The Effect of Soxhlet Extraction and Synthesis Temperature on Properties of Polyaniline
Polyaniline was synthesized by the chemical oxidation of aniline in an HCl solution by ammonium persulfate. The temperature of the synthesis was varied to increase the molecular weight of the polymer from 37,600 Da to 52,400 Da. Soxhlet extraction using methanol as the primary solvent was performed on half the sample in order to further increase the average molecular weight by approximately 3150 Da and decrease the polydispersity index by 0.8. Films were cast from
... -2(1H)-pyrimidinone (DMPU) and N-methyl-2-pyrrolidone (NMP) and then doped with trifluoromethanesulfonic acid. The effect of the soxhlet extraction and synthesis temperature on the polymer's properties were analyzed. Among the properties examined were thermal events, crystallinity, molecular weight, electrical conductivity, and infrared spectra. Soxhlet extraction proved useful in rinsing out not only short chain oligomers, but also residual solvent and dopant. This lack of solvent and oligomers proved advantageous to the properties of the polyaniline films, resulting in films that were more conductive and of higher crystallinity since higher molecular weight polymers have larger crystal domains. An unexpected result of the extraction was that the polymer powder showed a resilience to solid state crosslinking, an irreversible process which occurs at approximately 150°C which decreases the conductivity of the polymer by orders of magnitude. Decreased synthesis temperature also proved advantageous, producing polymer of significantly higher molecular weight. Films made from polymer synthesized at room temperature resulted in films which were brittle and cracked easily upon doping. This mechanical shortcoming resulted in films which were much less conductive than films cast from polymer synthesized at low temperature. The increased temperature and heat required for crosslinking was also observed for samples synthesized at lower temperatures.