Article pubs.acs.org/JPCB
Nucleation of Polyaniline Nano-/Macrotubes from Anilinium Composed Micelles Ruijuan Wang,† Chensen Wang,† Kong Liu, Fengli Bei,* Lude Lu, Qiaofeng Han, and Xiaodong Wu* Key Laboratory for Soft Chemistry and Functional Materials, Nanjing University of Science and Technology, Nanjing 210094, P. R. China S Supporting Information *
ABSTRACT: A mechanistic study on the nucleation of polyaniline nanotubes (PANI-NT) through template-free method is explored by in situ solution-state 1H NMR experiments via a careful analysis of the spectral evolution of the major species in the course of the reaction. Before polymerization, aniline and salicylic acid have assembled into loosely packed micelles due to electrostatic interactions and the proton exchange reaction between aniline and anilinium. A three-stage polymerization with a formation, accumulation of aniline dimers, as well as a generation of phenazine-like oligomers is observed, which can be attributed to the monomer transformation from neutral aniline molecules to anilinium cations and the significantly lowered pH in the reaction. Strong π−π stacking interactions from the phenazine-like oligomers facilitate the intermolecular aggregation which initiates the formation of PANI-NT. At first, such aggregates, locating at the outermost layer of anilinium composed micelles, shield in situ formed protons from releasing into the aqueous bulk but into the micelle instead. Due to the continuously increased charge in the micelle, a sphere-to-rod structural transition occurs which leads the oligomer aggregates to be sheathed at the exterior of the rod. Further consumption of anilinium in the micelle leaves the internal cavity while the fusion between the micelles elongates the length of the tubes. Our work demonstrates that (i) loosely packed anilinium composed micelles, highly mobile monomers within the micelle, and efficient blockage of the proton-releasing to the aqueous bulk are three key factors for the generation of tubular structures; and (ii) dynamic NMR line shape analysis provides a new perspective for resolving the formation profile of nanostructured polymers.
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INTRODUCTION
Though facile and easily operated, this method is encumbered by its high sensitivity to the experimental conditions and a simultaneous production of fibrous products. To improve the yield and the geometrical control of the tubular structure, the effect of the preparation conditions on the morphology of the products has been well studied.7,8,11−13 Meanwhile, understanding the formation mechanism of this structure is another factor to consider. Up to now, two mechanisms have been put forward.6,14 First, Wan and coworkers assume that the monomer should be self-assembled into micelles before the reaction because some of aniline molecules exist in the form of amphiphilic anilinium cations upon interacting with the acid, with the ionic part being hydrophilic and the aromatic one hydrophobic. This assumption was later confirmed experimentally by the same group using freeze-fracture electronic microscopy (FFEM).15 In the experiment, they also observed that the spherical micelles were changed into rodlike ones and a continuous fusion
Characterized by rapid diffusion rate for electrolytes, high specific area, and controllable conductivity, conducting polymers in nano- and microtubular structures are ideal components for fabricating highly integrated electronic devices and are extraordinary platforms for catalysts, drugs, and even tissues; therefore, they have attracted widespread attention.1−5 Of particular interest are polyaniline nanotubes (PANI-NTs) because of their facile synthesis, environmental stability, and proton dopability.6,7 Several chemical approaches including hard template method, solution based soft template method, and the template-free method have been developed to synthesize PANI-NTs and their nanocomposite derivatives.8−10 Among these approaches, the template-free one is most attractive due to its mild reaction conditions, simple purification procedures, and low cost. In this approach, the doping acid used is of fundamental importance and functions as the structural directing agent at the same time. With good control of the molar ratio of aniline and the acid, the oxidant, as well as the reaction conditions such as temperature and pH, PANI-NTs can be prepared successfully.7,8,11−13 © 2014 American Chemical Society
Received: November 15, 2013 Revised: January 26, 2014 Published: January 30, 2014 2544
dx.doi.org/10.1021/jp411235u | J. Phys. Chem. B 2014, 118, 2544−2552
