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

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

Shanju Zhang 1,*,†, Uwe H. F. Bunz 2,‡ and David G. Bucknall 1,*,§

1 School of Polymer, Textile and Fiber Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA 2 School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332, USA; [email protected] * Correspondence: [email protected] (S.Z.); [email protected] (D.G.B.) † Current address: Department of Chemistry and Biochemistry, California Polytechnic State University, San Luis Obispo, CA 93407, USA. ‡ Current address: Organisch-Chemisches Institut, Universitat Heidelberg, 69120 Heidelberg, Germany. § Current address: School of Engineering and Physical Science, Heriot-Watt University, Edinburgh EH14 4AS, UK.

Abstract: 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 Citation: Zhang, S.; Bunz, U.H.F.; of nanocomposite solutions display significant quenching of the fluorescence intensity and the Stern– Bucknall, D.G. Chromatic Conductive Volmer model is used to analyze fluorescence quenching. Electron microscopy of the chromatic solid Polymer Nanocomposites of Poly (p-Phenylene Ethynylene)s and films of high mass fraction PPE/SWNT nanocomposites obtained by vacuum filtration reveals the Single-Walled Carbon Nanotubes. J. debundled SWNTs in the PPE matrix. The tensile strength and Young’s modulus of these PPE/SWNT Compos. Sci. 2021, 5, 158. https:// nanocomposite films are as high as 150 MPa and 15 GPa, respectively. The composite films exhibit doi.org/10.3390/jcs5060158 remarkably high conductivities, ranging from ~1000 S/m to ~10,000 S/m for 10 wt% and 60 wt% SWNT nanocomposites, respectively. Academic Editor: Francesco Tornabene Keywords: conductive polymer nanocomposites; carbon nanotubes; conjugated polymers; chromatic materials Received: 28 May 2021 Accepted: 11 June 2021 Published: 14 June 2021 1. Introduction Publisher’s Note: MDPI stays neutral Single-walled carbon nanotubes (SWNTs) are promising for various potential appli- with regard to jurisdictional claims in cations such as electronic and biomedical devices and advanced conductive composite published maps and institutional affil- materials because of their unique 1D nanostructure, excellent electrical conductivities iations. and mechanical properties [1–5]. 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 surface- functionalization and molecular physical interactions have been successfully developed Copyright: © 2021 by the authors. to increase repulsive inter-tube interactions in order to disperse SWNTs down to the in- Licensee MDPI, Basel, Switzerland. dividual tubes. These methods include acid oxidation, [6] covalent functionalization, [7] This article is an open access article surfactant adsorption, [8] and polymer wrapping [9]. The latter is the focus of this work, distributed under the terms and which has the advantage that conductive polymer nanocomposites can potentially be pro- conditions of the Creative Commons duced directly by vacuum filtration from a solution without altering the chemical structures Attribution (CC BY) license (https:// of the pristine SWNTs. creativecommons.org/licenses/by/ 4.0/).

J. Compos. Sci. 2021, 5, 158. https://doi.org/10.3390/jcs5060158 https://www.mdpi.com/journal/jcs J. Compos. Sci. 2021, 5, 158 2 of 10

Conjugated polymers are organic semiconductors, which are extensively used in the fabrication of organic electronic devices such as solar cells, light-emitting diodes, sensors and field-effect transistors [10–12]. It has been recognized that the π-conjugated backbone of conjugated polymers strongly interacts with the π-electron surface of SWNTs and solubilizing alkyl side chains of conjugated polymers enables exfoliation and dispersion of the SWNTs in organic solvents [9]. Highly concentrated SWNT dispersions (>1 mg/mL) in common organic solvents have been reported when using poly(3-alkyl thiophene)s (P3AT), [13] poly(arylene ethynylene)s (PAEs), [14] poly(9,90-diakylfluorene)s (PFx), [9] and poly(phenylene vinylene)s (PPVs) [15]. The resulting conductive polymer nanocomposites exhibit synergistic optoelectronic effects. The combination of conjugated polymers and SWNTs can dramatically improve the electrical conductivities and charge mobilities [16–18]. It has been shown that SWNT-based conductive polymer composites as device active layers can efficiently improve the overall efficiency of photovoltaic cells [19–21] and device performance of field effect transistors [22–24]. Poly(p-phenylene ethynylene)s (PPEs) are a fascinating class of linear conjugated polymers with rigid backbone conformation [25–27]. PPEs are sensitive to their surroundings, including changing their color due to torsion-dependent modulation of their electronic properties, with strong thermo- and solvatochromic behavior observed in the dialkyl-PPEs and dialkyloxy-PPEs [27–29]. It is thought that planarization of the linear conjugated backbone is responsible for the unusual optical properties [29]. PPEs have been used to solubilize SWNTs in various organic solvents [30–32]. The rigid, linear-shaped backbone conformation of the PPEs has shown unique effectiveness in exfoliating SWNTs down to individual tubes. For instance, complete dispersion in chloroform up to 2.2 mg/mL SWNT has been reported [33]. The PPEs are highly chromatic materials; however, to the best of our knowledge, there are no reports of chromatic materials of PPE/SWNT nanocomposites to date. In this work, we report chromatic PPE/SWNT nanocomposites generated via simple solution mixing. The significant change in the solution color occurs when the SWNTs are debundled and dispersed in the organic solvents due to PPE wrapping. Absorption and emission spectra of dispersions are analyzed to explain such chromatism. After vacuum filtration, high mass fraction PPE/SWNT nanocomposites are obtained and they exhibit excellent mechanical performance and extremely high electrical conductivities.

