Comparison between Inorganic Geomimetic Chrysotile ... - Springer Link

Fibers and Polymers 2015, Vol.16, No.2, 426-433 DOI 10.1007/s12221-015-0426-x

ISSN 1229-9197 (print version) ISSN 1875-0052 (electronic version)

Comparison between Inorganic Geomimetic Chrysotile and Multiwalled Carbon Nanotubes in the Preparation of One-dimensional Conducting Polymer Nanocomposites Filippo Pierini1,2*, Massimiliano Lanzi3, Isidoro Giorgio Lesci2, and Norberto Roveri2 1

Department of Mechanics and Physics of Fluids, Institute of Fundamental Technological Research, Polish Academy of Sciences, Warsaw 02-106, Poland 2 Department of Chemistry (G.Ciamician), Alma Mater Studiorum, University of Bologna, Bologna 40126, Italy 3 Department of Industrial Chemistry (Toso Montanari), Alma Mater Studiorum, University of Bologna, Bologna 40136, Italy (Received May 20, 2014; Revised October 7, 2014; Accepted October 23, 2014) Abstract: The aim of this study was to examine the role of the nanofillers spatial arrangement in the electrical properties of hybrid organic-inorganic fibers. In this paper, we have presented experimental results for preparation of fibers with a nanometric diameter based on a polyaniline/poly(ethylene oxide) doped blend and geomimetic chrysotile nanotubes. The nanostructured material was prepared using electrospinning techniques. Electrospun fibers made by pristine polymers and by the same blend loaded with carbon nanotubes were used as reference materials to compare the structural, and electrical properties of the novel organic-inorganic material. Generally, electrical properties were improved by the addition of materials that have high conductivity. Electrospun fibers filled with a traditional insulator like chrysotile have shown higher electrical conductivity than the pristine materials. In order to fully understand how structural variations impact upon the electrical conductivity the materials were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy (RS), differential scanning calorimetry (DSC) and four-point probe method. The results suggest that the occurred electrical conductivity gain could be attributed to parallel orientation of the chrysotile nanotubes and higher crystallinity induced by the one-dimensional nanostructured filler materials. The obtained results bring us one step closer to using intrinsically conducting polymers (ICPs) in the creation of functionalized polymeric nanocomposites for nanotechnology. Keywords: Nanocomposites, Conductive polymer, Electrospinning, Chrysotile, Carbon nanotubes

Chen et al. [15] have shown that even if undoped emeraldinebase polyaniline (PANI) is an insulator (σ=10-10 ÷10-8 S/cm), an individual PANI nanotube doped with camphor-10sulphonic acid (CSA) can reach a specific electrical conductivity of 30.5 S/cm. Moreover, the addition of single-walled carbon nanotubes (SWCNTs) into conducting polymers can strongly enhance their thermal and electrical properties [16]. The wet-spinning of polyaniline-carbon nanotubes composite nanofibers showed that fibers containing only 2 % (w/w) of carbon nanotubes have superior mechanical and electrical properties when compared with neat PANI fibers; this effect was attributed to the strong interactions between polymeric and inorganic materials, thus enhancing the effective degree of electronic delocalization [17]. In recent years conductive polymer-inorganic nanocomposites have received much attention and several ICPs have been used as a matrix in composites based on inorganic nanotubes [18,19]. Intrinsically conducting polymers filled with inorganic nanotubes have shown higher mechanical [20], thermal and electrical properties [21] than the pristine materials due to the tubular structure and composition of the fillers and also to the improved crystalline polymer structure [20]. Conductive polymer-inorganic nanocomposites have been used in many applications, such as membrane engineering [22], gas storage [23] and electrochemical energy storage [24].

