Membrane Formation and Modification - American Chemical Society

Chapter 6

Effect of Surfactant as an Additive on the Formation of Asymmetric Polysulfone Membranes for Gas Separation 1

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A . Yamasaki , R. K. Tyagi , A . E . Fouda , K. Jonnason , and T. Matsuura Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 24, 2013 | http://pubs.acs.org Publication Date: December 29, 1999 | doi: 10.1021/bk-2000-0744.ch006

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National Institute of Materials and Chemical Research, 1-1 Higashi Tsukuba 305, Japan Institute of Chemical Process and Environmental Technology, National Research Council of Canada, Ottawa K1A 0R6, Canada Industrial Membrane Research Institute, Department of Chemical Engineering, University of Ottawa, Ottawa K1N 6N5, Canada

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The addition of a surfactant, sodium dodecyl sulfate (SDS), to the casting solution increased the oxygen permeance through asymmetric polysulfone membranes significantly, while maintaining relatively high selectivity of oxygen over nitrogen. A phase inversion technique with partial solvent evaporation (0 to 15 min at 95°C) and a dual coagulation step using 2-propanol and water was used to prepare the asymmetric polysulfone membranes. The content of SDS in the solution was varied from 0 to 2.0 wt.%. The oxygen permeance showed a maximum at 1.0 wt.% of SDS for a given solvent evaporation time. Scanning electron microscopy (SEM) indicated that the skin layer growth was very limited with an increase in the solvent evaporation time for the membranes prepared with SDS.

The phase inversion technique is a common method for the preparation of asymmetric membranes (1-4). Phase inversion membranes for gas separation are generally comprised of an ultrathin, dense skin layer and a porous substructure. The gas permeation properties of an asymmetric membrane are determined primarily the skin layer because most of the permeation resistance is concentrated in the skin layer. For an ideal phase inversion membrane, the skin layer should be as thin as possible to obtain a high flux. In addition, the skin layer must be defect-free to obtain the intrinsic selectivity of the membrane material. Optimization of phase inversion membranes is very difficult because many parameters are involved in the membrane formation process (1-4). In previous work (5), we studied the growth of the skin layer during the phase inversion process. A two-step coagulation process using 2-propanol and water, respectively, and partial solvent evaporation before coagulation, were used for membrane formation. The two-step coagulation process is based on the concept

© 2000 American Chemical Society

In Membrane Formation and Modification; Pinnau, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 24, 2013 | http://pubs.acs.org Publication Date: December 29, 1999 | doi: 10.1021/bk-2000-0744.ch006

88 proposed by van't Hof et al. (6), except for the additional solvent evaporation step. The growth of the skin layer was observed with scanning electron microscopy (SEM). It was found that the skin layer thickness increased with increased coagulation time in the 2-propanol bath or increased solvent evaporation time. With an increase in the skin layer thickness, the oxygen permeance decreased and the oxygen/nitrogen selectivity increased. This result suggested that the density of the skin layer as well as its thickness increased with an increase in the coagulation time and/or the solvent evaporation time. One option for controlling the membrane formation process may be using an additive in the casting solution. For example, the addition of an inorganic salt such as C a C l (7), a polymer such as poly(vinyl pyrrolidone) (8), or an organic acid such as propionic acid (9,10), can change the structure of phase inversion membranes, especially the density of the skin layer. These additives are assumed to change the polymer configuration in the casting solution, which could result in morphological changes in the skin layer. It is known that some polymers form a polymersurfactant complex in aqueous solution (11-15) through hydrophobic interactions between polymer and surfactant. Because of the complex formation, the properties of the polymer solution can be changed significantly. Therefore, the addition of a surfactant to the casting solution for a phase inversion membrane should greatly influence the polymer configuration, and consequently, the final membrane structure. In this study, a typical surfactant, sodium dodecyl sulfate (SDS), was used as an additive in the casting solution for the preparation of asymmetric polysulfone membranes and its effect on the membrane structure and gas permeation properties was investigated. 2