2. 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 Scheme1. 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). J. Compos. Sci. 2021, 5, 158 3 of 11 J. Compos. Sci. 2021, 5, 158 3 of 10

Scheme 1. Chemical structures of poly(p-phenylene ethynylene)s (PPEs). Dioctyl-PPE denoted as 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. PPEC8 has 136 repeated units while dioctyloxy-PPE denoted as PPEOC8 has 45 repeated units. 3. Results 3.1. Chromatic Dispersion of SWNT-PPE Nanocomposites 3. Results The solubilities of SWNTs in various organic solvents utilizing PPEC8 and PPEOC8 in3.1. 1:1 Chromatic mass ratios withDispersion the SWNT of SWNT-PPE are shown in Table Nanocomposites1. The organic solvents used ( o- dichlorobenzene (DCB), chloroform and toluene) were selected as they are good solvents for PPEs.The Although solubilities SWNTs of are SWNTs insoluble in in various chloroform organic and toluene, solvents and only utilizing weakly PPEC8 and PPEOC8 dispersiblein 1:1 mass in DCB, ratios they with became the highly SWNT soluble are when show addedn in to theTable PPE 1. solutions. The organic The solvents used (o- dispersionsdichlorobenzene of SWNTs/PPE (DCB), remained chloroform fully homogeneous and toluene) without were any signs selected of separation as they are good solvents or precipitation for several weeks after mixing. This observation conﬁrms PPEs act as good dispersingfor PPEs. agents Although for solubilizing SWNTs SWNTs are in insoluble organic solvents in chloroform [14]. The higher and dispersion toluene, and only weakly concentrationdispersible ofin SWNTs DCB, observed they became with PPEC8 highly compared soluble with when PPEOC8, added demonstrates to the PPE solutions. The thatdispersions SWNTs have of stronger SWNTs/PPE interactions remained with the PPEC8. fully ho Thismogeneous phenomenon without is believed any to signs of separation be attributed to a higher degree of polymerization of the PPEC8 [34]. The higher degree ofor polymerization precipitation of PPEC8for several produces weeks a longer after conjugated mixing. backbone This andobservation consequently confirms PPEs act as largergood contacts dispersing per chain agents with nanotubesfor solubilizing via π–π interactions. SWNTs in The organic highest SWNTsolvents dis- [14]. The higher dis- persionpersion concentrations concentration of ~2.5 of mg/mL SWNTs are obtainedobserved with with PPEC8 PPEC8 in DCB. compared This value can with PPEOC8, demon- be favorably compared to the previously reported values of 2.2 mg/mL of SWNTs with PPEsstrates in chloroform that SWNTs [33] and have 2.75 stronger mg/mL of interactions SNWTs with P3ATs with in the DCB PPEC8. [13]. As mayThis phenomenon is be- believed expected, to be higher attributed concentration to a SWNT/PPEhigher degree solutions of polymerization have higher viscosities of inthe DCB; PPEC8 [34]. The higher however,degree theof SWNT/PPEpolymerization dispersions of PPEC8 in this work produces did not forma longer a gel in conjugated contrast to the backbone and conse- literature report [35]. quently larger contacts per chain with nanotubes via π–π interactions. The highest SWNT Tabledispersion 1. Maximum concentrations solubility (mg/mL) of of ~2.5 SWNTs mg/mL in various are solvents obtained by PPE wrapping with PPEC8(a). in DCB. This value can

