Blends of a Perfluorosulfonate lonomer with Poly(vinylidene fluoride): Effect of Counterion Type on Phase Separation and Crystal Morphology

The type of counterion present in the Nafion component of a blend with poly(vinylidene fluoride) (PVDF) is shown to affect phase separation and the crystalline polymorphism of the PVDF component. Nafion neutralized with an alkali metal counterion, Na+, results in blends displaying largescale phase separation upon heating to temperatures above the melting point of PVDF. In contrast, when the Nafion counterion is changed to a larger tetrabutylammonium counterion (TBA⁺), the melt is homogeneous, and with the exception of the PVDF crystallites, large-scale phase separation is not observed after the blend is cooled to room temperature. For the blends containing Na⁺-form Nafion, the crystalline morphology of the PVDF component develops in predominately the α crystal form, similar to pure PVDF crystallized from the melt. However, for blends containing TBA⁺-form Nafion, the PVDF component crystallizes with a higher content of the β- and/or γ-crystal forms. This effect of counterion type on the phase separation behavior and crystal morphology is attributed to the strength of the electrostatic crosslinks within the Nafion component. Strong electrostatic cross-links induce gelation of the Nafion component leading to phase separation at elevated temperatures, whereas weak electrostatic cross-links provide a free-flowing melt that allows for a more favorable mixing with the PVDF component and is thus capable of influencing the crystallization process.