Dielectric Properties of Nanocomposites

Transcription

1 Chapter 9 Dielectric Properties of Nanocomposites Abstract The dielectric properties such as dielectric constant, volume conductivity, dielectric loss factor and dissipation factor of the nanocomposites were examined in the frequency range of 500 Hz - 5 MHz and as a function of filler content. The dielectric properties were found to be higher for layered silicate filled nanocomposites due to the conductive network formed as a result of the strong interaction of fillers with the polymer matrix. A comparison of the dielectric properties of the different nanoclay/ chlorobutyl rubber systems was done. The results of this chapter have been communicated to Composite science and Technology.

2 9.1 Introduction Polymer nanocomposites with better dielectric and electrical insulation properties are slowly emerging as excellent functional materials for dielectrics and electrical insulation application and the term nanodielectrics for such materials is increasingly becoming popular. Although the technology of addition of fillers to polymers to enhance a particular dielectric property has been in existence for several decades [1-3], the effect of filler type on the dielectric property of the polymer composites has not been understood fully. It is with the advent of nanotechnology leading to the availability and commercialization of nanoparticles that polymer nanocomposite technology started to gain momentum. Polymer nanocomposites have been found to exhibit enhanced physical, thermal and mechanical properties when compared to the traditional polymer materials and that too at low nano-filler concentrations (1-10%) [4-6]. But it is only recently that the dielectric properties of such polymer nanocomposites were looked into and limited research results demonstrate very encouraging dielectric properties for these materials. Irrespective of the type of base polymer material (thermoplastic or thermoset), significant enhancements in several physical properties, like thermal conductivity (with conducting fillers) or dielectric properties like resistivity, permittivity, dielectric strength, tracking and partial discharge resistant characteristics (with insulating fillers) were observed when compared to similar properties in traditional polymer micro composites [7-9]. These observations were mainly attributed to the unique properties of nanoparticles and the large interfacial area in polymer nanocomposites [10, 11]. It has been suggested that addition of inorganic fillers with high polarizability to dielectric elastomers may enhance the dielectric permittivity of the elastomer, leading to a reduction of the required electric field intensity in the actuation process.

3 A careful analysis of literature reveals that non-conducting rubber can be made conductive by the addition of conductive fillers such as carbon black, graphite fibre, metal oxides, metal powders and fibers [12-16]. The electrical properties of various blend systems were reported by different researchers.[17,18] The available literature suggest that the dielectric properties of blend system in general depend on structure, crystallinity, morphology and presence of filler or other additives [19-21]. Zhang and co-workers [22] found that with increasing content of silica, carbon black (CB) particles are optimally dispersed, contributing to the generation of a conductive network between CB particles via direct particle contact and a tunnelling effect. Maximum conductivity for the epoxy resin-cb-silica nanocomposite occurs at a ratio of 0.6:1.0 (SiO 2 :CB). The dielectric properties of nitrile rubber based blends were studied by George et al. [23] and they concluded that the dielectric properties depend on the composition of polar rubber in the blend. According to them, the addition of lithium salt further enhanced the ionic conductivity to about 10-4 S cm -1 without spoiling mechanical properties. 9.2 Results and discussion Dielectric permittivity It is generally believed that dielectric data is characterized by superposition of two processes: a conductivity contribution that produces an increase of both real part ε' and the imaginary part ε'' of the dielectric function on decreasing frequency and a relaxation process exhibiting a maximum in є'' that shifts higher frequency side with increase in temperature. Fig 9.1 shows the variation of imaginary part of dielectric permittivity (ε'') with frequency for various loadings of cloisite 15 A. It is found that at room temperature, ε'' decreases with increasing frequency. The higher value of dielectric loss (ε'') at low frequency is due to the free charge motion within the materials.

4 Permittivity imaginery part 1.0x x10 6 CIIR CIIR/15C 5 CIIR/15C 10 CIIR/15C x x x Frequency KHz Fig 9.1 Variation of permittivity (imaginary) with frequency in cloisite 15 A filled CIIR nanocomposites Figure 9.2 shows the variation of real part of dielectric permittivity (ε') with frequency at room temperature for different loadings of cloisite 15 A. It is seen that the dielectric constant is greater in the case of cloisite 15 A filled nanocomposites than in the case of pristine CIIR matrix. OMMT is a type of filler with a layered structure containing mobile ions on the platelets and counter-ions between them. They can easily polarize along their silicate sheets due to their ionic nature, and external charges can accumulate around silicate platelets due to the large conductivity mismatch with the rubber matrix. Similar results were observed in the case of silicone rubber filled with OMMT clay by Razzaghi et.al.[24].

