Study On Perfluorosulfonic Membrane Fabrication Mechanism

Solution-casting is an important method to prepare perfluorosulfonic membranes, yet fabrication process and mechanism of solution-cast perfluorosulfonic membranes is not clear. In this paper, we made systematic study on morphological evolution of perfluorosulfonic ionomer (PFSI) molecules in N,N-dimethylformamide (DMF) at different concentrations. Process of membrane fabrication, microstructure and properties of perfluorosulfonic membranes were studied to find out the fabrication mechanism of solution-cast PFSI membranes.Morphological evolution of PFSI molecules in DMF was studied in detail. It was found that PFSI molecules shrink and aggregate in DMF gradually with increasing concentration, accordingly, mixture of PFSI and DMF changes from solutions to dispersions and gels. For the typical perfluorosulfonic ionomer with ion exchange capacity of 0.95meq/g, the prepared PFSI/DMF mixture changes from solutions to dispersions at PFSI concentration of 0.5mg/g, and the mixture changes to be gels from dispersions gradually at PFSI concentrations of 208-336mg/g. Understanding of properties of the PFSI/DMF mixture provided important reference for further study on membrane fabrication mechanism.It was found that aggregation of the PFSI particles in the dispersions and gels is due to the interaction of backbones of the PFSI molecules. With shrinkage and interaction of the PFSI backbones, the segments of the main chains may arrange regularly to form crystals similar with the ones in polytetrafluoroethylene.Morphology of the DMF-based solution-cast PFSI membranes is different from that of thermo-extruded membranes. The solution-cast membranes are more isotropic. Crystallites in the solution-cast membranes seem to be randomly distributed over the amorphous phase. Hydrophilic zone in the membranes is composed of continuous ionic thin channels and nanoparticles that are smaller than the clusters in the thermo-extruded membranes. The different microstructure of the two kinds of membranes is the reason that the thermo-extruded PFSI membranes are stronger and conductivity of the solution-cast membranes is bigger. The solution-cast PFSI membrane is assembled by the PFSI aggregates formed in the dispersion. The PFSI molecules shrink to form about 20nm particles in dilute dispersions, and the particles aggregate to form bigger PFSI aggregates in the concentrated dispersions. The PFSI particles and aggregates fuse into each other with heat, capillary force and Van der Waals' force to form membranes.Effect of casting temperature, casting time and change of temperature on morphology and properties of PFSI membranes was studied. It is proved that casting temperature has important effect on the membranes. Increasing casting temperature may increase movement ability of the PFSI segments, volatilization of the solvent and driving force of fusion of the aggregates, so morphology and property of the membranes are different when prepared at different temperatures. With increasing casting temperature, the perfluorosulfonic aggregates fuse into each other deeper to form denser membranes, so density of the membranes increases and crystals in the membranes turns bigger and more perfect. Accordingly, proton conductivity, water uptake, methanol permeability and solubility in organic solvents of the membranes decrease with increasing casting temperature, whereas tensile stress at break of the solution-cast membranes increases.