Reaction Mechanism to Morphology Control of ...

Reaction Mechanism to Morphology Control of Polyaniline Nanomaterials Zhongfen Ding,1 DaliYang,1 Stephen Obrey,2 Robert Currier,2 Yusheng Zhao3 1

Materials Science & Technology Division, 2Chemistry Division, 3Los Alamos Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, NM 87545

Introduction The conducting polymer polyaniline (PANi) has very unique applications. For example, its electrical conductivity can be tuned over a wide range by controlled acid doping and base dedoping. PANi nanomaterials are finding applications in chemical sensing and microelectronics due to the high surface area and ease of processing.1 Different approaches have been explored to make PANi nanofibers (NFs) and nanotubes (NTs). In recent years, many research groups found that simple chemical oxidative polymerization under specific conditions can lead to PANi NFs2,3 or NTs4. Reaction temperature, reactant concentration/ratios, mechanical disturbances, and types of dopant acids etc. have been studied extensively for PANi NFs and NTs synthesis. In this study, we found that an apparent change in reaction mechanism can be directly related to different PANi nanomorphologies. Specifically, the reaction intermediates formed during the polymerization process can lead directly to formation of a different morphology (NTs vs NFs). Experimental Materials. Aniline was vacuum distilled before use. All other reagents were used without further purification. Aniline (An), Ammonia persulfate (APS) (ACS regent, 98+%), Camphor-10-sulfonic acid (CSA) (98%), and HPLC grade N-Methyl-2-pyrrolidone (NMP) were purchased from AlfaAldrich. 1N Hydrochloric Acid (HCl), and Ammonia were purchased from Fisher Scientific. DI water was used for all reaction and washing purpose. Synthesis. The An was dissolved in an aqueous acid solution. APS was dissolved in another aqueous solution. The two solutions were then quickly mixed well and sat for reaction at room temperature.2 An aliquot of the reaction mixture was taken out intermittently for immediate UV-Vis measurements. In order to record the UV-Vis in a timely manner, two reaction mixtures were chosen to represent the study results. The first one was a mixture of 0.025M An and 0.025M APS in 0.25M HCl acid (hereafter referred to as H+/An=10). The second one was 0.1M An and 0.1M APS in 0.1M HCl acid (hereafter referred to as H+/An=1). In both cases, the An to oxidant ratio was kept at 1/1. Separate reaction mixtures were made with the same chemical composition for open circuit potential (OCP) and pH measurements, as part of monitoring the reaction process. The reaction products were centrifuged to separate PANi from the colloids. The top waste was decanted and 30ml of DI water was then used to disperse the PANi which was then centrifuged again to wash PANi. The spinning rate was set at 2200 turns/second and each centrifugation was for 30 minutes. Each sample was washed 3 times. The samples were deposited on Si wafers for Scanning Electron Microscopy (SEM) measurements or on 200 mesh Carbon Type A Grid (Ted Pella, Inc.) for Scanning Transmission Electron Microscopy (STEM) measurements. The washed samples were de-doped with ammonia and washed again. The de-doped PANi was vacuum-dried at room temperature and then dissolved in NMP (with 0.01M LiBF4)5 for Gel Permeation Chromatography (GPC) measurements to determine molecular weights, molecular weight distributions, and polydispersity (Mw/Mn), of the polymer samples. Instrumentation. A Cary 5000 spectrophotometer (Varian) was used for UV-Vis measurements. The sample aliquots were diluted appropriately for the UV-Vis measurements.6 An Inspect™ F SEM (FEI) was used for SEM (5kev) and STEM (30kev) measurements. An Accumet® AR20 pH meter was used to monitor pH changes during the reaction. A Model 620B Electrochemical Analyzer (CH Instruments) was used for OCP measurements using a Pt working electrode and an Ag/AgCl reference electrode. The GPC consisted of an Alliance 2690 pump equipped with a Wyatt Rex Differential Refractive Index Detector and two Polymer Labs PL Mixed B GPC Columns at 60°C at a flow rate of 1.0 mL/min. The molecular weights were calculated relative to the retention times of polystyrene standards using Waters Corporation’s Empower software.

Results and Discussion Based on UV-Vis interrogation of a wide variety of PANi reaction systems, we found what appeared to be a change in reaction mechanism as presented below.6 We chose two samples to illustrate the salient points. The H+/An=10 system led to PANi NFs formation (Figure 1, left), while the H+/An=1 system led to PANi NTs formation (Figure 1, right). The NFs formed were homogeneous, thin, with a diameter of about 40nm. The NTs formed were often accompanied by nanoparticles, thick, with a diameter ranging from 100nm to 400nm, depending on reaction conditions. As synthesized, the PANi NTs were often inhomogeneous. The thick tubes were often rectangular as shown in the SEM image (Figure 2, left). STEM images (Figure 2, right) clearly showed that the NTs have hollow structures. However, when the NTs diameters were smaller (~100nm), the NTs may have tiny round inner holes or may even be solid.6 It is quite puzzling as to how the aniline polymerization can lead to this kind of tubular structures. However, the reaction mechanism study found a general trend for the PANi NTs formation process.

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Figure 1. The SEM images of PANi NFs (left) and NTs (right) synthesized under different reaction conditions.

