Influence of Prior Thermal History and CO2 Exposure On Physical Aging In Thin Matrimid® Films

Glassy polymers can provide excellent permeability-selectivity properties for application as gas separation membranes; however, glassy polymers are non-equilibrium materials and constantly evolve towards equilibrium with time, leading to changes in physical properties (density, refractive index, permeability, etc). The rate of this “physical aging” has been found to be much more rapid in sub-micron thin films < 1 µm, typical thickness for commercial membranes, than thick (or bulk) films.  The rate of physical aging is also affected by the thermal history of the films and the method of preparation. The effect of prior history on the physical aging behavior of thin Matrimid^® 5218 films (~ 200 nm) was tracked using gas permeability measurements. As-cast films were heated above Tg to erase prior history. These films were then annealed at various sub-Tg temperatures for different times. The permeability decreased with the increased annealing time and as the annealing temperature reaches closer to the Tg of Matrimid^® 5218. The extent of physical aging and increase in selectivity is lower for films annealed at 250 ºC compared to those annealed at 90 ºC and 200 ºC for 120 hr. Some of the sub-Tg annealed films were also exposed to CO₂ at fixed pressures to see its plasticizing effect on permeability and selectivity.  Films annealed below Tg showed higher resistance to plasticization by CO₂.  Spin-coated thin films as-cast show lower permeability and relatively slower aging rate than annealed films. The effect of various CO₂ exposure protocols also have significant plasticization effects on as-cast films compared to annealed films.