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between the rodlike micelles generates the final structure. On the basis of these results, they attribute such in situ formed rodlike micelles to be the actual template for nanotubes, whose external size determines the outer diameters of the tube, and a continuous consumption of the monomer in the micelle accounts for the cavity inside.16,17 Second, Stejskal et al. have performed a systematic study on the structures of the oligomers generated at the first stage of the polymerization.18 They found that a minor change of the reaction condition might cause a significant structural variation of the oligomers produced. Such large structural differences introduce a wide range of intermolecular interactions in varied strength, leading the nanostructured products to exhibit diverse morphologies. In the case of PANI-NT, phenazine is the only compound detected which is undetectable in the control experiment like the preparation of PANI nanofibers.19 Moreover, phenazines have a strong tendency to be aggregated into highly ordered products due to π−π stacking interactions. Consequently they concluded that phenazines should be the nuclei to initiate the oligomers rolling up into tubular structure.14,20−22 Although a number of aspects have been interpreted by these models, there are still some fundamental issues that remain unresolved. In the micelle model, even though the profile of the structural evolution has been elucidated clearly, the molecular origin to induce these transitions is still unknown. In particular the main driving force for this transition and the major factors affecting this process are still unsolved, which prohibit it from developing efficient strategies for the design of high performance PANI-NTs. On the other hand, although the phenazinelike oligomer based model provides a good interpretation of issues such as the significant structural difference and the relatively lower conductivity of PANI-NT compared with PANI in bulk, detailed stepwise procedures for this process have not been identified. According to this model, phenazine can be only produced after a formation of a large amount of aniline trimers. Then the predominant species in the system would be aniline trimer, unreacted anilinium cation, water, and the nascent phenazine. Rolling up phenazines into tubular structure could be accomplished either via its cooperation with the other compounds or just simply by itself. If other species were involved, how could these compounds work together in a concerted way? If it were otherwise, how could phenazine prevent the interruptions from the surroundings? To answer these questions, Supporting Information provided by some other technique is required. Over the past few years, predominant approaches adopted in the mechanistic study are the following: (1) direct recording of the morphology evolution of the product by electronic microscopies, such as TEM and SEM, at varied reaction conditions,23,24 and (2) analyzing the FTIR, Raman, or UV−visible spectral changes in the course of the reaction.25−27 Any object observable by microscopes is large enough and thus must have already passed the nucleation stage; nevertheless, understanding the structural transition, the distribution, and the mobility of the major compounds, as well as the interplay between these species in the nucleation period, is critical to appreciate the characteristic physical environment established and eventually the product with desired morphology. Unable to achieve this type of information is one of the most pronounced shortcomings of these techniques. On the other hand, multiple inter- and intramolecular interactions as induced by the presence of a large number of oligomers with different structures will complicate the spectral interpretation in FTIR, Raman, or UV−vis
experiments, rendering it hard to have a clear identification of the role of each component. Fortunately in situ solution-state 1 H NMR spectroscopy is a good complementary. This is because (i) NMR spectra are generally simpler and easier to interpret than vibrational and electronic spectra, so the standard of proof is likely to be much better; (ii) most species in the nucleation period are still mobile enough, favorable to obtain a spectrum of moderately high resolution. Most importantly, our recent work has demonstrated that water molecules in the hydration layer of the micelles are the most sensitive probe in detecting the nucleation process after growing the micelle into larger sizes, and thus a careful analysis of the signal evolution of the major species, including water molecules, monomers, in situ formed oligomers, as well as the structural directing agents, paying close attention to their relationship in time sequence, enables us to link each separate event into a complete nucleation scenario.28 In this paper, self-assembled aniline−salicylic acid (SA) micelles were explored by NMR techniques including 1H NMR, diffusion ordered spectroscopy, and spin−lattice relaxation time (T1H) measurements at first. The possible driving forces to form such aggregates as well as the properties differentiating them from the conventional micelles are proposed. Afterward a series of in situ 1H NMR experiments were utilized, an analysis of the spectral evolutions of the major species in the course of the reaction allows us to derive the stepwise procedures for the formation of PANI-NT, and the key parameters affecting the formation of this nanostructure are discussed.
