Carbon nanotubes are of great interest as a filler for polymer composites due to their unique properties such as high electrical and thermal conductivity, ultrahigh mechanical strength, high ratio length/diameter (~1000) at nanosized value of diameter. Several recent reviews summarizing different aspects of the composite investigations display a broad spectrum of properties useful for production of sensitive electrodes, sensors for chemical vapours, electromagnetic radiation shielding materials, electrical heaters, as well as pressure, deformation and temperature sensors and photovoltaic cells (Thostenson et al., 2001; Popov, 2004; Breuer & Sundararaj, 2004; Ramirez, 2005; Moniruzzaman & Winey, 2006; Baibarac & Gomez-Romero, 2006; Rajesh et al., 2009; Spitalsky et al., 2010). More substantially the methods of preparation and properties of CNT based polymer composites are described in new book (Mittal, 2010). Specials interest is represented by electrical properties of the composites because of the variety of potential applications.
Such systems are represented in a form of the polymer matrix containing conductive filler creating the conductive network. It seems promising to prepare composite materials with small conductive filler content, which preserves mechanical properties of a polymer along with high electrical conductivity. Random filler distribution typically shows the value of percolation threshold (i.e. critical filler concentration at which a conversion from nonconductive to conductive state occurs) within 10–30% for dispersed metals and 5-15 % for carbon black, for example (Foulger, 1999; Mamunya et al., 2002a). Selectively localized conductive particles in a polymer matrix can form the ordered network of conductive phase creating so-called segregated systems. Considerably lower value of percolation threshold
Segregated polymer composites can be formed either by technological methods in the composites processed by hot compacting or in the polymer blends by filler localization inside one polymer component and on the interface. In this chapter the conditions of creation of the ordered distribution of conductive filler (namely carbon nanotubes) in polymer matrix and electrical/physical properties of the segregated composites prepared by hot compacting method have been considered and analyzed.
2. Development of the segregated system approaches
First such a term “segregated distribution” was proposed in ref. (Malliaris & Turner, 1971) where the authors investigated electrical conductivity of the system based on PE powder and metal (Ni) powder. The scheme of processing of the polymer-metal composite with ordered, segregated distribution of conductive filler is presented in Fig.1. On the first stage of processing the “shells” of small metal particles around randomly distributed large polymer particles are created in the initial mechanical mixture at condition
After hot compacting (compression at temperature of the polymer softening) the initial distribution of metal particles remains essentially unchanged on the boundaries between polymer grains and forms observed pattern of the segregated structure. While the polymer particles are deformed under pressure and conglomerated creating solid polymer matrix. The Malliaris & Turner model predicts appearance of conductivity (percolation threshold) due to formation of continuous chains of the metallic particle contacts in a monolayer at a volume percent of
Evolution of the conductive phase structure as a result of growth of the filler content is shown in Fig. 2. At very low volume filler content
Such a type of system can be characterized by two values of filler concentration, the average concentration
Very important for attainable electrical parameters of segregated composites are the conditions of processing. Increasing of mixing time of the powders mixture from 25 to 100 min gives a value of the composites resistivity four orders of magnitude lower due to better distribution of small conductive particles on the surface of polymer particles and breakdown of their aggregates. Increasing of time and pressure of hot compacting promotes the lower values of resistivity as well (Bouchet et al., 2000). In contrast of this, in PE/carbon black composites the pressure did not influence on the composites morphology whereas higher temperature and longer time of pressing led to the penetration of particles of carbon black into polymer grains and increased resistivity of composites (Chan et al., 1997). Mechanical and rheological properties of polymer matrix influence on segregated structure as well. In composites based on methylmethacrylate, buthylacrylate and carbon black the value of
Since first detailed study of Malliaris & Turner, 1971 many investigations of segregated systems based on metal (and ceramic) particles as conductive filler were fulfilled, for example (Boushet et al., 2000; Bridget et al., 1990; Kusy, 1977; Kusy & Corneliussen, 1975; Lebovka et al., 2006; Mamunya et al., 2002b, 2002c; Privalko et al., 2000; Yacubowicz et al., 1990b). Extensive study of segregated polymer/metal systems and their possible applications were presented elsewhere (Kusy, 1986) where the prominent achievements in this area have been summarized. The polymer/carbon black composites were thoroughly studied as well (Chan et al., 1997; Chiteme & Mclachlan, 2000; Yacubowicz et al., 1990a; Youngs, 2003; Zhang et al., 2005). Segregated structure of conductive filler was also obtained in emulsions where the particles of conductive filler surround the particles of emulsified polymer (Bridge et al., 1990; Grunlan et al., 2001; Kim et al., 2008; Miriyala et al., 2008). The values of percolation threshold in poly(vinyl acetate)/carbon black composites prepared with using a solution (random distribution of filler) or an emulsion (latex) were 8.18 and 1.2 %, respectively (Miriyala et al., 2008).
