Ind. Eng. Chem. Res. 1995,34,3170-3172
3170
Update on the Aging of a Thin Polycarbonate-Ceramic Composite Membrane Mary E. Rezac' School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100
The gas transport properties of a thin polycarbonate-ceramic composite membrane over a nearly 4 year period are reported. This Research Note provides an update on the activities first reported 2 years ago. In the interim, transport rates through the film have continued to decrease and heliudnitrogen and heliudoxygen selectivities continue to increase. The rate of flux decay was quite rapid during the first month following manufacture. During this time, the helium flux decreased nearly 30%. Yet, the flux decay rate decreased considerably following that period with only a n additional 12% drop in the helium flux in the following 45 months. This behavior is consistent with a loss in free volume of the sample.
Introduction Two years ago, we reported on the aging behavior of thin polycarbonate-ceramic and polyimide-ceramic composite membranes (Rezac et al., 1993). In that publication, it was shown that the gas transport properties of the aged polymeric membrane changed with time. These changes were not attributable to changes in the porous structure of the substrate, addition or loss of chemical solvents, or other outside influences. Rather, the changes observed were attributed to physical aging of the polymer network and its slow drift to an equilibrium conformation. Since our last publication, Pfromm and Koros have reported an analysis of the physical aging of thin polymer films at temperatures far below their glass transition temperatures (1995). The densities of both polyimide and polysulfone samples were shown to increase with time (Pfromm, 1994). The permeation coefficients for the aged polymer films were independently measured and were shown to decrease with time. Pfromm further showed that the aging rate for these polymers was dependent upon the sample thickness. Unsupported polymer samples of less than 0.5 pm thickness aged at a greatly accelerated rate when compared to samples of 2.5 and 25 pm thickness. The transport rates of gases through the thinnest samples continued to decrease over the 250 days studied (Pfromm, 1994). Evaluation of polymer-agingbehavior over longer time periods was not reported. This Note documents the behavior of a single polycarbonate-ceramic composite membrane for a period of nearly 4 years. Although the transport properties continue to change over this entire period, the rate of change decreases steadily with time. After the 4 year interval, the properties can be considered approximately t i m e i n v a r i a n t for design purposes. However, the longterm aging data presented here may be of significant importance in the analysis of this complex process.
Experimental Section Complete experimental details were previously reported (Rezac et al., 1993). A polycarbonate-ceramic composite membrane was prepared by the application of a controlled volume of approximately 0.1 wt % polymer solution to the surface of the ceramic followed Phone: (404) 894-1255. FAX: (404) 894-2866. E-mail:
[email protected].
Figure 1. Structure of the tetramethylhexafluorobisphenol-A polycarbonate [TMHFPC] monomer.
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Figure 2. Percent of pressure-normalized flux of helium (01, oxygen (A),and nitrogen (W) for a TMHFPC-ceramic composite membrane. Percent of pressure-normalized flux = (flux on day of testing/original flux) x 100%. Ap = 50 psi, T = 25 "C.
by evaporative drying. In the process, a thin defectfree layer of polymer is deposited on the surface of the ceramic that serves as a mechanical support. Estimates of the thickness of this film have been made based on gas flux measurement and on a material balance. Effective thicknesses for the separating layers on membranes are commonly made by dividing the nitrogen permeability of the polymer by the pressure-
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Figure 3. Pure-gas heliudnitrogen, heliudoxygen, and oxygednitrogen selectivity (from top to bottom). Ap = 50 psi, T = 25 "C. Dense film selectivity measured on a 3-month-old film.
normalized nitrogen flux (Mulder, 1991). For the data reported here, the pressure-normalized flux is changing with time and thus, the calculated thickness will increase with time. The selectivities of the membrane and the thick isotropic film are nearly identical at the beginning of the test period, but at the end, there are significant differences. Therefore, the properties of the membrane appear to be most consistent with the thick film at the beginning of the test period. The estimated thickness of the polymer layer was 4700 based on nitrogen gas flux on the first day of testing. As a secondary estimate, the thickness has been calculated by simple material balance methods. In this calculation, the total volume of the applied solution, its polymer concentration, and the dry polymer density are known. The calculated thickness, assuming no loss of polymer into the substrate, is approximately 7500 A. This is in acceptable agreement with the 4600 A previously calculated and provides an upper limit on the thickness. For reference, the thickness calculated on the basis of the flux of the membrane after 1400 days is approximately 29 000 A. Clearly, this value is unreason-
able in comparison with the mass balance calculations. Rather, it is believed that the permeability of the thin aged membrane can no longer be represented by that of a much thicker "dense isotropic'' film. The polymer employed was a tetramethylhexafluoropolycarbonate, TMHFPC. Its structure is shown in Figure 1. The permeabilities of helium, oxygen, and nitrogen at 25 "C measured on a dense, isotropic film were 176,28.5, and 6.6 barrer, respectively (1barrer = cm3(STP)cm/(cm2s cmHg)). These values were obtained for a 75 pm thick film that was 3 months old at the time of testing. Gas transport measurements were made using a constant pressure-variable volume permeation system. All measurements were made at 22-26 "C and corrected to 25 "C using measured activation energies for permeation. Since the presence of even a small quantity of casting solvents can greatly influence the permeation of gases through the polymer layer, the membranes tested were thoroughly analyzed to ensure that all detectable solvent had been removed. A detailed discussion of the
3172 Ind. Eng. Chem. Res., Vol. 34, No. 9, 1995 process employed and the analysis completed was presented in the original paper (Rezac et al., 1993). All tests were completed in a stainless steel test cell (Millipore Corporation, Bedford, MA) sealed with a Viton O-ring. The gases were all ultrahigh purity grade and were used as received without further purification.
