateau phenomenon at low frequencies can be observed for the polycarbonate muti-walled carbon nanotube composites and proposed that this plateau phenomenon originates from the interconnected structure of carbon nanotubes. tan d ¼ G0 =G00 is sensitive to the structural changes of the materials and can be widely used to study the damping characteristics of the nanocomposites [17,19]. It is clearly seen from Fig. 6d, that tan d for the nanocomposites with low contents of Al nanofillers (less than 8 wt.%) monotonously decreases in the total frequency range while the plots of tan d versus frequency at low frequency range (less than 1 Hz) tends to be gradually flatten as the nanofiller contents further increase, showing the nanocomposites become more elastic [19]. When the content of the Al nanofiller exceeds 16 wt.%, the dependence of tan d on frequency can be hardly found in the total range of frequency. 3.4. Mechanical property and DMA
Fig. 5. A HRTEM image of an Al nanoparticle with a self-passivated oxide layer which is typically 5–7 nm thick.
The mechanical properties of the composites with different loadings of Al nanoparticles are shown in Fig. 7. The tensile strength is found to decrease slightly with the content of the Al nanoparticle increasing up to 24 wt.%, as shown in Fig. 7a, and the ultimate stress strength is 15.23 MPa, this being 82% of that of the neat PE. The elongation at break (Eb) is not changed when the content of Al nanoparticles increasing up to 4 wt.%. At higher nanoparticle contents, Eb decreases rapidly from 1106% for PE-8.0
X. Huang et al. / Composites Science and Technology 68 (2008) 2134–2140
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Fig. 6. Complex viscosity (a), storage modulus (b), loss modulus (c) and tan d (d) versus frequency for the nanocomposites.
Fig. 7. Tensile strength and elongation at break (a) and storage modulus versus temperature (b) for the nanocomposites.
to 200% for PE-24.0. These results are very important for the practical application because the nanocomposites with high dielectric constant still have good mechanical properties. It should be noted that the deviations of Eb for PE-12.0 and PE-16.0 are relatively larger, which may be related to the pores caused by the solvent residue and the agglomeration of the nanoparticles inside the nanocomposites. The data pertinent to the tensile strength and Eb also mean that the interfacial interactions between PE and the Al nanoparticles in the nanocomposites are at such a low level that it would have the weak effects on the bulk mechanical properties of the PE. Fig. 7b presents the temperature dependence of the
storage modulus, which shows that the storage moduli of the nanocomposites are lower than that of the neat PE at lower temperatures from 150 to about 20 °C, while above 20 °C the values of the storage moduli for all the composites with the loading levels of Al nanoparticles higher than 1 wt.% are greater than that of PE alone. There might be three factors to influence the temperature dependence of the storage modulus at the low-temperature range (620 °C), and the most influential factor among these three ones likely determines the low temperature behaviors of DMA for neat PE and PE/Al nanocomposites. These three factors suggested above are illustrated as follows: first, the nanoparticles can restrict the
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movement of the macromolecular chains of the interphase formed by the physical and chemical adsorption on the particle’s surface, and this effect can cause an increase of the storage modulus; second, the solvent residue and the agglomeration of the nanoparticles inside the PE matrix can introduce some voids and pores during the sample processing, which can cause a decrease of the storage modulus; third, the incorporation of Al nanoparticles may decrease the crystallinity of the PE due to the presence of imperfect crystals nucleated by the particles. At lower temperatures, the contribution of the interphase to the storage modulus is less than that of the others because of the high modulus of the matrix, and therefore, the storage modulus of polymer matrix is higher than those of the composites. At higher temperature (above 20 °C), the increase of the storage moduli for composites, on one hand, could be ascribed to the a relaxation of PE. According to our temperature dependence results of the loss modulus (not here), all the PE-based composites exhibited the a relaxation in the range of 20–100 °C, and the position of the a relaxation peaks shifted to lower temperature as the nanoparticle loading level increases. The relaxation firstly brings the crystals to perfection and hence improves the storage modulus. On the other hand, the nanoparticles in the interphase also improve the crystal perfection and hence their contribution to the modulus of the composites is much higher at higher temperatures than at lower temperatures. It can be clearly seen that, there