Advanced Materials for Membrane Separations - American Chemical

Chapter 18

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 3, 2017 | http://pubs.acs.org Publication Date: April 20, 2004 | doi: 10.1021/bk-2004-0876.ch018

Formation of Composite Membranes with Ultrathin Skins Using New Methods of Organic Film Formation: Gas-Selective Membranes Merlin L . Bruening Department of Chemistry, Michigan State University, East Lansing, MI 48824

This chapter describes the formation of ultrathin (< 50 nm) gas-selective skins on porous alumina supports using either graft-on-graft deposition of hyperbranched poly(acrylic acid) (PAA) or alternating polyelectrolyte deposition (APD). The highly branched structure of the P A A films allows coverage of substrate pores with diameters as large as 0.2 µm, and derivatization of P A A with H N C H ( C F ) C F results in an O / N selectivity of 2.3, showing that films are free of defects. Formation of gas-selective membranes using A P D involves alternating adsorption of poly(amic acids) and protonated poly(allylamine). Subsequent imidization by heating yields membranes with the same selectivities as bulk polyimides, even when skins are only 35-40 nm thick. Because P A A can be widely derivatized, and nearly any polyelectrolyte is suitable for A P D , these techniques provide versatile methods for forming functional, ultrathin membranes. 2

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© 2004 American Chemical Society

Pinnau and Freeman; Advanced Materials for Membrane Separations ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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Introduction Although membrane separations are attractive because of their simplicity and low energy costs, applications of membranes are often limited by insufficient flux, selectivity, or stability. This problem is especially challenging because the permeability of a material is frequently inversely related to its selectivity (1,2). The most common means for increasing flux while maintaining selectivity is to prepare a membrane containing a selective, ultrathin skin on a highly permeable support. Loeb and Sourirajan first demonstrated this concept by forming asymmetrically skinned cellulose acetate membranes (3). Subsequent development of composite membranes that consist of an ultrathin skin deposited on a separate porous support further increased the attractiveness of skinned membranes, because in these systems, only a small amount of selective material is needed. Thus, more expensive, high-performance materials can be used as membrane skins (4). Processes used to form skins for composite membranes include casting, interfacial polymerization, plasma grafting, and deposition of films from the air-water interface (5-9). However, in spite of successes in this area, depositing membrane skins that are selective, defect-free, and ultrathin (< 50 nm) remains difficult. Formation of defect-free membrane skins is critical to separations because even a small number of defects can negate selectivity (10). This chapter reviews the use of two recently developed techniques for formation of ultrathin membrane skins. The first is the deposition of hyperbranched poly(acrylic acid) (PAA) (11,12). This technique is attractive because the hyperbranched structure of these films should allow coverage of relatively large pores as shown in Figure 1. Additionally, the - C O O H groups of

Solid Substrate Figure 1. Idealized, schematic diagram of a hyperbranched PAA film covering a substrate pore. Reproduced with permission from reference 12. Copyright 2000 American Chemical Society.



271 P A A can be readily derivatized to provide specific functionality (13). Although this chapter focuses on the formation of gas separation membranes, P A A is also potentially attractive for its interactions with biomolecules such as proteins (14,15). The second technique for membrane formation discussed herein is alternating polyelectrolyte deposition (APD) (16). Formation of membranes occurs simply through alternating immersions of a charged support into solutions containing polycations and poly anions as shown in Figure 2. We recently reviewed the formation of ion-selective membranes using this technique (17). This chapter focuses on the deposition of selective gas separation membranes through formation of films containing poly(amic acids) and subsequent imidization. B y carefully controlling deposition conditions, A P D allows formation of selective polyimide membrane skins with thicknesses as low as 35-40 nm (75). Future exploitation of this technique may allow further reductions in the thickness of the selective layer.

1. Immersion in positively c h a r g e d polyelectrolyte ^ 2. R i n s e

Figure 2. Schematic diagram of alternating polyelectrolyte deposition (16). Repetition of the procedure yields multilayer films. Intertwining of neighboring layers is not shown for figure clarity.

Hyperbranched Poly(acrylic acid) Skins Synthesis of P A A films occurs as shown in Scheme 1 (77). To prepare a film on a porous alumina substrate, we first deposit 5 nm of gold on the surface and then immerse the substrate in a solution containing mercaptoundecanoic acid to form a monolayer with - C O O H groups. Activation of the - C O O H groups to anhydrides using ethyl chloroformate followed by reaction with amino-terminated poly(tert-butyl acrylate) (PTBA) yields a grafted layer of P T B A on the surface, and subsequent hydrolysis results in grafted P A A . Further grafting on each previous graft using the same activation, reaction, and hydrolysis procedure yields a hyperbranched P A A film.



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PAA= f

Advanced Materials for Membrane Separations - American Chemical

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