be favorablyPolymer compared DCBto the previously Chloroform reported values Toluene of 2.2 mg/mL of SWNTs with PPEs inPPEC8 chloroform [33] 2.5and 2.75 mg/mL 2.2of SNWTs with P3ATs 0.5 in DCB [13]. As may be expected,PPEOC8 higher concentration 1.5 SWNT/PPE 1.5 solutions have higher 0.3 viscosities in DCB; how- (a)ever,The mass the ratio SWNT/PPE of SWNTs to PPE wasdispersions set to be 1:1. in this work did not form a gel in contrast to the litera- tureAs report a consequence [35]. of solubilization of the SWNTs by PPE wrapping, we observed a simultaneous color change in the PPE solution. Figure1 shows the color change of PPEs in DCB by adding various amounts of SWNTs. The starting PPE solution concentration is set (a) asTable 1 mg/mL. 1. Maximum Adding 0.625 solubility wt% SWNTs (mg/mL) (relative of SWNTs to PPE amount) in various into solvents the PPE solution by PPE wrapping . results in color changes from yellow to blue–yellow for PPEC8 (Figure1a) and from orange to green–orangePolymer for PPEOC8 (Figure1b). WithDCB increasing SWNT loading,Chloroform the chromatic so- Toluene lutions becomePPEC8 darker. A similar chromatic2.5 behavior has been reported in 2.2 the PPV/SWNT 0.5 nanocomposites [15]. The color change is attributed to the conformation change of the conjugatedPPEOC8 backbone as a result of interaction1.5 with SWNTs [15]. It is well1.5 known that 0.3 nanotubes(a) The mass can ratio act as of atemplate SWNTs for to polymerPPE was orientation set to be across1:1. nanotube–polymer interfaces [36–38]. At a molecular level, the polymer chains are aligned along the tube long axis, formingAs a anconsequence ordered interfacial of solubilization polymer structure of on the nanotube SWNTs surfaces by[ 39PPE,40]. wrapping, In the we observed a conjugated polymers, this ordered polymer structure yields the extended π-electron delo- 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 [36–38]. 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 delocalization [33]. As such the nanotubes act as a doping agent for enhancing

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calization [33]. As such the nanotubes act as a doping agent for enhancing the stabilization the stabilization of the excited states of conjugated polymers, resulting in a drastic change theof thestabilization excited states of the of excited conjugated states polymers, of conjugated resulting polymers, in a drastic resulting change in a drastic in the solutionchange in the solution color [15]. incolor the solution [15]. color [15].

Figure 1. Images of (a) PPEC8/SWNT and (b) PPEOC8/SWNT nanocomposites in DCB. In each image, SWNT loading Figure 1. Images of (a) PPEC8/SWNT and (b) PPEOC8/SWNT nanocomposites in DCB. In each image, SWNT loading Figurerelative 1. toImages PPE amount of (a) PPEC8/SWNT in the vials from and left (b to) PPEOC8/SWNTright is 0 wt%, 0.625 nanocomposites wt%, 1.25 wt%, in 2.5 DCB. wt%, In each5 wt% image, and 10 SWNT wt%. The loading PPE relative to PPE amount in the vials from left to right is 0 wt%, 0.625 wt%, 1.25 wt%, 2.5 wt%, 5 wt% and 10 wt%. The PPE relativeconcentration to PPE is amount set as 1 in mg/mL. the vials from left to right is 0 wt%, 0.625 wt%, 1.25 wt%, 2.5 wt%, 5 wt% and 10 wt%. The PPE concentration is set as 1 mg/mL. concentration is set as 1 mg/mL. The SWNT doping of PPE is further confirmed by the optical properties. Typically, TheThe SWNT SWNT doping doping of of PPE PPE is is further further confirmed conﬁrmed by by the the optical optical properties. properties. Typically, Typically, the PPE/SWNT nanocomposites in DCB were diluted 30 times for investigation of optical thethe PPE/SWNT PPE/SWNT nanocomposites nanocomposites in in DCB DCB were were diluted diluted 30 30 times times for for investigation investigation of of optical optical properties. Figure 2 shows the UV-Vis absorption spectral features of PPEC/SWNT and properties.properties. Figure Figure 22 shows the UV-Vis absorptionabsorption spectralspectral featuresfeatures ofof PPEC/SWNTPPEC/SWNT and PPEOC8/SWNT nanocomposites in DCB with different SWNT loading. Pure PPEC8 and PPEOC8/SWNTPPEOC8/SWNT nanocomposites nanocomposites in inDCB DCB with with different different SWNT SWNT loading. loading. Pure PurePPEC8 PPEC8 and PPEOC8 show a broad absorption band of π-conjugation centered at λ~400 nm and λ~450 PPEOC8and PPEOC8 show showa broad a broadabsorption absorption band of band π-conjugation of π-conjugation centered centered at λ~400 at nmλ~400 and nm λ~450 and nm, respectively. Adding SWNTs into the PPE solution produced a significant change in nm,λ~450 respectively. nm, respectively. Adding Adding SWNTs SWNTs into the into PPE the solution PPE solution produced produced a significant a signiﬁcant change change in the intensity of the π-conjugation band as well as appearance and increase in intensity of thein theintensity intensity of the of theπ-conjugationπ-conjugation band band as well as well as appear as appearanceance and and increase increase in intensity in intensity of a new absorption band at a higher wavelength. The new absorption band is located at aof new a new absorption absorption band band at ata higher a higher wavelength. wavelength. The The new new absorption absorption band band is is located located at at λλ~450 nm and λλ~510 nm for PPEC8/SWNT and PPEOC8/SWNT, respectively. Appearance λ~450~450 nm nm and and λ~510~510 nm nm for for PPEC8/SWNT PPEC8/SWNT and and PPEOC8/SWNT, PPEOC8/SWNT, respectively. respectively. Appearance Appearance of new absorption bands at a higher wavelength in the PPE solution upon adding SWNTs ofof new new absorption absorption bands bands at at a ahigher higher wavelength wavelength in in the the PPE PPE solution solution upon upon adding adding SWNTs SWNTs indicatesindicates SWNTSWNT dopingdoping ofof PPEPPE viavia π––ππ interactionsinteractions betweenbetween PPEPPE backbonesbackbones andand SWNTs, indicates SWNT doping of PPE via π–π interactions between PPE backbones and SWNTs, producingproducing thethe change change in in the the solution solution color. color. This This observation observation is in is goodin good agreement agreement with with the producing the change in the solution color. This observation is in good agreement with literaturethe literature report report on the on PPV/SWNTthe PPV/SWNT nanocomposites nanocomposites in chloroform in chloroform [15]. [15]. the literature report on the PPV/SWNT nanocomposites in chloroform [15].

