Crystallite formation effect on the physicochemical properties of SPEEK membranes for fuel cell application

Crystallite formation in membranes cast from sulfonated poly(etheretherketone) (SPEEK)
dispersions in dimethylacetamide has been investigated by differential scanning calorimetry.
The crystallites melt at 220°C and the melting enthalpy (DHm) is strongly influenced
by environmental conditions of solvent evaporation. When solvent is evaporated in
air, DHm decreases with increasing the evaporation temperature so that no crystallites are
formed above 45 °C. The study of crystallite formation at 30 °C, under controlled relative
humidity (RH) and solvent evaporation rate, has shown that DHm decreases i) with
decreasing RH from 75% to 5%, at constant evaporation rate, and ii) with increasing
evaporation rate, especially at low RH. The exchange of protons with sodium ions results in
an increase of the crystallite melting temperature thus indicating the presence of sulfonic
groups in the crystallites. In comparison with crystallite-free SPEEK membranes, the
presence of crystallites turns out in enhanced membrane mechanical properties, reduced
hydration in liquid water above 75 °C, strongly anisotropic swelling (occurring mainly in
the through-plane direction) and better performance in H2/air fuel cell at 70 °C.