Introduction In the last decade the preparation and characterization of one-dimensional (1D) structured conducting polymer nanocomposites has become an active field of research due to the unique properties and multiple applications of this new class of materials [1]. These materials can be easily prepared using the electrospinning technique: this is an effective and widely used method for fabricating long continuous monodimensional polymer fibers with diameters ranging from microns to nanometers [2]. One-dimensional nanofibers made of intrinsically conducting polymers (ICPs) have received much attention because of their applications in electronic devices such as chemical and biological sensors [3-7], transistors [8], Schottky diodes [9], solar cells [10] and new catalytic systems [11]. Recently, nanofibers and nanowires of poly(3-buthylthiophene) (P3BT) have been prepared by template electrochemical synthesis [12], whisker method [13], and using an electrospinning technique [14]. In the latter, the ICPs nanofibers were obtained by adding a small amount of poly(ethylene oxide) (PEO) leading to abundant and continuous nanofibers with many optoelectronic properties. *Corresponding author: [email protected] 426

Conductive Electrospun PANI/Nanotubes Fibers

In order to obtain new one-dimensional conducting polymer nanocomposites we used electrospinning method to prepare PANI nanofibers to which we added inorganic geomimetic synthetic chrysotile nanotubes. In particular, we investigated the effects of adding synthetic inorganic chrysotile nanotubes to PANI solutions to prepare one-dimensional conducting polymer nanocomposites through electrospinning. Traditionally, inorganic geomimetic chrysotile nanotubes have been prepared by hydrothermal synthesis, primarily to obtain pure and stoichiometric chrysotile nanocrystals as a reference standard for the ongoing investigation into mineral asbestos cytotoxicity and carcinogenicity [25-34]. They have effectively been used also as new synthetic inorganic nanotubes with a high potential to be applied in the developing field of nanotechnology [35-38]. Synthetic geoinspired chrysotile nanocrystals with Mg3Si2O5(OH)4 stoichiometry are inorganic and insulating nanotubes whose radial dimensions are similar to those of multiwalled carbon nanotubes. In fact, they can be prepared with 7 nm and about 20 nm of inner and outer diameter respectively, and with a length similar to that of normally used carbon nanotubes. These characteristics make synthetic chrysotile nanotubes excellent candidates for the preparation of innovative 1D structured inorganic conducting polymer nanocomposites. Polyaniline-chrysotile nanotubes electrospun composite exhibit promising thermal stability and improved electrical conductivity that differ from those of the neat materials because of their composition, morphology and structure. Obviously, improvement of the electrical conductivity can’t be attributed to the electrical properties of the fillers because chrysotile is an insulator. The reason of the higher composite electrical conductivity than the polymer is that the presence of nanotubes directly affects the polymer chains arrangement and thus the electrical properties of the materials. In fact, the adequate dispersion of filler in the matrix is considered a crucial factor that affects the performance of nanocomposites [39]. Mechanical and electrical properties of nanocomposite based on polymer and one-dimensional (1D) nanostructured disperse material strongly depended on the orientation of nanotubes [40,41]. Several recent research studies have focused on the influence of the nanotube alignment on the nanocomposite properties and on their applications [42,43] and have suggested that the performance of photovoltaic devices based on intrinsically conducting polymers can be improved using nanotubes as an additive to increase the crystallinity by ordering the polymer chains along the wall of the nanotube [44]. The structural and electrical properties of the new one dimensional conducting polymer nanocomposites constituted by PANI nanofibers and inorganic geomimetic synthetic chrysotile nanotubes have been compared with those observed in a reference nanocomposite obtained by substituting geomimetic synthetic chrysotile nanotubes with MWCNTs and prepared under the same experimental conditions.