FigureFigure 2.2. UV-VisUV-Vis solution solution spectra spectra of of ( a(a)) PPEC8/SWNT PPEC8/SWNT andand ((bb)) PPEOC8/SWNTPPEOC8/SWNT inin DCB.DCB. TheThe SWNTSWNT loadingloading relativerelative toto PPEPPE Figure 2. UV-Vis solution spectra of (a) PPEC8/SWNT and (b) PPEOC8/SWNT in DCB. The SWNT loading relative to PPE amount is 0 wt%, 0.625 wt%, 1.25 wt%, 2.5 wt%, 5 wt% and 10 wt%. The PPE concentration is set as 1 mg/30 mL. amountamount is is 0 0 wt%, wt%, 0.625 0.625 wt%, wt%, 1.25 1.25 wt%, wt%, 2.5 2.5 wt%, wt%, 5 5 wt% wt% an andd 10 10 wt%. wt%. The The PPE PPE concentrat concentrationion is is set set as as 1 1 mg/30 mg/30 mL. mL. FigureFigure3 3shows shows ﬂuorescence fluorescence spectral spectral features features of of PPEC8/SWNT PPEC8/SWNT and and PPEOC8/SWNT PPEOC8/SWNT Figure 3 shows fluorescence spectral features of PPEC8/SWNT and PPEOC8/SWNT nanocompositesnanocomposites in DCB DCB as as well well as as control control polymers polymers PPEC8 PPEC8 and and PPEOC8. PPEOC8. Apparently, Apparently, the nanocomposites in DCB as well as control polymers PPEC8 and PPEOC8. Apparently, the theaddition addition of SWNTs of SWNTs to PPE to PPE solutions solutions results results in the in quenching the quenching of PPE of PPEfluorescence ﬂuorescence for both for addition of SWNTs to PPE solutions results in the quenching of PPE fluorescence for both PPEC8 and PPEOC8. The decrease in PPE fluorescence intensity with increasing SWNTs PPEC8 and PPEOC8. The decrease in PPE fluorescence intensity with increasing SWNTs

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both PPEC8 and PPEOC8. The decrease in PPE fluorescence intensity with increasing SWNTsisis attributedattributed is attributed toto aa decreaseddecreased to a decreased quantityquantity ofof quantity freefree PPEPPE of fluorophore.fluorophore. free PPE fluorophore. ThisThis fluorescencefluorescence This fluorescence quenchingquenching quenchingofof PPE/SWNTPPE/SWNT of PPE/SWNT nanocompositesnanocomposites nanocomposites isis believedbelieved is toto believed bebe causedcaused to bebyby caused energyenergy by transfertransfer energy fromfrom transfer thethe fromPPE-PPE- theconjugatedconjugated PPE-conjugated backbonebackbone backbone asas anan energyenergy as an donordonor energy toto thethe donor SWNTsSWNTs to the asas SWNTs anan energyenergy as anacceptor,acceptor, energy indicatingindicating acceptor, indicatingthatthat PPE/SWNTPPE/SWNT that PPE/SWNT nanocompositesnanocomposites nanocomposites involveinvolve strongstrong involve ππ strong––ππ interactionsinteractionsπ–π interactions betweenbetween between PPE-conju-PPE-conju- PPE- conjugatedgatedgated backbonesbackbones backbones andand SWNTsSWNTs and SWNTs inin DCBDCB in [41].[41]. DCB ThisThis [41 observation].observation This observation isis inin goodgood is inagreementagreement good agreement withwith thethe withliteratureliterature the literature reportsreports onon reports PPE/SWNTPPE/SWNT on PPE/SWNT nanocomposites.nanocomposites. nanocomposites. [42,43].[42,43]. [42,43].

