Carbon nanotubes in multiphase polymer blends
Nowadays, melt blending of two or more polymers in a conventional extruder is one of the cheapest and fastest ways to tailor the material’s properties according to the requirements of novel applications. By combining the various blend phase structures and property profiles with a conductive, high aspect ratio nanofiller such as carbon nanotubes (CNTs), specific morphological structures can be achieved that offer a much higher potential for the development of new functional materials than single-polymer composites. Thus, understanding and control of CNT localization inside the developing phase morphology during melt mixing are the keys to the desired structures and thus to new high performance materials.
Around the turn of the millennium, polymer blends constituted 36% of the total polymer production and are still growing far above average (Utracki, 2002). The combination of conductive, high aspect ratio nanofillers such as carbon nanotubes (CNTs) with the various phase structures and property profiles of polymer blends offers, compared to single-polymer composites, a much higher potential for the development of CNT-based functional materials for commercial applications.
This chapter introduces the most important phenomena regarding melt processing of thermoplastic polymer blends with CNTs and the connected morphological structures and thus the general properties of the resulting blends. The discussion is focused on melt processing as it is by far the most common and easiest production method for polymer blends and thermoplastics.
Generally, the field of CNT-filled polymer blends is closely connected to the other topics of CNT research. CNT production processes and the related surface characteristics, but also the aspect ratios, determine the localization behaviour in multiphase mixtures. The preference of CNTs for specific blend phases provides information about the interactions and interfacial energies of CNTs and polymers in the blend. These are, in turn, crucial factors for the strength of the polymer–CNT interface and the dispersability of primary CNT agglomerates (see Chapter 4).
Understanding and control of CNT localization inside the phase structure of molten polymers during melt blending are the keys to adjusting the macroscopic properties according to the requirements and thus achieve new commercial applications. Therefore, this topic is most comprehensively covered in this chapter, including the state of the art (Section 19.2). The present state of the discussion about the determination and importance of thermodynamic driving forces during melt blending is discussed next (Section 19.3.1). The explicit and fast selective localization of CNTs in one specific blend phase that is a peculiarity of CNTs and can include the CNT transfer form one phase to the other during a typical extrusion process is subsequently described with the help of an example (Section 19.3.2). Subsequently, the impact of the CNT aspect ratio on the peculiarities of the transfer and the localization within the blend morphology is discussed (Section 19.3.3).
Implications of the effect of selectively CNT-filled phases, that are typically obtained during and after melt blending on the final blend morphologies and the melt rheology are addressed in Section 19.3.4. Moreover, the present knowledge about the dynamics and the peculiarities of the CNT transfer from one thermodynamically unfavourable phase to another that can occur in extruders, kneaders or microcompounders is discussed (Section 19.3.5). The peculiarities of melt mixed CNT-filled blends are summarized in Section 19.3.6. A brief look at the possibilities of tailoring the localization of CNTs by utilizing chemically modified CNTs and reactive blend components is also presented (Section 19.4).
Finally, as an example of PC–ABS-CNT blends, the possibility of exploiting the discussed features of blends with CNTs in the manufacture of new functional materials is shown (Section 19.5). At the end of the chapter, a brief future outlook for the further potential of CNT blend nanocomposites is sketched, concerning the properties and the potential of so far unrealized morphologies of blends with carbon nanotubes (Section 19.6).
The focus of this chapter is to summarize the experiences and approaches regarding the morphological features and the localization behaviour of CNTs in the blend phases during melt blending. The arrangement of morphological structures directly determines the macroscopic properties, and therefore understanding and control of CNT localization in typical melt mixing processes such as twin-screw extrusion opens the door to new commercial applications of functional polymer blends. Figure 19.1 shows possible combinations of phase morphologies and nanofiller localizations in a dual-phase polymer blend. These morphologies can also be adjusted with fillers that are not nanoscaled if those are sufficiently small, compared to the blend morphology. For commercial blends with a balanced property profile and sufficient compatibility, the phase size is below the micro-level and thus only nanofillers will fulfil the precondition. For CNTs, due to the greatly differing diameters and lengths that result from the various production processes, a differentiated view is needed in this context. Typical diameters are from 1–50 nm and from 1 to more than 1000 μm length. Therefore, long tubes can be inhibited from penetrating in one blend phase or from positioning at the interface, whereas the diameters are always sufficiently small. In order to utilize combined approaches like those displayed in Fig. 19.1, the sizes of the blend system and the CNTs have to be appropriately selected. Furthermore, sufficient dispersion of the CNT primary agglomerates is also an essential precondition. If these major requirements are met, the combination of nanofillers with new or existing combinations of polymers will enable the rapid design and manufacture of new materials with properties that can be tailored according to the needs of the application.
19.1 Possible combinations of phase morphologies in a dual-phase polymer blend with one nanoscaled filler. Even though the displayed schemes describe the most simple case, as only two phases and one filler are present, the combination of different blend morphologies and filler localizations results in a high variety of possible morphological structures. Sea-island morphologies with different nanofiller localizations might have interesting mechanical properties (a–d) but the potential is limited in terms of electrical conductivity. Co-continuous blends with unique filler distribution in both blend phases (e) can combine the advantages of both blend phases, while the filler provides certain properties or functionality. Double percolated blend structures (f, g) and co-continuous blends with filler-saturated interface (h) are presently the structures with the highest potential for commercial applications, as they can provide maximum blend functionality for minimum filler contents.
Thus, due to the greatly increasing production capacities and decreasing costs of multi-walled carbon nanotubes (MWCNTs), the melt processing of polymer blends with different grades of MWCNTs has attracted significant interest in recent years. The promising experimental results of already published studies that indicate the possibility of manufacturing conductive blends with very low contents of CNTs are one of the reasons that motivate increasing research activity in the field of CNTs in polymer blends (Fig. 19.2). High economic potential can be expected when research leads to understanding and finally enables control of the CNT localization during extrusion. Presently, it is too early for generally valid conclusions and the research is focused on describing the phenomenology of blends with CNTs.
