Influence of electrostatic interactions on the morphology and properties of blends containing perfluorinated ionomers
The first goal of this research project was to investigate the influence of the electrostatic interactions within the ion-containing domains of Nafion ® perfluorosulfonate ionomer (PFSI) on the morphology and resultant properties of blend systems with poly(propylene imine) dendrimers of a variety of generational sizes and poly(vinylidene fluoride) (PVDF). Perfluorosulfonate ionomers (PFSIs) are a commercially successful class of semi-crystalline, ion-containing polymers whose most extensive application is in use as a polymer electrolytic membrane in fuel cell applications. Nafion ® was blended and high temperature solution processed with poly(propylene imine) dendrimer as the minor component in order to increase the efficiency of direct methanol fuel cells by decreasing methanol crossover without significant loss of protonic conductivity. The preferential insertion of the dendrimer into the ionic cluster due to proton transfer reactions and the creation of ammonium-sulfonate ion pairs served to alter the transport properties through the ionic network of the membrane. In the second major system investigated, blends of poly(vinylidene fluoride) (PVDF) with Nafion® , a perfluorosulfonate ionomer, have been prepared and examined in terms of the crystallization kinetics and crystal morphology of the PVDF component in the blend. DSC analysis showed faster rates of bulk crystallization when PVDF was crystallized in the presence of Na + -form Nafion® suggesting a high degree of phaseseparation in this blend system and an increase in the nucleation density. Nafion® neutralized with alkylammonium-form counterions display an increase in blend compatibility with PVDF with an increase in the alkylammonium counterion size. As the alkylammonium counterion size increases, the strength of the electrostatic network within the ionic domains of Nafion ® decrease resulting in a reduction in the driving force for ionic aggregation. Thus, a decrease is observed in the crystal growth rate and nucleation density of the PVDF component of the blend as the size of the alkylammonium-form counterion increases. This study demonstrates that the type of neutralizing counterion of the Nafion ® component has a dramatic impact on blend compatibility and the crystallization kinetics of the PVDF component within Nafion® /PVDF blends. In addition, higher phase mixing with the alkylammonium-form Nafion® component leads to an increase in the PVDF polar polymorphs.