17592
J. Phys. Chem. C 2008, 112, 17592–17595
New Crystalline Phase Induced by Boron Nitride Nanotubes in Polyaniline Chunyi Zhi,*,† Lijuan Zhang,‡ Yoshio Bando,§ Takeshi Terao,† Chengchun Tang,†,§ Hiroaki Kuwahara,| and Dmitri Golberg†,§ Nanoscale Materials Center, National Institute for Materials Science, Namiki1-1, Tsukuba, Ibaraki 305-004, Japan, Polymer Electronics, Research Centre, Department of Chemistry, UniVersity of Auckland, PriVate Bag 92019, Auckland, New Zealand, World Premier International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, and InnoVation Research Institute, Teijin Limited, 2-1, Hinode-cho, Iwakuni, Yamaguchi 740-8511, Japan ReceiVed: August 20, 2008; ReVised Manuscript ReceiVed: September 19, 2008
We report on a new method and related mechanism triggering new crystalline phase appearance in polyaniline (PANI), the useful conducting polymer. Different from a standard route toward PANI crystallinity improvement through inserting anions between PANI chains and thus enhancing Coulomb forces, it is found that the strong interactions between multiwalled boron nitride nanotubes (BNNTs) and PANI trigger new crystal phase appearance in the initially amorphous polymer. In addition, embedding BNNTs in PANI matrix induces notable variation of oxidation states or doping level, as revealed by Fourier transform infrared spectroscopy, and dramatically modifies wettability of PANI. 1. Introduction Conducting polymers have been of interest for years because of high conductivities and comprehensive applications.1 Among them, polyaniline (PANI) has been particularly well studied due to its remarkable chemical and environmental stability under various conditions.2 The crystalline structure and its evolution are primary important since it is closely related to the electronic states of PANI. However, except utilizing structural effect of dopants,3 sophisticated methods to modify the crystallinity of PANI have been overlooked in the literature3 and surely deserve new detailed studies. In recent years, studies on nanomaterials have become more and more attractive and many novel phenomena at nanoscale dimensions have been discovered.4 Numerous nanomaterials were synthesized, and some of them were found to interact with organic molecules via various weak interactions, such as π-stacking interactions or cation-π interactions, etc.5 Boron nitride nanotubes (BNNTs) possess a π electronic structure,6 which can be an ideal platform for establishing weak interactions with various molecules.7 Moreover, a BNNT is unique due to its constant wide band gap.8 It is also worth noting that BNNTs are chemically inert and structurally stable, which make them fully capable to interact with molecules, while preserving the molecule intrinsic properties.9 In addition, the B-N bonds are of an ionic-like type due to polarization and charge-transfer phenomena,10 which may enhance the regarded interactions with organic molecules also possessing π electronic structures. In fact, some reports have indicated that BNNTs can interact with some conjugated polymers,7 including PANI,11 and the solubility of BNNTs in common organic solvents was remarkably improved through these interactions. * Corresponding author. † Nanoscale Materials Center, National Institute for Materials Science. ‡ University of Auckland. § World Premier International Center for Materials Nanoarchitectonics, National Institute for Materials Science. | Teijin Limited.