It is important to note that of the various possible combined morphological arrangements (Fig. 19.1), each has unique properties, but only a very few have desirable material properties. Superior properties can be expected specifically from two scenarios, both of them requiring co-continuous blend morphologies: on the one hand, the localization of fillers directly at the interface (Fig. 19.1 (h)), on the other, the selective filling of only one of the blend phases (Fig. 19.1 (g)). Whereas the former is believed to be the ideal scenario to achieve the lowest possible percolation concentrations, it is difficult to undertake and stabilize for high-aspect fillers such as carbon nanotubes. The reasons for this will be discussed in Section 19.3.3. In contrast, the implementation of blends with selective CNT-filled phases is feasible for different blend systems. In this context, the term of double percolation (Fig. 19.1 (g)) was initially introduced by (Sumita et al., 1991) for immiscible blends where the percolation of a selective carbon-black filled blend phase (first percolation) in combination with the formation of a network of carbon black particles within the percolated blend phase (second percolation) enabled the formation of conductive pathways with significantly reduced contents of conductive filler. Since then, the occurrence of double percolation has been described for several blend systems.
Among these, co-continuous, compatible blends of PC and SAN are so far closest to the model scenario due to the almost ideal selectivity and continuity of the structures (Göldel et al., 2009a). Therefore, and as they are unaffected by crystallization phenomena, they are very suitable to discuss as examples of the peculiarities of MWCNTs in polymer blends. Thus, these blends will be addressed and described in different contexts in the following, completing the available contributions from the literature for the specific topics. Other examples of selective localization of MWCNTs in polymer blends with commercially important thermoplastics after melt mixing can be found for HDPE–PC, HDPE–PA6, PET–PVDF, PC–PE; PC–PP, PPS–PA66, PCL–PLA, PVDF–PA6, and PA6–ABS (Meincke et al., 2004; Wu and Shaw, 2004; Pötschke et al., 2005; Pötschke et al., 2007; Pötschke et al., 2008; Zou et al., 2006; Li and Shimizu, 2008; Wu D, 2009) and are summarized in Table 19.1. For SWCNT, presently no related literature is available, most probably as melt blending requires relatively high quantities of CNTs, thus making melt mixing experiments presently extremely expensive.
Note: *The wetting coefficient column denotes whether the wetting coefficient was discussed/whether the prediction of the wetting coefficient is consistent with the observed CNT localization (Y/Y, Y/N).
In the listed literature, commonly MWCNTs are melt-blended with two immiscible polymers, and some specific material properties are discussed and correlated with the observed morphological structures. Versatile proceedings are described and the topic is approached from many different viewpoints, therefore a direct comparison of the different trends that have been observed so far is very difficult. Thus, it is too early to derive general correlations for many important issues. Representatively, and as an issue of central importance, the calculation or measurement of thermodynamic driving forces that determine the CNT transfer during melt mixing is still subject to serious uncertainties and inexplicable exceptions, where existing concepts like the wetting coefficient fail. The possibility of being misguided by the predictions of this equation increases with the decreasing differences in surface tensions and related polar contributions between the two polymer partners involved. As those differences are large for most of the listed examples, specifically for polyolefins with polar polymers, relatively good consistencies of predictions and localization behaviour are obtained. It appears that MWCNTs strictly avoid e.g. the polyolefin phases if another, more polar phase is present during blending. This localization behaviour strongly indicates that most MWCNTs reveal surface polarities that are significantly above those of the neat and ideal graphitic structures. One reason for this is the fact that, due to the specific multi-walled feature of MWCNTs, a small mass percentage of oxygen or nitrogen within the overall carbon content results specifically for a high number of walls in a relatively high density of these elements on the outer tube surface and an increased surface polarity. Consequently, SWCNTs with identical mass fractions of these elements reveal much lower densities on the tube surface and therefore can behave significantly differently. Furthermore, lower tube diameters are supposed to result in more extreme wetting angles, thus reinforcing either the repulsive or the attractive forces towards the respective polymer for SWCNTs.
Actual research reveals two new viewpoints beyond the traditional concepts to describe the localization behaviour of nanofillers. First, the impact of the particle aspect ratio on the speed of the CNT transfer and the stability at the blend interface (Section 19.3.3), and, second, the irreversible absorption of specific polymers on the CNT surface. This phenomenon accounts for so far the only example in literature, where the localization of a significant amount of MWCNTs at the interface of a polymer blend is stable during mixing (Fig. 19.3) and occurs in the absence of any chemical reaction (Baudouin et al., 2010a; Baudouin et al., 2010b). Irreversible absorption of polymers on fillers is found to occur only for a few specific polymers and is a field of science that is not directly connected to the discussion of filler localizations during blending. Nevertheless, if present, it can have a strong impact, as the sections of the tube where the polymer chains are adsorbed are efficiently anchored at the respective blend phase. Attempts are made to decouple the CNT localization from the thermodynamical driving forces by using modified CNTs and a chemically active additive in the blend (Section 19.4).
19.3 MWCNTs in a blend of copolyamide 6/12 (PA6/12) with a copolymer of ethylene and methyl acrylate (EMA). For these polymers, the MWCNTs show a strong deviation from their regular localization in one of the blend phases and a significant amount of MWCNTs can be found at the interface of the polymer blend during melt mixing. Reprinted with permission from Elsevier (Baudouin et al., 2010a).
Summarizing the present state of the art, a dynamic development is presently observable in the field of polymer blends with CNTs. At the moment, the possibilities of designing new desirable structures have not been fully exploited. Several phenomena that determine the morphological features have not been completely understood so far. Furthermore, experimental skill and labour will be needed to characterize, e.g. the mechanical properties of CNT blend structures independent of the CNT dispersion and the CNT-induced changes of the polymer phases such as crystallization or compatibilization. The literature provides the first indications of structure–property relationships of these special nanocomposite blends, whereas it is too early to derive generally valid conclusions in this field.
Nevertheless, it is now already possible to significantly reduce electrical percolation thresholds by employing the selective localization of CNTs during blending. In the medium term, by understanding and tailoring the morphology development, antistatic materials with ultralow percolation thresholds seem to be feasible. Thus, functional multiphase systems could be developed, that by far exceed the properties of the presently available materials and those of other CNT-filled bulk materials.
