Advanced Materials for Membrane Separations - American Chemical

Chapter 20

Effect of Microwave Irradiation on C0 Permeability in Cellulose Acetate Membranes Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: April 20, 2004 | doi: 10.1021/bk-2004-0876.ch020

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Y. Tsujita, Y. Nakai, H . Yoshimizu, and M . Yamauchi Polymeric Materials Course, Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan

2.45 G H z microwaves are widely used for heating and drying materials. Water molecules i n foods are heated up spontaneously by microwave radiation. This concept is applied to polymers containing O H groups to generate locally enhanced molecular motion of O H groups. Locally enhanced molecular motion might increase gas permeation and gas diffusion due to an increase of free volume accompanying the enhanced molecular motion. To investigate the ability to control gas permeation using microwave radiation, CO permeation and diffusion coefficients in cellulose acetate were measured under constant microwave irradiation at 2.45GHz. Both permeability and diffusivity increased with increasing microwave power, up to 500W. In contrast, the solubility coefficient decreased as microwave power increased. 2

Separation of low molecular weight substances using chemical engineering process such as distillation can require large amounts of energy. Much attention has focused on gas separation membranes as a low energy separation technology (7, 2). Polymers are widely used in such applications because they can be fabricated in large surface areas as very thin, defect free membranes. The effect of polymer chemical structure on gas separation properties has been studied for a long time (J, 2). Recently, many polymeric membranes have been developed that contain high levels of free volume (3-14). Typical examples include poly(l-

300

© 2004 American Chemical Society

In Advanced Materials for Membrane Separations; Pinnau, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: April 20, 2004 | doi: 10.1021/bk-2004-0876.ch020

301 trimethylsilyl- 1-propyne) and perfluorodioxole copolymers, which contain fractional free volumes of 0.343 and 0.30-0.33, respectively. These polymeric membranes are especially highly gas permeable, but they exhibit low permselectivity for the separation o f permanent gases such as oxygen and nitrogen. However, not only high permeation rates but also high selectivity is required for gas separation membranes. On the other hand, physical modifications o f membranes have been explored as a route to enhance permeation properties. Physical modifications such as thermal quenching and pressure conditioning from the liquid state were tried for glassy polymers (15-17). Such processes effectively alter the physical rather than the chemical structure of the polymer. That is, they change the microvoid content, fractional free volume, and unrelaxed volume of glassy polymers, and variations in these properties can influence gas permeation performance. We have studied the gas permeability of electro-conductive polypyrole membranes in the presence of external fields such as electric fields. Application of a direct electric field in the direction of the membrane thickness could change the O 2 permeability coefficient, and the amount of change in permeability depended on the amount of electric power applied to the membrane. Due to electroconductivity, the O2 permeability coefficient was enhanced by applying a direct electric field to the membrane i f the O 2 permeation and hole mobility were in the same direction. On the other hand, the O2 permeability coefficient was reduced by reversing the direction of the direct electric field. It is probable that the interaction between the holes in the polypyrole membrane and 62 gas was of a radical nature. 2.45 G H z microwave radiation has been widely used for heating and drying materials such as rubbers, foods, and prints (18). In particular, it has been applied to vulcanization of rubbers, foam processing, polymerizationsolidification, adhesion, and so on. Water or water vapor in foods is heated spontaneously upon irradiation by 2.45 G H z microwaves. This concept is applied to polymers, such as cellulose and cellulose acetate, which contain O H groups. In principle, the microwave radiation can induce locally enhanced molecular motion of the O H groups attached to the polymer backbone. It is expected that the locally enhanced molecular motion might increase gas permeation and gas diffusion by increasing the free volume.

Experimental Samples A cellulose film 35.5 pm thick was kindly provided by Prof. E . Nakanishi. It was prepared by casting from a solution of L i C l and dimethylacetamide (DMAc).


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Cellulose acetate was purchased from Aldrich Co. The molecular weight was 30,000, and the acetyl content was 39.8 wt% (degree of substitute 2.4). Tetrahydrofuran solutions of cellulose acetate were cast onto Petri dishes at room temperature and dried thoroughly at 30 °C for 24hrs or more and then annealed at 190 °C under vacuum for 6hrs. Transparent cellulose acetate films were obtained. Their thickness was about 45 ^im, and their glass transition temperature was 180 °C.

Methods Dielectric Dispersion Dielectric dispersion curves were obtained at frequencies from 100Hz to 100kHz. Temperature was varied from -100 °C to 100 °C. The apparatus used was a dynamic dielectric spectrometer manufactured by Seiko Instrument (DES -100).