Figure 3. Fluorescence spectra (excitation at λ = 350 nm) of (a) PPEC8/SWNT and (b) PPEOC8/SWNT in DCB. The SWNT FigureFigure 3.3. FluorescenceFluorescence spectraspectra (excitation(excitation at atλ λ= =350 350 nm) nm) of of ( a(a)) PPEC8/SWNT PPEC8/SWNT andand ((bb)) PPEOC8/SWNTPPEOC8/SWNT inin DCB.DCB. TheThe SWNTSWNT loadingloading relativerelative toto PPEPPE amountamount isis 00 wt%,wt%, 0.6250.625 wt%,wt%, 1.251.25 wtwt%,%, 2.52.5 wt%,wt%, 55 wt%wt% andand 1010 wt%.wt%. TheThe PPEPPE concentrationconcentration isis setset loading relative to PPE amount is 0 wt%, 0.625 wt%, 1.25 wt%, 2.5 wt%, 5 wt% and 10 wt%. The PPE concentration is set as asas 11 mg/30mg/30 mL.mL. 1 mg/30 mL. FigureFigure 4 44 shows showsshows changeschanges of of thethe the fluorescencefluorescence fluorescence intensityintensity intensity ofof of PPEPPE PPE versusversus versus SWNTSWNT SWNT concentra-concentra- concen- trationtiontion forfor for PPEC8/SWNTPPEC8/SWNT PPEC8/SWNT andand and PPEOC8/SWNTPPEOC8/SWNT PPEOC8/SWNT inin inDCB.DCB. DCB. AccordingAccording According toto to thethe Stern–VolmerStern–VolmerStern–Volmer model,model, [[41][41]41] thethethe fluorescencefluorescencefluorescence intensityintensityintensity I IIis isis express expressexpress by byby ( I (0(II0−0 −−I )/II)/)/II === kkkSVSVSV [Q],[Q], wherewhere II00 isis thethethe intensityintensity withoutwithout SWNTs,SWNTs, kkkSVSVSV isisis aa a Stern–VolmerStern–Volmer Stern–Volmer interactioninteraction parameter,parameter, and and [Q][Q] isis the the con- con- centrationcentration ofof thethe quencherquencher (in(in thisthis casecase thethe concentrationconcentration ofof thethe SWNT).SWNT). LinearLinear fitsfitsfits tototo thethethe fluorescencefluorescencefluorescence data datadata using usingusingthe thethe Stern–Volmer Stern–VolmerStern–Volmer model modelmodel gives givesgives a kaaSV kkSVSV~490~490~490 mL/mg mL/mgmL/mg for forfor PPEC8/SWNT PPEC8/SWNTPPEC8/SWNT nanocompositesnanocomposites and and a a kkSVSV~147~147~147 mL/mgmL/mg mL/mg forfor for PPEOC8/SWNTPPEOC8/SWNT PPEOC8/SWNT nanocomposites.nanocomposites. nanocomposites. TheThe The largerlarger larger kkSVSV kforforSV thethefor PPEC8/SWNTPPEC8/SWNT the PPEC8/SWNT thanthan that thanthat inin that PPEOC8/SWNTPPEOC8/SWNT in PPEOC8/SWNT maymay bebe may attributedattributed be attributed toto thethe largerlarger to the degreedegree larger degreeofof polymerizationpolymerization of polymerization inin PPEC8/SWNTPPEC8/SWNT in PPEC8/SWNT ((nn == 136)136) (n than=than 136) thatthat than inin that PPEOC8/SWNTPPEOC8/SWNT in PPEOC8/SWNT ((nn == 45).45). (n TheseThese= 45). Theseresultsresults results indicateindicate indicate thatthat thethe that SWNTsSWNTs the SWNTs havehave strongerstronger have stronger ππ––ππ interactionsinteractionsπ–π interactions withwith PPEC8PPEC8 with thanthan PPEC8 thatthat thanwithwith thatPPEOC8.PPEOC8. with PPEOC8.ThisThis observationobservation This observation isis consistentconsistent is consistent withwith thethe withSWNTSWNT the solubilitysolubility SWNT solubility (Table(Table 1)1) (Table andand 1opticaloptical) and opticalabsorptionabsorption absorption datadata (Figure(Figure data (Figure 2).2). 2).

FigureFigure 4.4. Stern–VolmerStern–Volmer plotsplots plots ofof of changechange change ofof of fluorescencfluorescenc ﬂuorescenceee intensityintensity intensity versusversus versus SWNTSWNT SWNT concentration.concentration. concentration. ((aa)) ( PPEC8-SWNTPPEC8-SWNTa) PPEC8-SWNT andand and ((bb)) (PPEOC8-SWNT.PPEOC8-SWNT.b) PPEOC8-SWNT.

