Influence of Process Parameters on Functional Properties of Slurry Derived PFSA Membranes

The processing of perfluorosulfonic acid (PFSA) ionomer membranes derived from polymer slurry is the focus of this study. PFSA molecules self-aggregate and form micelles in a slurry as a consequence of its chemical structure. The gradual coalescence of polymer micelles occurs upon membrane formation processes. The particular application of PFSA membranes is proton conducting solid electrolyte for fuel cells, which operate at high temperatures up to 120°C and low relative humidity. The synthesis of PFSA is not a subject of this work. The polymer in a water-alcohol based slurry was provided by 3M Germany for the purpose of a scientific project. The main focus of this study was the characterization of membrane properties derived from a slurry as a consequence of variable process steps. The functional properties of PFSA membrane depend on process conditions. The investigated process parameters were as follows: homogenization and deareation of PFSA slurry, thermal treatment for drying and annealing, protonation for equilibration of ion content in the membrane.It can be concluded that the process steps influence functional properties of PFSA membrane including mechanical features. Therefore, the physical and chemical aspects of membrane formation were investigated. The following steps were defined as physical aspects: the energy input in the polymer slurry upon homogenization, casting parameters i.e. shear rate at constant humidity and temperature conditions, temperature and humidity during drying and annealing.The chemical features distinguished in processing were as follows: the chemical nature and concentration of ternary solvent, the acids utilized for membrane protonation.The investigation of PFSA slurry parameters was conducted by microscopic and rheological analysis. The variation in size and shape of polymer micelles was studied as a function of ternary solvent and energy input upon homogenization. It was concluded that energy input contributes to the formation of micelle agglomerates without differences in size of primary PFSA micelles, based on LSM study. Moreover, 3M PFSA in water and n-propanol shows shear thinning behaviour where dynamic viscosity decreases with an increase of shear rate based on rheological studies. The combination of dilution, solvent change and homogenization on polymer micelles was investigated by cryo-SEM. The energy input into binary solution did not lead to demixing of polymer with solvents in two separate phases. Such demixing was identified for PFSA slurry in ternary solvent system namely in water, n-propanol and diethylene glycol monoethyl ether. However, the size of primary micelles remained constant. It was clearly visible that the addition of ternary solvent inhibits the formation of micelles agglomerate upon homogenization in contrast to PFSA in binary solvent slurries. In addition, the influence of ternary solvent was visible also viscous behaviour of polymer slurry at low shear rates. All in all, the demixing of PFSA slurries had no negative impact on the thin film casting procedure. It was empirically proven that the defect-free membrane was obtained from PFSA slurry in the ternary solvents system in contrast to the membrane derived from H2O and C3H7OH in the same process route. Furthermore, solvents with dipole moment higher than 2 Debye may inhibit the micelle agglomeration in PFSA slurry upon homogenization based on a screening test of several organic solvents. It can be explained that the tendency to inhibit formation of vast micelle-agglomerates, which was found by cryo-SEM, was caused by solvent effect.Upon drying of the PFSA slurry, the gradual coalescence of polymer micelles led to formation of a gel-like membrane, that's structure is not yet networked where water and solvent molecules were embedded within PFSA micelles. Based on cryo-SEM studies of PFSA slurry and dried polymer, it was concluded that at 20°C only a few agglomerates in size of ~1 µm exist