5 Permitivity real part 1.4x x x x10 6 CIIR CIIR/15C 5 CIIR/15C 10 CIIR/15C x x x Frequency KHz Fig 9.2. Variation of permittivity (real) with frequency in cloisite 15 A filled CIIR nanocomposites. The effective permittivity in nanocomposites is determined by dielectric polarization mechanisms in the bulk of the material. In the present case, they are polarizations associated with CIIR and organoclay and interfacial polarizations at the CIIR-nanoparticle interfaces. It is well known that apart from the nanoparticles, nanocomposites have a large volume fraction of interfaces where interfacial polarizations are most likely to occur. The present investigations utilize uncoated nanoparticles for the experiments which further enhances the prospects of interfacial polarization. But in the range of high frequencies used in the present experiments, interfacial polarizations are unlikely to occur in the nanocomposites. Interfacial or space charge polarizations occur due to the accumulation of space charges at interface boundaries. When an electric field of high frequency as in the present case is applied, the probabilities of these space charges to drift and accumulate at polymer-nanoparticle interfaces become highly remote. Apart from this, most of

6 these space charges usually have large time lags for mobility which makes it even more difficult for them to drift at such high frequencies. Usually, occurrences of interfacial polarizations are observed at lower frequencies of dielectric measurement. Theoretical models have also been extended to consider the influence of interface on nano dielectrics. Tanaka et al. [25] proposed a multi-core model to understand the dielectric properties of polymer nanocomposites. They proposed that the interface of a spherical inorganic filler particle embedded in polymer matrix consists of three distinct regions: a bonded layer (first layer), a bound layer (second layer) and a loose layer (third layer), with an electric double layer overlapping the three layers as shown in figure 9.3. The first layer corresponds to a contact layer where the polymer is in intimate contact with the filler surface, and the second layer corresponds to the interfacial region. Finally, there is the third layer, where the properties of the bulk polymer are approached. The second layer contributes to the reduction in the permittivity by disturbing the motion of dipoles originating from some polar groups. The free volume in the composite, mostly associated with the third layer, also causes a reduction in the dielectric constant of the composite. Though the first layer directly links the particle to the polymer and establishes direct contact, the second and the third layers are suggested to be most influential in affecting the dielectric properties of the polymer composite. Fig 9.3. Indicative picture of the interface in the nanocomposite (Dual layer model) [25].

7 By affixing suitable organic groups to the filler particle, one can directly impact the interfacial region. The selection of the surface organic groups, in terms of polarity, polarizability, mobility, and size could have a major impact on the dielectric properties of the polymer nanocomposites. Due to the presence of different layers, a gradient in the charge mobility is established. The differences in the Fermi levels between the nanoparticles and the polymer gives rise to a surface charge on the nanoparticle. In order to maintain charge neutrality at the interface, a redistribution of charge occurs at the interface leading to a Helmholtz or Stern layer. A diffuse double layer of charge in the matrix polymer exists far away from the interface. In order to understand how the above description of interfacial structure has a positive effect on the dielectric properties of nanocomposites, the following idea has been proposed. The diffuse double layer in the polymer is a region of higher charge mobility and strongly influences the dispersion and dielectric properties of the composite. This double layer, in turn, depends on the charge in the Stern layer. Suitable alteration of the interface results in changes in mobility, free volume and trap sites for charge carriers (in this case, electron). This could explain the higher breakdown strength observed in nanocomposites developed by Ma et al. [26]. They modified titanium dioxide with a polar silane coupling agent and observed a decrease in the mobility of charge carriers in their nanocomposites. Fig 9.4. Dual layer model as applied to nanoclay platelets

8 Tan delta Figure 9.4 shows the dual layer model as applied in the case of layered silicates. The presence of filler in low concentrations eliminates overlapping of local conductive regions and thus prevents premature global breakdown in specimens. Fig 9.5 shows the variation of tan delta with frequency in the case of nanocomposites containing different filler loadings of cloisite 15 A. It is seen that the tan delta values are found to increase with increase in the filler loading. Since interfacial phenomenon is an additional polarization mechanism apart from ionic, electronic and dipolar mechanisms, their occurrence in a system is usually associated with distinct variations in the trends (a steep rise) of tan delta and effective permittivity with respect to frequency, especially at high filler concentration. In the present study, such trends in the variations of the dielectric properties are seen as shown in figure CIIR CIIR/15C 5 CIIR/15C 10 CIIR/15C Frequency KHz Fig 9.5 Variation of dissipation factor (Tan delta) with frequency Surface functionalization of nanofillers does not always improve the dispersion of fillers, as observed by Ma et al. in the same work. The advantage of adding nano-