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J. Compos. Sci. 2021, 5, 158 3.2. Chromatic Nanocomposites of PPE/SWNT 6 of 10 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 J. Compos. Sci. 2021, 5, 158 6 of 11 obvious3.2. Chromatic mechanical Nanocomposites damage. ofFigure PPE/SWNT 5 shows typical photographs of chromatic films of PPE/SWNT nanocomposites with 50 wt% SWNTs. Apparently, PPEC8/SWNT nanocom- High mass fraction PPE/SWNT nanocomposite films were received via vacuum posite films are purple–blue while PPEOC8/SWNT nanocomposite films are green. We 3.2.filtration. Chromatic TheNanocomposites materials of PPE/SWNT were mechanically robust and folding them 10 times resulted prepared various PPE/SWNT nanocomposites with SWNT loading in the range between in noHigh obvious mass fraction mechanical PPE/SWNT damage. nanocomposite Figure films5 shows were received typical via photographsvacuum fil- of chromatic 10 and 60 wt%. With increasing SWNT loading, the chromatic PPE/SWNT nanocompo- tration.films ofThe PPE/SWNT materials were nanocompositesmechanically robust withand folding 50 wt% them SWNTs. 10 times resulted Apparently, in no PPEC8/SWNT sitesobvious became mechanical darker. damage. Figure 5 shows typical photographs of chromatic films of nanocomposite films are purple–blue while PPEOC8/SWNT nanocomposite films are PPE/SWNT nanocomposites with 50 wt% SWNTs. Apparently, PPEC8/SWNT nanocom- positegreen. films We are prepared purple–blue various while PPEOC PPE/SWNT8/SWNT nanocomposite nanocomposites films are with green. SWNT We loading in the preparedrange between various PPE/SWNT 10 and 60 nanocomposites wt%. With increasing with SWNT SWNT loading loading,in the range the between chromatic PPE/SWNT 10nanocomposites and 60 wt%. Withbecame increasing darker. SWNT loading, the chromatic PPE/SWNT nanocomposites became darker.

FigureFigure 5. 5. 5.Photographs PhotographsPhotographs of chromatic of of chromatic nanocomposites nanocomposites with 50 wt% with withSWNTs. 50 ( wt%a) PPEC8/SWNT SWNTs. (anda)) PPEC8/SWNTPPEC8/SWNT and and ((bb) PPEOC8/SWNT.PPEOC8/SWNT. (b) PPEOC8/SWNT. Figure 6 shows typical SEM images of the PPEC8/SWNT and PPEOC8/SWNT nano- compositesFigureFigure containing 6 showsshows 50 typical wt% typical SWNTs. SEM SEM Weimages images can see of that ofthe the the PPEC8/SWNT entangled PPEC8/SWNT SWNTs and and PPEOC8/SWNT and bun- PPEOC8/SWNT nano- compositesdlesnanocomposites are shown containing as nanofibrils. containing 50 The wt% SWNTs 50 SWNTs. wt% are SWNTs.seen We to be can homogeneously We see can that see the that dispersed entangled the entangled in the SWNTs SWNTs and bun- and dlesPPEbundles matrixare shown are with shown noas significantnanofibrils. as nanofibrils. aggregations The SWNTs The observed SWNTs are seenin are either seen to bethe to homogeneously PPEC8/SWNT be homogeneously or dispersed dispersed in the in PPEPPEOC8/SWNT matrix with nanocomposites. no significant This morpho aggregationslogy supports observed that SWNTs in areeither debundled the PPEC8/SWNT or bythe PPE PPE wrapping, matrix which with nois in significant good agreemen aggregationst with solubility observed data and inoptical either absorp- the PPEC8/SWNT or PPEOC8/SWNTtion/fluorescencePPEOC8/SWNT spectra nanocomposites. nanocomposites. as discussed above. This This morpho morphologylogy supports supports that that SWNTs SWNTs are are debundled debundled byby PPE PPE wrapping, wrapping, which which is is in in good good agreemen agreementt with with solubility solubility data data and and optical optical absorp- absorp- tion/fluorescencetion/fluorescence spectra spectra as as discussed discussed above. above.

Figure 6. Typical scanning electron micrographs of PPE/SWNT nanocomposites containing 50 wt% SWNTs. (a) PPEC8/SWNT and (b) PPEOC8/SWNT. The SWNTs are shown as entangled nanofibrils in the polymer matrix.

FigureFigure 6. 6. TypicalTypical scanning scanning electron electron micrographs micrographs of ofPPE/SW PPE/SWNTNT nanocomposites nanocomposites containing containing 50 wt% 50 wt%SWNTs. SWNTs. (a) PPEC8/SWNT(a) PPEC8/SWNT and and(b) PPEOC8/SWNT. (b) PPEOC8/SWNT. The SWNTs The SWNTs are shown are shown as enta as entangledngled nanofibrils nanoﬁbrils in the in polymer the polymer matrix. matrix.

The mechanical performance of high mass fraction PPE/SWNT nanocomposites was quantiﬁed by tensile testing. Figure7 shows typical stress–strain curves of PPEC8/SWNT J. Compos. Sci. 2021, 5, 158 7 of 11 J. Compos. Sci. 2021, 5, 158 7 of 11