In the literature, localization of fillers in thermoplastic polymer blends most commonly is explained by the general tendency of interfacial energy minimization, which results in a more or less pronounced driving force to arrange the fillers in that particular blend phase that is energetically preferred. This means that fillers that are incorporated in the thermodynamically unfavourable phase will be transferred to the most favourable phase of the blend during mixing. The transfer is a process that is dependent on time and mixing intensity, therefore nonequilibrium transition states can be obtained that are stabilized during solidification of the polymer blend after mixing. Preferential localization of fillers in one of the blend phases is most commonly correlated with the differences in interfacial energies of filler and the respective polymers that originate in the differing polarities and surface energies. To quantify the driving forces (Sumita et al., 1991) introduced the wetting coefficient œ from Young’s equation that describes the adjustment of wetting angles on ideally plane surfaces (Equation 19.1) This is presently the most common approach to predict the filler localization after melt blending:
Based on the different interfacial energies γ between the liquid phases and fillers, the equilibrium filler localization can be calculated. The wetting coefficient most commonly is interpreted in a way that for ω < − 1, the filler is predicted to be in polymer 1 and for ω > − 1 in polymer 2. In the interval between 1 and − 1, fillers are predicted to be at the interface. In Section 19.3.3, the influence of the filler geometry on the range of this interval and the consequences for melt blending of nanoscaled fillers are described.
The fact that a reliable prediction requires the exact calculation of the wetting coefficient and thus the exact measurement of the interfacial tensions between the blend polymers and the filler, can be considered the main drawback of the concept, which is designed for ideally plane surfaces. Unfortunately, typical nanoscaled fillers reveal macro-, micro-, and nanostructured surfaces that, according to the Lotus Effect, have a strong impact on the apparent wetting angle. Therefore, the standard measurement methods that are based on the measurement of the contact angle between nanofiller and polymer fail. These measurement problems can be avoided by calculating the interfacial energies from the individual surface energies of the polymers and the nanofillers. Thus, the thermodynamic preferences can be obtained for combinations of CNTs and polymers that so far have not yet been investigated. Nevertheless, reliable surface energy values are required. For CNTs, and despite the enormous experimental efforts by different research groups, the suitability of methods and the obtained surface energies are still the topic of controversial discussion. The main challenge is, as mentioned above, to avoid any measurement disturbance by the surface geometries. One example of how to overcome this problem is the studies of (Nuriel et al., 2005) and (Barber et al., 2004) that determined the interactions of isolated carbon nanotubes with well-known liquids and polymers. It is obvious that these procedures that include the operation of individual MWCNTs need enormous experimental skills and labour. Very recently, attempts were made to access the surface properties of CNTs by studying their gas adsorption behaviour using inverse gas chromatography (Menzel et al., 2009). This method results in surface energies that are much higher than the above-mentioned values from the group of H. D. Wagner.
Another very recent approach is to access the interfacial energies between polymers and CNTs by implementing modulated differential scanning calorimetry. Thus, the influence of carbon nanotubes on the change of the heat capacity at the glass transition temperatures can be evaluated (Miltner et al., 2007). As well as these measuring concepts, there are attempts to access the interactions of different polymers and carbon nanotubes by molecular dynamics simulation (Yang et al., 2005). It is the task of future studies to evaluate which concepts will produce the most reliable predictions.
The results of the different methods show a very broad spectrum of CNT surface energies and polarities which might be due to the method itself or to the different characteristics and treatment histories of the investigated CNTs. Therefore, there is an urgent need for comparative investigations and standardization, and the literature sources for the calculation of interfacial tensions have to be carefully selected. Generally, predictions of filler localizations can be performed based on different concepts; for polymer blends with conductive fillers, most commonly the wetting coefficient is calculated.
The wetting coefficient (Equation 19.1) requires the interfacial energies in the three-component system, which, when applying the concept of polar and dispersive shares of surface energies, can be obtained by the utilization of the harmonic-mean (Equation 19.2) or the geometric-mean equation (Equation 19.3). The geometric-mean equation is described in the literature as more suitable for high surface energies (Wu, 1982):
where: γ1 γ2 are the surface energies of components 1 and 2; γ1d, γ2d are the disperse part of the surface energy of components 1 and 2; γ1P, γ2P are the polar part of the surface energy of components 1 and 2. Surface energies, disperse and polar contributions of different thermoplastic polymers can be found in online databases (e.g. Holz, 20 Nov. 2007). Some of the studies in Table 19.1 compared the predication of the wetting coefficient or related equations with the localization of carbon nanotubes in an immiscible two-phase blend with their experimental observations (Wu and Shaw, 2004; Pötschke et al., 2008; Göldel et al., 2009a; Baudouin, 2010a; Baudouin, 2010b; Zhang et al., 2009). Generally, it appears that the reliability of the predictions is higher, the more the surface properties of the polymers differ from each other. The prediction is determined by the calculation procedure itself and the literature values for CNTs and polymers. Also for the latter, the insecurities concerning the respective surface energies should not be underestimated. Commercial polymers can exhibit differing polymerization procedures and structural differences while they are still assigned to the same polymer group. Furthermore, different measuring procedures can also result in deviating values. Thus, for the calculation of the wetting coefficient, only consistent literature values should be employed. If not available, the surface characteristics of the respective blend partners should be measured with one measuring method and ideally by the same operator. For CNTs, it appears that the more reliable predictions are obtained, when the selected values reflect to some degree the influence of the polar surface groups on the outer tube surface.
The selective localization of CNTs in one phase of an immiscible polymer blend due to thermodynamic driving forces can occur for various blend systems during melt processing. The speed of CNT transfer and the selectivity of CNT localization presently appear to be superior compared to more traditional fillers such as carbon black or MMT. Whereas these peculiarities will be explained in Section 19.3.3, this chapter describes their exploitation in the manufacture of double percolated blend structures. Ideally, distinct co-continuous morphologies are combined with the highly selective filling of one of the phases with a percolated CNT network like the structure displayed schematically in Fig. 19.4. There, the continuous, selectively filled phase ensures the electrical conductivity, whereas the other continuous phase is completely free of tubes, but also contributes to thermo-mechanical, mechanical and physical properties and the solvent resistance of the blend. To give an example, the concept can be exploited for PC–SAN blends that are frequently studied to understand processes within the more complex commercial PC–ABS blends.
