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Morphology characterization of polyaniline nano- and microstructures
Synthetic Metals 158 (2008) 594–601
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Morphology characterization of polyaniline nano- and microstructures Alan R. Hopkins a,∗ , Russell A. Lipeles a , Son-Jong Hwang b a b
Materials Science Department, Space Materials Laboratory, The Aerospace Corporation, P.O. Box 92957, Mail Stop M2-242, Los Angeles, CA 90009-2957, USA Division of Chemistry and Chemical Engineering, California Institute of Technology, 210-41, Pasadena, CA 91125, USA
a r t i c l e
i n f o
Article history: Received 15 October 2007 Received in revised form 12 March 2008 Accepted 15 April 2008 Available online 16 June 2008 Keywords: Polyaniline emeraldine base (PANI-EB) Nanofibers Microtubes Solid-state nuclear magnetic resonance Wide-angle X-ray scattering Small-angle X-ray scattering
a b s t r a c t Small-angle neutron scattering (SANS), nuclear magnetic resonance (NMR), wide-angle and small-angle X-ray scattering (WAXS and SAXS) measurements were carried out to investigate the three morphological forms of polyaniline emeraldine base (PANI-EB): unstructured, microtubes, and nanofibers. Although the chemical backbone between these two materials is quite similar, their solid structures are quite different, showing differences in the molecular conformation and supramolecular packing. Detailed solid-state 13 C and 15 N NMR characterization of PANI nanofibers (compared to the unstructured, granular form) revealed a slight variation in the structural features of the polymer that led to some differences in the chemical environments of the respective nuclei. The presence of two extra-sharp peaks at 96.5 and 179.8 ppm is a distinct feature found exclusively in the nanofiber spectra. Moreover, the crosspolarization (CP) dynamics study disclosed the presence of a complete set of sharp NMR peaks that are responsible for the presence of a more ordered morphology in the nanofiber. Small-angle neutron scattering indicated very sharp interfaces in the PANI fibers, which are well organized and have extremely sharp domains within the length scales probed (∼10–1 nm). Overall, the X-ray scattering and spectroscopy data suggest that the nanofiber form is structurally different from the unstructured, PANI-EB powder. These differences are manifested, in part, by the additional chemistry occurring during the synthesis of the nanofibers. Published by Elsevier B.V.
1. Introduction The ability to combine the electrical properties of semiconductors such as silicon with the versatility and processability of polymers is the basic concept behind intrinsically conducting polymers (ICPs). Within this class of ICPs, polyaniline (PANI) is one of the most versatile materials that has evolved from the field of organic electronics in the past 30 years in terms of its stable mechanical and electrical properties in air and its processability in common solvents. Until 1997, the morphology of the as-synthesized form of PANI was assumed to be featureless, aggregated, granular clumps of polymer. However, in the past decade, “new” macrogeometries of this polymer have been reported in literature [1–5] due primarily to tighter stoichiometric control of polymerization conditions and some clever observations of the resulting polymer using scanning electron microscopy (SEM). As a result, PANI nanowires and fibers with 2–4 nm diameters and microtubes with 1–10 m diameters have been successfully synthesized, in stark contrast to the standard unstructured material that is typically seen in the polymer. Among all these different possible conformations,
∗ Corresponding author. Tel.: +1 310 336 5664. E-mail address: [email protected] (A.R. Hopkins). 0379-6779/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.synthmet.2008.04.018
the self-organization of PANI chains into rod or tube-like structure is of fundamental importance [6]. This new molecular configuration has the potential for producing an organic nanowire that is compatible with sensor fabrication [7–11] and thin film light electrochromic displays [12]. Moreover, density and diameter control of PANI nanofibers grown in the presence of an insulating host material offers the promise of forming more interconnected nanofiber networks that would lower the percolation threshold of a composite. To date, several techniques for creating these PANI nanostructured fibers have been developed, including the use of electropolymerization [13–15] and synthetic, template-free syntheses [16–23]. Most notably, the team of Huang and Kaner [2–4,17] discovered PANI’s unique ability to physically form tubes or fibers by controlling either the stoichiometry of the starting materials or the dynamics of the monomer and catalysts. Recently, Rhiou et al. demonstrated the controlled growth of extra-long PANI nanofibers [23] and Wei and co-workers showed quantitative control of the diameter by changing the redox potential of the oxidants [24]. Moreover, Wan and co-workers [25] proposed formation mechanisms of these self-assembled micro- and nanotubes that showed a direct correlation of tube diameter and room temperature conductivity with the molar ratio of dopant to aniline, but the formation mechanism is still elusive and has not been fully
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understood. Most of these investigations of PANI nanofibers have focused on controlling the formation of PANI nanostructures, but little has been dedicated to completely characterizing how the supramolecular organization of these PANI chains forms the fibrous structures. The purpose of this work is to use scattering techniques such as small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS) and nuclear magnetic resonance (NMR) to study how these PANI chains are spatially arranged within these self-assembled PANI nano- and microstructures and to compare and contrast these two forms (both undoped) with the aggregated, unstructured (i.e. granular) analog structure. Using a common interfacial, synthetic technique [25] that yields both types of tubes (nano and micro), we use the aforementioned non-destructive evaluation techniques to characterize the structures of these self-assembled polymers. The resulting spectra from these techniques and proposed orientation of these PANI chains are discussed herein.
2. Experimental 2.1. Materials and synthesis Aniline from Aldrich Chemical was vacuum distilled over zinc metal before use. 1R-(−)10-camphorsulfonic acid (CSA) from Aldrich Chemical was used as received. The nanofiber form of PANI, designated as nanofiber PANI, was successfully polymerized (using a synthesis described in detail elsewhere [25]) from enriched monomer, allowing parts of the PANI chains to be “tagged” using three different isotopes of the aniline monomer: (a) carbon-13 (13 C6 , 99%), (b) nitrogen-15 (15 N, 98%) and (c) deuterated hydrogen (D7 , 98%) (Cambridge Isotopes). This molecular “tagging” was used in selected areas of the polymer backbone structure to improve the sensitivity and selectivity of the solid-state nuclear magnetic resonance (SSNMR) spectroscopy. The deuterated aniline monomer (D7 , 98%) in the resulting PANI was used to enhance sample scattering and minimize background scattering contributions for SANS. The formation of nanotubes was achieved by using a ratio of [CSA]/[aniline] equal to 0.2:1 and microtubes were formed using the ratio of 3:1. Using this mole ratio of 1R-(−)10-camphorsulfonic acid (CSA dopant) to aniline monomer [26] the polymerization is autocatalyzed by the presence of the doping acid and proceeds at a rapid rate. During the reaction, aniline monomer diffuses from the organic layer (bottom) to the interface, gets protonated by the acidic aqueous layer to form an anilinum cation (stabilized by the phenyl group), and then connects to the ‘tail’ end of an oligomer. Nanofibers form in or near the interface, which suggests that species or conditions at the interface influence polymer morphology. In this template-free synthesis, the driving force(s) involved in forming nanofibers is unclear but it is hypothesized that there may be preferential electrostatic interaction [27] between the aniline monomer and the growing PANI chains that favor nanosized fiber formation. The tubes were filtered and washed with deionized (DI) water and methanol, deprotonated to the emeraldine base (EB) form and subsequently dried in vacuum at room temperature. The unstructured analog of the more ordered nanofiber and microtube PANI-EB forms was synthesized using the original procedure employed by MacDiarmid et al. [28]. The polymerization of PANI utilized (1R)(−)10-CSA as the doping acid to form the salt. This conducting form was subsequently dedoped with 1 M ammonium to yield the EB, which is designated in this work as unstructured PANI-EB.
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2.2. SEM experiment Morphology of nanofibers and microtubes was examined by a JSM-6460LV (JEOL) field emission scanning electron microscope.
2.3. SANS experiments The small-angle neutron scattering data were obtained on an 8 m instrument (in a 2–4 m configuration). The approximate range of the measured scattering vector, Q, scanned was 0.008–0.1 A˚ −1 , to probe a range of length scales from 100 to 1 nm at the SANS Facility of National Institute of Standards and Technology (NIST).
