Mechanical Properties of Perfluorosulfonated Ionomers: The Role of Temperature and Solute Activity

Water sorption and mechanical properties of Hyflon Ion have been measured and compared with Nafion under environmentally controlled conditions from 25 – 120 ºC and water activities of 0-0.95.  At water activity 0 Nafion and Hyflon Ion perfluorosulfonated ionomers (PFSI) undergo thermal transitions where the elastic modulus decreases by a factor of 100, at 50 ºC and 90ºC, respectively.  Below the thermal transition temperature water sorption plasticizes both PFSIs; the elastic moduli decrease with increasing water activity.  Above the thermal transition temperatures water sorption stiffens both ionomers; increasing the water activity from 0 to 0.01 increases the elastic moduli by a factor of >10.  The stiffening effect is also seen with sorption of methanol and acetonitrile. These results have been explained in terms of the thermal transition being a clustering transition. The clusters make cross-links that stiffen the PFSI.  At low temperature sulfonic acid groups cluster to form hydrophilic domains that minimize the interfacial interactions between the hydrophobic polymer backbone and the hydrophilic sulfonic acid groups.  At high temperature entropy drives the sulfonic acid groups to disperse in the hydrophobic matrix.  Water sorption causes the interfacial area of the hydrophilic domains to increase causing them to cluster.