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Infrared spectroscopic measurement of water permeability in polymer films exposed to liquid water
ARTICLE IN PRESS
POLYMER TESTING Polymer Testing 25 (2006) 642–649 www.elsevier.com/locate/polytest
Test Method
Infrared spectroscopic measurement of water permeability in polymer films exposed to liquid water William F. Bauer, Mark L. Stone, Christopher J. Orme, Mason K. Harrup, Thomas A. Luther Idaho National Laboratory, Idaho Falls, ID 83415-2208, USA Received 25 February 2006; accepted 1 April 2006
Abstract The details of a method for measuring the water vapor transport rate through polymer films utilizing Fourier transform infrared spectroscopy (FTIR) are given. Water flows across one side of the membrane and a nitrogen sweep gas carries the water vapor into the flow-through cell of the spectrometer for measurement. The method was used to test two polymers that have literature values and four new polymers. The resulting FTIR data was analyzed with a partial least-squares method. Experimentally the method is easy to apply and very accurate. r 2006 Elsevier Ltd. All rights reserved. Keywords: FTIR; Permeability; Water vapor transport
1. Introduction Common capacitive or resistive humidity sensors are only capable of measuring the humidity at 41% relative humidity (RH) and really only in the range of 20–99% RH to within 73% RH [1,2]. Measuring low humidity accurately is usually done using chilled mirror or surface acoustic wave (SAW) hygrometers. These chilled devices actually measure the dew point or the frost point, are quite accurate and generally do not need recalibration. To reach very low humidity requires devices with multistage thermoelectric coolers. A five-stage chilled mirror device can measure water vapor concentrations at Corresponding author.
E-mail address: [email protected] (W.F. Bauer). 0142-9418/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2006.04.001
less than 1 ppmv. A more typical four-stage SAW based device can measure to o2 ppmv and costs at least $15 k. It is also possible to measure water in gas streams to sub-ppmv levels with infrared (IR) spectroscopy [3,4]. For the measurement of compounds transported/permeating through a membrane into a gas phase, IR spectroscopy is desirable for some applications because many compounds may be measured at one time. So, to simply measure water permeability alone, IR spectroscopy is a reasonable alternative choice to the chilled devices mentioned above because there will be no interferences and, if there are, they can be resolved. An additional benefit is that infrared spectrometers are readily available in chemical laboratories. Because of the ready availability and the ability to match or exceed the lower detection capabilities of the chilled
ARTICLE IN PRESS W.F. Bauer et al. / Polymer Testing 25 (2006) 642–649
devices, IR spectroscopy was chosen to measure the transport and break through times of water through selected polymer membranes being developed at the Idaho National Laboratory (INL).
2. Materials and methods 2.1. Synthesis of polymers A series of phosphazene polymers were synthesized and then tested using this FTIR method. Structures of the ligands and polymers T2, T3, T4 and T7 are given in Table 1 and Fig. 1. All of these polymers were synthesized in a similar manner. The synthesis for polymer T2 follows. In a 2 L round bottom flask was installed a condenser, mechanical stirrer, thermometer and a nitrogen purge. To this flask was added Igepal CA-210s (82.2 g, 0.28 mol), 1 l of dry 1,4-dioxane and freshly cut sodium metal (6.15 g, 0.27 mol). The resulting mixture was stirred at reflux for approximately 48 h upon which the sodium was consumed. To this solution, poly(bischlorophosphazene) (14.0 g, 0.12 mol) was added as a solution in toluene (200 ml). The final mixture was stirred at 75 1C for 1.5 h at which it was determined to be complete using 31P NMR spectroscopy. Purification of the product was accomplished through division of the mother liquor into two portions. Each portion was poured into a mixture of 2-propanol (1875 ml) and water (625 ml) to precipitate the product as a swollen white solid. The product was then collected and dissolved into THF (1 l). The THF solution was divided into two portions and each portion was poured into 3 L of water. The product was collected and dried, followed by dissolution into THF (1 l). A final precipitation was performed in 3 L of methanol to obtain an amber colored rubber upon drying under vacuum. Table 2 gives the characterization data for the polymers.