In this paper, we discovered a new method to modify the crystallinity of PANI by utilizing the strong interactions between PANI and multiwalled BNNTs and uncovered the related mechanism of such modification. A novel PANI/BNNTs composite was fabricated by in situ polymerization, and various pioneering tests were performed on it. These indicated that, different from a normal amorphous state of PANI, natural for this method, novel crystalline phases were formed after adding BNNTs. BNNTs are suggested to play a role of a core for the “growth” of PANI molecules, and resulting interactions trigger the new ordering of PANI molecules. Another consequence of such interactions is the notable variation in a vibrational behavior of PANI and its BNNTs’ composites. 2. Experimental Section A typical process for the fabrication of PANI was as follows: Aniline (0.1 M) was mixed with 20 mL of distilled water under sonication for 20 min. Then 20 mL of APS ((NH4)2S2O8, 0.1 M) was added to the solution, and the resulting mixture was allowed to react for 24 h. For the fabrication of a PANI/BNNTs composite, a lump of 15 mg of BNNTs was used to absorb several drops of aniline (around 0.05 M). Then the BNNTs were put in an aqueous solution of APS (around 0.05 M) to react for 24 h. The products of PANI and PANI/BNNTs composite were filtered and multiply washed with water and ethanol and then dried in a vacuum at 50 °C for 10 h. A scanning electron microscope (SEM, JEOL SM67F) was used to characterize the products. A fine structure was investigated by means of a JEOL-3000F high-resolution field emission transmission electron microscope (TEM) operated at 300 kV. An SII Exstar DSC 6000 machine was used for the differential scanning calorimetry (DSC) measurements. The heating rate was set at 10 °C/min. A Reinshaw 2000 microRaman system with a 30 mW Ar+ laser of wavelength of 514 nm was employed for the study of Raman spectra. Fourier transform infrared (FTIR) spectra were collected by using a Perkin-Elmer FTIR spectrometer with a laser wavelength of
10.1021/jp807443y CCC: $40.75 2008 American Chemical Society Published on Web 10/18/2008
Boron Nitride Nanotubes/Polyaniline Composite
Figure 1. Image of a BNNTs/PANI composite (a) and an SEM image of the composite surface (b).
1024 nm. X-ray diffraction (XRD) patterns of the samples were recorded on a RINT2200 X-ray diffractometer with standard Cu KR radiation. The PANI was synthesized by in situ chemical oxidative polymerization.12 APS was used as an oxidant, and no dopant reagents were exploited in order to avoid the effects of anions on crystallization behavior. To make the PANI/BNNTs composites, a lump of BNNTs13 was used to absorb aniline during the polymerization process. Such strategy prevented the appearance of large quantities of extra PANI whose formation is not affected by BNNTs. 3. Results and Discussion Figure 1a shows an image of a lump of PANI/BNNTs composite. BNNTs are of a pure white color,14 whereas the composite displays a dark-green and brownish color, which indicates the formation of PANI on the BNNT surface. An SEM image of the composite is presented in Figure 1b. The rough and uncontinuous surface of the composite is similar to that of pure BNNTs shown in Figure 1c, which implies that most of the PANI molecules are attached to the surfaces of BNNTs and that there are not so many of them being filled in the space between the nanotubes. This guarantees that the interactions between BNNTs and PANI can actually influence the growth of PANI.
J. Phys. Chem. C, Vol. 112, No. 45, 2008 17593 The thickness of a PANI layer on BNNTs varies from tube to tube;11 on some nanotubes these layers are not continuous, as illustrated in Figure 2a. The morphology of a surface PANI layer is clearly visible on a high-resolution TEM image, Figure 2b. Its thickness ranges from 2 to 5 nm. The thickness can be easily increased up to 20 nm by introducing an extra amount of aniline before polymerization, but the phenomena discussed below become invisible in the latter case since the samples mainly contain aniline whose formation is not affected by BNNTs. Some nanotubes are filled with PANI, as displayed in Figure 2, parts b and c. The aniline molecules and APS aqueous solution may be absorbed inside BNNTs due to siphonage and react with the inner tube surfaces to form PANI-filled BNNTs. A thermogravimetry analysis (TGA) was further carried out to investigate the weight ratio between