Nanostructred Polymeric Membranes for Selective CO2 Removal from Light Gas Mixtures
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Date
2004-06-27
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Abstract
Two primary materials strategies have been developed to produce nanostructured polymer membranes for selective CO2 removal from mixed light-gas streams. In one approach, a microphase-ordered poly(styrene-b-ethylene oxide-b-styrene) (SEOS) triblock copolymer and its miscible blends with poly(ethylene glycol) (PEG) differing in molecular weight have been investigated to establish structure-transport property relationships. These membranes exhibit high CO2/H2 selectivity due to the affinity of CO2 for the ether moiety in the copolymer/homopolymer backbone. Crystalline regions in the EO microphase or introduced by relatively high-molecular-weight PEG serve as impermeable barriers to penetrating gas molecules and therefore compromise membrane performance. This drawback can be overcome through the physical addition of low-molecular-weight PEG, which behaves as a diluent. Upon PEO crystal melting at elevated temperatures, the CO2/H2 selectivity undergoes an abrupt increase consistent with the hypothesis that only amorphous regions can participate in penetrant transport.
An alternative approach to near-equilibrium block copolymer/homopolymer blends is the introduction of a B-compatible homopolymer into a swollen ABA triblock or higher-order multiblock copolymer. The resultant "mesoblends" are reproducible, nonequilibrium blends that do not undergo the same morphological transitions induced in the near-equilibrium blend analogues. This procedure has been adopted here to generate novel morphologies in the SEOS triblock copolymer and a poly(amide-b-ethylene glycol) (AEG) multiblock copolymer with PEG homopolymers. Solvent quality, solution concentration and temperature have a profound impact on PEG solubility within the copolymer. Incorporation of amorphous PEG into the AEG copolymer is found to enhance CO2 permeability, as well as CO2/H2 selectivity.
The second approach examined here relies on chemically crosslinked PEG diacrylate (PEGda) oligomers differing in molecular weight, as well as their nanocomposites prepared with up to 10 wt% methacrylate-functionalized fumed silica (FS) or an organically-modified nanoclay. The mechanical, thermal and morphological characteristics of these membranes have been probed by dynamic rheology, thermal gravimetric analysis (TGA) and transmission electron microscopy (TEM), respectively. These PEGda membranes exhibit exceptionally high acid-gas selectivity coupled with high gas permeabilities that tend to increase with increasing oligomer molecular weight. Addition of FS results in improved mechanical properties without deteriorating transport properties. Temperature-dependent permeation studies demonstrate Arrhenius behavior with considerably lower activation energy of permeation for CO2. The polarity of the matrix, represented by PEGda oligomer molecular weight, and the transmembrane pressure allow systematic tuning of CO2/H2 selectivity and CO2 permeability.
Crosslinked poly(propylene glycol) diacrylate (PPGda) membranes with various additives have also been synthesized due to their reportedly higher CO2 solubility. Gas transport and rheological properties are extremely sensitive to the molecular weight of oligomer, as in the case of the corresponding PEGda membranes. The major difference between these two membranes is the higher CO2 permeability, but lower CO2/H2 selectivity, in the PPGda membranes. Gas transport properties vary according to the rule of mixtures in PPGda/PEGda membranes blended prior to chemical crosslinking.
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Keywords
Crosslinking, Mesoblends, Block copolymer, Separation, Polymer membrane, Nanocomposite
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Degree
PhD
Discipline
Chemical Engineering
