Preparation, Characterization and Dielectric Properties of Epoxy and Polyethylene Nanocomposites

Extended Summary 本文は pp.1105-1111

Chao Zhang Non-member (GnoSys UK Ltd, University of Surrey, [email protected]) Ralf Mason Non-member (University of Surrey, [email protected]) Gary Stevens Non-member (GnoSys UK Ltd, University of Surrey, [email protected])

Keywords : nanocomposites, dielectric spectroscopy, characterization, alumina, polyethylene, epoxy

In the present research, two kinds of base polymers were insulative, and acts as traps of carriers under an electric field. So intentionally chosen to be very different, one polar, the one there is a distinct dielectric dissipation factor peak in the low non-polar. Bisphenol-A epoxy resin with anhydride hardener is a frequency due to the interfacial polarization. This peak is sensitive thermosetting resin which is widely used in rotating machine, to the dispersion information of the particles and clusters. The switchgear systems, and insulators. It is a polar polymer, and has a polyethelene nanocomposite with high nano-alumina content has a low to medium viscosity before cure. Linear low density wider tand peak than that with low nano-alumina content. In polyethylene (LLDPE) is a thermoplastic which may be used in epoxy nanocomposite, the interface is supposed to have a higher power cable insulation. It is a non-polar polymer and has a high electrical conductivity than either the epoxy or alumina bulk. The viscosity in the melt phase. A nanometric size aluminium oxide mobile carriers in the conductive interfacial layer may move away filler was chosen as a filler. Conventional alumina is commonly along the overlaped conductive interfacial layers in the used as filler to improve electrical, mechanical and thermal filamentary nanoparticle clusters, or neutralise with properties in insulating composites, however, there is little in the counter-charges. As a result, no significant blocked charges stay in published literature about the application of nanometric size the interfacial region, and no Maxwell-Wagner-Sillars polarization alumina in electrical insulating composites. With these two is induced. different base polymers and the same nano filler, the different dielectric behaviors due to different filler-matrix interactions are -3 expected to be revealed. 15x10 Figure 1 shows the SEM micrographs of the fracture surfaces of the cured epoxy composites with 5 wt% nano-alumina (×5k). The 10

alumina clusters in the fracture surface of the composite are δ clearly seen. The size of clusters is below 200 nm. The picture Tan Tan suggests a fine dispersion of the alumina fillers in the epoxy resin. 5 The dielectric measurement results show that the incorporation of alumina has very different effects on the dielectric behaviours of the two polymer systems. In polyethelene nanocomposites, 0 -2 -1 0 1 2 3 4 5 significant interfacial polarization is clearly seen. However, in 10 10 10 10 10 10 10 10 epoxy nanocomposites, no obvious interfacial polarization is f (Hz) found. The phenomena are explained by the different nature of the interface in the two kind of polymer nanocomposites. In (a) polyethelene nanocomposite, the interface is supposed to be high -3 15x10

10 δ Tan Tan 5

0 -2 -1 0 1 2 3 4 5 10 10 10 10 10 10 10 10

f (Hz) (b) Fig. 2. Frequency dependence of the dissipation factor of Fig. 1. SEM micrograph of the fracture surfaces of Epoxy-5 epoxy nanocomposites at selected temperature: (a) Neat epoxy; (×5k) (b) Epoxy with 5 wt% alumina

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Paper

Preparation, Characterization and Dielectric Properties of Epoxy and Polyethylene Nanocomposites

＊＊ Chao Zhang Non-member ＊ Ralf Mason Non-member ＊＊ Gary Stevens Non-member

Two very different kinds of polymer nanocomposites have been prepared, characterized and investigated by dielectric spectroscopy to investigate the effects of polymer-nanofiller matrix difference on the dielectric response of nanodielectric composites. Linear low density polyethylene (LLDPE) is a non-polar thermoplastic which has a high viscosity even in the melt phase and bisphenol-A epoxy resin with an anhydride hardener is a polar low viscosity thermosetting resin. Nanometric sized aluminium oxide filler was chosen as the common inorganic phase for both nanodielectrics. Generally, nanoparticles aggregate easily and are difficult to separate due to strong surface interactions. In this study various mixing methods were employed from ultrasonic liquid processing to controlled shear flow mixing to investigate the dispersion of the nanofillers. The resultant epoxy and polyethylene nanocomposites were characterized with SEM, TEM, and DSC. The dielectric properties and frequency response of the nanocomposites were measured in the frequency domain from 10-2 Hz to 106 Hz at different temperatures. In polyethylene nanocomposites, significant interfacial polarization is clearly seen. However, in epoxy nanocomposites, no obvious interfacial polarization is found. The results are discussed in terms of the difference in the electrical characteristics of the interfacial region between the polymers and the nano-alumina.

