The influence of heat treatment on the semi-crystalline structure of polyaniline Emeraldine-salt form

Polyaniline emeraldine-salt form (PANI-ES) was chemically synthesized using hydrochloric acid and subjected to heat treatment for 1 h at 50, 100, 200 and 300 °C. X-ray Diffraction (XRD), Le Bail method structural refinement, Infrared-transform Fourier Spectroscopy (FTIR), Small-angle X-ray Scattering (SAXS), Scanning Electron Microscopy (SEM) and Electrical Conductivity measurements were used to evaluate the influence of heat treatment on the semi-crystalline structure of PANI. The heat treatment has resulted in a progressive decrease of crystallinity from 50 to 22%. A crosslinking process during heat treatment was observed by FTIR at 200 °C, revealing some chemical changes in molecular structure of PANI such as elimination of HCl on the imino groups and the simultaneous chlorination of the aromatic rings. Le Bail method showed that crystal structure of the unheated ES-PANI is strongly dependent on the molecular size of the counter ion, so the unit cell volume needed to be increased for their accommodation in the polymer structure. The refined parameters suggested a decomposition from tetrameric to dimeric-folded chains, accompanied by a decrease in the crystallite anisotropy and average size and shape, which reduced from 36 Å to 16 Å and acquired oblate shape. The pair-distance distribution function (p(r)) curves suggested particles tending from oblate to prolate form over heat treatment. Well-defined nanofibers were observed in unheated ES-PANI, which decreased and lost progressively their initial morphology over heat treatment. Electrical conductivity showed a decreasing of about 90% due to the loss of emeraldine sequences and removal of chloride ions.