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Effects of Solvent-Casting Conditions on the Morphology and Properties of Disulfonated Poly(arylene ether sulfone) Copolymer Films for Polymer Electrolyte Membranes

Myoungbae Lee1, Ozma Lane2, Hae-Seung Lee2, James E. McGrath2, and Donald G. Baird1. (1) Chemical Engineering, Virginia Tech, Blacksburg, VA 24061, (2) Macromolecules and Interfaces Institute, Virginia Tech, Blacksburg, VA 24061

Disulfonated random and segmented multi-block copolymers based on poly(arylene ether sulfone) have been shown to be promising for fuel cell proton exchange membranes via a lab-scale batch solution casting followed by a mild drying. This batch casting approach is not appropriate for commercial production mainly because of its low drying rate. Thus, the effects of solution-casting conditions such as drying temperature, solvent type, etc. on the final morphology and properties were investigated for commercial production using a homemade batch convection drying apparatus installed to generate industrial-level drying conditions in the lab. In order to see the effect of molecular structure as well as casting conditions, a random copolymer and two different multi-block copolymers with different sequence lengths (5k and 15k) were prepared.

Despite a similar amount of proton-conducting moiety, the longer the block length was, the higher proton conductivity was achieved owing to more phase separation between sulfonated and non-sulfonated segments and subsequent well-defined continuous microstructure. No considerable effects of casting conditions on the microstructure and properties of the random copolymer were observed, whereas noticeable variations of the multi-block copolymers were induced; for example, the proton conductivity and water uptake of the multi-block copolymer films cast with a selective solvent (dimethylacetamide) were substantially higher than those of the equivalent films cast with a non-selective solvent (N-methylpyrrolidone). It was also observed that the multi-block copolymer with longer sequence length (15k) was more sensitive to the casting conditions. Our preliminary study also showed that the final morphology and properties also depended on substrate type, rate of solvent removal, and so on. Further investigation will be carried out in order to clarify the interrelationship among casting conditions, microstructures and properties. Also, this study will be extended to the development of measurement scheme of rheological properties (G' and G'') of copolymers in solution-cast films during drying process for the study of transformation and kinetics of the copolymers in solution-cast films.