Permeability coefficient A gas permeation apparatus, which could operate in the presence o f microwave irradiation, was constructed from a conventional permeation apparatus and a microwave generator (see Figure 1). Gas permeation experiments were conducted under constant microwave irradiation at 2.45GHz. In a typical experiment, a flat membrane having an area of 1.54 cm was set into the permeation cell. The total system, including the membrane, was evacuated under vacuum for 24 hours. The upstream pressure was fixed at 76 cmHg o f CO2. The downstream (i.e., low) pressure was recorded as a function of time while the membrane was exposed to constant microwave irradiation. The permeability coefficient was evaluated from the slope of the permeation curve after reaching steady state. We have taken care to homogeneously irradiate the polymeric membrane. To this end, a so-called "dummy road" (see Figure 1) was installed to prevent reflected radiation from reaching the membrane. 2

Diffusion coefficient Diffusion coefficients were evaluated using the classic time lag method. After thorough drying, the polymeric membrane was subjected to constant irradiation. C 0 at 76 cmHg was introduced to the upstream side of the membrane, and the time lag was determined after reaching steady state. The 2

average diffusion coefficient (D) was calculated from the time lag ( 9) obtained using e q ( l ) :



303

High Pressure

Low Pressure

Transducer

Transducer

Figure 1. Apparatus for measuring gas permeability in the presence of microwave irradiation.


304 2

D=L /60

(1)

where L is the membrane thickness.

Results and Discussion


Dielectric Dispersion Figure 2 presents the dielectric dispersion transition map of cellulose acetate in the form of an Arrhenius plot. According to this transition map, the dielectric loss peak at 2.45 G H z (i.e., the microwave frequency) is expected to appear at around 170 °C. However, die dielectric dispersion is quite broad, and it is likely that even at 25°C, some molecular motion is excited by exposing the sample to 2.45 G H z microwave radiation, and this molecular motion is probably related to the effect of microwave irradiation on the diffusion and permeability coefficients.

Permeation C 0 permeation curves of cellulose acetate membranes being irradiated with microwaves at power levels of 100, 300, and 500W are shown in Figure 3. One can observe enhancement of C Q 2 permeation during microwave irradiation, indicative of a remarkable instantaneous response of permeation to microwave irradiation. The permeability enhancement increases with power, suggesting that one might control permeability by varying microwave power. The permeation curve reduced rapidly to the original permeation curve when the microwave radiation was stopped. These changes in permeation rates upon microwave irradiation were not observed in a polystyrene film, presumably because it does not contain any highly polar, strongly microwave responsive groups. Figure 4 presents the effect of microwave power level on average C O 2 gas permeability coefficient in a C A membrane at 76 cmHg and 25 °C. The permeability coefficient changes parabolically with microwave power, indicating a spontaneous response of local molecular motion of groups such as O H groups to the microwave radiation. The increased molecular motion resulting from microwave irradiation enhanced gas permeability coefficients. A s shown in Figure 5, the C O 2 permeability o f cellulose increased by approximately one order of magnitude as microwave power increased from 0 to 500 W . Cellulose, which contains 3 0 H groups per repeat unit, exhibited a much stronger effect of microwave irradiation on permeability than cellulose acetate (see Figure 4). These results suggest that molecular motion of O H groups in the 2



305

1000/T (K* ) 1

Figure 2. Dielectric dispersion transition map of cellulose acetate.

Figure 3. CO2 gas permeation curve of cellulose acetate during microwave irradiation at 76cmHgand 25°C; (a) 100W, (b) 300W, and (c) 500W.



306

0

100

200

300

400

SOO

600

Microwave power (watt)

Figure 4. The effect of microwave power on average CO2 permeability coefficient in a cellulose acetate membrane at 76cmHg and 25°C.

0

100

200

300

400

SOO

600

Microwave power (watt)

Figure 5. The effect of microwave power on average CO2 permeability coefficient in a cellulose membrane at 76cmHg and 25°C.


307 polymer repeat unit can be activated by microwave irradiation, and the molecular motion of these groups can enhance gas permeation and diffusion.

Diffusion and Solubility Diffusion coefficients during microwave irradiation were obtained by the time lag method. Figure 6 presents the effect of microwave power on average C 0 gas diffusion coefficients in C A membranes at latm and 25 °C. Relative to the permeation data presented in Figure 4, there is a much stronger effect of microwave power on diffusivity. Consistent with the results presented in Figure 2, this increase in diffusivity is basically related to the enhanced local molecular motion of the O H groups in cellulose acetate during microwave irradiation. One can calculate the effect of microwave irradiation on solubility coefficients (S) by using eq (2) and the permeability and diffusion coefficients obtained with and without microwave irradiation. The permeability coefficient is determined by the product of the diffusion coefficient and the solubility coefficient according to the solution-diffusion mechanism:


2

P=DS

(2)

Solubility was reduced when the membrane was subjected to microwave irradiation. This might be explainable in terms of enhanced local molecular motion during microwave irradiation. This observation corresponds to lower gas solubility in the polymer which is in a state of more activated molecular motion, e.g., higher temperature. However, as shown below, the temperature increase in the polymer due to microwave irradiation is very small. We are interested in the effect of microwave irradiation on permeability, diffusivity, and solubility. For a microwave radiation power of 500W, diffusivity increases by about 40% and solubility decreases by about 7%, which leads to an increase in permeability o f about 30%. Assuming that the temperature dependence of solubility obeys an Arrhenius relation and using the reported heat of solution of CCfc in a C A membrane (-6.5 kcal/mol (19)), a 7% increase in solubility corresponds to a temperature increase of only about 2 Kelvin. During microwave irradiation, resistance thermometer sensors generally indicate temperatures higher than the real temperature because the microwave electric field concentrates at the tip of the resistance thermometer sensor. Nevertheless, we measured the surface temperature of C A and Cellulose membranes using a metal-sheathed, mineral insulated platinum resistance thermometer sensor. For irradiation of 500W, the temperature increase of both membranes was about 5 Kelvin. Therefore, there is no remarkable increase in temperature due to exposing the polymer membranes to microwave radiation.