2.2. Spectrometer IR spectra were acquired on a Bomem MB100 series Fourier transform infrared spectrometer (FTIR) equipped with a gas cell and deuterated triglycine sulfate (DTGS) detector. The gas cell has an optical path length of 20 cm, an internal volume of 250 ml and 4-mm-thick ZnSe windows. The interferometer is operated at 1 cm1 resolution and each spectrum typically consists of 20 co-added scans collected at a rate of 11 scan/min. The entire optical path of the spectrometer is purged with the same nitrogen used for the permeate purge stream to assure that there is no influence from atmospheric water. The spectrometer is controlled by a data station running Bomem Grams/AI (ABB Bomem and Galactic Industries). 3. Experimental apparatus and procedure Fig. 2 shows the basic configuration of the system to measure water transported through a membrane. Nitrogen supplied from a liquid nitrogen boil-off system is used to purge the spectrometer, as the reference gas and to provide the permeate sweep flow. A standard gas containing 100 ppmv of water in nitrogen (Scott Specialty Gases) is used to check the system. A constant flow of nitrogen or the standard gas is maintained with a mass flow controller (MKS Instruments, Inc.). A typical analysis consists of the following steps: (1) The first step is to purge the gas cell. V1 and V3 are in the closed positions, V2 is open and the three-way valve, V4, is turned to flow nitrogen through the cell. (2) The second step is to collect the background reference spectrum while still purging with nitrogen. After sufficient time to assure that the IR gas cell is dry, a background reference
Table 1 Ligand designations and chemical structures Material designation
Ligand trade name
T2
Igepal-CA210s
T3
Igepal-CO210s
T4
Brij-30s
T7
Brij-72s
643
Chemical structure of the ligands
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Fig. 1. General structures of the polymers.
Table 2 Characterization data for polymers T2, T3, T4 and T7 Polymer Polyether chain length (x)a Aliphatic chain (CmHm+1) (m)a T2 T3 T4 T7 a
1.52 1.63 4.89 2.36
8.31b 9.19 12.19 18.21
31
P NMR, s (ppm) Tg (1C)
6.3 6.7 7.3 7.2
11.0 11.0 45.0 (Tm5.0) 39.0 (Tm 22.0)
Mw (g/mol) Density (g/ml) 1.4 106 2.2 106 2.5 106 3.7 105
1.07 1.04 1.02 0.99
Refer to Fig. 1. Branched tert-octyl groups.
b
Fig. 2. Schematic of the FTIR-based water permeation system.
spectrum is collected. The time needed to purge the cell is variable and can be monitored with the FTIR if necessary. (3) The third step is to collect several ‘‘blank spectra’’; typically at least five to assure that the reference spectrum is good.
(4) The fourth step is to collect several absorbance spectra of the 100 ppmv water standard. Prior to actually flowing the standard through the system, it is advantageous to purge the line connecting the standard to the apparatus in order to reduce the equilibration time. This is
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done by simply opening V5 and V6 while V4 is still in the position to flow nitrogen. This should be done at least 20 min prior to actually flowing the standard to the gas cell. To actually flow the standard to the flow cell, V1 and V3 remain closed, V2 remains open, V6 is closed, V5 is open and V4 is turned to flow the 100 ppmv water standard. The actual time necessary for the 100 ppmv water standard to reach equilibrium in the gas cell will vary with the flowrate setting. At 90 ml/min, the equilibration time will be at least 10–15 min. Absorbance spectra are typically recorded at a rate of 1 every 2 min, which makes it easy to tell if the standard has equilibrated. (5) The fifth step is to purge the permeate side of the membrane with dry nitrogen. In order to acquire breakthrough data, the membrane should be as dry as possible. Therefore, prior to connecting it to the apparatus in Fig. 2, both side of the membrane are purged with dry air. Initially only the permeate side of the membrane cell is connected to the apparatus and that side of the membrane is then purged by setting V4 to flow nitrogen, closing V2 and opening V1 and V3. Simultaneously, the other side of the membrane can be purged with a separate air stream. (6) The last step is to flow water on the ‘‘feed’’ side of the membrane. ‘‘Blank’’ levels of water vapor in the permeate stream can rarely be achieved by purging with nitrogen because many of the membranes appear to retain some water that only slowly escapes the polymer. Therefore, when the rate of change in the permeate water vapor concentration appears to ‘‘level’’, the ‘‘feed’’ side of the membrane is connected to the water reservoir and pump and the flow initiated at 50 ml/min. Absorbance spectra are typically recorded at a rate of 1 every 2 min for 2–3 h. Longer or shorter monitoring periods can be used as appropriate. For simplicity and because the progress of the equilibration steps can be monitored easily, spectra are recorded by the software continuously into a single large file during steps 3–6. 4. Quantitative analysis Early quantitative analysis attempts utilized peak heights or areas of single peaks, areas of entire
645
spectral regions and spectra recorded at lower resolution. The precision, and thus the obtainable detection limits, for these procedures produced less than desirable results. These quantitative analyses were influenced by narrow natural line widths of water, which are less than the resolution of the instrument, and were easily influenced by even small variations in the shape of the spectral background. Spectral background shapes arise from many different sources ranging from slight changes of the operating temperature to the shape of the noise pattern in the background reference spectrum. To eliminate the influence of these various backgrounds, quantitative analysis is now performed utilizing a full spectrum method that essentially extracts the contributions of water from the sample spectrum. Partial least squares (PLS) [5] is a full spectrum method that can handle more than one source of variability in the spectra. PLS is a spectral decomposition method that identifies sources of variance that will