Physical aging, CO2 sorption and plasticization in thin films of polymer with intrinsic microporosity (PIM-1)

Abstract

Physical aging of both thick and thin films of “high free-volume” glassy polymer, PIM-1 (polymer with intrinsic microporosity) was studied by monitoring changes in pure gas permeability of O2, N2 and CH4 at a fixed temperature of 35°C. Permeability of PIM-1 is very sensitive to film thickness, post-treatment (methanol soak, water soak or dried) and casting solvent (chloroform and ortho-dichlorobenzene). Thin films of PIM-1 undergo accelerated aging, leading to initial permeability lower by an order of magnitude compared to thick films. The relative permeability for thin films of PIM-1 was decreased by 67% compared to 53% decrease for thick films at 1000h of aging due to the higher aging rates in thin films. The effect of casting solvent (vapor pressure and boiling point) is more pronounced on aging and selectivity of thin films with initial permeability almost two times higher for the film casted from chloroform (CHCl3) compared to that prepared from ortho-dichlorobenzene (o-DCB) solvent. The effect of casting solvent on the initial permeability is less prominent for thick films. The film thickness, casting solvent and CO2 exposure protocols also have significant effect on the absolute CO2 permeability and plasticization behavior of both thick and thin films. The thin films undergo significant increase in the relative permeability upon plasticization compared to thick films and permeability was found to increase even during CO2 depressurization. The long time, constant pressure CO2 exposure study revealed dominant aging effect in thin film where aging overcomes the CO2 plasticization effect just within 1.2min of the CO2 exposure compared to 10h for thick film. The ellipsometry measurements showed that the CO2 solubility in thin films decreases with increases in the aging time and temperature, and with decrease in the molecular weight. A strong correlation was observed for the Langmuir sorption capacity, CH' and solubility at infinite dilution, S0 with the aging time, temperature and molecular weight of PIM-1.