(694g) CO2 Induced Plasticization in Glassy Polymeric Membranes for Gas Separation

The plasticization phenomena of glassy polymers induced by gases or vapors in glassy polymeric materials are of crucial importance in the field of gas separation membranes, and are typically recognized as an increase in gas permeability as the upstream pressure increases. The mechanism is often associated to the sorption of large amounts of CO₂, or other low molecular weight species (present e.g. as impurities in the feed streams), having the ability to produce significant swelling of the polymer matrix, and to reduce its mechanical rigidity, with a consequent depression of the glass transition. The transport properties of the penetrating gases can be thus altered, eventually leading to a loss of the separation performances of the membrane material.

This work aims to address the CO₂ induced plasticization effect on glassy polymers from different points of view, looking for a clear understand of this phenomenon and of the mechanism related. A commercial polyimide (Matrimid) has been thus selected as an interesting test case material, suitable for CO₂/CH₄ separation, which shows an apparent non-monotonous behavior in the CO₂ permeability going through a minimum point (the so-called plasticization pressure).

The influence of CO₂ sorption on polymer material properties has been investigated by direct dynamic mechanical analysis (Rheometrics DMTA 3E) in isothermal frequency sweep operation at 35°C on the CO₂-containing polymer sample at gas pressures in the range 0-30 atm. The permanent effects produced by the CO₂ at high pressure have been also analyzed by temperature sweep tests on Matrimid samples before and after gas conditioning. Sorption tests, carried out in a pressure decay apparatus, provided the required penetrant loading in the polymer sample at the various pressures, as well as the evaluation of its diffusion coefficient. Finally, the CO₂ transport properties of the membrane have been characterized by permeation experiments in the same pressure range.

The DMA analysis showed that CO₂ is able to lower appreciably but not dramatically the elastic modulus (approximately 10% decrease), while the relaxation dynamics revealed a significant tan d increase, which almost doubles its value. However, the very high glassy transition point of such polyimide (T_g = 313 °C) prevented any second order transition even at the maximum pressure investigated. These results are then compared with the previous data obtained on conventional polymer glasses such as polycarbonate (PC) and polymethylmethacrylate (PMMA), which presented opposite CO₂ permeation behavior, as they showed decreasing and increasing trend, respectively.

Nonetheless, the minimum in CO₂ permeability is clearly identified at 15 bar, and further increasing the upstream pressure significantly enhanced the gas transport, while the gas diffusion coefficient, calculated both from transient sorption and from permeation tests, increased monotonously.

Therefore, the obtained results suggested that increasing permeability behaviors, as well as non-monotonuos trends, are not directly related to softening effect produced by the large CO₂ uptake on the polymeric matrix, while are most likely correlated with the behaviors of solubility and diffusivity coefficient of the penetrant itself.