influence the quantitation of the analyte of interest. Even if an unknown interference is encountered, PLS can develop a calibration that can accommodate this interference if it is possible to include it in the training set. As with all methods employing absorbance spectroscopy, once developed, a calibration generated by PLS from absorbance spectra should be universal and there should be no need to recalibrate. For the determination of water in the gas phase, stable standards are difficult to come by. The current PLS calibration is based upon 323 spectra of ‘‘blanks’’ and the 100 ppm standard collected as described above. These spectra were recorded with several different background reference spectra and spectra that were recorded at multiple time points after the background reference spectrum was recorded. As such, these cover a range of background spectral shapes caused by environmental factors and by the ratio of the run with its particular reference spectrum. Each spectrum is background corrected with a two-point baseline at 2053 and 1205 cm1. PLS ‘‘calibrations’’ were then generated for the spectral regions from 2053 to 1205 cm1 and from 1874 to 1412 cm1. Additional ‘‘calibrations’’ were generated using the first derivative spectra generated by the 5 point derivative smoothing algorithms described by Savitsky and Golay [6]. Calibrations generated employing the derivative spectra had one significant factor while those without the derivative had two significant
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W.F. Bauer et al. / Polymer Testing 25 (2006) 642–649
factors. Using any of the methods, the precision is such that detection limits o5 ppmv water are obtainable. 5. Discussion 5.1. Effect of permeate flowrate on determining breakthrough One of the early stated objectives was to have the ability to evaluate the water break through times for a particular membrane. The major drawback to the system as described above is the volume of the IR gas cell. With an internal volume of 250 ml, at least 3 min are required to fill the cell with the purge gas sample when flowing at 100 ml/min. Somewhat more is required to reach equilibrium. At high flowrates, this is not a major problem since data collection takes nearly 2 min anyway. Fig. 3 depicts
the influence of purge gas flowrate on the time necessary to reach equilibrium. The data were acquired from a 180-mm-thick T3 membrane previously equilibrated with water for several days. Only the flowrate of the permeate purge gas was changed. From the plots in Fig. 3, it can be readily noted that, even at 90 ml/min, 47.5 min are required to reach 490% of the equilibration value. Also notable in Fig. 3 is the S/N ratio. At the higher flowrates, the S/N is somewhat less because the concentration of water in the permeate stream is only in the range 200–300 ppmv. At 5 ml/min the water concentration is several thousand ppmv, however, 490% of equilibrium concentration is not reached within 3 h. Therefore, to assess breakthrough times, high permeate gas flowrates are required, however, sensitivity for determining low flux rates can be enhanced by operating at a lower permeate flowrate.
Fig. 3. Influence on purge gas flow rate on the response measured at the FTIR.
ARTICLE IN PRESS W.F. Bauer et al. / Polymer Testing 25 (2006) 642–649
5.2. Effect of permeate flowrate on determining flux The influence of flowrate on the final determined flux rate was also assessed to assure that the flowrate did not have an effect on the determined flux. Table 3 shows the water concentration in ppmv of the permeate stream and the associated calculated water flux for the membrane at equilibrium. Also shown are the fluxes for each flowrate calculated with a detection limit assumed to be 10 ppmv. Even using the flux values calculated at permeate flowrates of 5 and 10 ml/min where it was questionable if equilibrium had actually been reached, the agreement is quite good. The experiments were run in the order presented in Table 3 and close examination reveals that the higher concentrations at the lower flowrates appear to effectively passivate the system and the flux values calculated as the flowrates increase were slightly more consistent. 6. Sample analysis Fig. 4 shows what a typical sample run looks like using the methodology described above. In this
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particular experiment, the flowing water on the feed side of the membranes was supplied via a direct connection to the laboratory water supply. Fig. 5 shows the extension of this particular experiment when the feed water flow was shut off. There is a significant difference in the calculated water transport through the membrane and, thus, in the calculated flux values. To avoid any issues associated with the water feed pressure, it was decided to conduct all further testing using a pump with a flowrate arbitrarily set at 50 ml/min. Table 4 shows a complete listing of the testing performed for the four synthesized polymers plus some conventional polymers run as a system check. The Mocon Mylar is a support material utilized by others using the ASTM Method F1249 and a flux number utilizing that method is known. In ASTM Method F1249 the feed side of the membrane is a static water vapor saturated headspace. In this work, the measure of water permeability had flowing liquid water on the feed side of the membrane. The pressure differential caused by the flowing liquid could easily account for the slightly higher permeability measured by FTIR.
Table 3 Flowrates and fluxes Flowrate setting (mL/min)
Mean ppmv at Flux at setting Flux at 10 ppmv equilibrium (g/h m–2) line ¼ DL (mL/min) (g/h m–2)
100 80 60 40 20 10 5 10 20 30 40 50 70 90
232 275 355 539 1008 1994 3860 2187 1157 764 559 464 337 264
0.541 0.514 0.497 0.504 0.471 0.466 0.451 0.511 0.540 0.535 0.522 0.542 0.551 0.555
Mean all Stdev %RSD
0.514 0.033 6.5%
Mean drop most questionable Stdev %RSD
0.530 0.020 3.7%
0.023 0.018 0.014 0.009 0.005 0.002 0.001 0.002 0.005 0.007 0.009 0.011 0.016 0.021
Fig. 4. Results for a typicial run to measure water breakthrough and flux from a 180-mm-thick T3 membrane.