2.4. NMR experiments The nuclear magnetic resonance measurements were achieved using a Bruker DSX-500 spectrometer operating at 500.2, 125.4, 76.8, and 50.7 MHz for 1 H, 13 C, 2 H, and 15 N nuclei, respectively, and a Bruker 4 mm magic angle spinning (MAS) probe. The use of (MAS) and 13 C cross polarization (CP) MAS were employed to circumvent the obstacles of low sensitivity and long relaxation times typical in polymers [29]. Sample spinning rates were varied in 5–14 kHz range depending on the direct or CP methods for multinuclear nuclear magnetic resonance measurements. Chemical shifts were referenced to tetramethylsilane (TMS) for 1 H and 13 C, D2 O, 2 H, NH4 + and 1 M NH4 Claq for 15 N NMR, and all the measurements were made at ambient temperature. Note that no precaution was made for sample packing in the NMR tube. Mainly Bloch-decay signals (i.e. single-pulse excitations) were observed in the spectra reported in this work. In order to prevent severe probe detuning during NMR measurements and to resolve the peaks associated with each PANI form, the respective enriched, doped, PANI was deprotonated (i.e. dedoped) in the presence of 1 M ammonium hydroxide to achieve the enriched, undoped, EB (PANI-EB) form. All NMR experiments used the undoped form of PANI, which is designated in this work as either nanofiber PANI-EB or microtube PANI-EB.
2.5. SAXS experiments Small-angle X-ray scattering was used to obtain the spacing between chains within the fiber/tube macrostructure. Data were collected on a 3-pinhole SAXS instrument, which operates on a rotating anode generator (Cu K␣ 40 kV, 60 mA). A wire area detector was used. The sample-detector distance was 30 cm. Silver Behenate was used to calibrate the scattering angle axis.
2.6. WAXS experiment Wide-angle X-ray was used to study the intramolecular distances (d-spacing) in the various PANI conformations of nanofibers, tubes and unstructured form. Measurements of the powders were performed on a Panalytical X’Pert Pro diffractometer with ˚ at 45 kV and 40 mA. The angular Cu Ka radiation ( = 1.5418 A) 2 range was 1–60◦ . The d-spacings corresponding to the large peak(s) in the respective curves were calculated from Bragg’s equation: = 2(d-spacing) sin where 2 is the X-ray scattering angle.
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Fig. 1. Scanning electron microscope (SEM) of polyaniline emeraldine base (PANI-EB) nanofibers (a–c) synthesized by an interfacial polymerization, which shows the sharp interfaces of these nanofibers with average outer diameter is ∼70 nm. As seen in d–f, the average PANI-EB microtube outer diameter ranges from 10 to 20 m. The unstructured PANI-EB morphology is seen in g–i.
3. Results and discussion
3.2. Small-angle neutron scattering
3.1. Scanning electron microscopy
The complicated interplay between formation of bulk polymer and the mechanism for the formation of nanofibers seems to be key factors in controlling nanofiber diameter. The following smallangle neutron scattering data give some insight into the structures of these self-assembled materials that form nanofibers and microtubes. As seen in Fig. 2, the scattered intensity of the deuterated and undeuterated samples show a linear dependence on the scattered wave vector, Q (inversely proportional to wavelength), in the range 0.1–1 nm−1 . Probing a range of length scales from ∼10 to 1 nm, the scattered intensity (I) exhibits a very high intensity of 1 × 105 , which is primarily caused by the existence of very sharp interfaces in the PANI fibers that are well organized and have extremely sharp domains within the length scales probed. In addition, the scattering intensity scales as Q−4 , which is characteristic of organized materials with very sharp interfaces between domains. Also noted in Fig. 2 is that scattered intensities exhibit a crossover from Q−4 to Q−3 at 0.1 nm−1 (i.e. 10 nm). After subtracting the background scattering, the −4 slope at Q range of 0.1–1 nm−1 is quite apparent, and a fairly convincing −3 slope above Q = 0.1 nm−1 is observed, indicating that the dominant structure is not have sharp interfaces as the length scale increases beyond 10 nm. Deviations from true Porod law (i.e. crossover between Q−4 to Q−3 ) behavior will arise [30] if the interface is diffuse (rough) as seen in the slope differences in Fig. 2. This heterogeneous morphology is consistent with SEM images of these enriched PANI nanofibers (Fig. 3). Pockets of tube aggregation with both rough and sharp interfaces are seen in micrographs (Fig. 2d–f), which pos-
As seen in Fig. 1a–f, both micro- and nanotube forms of PANI were synthesized with average diameters ranging from 10 to 20 m for the microstructures to 70 nm for the nanotubes. As a comparison, the unstructured analog of PANI (granular) was also synthesized and their SEM images are seen in Fig. 1g–i. As well illustrated in Fig. 1b, the as-synthesized nanofibers are on the order of a few microns in length and exhibit a highly branched geometry. The conversion of monomer to tube structure in the synthetic route seems to be energetically favored since very little non-fibrous material was formed. The microfibers in Fig. 1d–f, on the other hand, show a much larger diameter with a rougher outer surface than the nanofibers. A cutaway view of these tubes shows small striations of tube-like structures supporting the outer shell of the hollow microtube. Note that the unstructured PANI contain cylindrical-like structures on either the nano or macrolevel (Fig. 1g–i). While the exact mechanism of formation of these nano- and microstructures is not completely understood, the combination of dopant concentration [25], rate of mixing the polymerization [19], oxidant concentration [23] and oxidant redox potential [24] appears to facilitate their formation. Moreover, the activation energy to form these fibrous structures using a template-free method is observed to be much lower than the case of the formation of carbon nanotubes. These results certainly implicate the ease with which the diameter of PANI can be altered, and indicate that the method has the potential to create multi-diameter fibers or mixed-morphology materials.
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Fig. 2. Small-angle neutron scattering (SANS) intensity profiles for powder samples of (a) deuterated polyaniline (PANI) nanofibers and (b) non-deuterated PANI nanofibers. After subtracting the incoherent background, the −4 slope at Q range of 0.1–1 nm−1 is quite apparent and a fairly convincing −3 slope above Q = 0.1 nm−1 is observed, indicating that the dominant structure is not all sharp interfaces as the length scale increases beyond 10 nm.
sibly contributes to the change in slope of the SANS observed in Fig. 2. As shown in Fig. 2, the scattered intensity of the PANI samples shows an exponential dependence on the scattered wave vector, Q (inversely proportional to wavelength) until larger length scales (smaller Q) are approached. Terminal slopes of Q−4 are observed in both the deuterated and undeuterated cases, indicating that the particles have smooth surfaces. If the materials were purely isolated cylinders, then the Q−4 slope would transition to a Q−1 dependence at the diameter of the cylinder. However, in both of these cases, there are transitions to slopes steeper than −1, at about d = 135 nm. Since there is not a clean transition to a Q−1 dependence, it is unlikely that the diameter of the rods is 135 nm. This result indicates that the fibrils are aggregated to form loose, perhaps fractal-like structures. Furthermore, the transitions in slope (shown in Fig. 2) occur at approximately similar length scales (63 nm for nanofibers and 78 nm for microtubes), indicating a common structural feature to both forms of PANI.
Fig. 3. Scanning electron micrograph (SEM) of polyaniline (PANI) nanofibers synthesized by an interfacial polymerization, which shows sharp interfaces of these nanofibers at size scales of about 100 nm. Pockets of tube aggregation are seen in the micrograph, possibly contributing to the change in slope of the small angle neutron scattering (SANS) seen in Fig. 2.
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Fig. 4. 13 C Bloch-decay (BD) MAS solid-state nuclear magnetic resonance (SSNMR) spectra of undoped 13 C-enriched polyaniline (PANI) in (a) nanofiber and (b) unstructured form. Peak signals to specific carbon sites are given assignments in the PANI repeat unit. Two additional peaks at 95 and 181 ppm are unique to the nanofiber structure.
A typical sample of PANI likely contains isolated tubes, bundles, and a significant fraction of aggregates that coexist in the solid state, and all contribute to the scattered intensity in different ways. Since the slope in Fig. 2 changes from −4 to −3, it is difficult to characterize these nanofibers as simply all random coils or rod-like structures based on the changing slope at such small length scales (1–10 nm). Furthermore, the lack of scattering data beyond the 10 nm length scale precludes prediction of the nature of the morphology of these materials on a larger scale without direct measurement of a larger Q range (i.e. Q = 0.005–0.1 nm−1 ). 3.3. Solid-state nuclear magnetic resonance (SS-NMR) Fig. 4 shows 13 C MAS spectra of PANI both in nanofiber and granular (unstructured) forms. The spectral line shape of the PANI granular material corresponds to 13 C NMR resonances of aromatic rings and quinoid groups which are very similar to that in the previous reports [31–33] as shown in the assignments. The nanofiber PANI powder, at a first glance, revealed the primary carbon resonances in 100–170 ppm range like the granular material while there are a few additional sharp peaks appear to be overlapped at 128.6, 138.7, and 146.4 ppm. The CP dynamics study yielded high values of CP time constants (TCH , T1 ) for these peaks, so further investigation was made in detail. The nature of these sharp components will be further discussed below. In addition, the presence of two extra-sharp peaks at 96.5 and 179.8 ppm is a distinct feature found exclusively in the nanofiber spectra. These two peaks indicate an additional chemical species is present with noticeable amount uniquely in the nanofiber structure. Note that this observation holds true for another batch of PANI nanofiber samples independently prepared. Their chemical shifts allow us to speculate the presence of oxide defects (–CH(OR)(OR )), carbonyl groups in either a carboxylic acid or ester that might be attached to framework phenyl rings. It is also possible that there may be other “defects” that give rise to these peaks. However, infrared analysis shows no evidence of these two functional groups in the nanofiber powder sample. The chemistry that leads to the formation of either of these two groups is not currently known. Fig. 5 shows stack plot of series of 13 C Bloch-decay MAS and CPMAS spectra obtained under various acquisition conditions
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Fig. 6. 13 C CPMAS NMR of deuterated polyaniline emeraldine base (PANI-EB) nanofibers.