Results The gas transport of helium, nitrogen, and oxygen through the TMHFPC membrane as a function of time is presented in Figure 2. For ease of evaluation, the data are presented on both linear and logarithmic time scales. The logarithmic time scale is used only for ease of viewing the data. No physical significance is suggested. Individual transport rates are normalized by the initial value and presented as a percentage of that rate. All transport rates decrease with time. A dramatic decrease was measured in the first 30 days following manufacture, and a less pronounced drift was subsequently observed. The rates for all gases continued to decrease over the entire 4 year period; however, the relative rates of flux loss varied with the gas tested. Therefore, the ideal selectivity calculated for helium relative t o nitrogen and oxygen increased steadily with time. Membrane selectivity for the separation of oxygen from nitrogen exceeds the value predicted from thick, dense film measurements on the second day of membrane testing. However, the value does not appear to be strongly time dependent. These results are presented in Figure 3. Reference values are presented in Figure 3 for the ideal selectivity measured from thick, dense films (Rezac et al., 1993). Analysis of Figure 2 indicates that there may be two mechanisms of importance in the aging of this thin polymer film: one causing large, rapid changes in properties that appears to dominate during the first 30 days of this evaluation and a second causing a slow drift in the properties of the polymer that appears to be present over the entire test period. The physical significance of these processes cannot be ascertained from the data available. Unfortunately, this _membranewas damaged by improper handling following this series of tests. Therefore, no further evaluation will be possible. The behavior of this membrane is in clear agreement with the work published by Pfromm and Koros (1995) in which the properties of thin polymer films showed dramatic changes in their gas transport properties over time. In their work, they proposed the elimination of free volume at the surface of the sample as the reason. As the free volume of the polymer film decreases, the permeability of gases through the film decreases causing the observed decrease in gas flux. Since the permeability is the result of both sorption and diffusion processes, this change in properties is penetrant specific and the ideal selectivities of the membrane change with
time. It is not possible to draw clear conclusions from the limited data available; however, for the three penetrants tested, the rate of loss of initial permeability increases as the solubility of the gas in the polymer increases. This might indicate that a loss in the solubility that would be associated with a decrease in the free volume of the polymer is the major contribution to a loss in permeation rate. This is consistent with the increase in selectivity. Independent measurement of both penetrant diffusivity and solubility would provide insight into the importance of changes in free volume (solubility) and chain packing (diffusivity) in this process.
Conclusions The transport properties for a defect-free polycarbonate-ceramic composite membrane have been reported as a function of aging time to nearly 4 years. The TMHFPC membrane had an effective thickness estimated from flux measurements of approximately 6000 A. The membrane experienced a decrease of nearly 65% in the oxygen and nitrogen transport rates, and over 40% in helium flux. The selectivity for helium over the other gases increased dramatically with time. Selectivities of the thin aged membrane greatly exceeded those of a thick, dense film of the polymer. Time-dependent physical changes in the properties of thin polymeric separating layer may be significant and must be considered in the design of membrane units. From the results presented here, these changes may be quite remarkable in the first few months following manufacture. Thereafter, the permeability of the membrane continues t o decrease, but at a significantly retarded pace. This slow drift in properties, while of great importance in understanding the underlying physical processes occurring, may be of a lesser concern in the design of industrial systems. Literature Cited Mulder, M. Basic Principles of Membrane Technology; Kluwer Academic Press: Boston, 1991. Pfromm, P. H. Gas Transport Properties and Aging of Thin and Thick Films Made from Amorphous Glassy Polymers. Ph.D. Dissertation, The University of Texas a t Austin, 1994. Pfromm, P. H.;Koros, W. J. Accelerated Physical Aging of Thin Glassy Polymer Films. Polymer 1995, in press. Rezac, M. E.; Pfromm, P. H.; Costello, L. M.; Koros, W. J. Aging of Thin Polyimide-Ceramic and Polycarbonate-Ceramic Composite Membranes. Ind. Eng. Chem. Res. 1993, 32, 1921.
Received for review March 7 , 1995 Accepted J u n e 26, 1995@ IE950163+
Abstract published in Advance ACS Abstracts, August 1, 1995. @