Fig. 5. Continuation of experiment in Fig. 4 showing the effects of flow rate.
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thickness variations between the individual membrane castings of the samples measured.
Table 4 Results of actual membrane testing Material
Thickness ppmv At (mm) equilibrium 22372
a
11.070.1
Feed water flow 50 mL/min
Mocon Mylar Mocon Mylar PDMS PDMS PDMS-a PDMS-a1 PDMS-a2 PDMS-b
23.4
8.9b
240 110 492 492 492 492
199279 185772 209375 192077
2139711 163475 2058710 191872 216276 198477
Static Static 50 mL/min Static 50 mL/min 50 mL/min
T2 T2 T2 T2 T2
80 78 42–96 100–143 94–135
27775 32073 41972 40276 41376
46.670.8 52.470.4 54.772 10371 99.470.7
50 mL/min 50 mL/min 50 mL/min 50 mL/min 50 mL/min
T3 T3
75–80 180
37773 24876
61.570.6 9472
T3 T3 T3 T3
180 30 75 75
12875 133327552 64273 656715
5472 840735 10171 10373
50 mL/min b50 mL/ min Static 50 mL/min 50 mL/min 50 mL/min
T4 T4 T4
11–12 30–52 26
1760271023 106957340 167537922
425725 921729 915750
50 mL/min 50 mL/min 50 mL/min
T7 T7 T7
35 35 35
2549732 2437723 2479718
18876 17972 18271
50 mL/min 50 mL/min 50 mL/min
a
23.4
g mm/h m–2
Measured via FTIR. Vapor per ASTM F1249.
b
Results from repeats experiments using the same and different membranes appear to be quite repeatable. For example see the results for the 75 mm T3 and the 492 mm poly(dimethyl siloxane) (PDMS) membranes. Given the variability of the various T3 membrane castings, the results are quite reproducible and hen repeating the test using the same T3 membrane (i.e. the 75 mm thick membrane), the results are identical. The PDMS membranes were cut from the same material with very reproducible results. The Mocon Mylar was the control. The reference value is slightly less than what was measured herein due to the fact that the current film had flowing water on the feed side as opposed to a saturated vapor in the method used to determine the reference values. The PDMS values are similar to those reported in the literature and were used to further validate the system. Variability in the flux determinations is primarily the result of
7. Conclusions A method for measuring the water transport through polymeric membranes utilizing an FTIR flow through gas cell is described. Experimentally the method is easy to apply and very accurate. The 20 cm cell used for this work gave detection limits of 5 ppmv. Replacing this cell with a 1 m cell could easily be done with a concomitant decrease in detection limits. Variations in the flux rates due to changes in the method of exposing the membrane to the water (either flowing or static) could be clearly differentiated. The only draw back to this FTIR flow through cell based system is that it takes a couple of minutes for the gas cell volume to come to equilibrium. Thus, it is difficult to determine break through times for highly water permeable membranes because the break though times are less than the time to establish equilibrium within the test cell. The fact that many laboratories are equipped with FTIR systems means that this method can be easily implemented. Unlike other detection methods [7], this same system could be calibrated and used to measure permeates other than water and for mixed or multiple permeates all measured at the same time. Acknowledgment The submitted manuscript has been authored by a contractor of the U.S. Government under DOE Contract DE-AC07-05ID14517. Accordingly, the U.S. Government retains a nonexclusive, royaltyfree license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes. References [1] B.M. Kulwicki, Humidity sensors, J. Am. Ceram. Soc. 74 (4) (1991) 697–708 (69 references). [2] G.J.W. Visscher, J.G. Kornet, Long-term tests of capacitive humidity sensors, Meas. Sci. Technol. 5 (1994) 1294–1302. [3] F 372-99, Standard Test Method for Water Vapor Transmission Rate Through Plastic Film and Sheeting Using a Modulated Infrared Sensor, American Society for Testing and Materials, 2001. [4] F 1249-01, Standard Test Method for Water Vapor Transmission Rate of Flexible Barrier Materials Using and Infrared Detection Technique, American Society for Testing and Materials, 1999.
ARTICLE IN PRESS W.F. Bauer et al. / Polymer Testing 25 (2006) 642–649 [5] D.M. Haaland, E.V. Thomas, Partial least-squares methods for spectral analyses. 1. Relation to other quantitative calibration methods and the extraction of qualitative information, E. V. T., Anal. Chem. 60 (1988) 1193–1201. [6] A. Savitzky, M.J.E. Golay, Smoothing and differentiation of data by simplified least squares procedures, Anal. Chem. 36 (1964) 1627.
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[7] M. Metayer, M. Labbe, S. Marais, D. Langevin, C. Chappey, F. Dreux, M. Brainville, P. Belliard, Diffusion of water through various polymer films: a new high performance method of characterization, Polym. Test. 18 (1999) 533–549.
