NAMS 2004 Conference
Honolulu, Hawaii 26 – 30th June
Abstracts of Attendees
Conor Braman’s Abstract
Water transport and fouling properties of crosslinked poly(ethylene glycol)
C. Braman, University of Texas at Austin
B. Freeman, University of Texas at Austin
All current ultrafiltration membranes are finely porous and are, therefore, subject to fouling by particulates, organics, and other wastewater components, resulting in a dramatic decline in the water flux (Ho 1999). Our approach to enhancing the severely limited fouling resistance of conventional ultrafiltration membranes is based on coating them with highly water permeable, nonporous, fouling resistant polymers. Crosslinked poly(ethylene glycol) (PEG) is used as the base material for the coatings because it is highly hydrophilic and has shown resistance to protein attachment (Ostuni 2001).
UV-induced radical polymerization of PEG diacrylate (PEGDA), which contains 13 PEO units, and PEG acrylate (PEGA), which contains 7 PEO units, was used to prepare crosslinked PEG films. The composition of the initial polymerization mixture used was between 20/80 and 100/0 for (PEGDA +PEGA)/water, with the focus being on those samples prepared with higher initial water concentration.
Dense, free-standing films of crosslinked PEGDA appear to show a non-linear relationship between transmembrane pressure difference and flux, and the theoretical basis for this result will be discussed. To characterize the properties of PEG films in crossflow experiments, a composite crosslinked PEG membrane was prepared. This composite membrane consists of a porous membrane support and a thin, approximately one micron, dense coating of crosslinked PEGDA. An interfacial polymerization strategy was used to prepare a thin, uniform film at the membrane surface. Water transport and fouling properties of these composites have been characterized and will be described. The utility of these composites is shown by comparing their performance with that of uncoated porous ultrafiltration membranes.
Emanuele Ostuni, R. G. C., R. Erik Holmlin, Shuichi Takayama, and and G. M. Whitesides (2001). “A Survey of Structure-Property Relationships of Surfaces that Resist the Adsorption of Protein.” Langmuir (17): 5605-5620. Ho, C.-C. Z., A. L. (1999). “Effect of membrane morphology on the initial rate of protein fouling during microfiltration.” Journal of Membrane Science (155): 261-275.[/tab]
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Scott Kelman’s Abstract
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Effect of Crosslinking on Gas Transport Properties of High Free Volume Gas Separation Materials
S. Kelman, University of Texas at Austin
B. Freeman, University of Texas at Austin
High free volume polymer membranes are often very weakly size-sieving and, consequently, can remove large gas or vapor molecules from a gas mixture with smaller molecules. This capability finds application in reverse-selective gas separations such as VOC removal from permanent gas streams and monomer recovery from the exhaust of polymerization reactors. Poly(1-trimethysilyl-1-propyne) (PTMSP) is a stiff chain, high free volume glassy polymer well known for its very high gas permeability [1]. PTMSP also has outstanding vapor/gas selectivity. For example, the n-C4H10/CH4 mixed gas selectivity is 30, which is the highest value ever reported for this gas pair effects [2]. This makes PTMSP an interesting material for vapor/gas separations.
However, gas permeabilities in PTMSP are sensitive to processing history and time [1]. PTMSP undergoes significant physical aging, which is the gradual relaxation of nonequilibrium excess free volume in glassy polymers [3]. PTMSP is also soluble in many organic compounds leading to potential dissolution of the membrane in the process streams where separation properties are of greatest interest. These processes compromise the practical utility of PTMSP. Studies have been performed to slow the aging process in PTMSP. For example, Jia et al. [4] crosslinked PTMSP with bis azides in an effort to stabilize the large excess free volume elements. They found that physical stability of crosslinked PTMSP was achieved at the expense of reduced O2 and N2 permeability.