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Experimental Reagents and Instrumentation All chemicals and solvents were purchased from commercial suppliers. Analytical grade magnesium chloride hexahydrate (MgCl2·6H2O Riedel-de-Haen), sodium hydroxide (NaOH, Aldrich), poly(ethylene oxide) (PEO, average MW=900000, Sigma-Aldrich), polyaniline emeraldine base (PANI, average MW=65000, Sigma-Aldrich), (±)-10-Camphorsulfonic acid (Fluka), chloroform (CHCl3, Sigma-Aldrich), multi-walled carbon nanotubes (MWCNTs, Nanocyl 3100), silicon dioxide Aerosil380 (SiO2, Degussa, surface area of about 380 m2/g) were used without further purification. Water was deionized using a Milli-Q Ultra-Pure-Water Purification 10 System (18.50 MΩ). Glassware and magnetic stir bars were cleaned with aqua regia, washed with distilled water and oven dried. Parr Stirred “Mini” reactor model 4652, equipped with 500 cm3 moveable vessel was used to synthesize stoichiometric chrysotile nanotubes. KD Scientific 200-CE syringe pump and Spellmann SL 150 high voltage power supplies were used to produce electrospun fibers. Infrared FT-IR spectra were recorded on a Thermo Scientific Nicolet 380 FT-IR Spectrometer using KBr pellets prepared by mixing 1 mg of the powdered samples with 150 mg of infrared grade KBr. NT-MDT NTEGRA system is used to characterize the Raman spectrum. The Raman scattering measurement was performed at room temperature using a 532 nm Nd:YAG laser as excitation source. TEM images were taken on a Philips TEM CM100 electron microscope at an accelerating voltage of 80 kV. The dispersed samples were deposited by dropcasting their solutions on Formvar/Carbon 200 mesh copper microgrids. X-ray diffraction (XRD) measurements were conducted on a Philips PW1050/81-PW1710 powder diffractometer. Thermogravimetric analyses were obtained using a Thermal Analysis Instrument SDT Q600 at a heating scan rate of 10 oC/min in a nitrogen atmosphere. A DSC TA Instruments 2920 was used to carry out thermal analyses. Temperatures ranged from -50 to 250 oC with a rate of 10 oC/min. SEM images were taken on a Zeiss EVO MA 10 scanning electron microscope and a on a Phenom™ microscope. Measurements were performed by the four-point probe method using a Keithley 2401 Source Meter connected to an Alessi Instruments four tip head. Electrical conductivity measurement were performed under ambient conditions. Synthesis of Chrysotile Nanotubes Stoichiometric chrysotile nanotubes were synthesized by a hydrothermal reaction under controlled conditions according to the previously reported procedure [26]. 1.07×10-1 mol of magnesium chloride hexahydrate was added under vigorous stirring to a solution of 60 ml of Aerosil 380 (7.10×10-2 mol) in water; the Si/Mg molar ratio was 0.68. The pH was regulated at 13.0 by adding an aqueous solution of sodium hydroxide (NaOH, 260 ml, 2.70×10-1 mol). The mixture was

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Table 1. Experimental parameters for the electrospinning proces Electrical Distance potential Rate collector-needle difference Polymer 8.0 kV 15 cm 0.02 ml/h Polymer and chrysotile 8.0 kV 15 cm 0.02 ml/h nanotubes Polymer and multi-wall 9.5 kV 8 cm 0.02 ml/h carbon nanotubes Composites

stirred for 1.5 h. The subsequent hydrothermal treatment was performed in a Parr reactor model 4652 operating at 82 atm and 300 oC for 6 h. The precipitate was removed from the solution, washed several times with water and then dried through lyophilization. Preparation of the Electrospinnable Solutions A solution of polyaniline (500 mg) and (±)-10-camphorsulfonic acid (645 mg) in 50 ml of CHCl3 was prepared in a flask. After stirring for 24 h, the solution was centrifuged and 500 mg of poly(ethylene)oxide were added to the supernatant. The solution was stirred again for 24 h. The supernatant was cooled down to 4 oC. A syringe was filled with 2 ml of the polymeric solution and inserted into the volumetric pump. Once the needle of the syringe and the metallic collector were connected to the high voltage power supply, the pump was turned on for electrospinning. The polymeric mat was recovered on a special slide at the center of the metallic collector. The preparation of reinforced blends was performed by adding 5 mg of nanomaterial - corresponding to 7.06 % of the total mass of the filler-polymer system - to 2 ml of polymer solution. The chrysotile suspension was shaken for 30 min and repeatedly sonicated for a few seconds. The carbon nanotubes suspension was sonicated for 30 minutes and shaken for 1 minute. Finally, suspensions were electrospun, according to the experimental parameters (Table 1).