Fig. 5. 13 C BDMAS and CPMAS spectra of 13 C-enriched polyaniline (PANI-EB) material in the solid state: (a) BD MAS spectrum of the unstructured form; (b) BD MAS spectrum of nanofiber form; (c) the broad components separated during the inversion recovery of the BD MAS spectrum of nanofiber PANI-EB; (d) the sharp components extracted by the CP-T1 filter method and (e) the fit of the sharp components. The bottom figure (f) is the 13 C CP MAS spectra of nanofiber PANI at various crosspolarization contact times. Spinning sidebands are marked by asterisk.
attempted with an aim of distinguishing the narrow line components uniquely present in the nanofiber of PANI prepared with 13 C-enriched aniline monomer material. As stated above, a simple comparison of these two BD spectra shows that there are two groups of peaks, sharp and broad at ca. 110–165 that are present in the nanofiber material. While three peaks are seen clearly distinguishable from broad peaks, it was predicted that more sharp peaks are present to explain carbon resonances of the benzenoid framework of the polymer. Note that up to 8 peaks were identified for granular form of the polymer [32]. Then the two sets of peaks (broad and sharp) with different characteristics can be correlated with the presence of two regions of phases. A rather clean extraction of sharp peaks from the broad components for all carbons was achieved when the crosspolarization contact time was increased to the point that the broad peaks decayed out due to their shorter life time in the rotating frame (T1 ), as illustrated in Fig. 5f. The 13 C CPMAS spectra with CP contact time longer than 30 ms may then represent the phase of sharp resonances, regardless of the identity. Also, as seen in Table 1, the spin–lattice relaxation time (T1 ) in each of two phases was found to be vastly different from the each other.
Using the difference in life times, it is also possible to make a clean separation of two phases (sharp and broad) by employing the CP-T1 filter method [34] and it is valuable to have the Bloch-decay spectrum split into two representing phases for quantitative purposes. The resulting two separate spectra are shown in Fig. 5c and d. By integrating the two spectra, the content of the sharp components for the nominal phenyl rings, excluding two 96.5 and 179.8 ppm peaks, was measured to be about 22% of total 13 C signal strength. The current result strongly implicates that high-resolution 13 C, solid-state NMR could address the structural deviation between the nanofibers and the unstructured forms. The broad components in the nanofibers appear fairly close to those observed in the unstructured PANI-EB although a subtle difference is noticeable. More specifically, the relaxation times T1 of broad components were comparable to those measured for the unstructured PANI-EB sample (∼10 ms) (see Table 1). The T1 values of the sharp components were found to be significantly longer (40–50 ms). The fact that T1 has been used for probing molecular motion [35] then allows us to conclude that the nanofiber PANIEB contains two morphologically different regions, low and high mobility. It was also noted that the difference in chain mobility is not high enough to affect 2 H MAS NMR spectra of perdeuterated PANIEB nanofiber. No difference was observed (not pictured) in 2 H MAS spectra between nanofiber and unstructured PANI-EB. Moreover, we do not see sharp components in the 13 C CP MAS NMR spectrum of perdeuterated nanofiber (Fig. 6), further proof that the morphology is not the same. The extraordinary resolution of the sharp components can be interpreted to represent at least 6 carbon sites in the 120–150 ppm range, probably carbons in phenyl rings, as shown in the fit of the spectrum (see Fig. 5e). The complete assignment of the sharp peaks is not available at press time. The CP time constant, TCH obtained from the CP dynamics experiment of Fig. 5f and listed in Table 1, allowed us to assign two peaks at 138.7 and 146.5 ppm to carbons bearing no direct C H bonds. The nitrogen (15 N) is an essential nucleus in the solid-state NMR study of polyanilines since it has a key location in the polymer’s backbone and is helpful in determining the structure from which the different forms of polyaniline are composed. In this solid-state study, the 15 N spectra were collected under the conditions of Bloch
Table 1 13 C NMR parameters of peaks found in 13 C-enriched nanofiber and unstructured PANI-EB samples where spin–lattice relaxation time is T1 , TCH is the carbon-proton cross polarization time constant and T1 is the rotating-frame spin–lattice relaxation time constant Peak (ppm)
Line width (Hz)
T1 (s)
TCH (ms) −2
T1 (ms)
Sharp peaks (nanofiber PANI-EB)
96.5 138.7 146.5 179.8
256 184 177 214
16
1.5 × 10 9.7 × 10−1 1.4 1.2
25.6 42.0 43 50
Broad peaks (nanofiber PANI-EB)
124 136 157.9
915 752 647
1.4
1.9 × 10−2 1.7 × 10−2 9.4 × 10−1
6.5 2 13
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Fig. 7. 15 N BDMAS solid-state NMR spectra of undoped 15 N enriched polyaniline (PANI) in (a) nanofiber and (b) unstructured form. Integration of both nitrogen groups (imine and amine) peaks yields a ratio of 1.4 for the nanofibers and 0.8 for unstructured PANI. Noted also is the presence of a lone peak at 32 ppm, which represents the –NH2 end group of the PANI chain that is not seen in the nanofiber material.
decay and magic angle spinning. Fig. 7 illustrates the spectrum of the nanofiber and unstructured forms of PANI-EB, which contain two main peaks at 320 and 60 ppm that arise from the imine and amine nitrogens, respectively. The imine peak at 320 ppm is slightly broader than that observed for standard PANI and appears to have several small shoulders, indicating that the imine nitrogens ( N ) in the PANI nanofiber backbone are in several slightly different environments. The amine nitrogens ( NH ) typically display a peak at 60 ppm and a peak at this position is observed in the unstructured PANI material. However, when the nanofiber N spectrum is blown up (not pictured here), there are additional peaks at 80 ppm and a very sharp shoulder peak at 92 ppm, indicating a different chemical environment for the nanofiber. Our interpretation of these peak shifts in the 15 N NMR spectra is that some additional chemistry is occurring in the polymerization of the PANI nanofibers.
3.4. Wide-angle and small-angle X-ray scattering Although NMR can provide information on sizes of domains, heterogeneities on the length scale of 0.5–50 nm [36–38], other characterization techniques such as wide-angle X-ray and smallangle X-ray scattering can be combined to give information on long-range order of these materials. The degree of order in these materials seems to be in step with typical unstructured PANI-EB [39], with the exception of the highly developed macrostructure of nanofibers. It is proposed that this supramolecular ordering of the PANI chains (as seen in the SEM) will have consequences in the formation of more ordered structures on longer length scales. WAXS yields information on both the short- and long-range order of polymer materials. As seen in Fig. 8c, the undoped, unstructured PANI-EB shows a broad band at a 2 value of ∼21◦ , which is the typical amorphous scattering in the PANI-EB material [40]. The (1 1 0) peak is seen to become broader with the formation of the microtube microstructure (Fig. 8a). However, under the conditions that form the fibrous nanostructure, some crystallinity is seen in Fig. 8b, which is typical of stretch-orientated or doped PANI [40]. Specifically, the Bragg diffraction peaks (as seen in Table 2) found in nanofibers have 2 values of 19.1◦ and 25.4,◦ which were assigned to the (1 1 0) and (2 0 0) crystallographic planes of the orthorhombic unit cell of PANI. Zhang et al. [41] have ascribed these peaks to the periodicity that is parallel and perpendicular to the PANI chains, respectively. However, this was observed in the doped state of PANI and not in the undoped form, indicating that these nanofibers retain their periodicity once dopant is removed. Table 2 Summary of wide-angle X-ray scattering (WAXS) peaks and corresponding distances of nanofibers, microtubes and unstructured forms polyaniline emeraldine base (PANI-EB)
Fig. 8. Wide-angle X-ray diffraction (XRD) patterns of polyaniline emeradine base (PANI-EB) (a) microtubes, (b) nanotubes and (c) unstructured polyaniline emeraldine base (PANI-EB) powder samples. Note the (1 1 0) and (2 0 0) reflections.
