Chromatic Conductive Polymer Nanocomposites of Poly ( p -Phenylene Ethynylene)s and Single-Walled Carbon Nanotubes

: We report on dispersions and thin ﬁlms of chromatic conductive nanocomposites of poly( p -phenylene ethynylene)s (PPEs) and single-walled carbon nanotubes (SWNTs) generated via solution mixing. The linear, conjugated PPEs with dialkyl- and dialkyloxy-side chain groups are shown to debundle and disperse high concentration (up to 2.5 mg/mL) SWNTs in various organic solvents. The solubilization of SWNTs and PPE wrapping is accompanied with the change in the solution color. Ultraviolet visible absorption spectra of nanocomposite solutions demonstrate a new absorption peak at a higher wavelength, supporting the observed chromatism. Fluorescence spectra of nanocomposite solutions display signiﬁcant quenching of the ﬂuorescence intensity and the Stern– Volmer model is used to analyze ﬂuorescence quenching. Electron microscopy of the chromatic solid ﬁlms of high mass fraction PPE/SWNT nanocomposites obtained by vacuum ﬁltration reveals the debundled SWNTs in the PPE matrix. The tensile strength and Young’s modulus of these PPE/SWNT nanocomposite ﬁlms are as high as 150 MPa and 15 GPa, respectively. The composite ﬁlms exhibit remarkably high conductivities, ranging from ~1000 S/m to ~10,000 S/m for 10 wt% and 60 wt% SWNT nanocomposites, respectively. ﬁlms fraction PPE/SWNT nanocomposite ﬁlms are close to those of the neat SWNT ﬁlms and conductivities versus SWNT mass fraction data display percolation-like scaling. Our work may provide a facile way to prepare highly conductive polymer nanocomposites for emerging applications.


Introduction
Single-walled carbon nanotubes (SWNTs) are promising for various potential applications such as electronic and biomedical devices and advanced conductive composite materials because of their unique 1D nanostructure, excellent electrical conductivities and mechanical properties [1][2][3][4][5]. However, processing difficulties limit the realization of their full potential because SWNTs readily form bundles, which derive from strong van der Waals attractions between nanotubes. Several methods including chemical surfacefunctionalization and molecular physical interactions have been successfully developed to increase repulsive inter-tube interactions in order to disperse SWNTs down to the individual tubes. These methods include acid oxidation, [6] covalent functionalization, [7] surfactant adsorption, [8] and polymer wrapping [9]. The latter is the focus of this work, which has the advantage that conductive polymer nanocomposites can potentially be produced directly by vacuum filtration from a solution without altering the chemical structures of the pristine SWNTs.

Materials and Methods
The SWNTs (Cheap Tubes Inc., Cambridgeport, VT, U.S.A.) were used as received without further treatment. The SWNTs had average diameters of around 1-2 nm. Two kinds of PPEs with dialkyl and dialkyloxy side chain groups were studied and their chemical structures are shown in Scheme 1. Mixtures of SWNTs and PPEs were sonicated in organic solvents (chloroform, dichlorobenzene (DCB) or toluene) with a typical solid concentration of 4.0 mg/mL for 30 min to generate stable dispersions. The mass ratio of SWNTs and PPEs was varied from 0.625:100 to 60:40. The SWNT/PPE dispersions were then dropped into the methanol and the thin films of nanocomposites were obtained by vacuum filtration. The resulting films were dried at room temperature under a vacuum and then they were peeled off from the filter paper for further investigation.
UV-vis absorption and fluorescence spectra of the solutions were acquired using a Shimadzu UV02401PC spectrophotometer and a PTI LPS-220B spectrofluorophotometer, respectively. The morphology of the solid films was examined with scanning electron microscopy (SEM, LEO 1530) operating at an accelerating voltage of 5 kV. The mechanical properties of the composite films cut to give a gauge width of 10 mm were determined using an RSA III solids analyzer (Rheometric Scientific, Co., Piscataway, NJ, U.S.A.) at room temperature at a strain rate of 0.05 mm/min. The electrical conductivities of nanocomposite films were conducted on a Keithley Instrument 2000 Source Meter in a four-point probe configuration and the specimen thickness was measured by a digital caliper (VWP).

