pubs.acs.org/Macromolecules Published on Web 10/27/2009 r 2009 American Chemical Society 326 Macromolecules 2010, 43, 326–333 DOI: 10.1021/ma901950u CO 2 -Philic Polymer Membrane with Extremely High Separation Performance Wilfredo Yave,* ,†,§ Anja Car, †,§ Sergio S. Funari, ‡ Suzana P. Nunes, †, ) and Klaus-Viktor Peinemann †, ) † Institute of Polymer Research and Institute of Materials Research, GKSS Research Centre Geesthacht GmbH, Max-Planck-Str. 1, 21502 Geesthacht, Germany, and ‡ Hasylab at DESY, Notkerstr. 85, 22603 Hamburg, Germany. § Present address: Institute of Materials Research, GKSS-Research Centre Geesthacht GmbH. ) Present address: 4700 KAUST, Thuwal 23955-6900, Saudi Arabia. Received September 2, 2009; Revised Manuscript Received October 14, 2009 ABSTRACT: Polymeric membranes are attractive for CO 2 separation and concentration from different gas streams because of their versatility and energy efficiency; they can compete with, and they may even replace, traditional absorption processes. Here we describe a simple and powerful method for developing nanos- tructured and CO 2 -philic polymer membranes for CO 2 separation. A poly(ethylene oxide)-poly(butylene terephthalate) multiblock copolymer is used as membrane material. Smart additives such as polyethylene glycol dibutyl ether are incorporated as spacers or fillers for producing nanostructured materials. The addition of these specific additives produces CO 2 -philic membranes and increases the CO 2 permeability (750 barrer) up to five-fold without the loss of selectivity. The membranes present outstanding performance for CO 2 separation, and the measured CO 2 flux is extremely high (>2 m 3 m -2 h -1 bar -1 ) with selectivity over H 2 and N 2 of 10 and 40, respectively, making them attractive for CO 2 capture. Introduction Global warming is mainly caused by the combustion of fossil fuels and other human activities, leading to carbon dioxide (CO 2 ) emissions. CO 2 sequestration is therefore needed 1 , and because CO 2 is always in a mixture of gases, an efficient separation technology is also required. The development of innovative materials, which efficiently separate CO 2 from other gases, is a big task in the membrane field. Nanotechnology might help us to master this challenge. Ethylene oxide (EO) containing block copolymers or, more generally, polyethers have been identified as outstanding CO 2 - selective materials. 2-5 Their high performance has been attrib- uted to the interaction between the EO unit and CO 2 . 6,7 By controlling the crystallinity of the copolymers by changing the content and molecular weight of the ethylene oxide segment and by altering its microstructure by chemical modification, block copolymers with improved properties can be synthesized. 8-12 However, the chemical modification can be expensive or for other reasons not feasible for large scale production of membranes. An important alternative is the simple incorporation of a second phase into existing polymers, which already have high perfor- mance. This modification can be done by blending with specific additives, which might be liquids or solids (nanoparticles). 7,12-16 The addition of polyethylene glycol (PEG) to the poly(amide- b-ethylene oxide) copolymer (Pebax) demonstrated that CO 2 permeability and selectivity over H 2 can be simultaneously increased. 13 This enhancement was attributed to the high CO 2 solubility in PEG, but a free volume increase was also taken into consideration because a decrease in density and glass-transition temperature (T g ) was observed. Later, the increase in total free volume and fractional free volume was demonstrated for the Pebax/PEG blend. 17 Poly(ethylene oxide)-poly(butylene terephthalate) (PEO- PBT) is another material with high CO 2 separation performance (known as Polyactive). By controlled addition of PEG into PEO-PBT, polymeric membranes were tailored. 9 High CO 2 permeability and high selectivity over H 2 and N 2 were obtained for the developed membranes. After these two works, we found that the addition of PEG had a limit on the permeability and selectivity improvement. The hydroxyl group of PEG tends to form hydrogen bonding with the ether groups in the copolymer, and thereby the number of free EO units decreases and the CO 2 solubility reaches a maximum; consequently, the permeability increase was limited. For increasing the CO 2 permeability with- out affecting the selectivity, other strategies were then considered to increase the total free volume, that is, the static and dynamic free volume of the polymer system, to a much higher extent than that observed for Pebax/PEG blends. 17-19 PEGs with different end groups such as methyl ether, ethyl ether, vinyl ether, phenyl ether, and so on have almost the same CO 2 solubility. Their densities however are different, and some of them have even lower density than PEG, indicating a high free volume. A careful selection of modified PEGs as spacer or filler to be added to the copolymer matrix was expected to increase the total free volume and, consequently, the permeability as well. The spacer or filler should hinder the hydrogen bonding between polymer chains and increase the space between them. At the same time, the glass-transition temperature should decrease (because of the increase in chain mobility and free volume) as well as the crystallinity, leading to higher fractional free volume and en- hanced CO 2 permeability. 7,17 Besides the effect of solubility diffusion in the polymer bulk, the nanostructured surface mor- phology of membranes, in the form of a disordered nanofiber- or “mikado”-like structure, seems to influence the separation per- formance as well. 7 On the basis of the works mentioned above and others previously reported by other groups, 3,5,20,21 we concluded that *Corresponding author. Tel: þ494152872403. Fax:þ494152872466. E-mail: Wilfredo.Yave.Rios@gkss.de.