Figure 1. TEM image of chrysotile nanotubes.

Figure 2. X-ray diffraction pattern of chrysotile.

Results and Discussion Fillers Stoichiometric chrysotile nanotubes were synthesized as a unique phase by hydrothermal reactions and their physicochemical characterization was performed by TEM, DRX and FT-IR analyses. Morphology was characterized by TEM, and the TEM image in Figure 1 clearly shows the nanotubular shape of synthesized stoichiometric chrysotile nanotubes. In fact, nanorods have a hollow core and are about 0.5-1.0 μm long. Their inner diameter is 7 nm, while the outer one is 21 nm. The nanotubes do not show a tendency to aggregate and the inner cavity is completely free. The X-ray diffraction pattern of synthesized nanotubes in Figure 2 shows a high crystallinity in the obtained samples. The diffraction peaks are ascribable to the chrysotile structure

Figure 3. FT-IR spectrum of a sample of chrysotile nanotubes.

(in particular 2θ =12 o) and confirm the presence of the latter as unique crystalline phase. The FT-IR spectrum in Figure 3 confirms the high purity of synthesized inorganic nanotubes, clearly showing the absorption peaks which are characteristic of chrysotile at 3692 cm-1 (in-phase outer Mg-OH stretching), 3646 cm-1 (in-phase inner Mg-OH stretching), 1082 cm-1 (out-of-plane symmetric Si-O stretching), 1005 cm-1 (in plane parallel to b axis Si-O stretching) and 958 cm-1 (in plane parallel to an axis Si-O stretching). Carbon nanotubes have been morphologically characterized


Figure 4. TEM image of carbon nanotubes.

Figure 5. FT-IR spectrum of a sample of carbon nanotubes.

Figure 6. X-ray diffraction pattern of carbon nanotubes.

by TEM, showing that nanotubes have a diameter of about 10 nm, a length of up to 1.0 μm, and a tendency to aggregate (Figure 4). The FT-IR spectrum in Figure 5 shows the nature of the MWCNTs sample. The wide absorption band at 3435 cm-1 can be ascribed to the presence of -OH groups deriving either from the water present in the nanotubes preserving environment or from the thermo-oxidative process that occurs during the purification of MWCNTs. The XRD pattern in Figure 6 shows a broad peak at about 2θ =25 o and two small ones at about 2θ =43 o and 2θ =53.5 o respectively. These peaks are ascribable to the hexagonal


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Figure 7. Raman spectrum of carbon nanotubes.