The effect of crosslinking PTMSP on aging behavior and transport properties of large organic molecules are presented. Crosslinking is successful in maintaining the permeability and vapor/gas selectivity of PTMSP over time. N2, O2, CH4 permeability values were constant over 100 days, while n-butane permeability increased for a crosslinked PTMSP membrane. The chemical resistance of PTMSP is strongly enhanced by crosslinking. For example crosslinked PTMSP is insoluble in common PTMSP solvents such as toluene and cyclohexane. The reaction between the bis azide crosslinker and PTMSP was observed using FTIR and XPS analysis. Crosslinking reduced the FFV of the polymer and therefore permeability decreased. Initial nitrogen permeability in crosslinked PTMSP was a factor of 4 less than pure PTMSP. Nanoparticles such as fumed silica and POSS particles were added to the crosslinked polymer to counteract the decrease in permeability.
[1] K. Nagai, T. Masuda, T. Nakagawa, B. D. Freeman and I. Pinnau, Poly(1-Trimethylsilyl-1-Propyne) and Related Polymers: Synthesis, Properties and Functions, Progress in Polymer Science, 26 (2001) 721-798.
[2] I. Pinnau and L. G. Toy, Transport of Organic Vapors through Poly(1-Trimethylsilyl-1-Propyne), J. Membrane Sci., 116 (1996) 199-209.
[3] L. C. E. Struik, Physical Aging in Amorphous Polymers and Other Materials, Elsevier, Amsterdam, 1978, pp. 7-9.
[4] J. Jia, PhD Thesis, Michigan State University, 1997.
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[tab label=”H.Lin”]
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Haiqing Lin’s Abstract
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Reverse-selective, highly branched polymers for purification of hydrogen and other light gases
H. Lin, University of Texas at Austin
B. Freeman, University of Texas at Austin
L. Toy, Research Triangle Institute
V. Bondar, Research Triangle Institute
R. Gupta, Research Triangle Institute
S. Pas, CSIRO Manufacturing Science and Technology
A. Hill, CSIRO Manufacturing Science and Technology
D. Dworak, The University of Akron
M. Soucek, The University of Akron
Polymer membranes are used in many applications, including gas separations, due to their inherently low energy requirements for molecular scale separations. Hydrogen, a potential energy source for fuel cells, is produced by steam reforming of hydrocarbons and requires removal of byproducts such as CO2 and H2S. Highly efficient membrane materials will be more permeable to large impurity molecules (e.g., CO2) than to H2 to produce purified H2 at high pressure; such behavior is opposite to that exhibited by most polymers, which sieve penetrants mainly based on relative molecular size, and are, therefore, more permeable to H2 than to CO2. We present results of studies aimed at separating molecules based on the relative affinity of the penetrants for the membrane. Based on an extensive survey of interactions between polar moieties in polymers and CO2, the polar ether units in ethylene oxide are promising candidates for preparing materials with high acid gas permeability and high selectivity for larger CO2 and H2S over smaller H2.
We have prepared and characterized a systematic series of polar, rubbery, branched hydrogels based on poly (ethylene glycol diacrylate) (PEGDA, which is a crosslinker) and the monomers poly(ethylene glycol methyl ether acrylate) (PEGMEA, which has a methyl ether end group) and poly (ethylene glycol acrylate) (PEGA, which has a hydroxyl end group). Crystallization of poly(ethylene oxide) can be completely suppressed by the choice of branch length (i.e., monomer molecular weight) and composition. Introducing PEGMEA into PEGDA improves CO2 permeability by 400% (from 110 to 570 Barrers) and CO2/H2 selectivity by 65% (from 7 to 12) at infinite dilution and 35C, as PEGMEA content increases from zero to 99 wt.%. However, copolymers of PEGDA and PEGA do not show any improvement in permeation properties relative to those of PEGDA alone. A copolymer containing 91 wt.% PEGMEA and the balance of PEGDA exhibits a mixed gas H2S permeability of 2,500 Barrers and H2S/H2 selectivity of 50 at 22C. Temperature could be manipulated to achieve better separation properties. For example, a copolymer containing 70 wt.% PEGMEA and the balance of PEGDA exhibits CO2 permeability of 52 Barrers and CO2/H2 selectivity of 40 at -20C, based on pure gas measurements at an upstream pressure of 4.4 atm. These materials exhibit the best CO2/H2 separation performance reported to date for solid non- facilitated transport membranes. Examples of the separation properties of these materials for CO2/CH4 and CO2/N2 separations will also be shown. The results are interpreted in terms of the effects of polymer chemical structure on free volume, as probed by density and positron annihilation lifetime spectroscopy (PALS), and glass transition temperature.