Keywords : nanocomposites, dielectric spectroscopy, characterization, alumina, polyethylene, epoxy

intentionally chosen to be very different, one polar, the one 1. Introduction non-polar. Bisphenol-A epoxy resin with anhydride hardener is a Electrical insulating polymers usually incorporate inorganic thermosetting resin which is widely used as a component material fillers to achieve specific electrical, mechanical, thermal in insulation systems in rotating machine, switchgear systems, and properties and reduce cost. For an insulating polymer composite insulators. It is a polar polymer, and has a low to medium composed of conventional inorganic fillers, the interface between viscosity before cure. Linear low density polyethylene (LLDPE) is the polymer matrix and individual particles or agglomerates is a thermoplastic which may be used in power cable insulation. It is usually large in size and irregular in shape due to the size of fillers, a non-polar polymer and has a high viscosity in the melt phase. A and along with surface wetting problems this can lead to electrical nanometric size aluminium oxide filler was chosen as a filler. weak spots. One potential solution is to apply nanoscale fillers as a Conventional alumina is commonly used as filler to improve partial or complete substitute for conventional fillers. However electrical, mechanical and thermal properties in insulating other enhancements in physical properties may occur as a result of composites, however, there is little in the published literature the large internal surface area presented by nanocomposites. about the application of nanometric size alumina in electrical Recently, polymer nanocomposites have attracted wide interest insulating composites. With these two different base polymers and as a method of enhancing polymer properties and extending their the same nano filler, it was anticipated that significantly different utility. Generally, in conventional filled polymer composites, the dielectric behaviors might be observed due to different constituents are immiscible, thereby producing a coarsely blended filler-matrix interactions and the amplification effect of the large macrocomposite. This results in poor physical interaction between internal surface area. the organic and inorganic components. In polymer nano- Generally, it is difficult to obtain the fine dispersion of composites, chemically dissimilar components are combined at the nanoparticles in a polymer matrix because nanoparticles aggregate nanometer scale, and stronger interactions between the polymer easily and are difficult to separate due to strong interparticle and nanoparticles produce markedly improved materials with interactions. In this research, various devices including an better electrical, mechanical, thermal, and rheological properties. ultrasonic liquid processor and a Brabender mixer have been used In recent years, polymer nanocomposites have been extensively to improve the dispersion of nano fillers in polymers. The studied, but there has been relatively little research into electrical prepared nanocomposites were chracterized by scanning electron properties(1)-(4). Currently, with some exceptions, few polymer microscopy (SEM), transmission electron microscope (TEM), and nanocomposites are actually used as electrical insulating materials. differential scanning calorimetry (DSC) in addition to dielectric In the present research, two kinds of base polymers were spectroscopy measurements.

* Polymer Research Centre, University of Surrey 2. Samples Preparation Guildford, Surrey GU2 7XH, UK ** The present address: GnoSys UK Ltd, University of Surrey All the samples used in this study are listed in Table 1, their Guildford, Surrey GU2 7XH, UK

