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

Abstract

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 large-scale 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 cross-links 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.