Chromatic Dispersion of SWNT-PPE Nanocomposites
The solubilities of SWNTs in various organic solvents utilizing PPEC in 1:1 mass ratios with the SWNT are shown in Table 1. The organic sol dichlorobenzene (DCB), chloroform and toluene) were selected as they are for PPEs. Although SWNTs are insoluble in chloroform and toluene, an dispersible in DCB, they became highly soluble when added to the PPE dispersions of SWNTs/PPE remained fully homogeneous without any sign or precipitation for several weeks after mixing. This observation confirm good dispersing agents for solubilizing SWNTs in organic solvents [14]. T persion concentration of SWNTs observed with PPEC8 compared with PP strates that SWNTs have stronger interactions with the PPEC8. This phen lieved to be attributed to a higher degree of polymerization of the PPEC8 [3 degree of polymerization of PPEC8 produces a longer conjugated backbo quently larger contacts per chain with nanotubes via π-π interactions. The dispersion concentrations of ~2.5 mg/mL are obtained with PPEC8 in DCB. be favorably compared to the previously reported values of 2.2 mg/mL o PPEs in chloroform [33] and 2.75 mg/mL of SNWTs with P3ATs in DCB [1 expected, higher concentration SWNT/PPE solutions have higher viscositie ever, the SWNT/PPE dispersions in this work did not form a gel in contra ture report [35]. As a consequence of solubilization of the SWNTs by PPE wrapping, simultaneous color change in the PPE solution. Figure 1 shows the color c in DCB by adding various amounts of SWNTs. The starting PPE solution c set as 1 mg/mL. Adding 0.625 wt% SWNTs (relative to PPE amount) into th Scheme 1. Chemical structures of poly(p-phenylene ethynylene)s (PPEs). Dioctyl-PPE denoted as PPEC8 has 136 repeated units while dioctyloxy-PPE denoted as PPEOC8 has 45 repeated units.

Chromatic Dispersion of SWNT-PPE Nanocomposites
The solubilities of SWNTs in various organic solvents utilizing PPEC8 and PPEOC8 in 1:1 mass ratios with the SWNT are shown in Table 1. The organic solvents used (odichlorobenzene (DCB), chloroform and toluene) were selected as they are good solvents for PPEs. Although SWNTs are insoluble in chloroform and toluene, and only weakly dispersible in DCB, they became highly soluble when added to the PPE solutions. The dispersions of SWNTs/PPE remained fully homogeneous without any signs of separation or precipitation for several weeks after mixing. This observation confirms PPEs act as good dispersing agents for solubilizing SWNTs in organic solvents [14]. The higher dispersion concentration of SWNTs observed with PPEC8 compared with PPEOC8, demonstrates that SWNTs have stronger interactions with the PPEC8. This phenomenon is believed to be attributed to a higher degree of polymerization of the PPEC8 [34]. The higher degree of polymerization of PPEC8 produces a longer conjugated backbone and consequently larger contacts per chain with nanotubes via π-π interactions. The highest SWNT dispersion concentrations of~2.5 mg/mL are obtained with PPEC8 in DCB. This value can be favorably compared to the previously reported values of 2.2 mg/mL of SWNTs with PPEs in chloroform [33] and 2.75 mg/mL of SNWTs with P3ATs in DCB [13]. As may be expected, higher concentration SWNT/PPE solutions have higher viscosities in DCB; however, the SWNT/PPE dispersions in this work did not form a gel in contrast to the literature report [35]. As a consequence of solubilization of the SWNTs by PPE wrapping, we observed a simultaneous color change in the PPE solution. Figure 1 shows the color change of PPEs in DCB by adding various amounts of SWNTs. The starting PPE solution concentration is set as 1 mg/mL. Adding 0.625 wt% SWNTs (relative to PPE amount) into the PPE solution results in color changes from yellow to blue-yellow for PPEC8 ( Figure 1a) and from orange to green-orange for PPEOC8 (Figure 1b). With increasing SWNT loading, the chromatic solutions become darker. A similar chromatic behavior has been reported in the PPV/SWNT nanocomposites [15]. The color change is attributed to the conformation change of the conjugated backbone as a result of interaction with SWNTs [15]. It is well known that nanotubes can act as a template for polymer orientation across nanotube-polymer interfaces [36][37][38]. At a molecular level, the polymer chains are aligned along the tube long axis, forming an ordered interfacial polymer structure on nanotube surfaces [39,40]. In the conjugated polymers, this ordered polymer structure yields the extended π-electron delo-calization [33]. As such the nanotubes act as a doping agent for enhancing the stabilization of the excited states of conjugated polymers, resulting in a drastic change in the solution color [15].  