is a peak of storage modulus at 50 °C and this stiffening effect, as mentioned above, should be ascribed to the a relaxation. As above-mentioned temperature dependence results of loss modulus, the same variation trend of the position of maximum storage modulus appeared and shifted to lower temperature as the nanoparticle loading level increases. It should be noted that, at higher temperatures, the modulus of the composites increases with the Al loading level increasing and the modulus of PE-8.0 is lower than that of PE-4.0 and PE-12.0. Considering the aggregation effect of the Al nanoparticles predominant over their dispersion effect, it can be expected that this phenomenon could be ascribed to the further reduction of the interphase for PE-8.0 compared to PE-4.0 and PE-12.0. 4. Conclusions PE/Al nanocomposites with the nanoparticle concentrations of 1 wt.% to 48 wt.% have been prepared by using the solution compounding method. It has been found from the morphological observation that the nanoscale dispersion is obtained when the content of Al nanoparticles is lower than 4 wt.%, and that agglom-
erates of Al nanofillers appear and increase in magnitude with the content of Al nanoparticle further increasing. The addition of Al nanoparticles results in much higher dielectric constant at low frequency range. The microstructure transition at 16 wt.% has been verified by using the rheological characteristics. At low frequencies, the curves of G0 or G0 versus frequency tend to approach a plateau in the composites with the contents of Al nanoparticles higher than the transition concentration, representing a ‘pseudo-solid-like’ behavior derived from the polymer–nanoparticle agglomerates network structure formed in the nanocomposites with high particle loading. The breakdown strength, ultimate tensile strength and elongation at break of the nanocomposites have been also investigated; it has been found that even at about microstructure transition concentration, the nanocomposites still keep the electrical and mechanical properties good enough to be used for practical applications. Acknowledgements The authors are grateful for the financial supports of the National Natural Science Foundation of China (No. 50677037). The authors also thank Prof. Yiu-Wing Mai (University of Sydney) and Prof. Sixun Zheng (Shanghai Jiao Tong University) for their useful comments on the manuscript. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
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Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Preparation, microstructure and properties of polyethylene aluminum nanocomposite dielectrics Xingyi Huang a,b, Pingkai Jiang a,b,*, Chonung Kim a,b,c, Qingquan Ke a,b, Genlin Wang a,b a
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Shanghai Key Laboratory of Electric Insulation and Thermal Aging, Shanghai 200240, China c Electrical Engineering Faculty, Kim Chaek University of Technology, Pyongyang, Democratic People’s Republic of Korea b
a r t i c l e
i n f o
Article history: Received 8 April 2007 Received in revised form 7 March 2008 Accepted 18 March 2008 Available online 25 March 2008 Keywords: A. Polymer–matrix composites (PMCs) B. Electrical properties B. Mechanical properties B. Microstructure
a b s t r a c t Linear low density polyethylene (PE) aluminum (Al) nanocomposites were prepared and their morphology and properties were investigated. The particular attention was given to the structure/property relationship of the nanocomposites and an equivalent electric circuit model was proposed to interpret the dependences of the dielectric behaviors of the PE/Al nanocomposites on the nanoparticle concentration and the measuring frequency. The increase of the dielectric constant shows the effectiveness of Al nanoparticles in altering the intrinsic dielectric properties of PE. The microstructure transition revealed by morphological investigations has been also verified at the same composition as that from the rheological data. And also, it has been found that the nanocomposites at high filler concentrations still keep good mechanical properties and breakdown strength, which is very important for practical applications. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Polyethylene is known to be one of the most widely used polymeric insulating materials. The high dielectric strength, very low electrical conductivity and remarkable mechanical properties allow it to be used as an outstanding insulation in cable applications [1]. However, its low dielectric constant makes it impossible for PE to be used for electrical stress control in cable terminations. At cable terminations, the electric field is concentrated at the cutback point, which may be great enough to cause the dielectric breakdown of the air, resulting in the surface and internal discharge of the cable insulation, which will ultimately destroy the cable insulation, causing premature failure. The use of high-dielectric-constant materials has been strongly recommended in order to reduce the high voltage stress below the levels at which the dielectric breakdown or partial discharge of air would occur in the insulation. The common approach to increase the dielectric constant of a polymer is known to adopt microscale ceramics of high dielectric constant, carbon fiber or nanotubes as fillers [2], e.g., BaTiO3 [3] and muti-walled carbon nanotube (MWCT) [4]. For the polymer/ ceramic composites, high filler loadings are usually needed in order to realize the values of dielectric constant high enough for