Whereas PC–SAN blends in principle can show LCST behaviour depending on the molecular structure of PC, the molecular weights and the AN content of the SAN phase, the literature as well as the transfer kinetics and the thermal analyses strongly indicate that both phases of the PC–SAN–MWCNT example blend are completely phase-separated during and after melt mixing. In these blends, PC is the thermodynamically preferred phase and contains almost all CNTs after 5 minutes melt mixing, independent of the mixing regime (Göldel et al., 2009a) (Fig. 19.5). Once in the PC phase, the inverse transfer from PC to SAN is inhibited by the interface (Fig. 19.5 (e) and (f)). Besides the displayed morphologies of PC–SAN, selective localization during and after melt mixing can occur in many blend systems. Double percolation in some cases can be achieved for blends that are adjusted at the very edge of the co-continuous interval as shown, e.g. for blends of PC with only 25 wt% PA6 (Fig. 19.6). Even though the number of electrical pathways in these blends is significantly lower as in those in Fig. 19.5, they are sufficient to manufacture antistatic composites. The CNT contents can be reduced to one-third or even one-quarter of that of the homopolymers. Also in this blend, the localization behaviour is consistent with the combined concept of wetting coefficient and the particle stability, according to the aspect ratio that is explained in Chapters 15 and 20.
19.5 Morphology of a co-continuous PC–SAN blend with MWCNT Baytubes® C150HP exclusively localized in the PC-phase after 5 minutes melt mixing at 100 rpm in a co-rotating microcompounder. Cutting direction was perpendicular to the die flow. The overall structure of PC– MWCNT (dark) and SAN (bright) can be seen in the transmission-light micrograph (a) and TEM images (b–f). The black lines in (b) and (c) are not a part of the morphology but artefacts from ultrathin cutting.
19.6 Another example of selective localization during melt mixing: MWCNTs (Baytubes® C150HP) selectively localized in the PA6 phase of a PA625–PC75 blend. Electrical percolation can be achieved for relatively low contents of the selectively filled phase (here 25 wt% PA6). In transmission light microscopy, the PA6 phase content appears to be beyond 25 wt% due to the cut thickness of 5 μm and the small domain size.
19.3.3 Aspect ratio dependence of nanofiller localization and the peculiar migration behaviour of CNTs
In contrast to low-aspect ratio fillers like carbon black, it seems that after melt mixing processes with an average mixing intensity, carbon nanotubes are commonly not, or only to a small extent, located at the interface of a polymer blend. Of that portion, only a small percentage is found to be orientated perpendicular to the interface while bridging both blend phases.
In contrast, the apparently higher stability of carbon black at the interface results, depending on the mixing regime and the thermodynamical driving forces, in less defined localization behaviour. Thus, scenarios where CB is at the interface of a non-reactive immiscible blend (Sumita et al., 1991; Gubbels et al., 1995; Calberg et al., 1999) and/or distributed within one or both of the blend phases after melt mixing (Sumita et al., 1991; Gubbels et al., 1995; Tchoudakov et al., 1996; Thongruang et al., 2002; Feng et al., 2003) are typically observed.
Whereas the surface tensions and polarities of CNTs and CBs can be in a related range and thus should be the interactions with the respective blend phases, their geometrical shapes exhibit a fundamental difference, namely the aspect ratio. The strong impact of this parameter on the equilibrium position of a solid, ellipsoidal particle at the interface of a liquid drop and a surrounding fluid (Fig. 19.7 (a)) was derived mathematically by (Krasovitski and Marmur, 2005).
19.7 Influence of different particle aspect ratios (1, 0.5, 0.1) on the equilibrium position at the interface of a liquid drop and a surrounding liquid. Reprinted with permission from Taylor & Francis (Krasovitski and Marmur, 2005). (a) Solid particle at the interface of a liquid drop and a surrounding liquid. (b) Correlation of penetration index and the contact angle for the attachment of an ellipsoidal particle to a spherical drop, assuming negligible line tension. The penetration index is defined as the share of the particle (0–1) that is penetrating one of the liquids. A penetration index of 0.5 therefore corresponds to equal penetration in both, whereas one or zero is equivalent to a complete penetration into either the liquid drop or the surrounding fluid.
An increase in the particle aspect ratio results in a sharper dependence of the penetration index on the contact angle (Fig. 19.7 (b)). Thus, for very high aspect ratios, the particles are almost completely located within the better wetting phase (for which the contact angle at the intersection of the three interfaces is less than 90 °), almost independent of the actual value of the contact angle. Although describing an equilibrium phenomenon, consequences can be derived for the speed of the particle transfer as well as the prediction of the most stable localization during typical melt blending processes in kneaders or extruders (Göldel et al., 2009b). First, the transfer of CB from one phase to the other is slower and the interfacial stability is higher than those of high aspect ratios CNTs. Second, the probability of reaching its lowest possible state of free energy by localizing the filler at the blend interface is relatively high for CB and very low for CNTs. Thus, the interval of wetting coefficient values, where the localization at the interface should occur is broad for CB (− 1 and 1) and strictly narrows with increasing effective aspect ratios. This can explain why melt blending of two or more immiscible thermoplastic polymers with CNTs commonly results in morphological structures, where the CNTs are almost completely located in the thermodynamically preferred phase even for low mixing intensities and short mixing times (Göldel et al., 2009b).
To transfer the CNTs from one phase to the other, direct contact with the blend interface is required. Generally, contact can be achieved by a flow field or by the motion of MWCNTs by diffusion, but the latter is very slow due to the MWCNT diameters that are much larger compared to other macromolecules, even though temperatures during melt blending can exceed 300 °C and thus support Brownian motion in the polymer melt. Therefore, during blending, the contact and thus the transfer will be mainly ensured by the flow field that is characteristic of the mixing process and typically ensures several shear-induced contact locations of any random segment with the interface.