The SWNT doping of PPE is further confirmed by the optical properties. Typically, the PPE/SWNT nanocomposites in DCB were diluted 30 times for investigation of optical properties. Figure 2 shows the UV-Vis absorption spectral features of PPEC/SWNT and PPEOC8/SWNT nanocomposites in DCB with different SWNT loading. Pure PPEC8 and PPEOC8 show a broad absorption band of π-conjugation centered at λ~400 nm and λ~450 nm, respectively. Adding SWNTs into the PPE solution produced a significant change in the intensity of the π-conjugation band as well as appearance and increase in intensity of a new absorption band at a higher wavelength. The new absorption band is located at λ~450 nm and λ~510 nm for PPEC8/SWNT and PPEOC8/SWNT, respectively. Appearance of new absorption bands at a higher wavelength in the PPE solution upon adding SWNTs indicates SWNT doping of PPE via π-π interactions between PPE backbones and SWNTs, producing the change in the solution color. This observation is in good agreement with the literature report on the PPV/SWNT nanocomposites in chloroform [15].   The SWNT doping of PPE is further confirmed by the optical properties. Typically, the PPE/SWNT nanocomposites in DCB were diluted 30 times for investigation of optical properties. Figure 2 shows the UV-Vis absorption spectral features of PPEC/SWNT and PPEOC8/SWNT nanocomposites in DCB with different SWNT loading. Pure PPEC8 and PPEOC8 show a broad absorption band of π-conjugation centered at λ~400 nm and λ~450 nm, respectively. Adding SWNTs into the PPE solution produced a significant change in the intensity of the π-conjugation band as well as appearance and increase in intensity of a new absorption band at a higher wavelength. The new absorption band is located at λ~450 nm and λ~510 nm for PPEC8/SWNT and PPEOC8/SWNT, respectively. Appearance of new absorption bands at a higher wavelength in the PPE solution upon adding SWNTs indicates SWNT doping of PPE via π-π interactions between PPE backbones and SWNTs, producing the change in the solution color. This observation is in good agreement with the literature report on the PPV/SWNT nanocomposites in chloroform [15].  The SWNT doping of PPE is further confirmed by the optical properties. Typically, the PPE/SWNT nanocomposites in DCB were diluted 30 times for investigation of optical properties. Figure 2 shows the UV-Vis absorption spectral features of PPEC/SWNT and PPEOC8/SWNT nanocomposites in DCB with different SWNT loading. Pure PPEC8 and PPEOC8 show a broad absorption band of π-conjugation centered at λ~400 nm and λ~450 nm, respectively. Adding SWNTs into the PPE solution produced a significant change in the intensity of the π-conjugation band as well as appearance and increase in intensity of a new absorption band at a higher wavelength. The new absorption band is located at λ~450 nm and λ~510 nm for PPEC8/SWNT and PPEOC8/SWNT, respectively. Appearance of new absorption bands at a higher wavelength in the PPE solution upon adding SWNTs indicates SWNT doping of PPE via π-π interactions between PPE backbones and SWNTs, producing the change in the solution color. This observation is in good agreement with the literature report on the PPV/SWNT nanocomposites in chloroform [15].    