POLYMER TESTING Polymer Testing 25 (2006) 642–649 www.elsevier.com/locate/polytest
Test Method
Infrared spectroscopic measurement of water permeability in polymer films exposed to liquid water William F. Bauer, Mark L. Stone, Christopher J. Orme, Mason K. Harrup, Thomas A. Luther Idaho National Laboratory, Idaho Falls, ID 83415-2208, USA Received 25 February 2006; accepted 1 April 2006
Abstract The details of a method for measuring the water vapor transport rate through polymer films utilizing Fourier transform infrared spectroscopy (FTIR) are given. Water flows across one side of the membrane and a nitrogen sweep gas carries the water vapor into the flow-through cell of the spectrometer for measurement. The method was used to test two polymers that have literature values and four new polymers. The resulting FTIR data was analyzed with a partial least-squares method. Experimentally the method is easy to apply and very accurate. r 2006 Elsevier Ltd. All rights reserved. Keywords: FTIR; Permeability; Water vapor transport
1. Introduction Common capacitive or resistive humidity sensors are only capable of measuring the humidity at 41% relative humidity (RH) and really only in the range of 20–99% RH to within 73% RH [1,2]. Measuring low humidity accurately is usually done using chilled mirror or surface acoustic wave (SAW) hygrometers. These chilled devices actually measure the dew point or the frost point, are quite accurate and generally do not need recalibration. To reach very low humidity requires devices with multistage thermoelectric coolers. A five-stage chilled mirror device can measure water vapor concentrations at Corresponding author.
E-mail address: [email protected] (W.F. Bauer). 0142-9418/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2006.04.001
less than 1 ppmv. A more typical four-stage SAW based device can measure to o2 ppmv and costs at least $15 k. It is also possible to measure water in gas streams to sub-ppmv levels with infrared (IR) spectroscopy [3,4]. For the measurement of compounds transported/permeating through a membrane into a gas phase, IR spectroscopy is desirable for some applications because many compounds may be measured at one time. So, to simply measure water permeability alone, IR spectroscopy is a reasonable alternative choice to the chilled devices mentioned above because there will be no interferences and, if there are, they can be resolved. An additional benefit is that infrared spectrometers are readily available in chemical laboratories. Because of the ready availability and the ability to match or exceed the lower detection capabilities of the chilled
ARTICLE IN PRESS W.F. Bauer et al. / Polymer Testing 25 (2006) 642–649
devices, IR spectroscopy was chosen to measure the transport and break through times of water through selected polymer membranes being developed at the Idaho National Laboratory (INL).
2. Materials and methods 2.1. Synthesis of polymers A series of phosphazene polymers were synthesized and then tested using this FTIR method. Structures of the ligands and polymers T2, T3, T4 and T7 are given in Table 1 and Fig. 1. All of these polymers were synthesized in a similar manner. The synthesis for polymer T2 follows. In a 2 L round bottom flask was installed a condenser, mechanical stirrer, thermometer and a nitrogen purge. To this flask was added Igepal CA-210s (82.2 g, 0.28 mol), 1 l of dry 1,4-dioxane and freshly cut sodium metal (6.15 g, 0.27 mol). The resulting mixture was stirred at reflux for approximately 48 h upon which the sodium was consumed. To this solution, poly(bischlorophosphazene) (14.0 g, 0.12 mol) was added as a solution in toluene (200 ml). The final mixture was stirred at 75 1C for 1.5 h at which it was determined to be complete using 31P NMR spectroscopy. Purification of the product was accomplished through division of the mother liquor into two portions. Each portion was poured into a mixture of 2-propanol (1875 ml) and water (625 ml) to precipitate the product as a swollen white solid. The product was then collected and dissolved into THF (1 l). The THF solution was divided into two portions and each portion was poured into 3 L of water. The product was collected and dried, followed by dissolution into THF (1 l). A final precipitation was performed in 3 L of methanol to obtain an amber colored rubber upon drying under vacuum. Table 2 gives the characterization data for the polymers.
2.2. Spectrometer IR spectra were acquired on a Bomem MB100 series Fourier transform infrared spectrometer (FTIR) equipped with a gas cell and deuterated triglycine sulfate (DTGS) detector. The gas cell has an optical path length of 20 cm, an internal volume of 250 ml and 4-mm-thick ZnSe windows. The interferometer is operated at 1 cm1 resolution and each spectrum typically consists of 20 co-added scans collected at a rate of 11 scan/min. The entire optical path of the spectrometer is purged with the same nitrogen used for the permeate purge stream to assure that there is no influence from atmospheric water. The spectrometer is controlled by a data station running Bomem Grams/AI (ABB Bomem and Galactic Industries). 3. Experimental apparatus and procedure Fig. 2 shows the basic configuration of the system to measure water transported through a membrane. Nitrogen supplied from a liquid nitrogen boil-off system is used to purge the spectrometer, as the reference gas and to provide the permeate sweep flow. A standard gas containing 100 ppmv of water in nitrogen (Scott Specialty Gases) is used to check the system. A constant flow of nitrogen or the standard gas is maintained with a mass flow controller (MKS Instruments, Inc.). A typical analysis consists of the following steps: (1) The first step is to purge the gas cell. V1 and V3 are in the closed positions, V2 is open and the three-way valve, V4, is turned to flow nitrogen through the cell. (2) The second step is to collect the background reference spectrum while still purging with nitrogen. After sufficient time to assure that the IR gas cell is dry, a background reference
Table 1 Ligand designations and chemical structures Material designation
Ligand trade name
T2
Igepal-CA210s
T3
Igepal-CO210s
T4
Brij-30s
T7
Brij-72s
643
Chemical structure of the ligands
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644
Fig. 1. General structures of the polymers.
