Polymers with intrinsic microporosity (PIMs) combine good thermal and chemical stability with a high free-volume, resulting in very high gas permeability, coupled with a solubility controlled permeation [1, 2]. PIM-1 is an organophilic, glassy polymer with no evidence of a glass transition below the degradation temperature [3]. Owing to its unique molecular structure, PIM-1 is one of the most interesting polymers for gas separation: its performance is close to or on Robeson's 2008 upper bound [4] for several important gas pairs (e.g., O2/N2, CO2/CH4) [5]. Glassy polymers are non-equilibrium materials and are prone to physical aging since they lose excess free volume over time. This process generally results in a reduced gas permeability and in an increased selectivity. Long-term aging may have a considerable influence on transport properties and could represent a major problem in the application of glassy polymer membranes. It can be particularly important in the thin films applied to gas separation systems [6]. The influence of solvent and thermal treatment on the behaviour of PIM-1 dense films will be discussed, elucidating their effect on the gas transport properties. Soaking of the membranes with alcohol was used to cancel the previous history of the samples and to remove residual solvent. A thermal treatment in the range of 25 to 125 °C was carried out after the ethanol soaking. Permeation tests carried out in the time lag mode provided data on the permeability, diffusion coefficient and solubility coefficient for six permanent gases. Sorption tests were independently carried out, using a McBain quartz spring balance. Results and discussion Permeability of all films decreases with aging time, consistent with a loss of fractional free volume. In particular, the diffusion coefficient is reduced, while the selectivity is enhanced. The thermal treatment results in a faster permeability reduction and in an increased gas solubility. Some representative results in terms of nitrogen permeability are reported in Figure 1 for films treated according to different protocols. The sample treated at 125 °C seems to be more stable against aging in the long term but has a lower initial permeability due to accelerated aging by the heat treatment. CO2 sorption isotherms are well described by the dual mode sorption model. There is a fairly good correspondence between the indirect estimation of the solubility coefficient from permeation tests and the values obtained from a sorption balance. Figure 1. Nitrogen permeability in PIM-1 films tracked over time after different membrane conditioning protocols. In this work the various factors influencing the aging phenomena in PIM-1 films will be investigated in detail, as well as the effect of the aging on sorption, permeation and free volume. Strategies to reduce time dependency of such high free-volume materials will be proposed as well. Acknowledgements The work leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement no. NMP3-SL-2009-228631, project DoubleNanoMem. References [1] P.M. Budd, B.S. Ghanem, S. Makhseed, N.B. McKeown, K.J. Msayib, C.E. Tattershall, Polymers of intrinsic microporosity (PIMs): robust, solution-processable, organic nanoporous materials, Chem. Commun. (2004) 230-231. [2] P.M. Budd, N.B. McKeown, Highly permeable polymers for gas separation membranes, Polym. Chem. 1 (2010) 63-68. [3] P.M. Budd, K.J. Msayib, C.E. Tattershall, K.J. Reynolds, N.B. McKeown, D. Fritch, Gas separation membranes from polymers of intrinsic microporosity, J. Membr. Sci. 251/1-2 (2005) 263-269. [4] L.M. Robeson, The upper bound revisited, J. Membr. Sci. 320 (2008) 390-400. [5] P. Bernardo, E. Drioli, G. Golemme, Membrane gas separation, A review / State of the art, Ind. Eng. Chem. Res. 48 (2009) 4638-4663. [6] Y. Huang, X. Wang, D.R. Paul, Physical aging of thin glassy polymer films: Free Volume interpretation, J. Membr. Sci. 277 (2006) 219-229.
Effect of post-treatment and physical aging on the gas transport properties of PIM-1 membranes
P Bernardo;F Bazzarelli;F Tasselli;JC Jansen;G Clarizia;
2012
Abstract
Polymers with intrinsic microporosity (PIMs) combine good thermal and chemical stability with a high free-volume, resulting in very high gas permeability, coupled with a solubility controlled permeation [1, 2]. PIM-1 is an organophilic, glassy polymer with no evidence of a glass transition below the degradation temperature [3]. Owing to its unique molecular structure, PIM-1 is one of the most interesting polymers for gas separation: its performance is close to or on Robeson's 2008 upper bound [4] for several important gas pairs (e.g., O2/N2, CO2/CH4) [5]. Glassy polymers are non-equilibrium materials and are prone to physical aging since they lose excess free volume over time. This process generally results in a reduced gas permeability and in an increased selectivity. Long-term aging may have a considerable influence on transport properties and could represent a major problem in the application of glassy polymer membranes. It can be particularly important in the thin films applied to gas separation systems [6]. The influence of solvent and thermal treatment on the behaviour of PIM-1 dense films will be discussed, elucidating their effect on the gas transport properties. Soaking of the membranes with alcohol was used to cancel the previous history of the samples and to remove residual solvent. A thermal treatment in the range of 25 to 125 °C was carried out after the ethanol soaking. Permeation tests carried out in the time lag mode provided data on the permeability, diffusion coefficient and solubility coefficient for six permanent gases. Sorption tests were independently carried out, using a McBain quartz spring balance. Results and discussion Permeability of all films decreases with aging time, consistent with a loss of fractional free volume. In particular, the diffusion coefficient is reduced, while the selectivity is enhanced. The thermal treatment results in a faster permeability reduction and in an increased gas solubility. Some representative results in terms of nitrogen permeability are reported in Figure 1 for films treated according to different protocols. The sample treated at 125 °C seems to be more stable against aging in the long term but has a lower initial permeability due to accelerated aging by the heat treatment. CO2 sorption isotherms are well described by the dual mode sorption model. There is a fairly good correspondence between the indirect estimation of the solubility coefficient from permeation tests and the values obtained from a sorption balance. Figure 1. Nitrogen permeability in PIM-1 films tracked over time after different membrane conditioning protocols. In this work the various factors influencing the aging phenomena in PIM-1 films will be investigated in detail, as well as the effect of the aging on sorption, permeation and free volume. Strategies to reduce time dependency of such high free-volume materials will be proposed as well. Acknowledgements The work leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement no. NMP3-SL-2009-228631, project DoubleNanoMem. References [1] P.M. Budd, B.S. Ghanem, S. Makhseed, N.B. McKeown, K.J. Msayib, C.E. Tattershall, Polymers of intrinsic microporosity (PIMs): robust, solution-processable, organic nanoporous materials, Chem. Commun. (2004) 230-231. [2] P.M. Budd, N.B. McKeown, Highly permeable polymers for gas separation membranes, Polym. Chem. 1 (2010) 63-68. [3] P.M. Budd, K.J. Msayib, C.E. Tattershall, K.J. Reynolds, N.B. McKeown, D. Fritch, Gas separation membranes from polymers of intrinsic microporosity, J. Membr. Sci. 251/1-2 (2005) 263-269. [4] L.M. Robeson, The upper bound revisited, J. Membr. Sci. 320 (2008) 390-400. [5] P. Bernardo, E. Drioli, G. Golemme, Membrane gas separation, A review / State of the art, Ind. Eng. Chem. Res. 48 (2009) 4638-4663. [6] Y. Huang, X. Wang, D.R. Paul, Physical aging of thin glassy polymer films: Free Volume interpretation, J. Membr. Sci. 277 (2006) 219-229.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
