Hierarchical Polyaniline Crystals from Bicontinuous Microemulsions: Roles of Interfacial Confinement and V2O5/HCl-Assisted Oxidative Polymerization

Abstract

Controlling the crystallization of polyaniline (PANI) into ordered hierarchical architectures remains challenging because rapid oxidative polymerization commonly produces fibrous or aggregated morphologies. Here, hierarchical PANI crystals were prepared through V2O5/HCl-assisted oxidative polymerization in a cyclohexane/water/Triton X-100/n-butanol bicontinuous microemulsion. Aniline was confined in cyclohexane-rich domains, whereas the acidic vanadium-containing oxidizing phase occupied the aqueous domains. Comparison of different regions of the ternary phase diagram showed that oil-in-water and water-in-oil microemulsions produced predominantly flat plate-like or ribbon-like PANI structures, whereas the optimized bicontinuous composition yielded elongated crystals decorated with aligned lamellar surface subunits. Replacement of V2O5/HCl with ammonium persulfate in the same bicontinuous system produced entangled PANI fibers, demonstrating that bicontinuous confinement alone is insufficient for hierarchical crystal formation. Ordered crystals were also obtained in an SDS-based bicontinuous microemulsion, indicating that the phenomenon is not exclusive to the Triton X-100 system. SEM revealed hierarchical organization from the microscale crystal habit to the aligned lamellar surface texture. XRD showed sharp reflections and a low-angle periodicity of approximately 12.6 Å, while Raman and FTIR spectra confirmed an emeraldine-like, partially protonated PANI structure. XPS showed a PANI-rich surface containing C, N, and O, with no detectable V 2p signal. Classical all-atom molecular dynamics simulations using a simplified interfacial model showed preferential association of Triton X-100 near the PANI/oil/water interfacial region. These results suggest that the cooperative action of bicontinuous interfacial confinement and controlled V2O5/HCl-assisted oxidation enables the formation of hierarchical PANI crystals.