Material
2
d (nm)
PANI-EB, nanofibers
6.4 19.1 25.4
0.140 0.046 0.035
PANI-EB, microtubes PANI-EB, unstructured
20.8 19.11
0.043 0.046
600
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Fig. 9. Small-angle X-ray scattering (SAXS) of (a) polyaniline emeraldine base (PANI-EB) nanofibers, (b) PANI-EB microtubes and (c) PANI-EB unstructured. The oriented PANI-EB crystals in the nanofiber sample produce a single scattering maxima at Q = 0.36 A˚ −1 .
With increasing diameter, the intensity of the broad amorphous peak had also increased, indicating that the microtubes have more amorphous content than the nanofibers. This observation is consistent with the CP MAS NMR data where the 13 C spectrum of the nanofiber shows a much narrower line width (Fig. 4a) than the unstructured PANI-EB (Fig. 4b), which is indicative of a more uniform ordered structure in the nanofiber morphology. Another peak seen in nanofiber material (Fig. 8b) is at a very low scattering angle of 6.4◦ which typically corresponds to the large spacing group induced when doped with a larger molecular weight molecule such as dodecyl-benzene sulfonic acid. Without the enhancement of solvent, chemical dopant or film stretching, it is unclear whether the synthetic conditions that caused this peak also enhanced the ordering of PANI chains in the nanofibers. Further investigation of this low-angle peak using SAXS shows a large single scattering (Fig. 9a). It is believed that the oriented PANI-EB domains in the nanofiber sample produce this single scattering maxima at Q = 0.36 A˚ −1 (where Q is the scattering vector whose module is 4 sin /), implying an intercrystal spacing of d ∼ 1.3 nm that has been assigned (0 0 1) crystal plane. Commonly in these conjugated materials, the position in the small Q region is strongly dependent on the molecular size of the respective dopant used to induce crystallinity and electrical conductivity in polyaniline [42]. However, since all forms of the PANI in this study were all thoroughly dedoped (i.e. deprotonated), this reflection is thought to be due to the periodical alternation of semi-crystalline lamellae and amorphous regions within the nanofiber. In this case, a lamellar self-assembly is induced by conditions of the synthesis which is similar to the dopant rendered lamellar structure previously reported [43] for PANI.
Fig. 10. Wide-angle X-ray scattering (WAXS) diffraction 2D photographs taken with a Debye–Scherrer (cylindrical) camera of polyaniline emeraldine base (PANI-EB) (a) nanofibers and (b) microtubes. Note the two equatorial reflections that correspond to (1 1 0) and (2 0 0) reflections in the PANI-EB nanofibers. Average domain size of 1.5 nm as determined by Debye–Scherrer line broadening in the PANI-EB microtubes, whereas the pattern of spacing in the nanofibers indicate large >5–10 m crystal domains.
To account for this single peak seen in Fig. 9a, a layered structure with many folds is proposed, with the microtubes forming many layers that would result in multiple reflections in the small Q region, whereas the nanofiber would have fewer folds and yield a predominate strong maximum peak. This layering effect is readily seen in Fig. 1e and f where the crosssection of a microtube shows many striations and layers of PANI-EB growth that make up the shell of
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the anisotropic structure. The unstructured PANI-EB should have no layering effect and yield a broad, diffuse scattering in the small Q range of 0.30–0.65 A˚ −1 which is what is observed in Fig. 9c. Both NMR and WAXS show a pattern of self-assembly of these chains which lead to some local order. Longer range ordering of PANI-EB is also evident in the SAXS data that shows these self-assembled domains are aligned during the synthesis to yield either the nanotube or microtube structures. Photographic X-ray diffraction patterns obtained by a cylindrical camera for both PANI-EB structures are seen in Fig. 10 Two equatorial reflections are apparent, which correspond to (1 1 0) and (2 0 0) indexes. The position of these diffraction peaks indicates a pseudohexagonal arrangement of the PANI-EB chains in the nanofiber structure, which is lost in the microtube form due to some proposed intermolecular packing disorder of the PANI-EB in the larger diameter tubes. Both of these structures have some short-range order involving neighboring PANI chains but only the nanofiber configurations in Figs. 9a and 10a show any signs of long-range order in the morphology. 4. Conclusions Three different morphological forms of polyaniline emeraldine base (PANI-EB), unstructured, microtubes and nanofibers were studied using small-angle neutron scattering, nuclear magnetic resonance, wide-angle and small-angle X-ray scattering. SANS indicated very sharp interfaces in the PANI fibers, which are well organized and have extremely sharp domains within the length scales probed (∼10–1 nm). Although SEM images confirmed that fiber diameters of 110 nm were outside the length scales probed in SANS, the nanosized structure that caused this scattering is unknown but may play a part in promoting fiber formation as opposed to unstructured formation. Detailed solid-state 13 C and 15 N NMR characterization of PANI nanofibers (compared to the unstructured, granular form) revealed a slight variation in the structural features of the polymer that led to some differences in the chemical environments of the respective nuclei. To account for these differences, there appears to be a substantial amount of additional chemistry occurring during the synthesis of the nanofibers, generating growth of new peaks and shifting others in the polymer nanofibers not seen in the unstructured material. This study has provided the first steps needed to fully understand the chain morphology and structure of the PANI nanofiber material. Acknowledgements This work was supported under The Aerospace Corporation’s Independent Research and Development (IRAD) program. The authors would like to specially thank Professor Matthew P. Espe of the University of Akron for helpful insight and discussion on NMR analyses. Also, we would like to thank Dr. Li-Piin Sung of the National Institute of Standards and Technology (NIST) for allow-
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ing use of their SANS instrument and interpretation of the data. The Caltech NMR facility was supported by the National Science Foundation (NSF) under Grant Number 9724240 and partially supported by the MRSEC Program of the NSF under Award Number DMR-0520565. Finally we would like to thank Dr. Wei H. Kao (The Aerospace Corporation) for many helpful discussions and support for investigating these nanomaterials. References [1] A.Z. Sadek, C.O. Baker, D.A. Powell, W. Wlodarski, R.B. Kaner, K. Kalantar-zadeh, IEEE Sens. J. 7 (1–2) (2007) 213–218. [2] S. Virgi, R.B. Kaner, B.H. Weiller, J. Phys. Chem. 110 (44) (2006) 22266–22270. [3] J.X. Huang, R.B. Kaner, Chem. Commun. 4 (2006) 367–376. [4] D. Li, R.B. Kaner, Chem. Commun. 26 (2005) 3286–3288. [5] D. Zhang, Y. Wang, Mater. Sci. Eng. B 134 (2006) 9–19. [6] A.E. Rowan, R.J.M. Nolte, Angew. Chem. Int. Ed. Eng. 37 (1998) 63. [7] S. Virgi, R.B. Kaner, B.H. Weiller, J. Phys. Chem. B 110 (2006) 22266–22270. [8] S. Virgi, R.B. Kaner, B.H. Weiller, Inorg. Chem. 45 (2006) 10467–10471. [9] S. Virgi, J.D. Fowler, C.O. Baker, J.H. Huang, R.B. Kaner, B.H. Weiller, Small 6 (1) (2005) 624–627. [10] S. Virgi, J. Huang, R.B. Kaner, B.H. Weiller, Nano Lett. 3 (4) (2004) 491–496. [11] S. Virgi, R.B. Kaner, B.H. Weiller, Chem. Mater. 17 (2005) 1256–1260. [12] A.A. Argun, J.R. Reynolds, J. Mater. Chem. 15 (2005) 1793–1800. [13] C.R. Martin, Acc. Chem. Res. 28 (1995) 61–68. [14] V.P. Menon, L. Lei, C.R. Martin, Chem. Mater. 8 (1996) 2382–2390. [15] S.-J. Choi, S.-M. Park, Adv. Mater. 12 (2000) 1547–1549. [16] L. Liu, M.J. Wan, Mater. Chem. 11 (2001) 404–407. [17] J. Huang, R.B. Kaner, J. Am. Chem. Soc. 126 (2004) 851–855. [18] J. Jang, J. Bae, B. Lim, Chem. Commun. 2006 (2006) 1622. [19] J. Huang, R.B. Kaner, Angew. Chem. Int. Ed. 43 (2004) 5817. [20] S.K. Pillalamarri, F.D. Blum, A.T. Tokuhiro, J.G. Story, M.F. Bertino, Chem. Mater. 