In the absence of supporting parameters like e.g. flow fields, CNTs that are not located at the interface will only be transferred on very long time scales. For the much lighter and more mobile SWCNTs, diffusion could significantly contribute, as their molecular dimensions are comparable to that of typical aromatic thermoplastics and their behaviour is presently discussed and correlated with that of macromolecules (Green et al., 2009).
It is well known that carbon nanotubes strongly interact with the matrix polymer due to their enormous surface area. In the literature, examples can be found of the strong influence of even small concentrations of CNTs on the melt viscosity and the viscoelastic properties of the matrix polymer in the low frequency range, as described first by (Pötschke et al., 2003). In the previous chapters it has been shown that MWCNTs tend to localize very selectively in one of the phases of an immiscible polymer blend. Thus, the rheological properties of this particular blend phase such as storage modulus and complex viscosity can be significantly changed, whereas those of the other blend phase(s) remain unaffected. The blend morphology development inside a kneader, compounder or extruder is again strongly dependent on the rheological interactions of the blend partners and thus can be affected by the incorporation of CNTs. Generally, the literature covers several correlations that have been used to discuss and predict the phase inversion composition and thus the centre of the co-continuous region based on rheological properties. Of those, the Utracki equation (Utracki, 1991) that predicts the phase inversion concentration employing the viscosity ratio of the two blend phases can be considered the most advanced model (Pötschke and Paul, 2003). The generally observable tendency of the higher viscous phase to be less continuous as the phase with the lower viscosity is combined in this advanced empirical model with an adaptation of the percolation theory. Figure 19.8 (a) shows the general interdependence of viscosity ratio and predicted phase inversion concentrations according to one of the first and simplest models (Miles and Zurek, 1988) and the more recent and complex Utracki model (Utracki, 1991). Especially for high viscosity ratios, the Miles equation is not suitable to describe the blend morphology, whereas good agreements can be achieved with the Utracki equation (see e.g. Göldel et al., 2008). According to these findings, any modification of the blend that specifically changes the viscosity of only one of the blend phases will consequently affect the phase inversion concentration (PI). If the CNTs increase the melt viscosity of the selectively filled phase, it can be expected that the unfilled phase will gain some continuity at the cost of the other. So far, no investigations dealing with this specific phenomenology have been described in the literature. Therefore, the influence of a selectively filled blend phase is studied for co-continuous (PC–MWCNT)–SAN blends. These are well described and suitable model systems, as the morphological features and the explicit localization of MWCNTs in the PC phase after short mixing times have been comprehensively approved by several independent methods (Göldel et al., 2009a). Therefore, the impact of MWCNTs on the viscoelastic properties of the blend during mixing and thus on the morphology development can be investigated by studying the shear-rate/frequency dependent properties of PC–MWCNT composites and of neat SAN, reflecting the situation in the blend. Addressing the broad spectrum of flows inside a microcompounder or extruder, the shear rate dependence of melt viscosity is considered, as can be seen in Fig. 19.8 (b) for different PC-MWCNT compounds. At low frequencies, the melt viscosity of PC is increased dramatically, even for low concentrations of MWCNTs. This means, that e.g. at 0.1 rad/s, the addition of 1% MWCNTs to PC results in viscosities that are a factor of 4 higher than those of the unfilled polymer, for 2% MWCNTs even a factor of 30 is obtained. This increase at very low frequencies is due to the long-range combined CNT-polymer networks and is a typical phenomenon of CNT-filled thermoplastics (Pötschke et al., 2002; Pötschke et al., 2004). Whereas this can result in strongly inhibited dripping of the polymer melt which might improve the flame resistance, for the formation of phase morphologies during melt blending much shorter excitation times and higher frequencies have to be considered. To correlate the angular speeds from the oscillatory measurements in the rheometer with the shear rates within microcompounders or extruders, for neat polymers the Cox–erz rule can be applied (Cox and Merz, 1958):
19.8 Rheological properties, viscosity ratios peff and prediction of phase inversion concentration ΦI PC–MWCNT for the immiscible phases in (PC–MWCNT)–SAN blends. (a) General prediction of the phase inversion concentration ΦI based on the blend phase viscosity ratio according to Utracki (1991) and Miles and Zurek (1988). (b) Complex viscosity of the immiscible blend phases for the example of selective localization of MWCNT within the PC phase as a function of the oscillatory frequency. (c) Respective viscosity ratio according to Equation 19.5. (d) Phase inversion concentration according to Equation 19.6.
This empirical relation allows comparison of the complex viscosities in oscillatory |η∗(ω)| and steady shear by equalizing the frequencies ω in rad/s and the rates of steady shear in s–1. If was proved to be valid for almost all homopolymers. Although not accurate for filled polymer systems, the data at high frequencies are nevertheless suitable to estimate the change of rheological behaviour inside the microcompounder due to the selective filling of the PC phase. As the shear rate regime typically observed during twin-screw extrusion ranges from 1 to 1000 s–1 (Schramm, 1994), the rheological data presented here can cover three of the four relevant decades. Due to the impossibility of measuring shear rates in an extruder and to the strong local variation of shear according to the actual position of the screw elements, a medium shear rate has to be assumed. A possible change of blend morphology by the addition of MWCNTs is discussed in the following for the meltblending process, where the (PC–MWCNT)–S AN blends revealed morphologies like those displayed in Fig. 19.5 by applying rather low rotational speeds of 100 rpm in a microcompounder. For this process, a medium shear rate of 100 rad/s is assumed, which corresponds to the upper limit of the oscillatory experiment’s measuring range.
Due to the characteristic features of such filler networks, the dependence of melt viscosity and CNT content strongly decreases for increasing frequencies (Figure 19.8 (b), see also Chapter 15). The same holds for the viscosity ratio peff (Equation 19.5) that is defined as the ratio between the melt viscosities of the dispersed phase and the matrix phase. The strong and the negligible impact of the MWCNT concentration within the PC phase on peff at low and high shear rates, respectively, can be seen in Fig. 19.8 (c) for the example of (PC-MWCNT)-SAN blends.