* Corresponding author. Address: School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. Tel.: +86 21 54740787; fax: +86 21 54746520. E-mail address: [email protected] (P. Jiang). 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.03.009
the purposed industrial applications. However, high filler loading is sure to deteriorate the mechanical and other electrical properties. Furthermore, the dielectric constant of BaTiO3 is strongly dependent on grain size and temperature [5]. Carbon fiber/nanotubes with the larger aspect ratios and the high conductivities can produce very low percolation threshold and high dielectric constant [6], while a very low value of percolation threshold gives a big challenge to obtain reproducible products for practical applications. Some novel metal particles, e.g., silver particles can be used to increase the dielectric constant of polymers [7], whereas their high cost is a main disadvantage for practical applications. It should be noted that, although several polymer/metal nanocomposites with very high dielectric constant have been prepared, few paper have dealt with the mechanical property, processing performance and dielectric strength of the nanocomposites up to now, which are regarded to be of a great importance from the viewpoint of the practical applications. In this paper, we report the preparation of PE/Al nanocomposites by using the solution compounding method and the investigations on the microstructure of the nanocomposites. Some characterizations for the prepared materials and their processing, including mechanical properties, dielectric and rheological characteristics are also investigated. Aluminum (Al) is selected because of its self-passivation characteristic. The self-passivated oxide layer forms a thin insulating boundary around the surface of its metallic core, allowing the polyethylene/Al composites to have a high dielectric constant [8].
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2. Experimental part 2.1. Materials The polyethylene used is linear low density polyethylene from ExxonMobil in Saudi Arabia. It has a Melt Flow Index (MFI) of 3.6 g/min by ASTM D1238 and a density of 0.924 g/cm3 by ASTM D4703/D1505. The Al nanoparticles (purchased from Hongwu nanomaterials Co. China) with an average diameter of 102 nm according to ISO/TS 13762 were used as received.
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than 5% was used, which was in the linear viscoelastic regime for all samples. The tensile tests were performed according to ASTM D 638-2003 by using an Instron series IX 4465 materials tester, with a grip separation rate of 250 mm/min, with 1 mm-thick dumbbell type specimens. DMA experiments were performed using the Netzsch 242C instrument. The deformation applied was the cantilever mode, and the mean dimensions of sample plaques were 40 mm 10 mm 0.92 mm. The temperature varied from 150 °C up to the melting temperature (about 120 °C). The frequency was 10 Hz and the heating rate 5 °C/min. The amplitude of deformation is 0.03%.
2.2. Preparation of PE/Al nanocomposites 3. Results and discussion Al nanoparticles were added to analytically pure trichloromethane (CHCl3) to give a concentration of 5 mg of Al/ml of CHCl3, and dispersed in a sonication bath (40 kHz, 250 W) for 2 h on the basis of the desired weight fraction of Al in the final composites at room temperature. At the same time, an appropriated quantity of PE was dissolved in chemically pure para-xylene (pX) to give a concentration of 10 mg of PE/ml of pX at 368 K. Then, the suspension of Al/CHCl3 was added to a solution of PE/pX, and the resulting mixture was stirred with an electric mixer (agitating speed is about 2500 r/min) at 368 K for 2 h. Afterward, the mixture were heated at 368 K for 6 h so as to evaporate the solvent. The resulting nanocomposites were dried in a vacuum oven at about 393 k for 24 h to evaporate the solvent completely. In order to make the results of the neat and filled polymer samples comparable with each other, the neat PE was treated with the same process as the filled polymer samples were done. Thin films with thickness of around 250 ± 10 lm were used for dielectric breakdown strength measurements. For the sake of convenience, the composites were denoted using the following notation: PE-particle weight, thus PE-2.0 indicates the composites with 2.0 wt.% Al nanoparticles. 