University of Connecticut |
Polymer Program SeminarRational Design of Polyelectrolytes: Rigid Rod Polymers and Copolymers Friday, October 21, 2005 11:00 am , IMS Room 20 The search for a Nafion replacement has concentrated on sulfonated poly(phenylene ether ketones or sulfones). These have many deficiencies: they conduct well only at 100% relative humidity and one must balance equivalent weight against excessive swelling. My group has identified sulfonic acid containing rigid rod, liquid crystalline polyelectrolytes as potential materials that could have properties superior to polyelectrolytes with other backbones. The rationale is: rigid rod polymers can organize into nematic liquid crystals during drying. Films cast from them then have local domains with all the chains parallel. If a small mole fraction of a copolymer with a bulky cross-section is incorporated, the chains are pushed apart along their whole length because the comonomer residues contact the neighboring chains first and prevent the backbones from touching each other.. This can create long nanopores lined with sulfonic acid groups that hold water very tightly. Such materials should have reasonable conductivity at low humidities, allowing them to function at 1 atmosphere pressure above 100oC. We studied two systems in detail. The first was a polyimide system; its structure/property relationships proved the concept. The materials had very good properties but the imide groups hydrolyzed slowly, even at room temperature. The second class of materials was poly(phenylene sulfonic acid) and its copolymers. While the homopolymer is water soluble, the copolymers are not and could function well as MEAs. The primary properties are exceptionally good. The solution properties and solid state structure demonstrate that the rods associate in solution to form planar micelles with sulfonic acid surfaces; these pack to form the final structure. If there is time, Nafion structure and properties will be discussed.
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