both PPEC8 and PPEOC8. The decrease in PPE fluorescence intensity with increasing SWNTs is attributed to a decreased quantity of free PPE fluorophore. This fluorescence quenching of PPE/SWNT nanocomposites is believed to be caused by energy transfer from the PPE-conjugated backbone as an energy donor to the SWNTs as an energy acceptor, indicating that PPE/SWNT nanocomposites involve strong π-π interactions between PPEconjugated backbones and SWNTs in DCB [41]. This observation is in good agreement with the literature reports on PPE/SWNT nanocomposites. [42,43]. is attributed to a decreased quantity of free PPE fluorophore. This fluorescence quenching of PPE/SWNT nanocomposites is believed to be caused by energy transfer from the PPEconjugated backbone as an energy donor to the SWNTs as an energy acceptor, indicating that PPE/SWNT nanocomposites involve strong π-π interactions between PPE-conjugated backbones and SWNTs in DCB [41]. This observation is in good agreement with the literature reports on PPE/SWNT nanocomposites. [42,43].  Figure 4 shows changes of the fluorescence intensity of PPE versus SWNT concentration for PPEC8/SWNT and PPEOC8/SWNT in DCB. According to the Stern-Volmer model, [41] the fluorescence intensity I is express by (I0 − I)/I = kSV [Q], where I0 is the intensity without SWNTs, kSV is a Stern-Volmer interaction parameter, and [Q] is the concentration of the quencher (in this case the concentration of the SWNT). Linear fits to the fluorescence data using the Stern-Volmer model gives a kSV~490 mL/mg for PPEC8/SWNT nanocomposites and a kSV~147 mL/mg for PPEOC8/SWNT nanocomposites. The larger kSV for the PPEC8/SWNT than that in PPEOC8/SWNT may be attributed to the larger degree of polymerization in PPEC8/SWNT (n = 136) than that in PPEOC8/SWNT (n = 45). These results indicate that the SWNTs have stronger π-π interactions with PPEC8 than that with PPEOC8. This observation is consistent with the SWNT solubility (Table 1)     , where I 0 is the intensity without SWNTs, k SV is a Stern-Volmer interaction parameter, and [Q] is the concentration of the quencher (in this case the concentration of the SWNT). Linear fits to the fluorescence data using the Stern-Volmer model gives a k SV~4 90 mL/mg for PPEC8/SWNT nanocomposites and a k SV~1 47 mL/mg for PPEOC8/SWNT nanocomposites. The larger k SV for the PPEC8/SWNT than that in PPEOC8/SWNT may be attributed to the larger degree of polymerization in PPEC8/SWNT (n = 136) than that in PPEOC8/SWNT (n = 45). These results indicate that the SWNTs have stronger π-π interactions with PPEC8 than that with PPEOC8. This observation is consistent with the SWNT solubility (Table 1)  is attributed to a decreased quantity of free PPE fluorophore. This fluorescence quenching of PPE/SWNT nanocomposites is believed to be caused by energy transfer from the PPEconjugated backbone as an energy donor to the SWNTs as an energy acceptor, indicating that PPE/SWNT nanocomposites involve strong π-π interactions between PPE-conjugated backbones and SWNTs in DCB [41]. This observation is in good agreement with the literature reports on PPE/SWNT nanocomposites. [42,43].  Figure 4 shows changes of the fluorescence intensity of PPE versus SWNT concentration for PPEC8/SWNT and PPEOC8/SWNT in DCB. According to the Stern-Volmer model, [41] the fluorescence intensity I is express by (I0 − I)/I = kSV [Q], where I0 is the intensity without SWNTs, kSV is a Stern-Volmer interaction parameter, and [Q] is the concentration of the quencher (in this case the concentration of the SWNT). Linear fits to the fluorescence data using the Stern-Volmer model gives a kSV~490 mL/mg for PPEC8/SWNT nanocomposites and a kSV~147 mL/mg for PPEOC8/SWNT nanocomposites. The larger kSV for the PPEC8/SWNT than that in PPEOC8/SWNT may be attributed to the larger degree of polymerization in PPEC8/SWNT (n = 136) than that in PPEOC8/SWNT (n = 45). These results indicate that the SWNTs have stronger π-π interactions with PPEC8 than that with PPEOC8. This observation is consistent with the SWNT solubility (Table 1)

Chromatic Nanocomposites of PPE/SWNT
High mass fraction PPE/SWNT nanocomposite films were received via vacuum filtration. The materials were mechanically robust and folding them 10 times resulted in no obvious mechanical damage. Figure 5 shows typical photographs of chromatic films of PPE/SWNT nanocomposites with 50 wt% SWNTs. Apparently, PPEC8/SWNT nanocomposite films are purple-blue while PPEOC8/SWNT nanocomposite films are green. We prepared various PPE/SWNT nanocomposites with SWNT loading in the range between 10 and 60 wt%. With increasing SWNT loading, the chromatic PPE/SWNT nanocomposites became darker.

