In this work, we studied CO2 sorption and transport in two series of aliphatic-aromatic copolyimide membranes, as a function of chemical formulation, pressure and temperature. Such materials are formed by two distinct phases with rather different properties: the first one is formed of rubbery polyether segments (poly(propylene oxide) (PPO) or poly(ethylene oxide) (PEO)), characterized by favorable energetic interactions with CO2 and high flexibility, which endows the copolymers with high CO2 permeability, suitable for capture processes. The second phase is formed by hard glassy polyimide blocks, randomly distributed at the microscopic level, which provide the necessary thermal, chemical and mechanical stability.
Previous studies indicate that CO2 permeability increases with increasing the amount of polyether phase in the copolymers; in the present work we investigate more deeply the interactions and synergies occurring between the two phases, by focusing separately on the CO2 solubility and diffusivity terms that contribute to permeability.
In particular, by studying the shape of the solubility isotherm, as well as the values of diffusivity, sorption enthalpy and activation energy, we were able to monitor the transition from a glassy-like to a rubbery-like behavior as the fraction of rubbery component in the copolymer increases. The data indicate that polyether enhances CO2 permeability by acting mostly on diffusivity, while the solubility contribution is less affected on a quantitative basis. However, the qualitative behavior of solubility allows understanding the nature of interactions between the two phases. In particular, by using a simple additive approach to estimate the CO2 solubility of the copolymer, and the Non-Equilibrium Lattice Fluid (NELF) to evaluate the CO2 solubility in the pure homopolymers, one concludes that the copolymers sorption behavior is “ideal”, i.e. purely additive, indicating a good combination of the two phases. The copolymer volume, on the other hand, shows a contraction upon combination of the two phases. The NELF modeling of solubility data allows attributing such a contraction only to the glassy phase, whose excess free volume is reduced in the presence of the rubbery portion in the copolymer, which possibly partly occupies such excess volume, indicating a strong interpenetration of the two phases.