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MATERIALS AND METHODS Materials. Deuterium oxide was purchased from Alfa Aesar and used as received. Aniline, salicylic acid, and ammonium persulfate (APS) were obtained from Sinopharm Chemical Reagent Co. Ltd. Aniline was distilled under reduced pressure before usage. Stock Solutions for NMR Measurements. A series of aniline/salicylic acid stock solutions in a molar ratio of 100:1, 33.3:1, 20:1, 5:1, 2.5:1, and 1:1 were prepared, with a fixed aniline concentration of 0.20 M. Both compounds were mixed well in a D2O solution with 0.05% of trimethylsilyl propanoic acid (TMSP). Synthesis of Polyaniline Nanotubes. The monomer was assembled with SA prior to initiating the reaction by APS. All other procedures followed the same steps according to the literature.15 Instrumentation. High-resolution transmission electron microscopic (HRTEM) images were obtained on a JEM-2100 microscope (JEOL). NMR Measurements. Before any NMR measurement, a total volume of about 0.5 mL of the stock solution was first filled in an NMR tube and kept at 25 °C for 15 min. All NMR experiments were performed on a Bruker DRX 300 Spectrometer (300.13 MHz for 1H), equipped with a BBI gradient probe with a maximum gradient strength of 52.7 G· cm−1. The measurements were performed at 25 °C, and the temperature was controlled to within ±0.1 °C with a Bruker DVT300 digital controller. The in situ experiment was designed so that the aniline and salicylic acid in varied molar ratio were polymerized in an NMR tube by adding APS, and a series of 1H NMR spectra were collected as a function of the reaction time. The line widths of high resolution 1H NMR spectra were measured directly at the half-height of the experimental peaks, 2545
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and a 1D Carr−Purcell−Meiboom−Gill pulse sequence was used to eliminate the effects from the inhomogeneous external field brought about in the nucleation. Each spectrum was acquired with 16 transients and 4 CPMG echoes with a duration time of 2.0 ms per transient. 1H NMR diffusion measurements were performed using the pulsed-field gradient (PFG) stimulated-echo (STE) procedure,29 with phase cycling of the radio-frequency pulses to remove unwanted echoes.30 The gradient strength was calibrated from the known diffusion coefficient of HDO at 25 °C.31 Typical parameters used in our experiments were a total diffusion encoding pulse duration δ of 3.5 ms and a diffusion delay Δ of 20 ms. The diffusion coefficient was yielded by monoexponential fitting of the echo intensities as a function of the echo time using the Bruker Topspin 1.3 software package. Typical parameters for 2D homonuclear J-resolved spectroscopy are as follows. The sweep widths were 3598.0 Hz in F2 dimension and 50.0 Hz in F1 dimension. The recycle time was 2.0 s between each transient. Sixteen transients were acquired for each t1 increment. The data matrix was consisted of 256 spectra of 2048 points each and zero-filled to 512 spectra of 4096 points.
Table 1. Determined Proton Chemical Shifts by J-Resolved Spectroscopy for Mixtures with 0.20 M of Aniline and Varied Concentration of SA assignment [SA] (M)
aniline (ortho)
aniline (meta)
aniline (para)
SA (H-3)
SA (H-4)
SA (H-5)
SA (H-6)
0 0.002 0.006 0.01 0.04 0.10 0.20
6.87 6.87 6.88 6.88 6.97 7.14 7.34
7.26 7.26 7.26 7.27 7.31 7.40 7.50
6.89 6.89 6.91 6.92 7.01 7.21 7.42
6.85 6.86 6.87 6.95 6.94 6.94
s 7.52 7.46 7.45 7.45 7.46 7.46
6.88 6.89 6.89 6.98 6.97 6.97
7.83 7.83 7.83 7.83 7.82 7.83
aniline indicates the presence of rapid proton exchange between aniline and anilinium. It should be noted that while Wan and co-workers have attributed the formation of AIM mainly to the electrostatic interactions and hydrogen bondings, our data suggest the proton exchange reaction is another important contribution because this reaction turns the previous one-to-one interaction into a style of one-to-many, attracting a large number of aniline molecules around SA and accelerating the formation of AIM. Eventually these species are aggregated into spherical micelles to reach into a state with minimum interfacial charge density. In this sense, the proportion of aniline in forms of cationic or neutral states is one of the key factors to determine the packing degree of the micelle, the reactivity of the monomer, and eventually the kinetics of the polymerization. In theory, a micelle with most aniline molecules in the form of anilinium will be more tightly packed due to stronger electrostatic interactions but less reactive because anilinium cations are rather inert for this polymerization. Considering that the chemical shift of aniline recorded for any AIM micelle is an average contribution of aniline in these two forms, the proportion of each can be determined directly once the resonance of aniline in each pure state is known. For this purpose, 0.20 M aniline solution and a mixture with equal amounts of SA and aniline (0.20 M for each) are designated as our two standards. The proportions of aniline relative to anilinium for micelles with SA concentration of 0.002, 0.006, 0.01, 0.04, and 0.10 M are determined to be 0.99, 0.98, 0.96, 0.76, and 0.42, respectively, indicating micelles with a SA concentration less than 0.04 M should be loosely packed. To evaluate the intermolecular interactions in each micelle, both proton spin−lattice relaxation time (T1H) and the diffusion coefficient (D0) of aniline were measured. The T1H for protons in aniline shortens significantly for micelles with high concentration of SA. For example, 9.56 and 10.74 s are observed for meta- and para-/ortho-CH for aniline in AIM containing 0.002 M SA, which are shortened to 6.52 and 6.63 s for micelles with an SA concentration of 0.10 M, indicating the presence of stronger dipolar interactions in the latter system. On the other hand, the diffusion coefficients for aniline within micelles having an SA concentration of 0.006, 0.01, 0.04, and 0.10 M are 8.84 × 10−10, 8.43 × 10−10, 8.34 × 10−10, and 7.40 × 10−10 m2/s, suggesting more tightly packed micelles are formed at a higher concentration of SA. It is worth mentioning that the diffusion coefficient for aniline at low concentration of SA (