17 (2005) 227. [21] X.L. Jing, Y.Y. Wang, D. Wu, L. She, Y. Guo, J. Polym. Sci., Part A: Polym. Chem. 44 (2006) 1014. [22] X.L. Jing, Y.Y. Wang, D. Wu, J.P. Qiang, Ultrason. Sonochem. 14 (2007) 75. [23] C.N. Rhiou, L.J. Lee, A.J. Epstein, Chem. Mater. 19 (2007) 3589–3591. [24] H. Ding, M. Wan, Y. Wei, Adv. Mater. 19 (2007) 465–469. [25] Z. Wei, Z. Zhang, M. Wan, Langmuir 18 (2002) 917–921. [26] J. Huang, S. Virgi, B.H. Weiller, R.B. Kaner, J. Am. Chem. Soc. 125 (2) (2003) 314. [27] W. Liu, A.L. Cholli, R. Nagaragan, J. Am. Chem. Soc. 121 (1999) 1345. [28] A.G. MacDiarmid, J.C. Chang, A.F. Richter, N.L.D. Somasiri, A.J. Epstein, in: L. Alcacer (Ed.), Conducting Polymers, Reidel Publishing Co., Holland, 1987, p. 105. [29] R.N. Smith, L. Reven, C.J. Barett, Macromolecules 36 (6) (2003) 1876–1881. [30] R.A. Pethrick, J.V. Dawkins, Modern Techniques for Polymer Characterization, John Wiley & Sons, Chichester, 1999. [31] T. Hjertberg, W.R. Salaneck, I. Lundsrom, N.L.D. Somasiri, A.G. MacDiarmid, J. Polym. Sci. Polym. Lett. 23 (1985) 503. [32] S. Kaplan, E.M. Conwell, A.F. Richter, A.G. MacDiarmid, J. Am. Chem. Soc. 110 (1988) 7647. [33] M.P. Espe, B.R. Mattes, J. Schaefer, Macromolecules 30 (20) (1991) 6307. [34] D.A. Torchia, J. Magn. Res. 30 (1978) 613. [35] F.A. Bovey, P.A. Mirau, NMR of Polymers, Academic Press, San Diego, 1996. [36] R.A. Assink, Macromolecules 11 (1978) 1233. [37] J. Clauss, K. Schmidt-Rohr, H.W. Spiess, Acta Polym. 44 (1993) 1. [38] M. Guo, TRIP 4 (1996) 238. [39] A.R. Hopkins, P.G. Rasmussen, R.A. Basheer, Macromolecules 29 (1996) 7838–7846. [40] W. Zheng, M. Angelopoulos, A.J. Epstein, A.G. MacDiarmid, Macromolecules 30 (1997) 7634–7637. [41] Z.M. Zhang, Z.X. Wei, M.X. Wan, Macromolecules 35 (2002) 5937. [42] A.R. Hopkins, P.G. Rasmussen, R.A. Basheer, B.K. Annis, G.D. Wignall, W.A. Hamilton, Synth. Met. 97 (1998) 47–51. [43] W.-Y. Zheng, R.-H. Wang, K. Levon, Z.Y. Rong, T. Taka, W. Pan, Makromol. Chem. Phys. 196 (1995) 2443.
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Morphology characterization of polyaniline nano- and microstructures Alan R. Hopkins a,∗ , Russell A. Lipeles a , Son-Jong Hwang b a b
Materials Science Department, Space Materials Laboratory, The Aerospace Corporation, P.O. Box 92957, Mail Stop M2-242, Los Angeles, CA 90009-2957, USA Division of Chemistry and Chemical Engineering, California Institute of Technology, 210-41, Pasadena, CA 91125, USA
a r t i c l e
i n f o
Article history: Received 15 October 2007 Received in revised form 12 March 2008 Accepted 15 April 2008 Available online 16 June 2008 Keywords: Polyaniline emeraldine base (PANI-EB) Nanofibers Microtubes Solid-state nuclear magnetic resonance Wide-angle X-ray scattering Small-angle X-ray scattering
a b s t r a c t Small-angle neutron scattering (SANS), nuclear magnetic resonance (NMR), wide-angle and small-angle X-ray scattering (WAXS and SAXS) measurements were carried out to investigate the three morphological forms of polyaniline emeraldine base (PANI-EB): unstructured, microtubes, and nanofibers. Although the chemical backbone between these two materials is quite similar, their solid structures are quite different, showing differences in the molecular conformation and supramolecular packing. Detailed solid-state 13 C and 15 N NMR characterization of PANI nanofibers (compared to the unstructured, granular form) revealed a slight variation in the structural features of the polymer that led to some differences in the chemical environments of the respective nuclei. The presence of two extra-sharp peaks at 96.5 and 179.8 ppm is a distinct feature found exclusively in the nanofiber spectra. Moreover, the crosspolarization (CP) dynamics study disclosed the presence of a complete set of sharp NMR peaks that are responsible for the presence of a more ordered morphology in the nanofiber. Small-angle neutron scattering indicated very sharp interfaces in the PANI fibers, which are well organized and have extremely sharp domains within the length scales probed (∼10–1 nm). Overall, the X-ray scattering and spectroscopy data suggest that the nanofiber form is structurally different from the unstructured, PANI-EB powder. These differences are manifested, in part, by the additional chemistry occurring during the synthesis of the nanofibers. Published by Elsevier B.V.
1. Introduction The ability to combine the electrical properties of semiconductors such as silicon with the versatility and processability of polymers is the basic concept behind intrinsically conducting polymers (ICPs). Within this class of ICPs, polyaniline (PANI) is one of the most versatile materials that has evolved from the field of organic electronics in the past 30 years in terms of its stable mechanical and electrical properties in air and its processability in common solvents. Until 1997, the morphology of the as-synthesized form of PANI was assumed to be featureless, aggregated, granular clumps of polymer. However, in the past decade, “new” macrogeometries of this polymer have been reported in literature [1–5] due primarily to tighter stoichiometric control of polymerization conditions and some clever observations of the resulting polymer using scanning electron microscopy (SEM). As a result, PANI nanowires and fibers with 2–4 nm diameters and microtubes with 1–10 m diameters have been successfully synthesized, in stark contrast to the standard unstructured material that is typically seen in the polymer. Among all these different possible conformations,
∗ Corresponding author. Tel.: +1 310 336 5664. E-mail address: [email protected] (A.R. Hopkins). 0379-6779/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.synthmet.2008.04.018
the self-organization of PANI chains into rod or tube-like structure is of fundamental importance [6]. This new molecular configuration has the potential for producing an organic nanowire that is compatible with sensor fabrication [7–11] and thin film light electrochromic displays [12]. Moreover, density and diameter control of PANI nanofibers grown in the presence of an insulating host material offers the promise of forming more interconnected nanofiber networks that would lower the percolation threshold of a composite. To date, several techniques for creating these PANI nanostructured fibers have been developed, including the use of electropolymerization [13–15] and synthetic, template-free syntheses [16–23]. Most notably, the team of Huang and Kaner [2–4,17] discovered PANI’s unique ability to physically form tubes or fibers by controlling either the stoichiometry of the starting materials or the dynamics of the monomer and catalysts. Recently, Rhiou et al. demonstrated the controlled growth of extra-long PANI nanofibers [23] and Wei and co-workers showed quantitative control of the diameter by changing the redox potential of the oxidants [24]. Moreover, Wan and co-workers [25] proposed formation mechanisms of these self-assembled micro- and nanotubes that showed a direct correlation of tube diameter and room temperature conductivity with the molar ratio of dopant to aniline, but the formation mechanism is still elusive and has not been fully
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understood. Most of these investigations of PANI nanofibers have focused on controlling the formation of PANI nanostructures, but little has been dedicated to completely characterizing how the supramolecular organization of these PANI chains forms the fibrous structures. The purpose of this work is to use scattering techniques such as small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS) and nuclear magnetic resonance (NMR) to study how these PANI chains are spatially arranged within these self-assembled PANI nano- and microstructures and to compare and contrast these two forms (both undoped) with the aggregated, unstructured (i.e. granular) analog structure. Using a common interfacial, synthetic technique [25] that yields both types of tubes (nano and micro), we use the aforementioned non-destructive evaluation techniques to characterize the structures of these self-assembled polymers. The resulting spectra from these techniques and proposed orientation of these PANI chains are discussed herein.