Consequently, a blend with 2 wt% MWCNT within the PC phase reveals a viscosity ratio that is almost 30 times higher than that of the neat blend at very low frequencies (0.1 rad/s) whereas this effect almost disappears at elevated frequencies where the viscosity ratios are almost independent of the MWCNT content in the PC phase (3.1 for the unfilled and 3.8 for the PC2wt% CNT–SAN blend, respectively). Finally, this characteristic feature is also reflected in the theoretical prediction of the phase-inversion concentration based on the Utracki equation (Utracki, 1991; Pötschke and Paul, 2003):
where ΦI denotes the volume content of the higher viscous phase, where phase inversion (PI) occurs, which is PC in our example. The required constants, in particular, the maximum packing density of spheres ϕm and the intrinsic viscosity [η] were selected according to (Utracki, 1991). The results of the calculation are displayed in Fig. 19.8 (d). It shows the shear rate-dependent respective contents of the PC–MWCNT phase (ΦI,PC–MWCNT) that are theoretically required to adjust the blend in the centre of the interval of co-continuous morphology by a melt blending process. Whereas significant shifts of the phase inversion concentration can be expected for blends processed at very low shear rates, in typical extrusion processes with assumed shear rates of 100 s–1, no or only very minor shifts are expected even for high MWCNT contents. The validity of this correlation and thus the similarity of morphology and continuity of a PC60–SAN40 blend with (2 wt%) and without MWCNTs in PC are demonstrated in Fig. 19.9. Nevertheless, for mixing or kneading processes with very low rotational speeds or shear rates, the influence of the selective MWCNT loading can be much higher. One can deduce that the selective localization of MWCNTs in PC will most probably enable the manufacturing of PC–SAN blends with phase continuities and thus properties, which have not so far been described. There, co-continuity could be achieved for increased contents of the PC phase that would result in PC droplet-SAN matrix structures without carbon nanotubes. The rheological behaviour of co-continuous PC60–SAN40 blend with 1 wt% Baytubes® C150HP selectively localized in the PC phase is shown in Fig. 19.10. Due to the double percolated blend structure, the increase of viscosity at low frequencies can be attributed to both the percolation of the CNT network inside PC and that of the PC phase within the co-continuous blend. For other blend systems, the influence of a selectively filled blend phase on the viscosity ratio and thus on the phase inversion concentration can be more pronounced. The impact is specifically high for blends where the selectively filled polymer reveals, in contrast to polycarbonate, a low melt viscosity. This holds for most of the semi-crystalline thermoplastics, as their melt and processing temperatures by far exceed the region of glass transition. Blends of PVDF–PA6 are good examples of this and of the inverse dependency of the phase viscosity and the continuity that is the base of the above-mentioned equations. In detail, a strong increase in the initially low melt viscosity of the PA6 phase can be observed by the selective localization of MWCNTs within this phase (Li and Shimizu, 2008). Therefore, the continuity of PA6 is decreased, whereas in return, that of the previously dispersed PVDF phase is increased in the final blend morphology. By the addition of a certain minimum amount (1.2 wt% MWCNTs), the sea island morphology of the unfilled blend can be converted into a co-continuous type of morphology. Furthermore, the electrical properties are improved compared to the bulk materials with the same MWCNT content. Thus, the addition of CNTs offers the possibility of tailoring morphological structures and simultaneously improving the electrical performance of the blend. Beyond the change of phase continuity, CNTs can significantly reduce the domain sizes in polymer blends (Wu et al., 2009), and thus affect the blend morphology like a compatibilizer. Unlike classical additives that compatibilize the phases mainly thermodynamically, for CNTs, the kinetic suppression of the selectively filled blend phase’s coalescence is of higher importance (Vermant et al., 2008; Bose et al. 2010). This mechanism would be most effective for the localization of CNTs at the blend interface according to the theory of solid particles in the emulsions, which is well described by Pickering (Pickering, 1907). The difficulties in achieving such a thermodynamic and mechanical compatibilization of polymer blends by selective localization of CNTs at the blend interface have been described in previous chapters in this volume.
19.9 Comparison of the phase continuity in PC60–SAN40 with Comparison of the phase continuity in PC60–SAN40 with (a) selectively MWCNT filled PC phase, TEM micrograph) and (b) the neat blend without CNTs (SEM picture PC phase was selectively hydrolysed (DSM 15 Microcompounder, 100 rpm, 5 min, Baytubes® C150HP).
Generally, the time-dependent morphology development in a phase-separated polymer blend includes an endless number of transition states and morphological shapes. It has been shown that carbon nanotubes can be transferred from one of the blend phases to the other during this process. In parallel, the dispersion of primary CNT agglomerates will occur in one or in both phases. Beyond that, agglomerates will also be transferred from a thermodynamically unfavourable phase into the other by passing the interface, according to dependence on the transfer speed on the aspect ratio (Section 19.3.3), significantly slower than separated CNTs. Independent of shape, the transfer requires contact with the interface as explained above and thus the transfer speed will be strongly dependent on the developing amount of interfacial area. The parallel evolution of macroscopic and microscopic structures (Fig. 19.11) also results in a broad range of CNT transfer states. This can be nicely seen when blending a SAN–CNT pre-compound with neat PC (Fig. 19.12). Whereas the macroscopic domains of neat PC do not contain any CNTs after 10 seconds kneading (Fig.19.12 (a), lower left corner), the transfer is quite advanced in directly neighbouring regions to the right. Intensive transfer is occurring in the regions where the interface of droplets and structures is visible, but where the phases themselves exhibit no contrast (grey). Therefore, the transfer can be highly advanced or almost completed in microstructured domains, whereas the macroscopic phases can still be in the initial state. In the very early mixing stages it is possible to observe even the thermodynamical highly unstable localization of separated CNTs at the blend interface (Section 19.3.5, Fig. 19.12 (b)).
19.11 Morphology development mechanism of an immiscible polymer blend as proposed by Scott and Macosko (1995). Reprinted with permission from Elsevier.