Chromatic Nanocomposites of PPE/SWNT
High mass fraction PPE/SWNT nanocomposite films were received via vacuum filtration. The materials were mechanically robust and folding them 10 times resulted in no obvious mechanical damage. Figure 5 shows typical photographs of chromatic films of PPE/SWNT nanocomposites with 50 wt% SWNTs. Apparently, PPEC8/SWNT nanocomposite films are purple-blue while PPEOC8/SWNT nanocomposite films are green. We prepared various PPE/SWNT nanocomposites with SWNT loading in the range between 10 and 60 wt%. With increasing SWNT loading, the chromatic PPE/SWNT nanocomposites became darker.  Figure 6 shows typical SEM images of the PPEC8/SWNT and PPEOC8/SWNT nanocomposites containing 50 wt% SWNTs. We can see that the entangled SWNTs and bundles are shown as nanofibrils. The SWNTs are seen to be homogeneously dispersed in the PPE matrix with no significant aggregations observed in either the PPEC8/SWNT or PPEOC8/SWNT nanocomposites. This morphology supports that SWNTs are debundled by PPE wrapping, which is in good agreement with solubility data and optical absorption/fluorescence spectra as discussed above.   Figure 6 shows typical SEM images of the PPEC8/SWNT and PPEOC8/SWNT nanocomposites containing 50 wt% SWNTs. We can see that the entangled SWNTs and bundles are shown as nanofibrils. The SWNTs are seen to be homogeneously dispersed in the PPE matrix with no significant aggregations observed in either the PPEC8/SWNT or PPEOC8/SWNT nanocomposites. This morphology supports that SWNTs are debundled by PPE wrapping, which is in good agreement with solubility data and optical absorption/fluorescence spectra as discussed above.

Chromatic Nanocomposites of PPE/SWNT
High mass fraction PPE/SWNT nanocomposite films were received via vacuum filtration. The materials were mechanically robust and folding them 10 times resulted in no obvious mechanical damage. Figure 5 shows typical photographs of chromatic films of PPE/SWNT nanocomposites with 50 wt% SWNTs. Apparently, PPEC8/SWNT nanocomposite films are purple-blue while PPEOC8/SWNT nanocomposite films are green. We prepared various PPE/SWNT nanocomposites with SWNT loading in the range between 10 and 60 wt%. With increasing SWNT loading, the chromatic PPE/SWNT nanocomposites became darker.  Figure 6 shows typical SEM images of the PPEC8/SWNT and PPEOC8/SWNT nanocomposites containing 50 wt% SWNTs. We can see that the entangled SWNTs and bundles are shown as nanofibrils. The SWNTs are seen to be homogeneously dispersed in the PPE matrix with no significant aggregations observed in either the PPEC8/SWNT or PPEOC8/SWNT nanocomposites. This morphology supports that SWNTs are debundled by PPE wrapping, which is in good agreement with solubility data and optical absorption/fluorescence spectra as discussed above.  The mechanical performance of high mass fraction PPE/SWNT nanocomposites was quantified by tensile testing. Figure 7 shows typical stress-strain curves of PPEC8/SWNT and PPEOC8/SWNT nanocomposite films with various nanotube loading of 10-60 wt%. The tensile strength is, respectively, 100-150 MPa and 25-80 MPa for PPEC8/SWNT and PPEOC8/SWNT while the Young's modulus is, respectively, 9-15 GPa and 2-10 GPa for PPEC8/SWNT and PPEOC8/SWNT (see Figure 8). PPEC8 possesses 136 repeated units which is three times the repeated units (n = 45) in PPEOC8. The longer conjugated backbone of PPEC8 yields stronger π-π interaction between PPE and SWNTs and thereafter the more efficient load transfer from PPE to SWNTs. The mechanical performance of high mass fraction PPE/SWNT nanocomposites was quantified by tensile testing. Figure 7 shows typical stress-strain curves of PPEC8/SWNT and PPEOC8/SWNT nanocomposite films with various nanotube loading of 10-60 wt%. The tensile strength is, respectively, 100-150 MPa and 25-80 MPa for PPEC8/SWNT and PPEOC8/SWNT while the Young's modulus is, respectively, 9-15 GPa and 2-10 GPa for PPEC8/SWNT and PPEOC8/SWNT (see Figure 8). PPEC8 possesses 136 repeated units which is three times the repeated units (n = 45) in PPEOC8. The longer conjugated backbone of PPEC8 yields stronger π-π interaction between PPE and SWNTs and thereafter the more efficient load transfer from PPE to SWNTs.  