(606c) Halogenated Blended Membranes Exhibiting Tunable Helium Permeability/Selectivity

Perfluorinated polymers have raised academic and industrial interest for gas and liquid separations due to their high permeability, stability, and moderate selectivity. Specifically, perfluorinated polymers have exceptional performance in helium separation from light gases compared to hydrocarbon-based materials. Although these materials have promise to replace current energy-intensive techniques, most perfluorinated polymers have selectivities too low to efficiently recover helium at the dilute concentrations of 0.1% - 7% occurring in natural gas. In industrial applications, the He selectivity needed is inversely proportional to the He concentration in the feed, therefore, to efficiently recover He from natural gas, membranes exhibiting tunable selectivity are needed. This study sets forth a new approach to fabricate membranes exhibiting these features.

Recently, new perfluorinated polymers exhibiting a chlorine substituent, poly(PFMMD-co-CTFE), have shown a He/CH₄ perm-selectivity as high as 900, which is exceptionally higher relative to the state of the art materials. They have also been shown to have high resistance to swelling and plasticization upon exposure to hydrocarbons. However, these materials suffer from extremely high brittleness, which even hampers the fabrication of self-standing membranes for lab tests. For this reason, its gas permeability was measured after supporting poly(PFMMD-co-CTFE) on a porous backing. To remediate this situation and take advantage of its exceptionally high selectivity, we blended poly(PFMMD-co-CTFE) with Teflon AF2400, a commercially available perfluoropolymer exhibiting much better mechanical stability. In this way, we plan to combine the high selectivity of the former (i.e., poly(PFMMD-co-CTFE)) with the high permeability of the latter (i.e., Teflon AF2400).

First, a detailed study of the miscibility of these two polymers was performed using differential scanning calorimetry and high-resolution light microscopy. It was discovered that the dipolar nature of the chlorotrifluorethylene (CTFE) comonomer in poly(PFMMD-co-CTFE) makes the two polymers totally immiscible in the entire composition range, with a morphology that depends on the blend and solvent composition. Defect-free, free-standing blended membranes could be prepared with different polymer-polymer composition and their morphology was systematically studied. The molecular origin of the polymers immiscibility was elucidated.

He, H₂ and CH₄ permeability was measured at 35°C in blends of varying composition, and a tunable selectivity-permeability behavior lying on the 2008 Robeson upper bound was observed, based on which blends exhibiting variable He permeability/selectivity combinations can be fabricated, depending on the amount of Helium in the feed. Mixing-rule models, permeation, and microscopy were used to build structure-property correlations and back out how poly(PFMMD-co-CTFE)and Teflon AF2400 interact under a variety of casting conditions.