2. Experimental 2.1. Materials and synthesis Aniline from Aldrich Chemical was vacuum distilled over zinc metal before use. 1R-(−)10-camphorsulfonic acid (CSA) from Aldrich Chemical was used as received. The nanofiber form of PANI, designated as nanofiber PANI, was successfully polymerized (using a synthesis described in detail elsewhere [25]) from enriched monomer, allowing parts of the PANI chains to be “tagged” using three different isotopes of the aniline monomer: (a) carbon-13 (13 C6 , 99%), (b) nitrogen-15 (15 N, 98%) and (c) deuterated hydrogen (D7 , 98%) (Cambridge Isotopes). This molecular “tagging” was used in selected areas of the polymer backbone structure to improve the sensitivity and selectivity of the solid-state nuclear magnetic resonance (SSNMR) spectroscopy. The deuterated aniline monomer (D7 , 98%) in the resulting PANI was used to enhance sample scattering and minimize background scattering contributions for SANS. The formation of nanotubes was achieved by using a ratio of [CSA]/[aniline] equal to 0.2:1 and microtubes were formed using the ratio of 3:1. Using this mole ratio of 1R-(−)10-camphorsulfonic acid (CSA dopant) to aniline monomer [26] the polymerization is autocatalyzed by the presence of the doping acid and proceeds at a rapid rate. During the reaction, aniline monomer diffuses from the organic layer (bottom) to the interface, gets protonated by the acidic aqueous layer to form an anilinum cation (stabilized by the phenyl group), and then connects to the ‘tail’ end of an oligomer. Nanofibers form in or near the interface, which suggests that species or conditions at the interface influence polymer morphology. In this template-free synthesis, the driving force(s) involved in forming nanofibers is unclear but it is hypothesized that there may be preferential electrostatic interaction [27] between the aniline monomer and the growing PANI chains that favor nanosized fiber formation. The tubes were filtered and washed with deionized (DI) water and methanol, deprotonated to the emeraldine base (EB) form and subsequently dried in vacuum at room temperature. The unstructured analog of the more ordered nanofiber and microtube PANI-EB forms was synthesized using the original procedure employed by MacDiarmid et al. [28]. The polymerization of PANI utilized (1R)(−)10-CSA as the doping acid to form the salt. This conducting form was subsequently dedoped with 1 M ammonium to yield the EB, which is designated in this work as unstructured PANI-EB.
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2.2. SEM experiment Morphology of nanofibers and microtubes was examined by a JSM-6460LV (JEOL) field emission scanning electron microscope.
2.3. SANS experiments The small-angle neutron scattering data were obtained on an 8 m instrument (in a 2–4 m configuration). The approximate range of the measured scattering vector, Q, scanned was 0.008–0.1 A˚ −1 , to probe a range of length scales from 100 to 1 nm at the SANS Facility of National Institute of Standards and Technology (NIST).
2.4. NMR experiments The nuclear magnetic resonance measurements were achieved using a Bruker DSX-500 spectrometer operating at 500.2, 125.4, 76.8, and 50.7 MHz for 1 H, 13 C, 2 H, and 15 N nuclei, respectively, and a Bruker 4 mm magic angle spinning (MAS) probe. The use of (MAS) and 13 C cross polarization (CP) MAS were employed to circumvent the obstacles of low sensitivity and long relaxation times typical in polymers [29]. Sample spinning rates were varied in 5–14 kHz range depending on the direct or CP methods for multinuclear nuclear magnetic resonance measurements. Chemical shifts were referenced to tetramethylsilane (TMS) for 1 H and 13 C, D2 O, 2 H, NH4 + and 1 M NH4 Claq for 15 N NMR, and all the measurements were made at ambient temperature. Note that no precaution was made for sample packing in the NMR tube. Mainly Bloch-decay signals (i.e. single-pulse excitations) were observed in the spectra reported in this work. In order to prevent severe probe detuning during NMR measurements and to resolve the peaks associated with each PANI form, the respective enriched, doped, PANI was deprotonated (i.e. dedoped) in the presence of 1 M ammonium hydroxide to achieve the enriched, undoped, EB (PANI-EB) form. All NMR experiments used the undoped form of PANI, which is designated in this work as either nanofiber PANI-EB or microtube PANI-EB.
2.5. SAXS experiments Small-angle X-ray scattering was used to obtain the spacing between chains within the fiber/tube macrostructure. Data were collected on a 3-pinhole SAXS instrument, which operates on a rotating anode generator (Cu K␣ 40 kV, 60 mA). A wire area detector was used. The sample-detector distance was 30 cm. Silver Behenate was used to calibrate the scattering angle axis.
2.6. WAXS experiment Wide-angle X-ray was used to study the intramolecular distances (d-spacing) in the various PANI conformations of nanofibers, tubes and unstructured form. Measurements of the powders were performed on a Panalytical X’Pert Pro diffractometer with ˚ at 45 kV and 40 mA. The angular Cu Ka radiation ( = 1.5418 A) 2 range was 1–60◦ . The d-spacings corresponding to the large peak(s) in the respective curves were calculated from Bragg’s equation: = 2(d-spacing) sin where 2 is the X-ray scattering angle.
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Fig. 1. Scanning electron microscope (SEM) of polyaniline emeraldine base (PANI-EB) nanofibers (a–c) synthesized by an interfacial polymerization, which shows the sharp interfaces of these nanofibers with average outer diameter is ∼70 nm. As seen in d–f, the average PANI-EB microtube outer diameter ranges from 10 to 20 m. The unstructured PANI-EB morphology is seen in g–i.
3. Results and discussion
3.2. Small-angle neutron scattering
3.1. Scanning electron microscopy
The complicated interplay between formation of bulk polymer and the mechanism for the formation of nanofibers seems to be key factors in controlling nanofiber diameter. The following smallangle neutron scattering data give some insight into the structures of these self-assembled materials that form nanofibers and microtubes. As seen in Fig. 2, the scattered intensity of the deuterated and undeuterated samples show a linear dependence on the scattered wave vector, Q (inversely proportional to wavelength), in the range 0.1–1 nm−1 . Probing a range of length scales from ∼10 to 1 nm, the scattered intensity (I) exhibits a very high intensity of 1 × 105 , which is primarily caused by the existence of very sharp interfaces in the PANI fibers that are well organized and have extremely sharp domains within the length scales probed. In addition, the scattering intensity scales as Q−4 , which is characteristic of organized materials with very sharp interfaces between domains. Also noted in Fig. 2 is that scattered intensities exhibit a crossover from Q−4 to Q−3 at 0.1 nm−1 (i.e. 10 nm). After subtracting the background scattering, the −4 slope at Q range of 0.1–1 nm−1 is quite apparent, and a fairly convincing −3 slope above Q = 0.1 nm−1 is observed, indicating that the dominant structure is not have sharp interfaces as the length scale increases beyond 10 nm. Deviations from true Porod law (i.e. crossover between Q−4 to Q−3 ) behavior will arise [30] if the interface is diffuse (rough) as seen in the slope differences in Fig. 2. This heterogeneous morphology is consistent with SEM images of these enriched PANI nanofibers (Fig. 3). Pockets of tube aggregation with both rough and sharp interfaces are seen in micrographs (Fig. 2d–f), which pos-
As seen in Fig. 1a–f, both micro- and nanotube forms of PANI were synthesized with average diameters ranging from 10 to 20 m for the microstructures to 70 nm for the nanotubes. As a comparison, the unstructured analog of PANI (granular) was also synthesized and their SEM images are seen in Fig. 1g–i. As well illustrated in Fig. 1b, the as-synthesized nanofibers are on the order of a few microns in length and exhibit a highly branched geometry. The conversion of monomer to tube structure in the synthetic route seems to be energetically favored since very little non-fibrous material was formed. The microfibers in Fig. 1d–f, on the other hand, show a much larger diameter with a rougher outer surface than the nanofibers. A cutaway view of these tubes shows small striations of tube-like structures supporting the outer shell of the hollow microtube. Note that the unstructured PANI contain cylindrical-like structures on either the nano or macrolevel (Fig. 1g–i). While the exact mechanism of formation of these nano- and microstructures is not completely understood, the combination of dopant concentration [25], rate of mixing the polymerization [19], oxidant concentration [23] and oxidant redox potential [24] appears to facilitate their formation. Moreover, the activation energy to form these fibrous structures using a template-free method is observed to be much lower than the case of the formation of carbon nanotubes. These results certainly implicate the ease with which the diameter of PANI can be altered, and indicate that the method has the potential to create multi-diameter fibers or mixed-morphology materials.