Figure 2. SEM (left) and STEM (right) images of NTs synthesized at H+/An=1 reaction condition with CSA as dopant acid. The UV-Vis spectra of reaction intermediates for the two model systems are shown in Figure 3. For the H+/An=10 (NFs) system (Figure 3, left), there was an initiation period of about 15 minutes when the reaction mixture was colorless. The solution showed a greenish-blue color and darkened very fast. The UV-Vis showed that it then absorbed at around 550nm-700nm (21-40 minutes). We believe that this corresponded to the period of pernigraniline (PNG) (~550nm) oxidative state in balance with the corresponding protonated states (radical cations, or dications) (~700nm). When the APS was consumed, PNG reacted with An to form emeraldine salt (ES) (47 minutes). On the other hand, for the H+/An=1 (NTs) system (Figure 3, right), there was no initiation period. The reaction mixture showed a yellow color immediately upon mixing. It absorbed at 410nm (5 minutes) indicating polyaniline dimer in oxidized form, i.e. N-Phenylquinonediimine (PQDI).7 PQDI formed and was slowly consumed at the initial stage of the reaction (up until ~20 minutes). Afterwards, the reaction proceeded through a protonated PNS stage (32-46 minutes), and finally formed ES (50 minutes). Other than the PQDI formation, the remainder of the reaction appeared to follow the same path as in the previous system (H+/An=10). It is worth mentioning that the reaction rate of NFs was much faster than that of NTs. We managed to keep the reaction time frame similar by using much diluted (1/4) solutions in H+/An=10 system to illustrate our points. For comparison, 0.025M An (and APS) in H+/An=1 system, the reaction needed more than 10 hours to finish, while a 0.1M An (and APS) in a H/An=10

Polymer Preprints 2009, 50(1), 350 Proceedings Published 2009 by the American Chemical Society

system, the reaction only needed about 7 minutes to finish. Both 0.1M and 0.025M An in H+/An=10 leads to PANi NFs, while both 0.1M and 0.025M An in H+/An=1 leads to PANi NTs (accompanied with nanoparticles). From the UV-Vis spectra, it was obvious that for the H+/An=1 system, PQDI formed in significant amounts. The PQDI aggregated to form inhomogeneous reaction mixture (visible precipitates). We speculate that the aggregated PQDI served as a template for further reaction and therefore formed tubular structure.

should correspond to oligmers) and a significant low molecular weight peak (>10%). The molecular weight distribution of the PANi NTs demonstrated that the PANi was not homogeneous, which might have resulted from the aggregation of the reaction intermediate, PQDI. PQDI aggregation correlated with, and apparently led to, the formation of the NTs structure.

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Figure 3. V-Vis spectra of reaction intermediates of H /An = 10 (0.025M An) and H+/An = 1 (0.1M An). For both samples, the An:APS ratio was kept at 1.

Figure 5. The GPC results of PANi eluents at different retention time for the two samples.

OCP was found to be a useful technique for electrochemically monitoring the aniline polymerization process.8 OCP and pH measurements were carried out in separate reaction mixtures. For the H+/An=10 system (Figure 4, left), a high potential existed throughout the reaction, which was consistent with the PNS radical cations or dications formation as monitored by the UV-Vis spectra. For the H+/An=1 system (Figure 4, right), the OCP increased slowly at the initial stage of reaction, which was related to the formation of PQDI (0-15 minutes). We suspect that the PQDI began to aggregate, which made the further protonation reaction very slow. As the PQDI was slowly protonated, further reactions formed oligmers and polymers, the OCP slowly increased. As a result, the reaction appeared to proceed through distinct stages. The pH change also showed the same pattern. One practical issue to consider is that the electrodes are subject to coating by PANi after use, both for the pH electrode and the OCP electrodes. The measurement vessel (glass) also could have trace amounts of PANi left at the bottom after cleaning. Any remaining PANi coating could potentially speed up the polymerization process. Comparing the time scale of Figure 4 to Figure 3, it was found that the increase in rate was much more obvious for the H+/An=10 system. We suspect that due to aggregation, PQDI protonation was a rate controlling process for the H+/An=1 system, therefore the selfaccelerating effect was not as pronounced in this system due to the limited supply of reaction intermediates.

Conclusions Aniline oxidative polymerization reaction intermediates were studied using UV-Vis spectrometry for two model systems that lead to PANi NFs or NTs formation. It was found that reaction mechanism was a fundamental driving force for forming different morphologies of PANi nanomaterials. The formation of a distinct reaction intermediate, PQDI, and its aggregation, led to a staged polymerization process, which directly led to PANi NTs formation. As a result, the final reaction product had a wider molecular weight distribution and contained significantly higher levels of dimer and oligmer. Acknowledgements. We would like to thank Dr. Ross E. Muenchausen (LANL MST-8) for UV-Vis access and Dr. Rob Dickerson and Dr. Rod McCabe (LANL MST-6) for SEM and TEM access and Dr. Debra Wrobleski for GPC access. This work was supported by the Laboratory Directed Research and Development program at Los Alamos National Laboratory.

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Figure 4. Open Circuit Potential (OCP) and pH vs. time for the corresponding reaction mixtures after initial mixing. The OCP and pH were measure in parallel in 2 vials. To explore the chemical composition of the PANi NFs and NTs, We measured the molecular weight distribution using GPC as described in previous section. The GPC traces were shown in Figure 5. The PANi NFs (H+/An=10) had a narrower molecular weight distribution (with a polydispersity of 2.93). The low molecular weight peak was trivial (