The removal of carbon dioxide, which is a major impurity in hydrogen streams, could benefit from application of reverse-selective membrane separation. While size-sieving polymers tend to favor the transport of H₂ over larger molecules such as CO₂, reverse-selective membranes preferentially remove CO₂ from mixtures with H₂. Since H₂ is often required to be at high pressure for further use, this approach minimizes or avoids the need for the expensive H₂ recompression associated with conventional membrane separation [1]. Reverse-selectivity can be achieved by increasing the membrane affinity for CO₂; for instance, introducing polar groups into the membrane will increase the solubility of CO₂, which is quadrupolar [2].

One of the more effective polar groups for increasing CO₂ affinity is ethylene oxide. However, high molecular weight poly(ethylene oxide) (PEO) has low permeability due to crystallization [3]. We have demonstrated that UV cross-linking of low molecular weight PEO diacrylates can produce a completely amorphous membrane with much higher permeability, close to the theoretical upper bound of Robeson's trade-off curve [4]. Not only does this membrane demonstrate reverse-selectivity for CO₂/H₂ separation, its diffusivity (and thus permeability) can be improved further by copolymerizing PEO diacrylates with certain mono-functionalized PEO [1]. This improvement has been attributed to an increase in fractional free volume of the polymer, as measured by various methods. The increase of fractional free volume leads to improved CO₂/H₂ selectivity due to a decrease in diffusivity selectivity.

In this study, we further examine diffusivity enhancement by strategic copolymerization of selected PEO diacrylates and monoacrylates. We are especially interested in assessing the influence of the monoacrylate end group on the free volume and transport properties of the resulting membranes. In addition to the measurement of transport properties via pure gas permeability and solubility experiments, the thermomechanical properties of the membranes have been characterized using dynamic mechanical analysis. The results underscore the strong correlation between network structure and gas transport in these crosslinked PEO copolymers.

[1] H. Lin, E. Van Wagner, B. D. Freeman, L. G. Toy, R. P. Gupta, Plasticization-Enhanced H₂ Purification Using Polymeric Membranes, Science 2006, 311, 639-642.

[2] H. Lin, B. D. Freeman, Materials Selection Guidelines for Membranes that Remove CO₂ from Gas Mixtures, Journal of Molecular Structure 2005, 739, 57-74.

[3] H. Lin, B. D. Freeman, Gas Solubility, Diffusivity and Permeability in Poly(ethylene oxide), Journal of Membrane Science 2004, 239, 105-117.

[4] H. Lin, T. Kai, B. D. Freeman, S. Kalakkunnath, D. S. Kalika, The Effect of Crosslinking on Gas Permeability in Crosslinked Poly(ethylene glycol diacrylate), Macromolecules 2005, 38, 8381-8393.