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preparation processes are described below. films were prepared with concentrations of alumina of 0, 1, 5, and 2.1 Polyethylene Nanocomposites The LLDPE used in 10 wt%, respectively. The films are uniform, flexible, nearly this study is a commercial linear low-density polyethylene from transparent, and no visual difference was found among the four Atofina. It has a density of 0.934 g/cm3, a melt index of 0.87 g/10 kinds of samples. The torque versus time curves for all the four min. Nanoparticles of aluminium oxide from Degussa with a samples during Brabender mixing are shown in Fig. 1(b). It particle size of about 13 nm were used as filler. This nano scale indicates that the viscosity of the system increases with the filler has a nearly spherical shape with a specific surface area of increasing content of nano-alumina, the torque of LLDPE about 100 m2/g. The filler was dried before using. increases by 10.7 % with addition of 10 wt% nano-alumina. Polyethylene nanocomposites were firstly prepared by melt 2.2 Epoxy Nanocomposites The epoxy resin system mixing at 150°C using a Brabender W50EHT mixer with chamber used in this study is a diglycidyl ether of bisphenol-A (DGEBA) size of 50 cm3 under a nitrogen atmosphere. The mixer has a high resin cured with methyl hexahydro-phthalic anhydride and and controlled shear force, the screw speed was controlled at 60 accelerated by tetraethylammonium bromide (TEAB). The same rpm, and the mixing time was 20 min for each sample, the mixing nanoparticles of aluminium oxide described above were used. chamber is shown in Fig. 1(a). The sheets of mixtures were then Epoxy nanocomposites were firstly prepared by mixing the extruded by using a Polylab Universal Moulding Machine at epoxy resin, anhydride hardener and filler by conventional 150°C. The nanocomposites were then prepared into films by hot mechanical stirring under degassing for 60 min at 60°C, then the melt pressing at 150°C. Four types of polyethylene nanocomposite composites were further mixed in an ultrasonic processor at 20 kHz. After that the composites together with TEAB were mixed Table 1. Specimens investigated by stirring under degassing for 30 min again, and finally, they were cast in moulds for curing. No exothermic peak was observed Specimens Composition in the cured samples between 20-300°C. Three types of epoxy PE-0 LLDPE nanocomposites were prepared with the concentration of alumina of 0, 1, and 5 wt %, respectively. They were prepared as discs with PE-1 LLDPE + 1wt% alumina a diameter of 35mm and thickness of about 0.5mm. These discs PE-5 LLDPE + 5wt% alumina are uniform, transparent, and no differences were found among the PE-10 LLDPE + 10wt% alumina three kinds of samples. Epoxy-0 Epoxy/Anhydride 3. Experimental Epoxy-1 Epoxy/Anhydride + 1wt% alumina 3.1 Characterization of Microstructure by SEM and Epoxy-5 Epoxy/Anhydride + 5wt% alumina TEM In this research, a Hitachi S4000 scanning electron microscopy (SEM) was used to investigate the dispersion of fillers in the polymer matrices. The specimens were fractured in liquid nitrogen, then gold coated and observed at an acceleration voltage of 20 kV. A Hitachi 7100 transmission electron microscope (TEM) was used to observe the dispersion of fillers in the polymer matrices. The specimens were sliced with a microtome before observation. 3.2 Differential Scanning Calorimetry Analysis The interaction between the fillers and the polymer matrices was investigated using standard differential scanning calorimetry (DSC). In this research, the DSC scans were performed with a TA 2920 MDSC, purged with a nitrogen flow of 40ml/min, and calibrated with indium. (a) For the polyethylene nanocomposites, the samples were firstly subjected to a heating procedure from -50 to 250°C at a rate of 50 10°C/min, followed by cooling to -120°C at a rate of 10°C/min, PE-0 PE-1 then a second heating to 250°C at a rate of 10°C/min was carried PE-5 out. The first heating wiped out the thermal history of the samples, 40 PE-10 the endotherms in the second heating were used to study the melting behavior of the samples. 30 The degree of crystallinity of a polymer can be determined by Torque (Nm)Torque DSC, defined as(5) ∆H(T) 20 X = fm ...... (1) 400 600 800 1000 1200 c oo ∆H(Tfm) Time (sec) where Xc is the weight fraction extent of crystallinity, ∆H(T)fm (b) is the specific enthalpy of fusion measured at the melting point oo Fig. 1. (a) The mixing chamber of Brabender W50EHT (Tm), and ∆H(T)fm is the specific enthalpy of fusion of the mixer; (b) The torque versus time curves during Brabender completely crystalline polymer measured at the equilibrium o oo mixing melting point ( Tm ). For polyethylene, ∆H(T)fm = 295.8 J/g was

1106 IEEJ Trans. FM, Vol. 126, No.11, 2006 Preparation, Characterization and Dielectric Properties of Epoxy

used(6). The melting point is also used to determine the thickness of the lamella with Gibbs-Thomson equation(7).

o 2Tγ m ...... L = o (2) ρ∆H(Tfm- T) m where L is the lamella thickness, γ is the lamella surface free o energy, ρ is the density of the polymer. For polyethylene, Tm =