A variety of models have been proposed to describe the electrical properties of segregated polymer systems. First model that represented the segregated structure of metal particles in a form of planar hexagonal, square and triangular lattices was not agreed well with experimental values of
The geometry of the shell structure model implies:
From this the following expression can be obtained for the percolation threshold
3. Carbon nanotubes in segregated systems
The investigations of segregated systems have got a new impulse with expansion of the carbon nanotubes area, namely as conductive filler in polymer composites. It is caused by possibility to reach still lower value of percolation threshold than for carbon nanotubes and segregated systems separately. Authors (Mierczynska et al., 2004) formed a segregated structure using UHMWPE and single-walled carbon nanotubes (SWCNT) and obtained the value of the percolation threshold equal to 0.5-1.5% depending on type of SWCNT. In next extensive paper the authors (Mierczynska et al., 2007) have shown the influence of CNTs type on the value of
was equal to 1.15 whereas a theory predicts the value of
Introduction of CNTs into polymer emulsions is of benefit to creation of the conductive segregated structure with low value of percolation threshold, for example
4. Segregated PVC/MWCNT and UHMWPE/MWCNT systems
An excursus in a history of development of the segregated systems indicates that polyvinyl chloride is the most acceptable polymer for creation of segregated composites processed by hot compacting due to its high viscosity, a variety of the powdered PVC types with different size of particles manufactured by industry, wide temperature range of softening (as a result of its amorphous structure) that facilitates a processing (Kusy, 1986). Second attractive polymer is UHWMPE which has very high viscosity, acceptable temperature range of the processing and good mechanical properties.
4.1. The processing features of segregated PVC/MWCNT composites
Electrical, thermal conductivity and dielectric properties of the PVC/MWCNT segregated system depending on concentration of the nanotubes in wide temperature and frequency range have been studied.
Polyvinyl chloride was used in the powder form with average size of particles of 100 μm and density of 1.37 g/cm3. Ultrahigh molecular weight polyethylene Hostalen GUR, type GHR 8110, in a powdered form with average size of particles about 100 μm and density of 0.95 g/cm3 was used. The multiwalled nanotubes were produced by TMSpetsmash (Ukraine) using CVD-method. The MWCNT typically had diameter
Fig. 5 represents a transformation of PVC/MWCNT structure from mechanical mixture of PVC powder and CNTs (a) to hot compacted segregated composite (c). There is only one distinction compared to Fig.1, namely the presence of intermediate stage (b) that provides the even distribution of tangled CNTs on the surface of polymer grains. It is indispensable condition of the hot compacted system having perfect conductive network without aggregates. In this case one can expect to obtain the conductive composite with low value of the percolation threshold. This has been reached by preliminary solution of MWCNT in ethanol
with following exposition during 20 min under ultrasonic action with frequency of 22 kHz. The sonicated MWCNT and PVC powder were homogenized by thorough grinding in a porcelain mortar to the visually homogeneous state. Homogenized composite was placed into a hot steel mold heated to 145º C and then pressed (hot compacted) during 5 min at 20 MPa with subsequent cooling of the mold in the air flow to room temperature. The samples of pressed composites used for dielectric and electrical measurements were produced as discs with 30 mm diameter and 1-1.5 mm height.
The DC electrical conductivity was measured using a two-contact scheme. The values of DC conductivity
4.2. Structure of segregated PVC/MWCNT system
The structure of hot compacted PVC/MWCNT system was studied by optical and electron (SEM) microscopy. Figs. 6 and 7 display the segregated structure of composites. Optical microscope images of the segregated PVC/MWCNT structure are presented in Fig. 6. It shows the evolution of the composite structure along increase of the MWCNT concentration from 0.02 % that is below percolation threshold
Hot pressing deforms polymer particles and results in formation of compacted continuous polymer phase, where conductive paths of CNTs are located on the boundaries between particles. Fig. 7 demonstrates the SEM images of PVC/MWCNT composite prepared by the fracture in liquid nitrogen. It is seen a granular structure of the polymer matrix with granules covered by nanotubes (Fig. 7-a). The presence of nanotubes on the surface between grains displays Fig. 7-b.