2.3. Characterization The morphology and dispersion of the Al nanoparticles in the PE were investigated using JEOL JEM-4701 field emission scanning electron microscopy (FE-SEM). Cylindrical samples were broken in the liquid nitrogen and sputtered with thin layers of gold to avoid accumulation of charges. The high resolution transmission electron microscopy (HRTEM) investigations were performed on a JEOL 2100F transmission electron microscope, which was operated at an accelerating voltage of 200 kV. The sample was prepared by placing a drop of a dilute dimethylformamide-dispersion of Al nanoparticles on the holey-carbon coated copper TEM grid. Dielectric spectroscopy measurements were performed by using a broadband dielectric spectrometer (Novocontrol GmbH, Germany). Sample thickness is about 1 mm in all cases. Dielectric constant (e0 ) was recorded in the discrete frequencies ranged from 0.1 Hz to 1 MHz. Dielectric breakdown testing was performed on an AHDZ-10/100 alternating-current dielectric strength tester (Shanghai Lanpotronics Corp., Shanghai, China) according to ASTM D 1492004. The specimens were immersed in the pure silicon oil, between two 10-mm-diameter copper ball electrodes opposing to each other. The lower electrode was connected to earth and an increasing AC voltage with a rate of 2 KV/s was applied to the upper electrode until the sample failed. Twenty breakdown tests were performed on each specimen. And 63.2% electrical breakdown characteristics were calculated from the experimentally obtained data by using Weibull statistical method [9]. A rotational rheometer, Gemini 200HR (Bohlin instruments, UK) with a parallel-plate geometry (diameter of 25 mm), was used for rheological measurements. Small amplitude oscillatory shear was performed in the frequency range 0.01–100 rad/s at 160 °C. A strain of less
3.1. Morphology In the case of nanoparticle-filled polymer composites, the nanofillers inside the polymer matrix generally exist in the two states: single particles and particle agglomerates (clusters). Metal-filled insulation systems can be simulated by means of a large number of equivalent elementary capacitors where the single particles or agglomerates (clusters) can act as electrodes when an external electric field is applied on the composites [10]. Since the above-mentioned elementary capacitors are considered be connected both in parallel and in series with one another, the dielectric constant of PE/Al nanocomposites can be affected by the following factors: the number of the elementary capacitors and the thickness of the intercapacitor layer. Furthermore, all of these two factors are directly related to the dispersion state of the fillers. Take into account this consideration, it is necessary to investigate the dispersion of the nanoparticles within the polyethylene matrix. Fig. 1a–f shows the FE-SEM micrographs of the cross section of the nanocomposites prepared by solution compounding. From the FE-SEM images, it can be seen that the dispersion in nanoscale is obtained when the content of Al nanoparticles is lower than 4 wt.%: the individually dispersed particles, even very small nanoparticles can be found in Fig. 1b. With further loading with Al nanoparticles, particle clusters begin to appear from place to place and their magnitudes are found to further increase. Starting at 12 wt.%, nanoparticle agglomerates larger than microscale can be seen (Fig. 1d), and the microstructure transition is observed to occur up to 16 wt.% (Fig. 1e). The transition mentioned here is defined as such a process that the unconnected topological agglomerates continue to grow until the polymer–nanoparticle agglomerates networks (a state that the nanoparticle clusters have formed and contact between one another) are formed (Fig. 1f). The dispersion of nanofillers into a polymer matrix is determined by the kinetics of composite preparation. The attractive van der Waals forces between nanoparticles are relatively weak because of the long particle-to-particle distance at low nanoparticle loading, the agglomerated nanoparticles are thought to be easily broken up by the shear force provided by the mixing device, which may be the main reason why the nanoscale dispersion is obtained for the nanocomposites with nanoparticle loading lower than 4 wt.%. With further increasing the Al nanoparticle loading, the particle-to-particle distance decreases exponentially and the more attractive van der Waals forces begin to create agglomeration of the particles from place to place. 3.2. Dielectric characteristics Fig. 2 presents the behaviors of the frequency dependences of the real part of the dielectric constant (e0 ) for different Al nanoparticle contents. It can be seen that the nanocomposites with the Al nanoparticle content lower than 12.0 wt.% are observed to have lit-
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Fig. 1. FE-SEM micrographs of (a) PE, (b) PE-4.0, (c) PE-8.0, (d) PE-12.0, (e) PE-16.0 and (f) PE-24.0.
