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Ind. Eng. Chem. Res. 2007, 46, 2342-2347
Effect of Film Thickness on the Gas-Permeation Characteristics of Glassy Polymer Membranes Y. Huang† and D. R. Paul* Department of Chemical Engineering and Texas Materials Institute, The UniVersity of Texas at Austin, Austin, Texas 78712
Films made from three glassy polymersspolysulfone, poly(2,6-dimethyl-1,4-phenylene oxide), and a commercial polyimide known as Matrimidswere prepared in thicknesses ranging from 0.4 µm to 60 µm, and their permeabilities to oxygen, nitrogen, and methane were monitored for more than a year. These films exhibited substantial decreases in permeability with time, because of volume relaxation that is due to physical aging, which is a reversible process. The observed permeability coefficients were originally greater than the literature values for thick, or so-called “bulk” films, but eventually decreased well below these values. The rate of the aging effect becomes greater the thinner the film. The implications of this observation for practical membrane gas separation processes and the selection of membrane materials based on thick film data are discussed. 1. Introduction Glassy polymers are generally the preferred materials for practical gas separation membranes, because of their inherently better permeability/selectivity balance than is typically the case for polymers above their glass-transition temperature (Tg).1-3 Of course, the structural rigidity provided by the glassy state is essential for membranes that must be self-supporting (e.g., asymmetric hollow fibers).3 Glasses are not in a state of equilibrium; therefore, their properties are dependent on the details of their fabrication and time-temperature history.4-13 Thus, it is not surprising to observe some variance in the reported properties, such as density, refractive index, gas permeability, etc., of glassy polymers. At least for macroscopic specimens, the variability seems to be within a range that is small enough that meaningful property tabulations can be made for glassy polymers, as observed in many handbooks.14-16 However, recent research has shown that this variability may be considerably more pronounced for thin films, because of their significantly more-rapid evolution toward the equilibrium state, which is a process known as physical aging, most often observed in terms of volume relaxation or densification.17 This densification affects properties that are sensitive to free volume, such as permeability, and the associated changes can be quite significant, on time scales of weeks to years.18-31 Practical membranes must be very thin to achieve the high fluxes needed for economical productivity. Indeed, developing processes to make membranes that have effectively defect-free “skins” or separating layers with thicknesses of the order of 0.1 µm (or 100 nm) with minimal defects were the essential breakthroughs required to make membrane separations that operate via a solution-diffusion mechanism a viable technology.2,3,32 However, such thin layers of glassy polymers can be greatly affected by the physical aging issues mentioned previously. This brings into question the widely practiced approach of using relatively thick films for screening or selecting polymers * To whom all correspondence should be addressed. Tel.: (512) 4715392. Fax: (512) 471-0542. E-mail:
[email protected]. † Current address: Membrane Technology Research, Inc., 1360 Willow Rd., Suite 103, Menlo Park, CA 94025.
as membrane materials. Indeed, the permeation properties of thick films are often used to calculate the effective thickness of the skin layer of asymmetric or composite membrane structures from observed fluxes. The purpose here is to compare the permeation response of thin glassy polymer films made from typical polymers used as membrane materials during aging over the course of ∼1 year to that of typical “bulk” property values. This comparison suggests that thick film data must be used with caution or only as a rough guide. 2. Background Figure 1 is a tentative attempt to classify glassy polymer films into different regimes of behavior according to thickness. To the far right of the thickness scale is the familiar case where properties, including those related to the departure from an equilibrium state, are expected to be independent of specimen size. This is clearly the expectation on the millimeter or centimeter size scale and probably extends down to several micrometers; we might call this the “bulk” regime. On the other extreme are ultrathin films where the thickness is of the same order of magnitude as the dimensions of the polymer chain coils (
