Morphological Manipulation and Plasticization of the Electrostatic Network in Perfluorosulfonate Ionomers
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Morphological Manipulation and Plasticization of the Electrostatic Network in Perfluorosulfonate Ionomers Alan K. Phillips and Robert B. Moore* Department of Polymer Science The University of Southern Mississippi 118 College Dr # 10076 Hattiesburg, MS 39406-001 ABSTRACT The addition of small molecule fluorosurfactants to perfluorosulfonate ionomers has been found to have a profound effect on the morphology and mechanical properties of solution processed membranes. Perfluorooctane sulfonates neutralized with both alkali metal and alkylammonium counterions have been incorporated into solution processed Nafion® membranes. Sodium neutralized fluorosurfactants were found to crystallize during membrane casting leading to the formation of a surface layer of crystalline material. Tetramethylammonium neutralized surfactants behave in a similar fashion with crystallization of the surfactant occurring during casting. The use of large tetrabutylammonium counterions is found to prevent crystallization of the surfactant and create a highly plasticized membrane. INTRODUCTION Perfluorosulfonate ionomers (PFSI’s), the most common being Nafion from DuPont, are copolymers of tetrafluoroethylene and a perfluorovinyl ether comonomer with a terminal sulfonic acid functionality, typically composing less than 15 mole percent of the copolymer.1 Phase separation between the hydrophobic chain matrix and the polar sulfonate groups create a nanophase separated morphology with ionic functionalities aggregating into “multiplets” held together by strong, Coulombic attractive forces.2 PFSI morphology is further complicated by the organization of backbone segments into “PTFE-like” crystalline regions in the matrix.1 Neutralization of the sulfonate functionality with either alkali metal or alkylammonium counterions has been shown to have profound effects on the morphology and mechanical properties through the formation of electrostatic cross-links and internal plasticization of the ionic aggregates, respectively.3 Prior studies on the addition of surfactants to ionomer materials have shown unique behaviors based on the polarity of the surfactant used. Nonpolar plasticizers have been shown to plasticize both the aggregate regions and the matrix leading to decreases in the glass transition of both the cluster phase and the matrix phase.4 In contrast, polar plasticizers have been shown to selectively plasticize the cluster phase resultant from the plasticizer being incorporated into the aggregate.5 In this work, we explore the addition of polar, perfluorinated plasticizers into Nafion®. EXPERIMENTAL Nafion® 112 membranes were initially cleaned using 4 M methanolic H2SO4 prior to being neutralized to the desired counterion form using NaOH, tetramethylammonium (TMA+)
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hydroxide and tetrabutylammonium (TBA+) hydroxide. Membranes were then dissolved in alcohol-water solvent systems using high temperature and pressure.6 Recast Nafion® is then recovered by room temperature evaporation of the alcohol-water solvents leaving dry, amorphous
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