Fig. 2. Frequency dependences of e0 for the PE nanocomposites.
tle dependences on frequency, resulting in nearly the same value of e0 in the total range of the measuring frequencies. Starting at 16 wt.%, the e0 values of the nanocomposites have significantly different dependences on frequency: a much significant decrease of
the dielectric constant can be observed when the frequency increases in the extra low frequency range, and the addition of Al nanoparticles results in much higher e0 value. For example, at 0.1 Hz, the e0 value increases from 2.35 for pure PE to 24.14 for PE-16.0. Such an increase of the e0 value shows the effectiveness of Al nanoparticles in altering the intrinsic dielectric properties of the base polymers, giving us a hint to overcome one of the main disadvantages of PE from the viewpoint of its application, e.g., low capability of charge storage. This increase of dielectric constant observed in the composites with higher loading levels is caused by the interfacial polarization, which is understood to occur in the dielectrics composed of two or more materials with significantly different electrical properties. The above-mentioned interfacial polarization can cause the formation of electric charge at the interfaces between different materials when voltage is applied to the materials [11]. At 32 wt.%, however, the dielectric constant reaches the maximum point at the low frequency range and begins to decrease as the filler fraction increases. It is also interesting to pay attention to that the dielectric constant transition at 24 wt.% only takes place at extra-low frequency range but not at the higher frequency range. As shown in Fig. 2, a moderate increase of the dielectric constant in the frequency range of 100 Hz to 1 MHz is observed as the Al weight fraction increases up to 24 wt.%, and then any significant changes of the dielectric permittivity can not be found over 24 wt.%. These features might be attributed to that
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there are different mechanisms for dielectric polarization of the PE/ Al nanocomposites considered here. A detailed description of the mechanisms was presented elsewhere [12]. The above-mentioned dependences of the dielectric behaviors of the PE/Al nanocomposites on the nanoparticle concentration and the measuring frequency can be interpreted by means of the equivalent electric circuit model in this study. As well known, a dielectric material can be generally modeled with the parallel or serial combination of the capacitor and the resistor: in the equivalent electric circuit, the pure polarization and the electric power loss in the dielectric material components can be characterized in terms of the equivalent parallel or serial capacitance (or dielectric permittivity) and resistance (or ac resistivity). In the case of the PE/Al nanocomposites considered here, the general equivalent electric circuit model can be supposed as shown in Fig. 3. In Fig. 3, CPE represents the average capacitance of PE domain between two electrodes; CAl, RAl are the equivalent average capacitances and ac resistances between the Al core and the internal interface of the oxide layer, respectively; C0, R0 are the equivalent average capacitance and ac resistance of the oxide layer, respectively; Ci, Ri are the equivalent average capacitance and ac resistance between nanoparticles (or between nanoparticles and the electrodes), respectively. When the nanofiller concentration is very low (below about 4 wt.% according to our experiment results), CAl and C0 can be neglected because the separate nanoparticles are so small and the nanoparticle-to-nanoparticle distance is so long that it could be impossible for the interfacial polarization to be formed, thus Ci being less different in magnitude from CPE, which leads to the conclusion that in the case of the relatively low Al nanofiller concentrations below 4 wt.% the dielectric behaviors of the PE/Al nanocomposites are very similar with those of the well-known nonpolar dielectrics (e.g., polyethylene, polypropylene, etc.), that is, dielectric permittivity of a nanocomposite keeps a defined low value in the total frequency range of 0.1 Hz to 1 MHz. The further increase of the nanofiller concentration causes both the increase of the effective average surface area of the nanoparticle and the decrease of the nanofiller-to-nanofiller space. It is well understood that the larger the surface of a dielectric specimen and the shorter the electrode-to-electrode space (the thickness of the dielectric material), the higher the capacitance of the system.