J. Compos. Sci. 2021, 5, 158 7 of 10 The mechanical performance of high mass fraction PPE/SWNT nanocomposites was 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 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%. and PPEOC8/SWNT nanocomposite films with various nanotube loading of 10–60 wt%. Theand tensile PPEOC8/SWNT strength is, nanocomposite respectively, 100–150 ﬁlms with MPa various and 25–80 nanotube MPa for loading PPEC8/SWNT of 10–60 wt%.and The tensile strength is, respectively, 100–150 MPa and 25–80 MPa for PPEC8/SWNT and PPEOC8/SWNTThe tensile strength while is, the respectively, Young’s modulus 100–150 MPais, respectively, and 25–80 MPa9–15 forGPa PPEC8/SWNT and 2–10 GPa and for PPEOC8/SWNT while the Young’s modulus is, respectively, 9–15 GPa and 2–10 GPa for PPEC8/SWNTPPEOC8/SWNT and while PPEOC8/SWNT the Young’s modulus(see Figure is, 8). respectively, PPEC8 possesses 9–15 GPa 136 and repeated 2–10 GPa units for PPEC8/SWNT and PPEOC8/SWNT (see Figure 8). PPEC8 possesses 136 repeated units whichPPEC8/SWNT is three times and PPEOC8/SWNTthe repeated units (see (n Figure= 45) in8 ).PPEOC8. PPEC8 possessesThe longer 136 conjugated repeated unitsback- which is three times the repeated units (n = 45) in PPEOC8. The longer conjugated back- bonewhich of is PPEC8 three times yields the stronger repeated π units–π interaction (n = 45) in PPEOC8.between PPE The longerand SWNTs conjugated and thereafter backbone bone of PPEC8 yields stronger π–π interaction between PPE and SWNTs and thereafter theof PPEC8 more efficient yields stronger load transferπ–π interaction from PPE between to SWNTs. PPE and SWNTs and thereafter the more theefﬁcient moreload efficient transfer load from transfer PPE from to SWNTs. PPE to SWNTs.

Figure 7. Stress–strain curves of the PPE/SWNT nanocomposites with various SWNT loadings. (a) PPEC8/SWNT and (b) Figure 7. Stress–strain curves curves of of the the PPE/SWNT PPE/SWNT nanoco nanocompositesmposites with with various various SWNT SWNT loadings. loadings. (a)( PPEC8/SWNTa) PPEC8/SWNT and and (b) PPEOC8/SWNT. The SWNT loading is shown in the plots. (PPEOC8/SWNT.b) PPEOC8/SWNT. The TheSWNT SWNT loading loading is shown is shown in the in theplots. plots.

Figure 8. MechanicalMechanical properties properties of of the the PPE/SWNT PPE/SWNT nanocomposites nanocomposites versus versus SWNT SWNT loading. loading. (a) Tensile (a) Tensile stress stress and and(b) Figure 8. Mechanical properties of the PPE/SWNT nanocomposites versus SWNT loading. (a) Tensile stress and (b) Young’s(b) Young’s modulus. modulus. Young’s modulus. The strength and Young’s modulus of both PPEC8/SWNT and PPEOC8/SWNT nanocompositesThe strength increaseand Young’s more modulus or less of linearly both PPEC8/SWNT with increasing and SWNT PPEOC8/SWNT loading. Thesenano- The strength and Young’s modulus of both PPEC8/SWNT and PPEOC8/SWNT nano- compositesresults are generallyincrease more associated or less with linearly good with load increasing transfer from SWNT the polymerloading. These matrix results to the composites increase more or less linearly with increasing SWNT loading. These results areSWNTs generally [44]. Theassociated slope of with the strength good load versus transfer SWNT from loading the polymer curve is, matrix respectively, to the 300SWNTs MPa are generally associated with good load transfer from the polymer matrix to the SWNTs [44].and 128The MPa slope for of PPEC8/SWNT the strength versus and PPEOC8/SWNT, SWNT loading curve while is, the respectively, slope of Young’s 300 MPa modulus and [44]. The slope of the strength versus SWNT loading curve is, respectively, 300 MPa and 128versus MPa SWNT for PPEC8/SWNT loading curve and is, PPEOC8/SWNT, respectively, 32 GPawhile and the 14slope GPa of for Young’s PPEC8/SWNT modulus ver- and 128 MPa for PPEC8/SWNT and PPEOC8/SWNT, while the slope of Young’s modulus ver- susPPEOC8/SWNT. SWNT loading The curve data furtheris, respectively, conﬁrm that 32 theGPa PPEC8 and 14 is moreGPa effectivelyfor PPEC8/SWNT reinforced and by sus SWNT loading curve is, respectively, 32 GPa and 14 GPa for PPEC8/SWNT and PPEOC8/SWNT.SWNTs than the PPEOC8.The data further confirm that the PPEC8 is more effectively reinforced PPEOC8/SWNT. The data further confirm that the PPEC8 is more effectively reinforced by SWNTsThe DC than electrical the PPEOC8. conductivity of chromatic PPE/SWNT nanocomposites (see Figure9 ) byshows SWNTs that than the conductivity the PPEOC8. 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 × 103 S/m for PPEC8/SWNT nanocomposites with 10–60 wt% SWNTs and 3–10 × 103 S/m for PPEOC8/SWNT nanocomposites with 10–60 wt% SWNTs), which is close to the in-plane conductivity (2 × 104 S/m)

