A water soluble polyelectrolyte, PPDSA [poly( p-phenylene-2,5-disulfonic acid)], was synthesized using Ullmann coupling between DBBDSA (1,4-dibromo-2,5-benzenedisulfonic acid). For the design of PPDSA, two concepts were adopted: highly sulfonated and rigid rod liquid crystalline polymer. It has a high ionic exchange capacity, 8.46meq/g, and the positions of the sulfonic acids on its backbone were well controlled.
Its solution properties were studied using viscosity, rheometric measurements, and optical polarizing microscopy. Due to the rigid rod structure, its reduced viscosities increased at low concentration under all conditions. PPDSA solutions showed shear rate dependent viscosity, and concentrated solutions were lyotropic with a typical nematic structure using crossed polarization.
PPDSA 1H-NMR spectra had many overlapping peaks that were analyzed by deconvolution. The highest number average degree of polymerization was 142. But, 1H-NMR spectra are not fully understood due to the local organization in solution. However, 13C-NMR showed a linear para-coupled PPDSA structure with only three 13C resonances.
PPDSA membrane properties were characterized as a function of temperature and relative humidity. Its water contents per acid group were about two water molecules more than that of Nafion between 15 and 75%RH at room temperature. DSC results showed that at least 8.45 H 2O per sulfonic acid were strongly bound in the polymer.
The PPDSA hexagonally packed nematic liquid crystal polydomain structure with frozen-in free volume is a possible reason for the high water retention. This was suggested based on WAXD, 2D X-ray and dimensional changes. The inter-chain distance increased from ~8 to ~11Å as humidity increased. The structure consists of nano-size channels lined with sulfonic acids. The free volume in the polymer was shown by the fact that the absorbed water had a density of about 1.23g/cc.
Its conductivities were extremely high at low humidities, ~0.01 S/cm at 15%RH and room temperature, and ~0.1 S/cm at 15%RH and 75 °C. PPDSA with more than 4.5 water molecules per sulfonic acid had higher conductivity than Nafion. PPDSA is very thermally stable (TGA decomposition temperature: ~ 304 °C at 10 °C/min). Stress-strain measurements showed that the film had reasonable elongation for a linear rigid rod polymer.