Experimental Membrane Preparation. Polysulfone (Udel-P1700, Amoco Performance Products) was used as the membrane-forming polymer, N,N-dimethylacetamide (DMAc) was used as a solvent, and sodium dodecyl sulfate (SDS) was used as a surfactant. The initial polymer concentration in the casting solution was 27.5 wt.% for all membranes in this study. The SDS concentration was varied from 0 to 2.0 wt.% in the casting solution. When the SDS content was larger than 2.0 wt.%, the surfactant precipitated in the casting solution. The membranes were cast on a glass plate with a thickness of 200 μηι (8 mils). The casting process was carried out at ambient conditions and the relative humidity in the casting chamber was controlled to be less than 10%. After casting, the solvent' was evaporated in a constant temperature oven at 95°C for a given period (0-15 min). Then, the film was immersed in a 2-propanol bath for 5 s at room temperature where the initial solidification of the membrane took place. Thereafter, the membrane was immersed in a water bath for more than 12 h, and subsequently washed with fresh water. The membrane was dried in a desiccator for several days before the gas permeation measurements were carried out. X-ray photoelectron spectroscopy (XPS) was used to detect any remaining SDS (sodium atom) in the membranes; no sodium was detected for all membranes studied within the detection limit; thus, SDS was completely extracted from the membranes.


89 The asymmetric polysulfone membranes were coated with a thin layer of polydimethylsiloxane (thickness 1-3 μηι) to eliminate any skin layer defects. The permeance of defect-free, silicone-coated polysulfone membranes was calculated based on the series-resistance model (5).


Gas Permeation Measurements. A constant-pressure/variable volume system was used for the gas permeation experiments. The gas permeation experiments were carried out with oxygen and nitrogen at 25°C at a feed pressure of 100 psig and atmospheric permeate pressure (0 psig). Results and Discussion The effect of the evaporation time on the oxygen permeance of the membranes is shown in Figure 1. For a given SDS content, the oxygen permeance decreased with an increase in the solvent evaporation time. When the SDS content was 1.0 wt.%, the oxygen permeance showed a maximum value for a given evaporation time, that is, nearly ten times larger than the permeance of the membrane prepared without SDS. The oxygen permeance of the membrane prepared with 2.0 wt.% SDS was generally lower than that made from 1.0 wt.% SDS, but still much larger than that made without SDS. The oxygen permeance of the membrane prepared with 0.5 wt.% SDS was almost equal to that made without SDS. The effect of the solvent evaporation time on the oxygen/nitrogen selectivity of the various membranes is shown in Figure 2. The oxygen/nitrogen selectivity increased with an increase in the solvent evaporation time for the membranes made without SDS up to about 10 min, where the selectivity was about 6, and leveled off thereafter. The selectivity of a dense, homogeneous polysulfone film was reported to be 6.2 (2). The oxygen/nitrogen selectivity of the membrane made from 1.0 wt.% SDS was slightly lower that made without SDS. For the membranes made from 0.5 and 2.0 wt.% SDS, on the other hand, the oxygen/nitrogen selectivity of 3 was essentially constant, which was always less than that made without SDS. Figures 3(a)-3(d) show the S E M images of the cross-sections of the membranes prepared without SDS at different evaporation times. The skin layer thickness increased with an increase in the solvent evaporation time from less than 1 μπι (0 min.) to about 3-4 μηι (15 min.). The increase in skin layer thickness was consistent with the decrease in the oxygen permeance as the solvent evaporation time increased. Figures 4(a)-4(d) show the change of the membrane structures with an increase in the solvent evaporation time for the membranes prepared with 1.0 wt.% SDS. The skin layer thickness was less than 1 μιη for all solvent evaporation times up to 10 min, and about 1 μιη when the solvent evaporation time was 15 min. This result suggests that the gas permeance of the membranes increased because the growth of the skin layer was reduced with the addition of SDS. The growth of the skin layer with the solvent evaporation time can be explained as follows. During evaporation of the solvent, a higher concentration of polymer is formed in the topmost part of the cast film. This region is converted into the skin layer during the solidification process when the polymer concentration exceeds a


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Membrane Formation and Modification - American Chemical Society

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