19.12 CNT and agglomerate transfer from a SAN–MWCNT precompound-phase to a unfilled PC-phase in a very early mixing stage (SAN–MWCNT)40–PC60. (a) 10 seconds kneading (transmission light microscopy). Black: MWCNT-agglomerate; different grey levels correspond to different MWCNT loadings. Unfilled polymer is transparent (both PC and SAN) and appears bright white. The macroscopic transparent (white) phase in the lower left corner is PC. (b) 30 seconds kneading (TEM). In this early stage, MWCNTs can frequently be observed during the transfer process, including even the highly unstable transition state of perpendicular orientation to the blend interface. From the visible characteristic features, the bright minority phase most likely corresponds to PC.
19.3.6 Summary: factors influencing transfer and localization of carbon nanotubes and other nanoscaled fillers
Nowadays, carbon nanotubes are received from the manufacturers in the shape of more or less compact agglomerates, which is due to the available production processes and the strong van der Waals interactions between the individual tubes. Earlier chapters of this book (Chapters 4–8) describe how the outstanding individual properties of CNTs can only be exploited in a polymer composite when it is possible to separate them from the agglomerates. Therefore, during melt blending of CNTs with polymer blends, usually a broad spectrum of different CNT shapes, sizes and aspect ratios is present inside the compounder, which is dependent on the intensity and state of the dispersion process. Furthermore, the transfer rate (e.g. CNTs/second) of CNTs through the interface is strongly coupled with the available interfacial area of the blend phases, and thus with morphology development from macroscopic granules to microscopic phase structures. Thus, if the carbon nanotubes are predispersed in a polymer that reveals high interfacial tensions towards the CNTs, during blending with an unfilled polymer of higher affinity, several processes will be present at the same time, specifically in the beginning of the blending process:
Therefore, the peculiarities of CNTs such as the fast interfacial transfer and the subsequent selective localization that have been described in earlier chapters are only significant for separated CNTs or small bundles that still exhibit high aspect ratios. For coiled CNTs or agglomerates, different transfer mechanisms are present that depend on the geometrical shape and size of the CNT object. Extrusion processes with medium to long residence times will, for most blend–CNT composites, result in highly advanced dispersion and almost complete transfer of well-dispersed CNTs. The remaining agglomerates can, depending on wetting and phase viscosities, be either coated by the preferred phase or remain in the initial carrier polymer (Fig. 19.13).
19.13 Peculiarities of CNT agglomerates that are big compared to the blend morphology. Depending on mixing conditions and characteristic features of the blend partners, the interaction with the blend can follow different mechanisms. Here: PC-SAN with MWCNTs (Baytubes® C150HP). (a) Large agglomerate still covered by worse wetting (and less viscous) phase. Once dispersed in the surrounding SAN-melt, CNTs are immediately transferred into PC (grey). Thus even in the regions directly neighbouring the agglomerate, most of the CNTs are within the PC phase. (b) The better wetting phase (here PC) completely covers the CNT agglomerates in the upper right corner. For the displayed example the dispersion of MWCNTs will be dominated by the PC–MWCNT dispersion mechanisms (Chapter 4).
In the earlier chapters it was shown that during melt mixing of thermoplastic polymer blends the selective localization in the better wetting blend phases occurs specifically for nanotubes or related nanofillers. Whereas those systems revealed in almost all cases particularly interesting electrical properties, the CNTs will always be in the thermodynamically preferred phase, in most cases, even independent of the processing parameters and mixing regime. For future applications, the possibility of tailoring those morphologies and the CNT localization is highly desirable, as this would enable the instant manufacture of new ‘tailor-made’ materials with superior properties. Presently, several manufacturers provide chemically modified carbon nanotubes, such as Nanocyl® with its 315 × series. This enables the reaction with suitable reactive groups, such as, e.g. maleic anhydride (MA), which is also commonly employed for reactive extrusion and compatibilization of polymer blends. There have been recent studies on PA–ABS blends with MWCNTs and a reactive blend component (Bose et al., 2007a, 2007b, 2009), but the impact of the potential reactions on blend morphology and CNT localization is still not completely clear. The common understanding is that those reactions or strong interactions will irreversibly couple the CNT versus their corresponding functional groups to the polymer. Considering the enormous length of typical CNTs it appears reasonable to imagine that some part of the CNT is thus coupled and covered by e.g. the thermodynamically unfavourable but reactive phase, while those areas that are not occupied are wetted by an energetically preferred polymer. This could result in the localization of the CNT at a blend interface. When large amounts of the surface are covered by the coupled polymer, the CNT will tend to be surrounded by more molecules of the reactive polymer or a compatible blend phase. This can be seen in a set of very recent experiments (Gültner et al., 2009; Gültner et al., 2011) that demonstrate the possibility of even inverting the phase selectivity by introducing groups of MA into the thermodynamically unfavourable phase (SAN). It was found that NH2-modified MWCNTs (Nanocyl® 3152) localize, equal to Baytubes® C150HP (Section 19.3.2), entirely within the PC phase of unmodified and compatible PC–SAN blends, whereas they are completely located within SAN when this phase is composed of SAN and a related miscible copolymer that contains a sufficient number of MA groups.