414.2 K, Wunderlich published 2γ/( ρ∆Hf ) = 0.627 nm (in the (8) original paper , the equation appears in 2γ / ∆Hf = 0.627, we think a density ρ should be included, also see Hohne(7)) with data collected by Illers and Hendus(9). For the epoxy nanocomposites, the samples were firstly subjected to a heating procedure from 20 to 250 °C at a rate of 10 °C/min, followed by cooling to -50 °C at a rate of 10 °C/min, then a second heating to 250 °C at a rate of 10 °C/min was carried out. The first heating erased the previous thermal history of the (a) samples, and the DSC spectra of the second heating were used to study the glass transition. The Tg was determined from the midpoint of the corresponding glass transition region. 3.3 Dielectric Spectroscopy Measurement The dielectric spectroscopy of the samples was measured in the frequency domain from 10-2 Hz to 106 Hz at different temperatures for the polyethylene nanocomposites, the epoxy nanocomposites, and the nano-alumina powder by using a Novocontrol ALPHA-A high resolution dielectric analyser. Prior to dielectric measurements, the gold electrodes were deposited onto both surfaces of the nanocomposite specimens by sputtering. The diameter of sputtered electrodes is 15mm for the polyethylene nanocomposites, and 30mm for the epoxy nanocomposites. For the nano-alumina powder, a Novocontrol parallel plate liquid sample cell BDS1308 was used. In order to get rid of the effect of absorbed water, all samples were dried before the dielectric measurements. The drying condition is 80°C, 4hrs, under nitrogen (b) gas flow for the polyethylene nanocomposites, and 130°C, 4hrs, Fig. 2. (a) SEM micrograph of the fracture surfaces of under nitrogen gas flow for the epoxy nanocomposites. Epoxy-5 (×5k); (b) TEM micrograph of Epoxy-5 (×50k)

4. Results and Discussion of nano-alumina increases to 5 wt% and 10 wt%. The decrease of 4.1 Microscopic Observation Figure 2(a) shows the the melting point and enthalpy of fusion by the incorporation of SEM micrographs of the fracture surfaces of Epoxy-5 (×5k). The the alumina can be seen clearly in Fig. 3(b). alumina clusters in the fracture surface of Epoxy-5 are clearly seen. As all the samples show one melting endotherm which suggests The size of clusters is below 200 nm. The picture suggests a fine a unimodal lamella population, we can calculate the degree of dispersion of the alumina fillers in the epoxy resin. crystallinity and the nominal thickness of the lamella using the Figure 2(b) show typical TEM micrographs of Epoxy-5 (×50k). enthalpy of fusion and melting point according to Equations 1 and It is seen that the primary alumina particles are very small, their 2, respectively. The results show that both the crystallinity or sizes are around 10 - 15 nm, but they aggregate to form dendritic lamella thickness decrease sharply with addition of 1 wt% nanoparticle clusters, the size of dense part of the clusters is about nano-alumina (shown in Fig. 4), after which they both tend to 200 nm or less. decrease more slowly with further increasing of nano-alumina. Unfortunately, we were unable to obtain good micrographs of The nano-alumina particles may play the dual role of the polyethylene nanocomposites by SEM and TEM due to the nucleating agent and space occupier during the crystallization flexibility of the material even at -195 °C. The dispersion state of process of the polyethylene nanocomposites. This results in the the samples can be seen from their good transparancy while the decrease of crystallinity and thickness of the lamella. Increasing transparancy is not good for the polyethylene composites with the content of nano-alumina from 1 wt% to 5 and 10 wt% does not microsized fillers. decrease the melting point, crystallinity and thickness of the 4.2 Differential Scanning Calorimetry Results Figure lamella very much, despite there being a significant increase in 3(a) shows the melting behavior of the polyethylene viscosity of the composite melt phase. nanocomposites in DSC scans. The neat LLDPE (PE-0) shows the Typical DSC spectra of the epoxy nanocomposites are presented highest melting point, which is observed at 133.0°C at the peak. in Fig. 5(a). The measured Tg values are plotted as a function of The melting point decreases sharply to 127.9°C with addition of the nano-alumina content as shown in Fig. 5(b). The results only 1 wt% nano-alumina, it then decreases slightly as the content indicate that 1 wt% nano-alumina increases the Tg by 4.2°C, and 5

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Epoxy-0 Epoxy-1 Epoxy-5

Heat Flow PE-0 PE-5 Flow Heat PE-10

0 40 80 120 160 200 100 120 140 160 180 200 o Temperature ( C) o Temperature ( C) (a) (a) 140 220 165

C) melting point

o 210 135 enthalpy of fusion 200 130 160 C) 190 o

125 Tg ( 180 155 Melting point ( point Melting 120 170 0 2 4 6 8 10 150 0 1 2 3 4 5 Alumina Content (wt%) (b) Alumina Content (wt%) Fig. 3. (a) Melting behavior of the polyethylene (b) nanocomposites; (b) Melting point and enthalpy of the Fig. 5. (a) DSC curves of the epoxy nanocomposites; (b) polyethylene nanocomposites as a function of alumina content Glass transition temperature of the epoxy nanocomposites as a function of alumina content 80 40 Lamella Thickness (nm)