9 fillers is that they conform to the chain length of the polymer and hence reduce Maxwell-Wagner-Sillar type interfacial polarization arising from the differences in dielectric permittivity of the polymer and filler. Depending on the interaction mechanism of the polymer with the nano filler, an interfacial polymer nanolayer is reported to form on the nano filler surface which is highly immobile due to the strong bonding of the polymer chains and the filler surface [27]. When these immobile nanolayer formations are extended to all the nano fillers in a polymer matrix, it can be expected that the mobility of the polymer segments or chains interacting with these nanoparticles in the nanocomposite are restricted. It has also been mentioned in some other studies that when the length scales of the polymer chain and the filler particles come closer, the interface wall-wall distances between filler particles become smaller and a secondary polymer chain network forms which causes entanglements [27,28]. These entanglements further reduce the mobility of polymer chains in the nanocomposite. The immobility and entanglement dynamics of the polymer chains are a function of the filler concentration and only those polymer chains which come in contact with the nano fillers will become immobile or entangled. The same theories as above can be extended to the case of CIIR nanocomposites in the present study too. In all probability, the CIIR polymer segments interact with the silicate nanofillers causing a restriction in the mobility of these polymer segments. This restriction in turn influences the occurrence of a lower effective permittivity in nanocomposites. In agreement with data reported for similar systems in the literature [7, 16], the large increase of both ε in Figure 9.1 and ε in Figure 9. 2 are to a large extent related to the formation of a percolation structure of the nanoparticles. The low frequency losses may be due to either the Maxwell-Wagner effect [29] as a result of an alternating current (ac) in phase with the applied potential or the direct current (dc) conductivity resulting from the increase of ion mobility, or both Conductivity

10 Conductance (S) At high volume fractions of fillers, the population of conductive particles is just right to bring about uniform dispersion and facilitate the flow of current. This can be explained on the basis of the formation of conductive network by layered silicates having polar groups which facilitates the conducting process. Another reason for the decrease of resistivity (increase of conductivity) with layered silicate addition is attributed to the incorporation of polar groups. This observation is due to the fact that layered silicate can make large conductive contacts as a result of its particle geometry. 2.45x x x10-4 CIIR CIIR/15C 5 CIIR/15C 10 CIIR/15C x x x x Hz 5000Hz 50000Hz Hz Hz Frequency Hz Fig 9.6. Variation of conductance of cloisite 15 A filled chlorobutyl rubber nanocomposites with frequency. In nano silicates, due to very high aspect ratio an effective interaction with the polymer matrix takes place which resulted in enhanced conductivity. As the filler loading increases conductivity also increases due to the enhancement in conductive

11 Conductance (S) interactions. Figure 9.6 presents the impact of filler addition on the conductivity of chlorobutyl rubber matrix and it indicates the increase in the conductivity of the polymer matrix with filler addition. At higher cloisite 15 A contents conductivity increases significantly. In consistency with the dielectric data presented the large increase of conductivity in these samples is attributed to the formation of a percolation structure. 2.5x x10-4 CIIR CIIR/15C CIIR/20C CIIR/10C 1.5x x x Hz 5000Hz 50000Hz Hz Hz -- Frequency KHz Fig 9.7. Variation of conductance of chlorobutyl rubber nanocomposites containing different types of clays (5 phr) with frequency. The effect of surfactants on the conductance of nanocomposites is shown in figure 9.7. The maximum conductance is shown by cloisite 10 A filled nanocomposites at high frequencies. This can be attributed to the presence of long chain aromatic modifier group present in cloisite 10 A Modeling of effective dielectric constant

12 Many theoretical approaches have been developed to predict the effective dielectric constants of polymer composite systems. The volume-fraction average is a simple method to estimate the effective dielectric constant of a polymer composite material: Є eff = φ 1 є 1 +φ 2 є 2 (1) where the subscripts 1 and 2 represent the polymer and the filler, respectively, and φ is the volume fraction of the constituents. According to the volume-fraction average model, the effective dielectric constant of the composite increases sharply at low volume fractions of the ceramic filler. Many studies involving both experiments [30, 31] and theory [32,33] disprove the trend predicted by the above equation. More realistic models are based on mean field theory. Є eff = Є 1 [Є Є 1 2(1-φ 1 ) (Є 1- Є 2 )] (2) [Є Є 1 + (1-φ 1 ) (Є 1- Є 2 )] The Maxwell equation is based on a mean field approximation of a single spherical inclusion surrounded by a continuous matrix of the polymer [34]. Thus Maxwell s equation is strictly valid only as the filler fraction goes to zero, i.e. infinite dilution. In two-phase models, both the constituents of the composite system are considered as different phases rather than considering one constituent of the composite as an inclusion in a continuous phase of another. Figure 9.8 shows the Comparison of experimental and theoretical values of effective dielectric constant with frequency for 5 phr cloisite 15 A filled chlorobutyl rubber nanocomposites. The experimental values are found to be in more agreement with the Maxwell model as suggested by the literature.

13 Frequency Hz 9x10 6 8x10 6 Experimental Volume fraction model Maxwell model 7x10 6 6x10 6 5x10 6 4x10 6 3x10 6 2x10 6 1x Effective dielectric constant Fig 9.8. Comparison of experimental and theoretical values of effective dielectric constant with frequency for 5 phr cloisite 15 A filled CIIR nanocomposites. 9.3 Conclusion The presence of layered silicate in chlorobutyl rubber leads to an overwhelming presence of polar groups giving rise to dipole or orientation polarizability. Maximum dielectric permittivity is shown at lower frequencies and higher filler loadings. The modeling of dielectric properties shows that the experimental values are in more agreement with Maxwell model. The effect of surfactants appear to be more pronounced at higher frequencies and cloisite 10 A filled nanocomposites show higher conductance values at higher frequencies.

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