The strength and Young's modulus of both PPEC8/SWNT and PPEOC8/SWNT nanocomposites increase more or less linearly with increasing SWNT loading. These results are generally associated with good load transfer from the polymer matrix to the SWNTs [44]. The slope of the strength versus SWNT loading curve is, respectively, 300 MPa and 128 MPa for PPEC8/SWNT and PPEOC8/SWNT, while the slope of Young's modulus versus SWNT loading curve is, respectively, 32 GPa and 14 GPa for PPEC8/SWNT and PPEOC8/SWNT. The data further confirm that the PPEC8 is more effectively reinforced by SWNTs than the PPEOC8. The mechanical performance of high mass fraction PPE/SWNT nanocomposites was quantified by tensile testing. Figure 7 shows typical stress-strain curves of PPEC8/SWNT and PPEOC8/SWNT nanocomposite films with various nanotube loading of 10-60 wt%. The tensile strength is, respectively, 100-150 MPa and 25-80 MPa for PPEC8/SWNT and PPEOC8/SWNT while the Young's modulus is, respectively, 9-15 GPa and 2-10 GPa for PPEC8/SWNT and PPEOC8/SWNT (see Figure 8). PPEC8 possesses 136 repeated units which is three times the repeated units (n = 45) in PPEOC8. The longer conjugated backbone of PPEC8 yields stronger π-π interaction between PPE and SWNTs and thereafter the more efficient load transfer from PPE to SWNTs.  The strength and Young's modulus of both PPEC8/SWNT and PPEOC8/SWNT nanocomposites increase more or less linearly with increasing SWNT loading. These results are generally associated with good load transfer from the polymer matrix to the SWNTs [44]. The slope of the strength versus SWNT loading curve is, respectively, 300 MPa and 128 MPa for PPEC8/SWNT and PPEOC8/SWNT, while the slope of Young's modulus versus SWNT loading curve is, respectively, 32 GPa and 14 GPa for PPEC8/SWNT and PPEOC8/SWNT. The data further confirm that the PPEC8 is more effectively reinforced by SWNTs than the PPEOC8. The strength and Young's modulus of both PPEC8/SWNT and PPEOC8/SWNT nanocomposites increase more or less linearly with increasing SWNT loading. These results are generally associated with good load transfer from the polymer matrix to the SWNTs [44]. The slope of the strength versus SWNT loading curve is, respectively, 300 MPa and 128 MPa for PPEC8/SWNT and PPEOC8/SWNT, while the slope of Young's modulus versus SWNT loading curve is, respectively, 32 GPa and 14 GPa for PPEC8/SWNT and PPEOC8/SWNT. The data further confirm that the PPEC8 is more effectively reinforced by SWNTs than the PPEOC8.
The DC electrical conductivity of chromatic PPE/SWNT nanocomposites (see Figure 9) shows that the conductivity of nanocomposites increases with increasing SWNT loading for both PPEC8/SWNT and PPEOC8/SWNT composites. Remarkably, all PPE/SWNT nanocomposites exhibit very high electrical conductivity (1-5 × 10 3 S/m for PPEC8/SWNT nanocomposites with 10-60 wt% SWNTs and 3-10 × 10 3 S/m for PPEOC8/SWNT nanocomposites with 10-60 wt% SWNTs), which is close to the in-plane conductivity (2 × 10 4 S/m) of pure SWNT films [45]. The high conductivity is attributed to the high mass fraction SWNT (10-60 wt%) in the PPE/SWNT nanocomposites [17,44]. Our results are comparable to PS/SWNT nanocomposites with 80 wt% SWNTs (7000 S/m) [17] and PAN/SWNT nanocomposites with 40 wt% SWNTs (1.5 × 10 4 S/m) [46]. While percolation scaling laws of electrical conductivities of SWNT-based polymer nanocomposites are only strictly valid near the percolation threshold, percolation-like scaling at high volume fractions has also been reported [17]. According to the percolation theory, the conductivity (σ) of a percolative system is predicted to scale with volume fraction (V) as σ = σ 0 where σ 0 is the conductivity of the pure filler (in this case the conductivity of SWNTs), V c is the percolation threshold, and n is the conductivity exponent. As the percolation threshold is very small in the SWNT-based polymer nanocomposites, it might be expected the approximation σ ≈ σ 0 V n to hold in high volume fraction nanocomposites. In this work, we approximated mass fraction (M) as equivalent to volume fraction (V) and obtained σ ≈ σ 0 M n . This equation was fitted to the conductivity versus SWNT mass fraction data in Figure 9. The power law fitting gives a value of σ 0~1 0 4 S/m, which is in good agreement with the in-plane conductivity (2 × 10 4 S/m) of pure SWNT films [45]. The power law exponent n = 0.6-0.9 which is characteristic of hopping in conductive materials [47].
