Characterization of Sulfonated Perfluorocyclobutane /Poly(Vinylidene Difluoride)-co-Hexafluoropropylene (PFCB/PVDF-HFP) Blends for Use as Proton Exchange Membranes

The research herein focuses on the characterization of a PFCB/PVDF-HFP (70:30 wt:wt) blend fuel cell membrane including the constitutive and morphological properties, how these properties predict the stresses incurred under fuel cell operating conditions, and how these properties change over time under fuel cell operating conditions. Characterization was performed to mimic temperature and moisture conditions found in operating fuel cells to understand how these materials will behave in service.  This included thermal and hygral expansion, mass uptake, and the stress relaxation modulus.  These constitutive properties were chosen for characterization such that a model could be created to predict the stresses incurred during fuel cell operation, and examine how these stresses may change under different operating conditions and over time.  Based on the results of this model, lifetime predictions were made resulting in recommendations to further extend the operating time of this membrane beyond the DOE 5000 hr requirement.

Stress predictions are useful, however if the material properties are changing over time under the fuel cell operating conditions, they may no longer be valid.  Therefore, PFCB/PVDF-HFP membranes were conditioned for different amounts of time under conditions similar to those commonly found in operating fuel cells.  These conditioned membranes were then characterized and compared with solvent exchanged membranes, the same materials used for previous material characterization.  The properties examined included stress relaxation modulus, bi-axial strength, mass uptake, water diffusion, and proton conductivity.  To further understand any changes noted in these properties after different environmental exposures, morphological analysis was performed.  This included small angle x-ray scattering, infrared spectroscopy, transmission electron microscopy, and differential scanning calorimetry.

It was initially found that the proton conductivity decreased severely when the material was immersed at high temperatures over short time periods.  This was consistent with changes noted in other properties, and morphological analysis showed a decrease in the ionic network as well as an increase in the phase separation of the PFCB block copolymer as well as the PVDF-HFP crystallinity.  These large morphological changes could be very detrimental while in service, resulting in early termination of the fuel cell.  However, it was also noted that if these materials are annealed at high temperature (140"C), the negative property changes are abated.  This abatement is again tied to the morphology of the material, as annealing the material at high temperature creates stronger physical crosslinks, and induces a small amount of chemical crosslinking via condensation of the sulfonic acid groups, thus allowing the stress predictions performed earlier to have greater validity.   Therefore, it is important to not only understand the properties of a material during characterization, but also the underlying polymer structure, and how this structure can change over time, as all of these items control the long term material performance while in service.