Solid-State (2) H NMR Determination of Poly(aniline) Conformation Within a MoO(3) Nanocomposite.

The focus of much recent research activity and interest, including our own, have been the synthesis, structure, and properties of conductive polymer/transition metal oxide nanocomposites. These hybrid materials comprise conductive organic polymers, such as poly(aniline) (PANI), interleaved between the layers of an inorganic oxide lattice, and are intriguing candidates for advanced material applications. In particular, the redox-active natures of both the polymer and inorganic components offer promising possibilities in electrochemical applications such as positive electrode materials in lithium batteries. While the materials demonstrate interesting bulk properties, little is understood about the microscopic interplay between the polymer and the oxide. Preliminary NMR studies have begun to provide details about the local structure of polymer and cations within the lattice. It is surmised that the conducting polymer can augment the electron transport properties, and enhance lithium diffusion within the oxide host by minimizing steric and electrostatic limitations to lithium transport. Several investigations into the electrochemical performance of these nanocomposites as cathodes in lithium batteries have indicated complex behavior. The primary concern addressed here is that of the interaction between the polymer and the oxide. Namely, we are trying to understand the role of the polymer, how it is ordered, and whether the polymer order affects the electrochemical response of the system. Characterization of these systems at the microscopic level is difficult. Large single crystals of polymer/oxides are almost non-existent, except for (PANI)xFeOCl, prepared by Kanatzidis et al. Although the locations of the polymer, and hence conformation of the PANI chains, could not be determined, their single-crystal X-ray studies showed that the PANI is intercalated in an ordered fashion that forms a superlattice commensurate with the FeOCl lattice. In all other interleaved polymer/oxide nanocomposites, however, information must be gleaned from powder X-ray methods (PXRD). Typically, the PXRD patterns show strong d00l reflections, and only weak mixed reflections. This provides limited information on the order of the oxide host itself, and cannot be used to probe order of the polymer within the oxide. Our current research involves H NMR to probe polymer conformation and dynamics in nanocomposites containing perdeuterated PANI between the layers of MoO3. Our one-dimensional (1D) and 2D experiments reported here unequivocally show that the PANI is organized within the interlayer region. Figure 1a shows the solid-state H NMR lineshape obtained for crushed powder samples of deuterated (PANI)0.24MoO3 [3] nanocomposites at 298 K. The H quadrupolar interaction dominates the two overlapping transitions in the NMR spectrum of this spin-1 nucleus. The polymer is rigid on the NMR time scale; the static value of the H quadrupolar coupling constant is 178 kHz with an asymmetry parameter of 0.05, values which are characteristic of poly(aniline). The high-temperature (370 K) H NMR spectrum of (PANI)0.24MoO3 (Fig. 1b) displays a set of inner doublets at ±17 kHz, indicative of phenyl deuterons undergoing two-site exchange due to 180 phenyl ring-flipping about the polymer chain axis. The degree of motion at this temperature is slightly greater than that of the corresponding bulk poly(aniline) in its emeraldine salt form, and less than that of the emeraldine base. In the latter, there is only physical constraint to phenyl ring flipping; in both the salt and polymer/oxide nanocomposite, the dynamics are influenced by the electrostatic interaction between the delocalized positively charged polaron on the PANI chain, and the negatively charged lattice or counteranion, albeit to different degrees. The relative chain rigidity allows us to examine polymer conformation within the oxide host. Solid-state H NMR is an ideal probe for investigating preferential orientation distributions; however, both the polymer and the host oxide lattice must first be macroscopically and microscopically ordered. FeOCl/PANI bronze, although highly crystalline, contains a relatively large fraction of paramagnetic sites, whose interaction with the polymer obscures the information being sought. In V2O5 itself, as well as in the corresponding PANI bronze, the inherent lack of structural order within the sheets (as is evident from the XRD patterns of these materials) inhibits study of the polymer order and makes it difficult to ascertain the degree of macroscopic order of the sheets. In contrast, the relatively high degree of both macroscopic and microscopic order attainable in MoO3 and its PANI bronze is apparent from the degree of order in oriented film XRD patterns obtained on suspensions of