Surprisingly, nominally unmodified MWCNTs (such as Nanocyl® 7000, Baytubes® C150P and Baytubes® C150HP) are found as well within the SAN–MA phase of PC–SAN–MA blends, although they are not expected to develop chemical bonds or strong irreversible interactions with MA. This behaviour that is similar to that of specifically surface-modified CNTs is an example of the present difficulties in characterizing and defining CNTs’ functionality and the chemical reactions within the melt. The direct quantification of the formation of chemical bonds between CNTs and polymers during melt mixing is, due to the low concentration of CNTs in the composite and that of reactive groups on the tube surface, very difficult to access analytically. One has to be aware that a typical composite with 3 wt% CNTs that are modified with e.g. 1% of reactive groups contains only 0.03 total wt% of surface active groups that will interact with the polymer. Even the characterization of reactive groups on the tube surface is delicate. The state of the art is to access the elements that can form potentially reactive groups (mainly from main group V and VI) via XPS. In the example, for both CNTs, modified and unmodified, sufficient amounts of nitrogen (Nanocyl® 3152) and oxygen (Nanocyl® 3150) for the formation of reactive groups could be detected using XPS. The presence and quantity of these elements and the respective groups that can determine the CNT localization within polymer blends are strongly coupled to the proceedings during the CNT production process, specifically to acid treatment and purification. In this context, it is important to note that the amount of nitrogen that is detected via XPS is most probably covalently attached to the CNT surface and thus forms possible reactions, whereas a certain amount of the detected oxygen can also be non-covalently adsorbed from the surrounding atmosphere and thus cannot covalently couple the CNT to any reactive component. Furthermore, defect sites on the CNT surface, e.g. introduced by ball milling, also correspond to local maxima of free energy and thus possibly develop specific interactions with different polymers. Thus, any impurity on the CNT surface can affect the CNT localization behaviour in a polymer blend. This correlation is highlighted when CNTs with high surface purities such as HiPCO–SWCNTs (the surface of these specific CNTs is not disturbed by the included impurities of 5% that are caused by remaining catalyst particles) are incorporated in the blends with the above-mentioned MA-modified SAN phase. These SWCNTs locate within the PC phase despite the behaviour of the investigated MWCNTs. Therefore, before denoting CNTs as ‘unfunctionalized/functionalized’ or ‘unmodified/modified’, their surface characteristics should be characterized very carefully to prevent misinterpretation.
Besides these difficulties, and counteracting the high expectations that go hand in hand with CNTs covalently or physically coupled at e.g. blend interfaces, some mechanism related to the formation of these bonds between polymer and CNT seems to significantly worsen the electrical conductivity. One possible explanation for this behaviour could be the encapsulation of CNTs by specific polymers or reactive components (Bose et al., 2007b).
Summarizing, highly sophisticated analytical methods will be needed to reliably describe the chemistry and surface interactions of CNTs in a melt blending process. The understanding of surface chemistry, reactions, localization behaviour and the resulting electrical properties could open up a new field of commercial applications for polymer blends manufactured by utilization of in-situ coupling during melt blending.
19.5 Utilization of selective localization: double percolated polycarbonate–acrylonitrile butadiene styrene (PC–ABS)-CNT blends
The peculiarities of CNTs such as the fast transfer to a thermodynamically preferred blend phase can be employed in various immiscible blends. The possibility of manufacturing conductive or antistatic composites with very low content of the still expensive CNTs, e.g. by the application of the double percolation concept, opens the door to several potential commercial applications. Specifically for automotive and electronic applications, e.g. for electrostatic painted parts, there is a strong market for antistatic composites. In these sectors, PC–ABS blends exhibit a very strong market position, specifically due to their outstanding impact toughness that is combined with high stiffness and surface quality.
Figure 19.14 shows the morphology of a PC–ABS -CNT blend with Baytubes® C150HP, in which the characteristics of the CNT transfer during melt blending were utilized. The MWCNTs are selectively localized in the continuous PC phase and not within the complex structured ABS, which is composed of two immiscible components, SAN and butadiene rubber (BR). Adjusted mixing conditions and regimes enable the manufacturing of PC–ABS blends that reveal electrical conductivities that almost directly follow the theoretical predictions of the ideal double percolation concept. This implies that, e.g. for a phase composition of 50 wt% PC and 50 wt% ABS, such a blend is electrically percolated at roughly half the CNT content as in bulk PC. Thus the blend reveals electrical conductivities that can be factors or decades higher than those of a PC–CNT composite with the same CNT content. Aiming at the implementation of this concept for commercial application, a suitable twin-screw extrusion process can be developed, that results in significantly improved electrical conductivities (Fig. 19.15) and well-developed double percolated blend structures (Fig. 19.16). For identical MWCNT mass fractions, the resistivity of the displayed PC50–ABS50 blend is a factor of 2000 lower than that of the masterbatch dilution of MWCNTs in neat PC.
In contrast to conventional reinforced composites with e.g. glass or carbon fibres, most grades of carbon nanotubes can freely localize within the typical morphologies of polymer blends, as they are small enough. The fast interfacial transfer and highly selective localization in the thermodynamically preferred blend phase of binary polymer blends are a peculiarity of such high-aspect ratio nanoscaled fillers, and are also expected to occur in ternary or quaternary polymer blends. Multiple morphologies are conceivable that can be tailored according to the requirements of the application of interest. Use will probably be found for both the fast CNT transfer as such and the selectively filled multiphase morphologies, respectively. The specific behaviour of CNTs in a molten multiphase blend structure already results now in material properties very close to application, such as the possibility of reducing the electrical percolation threshold by a factor of two and more by the application of the double percolation concept. These concepts work hand in hand with the advances in CNT production and the progress in dispersion and processing. This implies that, independently from what can be achieved e.g. in reducing the percolation thresholds of homopolymer CNT composites, they can subsequently be improved even more by a sophisticated blending step. The potential is also reflected in the strong increase in research activity during the last few years. Further research is needed on the investigation of CNT–polymer interactions and interfacial properties. Also the correlation of morphological features and mechanical properties is an issue of high importance for most technical applications and has to be evaluated independently of the influence of CNTs on the polymer itself and of CNT dispersion. In general, the properties of multiphase blends with CNTs are linked to the advances in CNT dispersion in homopolymers, and already now can push any progress in this field even one step further, in spite of the fact that the morphological potential so far has been only partially exploited. Commercial applications based on the reported technological achievements can be expected in the near future. Nevertheless, and although control of structural features in mixing processes is very difficult, the potentially possible ultra-low percolation thresholds of tailored blends promise a real breakthrough in the market of conductive thermoplastic nanocomposites with carbon nanotubes.
Part of this work was performed within the Framework Concept ‘Research for Tomorrow’s Production’ (fund number 02PU2392) managed by the Project Management Agency Karlsruhe (PTKA) that was funded by the German Federal Ministry of Education and Research (BMBF).
The authors thank Prof. Abraham Marmur from Technion Israel Institute of Technology (Haifa, Israel) for finding the key to correlate the localization behaviour of CNTs and other nanoscaled fillers to their aspect ratio.
The contribution of Maren Gültner to Section 19.4 concerning the use of functionalized CNTs in blends of PC–SAN is acknowledged.
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