Table 2 Characterization data for polymers T2, T3, T4 and T7 Polymer Polyether chain length (x)a Aliphatic chain (CmHm+1) (m)a T2 T3 T4 T7 a
1.52 1.63 4.89 2.36
8.31b 9.19 12.19 18.21
31
P NMR, s (ppm) Tg (1C)
6.3 6.7 7.3 7.2
11.0 11.0 45.0 (Tm5.0) 39.0 (Tm 22.0)
Mw (g/mol) Density (g/ml) 1.4 106 2.2 106 2.5 106 3.7 105
1.07 1.04 1.02 0.99
Refer to Fig. 1. Branched tert-octyl groups.
b
Fig. 2. Schematic of the FTIR-based water permeation system.
spectrum is collected. The time needed to purge the cell is variable and can be monitored with the FTIR if necessary. (3) The third step is to collect several ‘‘blank spectra’’; typically at least five to assure that the reference spectrum is good.
(4) The fourth step is to collect several absorbance spectra of the 100 ppmv water standard. Prior to actually flowing the standard through the system, it is advantageous to purge the line connecting the standard to the apparatus in order to reduce the equilibration time. This is
ARTICLE IN PRESS W.F. Bauer et al. / Polymer Testing 25 (2006) 642–649
done by simply opening V5 and V6 while V4 is still in the position to flow nitrogen. This should be done at least 20 min prior to actually flowing the standard to the gas cell. To actually flow the standard to the flow cell, V1 and V3 remain closed, V2 remains open, V6 is closed, V5 is open and V4 is turned to flow the 100 ppmv water standard. The actual time necessary for the 100 ppmv water standard to reach equilibrium in the gas cell will vary with the flowrate setting. At 90 ml/min, the equilibration time will be at least 10–15 min. Absorbance spectra are typically recorded at a rate of 1 every 2 min, which makes it easy to tell if the standard has equilibrated. (5) The fifth step is to purge the permeate side of the membrane with dry nitrogen. In order to acquire breakthrough data, the membrane should be as dry as possible. Therefore, prior to connecting it to the apparatus in Fig. 2, both side of the membrane are purged with dry air. Initially only the permeate side of the membrane cell is connected to the apparatus and that side of the membrane is then purged by setting V4 to flow nitrogen, closing V2 and opening V1 and V3. Simultaneously, the other side of the membrane can be purged with a separate air stream. (6) The last step is to flow water on the ‘‘feed’’ side of the membrane. ‘‘Blank’’ levels of water vapor in the permeate stream can rarely be achieved by purging with nitrogen because many of the membranes appear to retain some water that only slowly escapes the polymer. Therefore, when the rate of change in the permeate water vapor concentration appears to ‘‘level’’, the ‘‘feed’’ side of the membrane is connected to the water reservoir and pump and the flow initiated at 50 ml/min. Absorbance spectra are typically recorded at a rate of 1 every 2 min for 2–3 h. Longer or shorter monitoring periods can be used as appropriate. For simplicity and because the progress of the equilibration steps can be monitored easily, spectra are recorded by the software continuously into a single large file during steps 3–6. 4. Quantitative analysis Early quantitative analysis attempts utilized peak heights or areas of single peaks, areas of entire
645
spectral regions and spectra recorded at lower resolution. The precision, and thus the obtainable detection limits, for these procedures produced less than desirable results. These quantitative analyses were influenced by narrow natural line widths of water, which are less than the resolution of the instrument, and were easily influenced by even small variations in the shape of the spectral background. Spectral background shapes arise from many different sources ranging from slight changes of the operating temperature to the shape of the noise pattern in the background reference spectrum. To eliminate the influence of these various backgrounds, quantitative analysis is now performed utilizing a full spectrum method that essentially extracts the contributions of water from the sample spectrum. Partial least squares (PLS) [5] is a full spectrum method that can handle more than one source of variability in the spectra. PLS is a spectral decomposition method that identifies sources of variance that will influence the quantitation of the analyte of interest. Even if an unknown interference is encountered, PLS can develop a calibration that can accommodate this interference if it is possible to include it in the training set. As with all methods employing absorbance spectroscopy, once developed, a calibration generated by PLS from absorbance spectra should be universal and there should be no need to recalibrate. For the determination of water in the gas phase, stable standards are difficult to come by. The current PLS calibration is based upon 323 spectra of ‘‘blanks’’ and the 100 ppm standard collected as described above. These spectra were recorded with several different background reference spectra and spectra that were recorded at multiple time points after the background reference spectrum was recorded. As such, these cover a range of background spectral shapes caused by environmental factors and by the ratio of the run with its particular reference spectrum. Each spectrum is background corrected with a two-point baseline at 2053 and 1205 cm1. PLS ‘‘calibrations’’ were then generated for the spectral regions from 2053 to 1205 cm1 and from 1874 to 1412 cm1. Additional ‘‘calibrations’’ were generated using the first derivative spectra generated by the 5 point derivative smoothing algorithms described by Savitsky and Golay [6]. Calibrations generated employing the derivative spectra had one significant factor while those without the derivative had two significant