PANI and BNNTs in the composites. As shown in Figure 3, PANI starts to lose weight at around 250 °C, which is due to degradation and carbonization of PANI molecules.15 Weight losses above 300 °C are believed to be induced by carbon oxidation within the carbonized PANI. The TGA results reveal that there is ∼20-30 wt % of PANI in the composites. X-ray diffractions from pure BNNTs, PANI, and their composites were taken, as shown in Figure 4. Pure BNNTs exhibit a hexagonal BN (h-BN) phase (two-layered repeating units) mixed with a rhombohedral BN phase (three-layered repeating units).16 In general, a polymer chain has both amorphous and crystalline domains within the matrix, and their ratio varies from one polymer to another. PANI has a highly rigid structure, reportedly being amorphous.12,17 As shown in Figure 4, pure PANI exhibits two broad peaks centered at 21° and 25°, which are characteristics of an amorphous polymer and are attributed to a periodicity parallel and perpendicular to a polymer chain, respectively.12 For the composites of PANI/ BNNTs, a peak at 25° cannot be identified because of its overlapping with the (002) reflection of BNNTs. Most interestingly, the former broad peak at 21° splits into three sharp peaks at 19.9°, 21.3°, and 22.1° with the corresponding d spacings of ∼4.46, 4.17, 4.02 Å, suggestive of a new molecular ordering within the composite. It was reported that an undoped PANI is an amorphous polymer, whereas its crystallinity increases with doping.3,17 This effect was explained by the insertion of anions between polymer chains which add Coulomb forces and drive the structure toward a crystalline state. However, in our experiments, no special anions (except some sulfate ions generated by APS) were adopted; only BNNTs were exploited. This implies that there must be a force which plays the same role as the Coulomb force induced by the anions in a composite. It is suggested that the strong π-stacking interactions between a BNNT surface and quinoid and benzene rings in PANI trigger the ordering of the first PANI molecule layer attached to BNNT.11 Due to this ordering, a large space hindrance induced by coiled chains dramatically reduces, which makes it possible for the second layer to easily approach the first layer. Then, such ordering propagates along the radial direction of a BNNT to form a wellcrystalline phase within PANI. To confirm the formation of a new phase, DSC was applied to the samples. They were heated up to 100 °C and kept for 30 min to surely remove water. As shown in Figure 5, the first exothermic peak of pure PANI under heating is seen at 186 °C, which is suggested to be induced by either recrystallization or a cross-linking reaction,15 since there is no corresponding weight loss at this temperature according to the TGA results of Figure 3. Accordingly, a considerable weight loss associated
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Figure 2. Low-magnification TEM image (a) and a high-resolution TEM image (b) of a BNNTs/PANI composite; (c) a TEM image depicting a BNNT filled with PANI.
Figure 3. Comparative TGA curves of pure PANI and a composite BNNTs/PANI sample.
Figure 5. DSC curves of pure PANI and a composite BNNTs/PANI sample.
Figure 4. XRD patterns of pure PANI, pure BNNTs, and a composite BNNTs/PANI sample.
Figure 6. Comparative FTIR spectra of pure PANI, pure BNNT, and a composite BNNTs/PANI sample.
with the second exothermic peak at ∼270 °C is likely induced by the decomposition and carbonization of PANI molecules. However, for a BNNTs/PANI composite, the first peak upon heating is an exothermic peak at 207 °C. This indicates that a phase transition occurs in PANI/BNNTs composites, and the latter is totally different from the case of pure PANI. The higher transition temperature implies the better heat resistance of ordered molecules due to the stronger interfacial interactions. It should be noted that the curves for a second thermal cycle were featureless for both PANI and its BNNT composites, which indicates that PANI molecules are destroyed at 400 °C, whereas BNNTs do not participate in any endothermic or exothermic processes at this temperature range due to their high thermal stability. Thus, all signals measured in the first cycle are due to PANI. The embedment of BNNTs in PANI should induce variations in vibration of bonds.18 Figure 6 displays FTIR spectra of
