(381ay) Pure and Mixed Gas Transport and Physical Aging in Glassy Polymers Exhibiting Configurational Free Volume

Plasticization and physical aging represent two major drawbacks for using glassy polymer membranes. These issues can be mitigated by using triptycene-based polymers. Triptycenes are formed by three aromatic rings arranged in a paddlewheel-like, 3-D structure. Once inserted into a polymer backbone, triptycene groups provide an internal free volume which greatly enhances gas permeability and selectivity. Unlike in conventional glassy polymers, the internal free volume provided by iptycene units is not related to the non-equilibrium, transient conformation, but to the molecular configuration. Such configurational free volume is intrinsic to the polymer structure and, as such, it is non collapsible, similar to the case of inorganic molecular sieves. Equally important, rigid triptycene units can accommodate penetrant molecules in their internal free volume without causing swelling and plasticization.

In this study, pure gas and vapor (He, N₂, CH₄, CO₂, C₂H₆, water vapor, hydrocarbons and alcohols) sorption and diffusion in a novel thermally rearranged co-polymer prepared from a co-polyimide precursor with controlled triptycene molar content, i.e., triptycene-dianhydride(0.25)-6FDA(0.75)-6FAP(1.0), was investigated in the temperature range 5-50°C and up to 32 atm. The experimental data were analyzed in the framework of the Dual Mode and Lattice Fluid models, and several meaningful structure/property correlations were identified. The dual mode parameters retrieved from the analysis of single penetrant sorption isotherms were used to predict the sorption behavior in mixed gas conditions. The mixed gas solubility-selectivity is significantly higher than ideal solubility-selectivity. The analysis of gas diffusion coefficients also disclosed precious information about the effect of the polymer structure on small molecule transport in this class of materials.

Finally, to accelerate physical aging, selected polymer samples were annealed at 220°C under vacuum for 10 days. Remarkably, transport properties were unaffected by thermal annealing, thus demonstrating the superior resistance to physical aging exhibited by these materials.