Therefore, CAl, C0 and Ci tend to further and further increase as the nanofiller loading becomes higher in the total frequency range under consideration, especially in the extra-low frequency range. It can be expected from the investigation of the dielectric behaviors that the increase of CAl and C0 could be predominant over that of Ci in the extra-low frequency range because they are closely related to the electrical conduction phenomena happened mainly in such low frequencies. In other words, the dielectric phenomena observed in the extra-low frequencies may be ascribed to the interfacial polarization due to the motion of the free electrical charge carriers both in the Al core and the oxide layer. In other hands, the higher the frequency becomes, the more difficult the motion of the free charge carriers, which may result in that the increase of Ci with the nanofiller loading level increasing becomes much more significant than those of CAl and C0 in the relatively high frequencies over 1 kHz. However, the increase of the nanofiller loading over about 24 wt.% begins to cause the significant agglomeration of the nanoparticles as mentioned above, resulting in the significant decrease of the effective surface area of the nanofiller and the increase of the equivalent average thickness of the oxide layer, which hints at the fact that CAl and C0 in the extra-low frequencies begin to decrease again beyond a relatively high loading level (RAl and R0 conversely begin to increase, causing the decrease of the dielectric loss tangent). This may be the main reason why in the extra-low frequency range the dielectric permittivity of the PE/Al nanocomposite shows the decreasing tendency again beyond the nanofiller loading level of about 24 wt.%. And also, little changes of the permittivity value in the comparatively high frequencies over 1 kHz can be seen beyond the nanofiller loading level of 24 wt.%, as can shown in Fig. 2, which suggests that although the nanoparticles becomes closer and closer with neighboring ones so as to contact with one another as the nanofiller concentration increases further and further, the complete contacts between nanoparticles cannot be realized and it is quite possible to suppose the existence of the apparent contact resistance between nanoparticles even in the high loading levels. Then, the physical meanings of Ci and Ri represented in the equivalent electric circuit shown in Fig. 3 are changed into the equivalent average capacitance and ac resistance in a nanofillerto-nanofiller contact point. Beyond 24 wt.%, further increase of the nanoparticle concentration no longer change the nanofiller-to-nanofiller contact spaces, allowing the permittivity of the PE/Al nanocomposite in the higher frequencies over 1 kHz to keep nearly unchanged. This mechanism can be supported by the previous research [13]. For dielectric application, the dielectric strength should be considered so as to check the ability to withstand high electric field. Fig. 4 presents the dependences of the breakdown strength on the content of Al nanoparticles. It can be seen that the breakdown strength tends to sharply decrease as Al nanoparticle loading level increases in the range of the comparatively low contents of nanofillers, whereas in the range of nanofiller contents above the microstructure transition concentration (16 wt.%), it is found to be kept approximately constant. According to Beale and Duxbury’s model used to describe dielectric breakdown in metal-loaded dielectric composites [14], the breakdown strength Ebre decreases logarithmically with the linear dimension L of the composites when the volume fraction of the metal pmetal is below the percolation threshold pc and behaves as Ebre ¼
Fig. 3. Schematic diagram (a) and equivalent electric circuit model (b) for a PE/Al nanocomposites.
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ðpc pmetal Þt ln L
ð1Þ
According to Eq. (1), the breakdown strength tends to zero if the metal fraction in the composites approaches the classical percola-
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Fig. 4. The breakdown strength corresponding to the cumulative failure probability of 63.2% for the nanocomposites at room temperature and 50 Hz.