4.3. Electrical properties of segregated PVC/MWCNT composites
The concentration dependence of DC conductivity
Insertion to Fig. 8 shows scaling of
The value of
Described above models (see section 2) predict decrease of the percolation threshold
Temperature dependencies of conductivity of the PVC/MCWNT composites are shown in Fig. 10. The increase of filler content changes the form of the curves. In pure PVC and composite with 0.04 % MCWNT the conductivity increases with rise of temperature (curves 1, 2). Therefore it is possible to assume ionic character of conductivity in these systems as a growing of conductivity with temperature is the feature of ionic conductivity and is caused by rise of the ionic mobility (Margolis, 1989; Blythe & Bloor, 2005;). The temperature dependence is found to be composed of two linear regions with a bend at temperature of glass transition Tg. The transition from glassy state to high-elastic state leads to stronger dependence of conductivity on temperature that can be related to lightened ionic mobility in the polymer state with heightened molecular mobility. These dependencies are represented by Arrhenius plot in Fig. 11:
Crossing glass transition temperature the value of activation energy sharply increases as a result of rise of the charge carrier mobility at temperature higher of Tg. Such values of the activation energy are typical for ionic conductivity, for example the close values of
Even insignificant excess of the filler concentration over percolation threshold leads to the change of the conductivity character, thus at MWCNT content equal to 0.054 % the conductivity
4.4 Dielectric properties of PVC/MWCNT and UHMWPE/MWCNT systems
Fig. 12 represents dielectric constant
The frequency dependence of dielectric parameters in two-phase conductive-insulating system can be considered with two models, such as intercluster polarization (IP) that implies polarization effects between clusters inside percolation system or anomalous diffusion (AD) within cluster (Song et al., 1986; Yoon & Lee, 1990; Youm & Lee, 1991). The IP model predicts the power-law dependence of
In the AD model the values of
Plot of conductivity versus frequency in a double logarithmic scale (Fig. 12) shows two cases of the frequency dependence of AC conductivity
For higher concentration of the filler in the composites (in the range of 0.201–0.672%) the values of
The comparison of conductivity and dielectric parameters (
Dielectric characteristics, measured at fixed frequency 1 kHz, demonstrate the percolation behaviour as well (Fig. 13). In the region above percolation threshold
It is seen from Fig. 13 that the interval of tan
A very low concentration of MWCNT in the composites and a specific type of their distribution as the segregated nanotube shell structure remains most part of the polymer in the neat state within framework. Consequently, one could expect the dielectric properties of polymer/MWCNT composites close to those ones in the unfilled polymer. However, the experimental data reveal very high contribution of conductive phase in the dielectric response. We can assume that majority of nanotubes creating the framework takes part in conductivity due to their high local concentration
4.5. Thermal conductivity of PVC/MWCNT composites
The thermal conductivity values of PVC/MWCNT composites and their associated uncertainties are plotted in Fig. 14 versus MWCNT volume content
Dependence of the thermal conductivity
the heat flow transport, thus resulting in the interfacial resistance. Second effect is the presence of the volume of a new phase with high thermal conductivity, which results in increase of the heat flow. Our results (Fig. 14) demonstrate that the first effect is predominant at low filler concentrations (
Indeed, in spite of a small content of filler in the polymer matrix at
Comparison of dependencies of the PVC/MWCNT electrical conductivity
Experimental results presented in Fig. 13 show that starting from 0.14 vol. % of MWCNT, the thermal conductivity slowly rises with increase of the filler content. For two-phase systems, the concentration dependence of the thermal conductivity can be described by a large number of equations (Progerlhof et al., 1976). All of them lie within the interval between the largest thermal conductivity
to the smallest thermal conductivity
The thermal conductivity of real two-phase systems lies within the interval between functions (13) and (14) and can not be higher than
Some authors (Nan et al., 2003) have proposed a simple equation for description of
Eq. (15) fits our experimental data quite well if the value of
The thermal conductivity of PVC/MWCNT composites, calculated according to eq. (16), is plotted in Fig. 13 for several
Lines 2, 3 and 4 in Fig. 13 represent data calculated according to eq. (14) for
Investigation of electrical conductivity
Frequency dependence of the dielectric parameters
The thermal conductivity k does not reveal any percolation behaviour in the vicinity of electrical percolation concentration φ =φc but exhibits minimum in this region. This effect can be explained by running of two opposite processes: first, the presence of interfacial resistance to heat flow on the polymer-MWCNT boundary that decreases the values of k; and a subsequent increase of the heat flow due to appearance of a noticeable concentration of the filler phase with high thermal conductivity. Lichtenecker’s equation allowed to predict the value of the thermal conductivity of MWCNT, kf = 55 Wm-1K-1. This value is much lower than the theoretically predicted one for nanotubes kf = 103 –104 Wm-1K-1. In the polymer materials filled with conductive particles, both polymeric and filler phases always take part in the heat transport. So, the thermal conductivity value of such a system depends on relative concentrations of the polymer and filler.
My deep gratitude to doctorant V.V. Levchenko for his help in preparation of the manuscript.
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