Crystallinity but moves very slightly to lower frequency with increasing Lamella thickness temperature above －10°C. This movement can be seen by the 70 30 decrease of tanδ at the right side of fp (the frequency of maximal tanδ) and the increasing of tanδ at the left side of fp; it is difficult 60 20 to distinguish the peak at temperatures above 10 °C. PE-5 shows a more broad tanδ peak at low frequency region as Crystallinity (%) Crystallinity shown in Fig. 6(c). This peak is thought to be superimposed by 50 10 two or more peaks at this frequency range. The dielectric spectra 0 2 4 6 8 10 remain almost unchanged in the temperature range of –50 to Alumina Content (wt%) –10 °C. The peak then moves very slowly to low frequency with Fig. 4. Crystallinity and lamella thickness of the polyethylene increasing temperature while the tanδ decreases at the right side of

nanocomposites as a function of alumina content fp and increases at the left side of fp. The loss peak of PE-5 is wider but lower than that in PE-1. wt% nano-alumina increases the Tg by 8.2 °C. PE-10 shows an even broader loss peak at low frequency as The increase of Tg may be explained by the interaction between shown in Fig. 6d. The dielectric spectra remain almost unchanged the nano-alumina and epoxy, which reduces the mobility of in the temperature range of –50 to –10°C. The peak then moves network molecular segments near the filler surface. very slowly to low frequency with increasing temperature while

4.3 Dielectric Properties of Polyethylene Nanocomposites the tanδ decreases at the right side of fp and increases at the left Figure 6(a) shows the dielectric dissipation factor of the neat side of fp. The tanδ peak of PE-10 is wider than the tanδ peak of LLDPE as a function of frequency at selected temperatures from PE-5. －50 to 70°C. No loss peak is observed, which shows, as expected, It is obvious that these loss peaks are due to interfacial no dielectrically active processes in the neat LLDPE. Dry polarization because no diploar response is evident in either the nano-alumina powder also does not show any loss peak in this pure polyethylene or nano-alumina powder. It should be pointed frequency range (not shown in here). out that all the loss peaks found in alumina filled polyethylene are In the LLDPE composite with 1 wt% nano-alumina (PE-1), the always broad in the frequency domain, which suggests they arise loss behavior is very different with that in the neat LLDPE as from a group of dielectric relaxations. The dispersion of the shown in Figure 6(b); a peak centred at about 2 × 10-2 Hz at interfacial polarization peak in the frequency domain is explained －50°C is observed. This peak does not change its shape and by the fact that interfacial relaxation is easily readily affected by position very much in the temperature range of －50 to －10°C, the nature of the interface and its local environment.

1108 IEEJ Trans. FM, Vol. 126, No.11, 2006 Preparation, Characterization and Dielectric Properties of Epoxy

-3 -3 10x10 10x10 o o -50 C -50 C o o 8 -20 C 8 -30 C o o 10 C -10 C o o δ 6 30 C δ 6 10 C o o 70 C 30 C

Tan Tan o 4 4 70 C

2 2

0 0 -2 -1 0 1 2 3 4 5 6 -2 -1 0 1 2 3 4 5 6 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10

f (Hz) f (Hz) (a) (c) -3 -3 10x10 10x10 o o -50 C -50 C o o 8 -30 C 8 -30 C o o -10 C -10 C o o

δ 6 10 C δ 6 30 C o o 30 C 50 C o

Tan o Tan 4 70 C 4 70 C

2 2

0 0 -2 -1 0 1 2 3 4 5 6 -2 -1 0 1 2 3 4 5 6 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 f (Hz) f (Hz) (b) (d) Fig. 6. Frequency dependence of the dissipation factor from –50 to 70oC; (a) PE-0; (b) PE-1; (c) PE-5; (d) PE-10

