Molecular Mechanism for Formation of Polyaniline Lamella from a Lyotropic Liquid Crystal: An NMR Study

Abstract

Polyaniline (PANI) microlamellas with an average interlamellar distance of 2.6 nm were prepared from a nematic lyotropic liquid crystal system composed by sodium dodecyl sulfate (SDS) aqueous solution. To reveal the formation mechanism of these lamellas, a series of NMR studies have been performed. At first, variable-temperature (VT) 13C NMR experiments have suggested that, prior to polymerization, anilines are predominantly located in the vicinity of the SDS polar head region with a limited mobility at low temperature, whereas they become more mobile and penetrate into the SDS hydrophobic domain at elevated temperature. Subsequent in situ 13C NMR measurements at 310 K have indicated that the overall polymerization can be taken place in two stages. In the beginning, the reaction sites are within the SDS micelles, resulting in the formation of oligomeric PANI species with benzenoid and quinoid structures. Interestingly, these oligomeric species fall off from the micellar hydrophobic domains and reorganize into layered structures with the support of SDS. In the second stage, further polymerization can be continued within the interlayers. This paper provides a good example in studying the roles of surfactants at the nucleation stage qualitatively during the synthesis of morphology-specific polymers with the application of NMR techniques, a period difficult to be examined by other approaches currently.