graphitic phase of the MWCNTs, in particular: 2θ =25 o (002), 2θ=43 o (100) and 2θ=53.5 o (004). The strong gradient between 2θ =0 o and 2θ =10 o is due to the presence of traces of amorphous carbon, while the peak at 2θ =26.3 o is due to the presence of a small amount of graphite. Raman spectrum (Figure 7) shows the characteristic peaks of carbon materials where two strong Raman peaks G (1588 cm-1) and D (1354 cm-1) band are seen. G-band (Graphite) is attributed to the in-plane vibrations of the C-C bonds and the D-band (Disorder) is due to the presence of defects caused by pentagons, heptagons, amorphous carbon and graphitic impurities. The Raman spectrum also presents the G’ band ascribable to the overtone of the D band at 2685 cm-1. The ratio between the D band and G band - which is a good indicator of the quality of sample - confirms both the previously obtained results and the presence of amorphous carbon traces deposited on the nanotubes surface. Composites In this work we examined the changes in the conducting properties and structural behavior of 1D-conducting polymer nanocomposites made with a polymeric blend of PANI and PEO and reinforced by two distinct inorganic nanotubes. The electrospinning procedure was used to obtain polymeric fibers with diameters in the nano-micron range, meaning that some important parameters needed to be respected, mainly concerning the viscosity of the solution, the temperature and the distance between the nozzle and the fiber collecting surface. With the aim of obtaining high quality fibers, the spinning solution in CHCl3 was added with the same amount of PEO as PANI weight. In fact, it is well known that the electrospinning of pure ICPs (PANI, P3BT and P3HT) is not possible due to the absence of chain entanglements, since these polymers exist in solution in the rigid-rod conformation, while their existence in the random-coil (entangled) form is a prerequisite for using the electrospinning method [45]. The addition of PEO to the PANI solution generally induces chain entanglements with positive effects on the viscosity

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Figure 9. TEM micrograph of PANI/PEO fiber filled with chrysotile nanotubes.

Figure 8. SEM image of electrospun PANI/PEO fibers.

and spinnability of the polymeric blend [46,47]. In order to increase solubility and electrical conductivity of PANI, camphorsulfonic acid was used as a dopant and the thermal, structural and electrical properties of the PANI/PEO electrospun fibers have been compared with those obtained with the addition of chrysotile nanotubes and MWCNTs respectively. Figure 8 shows the SEM image of a sample of electrospun PANI/PEO fibers. The surface of the fibers appears quite smooth with limited imperfections such as beads or blobs. The TEM analysis reveals fiber diameters ranging from 0.2 to 0.8 μm. No phase separation between the two polymers is visible during the electron microscopy characterization. Figure 9 shows the TEM image of fibers filled with chrysotile nanotubes. The anisotropic distribution of chrysotile nanotubes in the fiber is clearly evident. Chrysotile nanotubes appear fastened with their parallel long axis aligned to the fiber axis. The diameter is still within the range observed in the case of unfilled PANI blend. Figure 10 shows the X-ray diffraction spectrum of unspun polymeric blend in film, together with those of its single components, i.e. PANI and PEO polymers and camphorsulfonic acid. The main diffraction peaks observed in blend diffractograms at 2θ =19 o and 24 o can be ascribed to the main crystallization peaks of PEO, as well as the three weak peaks appearing in the 2θ =25÷29 o range. The X-ray diffraction pattern of PANI/PEO nanofibers is displayed in Figure 11. The diffractogram is different from that shown in Figure 10 which refers to the blend in bulk. Electrospun fibers exhibit an XRD spectrum in which diffraction peaks can be related to a more structured polymeric sample, where molecular orientation has been induced by the electrical field during the electrospinning procedure, in agreement with previous

Figure 10. XRD diffractogram of polymer blend components.

Figure 11. X-ray diffraction pattern of PANI/PEO nanofibers.

observations [48]. Both the intense reflection at 2θ =19 o and the broader peak at about 2θ =23 o can be ascribed to PEO chains. The addition of chrysotile nanotubes causes a variation in the intensity of the reflections ascribable to



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Table 2. Electrical conductivities of the prepared samples Sample PANI/PEO unspun PANI/PEO electrospun fibers PANI/PEO chrysotile nanotubes composite electrospun fibers PANI/PEO MWCNTs composite electrospun fibers

Figure 12. X-ray diffraction pattern of PANI/PEO fibers filled with chrysotile nanotubes.

Figure 13. SEM micrograph of PANI/PEO fibers filled with MWCNTs.

Figure 14. X-ray diffraction pattern of PANI/PEO fibers filled with MWCNTs.