It is difficult to obtain the fine dispersion of individual temperatures from -50 to 90°C. A broad loss peak in the low nanoparticles in a polymer matrix like the LLDPE because the frequency region is clearly seen in all Epoxy-0 (Fig. 7(a)), LLDPE has high melt viscosity and nanoparticle aggregation is Epoxy-1 (Fig. 7(b)) and Epoxy-5 (Fig. 7(c)), and no obvious strong, and this is more serious at high alumina content as this difference in position, shape and intensity of this peak is found creates higher melt viscosity in the system (see Fig. 1(b)). As a among all these three samples at each temperature. Because consequence, we can expect a distribution of aggrgated filler interfacial polarization appears in this frequency region, the results structures with various interfacial regions having different suggest no significant interfacial polarization induced by dimensions and shapes etc. which results in the broad dispersion incorporating the nano-alumina in epoxy resin. of the interfacial polarization peak. It is easy then to understand This dissipation factor peak found in the epoxy nanocomposites that the width of the loss peak increases with increasing alumina appears in the low frequency region at about 2-3 Hz at –50°C, and content. it moves to a higher frequency with increasing temperature to It is very interesting that dielectric spectroscopy of this type 30°C (the peak is difficult to distinguish above 30°C). This polymer nanocomposite reveals information on particle dispersion. relaxation process is well described by the Havriliak-Negami This suggests that the dispersion of nanoparticles and clusters (HN) functional form(11), and has Arrhenius temperature could be evaluated more critically using dielectric measurements. dependence with an activation energy of 67 kJ/mol. It is ascribed These interfacial polarization peaks do not show the usual to the local motions of the epoxy network β relaxation. A β Arrhenius-like shifts to high frequency with increasing relaxation process at similar frequencies with an activation energy temperature; this is currently not understood. However, the peak of 60 kJ/mol on a DGEBA/diethylene triamine system was moving to low frequency with increasing temperature above reported by Mijovic and Zhang(12). They suggested the β process -10°C is due to the existence of adsorbed water in the is associated with hydroxyl groups. The β relaxation is also nanocomposites. Bohning et al. found that this unusual dielectric reported to be related to the hydroxyether groups phenomena can be explained by a redistribution of water –CH2-CHOH-CH2-O- or to the diphenylpropane units (13)~(14) moleculaes between a locally bonded state in the interfacial region –O-C6H4-C(CH3)2-C6H4- by other researchers . The DGEBA and a more freely dispersion in the polymer matrix(10). The water resin used in this research has a low concentration of hydroxyl content of the PE nanocomposites after drying treatment (see groups as the initial resin is a low molecular weight diepoxide. Experimental Section) is roughly around 0.06 % as determined by 4.5 Difference in Dielectric Properties between thermogravimetric analysis (TGA). Polyethylene and Epoxy Nanocomposites The results show 4.4 Dielectric Properties of Epoxy Nanocomposites that the incorporation of alumina has very different effects on the Figure 7 shows the dielectric loss behaviour of the epoxy dielectric behaviours of the two polymer systems. In nonpolar nanocomposites as a function of frequency at selected polyethelene nanocomposites, significant interfacial polarization is

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-3 15x10 Therefore, these two materials are very dissimilar. Although the alumina particles may act as a nucleating agent in the crystallization process of the polyethylene nanocomposites, there 10 is no obvious bond between the alumina particles and δ polyethylene matrix. Thus, it is easy to speculate that the

Tan Tan interfacial region in the polyethylene nanocomposites is highly 5 electrically insulating because neither the alumina nor polyethylene has a high volume or surface conductivity. When the polyethylene nanocomposite is subjected to an ac electric field, the 0 -2 -1 0 1 2 3 4 5 free charges, which are injected from electrodes or ionized 10 10 10 10 10 10 10 10 impurities in the polyethylene, move to the vicinity of the alumina f (Hz) particles, and are trapped in the interfacial area. This may happen (a) easily because the alumina particles are typical polar compounds -3 which attract the counter free charges. These charges may be 15x10 captured or bonded in a limited area because of the environments and the high insulating nature of the interface. Chin et al.

suggested that the surface hydroxyl groups of TiO2 specimens 10 (15)

δ acted as the traps of the holes in photoconduction . The captured free charges in the nanocomposite will be countered by charges Tan Tan 5 and form dipoles at ther interface which result in additional Maxwell-Wagner-Sillars polarization. As a consequence, the polarization in the polyethylene nanocomposite is much enhanced. 0 This is why the interfacial polarization is more easily seen in the -2 -1 0 1 2 3 4 5 10 10 10 10 10 10 10 10 polyethylene nanocomposite. In contrast, the epoxy nanocomposite is very differnet. The f (Hz) hydroxyl groups on the alumina particle surface may form (b) hydrogen bonds with the polar groups in epoxy resin such as the -3 15x10 ether groups, carbonyl groups, hydroxyl groups and ester groups. Therefore, there are less free surface hydroxyl groups which act as holes traps, and there is a high density of hydrogen bonds in the 10 interfacial region, which leads to a very different interfacial δ structure from that in the polyethylene case. Work on phosphoric

