Blends of Perfluoropolymers: Fabrication, Structural Characterization and Performance in Helium Recovery from Stranded Natural Gas

Thermal processes, which are the standard industrial practice for molecular separations, are energy intensive and make up approximately 15% of the energy consumption in the United States. Commercial perfluorinated polymer membranes have raised interest in molecular separations, due to their high permeability and outstanding long-term stability, coupled with moderate selectivity. Interestingly, this class of materials exhibits excellent performance in helium separation from light gases. Helium has significant industrial value because of its use in MRI, semiconductor manufacturing, and analytical chemistry. There are trace amounts of helium ranging from 0.01% to 7% in stranded natural gas, which is difficult to extract effectively through distillation. Perfluorinated polymers could offer cost-effective recovery of helium, but current commercial materials have a selectivity too low for dilute extraction. Combining the high permeability of Teflon AF2400 with the high helium/methane selectivity of poly(PFMMD-co-CTFE), as high as 900, offers a potential solution for cost-effective recovery of helium. Poly(PFMMD-co-CTFE) is a recently discovered, extremely brittle perfluorinated polymer characterized by its average permeability and one of the highest He/ CH₄ selectivities found to date. Blending of these two polymers is expected to provide high permeability, selectivity, mechanical durability, - as well as high stability against problems such as physical aging and plasticization. However, before proceeding with a detailed study of gas transport in these materials, a detailed study of miscibility among perfluoropolymers is needed. While miscibility among hydrocarbon-based polymers is a well-established topic in materials science, the issue of miscibility among halogenated polymers is a still unanswered question. The challenge with blending these two polymers revolves around the two halogens present in the perfluorinated and chlorinated polymers. Fluorine and chlorine exhibit substantially different physicochemical properties, in terms of polarity, hydrophilicity, and interaction pattern. Therefore, before being able to fabricate blends exhibiting optimal combinations of transport properties and long-term stability, it is essential to study the fundamental miscibility of perfluoropolymers with chlorofluoropolymers. After preparing several blends by systematically changing the amount of the two polymers, the permeability of CH₄, He, H₂ and CO₂ was measured at 35°C and compared with state-of-the-art materials. Noteworthy, the blends’ performance lies on the 2008 Robeson upper bound. Permeability data, high-resolution light microscopy, UV-vis, and thermal analysis is used to develop fundamental structure-property correlations for these novel blends. The possibility of achieving better CH₄/He separation is a top priority in Oklahoma’s economy, as it would help recover helium, a valuable impurity, from stranded natural gas.