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factors. Using any of the methods, the precision is such that detection limits o5 ppmv water are obtainable. 5. Discussion 5.1. Effect of permeate flowrate on determining breakthrough One of the early stated objectives was to have the ability to evaluate the water break through times for a particular membrane. The major drawback to the system as described above is the volume of the IR gas cell. With an internal volume of 250 ml, at least 3 min are required to fill the cell with the purge gas sample when flowing at 100 ml/min. Somewhat more is required to reach equilibrium. At high flowrates, this is not a major problem since data collection takes nearly 2 min anyway. Fig. 3 depicts
the influence of purge gas flowrate on the time necessary to reach equilibrium. The data were acquired from a 180-mm-thick T3 membrane previously equilibrated with water for several days. Only the flowrate of the permeate purge gas was changed. From the plots in Fig. 3, it can be readily noted that, even at 90 ml/min, 47.5 min are required to reach 490% of the equilibration value. Also notable in Fig. 3 is the S/N ratio. At the higher flowrates, the S/N is somewhat less because the concentration of water in the permeate stream is only in the range 200–300 ppmv. At 5 ml/min the water concentration is several thousand ppmv, however, 490% of equilibrium concentration is not reached within 3 h. Therefore, to assess breakthrough times, high permeate gas flowrates are required, however, sensitivity for determining low flux rates can be enhanced by operating at a lower permeate flowrate.
Fig. 3. Influence on purge gas flow rate on the response measured at the FTIR.
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5.2. Effect of permeate flowrate on determining flux The influence of flowrate on the final determined flux rate was also assessed to assure that the flowrate did not have an effect on the determined flux. Table 3 shows the water concentration in ppmv of the permeate stream and the associated calculated water flux for the membrane at equilibrium. Also shown are the fluxes for each flowrate calculated with a detection limit assumed to be 10 ppmv. Even using the flux values calculated at permeate flowrates of 5 and 10 ml/min where it was questionable if equilibrium had actually been reached, the agreement is quite good. The experiments were run in the order presented in Table 3 and close examination reveals that the higher concentrations at the lower flowrates appear to effectively passivate the system and the flux values calculated as the flowrates increase were slightly more consistent. 6. Sample analysis Fig. 4 shows what a typical sample run looks like using the methodology described above. In this
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particular experiment, the flowing water on the feed side of the membranes was supplied via a direct connection to the laboratory water supply. Fig. 5 shows the extension of this particular experiment when the feed water flow was shut off. There is a significant difference in the calculated water transport through the membrane and, thus, in the calculated flux values. To avoid any issues associated with the water feed pressure, it was decided to conduct all further testing using a pump with a flowrate arbitrarily set at 50 ml/min. Table 4 shows a complete listing of the testing performed for the four synthesized polymers plus some conventional polymers run as a system check. The Mocon Mylar is a support material utilized by others using the ASTM Method F1249 and a flux number utilizing that method is known. In ASTM Method F1249 the feed side of the membrane is a static water vapor saturated headspace. In this work, the measure of water permeability had flowing liquid water on the feed side of the membrane. The pressure differential caused by the flowing liquid could easily account for the slightly higher permeability measured by FTIR.
Table 3 Flowrates and fluxes Flowrate setting (mL/min)
Mean ppmv at Flux at setting Flux at 10 ppmv equilibrium (g/h m–2) line ¼ DL (mL/min) (g/h m–2)
100 80 60 40 20 10 5 10 20 30 40 50 70 90
232 275 355 539 1008 1994 3860 2187 1157 764 559 464 337 264
0.541 0.514 0.497 0.504 0.471 0.466 0.451 0.511 0.540 0.535 0.522 0.542 0.551 0.555
Mean all Stdev %RSD
0.514 0.033 6.5%
Mean drop most questionable Stdev %RSD
0.530 0.020 3.7%
0.023 0.018 0.014 0.009 0.005 0.002 0.001 0.002 0.005 0.007 0.009 0.011 0.016 0.021
Fig. 4. Results for a typicial run to measure water breakthrough and flux from a 180-mm-thick T3 membrane.