Tan Tan acid doped polymide carried out by Pu et al. suggested that the 5 hydrogen bonds played an important role in the electrical conductivity(16). It is reasonable to assume that the interfacial 0 region in the epoxy nanocomposites has a much higher electrical -2 -1 0 1 2 3 4 5 conductivity than either the epoxy or alumina bulk. When the 10 10 10 10 10 10 10 10 epoxy nanocomposite is subjected to an ac electric field, free f (Hz) charges, which are injected from electrodes or from ionized (c) impurities in the epoxy, likely migrate to the vicinity of the alumina particles, the interfacial area. However, the interfacial Fig. 7. Frequency dependence of dielectric loss of epoxy region is much more conductive in this case. The free charges are nanocomposites at selected temperatures: －50, －30, －10, more mobile in the interfacial layer under the external field o 10, 30, 50, 70, and 90 C (from left): (a) Epoxy-0; (b) Epoxy-1; (overcoming the other forces), they may move away along the (c) Epoxy-5 overlaping conductive interfacial layers in the dendritic filamentary nanoparticle clusters (see TEM), or neutralise with clearly seen. However, in polar epoxy nanocomposites, no obvious counter-charges in the interface layer. As a result, no blocked interfacial polarization is found. The different nature of the charges stay in the interfacial region, and no significant interfacial region is thought to be the reason. In the following, a Maxwell-Wagner-Sillars polarization can be found. This simple model based on the electrical conductivity of the interface is explanation now needs to be modelled and tested . proposed to explain the experimental phenomena. 5. Conclusions Nanometric size particles have high surface-to-volume ratio, hence the nanoparticle-filled polymer has a much higher volume Fine dispersions of nanometric size aluminium oxide in both fraction of the interfacial region than the composites filled with LLDPE and epoxy/anhydride resins have been achieved but conventional fillers at the same loading. The dielectric properties aggregated nanoparticle structures persists. In epoxy of the interfacial region play an important role in the overall nanocomposites, the primary alumina particles are very small with dielectric properties of the nanocomposites. the size around 10-15nm and they aggregate to form filamentary In polyethelene nanocomposites, the alumina particle has nanoparticle clusters with a size up to 300 nm. The DSC results hydroxyl groups at its surface and the polyethylene has no polar show that the melting point and enthalpy of fusion of the groups in its structure except for impurities and oxidation centres. polyethylene nanocomposites decrease with the incorporation of

1110 IEEJ Trans. FM, Vol. 126, No.11, 2006 Preparation, Characterization and Dielectric Properties of Epoxy

(11) S. Havrilia and S. Negami : “A Complex Plane Representation of Dielectric nano-alumina. The DSC results also show that the Tg of the epoxy nanocomposites with up to 5 wt% nanoalumina increases by up to and Mechanical Relaxation Processes in Some Polymers”, Polymer, Vol.8, No.4, p.161-210 (1967) 8.2°C compared with the unfilled epoxy resin. These results (12) J. Mijovic and H. Zhang : “Local dynamics and molecular origin of suggest strong interactions exist between the nano-alumina and polymer network-water interactions as studied by broadband dielectric polymer. relaxation spectroscopy, FTIR, and molecular simulations”, Macromolecules, Vol.36, No.4, pp.1279-1288 (2003) Dielectric measurements show that the incorporation of alumina (13) E. Butta, A. Livi, G. Levita, and P. A. Rolla : “Dielectric Analysis of An has very different effects on the dielectric behaviour of the two Epoxy-Resin During Cross-Linking”, J. Polymer Sci. Part B-Polymer Phys., polymer systems. In nonpolar polyethelene nanocomposites, Vol.33, No.16, pp.2253-2261 (1995) significant interfacial polarization is clearly seen. However, in (14) J. M. Pochan, R. J. Gruber, and D. F. Pochan : “Dielectric-Relaxation Phenomena in a Series of Polyhydroxyether Co-Polymers of Bisphenol-A - polar epoxy nanocomposites, no obvious interfacial polarization is Endcapped Polyethylene-Glycol with Epichlorohydrin”, J. Polymer Sci. found. This is explained by the different nature of the interface in Part B-Polymer Phys., Vol.19, No.1, pp.143-149 (1981) the two kinds nanocomposites. In polyethelene nanocomposites, (15) J. Chin, S. Scierka, T. Kim, and A. Forster : “A photoconductivity technique for the assessment of pigment photoreactivity”, Proc. of the 81th Annual the interface is highly insulating, and acts as a trap for carriers Meeting of the FSCT (2003) under an electric field. This gives a distinct dielectric loss peak in (16) H. T. Pu, L. Qiao, Q. Z. Liu, and Z. L. Yang : “A new anhydrous proton the low frequency due to resulting interfacial polarization. We conducting material based on phosphoric acid doped polyimide”, European Polymer J., Vol.41, No.10, pp.2505-2510 (2005) believe this peak is sensitive to the aggregation structure of nanoparticles and clusters and with increasing concentration of nanfiller the loss peak becomes more significant and broader. In epoxy nanocomposites, the interface is considered to have a higher electrical conductivity than either the epoxy or alumina bulk. The mobile carriers in the conductive interfacial layer may move along Chao Zhang (Non-member) obtained his BEng and MEng degrees the overlaping conductive interfacial layers in the filamentary in electrical engineering from Xi’an Jiaotong University nanoparticle clusters, or neutralise with counter-charges. As a in China, and Ph.D. degree in electrical engineering result, no significant charge trapping occurs in the interfacial from Nagoya University in Japan, in 1984, 1987 and region, and no Maxwell-Wagner-Sillars polarization is observed. 2002, respectively. After his MEng he worked as an (Manuscript received Feb. 3, 2006, revised July 6, 2006) academic staff at Sichuan University in China where his work involved researching and teaching of electrical References materials. Chao joined the PRC in 2004 and is currently working on high performance insulating nanocomposites. (1) T. J. Lewis : “Interfaces are the dominant feature of dielectrics at the