J. Compos. Sci. 2021, 5, 158 8 of 11

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 J. Compos. Sci. 2021, 5, 158 nanocomposites exhibit very high electrical conductivity (1–5 × 103 S/m for PPEC8/SWNT8 of 10 nanocomposites with 10–60 wt% SWNTs and 3–10 × 103 S/m for PPEOC8/SWNT nanocomposites with 10–60 wt% SWNTs), which is close to the in-plane conductivity (2 × 104 S/m) of pure SWNT films [45]. The high conductivity is attributed to the high mass fraction SWNTof pure (10–60 SWNT wt%) films in the [45]. PPE/SWNT The high conductivitynanocomposites is attributed [17,44]. Our to results the high are mass comparable 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 nano- to PS/SWNT nanocomposites with 80 wt% SWNTs (7000 S/m) [17] and PAN/SWNT composites with 40 wt% SWNTs (1.5 × 104 S/m) [46]. While percolation scaling laws of nanocomposites with 40 wt% SWNTs (1.5 × 104 S/m) [46]. While percolation scaling laws electrical conductivities of SWNT-based polymer nanocomposites are only strictly valid 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 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 percola- been reported [17]. According to the percolation theory, the conductivity௡ (σ) of a percola- ௏ି௏೎ n௡ tive system is predicted to scale with volume fraction (V) as ߪൌߪ଴ ቀ ቁ Vൎߪ−Vc ଴ܸ , wheren tive system is predicted to scale with volume fraction (V) as σ =ଵି௏σ೎ ≈ σ V , 0 1−Vc 0 ߪ଴ is the conductivity of the pure filler (in this case the conductivity of SWNTs), Vc is the where σ0 is the conductivity of the pure filler (in this case the conductivity of SWNTs), percolation threshold, and n is the conductivity exponent. As the percolation threshold is Vc is the percolation threshold, and n is the conductivity exponent. As the percolation verythreshold small isin verythe SWNT-based small in the SWNT-basedpolymer nanocomposites, polymer nanocomposites, it might be expected it might the be expectedapprox- ௡ imation ߪൎߪ଴ܸ to hold inn high volume fraction nanocomposites. In this work, we ap- the approximation σ ≈ σ0V to hold in high volume fraction nanocomposites. In this work, proximatedwe approximated mass fraction mass fraction (M) as (Mequivalent) as equivalent to volume to volume fraction fraction (V) and (V )obtained and obtained ߪൎ ߪ ܯ௡. Thisn equation was fitted to the conductivity versus SWNT mass fraction data in σ଴ ≈ σ0 M . This equation was fitted to the conductivity versus SWNT mass fraction data in Figure 9. The power law fitting gives a value of ߪ ~104 S/m, which is in good agreement Figure9. The power law fitting gives a value of σ0଴~10 S/m, which is in good agreement withwith the the in-plane in-plane conductivity conductivity (2 (2 × ×10104 S/m)4 S/m) of pure of pure SWNT SWNT films films [45]. [ 45The]. Thepower power law ex- law ponentexponent n = n0.6–0.9= 0.6–0.9 which which is characteristic is characteristic of hopping of hopping in conductive in conductive materials materials [47]. [47 ].

Figure 9. Data of electrical conductivities versus SWNT mass fraction for PPEC8/SWNT and FigurePPEOC8/SWNT 9. Data of nanocomposites. electrical conductivities The dashed versus lines SWNT are a power-law mass fraction fit. for PPEC8/SWNT and PPEOC8/SWNT nanocomposites. The dashed lines are a power-law fit. 4. Conclusions 4. ConclusionsWe have reported the dispersions and films of chromatic nanocomposites of poly(p- phenyleneWe have ethynylene)s reported the (PPEs) dispersions and single-walled and films of carbonchromatic nanotubes nanocomposites (SWNTs). of While poly( thep- phenylenePPEs act as ethynylene)s a dispersing (PPEs) agent and to solubilize single-walled pristine carbon SWNTs nanotubes in organic (SWNTs). solvents, While SWNTs the PPEsserve act as aas doping a dispersing agent toagent enhance to solubilize the stabilization pristine ofSWNTs the excited in organic states ofsolvents, chromatic SWNTs PPEs. serveBy adding as a doping SWNTs, agent the to PPE enhance solution the stabili colorzation changes of the from excited yellow states to blue of chromatic for the dialkyl- PPEs. 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 J. Compos. Sci. 2021, 5, 158 9 of 10

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.

Author Contributions: Conceptualization, S.Z. and D.G.B.; methodology, S.Z.; validation, S.Z.; formal analysis, S.Z.; investigation, S.Z.; resources, U.H.F.B.; writing—original draft preparation, S.Z.; writing—review and editing, S.Z. and D.G.B.; supervision, D.G.B.; funding acquisition, D.G.B. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Science Foundation (CMMI-0653337). Acknowledgments: S.Z. acknowledges the financial support from American Chemical Society- Petroleum Research Fund (53970-UR7). The authors would like to thank Andrew J. Zappas II and Wei Lin for PPE samples and conductivity measurements, respectively. Conflicts of Interest: The authors declare no conflict of interest.

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