Modification of Polymer Membranes: A Study of Crosslinking and In-Situ Growth of Palladium-Containing Nanoparticles in Polymer Matrices.
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2008-08-21
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Abstract
A crucial step in obtaining pure hydrogen is separating it from other compounds—mainly CO2—that often accompany hydrogen in industrial chemical reactions. Advanced membrane technology may prove to be the key to the successful, economical production of molecular hydrogen for the eventual consumer market. Size-sieving glassy polymer membranes can separate H2 on the basis of its small size. Alternatively, reverse-selective rubbery polymers can expedite the passage and, hence, removal of CO2 due to its relatively high solubility in such membranes alone or in conjunction with dissociative chemical reactions. Transition-metal membranes and their alloys can adsorb H2 molecules, dissociate the molecules into H atoms for transport through interstitial sites, and subsequently recombine the H atoms to form molecular H2 again on the opposite membrane side. Microporous amorphous silica and zeolite membranes comprising thin films on a multilayer porous support exhibit good sorption selectivity and high diffusion mobilities for H2, leading to high H2 fluxes. Finally, carbon-based membranes, including carbon nanotubes, may be viable for H2 separation on the basis of selective surface flow and molecular sieving. One approach to achieve higher gas selectivity is to cross-link polymer membranes, thus restricting the ability of gases of various sizes to readily permeate at an unimpeded rate. Cross-linking can occur through a number of means: UV and ion irradiation, plasma treatment, or chemical and thermal techniques. In this study, a chemical technique has been chosen to cross-link the polyimide Matrimid®. Polyimides are well-established as gas-separation membranes due to their intrinsically low free-volume and correspondingly high H2 selectivity relative to other gases such as CO2. Prior studies have established that H2⁄CO2 selectivity can be improved by cross-linking polyimides with diamines differing in spacer length. In this first set of work, we follow the evolution of macroscopic and microscopic properties of a commercial polyimide over long cross-linking times (tx) with 1,3-diaminopropane. According to spectroscopic analysis, the cross-linking reaction saturates after ˜24 h, whereas tensile, nanoindentation and stress-relaxation tests reveal that the material stiffens, and possesses a long relaxation time that increases, with increasing tx. Although differential scanning calorimetry shows that the glass transition temperature decreases systematically with increasing tx, permeation studies indicate that the permeabilities of H2 and CO2 decrease, while the H2⁄CO2 selectivity increases markedly, with increasing tx. At long tx, the polyimide becomes impermeable to CO2, suggesting that it could be used as a barrier material. Alternatively, polymer nanocomposites continue to receive considerable attention as multifunctional hybrid materials, with most nanocomposites fabricated by physical dispersion of surface-functionalized nanoscale objects. In the second study, we explore the viability of growing Pd-containing nanoparticles from Na2PdCl4 in two different polymers —hyper-cross-linked polystyrene (HPS) and an aromatic polyimide (PIm). In HPS, single Pd-containing nanoparticles possessing a relatively narrow size distribution (ca. 1-4 nm) are observed to form upon reduction of the divalent PdCl4-2 ions and cluster more readily if the reducing agent is introduced as a liquid. Single nanoparticles with a broad size distribution ranging from ˜2 to 16 nm develop in PIm, which simultaneously undergoes chemical cross-linking during ion reduction. The conditions yielding Pd incorporation in PIm are explored through the use of instrumental neutron activation analysis. Such Pd-containing hybrid materials hold promise in molecular catalysis and gas separations. Results from these studies give prospect that these materials, with a great deal of future research, could be developed for H2 separations applications.
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crosslinking, membrane, polymer, H2, hydrogen, gas separation, polyimide, nanoparticles, palladium
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Degree
MS
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Chemical Engineering