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Fig. 2. Small-angle neutron scattering (SANS) intensity profiles for powder samples of (a) deuterated polyaniline (PANI) nanofibers and (b) non-deuterated PANI nanofibers. After subtracting the incoherent background, the −4 slope at Q range of 0.1–1 nm−1 is quite apparent and a fairly convincing −3 slope above Q = 0.1 nm−1 is observed, indicating that the dominant structure is not all sharp interfaces as the length scale increases beyond 10 nm.
sibly contributes to the change in slope of the SANS observed in Fig. 2. As shown in Fig. 2, the scattered intensity of the PANI samples shows an exponential dependence on the scattered wave vector, Q (inversely proportional to wavelength) until larger length scales (smaller Q) are approached. Terminal slopes of Q−4 are observed in both the deuterated and undeuterated cases, indicating that the particles have smooth surfaces. If the materials were purely isolated cylinders, then the Q−4 slope would transition to a Q−1 dependence at the diameter of the cylinder. However, in both of these cases, there are transitions to slopes steeper than −1, at about d = 135 nm. Since there is not a clean transition to a Q−1 dependence, it is unlikely that the diameter of the rods is 135 nm. This result indicates that the fibrils are aggregated to form loose, perhaps fractal-like structures. Furthermore, the transitions in slope (shown in Fig. 2) occur at approximately similar length scales (63 nm for nanofibers and 78 nm for microtubes), indicating a common structural feature to both forms of PANI.
Fig. 3. Scanning electron micrograph (SEM) of polyaniline (PANI) nanofibers synthesized by an interfacial polymerization, which shows sharp interfaces of these nanofibers at size scales of about 100 nm. Pockets of tube aggregation are seen in the micrograph, possibly contributing to the change in slope of the small angle neutron scattering (SANS) seen in Fig. 2.
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Fig. 4. 13 C Bloch-decay (BD) MAS solid-state nuclear magnetic resonance (SSNMR) spectra of undoped 13 C-enriched polyaniline (PANI) in (a) nanofiber and (b) unstructured form. Peak signals to specific carbon sites are given assignments in the PANI repeat unit. Two additional peaks at 95 and 181 ppm are unique to the nanofiber structure.
A typical sample of PANI likely contains isolated tubes, bundles, and a significant fraction of aggregates that coexist in the solid state, and all contribute to the scattered intensity in different ways. Since the slope in Fig. 2 changes from −4 to −3, it is difficult to characterize these nanofibers as simply all random coils or rod-like structures based on the changing slope at such small length scales (1–10 nm). Furthermore, the lack of scattering data beyond the 10 nm length scale precludes prediction of the nature of the morphology of these materials on a larger scale without direct measurement of a larger Q range (i.e. Q = 0.005–0.1 nm−1 ). 3.3. Solid-state nuclear magnetic resonance (SS-NMR) Fig. 4 shows 13 C MAS spectra of PANI both in nanofiber and granular (unstructured) forms. The spectral line shape of the PANI granular material corresponds to 13 C NMR resonances of aromatic rings and quinoid groups which are very similar to that in the previous reports [31–33] as shown in the assignments. The nanofiber PANI powder, at a first glance, revealed the primary carbon resonances in 100–170 ppm range like the granular material while there are a few additional sharp peaks appear to be overlapped at 128.6, 138.7, and 146.4 ppm. The CP dynamics study yielded high values of CP time constants (TCH , T1 ) for these peaks, so further investigation was made in detail. The nature of these sharp components will be further discussed below. In addition, the presence of two extra-sharp peaks at 96.5 and 179.8 ppm is a distinct feature found exclusively in the nanofiber spectra. These two peaks indicate an additional chemical species is present with noticeable amount uniquely in the nanofiber structure. Note that this observation holds true for another batch of PANI nanofiber samples independently prepared. Their chemical shifts allow us to speculate the presence of oxide defects (–CH(OR)(OR )), carbonyl groups in either a carboxylic acid or ester that might be attached to framework phenyl rings. It is also possible that there may be other “defects” that give rise to these peaks. However, infrared analysis shows no evidence of these two functional groups in the nanofiber powder sample. The chemistry that leads to the formation of either of these two groups is not currently known. Fig. 5 shows stack plot of series of 13 C Bloch-decay MAS and CPMAS spectra obtained under various acquisition conditions
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Fig. 6. 13 C CPMAS NMR of deuterated polyaniline emeraldine base (PANI-EB) nanofibers.
Fig. 5. 13 C BDMAS and CPMAS spectra of 13 C-enriched polyaniline (PANI-EB) material in the solid state: (a) BD MAS spectrum of the unstructured form; (b) BD MAS spectrum of nanofiber form; (c) the broad components separated during the inversion recovery of the BD MAS spectrum of nanofiber PANI-EB; (d) the sharp components extracted by the CP-T1 filter method and (e) the fit of the sharp components. The bottom figure (f) is the 13 C CP MAS spectra of nanofiber PANI at various crosspolarization contact times. Spinning sidebands are marked by asterisk.
attempted with an aim of distinguishing the narrow line components uniquely present in the nanofiber of PANI prepared with 13 C-enriched aniline monomer material. As stated above, a simple comparison of these two BD spectra shows that there are two groups of peaks, sharp and broad at ca. 110–165 that are present in the nanofiber material. While three peaks are seen clearly distinguishable from broad peaks, it was predicted that more sharp peaks are present to explain carbon resonances of the benzenoid framework of the polymer. Note that up to 8 peaks were identified for granular form of the polymer [32]. Then the two sets of peaks (broad and sharp) with different characteristics can be correlated with the presence of two regions of phases. A rather clean extraction of sharp peaks from the broad components for all carbons was achieved when the crosspolarization contact time was increased to the point that the broad peaks decayed out due to their shorter life time in the rotating frame (T1 ), as illustrated in Fig. 5f. The 13 C CPMAS spectra with CP contact time longer than 30 ms may then represent the phase of sharp resonances, regardless of the identity. Also, as seen in Table 1, the spin–lattice relaxation time (T1 ) in each of two phases was found to be vastly different from the each other.
Using the difference in life times, it is also possible to make a clean separation of two phases (sharp and broad) by employing the CP-T1 filter method [34] and it is valuable to have the Bloch-decay spectrum split into two representing phases for quantitative purposes. The resulting two separate spectra are shown in Fig. 5c and d. By integrating the two spectra, the content of the sharp components for the nominal phenyl rings, excluding two 96.5 and 179.8 ppm peaks, was measured to be about 22% of total 13 C signal strength. The current result strongly implicates that high-resolution 13 C, solid-state NMR could address the structural deviation between the nanofibers and the unstructured forms. The broad components in the nanofibers appear fairly close to those observed in the unstructured PANI-EB although a subtle difference is noticeable. More specifically, the relaxation times T1 of broad components were comparable to those measured for the unstructured PANI-EB sample (∼10 ms) (see Table 1). The T1 values of the sharp components were found to be significantly longer (40–50 ms). The fact that T1 has been used for probing molecular motion [35] then allows us to conclude that the nanofiber PANIEB contains two morphologically different regions, low and high mobility. It was also noted that the difference in chain mobility is not high enough to affect 2 H MAS NMR spectra of perdeuterated PANIEB nanofiber. No difference was observed (not pictured) in 2 H MAS spectra between nanofiber and unstructured PANI-EB. Moreover, we do not see sharp components in the 13 C CP MAS NMR spectrum of perdeuterated nanofiber (Fig. 6), further proof that the morphology is not the same. The extraordinary resolution of the sharp components can be interpreted to represent at least 6 carbon sites in the 120–150 ppm range, probably carbons in phenyl rings, as shown in the fit of the spectrum (see Fig. 5e). The complete assignment of the sharp peaks is not available at press time. The CP time constant, TCH obtained from the CP dynamics experiment of Fig. 5f and listed in Table 1, allowed us to assign two peaks at 138.7 and 146.5 ppm to carbons bearing no direct C H bonds. The nitrogen (15 N) is an essential nucleus in the solid-state NMR study of polyanilines since it has a key location in the polymer’s backbone and is helpful in determining the structure from which the different forms of polyaniline are composed. In this solid-state study, the 15 N spectra were collected under the conditions of Bloch
Table 1 13 C NMR parameters of peaks found in 13 C-enriched nanofiber and unstructured PANI-EB samples where spin–lattice relaxation time is T1 , TCH is the carbon-proton cross polarization time constant and T1 is the rotating-frame spin–lattice relaxation time constant Peak (ppm)
Line width (Hz)
T1 (s)
TCH (ms) −2
T1 (ms)
Sharp peaks (nanofiber PANI-EB)
96.5 138.7 146.5 179.8
256 184 177 214
16
1.5 × 10 9.7 × 10−1 1.4 1.2
25.6 42.0 43 50
Broad peaks (nanofiber PANI-EB)
124 136 157.9
915 752 647
1.4
1.9 × 10−2 1.7 × 10−2 9.4 × 10−1
6.5 2 13
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599
Fig. 7. 15 N BDMAS solid-state NMR spectra of undoped 15 N enriched polyaniline (PANI) in (a) nanofiber and (b) unstructured form. Integration of both nitrogen groups (imine and amine) peaks yields a ratio of 1.4 for the nanofibers and 0.8 for unstructured PANI. Noted also is the presence of a lone peak at 32 ppm, which represents the –NH2 end group of the PANI chain that is not seen in the nanofiber material.
decay and magic angle spinning. Fig. 7 illustrates the spectrum of the nanofiber and unstructured forms of PANI-EB, which contain two main peaks at 320 and 60 ppm that arise from the imine and amine nitrogens, respectively. The imine peak at 320 ppm is slightly broader than that observed for standard PANI and appears to have several small shoulders, indicating that the imine nitrogens ( N ) in the PANI nanofiber backbone are in several slightly different environments. The amine nitrogens ( NH ) typically display a peak at 60 ppm and a peak at this position is observed in the unstructured PANI material. However, when the nanofiber N spectrum is blown up (not pictured here), there are additional peaks at 80 ppm and a very sharp shoulder peak at 92 ppm, indicating a different chemical environment for the nanofiber. Our interpretation of these peak shifts in the 15 N NMR spectra is that some additional chemistry is occurring in the polymerization of the PANI nanofibers.
