In this study, emeraldine base (EB)-form polyaniline (PANI) powder was chemically prepared in 1M HNO3 aqueous solution. The thermal characteristics and chemical structures of this powder were studied by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). A polarizing optical microscope was also used to examine the crystalline morphology of this sample. The results indicated that the EB-form PANI powder had a discernible moisture content. Moreover, in the first run of DSC thermal analysis, the exothermic peak at 170-340degreesC was due to the crosslinking reaction occurring among the EB-form PANI molecular chains. FTIR and XRD examinations further confirmed the chemical crosslinking reaction during thermal treatment. TGA results illustrated that there were two major stages for weight loss of the EB-form PANI powder sample. The first weight loss, at the lower temperature, resulted from the evaporation of moisture. The second weight loss, at the higher temperature, was due to the chemical structure degradation of the sample. The degradation temperature of the EB-form PANI powder was around 420-450degreesC. The degradation temperature of emeraldine salt (ES)-form PANI powder was lower (around 360-410degreesC) than that of the EB form (around 420-450degreesC). From the TGA results, I roughly estimated that 2.74 aniline repeat units, on average, were doped with 1 HNO3 molecule in the ES-form PANI. I found a single crystalline morphology of EB-form PANI, mostly like a conifer leaf. More complex, multilayered dendritic structures were also found. (C) 2003 Wiley Periodicals, Inc.