The effect of temperature on the transport properties of high performance polymers
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The transport properties of high-performance polymers are typically studied near ambient conditions. However, many separations are performed at temperatures either above or below ambient conditions. The permeability and solubility of light gases in thermally rearranged (TR) polymers were measured as a function of temperature. Solubilities decreased with increasing temperature for all samples. At low TR conversion, the sorption process initially becomes less exothermic. However, enthalpies of sorption do not significantly change with TR conversion after thermal rearrangement. Permeabilities increase with increasing temperature for all but CO2 at the highest TR conversion. As extent of thermal rearrangement increases, activation energies increase slightly before decreasing significantly at higher TR conversions. Activation energies of diffusion decreased with increasing TR conversion while enthalpies of sorption remained mostly constant. At the highest TR conversion, decreases in temperature move the polymer toward the upper right on the upper bound. Polybenzimidazoles have been the focus of increasing amounts of study due to their good H2/CO2 separation properties and high thermal stability. Gas transport properties of a novel series of polybenzimidazoles based on a new tetraaminodiphenylsulfone (TADPS) monomer have been characterized at temperatures from 35 to 190 °C. Permeability increases with increasing temperature for all gases. Separations with TADPS-based PBIs are strongly size selective, with CO2/N2, CO2/CH4, and N2/CH4 selectivities decreasing with increasing temperature. However, H2/CO2 selectivities increase with increasing temperature due to a lower activation energy of permeation for CO2 than for H2.