Origin of Bulk Nanoscale Morphology in Conducting Polymers

The direct one-step chemical transformation of a liquid monomer to bulk conducting polymer nanofibers has opened new opportunities in the field of energy storage and chemical/ biological sensors. The conducting polymer polyaniline has evolved as a prototype system in the study of this transformation, and there are now several empirical methods to convert aniline directly to bulk conducting polyaniline nanofibers. The key to broadening this phenomenon to other conducting and classical polymers would be in uncovering the mechanism of nanofiber formation. Here we report that surfaces such as the walls of the reaction vessel and/or intentionally added surfaces play a dramatic role in the evolution of nanofibrillar morphology. Nucleation sites on surfaces promote the accumulation of aniline dimer that reacts further to yield aniline tetramer, which (surprisingly) is entirely in form of nanofibers and whose morphology is transcribed to the bulk by a double heterogeneous nucleation mechanism. This unexpected phenomenon could form the basis of nanofiber formation in all classes of precipitation polymerization systems. The conventional chemical oxidative polymerization of aniline using ammonium peroxydisulfate oxidant in dilute aqueous acids yields a granular polyaniline powder having very little nanoscale morphology. The reaction is characterized by an induction period that can last from a few minutes to hours depending on temperature, concentration, etc., which is followed by a rapid precipitation of a dark-green polyaniline powder. By altering the synthetic conditions during the induction period, it is possible to completely change the morphology of polyaniline from granules to nanofibers. For example, nanofibers are obtained by carrying out the polymerization at the interface of two immiscible solvents or by simply adding catalytic (seed) quantities of nanofibers of known composition of any kind during the polymerization (called nanofiber seeding). Adding aniline oligomers to the reaction or simply diluting the reaction mixture by a factor of 20 also yields polyaniline nanofibers. However, the nanofibers obtained in most systems look strikingly alike, i.e., a nonwoven mesh of fibers that are microns long and 30-70 nm in diameter, suggesting a common underlying mechanism. We suspected that available surfaces, such as the walls of the reaction vessel, magnetic stir bar, etc., could be playing an important role in nanofiber formation. This was based on laser light scattering studies that showed that rodlike aggregates are formed in systems that yielded nanofibers (spherical aggregates yielded granules). Since these aggregates would be highly hydrophobic, we suspected that they would deposit on available surface sites and initiate polymerization from these sites. To increase the hydrophobicity of the surface, we carried out a conventional polyaniline synthesis in a glass vessel (no nanofibers, Figure 1a, inset) containing a rolled-up sheet of commercial poly(ethylene terephthalate) (PET). The reaction rate increases significantly, and surprisingly, the morphology changes dramatically from granules to entirely nanofibers (Figure 1a). The reaction is initiated on the PET surface as evidenced by the dark blue color of in situ deposited pernigraniline film that is much thicker than corresponding films on a glass surface (control). Conversely, it is also possible to obtain granules under conditions that typically yield nanofibers, e.g., when the reaction is carried out at 20× dilution in a glass beaker. If the reaction is carried out in a stainless steel vessel, nanofiber growth is completely suppressed and only granules are obtained (Figure 1b). A stainless steel surface is mildly reactive under the acidic reaction conditions and would not permit the deposition of hydrophobic species from solution. If a PET sheet is immersed in the stainless steel vessel, nanofiber formation is restored, showing that surface effects are playing an important role in orchestrating nanofibrillar growth (Figure 1c). In contrast, a recent report describes nanofiber formation in an aniline dimer promoted reaction where no surface films are formed. To unequivocally elucidate the effect of surfaces in nanofiber formation, we identified a reaction vessel that would, in essence, have “no surface”. We found that a flask made entirely of ice satisfies the condition in that its surface would constantly be in equilibrium with the reaction mixture and would not allow any permanent surface deposition to take place. We carried out the reaction in an ice vessel at 8× dilution and found no nanofibers (Figure 2a). Under these conditions nanofibers are readily formed in a glass vessel (Figure 2a inset), suggesting that when there is no surface to promote heterogeneous nucleation, homogeneous nucleation is the only available pathway, and this favors granular polymer growth. Importantly, when a PET sheet is immersed in the ice vessel, nanofiber formation is restored which is consistent with a change back to heterogeneous nucleation (Figure 2b). We believe that the formation of aniline dimer on surfaces is playing a key role in bulk nanofiber formation. We reported earlier that when a small amount of aniline dimer is added during the induction period, the reaction rate increases and nanofibers are obtained. This is qualitatively similar to what is observed when a PET sheet is immersed in the reaction. Similar results using aniline dimer have recently been reported, although it is to be noted that aniline dimer is an intermediate regardless of bulk polyaniline morphology, and something strikingly different is taking place when solid aniline dimer is added to the reaction. In a control experiment, aniline dimer alone was treated with peroxydisulfate in aqueous 1.0 M HCl. Contrary to previously published results on aniline dimer oxidation, we observe a near-instantaneous formation of a blue-green powder that is analytically and spectroscopically consistent with hydrochloridedoped aniline tetramer and not polyaniline (see Supporting Information Section). The yield is quantitative based on oxidant, and the powder displays a four-probe pressed pellet electronic conductivity σRT ∼10-2 S/cm. To our surprise, the tetramer powder is composed entirely of very long 40-80 nm diameter nanofibers (no granules, Figure 3a). The near-instantaneous oxidative dimerization of aniline dimer coupled with what appears to be simultaneous nanofiber formation is a new phenomenon and shows that extremely short * Corresponding author: e-mail sanjeev_manohar@uml.edu; Ph 978-9343162. 1792 Macromolecules 2009, 42, 1792-1795