Chemical Effects of a Solvent-Limited Approach to HCl-Doped Polyaniline Nanopowder Synthesis

The unconventional solvent-limited synthesis approach for conductive polymer production is analyzed for the well-known system of HCl-doped polyaniline (PAN) to elucidate the impact of nearly solvent-free reactions on chemical and electronic structures including chain conformation, specifically, branching and cross-linking. Moreover, the dependence of PAN polymorphism on the synthesis approach was demonstrated. A careful comparison is established with conventionally solvent-rich syntheses for rapid and extremely slow oxidations. Detailed Raman and UV–vis spectroscopy analyses demonstrate that the “solvent-free” product has the lowest cross-linking density with enhanced charge delocalization as a result of more expanded-coil chains and achieves an optimal oxidation level for the highest conductivity. This is in contrast to conventional solvent-rich polymerization for more compact coils and cross-links producing lower conductivities. The importance of tuning synthesis conditions toward delocalized polaron electronic structure relative to bipolarons and/or localized polarons is exemplified. These findings further support previous results from electromagnetic shielding effectiveness using the three synthesis-varied HCl-PAN nanopowders in the microwave band as well as explain the apparent independence of the polymer conductivity and crystallinity from previous studies.