3.4. Wide-angle and small-angle X-ray scattering Although NMR can provide information on sizes of domains, heterogeneities on the length scale of 0.5–50 nm [36–38], other characterization techniques such as wide-angle X-ray and smallangle X-ray scattering can be combined to give information on long-range order of these materials. The degree of order in these materials seems to be in step with typical unstructured PANI-EB [39], with the exception of the highly developed macrostructure of nanofibers. It is proposed that this supramolecular ordering of the PANI chains (as seen in the SEM) will have consequences in the formation of more ordered structures on longer length scales. WAXS yields information on both the short- and long-range order of polymer materials. As seen in Fig. 8c, the undoped, unstructured PANI-EB shows a broad band at a 2 value of ∼21◦ , which is the typical amorphous scattering in the PANI-EB material [40]. The (1 1 0) peak is seen to become broader with the formation of the microtube microstructure (Fig. 8a). However, under the conditions that form the fibrous nanostructure, some crystallinity is seen in Fig. 8b, which is typical of stretch-orientated or doped PANI [40]. Specifically, the Bragg diffraction peaks (as seen in Table 2) found in nanofibers have 2 values of 19.1◦ and 25.4,◦ which were assigned to the (1 1 0) and (2 0 0) crystallographic planes of the orthorhombic unit cell of PANI. Zhang et al. [41] have ascribed these peaks to the periodicity that is parallel and perpendicular to the PANI chains, respectively. However, this was observed in the doped state of PANI and not in the undoped form, indicating that these nanofibers retain their periodicity once dopant is removed. Table 2 Summary of wide-angle X-ray scattering (WAXS) peaks and corresponding distances of nanofibers, microtubes and unstructured forms polyaniline emeraldine base (PANI-EB)
Fig. 8. Wide-angle X-ray diffraction (XRD) patterns of polyaniline emeradine base (PANI-EB) (a) microtubes, (b) nanotubes and (c) unstructured polyaniline emeraldine base (PANI-EB) powder samples. Note the (1 1 0) and (2 0 0) reflections.
Material
2
d (nm)
PANI-EB, nanofibers
6.4 19.1 25.4
0.140 0.046 0.035
PANI-EB, microtubes PANI-EB, unstructured
20.8 19.11
0.043 0.046
600
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Fig. 9. Small-angle X-ray scattering (SAXS) of (a) polyaniline emeraldine base (PANI-EB) nanofibers, (b) PANI-EB microtubes and (c) PANI-EB unstructured. The oriented PANI-EB crystals in the nanofiber sample produce a single scattering maxima at Q = 0.36 A˚ −1 .
With increasing diameter, the intensity of the broad amorphous peak had also increased, indicating that the microtubes have more amorphous content than the nanofibers. This observation is consistent with the CP MAS NMR data where the 13 C spectrum of the nanofiber shows a much narrower line width (Fig. 4a) than the unstructured PANI-EB (Fig. 4b), which is indicative of a more uniform ordered structure in the nanofiber morphology. Another peak seen in nanofiber material (Fig. 8b) is at a very low scattering angle of 6.4◦ which typically corresponds to the large spacing group induced when doped with a larger molecular weight molecule such as dodecyl-benzene sulfonic acid. Without the enhancement of solvent, chemical dopant or film stretching, it is unclear whether the synthetic conditions that caused this peak also enhanced the ordering of PANI chains in the nanofibers. Further investigation of this low-angle peak using SAXS shows a large single scattering (Fig. 9a). It is believed that the oriented PANI-EB domains in the nanofiber sample produce this single scattering maxima at Q = 0.36 A˚ −1 (where Q is the scattering vector whose module is 4 sin /), implying an intercrystal spacing of d ∼ 1.3 nm that has been assigned (0 0 1) crystal plane. Commonly in these conjugated materials, the position in the small Q region is strongly dependent on the molecular size of the respective dopant used to induce crystallinity and electrical conductivity in polyaniline [42]. However, since all forms of the PANI in this study were all thoroughly dedoped (i.e. deprotonated), this reflection is thought to be due to the periodical alternation of semi-crystalline lamellae and amorphous regions within the nanofiber. In this case, a lamellar self-assembly is induced by conditions of the synthesis which is similar to the dopant rendered lamellar structure previously reported [43] for PANI.
Fig. 10. Wide-angle X-ray scattering (WAXS) diffraction 2D photographs taken with a Debye–Scherrer (cylindrical) camera of polyaniline emeraldine base (PANI-EB) (a) nanofibers and (b) microtubes. Note the two equatorial reflections that correspond to (1 1 0) and (2 0 0) reflections in the PANI-EB nanofibers. Average domain size of 1.5 nm as determined by Debye–Scherrer line broadening in the PANI-EB microtubes, whereas the pattern of spacing in the nanofibers indicate large >5–10 m crystal domains.
To account for this single peak seen in Fig. 9a, a layered structure with many folds is proposed, with the microtubes forming many layers that would result in multiple reflections in the small Q region, whereas the nanofiber would have fewer folds and yield a predominate strong maximum peak. This layering effect is readily seen in Fig. 1e and f where the crosssection of a microtube shows many striations and layers of PANI-EB growth that make up the shell of
A.R. Hopkins et al. / Synthetic Metals 158 (2008) 594–601
the anisotropic structure. The unstructured PANI-EB should have no layering effect and yield a broad, diffuse scattering in the small Q range of 0.30–0.65 A˚ −1 which is what is observed in Fig. 9c. Both NMR and WAXS show a pattern of self-assembly of these chains which lead to some local order. Longer range ordering of PANI-EB is also evident in the SAXS data that shows these self-assembled domains are aligned during the synthesis to yield either the nanotube or microtube structures. Photographic X-ray diffraction patterns obtained by a cylindrical camera for both PANI-EB structures are seen in Fig. 10 Two equatorial reflections are apparent, which correspond to (1 1 0) and (2 0 0) indexes. The position of these diffraction peaks indicates a pseudohexagonal arrangement of the PANI-EB chains in the nanofiber structure, which is lost in the microtube form due to some proposed intermolecular packing disorder of the PANI-EB in the larger diameter tubes. Both of these structures have some short-range order involving neighboring PANI chains but only the nanofiber configurations in Figs. 9a and 10a show any signs of long-range order in the morphology. 4. Conclusions Three different morphological forms of polyaniline emeraldine base (PANI-EB), unstructured, microtubes and nanofibers were studied using small-angle neutron scattering, nuclear magnetic resonance, wide-angle and small-angle X-ray scattering. SANS indicated very sharp interfaces in the PANI fibers, which are well organized and have extremely sharp domains within the length scales probed (∼10–1 nm). Although SEM images confirmed that fiber diameters of 110 nm were outside the length scales probed in SANS, the nanosized structure that caused this scattering is unknown but may play a part in promoting fiber formation as opposed to unstructured formation. Detailed solid-state 13 C and 15 N NMR characterization of PANI nanofibers (compared to the unstructured, granular form) revealed a slight variation in the structural features of the polymer that led to some differences in the chemical environments of the respective nuclei. To account for these differences, there appears to be a substantial amount of additional chemistry occurring during the synthesis of the nanofibers, generating growth of new peaks and shifting others in the polymer nanofibers not seen in the unstructured material. This study has provided the first steps needed to fully understand the chain morphology and structure of the PANI nanofiber material. Acknowledgements This work was supported under The Aerospace Corporation’s Independent Research and Development (IRAD) program. The authors would like to specially thank Professor Matthew P. Espe of the University of Akron for helpful insight and discussion on NMR analyses. Also, we would like to thank Dr. Li-Piin Sung of the National Institute of Standards and Technology (NIST) for allow-
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