308

Figure 6. The effect of microwave power on average CO2 diffusion coefficient in a cellulose acetate membrane at 76cmHg and 25°C.


309


Conclusion 2.45 G H z Microwave irradiation of polymer membranes containing strongly microwave responsive functional groups, such as O H groups, activates local molecular motion of these functional groups and enhances both gas permeability and diffusivity. The effect of microwave irradiation on transport properties is greater for cellulose than for cellulose acetate. There is little effect of microwave irradiation on polystyrene (which has no strongly microwaveresponsive functional groups). Permeability and diffusion coefficients o f cellulose acetate increased with microwave power up to 500W. In contrast, solubility decreased with increasing microwave power. More research is needed to better understand the effect of microwave radiation on gas permeation and separation properties. Further studies of the frequency dependence o f microwave radiation on transport properties are planned. Additionally, the influence o f microwave radiation on polymers containing functional groups other than O H groups is not understood. Mixture gas separation properties of cellulose acetate membranes during microwave irradiation are expected to be different from those not subject to microwave irradiation, but this needs to be demonstrated.

Acknowledgement We greatly acknowledge active discussions with Dr. M . Miyamoto, Central Research Laboratory, Nissan Chemical Co. Japan. Partial financial support was provided by a Grant-in-Aid for Scientific Research in Priority Area (B), "Novel Smart Membrane with Controlled Molecular Cavity", No. 13133202 (2001) from the Ministry of Education, Culture, Sports, Science and Technology and for Scientific Research (B), No. 11695047 (1999) from the Japan Society for the Promotion of Science.

References 1. 2. 3. 4. 5.

Kesting, R. E . Synthetic Polymeric Membranes; A Structural Perspective Second Edition; Wiley-Interscience: New York, 1985. Osada, Y.; Nakagawa, T., Eds.; Membrane Science and Technology; Marcel Dekker, Inc.: New York, 1992. Morisato, A.; Pinnau, I. J. Membrane Sci., 1996, 121, 229-242. Morisato, A.; Freeman, B . D.; Pinnau, I.; Casillas, C. G . J. Polymer Sci., 1996, 34, 1925-1934. Pinnau, I.; Casillas, C. G.; Morisato, A.; Freeman, B . D . J. Polymer Sci., 1996, 34, 2613-2621.


310 6. 7. 8. 9.


10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

Pinnau, I.; Casillas, C. G.; Morisato, A.; Freeman, B . D . J. Polymer Sci., 1997, 35, 1483-1490. Merkel, T. C.; Bonder, V.; Nagai, K.; Freeman, B . D . Macromolecules, 1999, 32, 370-374. Toy, L . G.; Freeman, B . D.; Spontak, R. J.; Morisato, A.; Pinnau, I. Macromolecules, 1997, 30, 4766-4769. Dixon-Garrett, S. V.; Nagai, K.; Freeman, B . D . J. Polymer Sci., 2000, 38, 1078-1089. Nagai, K.; Toy, L . G.; Freeman, B . D.; Teraguehi, M.; Masuda, T.; Pinnau, I.J. Polymer Sci., 2000, 38, 1474-1484. Bondar, V . I.; Freeman, B . D.; Yampolskii, Y u . P. Macromolecules, 1999, 32, 6163-6171. Morisato, A.; Shen, H . C.; Sankar, S. S.; Freeman, B . D.; Pinnau, I.; Casillas, C. G. J. Polymer Sci., 1996, 34, 2209-2222. Nagai, K.; Mori, M.; Watanabe, T.; Nakagawa, T. J. Polymer Sci., 1997, 35, 119-131. Merkel, T. C.; Bondar, V.; Nagai, K.; Freeman, B . D . J. Polymer Sci., 2000, 38, 273-296. Tsujita, Y.; Hachisuka, H.; Imai, T.; Takizawa A.; Kinoshita, T. J. Membrane Sci., 1991, 60, 103-111. Hachisuka, H.; Takizawa, H.; Tsujita, Y.; Takizawa A.; Kinoshita, T. Polymer, 1991, 32, 2382-2386. Hachisuka, H.; Tsujita, Y.; Takizawa, A.; Kinoshita, T. J. Polymer Sci., 1991, 29, 11-16. Shibata, C. Industrial Microwave Power Engineering, Dennkisyoinn, Japan, 1986. Stem, S. A.; De Meringo, A . H . J. Polymer Sci. Polym. Phys. Ed., 1978, 16, 735-751.


Advanced Materials for Membrane Separations - American Chemical

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