BNNTs, PANI, and a BNNTs/PANI composite. A typical FTIR spectrum of BNNTs reveals two dominant peaks at ∼820 and ∼1366 cm-1, which are assigned to be A2u (BN vibration perpendicular to the tube axis) and E1u (BN vibration parallel to the tube axis) modes.19 The FTIR spectrum of PANI within the range of 700-1800 cm-1 exhibits four dominant peaks at 1147, 1302, 1494, 1574 cm-1, respectively. The peak at 1302 cm-1 is due to a benzenoid-amine-stretching vibration, and the intense vibrational mode at 1494 cm-1 originates from a C-C ring-stretching mode.20 The 1147 cm-1 mode is assigned to either C-H in-plane ring deformation or ring-amine-stretching vibration. This normally infrared-inactive mode becomes infraredactive when different substitutes (amine nitrogens or imine nitrogens) are placed in the para-disubstituted benzenes, resulting in the broken symmetry along a chain. The 1574 cm-1 mode has two sources: a C-C ring-stretching vibration of the quinoid
Boron Nitride Nanotubes/Polyaniline Composite
J. Phys. Chem. C, Vol. 112, No. 45, 2008 17595 4. Summary In summary, different from a normal route toward crystallinity modification of PANI by introducing anions, we discovered a new method to trigger an ordered phase appearance in PANI via strong π interactions between multiwalled BNNTs and PANI. Through in situ polymerization, a novel PANI/BNNTs composite film was fabricated. Novel sharp peaks appeared in the XRD spectra; these indicated that PANI molecules tend to orderly pile on the BNNT surfaces, as additionally confirmed by the DSC data. In addition, the embedment of BNNTs in PANI induces the notable variations in vibrational spectra and wettability of PANI due to modified functional groups and/or surface morphology. This may also change the electronic structures and transport properties whose prominent changes are the subject of ongoing calculations and experiments. Acknowledgment. The authors thank Drs. Y. Uemura, M. Mitome, and Mr. K. Kurashima for cooperation and fruitful discussions. References and Notes
Figure 7. Photographs of a water drop on a pure PANI film (a) and on a composite BNNTs/PANI film (b).
groups and a ring-stretching mode of the benzenoid groups. Again, this Raman mode becomes infrared-active because of the symmetry-breaking of the quinoid groups. The relative intensity of modes at 1147 and 1574 cm-1 can be used as a measure of the oxidation state in PANI.21 According to these spectra, the pure PANI is of a half-oxidized emeraldine base (EB) form. However, in a BNNTs/PANI composite, the three peaks at around 1302, 1494, and 1574 cm-1 can be clearly identified, while the peak at 1147 cm-1 becomes much broader and weaker. This reveals that, possibly, different from a pure PANI, the PANI within the formed composite is closer to a fully reduced leucoemeraldine base (LB) form compared with the pure PANI discussed above.18 The other possibility is that the variation of the peak at 1147 cm-1 is induced by different doping levels since this peak can be a measure of the degree of delocalization of electrons.21 The interactions between the BNNTs’ surface and PANI provide a different chemical environment for the growth of PANI chains. This, in turn, induces a different oxidation state or doping level at the same condition of chemical reaction. The hydrophilicity or hydrophobicity is the natural property of materials. PANI normally exhibits hydrophilicity due to hydrophilic functional groups, such as -SO3H, -SO4, etc., whereas BNNTs are known to be hydrophobic.12 In comparison with pure PANI, the BNNTs/PANI composites become more hydrophobic. Figure 7 illustrates the typical appearances of a water drop on a PANI film and a PANI/BNNTs film. A contact angle is dramatically increased from ∼43° to ∼97°. There are two possible reasons behind the regarded wettability variations. One possibility is that the embedment of BNNTs modifies the PANI structure and leads to different functional group coverage of its surface. The other possibility is that the surface morphology of PANI/BNNTs, shown in Figure 1b, is rougher than that of the pure PANI’s surface. In fact, it was reported that a fractal surface can be repellent to a liquid due to enlarged surface area.22 The rougher surface of a PANI/BNNTs film compared with that of a pure PANI film may be responsible for the observed phenomenon. This result paves the way to the delicate and smart adjustment of PANI wettability in many technological applications.
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