Sulfonic acid doped thermoreversible polyaniline gels. 2. Influence of sulfonic acid content on morphological, thermodynamical, and conductivity properties

Thermoreversible gelation behavior of polyaniline (PANI) in the presence of varying concentrations of sulfonic acids are studied. Four different sulfonic acids are used, e.g., dinonylnaphthalene sulfonic acid (DNNSA), dinonylnaphthalene disulfonic acid (DNNDSA), (+/-)-camphor-10-sulfonic acid TSA), and n-dodecyloxo sulfonic acid (DOSA). The composition range studied here is W-PANI = 0.05-0.80 (W-PANI is the weight fraction of PANI in the gel). It has been found from the SEM study that for all the sulfonic acids in the composition range W-PANI = 0.05-0.40, fibrillar network is present. The TEM study also supports the above viewpoint. The thermodynamic study of the gels has been done by DSC-7, and for all the systems broad peaks consisting of two fused gel melting/gel formation peaks are observed. After proper deconvolution of the two peaks the gel melting/gel formation temperatures are measured. When they are plotted with W-SO3H (weight fraction of sulfonic acid), typical phase diagrams consisting of two almost parallel curves are found. They are explained by considering the lamellar model where the bilayer and monolayer portions of the surfactant form different crystalline domains. The higher melting point curve is due to the former and the lower one is due to the later in each phase diagram. The lowering of gel melting/gel formation points with increase in PANI concentration of each curve has been attributed to the dilution effect of PANI in the gel. The corresponding enthalpy values (DeltaH also show similar decrease with increasing PANI concentration and are explained also from the dilution effect of PANI. The conductance of these gels varies with weight fraction of PANI (Wp-ANI), showing a maximum, and it has been explained by considering that conductivity in the gel is due to both intrachain and interchain contributions. Attempt is made to discuss the above results by using a molecular model of the PANI-DOSA system with the help of MMX program.