nanometric level”, IEEE Trans. Dielect. Elect. Insulation, Vol.11, No.5, pp.739-753 (2004) (2) T. Tanaka, G. C. Montanari, and R. Mulhaupt : “Polymer nanocomposites as Ralf Mason (Non-member) obtained his PhD in Polymer Science in dielectrics and electrical insulation-perspectives for processing technologies, the United States having previously studied chemical material characterization and future applications”, IEEE Trans. Dielect. Elect. Insulation, Vol.11, No.5, pp.763-784 (2004) engineering in the UK. On returning to the UK he (3) J. K. Nelson and J. C. Fothergill : “Internal charge behaviour of joined the PRC in 2002 working on polymer nanocomposites”, Nanotechnology, Vol.15, No.5, pp.586-595 (2004) nanocomposites based on polyethylene and layered (4) M. Kozako, N. Fuse, Y. Ohki, T. Okamoto, and T. Tanaka : “Surface degradation of polyamide nanocomposites caused by partial discharges silicate clays. He is currently developing tools for the using IEC(b) electrodes”, IEEE Trans. Dielect. Elect. Insulation, Vol.11, efficient qualification and recycling of engineering No.5, pp.833-839 (2004) thermoplastics and he manages a Network on the (5) Y. Kong and J. N. Hay : “The measurement of the crystallinity of polymers Sustainable Use of Materials in Electrical and Electronic Products - by DSC”, Polymer, Vol.43, No.14, pp.3873-3878 (2002-6) (6) J. Mark, Ed. : Physical Properties of Polymers Handbook, Woodbury: SUMEEPnet. American Institute of Physics (1996) (7) G. W. H. Hohne : “Another approach to the Gibbs-Thomson equation and the melting point of polymers and oligomers”, Polymer, Vol.43, No.17, pp.4689-4698 (2002-8) Gary Stevens (Non-member) obtained his PhD in Solid State (8) B. Wunderlich and G. Czornyj : “Study of Equilibrium Melting of Polymer Physics from Queen Mary College in London Polyethylene”, Macromolecules, Vol.10, No.5, pp.906-913 (1977) in 1975. After working for the UK power industry in (9) K. H. Illers and H. Hendus : “Melting Point and Crystal Size in dielectrics research and other topics from 1975 to 1994 Polyethylenes Crystallized from Melts and Solution”, Makromolekulare Chemie-Macromolecular Chemistry & Phys., Vol.113, No.APR, p.1-22 he established the Polymer Research Centre at the (1968) University of Surrey in 1994. He is Director of the PRC (10) M. Bohning, H. Goering, A. Fritz, K. W. Brzezinka, G. Turky, A. Schonhals, and also Professor of Polymer Science at the University and B. Schartel : “Dielectric study of molecular mobility in poly (propylene-graft-maleic anhydride)/clay nanocomposites”, Macromolecules, of Surrey. He is also Chairman of the Dielectrics Group Vol.38, No.7, pp.2764-2774 (2005) of the UK Institute of Physics.

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