tion threshold, whereas the nanocomposites with the nanoparticle contents above the microstructure transition concentration are observed to still have 40% breakdown strength of the neat PE, which may be attributed to that (i) Al nanoparticle has the passivated oxide layer (a kind of insulating layer) around its core surface, as shown in Fig. 5, which can act as the electrical barrier governing the tunneling current between the neighboring Al cores and (ii) the apparent contact resistance may be found between nanoparticles in the high nanoparticle concentrations because the existence of the undesirable voids and the solvent residue make it impossible for the complete contacts between the nanoparticles to be realized. It is worth noting that the nanocomposites with high particle loadings still have good dielectric breakdown characteristics, this being of a great significance for practical applications. 3.3. Melt rheological characteristics The rheological characteristics of polymer nanocomposites have both a practical significance related to the processing of compos-
ites and a scientific importance as a probe for dynamics and microstructures of the composites [15–17], and thus the nanocomposites have been further investigated from the viewpoint of their dynamic rheological properties. Fig. 6 is the experimental results for the various rheological parameters. It is clearly seen in Fig. 6a that the nanocomposites, especially those with high contents of Al nanoparticles have much greater viscosities than the neat PE. A Newtonian plateau at low frequencies is quite apparent for the nanocomposites with Al nanoparticle contents up to 12 wt.%. When the content of Al nanoparticle is larger than 16 wt.%, the plateau disappears and the nanocomposites show the strong shear thinning effect (the complex viscosity almost decreases linearly with the increasing frequency), and the reduction gradient of complex viscosity becomes greater as the nanoparticle content increases. Fig. 6b shows that the storage module G0 of the nanocomposite increases monotonously at all the measurement frequencies as the concentration of Al nanoparticle increases, and such an increase in the storage module is consistent with an increase in complex viscosity with the content of Al nanoparticle increasing. Much more significant increases in the complex viscosity, storage and loss module are observed at low frequencies: in our case, when the nanoparticle contents are ranged from 12 to 16 wt.% and from 16 to 24 wt.%, respectively, the differences of complex viscosities, storage and loss modules of the composites are found to reach one or two orders of magnitude at 0.01 rad/s, whereas they are found to be nearly unchanged or insignificantly changed both below 12 wt.% and above 24 wt.%, leading to the conclusion that 16 wt.% may be the critical point for the microstructure transition in the nanocomposites, as discussed above in more detail. A transition of the curve slopes can be found in Fig. 6b–c; the higher the content of nanofillers in nanocomposites, the further the curves of storage modulus G0 versus frequency or loss modulus G0 versus frequency are flattened, respectively; and a plateau would appear at low frequencies in the composites with Al nanoparticle loading higher than 24 wt.%. The low-frequency plateau is indicative of a ‘pseudo-solid-like’ behavior) [18]. As seen from Fig. 1f, the ‘pseudo-solid-like’ behavior may be derived from the polymer–nanoparticle agglomerates network structure formed in the nanocomposites with high particle loading. Pötschke et al. [16] have reported that the plateau phenomenon at low frequencies can be observed for the polycarbonate muti-walled carbon nanotube composites and proposed that this plateau phenomenon originates from the interconnected structure of carbon nanotubes. tan d ¼ G0 =G00 is sensitive to the structural changes of the materials and can be widely used to study the damping characteristics of the nanocomposites [17,19]. It is clearly seen from Fig. 6d, that tan d for the nanocomposites with low contents of Al nanofillers (less than 8 wt.%) monotonously decreases in the total frequency range while the plots of tan d versus frequency at low frequency range (less than 1 Hz) tends to be gradually flatten as the nanofiller contents further increase, showing the nanocomposites become more elastic [19]. When the content of the Al nanofiller exceeds 16 wt.%, the dependence of tan d on frequency can be hardly found in the total range of frequency. 3.4. Mechanical property and DMA
Fig. 5. A HRTEM image of an Al nanoparticle with a self-passivated oxide layer which is typically 5–7 nm thick.
The mechanical properties of the composites with different loadings of Al nanoparticles are shown in Fig. 7. The tensile strength is found to decrease slightly with the content of the Al nanoparticle increasing up to 24 wt.%, as shown in Fig. 7a, and the ultimate stress strength is 15.23 MPa, this being 82% of that of the neat PE. The elongation at break (Eb) is not changed when the content of Al nanoparticles increasing up to 4 wt.%. At higher nanoparticle contents, Eb decreases rapidly from 1106% for PE-8.0
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Fig. 6. Complex viscosity (a), storage modulus (b), loss modulus (c) and tan d (d) versus frequency for the nanocomposites.