Electrical conductivity (S/cm) 4.2 10-9 1.2 10-8 5.5 10-8 8.3 10-7

polymers, since the presence of the filler in the polymer structure leads to the reorganization of the macromolecular crystalline structure. In this case, the peak at 2θ =12 o due to the chrysotile structure is clearly evident (Figure 12). Figure 13 shows the SEM image of the PANI/PEO composite filled with MWCNTs. In this case nanofibers have submicrometric dimensions and morphological characteristics similar to those highlighted for the unfilled PANI/PEO electrospun fibers, but with a higher content of imperfections in the final mat. The X-ray diffraction pattern of the PANI/PEO composite electrospun fibers with MWCNTs is shown in Figure 14. Contrary to expectations, it appears very similar to the pattern of the PANI-PEO electrospun fibers without any diffraction maxima due to the revealing of carbon nanotubes. Table 2 shows the specific electrical conductivities of the measured samples. The electrical conductivity on the surface of nanofiber mats was measured under ambient conditions. The electrical conductivity of the prepared nanofibers is influenced by the type of filler employed. Even if the presence of PEO adversely affects the electrical properties of the examined samples - as PANI films prepared from chloroform/camphorsulfonic acid solutions showed specific electrical conductivity around 10-5 S/cm - conductivity increases approximately threefold passing from the unspun polymer to electrospun nanofibers and increases further when adding nanofillers. The effect of chrysotile nanotubes seems to be mainly conformational, as they act directly on the polymer morphology. As a matter of fact, nanotubes are able to improve the molecular orientation and crystallinity of nanofibers by their arrangement parallel to the fiber axis, along the direction in which they stretch [49]. The best results are obtained with MWCNTs, which is capable of increasing the electrical conductivity of the blend up to two orders of magnitude, if compared with the unspun sample. Figure 15, 16 and 17 show the DSC curves - recorded under nitrogen at a heating/cooling rate of 10 oC/min - of PANI/PEO electrospun fibers as well as of the electrospun fibers with added chrysotile nanotubes and MWCNTs, respectively. Both recorded main transition temperatures and the related enthalpies of fusion (∆Hm) are shown in Table 3. Even if glass transition is hard to find, all the samples show

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reported that nanotubes can have positive effects on the crystallization of electrospun polymers [51].

Conclusion

Figure 15. DSC curve of electrospun PANI/PEO fibers.

Figure 16. DSC curve of electrospun PANI/PEO fibers filled with chrysotile nanotubes.

PANI/PEO nanofibers and PANI/PEO nanofiber composites with chrysotile nanotubes were prepared from a chloroform/ camphorsulfonic acid solution through electrospinning. The fibers obtained were long and straight, with uniform diameter and devoid of conglutination and macroscopic defects, proving that the reinforcing fillers were dispersed well in the final composites. The electrical conductivity of fibers was higher than the polymeric blend in film and, among the electrospun materials, the composite showed higher values of specific conductivity than the non-filled fibers. Even if the final conductivity values of the obtained fibers are not particularly high in absolute terms, they could be enhanced by acting on the amount and type of the added filler. In summary, this outcome suggests the great potential of electrospun PANI/PEO nanofiber composites with chrysotile inorganic nanotubes and MWCNTs in obtaining new strategic materials for molecular electronics.

Acknowledgements We thank the University of Bologna (funds for selected research topics), the Italian Ministry of Education, Universities and Research (MIUR) (PRIN 2009RR5KCE), and the Inter University Consortium for Research on Chemistry of Metals in Biological Systems (C.I.R.C.M.S.B). We are also grateful to Francesco de Laurentiis for his technical assistance.

References Figure 17. DSC curve of electrospun PANI/PEO fibers filled with MWCNTs.

Table 3. Main thermal transitions of electrospun samples Sample PANI/PEO PANI/PEO+chrysotile PANI/PEO+MWCNTs

Tm1 (oC) 50 64 53

∆Hm1 (J/g) 14 57 34

Tm2 (oC) 96 98 112

∆Hm2 (J/g) 151 28 235

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