Effect of Moisture on Mechanical Properties of an Amphiphilic Block Copolymer Membrane

Applications such as CO₂ capture and flue gas dehydration require membranes that are mechanically robust at elevated temperatures and that have high selectivity for CO₂ and/or water over other gases. Polystyrene-b-poly(ethylene oxide) is an amphiphilic block copolymer. In the phase-separated state (e.g. for high molecular weight polymer) glassy polystyrene (PS) domains act as mechanical crosslinks resulting in exceptional toughness. Hydrophilic poly(ethylene oxide) (PEO) domains confer selectivity for polar compounds such as water and carbon dioxide. An important consideration that has not been investigated is the impact of moisture on the mechanical properties of this material. We report our findings on the effect of moisture on the glass transition temperature of PS and the resulting impact on the mechanical properties of the block copolymer using humidity-controlled differential scanning calorimetry and dynamic mechanical analysis. In order to understand how mechanical properties are transmitted from the length scale of block copolymer phase separation to macroscopic lengths important for membrane performance, we have investigated local measurements of block copolymer dynamics using a time-resolved scattering technique, x-ray photon correlation spectroscopy.