Fig. 7. Tensile strength and elongation at break (a) and storage modulus versus temperature (b) for the nanocomposites.
to 200% for PE-24.0. These results are very important for the practical application because the nanocomposites with high dielectric constant still have good mechanical properties. It should be noted that the deviations of Eb for PE-12.0 and PE-16.0 are relatively larger, which may be related to the pores caused by the solvent residue and the agglomeration of the nanoparticles inside the nanocomposites. The data pertinent to the tensile strength and Eb also mean that the interfacial interactions between PE and the Al nanoparticles in the nanocomposites are at such a low level that it would have the weak effects on the bulk mechanical properties of the PE. Fig. 7b presents the temperature dependence of the
storage modulus, which shows that the storage moduli of the nanocomposites are lower than that of the neat PE at lower temperatures from 150 to about 20 °C, while above 20 °C the values of the storage moduli for all the composites with the loading levels of Al nanoparticles higher than 1 wt.% are greater than that of PE alone. There might be three factors to influence the temperature dependence of the storage modulus at the low-temperature range (620 °C), and the most influential factor among these three ones likely determines the low temperature behaviors of DMA for neat PE and PE/Al nanocomposites. These three factors suggested above are illustrated as follows: first, the nanoparticles can restrict the
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movement of the macromolecular chains of the interphase formed by the physical and chemical adsorption on the particle’s surface, and this effect can cause an increase of the storage modulus; second, the solvent residue and the agglomeration of the nanoparticles inside the PE matrix can introduce some voids and pores during the sample processing, which can cause a decrease of the storage modulus; third, the incorporation of Al nanoparticles may decrease the crystallinity of the PE due to the presence of imperfect crystals nucleated by the particles. At lower temperatures, the contribution of the interphase to the storage modulus is less than that of the others because of the high modulus of the matrix, and therefore, the storage modulus of polymer matrix is higher than those of the composites. At higher temperature (above 20 °C), the increase of the storage moduli for composites, on one hand, could be ascribed to the a relaxation of PE. According to our temperature dependence results of the loss modulus (not here), all the PE-based composites exhibited the a relaxation in the range of 20–100 °C, and the position of the a relaxation peaks shifted to lower temperature as the nanoparticle loading level increases. The relaxation firstly brings the crystals to perfection and hence improves the storage modulus. On the other hand, the nanoparticles in the interphase also improve the crystal perfection and hence their contribution to the modulus of the composites is much higher at higher temperatures than at lower temperatures. It can be clearly seen that, there is a peak of storage modulus at 50 °C and this stiffening effect, as mentioned above, should be ascribed to the a relaxation. As above-mentioned temperature dependence results of loss modulus, the same variation trend of the position of maximum storage modulus appeared and shifted to lower temperature as the nanoparticle loading level increases. It should be noted that, at higher temperatures, the modulus of the composites increases with the Al loading level increasing and the modulus of PE-8.0 is lower than that of PE-4.0 and PE-12.0. Considering the aggregation effect of the Al nanoparticles predominant over their dispersion effect, it can be expected that this phenomenon could be ascribed to the further reduction of the interphase for PE-8.0 compared to PE-4.0 and PE-12.0. 4. Conclusions PE/Al nanocomposites with the nanoparticle concentrations of 1 wt.% to 48 wt.% have been prepared by using the solution compounding method. It has been found from the morphological observation that the nanoscale dispersion is obtained when the content of Al nanoparticles is lower than 4 wt.%, and that agglom-
erates of Al nanofillers appear and increase in magnitude with the content of Al nanoparticle further increasing. The addition of Al nanoparticles results in much higher dielectric constant at low frequency range. The microstructure transition at 16 wt.% has been verified by using the rheological characteristics. At low frequencies, the curves of G0 or G0 versus frequency tend to approach a plateau in the composites with the contents of Al nanoparticles higher than the transition concentration, representing a ‘pseudo-solid-like’ behavior derived from the polymer–nanoparticle agglomerates network structure formed in the nanocomposites with high particle loading. The breakdown strength, ultimate tensile strength and elongation at break of the nanocomposites have been also investigated; it has been found that even at about microstructure transition concentration, the nanocomposites still keep the electrical and mechanical properties good enough to be used for practical applications. Acknowledgements The authors are grateful for the financial supports of the National Natural Science Foundation of China (No. 50677037). The authors also thank Prof. Yiu-Wing Mai (University of Sydney) and Prof. Sixun Zheng (Shanghai Jiao Tong University) for their useful comments on the manuscript. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
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