Synthesis and characterization of soluble polypyrrole and polypyrrole/organoclay nanocomposites

As one of the novel conducting polymers, polypyrrole (PPY) has been widely studied due to its good thermal and environmental stability as well as its superior electric conductivity [1, 2], and electrorheological applications [3]. However, PPY is generally insoluble in typical organic solvents because of its strong interand intra-molecular interactions and the formation of a crosslinked structure. Therefore, to improve its solubility and processability, efforts [4–6] have been applied to the development of soluble and dispersible PPY. Examples include a synthesis of PPY containing a long chain of an alkyl or alkyl sulfonate substituent. Poly(3-alkylpyrrole) was found to be readily soluble in common solvents and water when the substituent bears a hydrophilic group such as SO3H. However, this method has some difficulties, especially in the synthesis of 3-substituted pyrrole monomer. On the other hand, a conducting polymer/clay nanocomposite system has been introduced to improve the applicability or physical properties over the pure conducting polymer, such as a PPY/montmorillonite nanocomposite synthesized [7] by an emulsion polymerization [8, 9] using dodecylbenzene sulfonic acid (DBSA) as an emulsifier and polyaniline/clay nanocomposites [10–12]. These nanocomposites consist of multilayered silicates and polymers, which intercalate between the inter layers of inorganic clay, and their electrical and ER characteristics were reported recently [9–12]. In this letter, we initially synthesized highly soluble PPYs via an in situ polymerization method, using sodium di-(2-ethylhexyl sulfosuccinate) (Na-DEHS) as a dopant, and then prepared a PPY intercalated organoclay nanocomposite via a solvent casting method using chloroform. This soluble conducting polymer was treated with clay to control its electric conductivity. The soluble PPY was polymerized by an in situ doping method using Na-DEHS as a secondary dopant. At first, pyrrole monomer (0.4 mol) and Na-DEHS (0.2 mol) were dissolved in distilled water at 0 ◦C, and 0.1 mol of ammonium peroxysulfate solution (APS) was dissolved in 10−7 m3 distilled water, separately. While keeping the temperature of pyrrole and Na-DEHS dispersion at 0 ◦C, the APS solution was added slowly over 3 min into the solution. The polymerization lasted for 24 h at 0 ◦C. After the reaction, synthesized PPY was filtered and washed 5–6 times with 6 × 10−6 m3 distilled water, and the resultant black cake from the filtering process was dried in a vacuum oven for 3 days. After the drying process, the dark powder of PPY was simply milled and sieved with 100 μm sieve. We then prepared the PPY-DEHS/clay nanocomposite via a solvent casting method using chloroform. As one of the intercalation methods of polymer into the intergallery of the clay, this solvent casting method has been widely applied for various polymer systems, such as polystyrene [13], poly(ethylene oxide) (PEO) [14], poly(methylmethacrylate)/PEO blend [15], biodegradable polymer [16], and polyaniline [17]. To prepare the polymer/clay nanocomposite, organocally modified montmorillonite (OMMT) (Cloisite 25A, Southern Clay Products, USA) was also swelled in the same organic solvent of chloroform for 24 h. Both PPY-DEHS and OMMT dispersed in chloroform were then mixed together and stirred for 24 h to ensure the complete intercalation of PPY-DEHS chains into multi-layered silicates of the OMMT. Finally, the product was filtered and dried to prepare the soluble PPY-DEHS/OMMT nanocomposites. For characterization of the synthesized nanocomposite, X-ray diffraction (XRD) measurements were performed using the Rigaku DMAX 2500 (λ = 0.154 nm) diffractometer. Using scanning electron microscopy (SEM, S-4300, Hitachi), the morphological characteristics were examined. A transmission electron microscope (TEM, CM200, 2DS DX-4, Phillips) was