Fig. 5. Continuation of experiment in Fig. 4 showing the effects of flow rate.
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thickness variations between the individual membrane castings of the samples measured.
Table 4 Results of actual membrane testing Material
Thickness ppmv At (mm) equilibrium 22372
a
11.070.1
Feed water flow 50 mL/min
Mocon Mylar Mocon Mylar PDMS PDMS PDMS-a PDMS-a1 PDMS-a2 PDMS-b
23.4
8.9b
240 110 492 492 492 492
199279 185772 209375 192077
2139711 163475 2058710 191872 216276 198477
Static Static 50 mL/min Static 50 mL/min 50 mL/min
T2 T2 T2 T2 T2
80 78 42–96 100–143 94–135
27775 32073 41972 40276 41376
46.670.8 52.470.4 54.772 10371 99.470.7
50 mL/min 50 mL/min 50 mL/min 50 mL/min 50 mL/min
T3 T3
75–80 180
37773 24876
61.570.6 9472
T3 T3 T3 T3
180 30 75 75
12875 133327552 64273 656715
5472 840735 10171 10373
50 mL/min b50 mL/ min Static 50 mL/min 50 mL/min 50 mL/min
T4 T4 T4
11–12 30–52 26
1760271023 106957340 167537922
425725 921729 915750
50 mL/min 50 mL/min 50 mL/min
T7 T7 T7
35 35 35
2549732 2437723 2479718
18876 17972 18271
50 mL/min 50 mL/min 50 mL/min
a
23.4
g mm/h m–2
Measured via FTIR. Vapor per ASTM F1249.
b
Results from repeats experiments using the same and different membranes appear to be quite repeatable. For example see the results for the 75 mm T3 and the 492 mm poly(dimethyl siloxane) (PDMS) membranes. Given the variability of the various T3 membrane castings, the results are quite reproducible and hen repeating the test using the same T3 membrane (i.e. the 75 mm thick membrane), the results are identical. The PDMS membranes were cut from the same material with very reproducible results. The Mocon Mylar was the control. The reference value is slightly less than what was measured herein due to the fact that the current film had flowing water on the feed side as opposed to a saturated vapor in the method used to determine the reference values. The PDMS values are similar to those reported in the literature and were used to further validate the system. Variability in the flux determinations is primarily the result of
7. Conclusions A method for measuring the water transport through polymeric membranes utilizing an FTIR flow through gas cell is described. Experimentally the method is easy to apply and very accurate. The 20 cm cell used for this work gave detection limits of 5 ppmv. Replacing this cell with a 1 m cell could easily be done with a concomitant decrease in detection limits. Variations in the flux rates due to changes in the method of exposing the membrane to the water (either flowing or static) could be clearly differentiated. The only draw back to this FTIR flow through cell based system is that it takes a couple of minutes for the gas cell volume to come to equilibrium. Thus, it is difficult to determine break through times for highly water permeable membranes because the break though times are less than the time to establish equilibrium within the test cell. The fact that many laboratories are equipped with FTIR systems means that this method can be easily implemented. Unlike other detection methods [7], this same system could be calibrated and used to measure permeates other than water and for mixed or multiple permeates all measured at the same time. Acknowledgment The submitted manuscript has been authored by a contractor of the U.S. Government under DOE Contract DE-AC07-05ID14517. Accordingly, the U.S. Government retains a nonexclusive, royaltyfree license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes. References [1] B.M. Kulwicki, Humidity sensors, J. Am. Ceram. Soc. 74 (4) (1991) 697–708 (69 references). [2] G.J.W. Visscher, J.G. Kornet, Long-term tests of capacitive humidity sensors, Meas. Sci. Technol. 5 (1994) 1294–1302. [3] F 372-99, Standard Test Method for Water Vapor Transmission Rate Through Plastic Film and Sheeting Using a Modulated Infrared Sensor, American Society for Testing and Materials, 2001. [4] F 1249-01, Standard Test Method for Water Vapor Transmission Rate of Flexible Barrier Materials Using and Infrared Detection Technique, American Society for Testing and Materials, 1999.
ARTICLE IN PRESS W.F. Bauer et al. / Polymer Testing 25 (2006) 642–649 [5] D.M. Haaland, E.V. Thomas, Partial least-squares methods for spectral analyses. 1. Relation to other quantitative calibration methods and the extraction of qualitative information, E. V. T., Anal. Chem. 60 (1988) 1193–1201. [6] A. Savitzky, M.J.E. Golay, Smoothing and differentiation of data by simplified least squares procedures, Anal. Chem. 36 (1964) 1627.
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[7] M. Metayer, M. Labbe, S. Marais, D. Langevin, C. Chappey, F. Dreux, M. Brainville, P. Belliard, Diffusion of water through various polymer films: a new high performance method of characterization, Polym. Test. 18 (1999) 533–549.