The DC electrical conductivity of chromatic PPE/SWNT nanocomposites (see Figure  9) shows that the conductivity of nanocomposites increases with increasing SWNT loading for both PPEC8/SWNT and PPEOC8/SWNT composites. Remarkably, all PPE/SWNT nanocomposites exhibit very high electrical conductivity (1-5 × 10 3 S/m for PPEC8/SWNT nanocomposites with 10-60 wt% SWNTs and 3-10 × 10 3 S/m for PPEOC8/SWNT nanocomposites with 10-60 wt% SWNTs), which is close to the in-plane conductivity (2 × 10 4 S/m) of pure SWNT films [45]. The high conductivity is attributed to the high mass fraction SWNT (10-60 wt%) in the PPE/SWNT nanocomposites [17,44]. Our results are comparable to PS/SWNT nanocomposites with 80 wt% SWNTs (7000 S/m) [17] and PAN/SWNT nanocomposites with 40 wt% SWNTs (1.5 × 10 4 S/m) [46]. While percolation scaling laws of electrical conductivities of SWNT-based polymer nanocomposites are only strictly valid near the percolation threshold, percolation-like scaling at high volume fractions has also been reported [17]. According to the percolation theory, the conductivity (σ) of a percolative system is predicted to scale with volume fraction (V) as , where is the conductivity of the pure filler (in this case the conductivity of SWNTs), Vc is the percolation threshold, and n is the conductivity exponent. As the percolation threshold is very small in the SWNT-based polymer nanocomposites, it might be expected the approximation to hold in high volume fraction nanocomposites. In this work, we approximated mass fraction (M) as equivalent to volume fraction (V) and obtained . This equation was fitted to the conductivity versus SWNT mass fraction data in Figure 9. The power law fitting gives a value of ~10 4 S/m, which is in good agreement with the in-plane conductivity (2 × 10 4 S/m) of pure SWNT films [45]. The power law exponent n = 0.6-0.9 which is characteristic of hopping in conductive materials [47].

Conclusions
We have reported the dispersions and films of chromatic nanocomposites of poly(pphenylene ethynylene)s (PPEs) and single-walled carbon nanotubes (SWNTs). While the PPEs act as a dispersing agent to solubilize pristine SWNTs in organic solvents, SWNTs serve as a doping agent to enhance the stabilization of the excited states of chromatic PPEs.

Conclusions
We have reported the dispersions and films of chromatic nanocomposites of poly(pphenylene ethynylene)s (PPEs) and single-walled carbon nanotubes (SWNTs). While the PPEs act as a dispersing agent to solubilize pristine SWNTs in organic solvents, SWNTs serve as a doping agent to enhance the stabilization of the excited states of chromatic PPEs. By adding SWNTs, the PPE solution color changes from yellow to blue for the dialkyl-PPE and from orange to green for the dialkyloxy-PPE. Optical properties of PPE/SWNT nanocomposites support chromatism and π-π interactions between PPEs and SWNTs.
By membrane filtration, chromatic films of PPE/SWNT nanocomposites are obtained with high mass SWNT loading. The tensile strength and modulus of resulting nanocomposite films increase more or less linearly with SWNT loading for both dialkyl-PPE and dialkyloxy-PPE. The effective stress transfer from PPEs to individual SWNTs contributes to the enhanced mechanical performance. High electrical conductivities of high mass fraction PPE/SWNT nanocomposite films are close to those of the neat SWNT films and conductivities versus SWNT mass fraction data display percolation-like scaling. Our work may provide a facile way to prepare highly conductive polymer nanocomposites for emerging applications.