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Scott Matteucci’s Abstract
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Interactions of basic nanoparticles with polyacetylenes and their influence upon gas transport and aging properties
S. Matteucci, The University of Texas at Austin
R. Raharjo, The University of Texas at Austin
S. Kelman, The University of Texas at Austin
B. Freeman, University of Texas at Austin
Due to increasing use of H2 in refining and as an expected fuel for fuel cells, there is growing interest in finding economically and industrially feasible methods of producing and purifying H2. Currently H2 is produced from steam reforming of hydrocarbons, which produces byproducts such as CO2, H2O, and CO.
Relative to current separation technologies for purifying H2, membranes offer advantages of compact size, modularity, low capital costs, and low environmental impact [1]. However most membranes separate gases based on molecular size, which causes smaller gases (e.g. H2) to permeate preferentially into the low pressure stream. Since H2 is the major component of the feed stream and since H2 is typically required at or above the feed pressures which would be available for membrane separators, there is significant interest in membranes that could remove the minor components (e.g. CO2) and maintain H2 at high pressure. High free volume glassy polymers such as poly(1-trimethylsilyl-1-propyne) [PTMSP] can selectively remove larger, more condensable gases from mixtures with smaller, less condensable species. Additionally the permeability of high free volume glassy polymers can be greatly increased by dispersing nanosized inorganic nonporous particles, such as fumed silicia [FS], in the polymer matrix [2].
Our goal has been to use nanoparticles to selectively improve the solubility of CO2 in nanocomposite membranes, thereby increasing the CO2 / H2 selectivity. We have found that PTMSP membranes containing 0 to 25 % by volume of basic nanoparticles (3-100 nm diameter) exhibit higher permeabilities for CO2 (up to 106,000 Barrers at 35oC) and permanent gases relative to the previously reported pure polymer or PTMSP/FS composites [2]. However, the nanocomposite selectivity is similar to that of the pure polymer. Physical aging properties are also reported for nanocomposites. While nanocomposite samples do age, their long-term permeation properties are substantially above those of PTMSP alone. Both polymer-particle and gas-particle interactions are believed to contribute to the altered transport properties. These interactions have been observed using FTIR, XPS, and AFM and will be discussed herein. These results will also be extended to other high free volume glassy polymers to study the generality of the observed changes in permeation and aging properties.
[1] A. Kohl and R. Nielsen, Gas Purification, 5th ed., Gulf Publishing Company, Houston, 1997, pp. 1238-1295.
[2] T. C. Merkel, B. D. Freeman, R. J. Spontak, Z. He, I. Pinnau, P. Meakin and A. J. Hill, Ultrapermeable, Reverse-Selective Nanocomposite Membranes, Science, 296 (2002) 519-522.
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Rajeev Prabhakar’s Abstract
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Plasticization-resistant membranes for CO2 removal from natural gas
R. Prabhakar, University of Texas at Austin
B. Freeman, University of Texas at Austin
I. Roman, MEDAL L. P. / Air Liquide /P>
Polymer membrane-based separation of CO2 from natural gas is gaining attention due to the compact size, low energy requirement, ease of use and scale-up, and potential for offshore installation of membrane systems. Current membranes, however, suffer losses in separation performance under field conditions due to plasticization of the polymer. This is caused mainly by sorption of CO2 and higher hydrocarbons into the polymer membrane in amounts sufficient to increase polymer chain mobility and reduce its ‘size-sieving’ ability. The result is a loss of the product, methane, from the feed stream into the low pressure permeate stream. This requires either a second membrane module to recover the lost product and recompress it to pipeline specifications, or to simply accept the loss – both of these can be expensive options.
Efforts have been made to suppress plasticization of current polymer membranes by blending with other polymers, thermal treatment of membranes and crosslinking of polymer chains.1-6 While some success has been achieved in delaying the onset of plasticization to higher partial pressures of the plasticizing penetrants, these approaches essentially “treat the symptom” rather than the fundamental cause. An alternate approach is to address the core issue of high solubility of higher hydrocarbon compounds by considering polymeric materials with inherently low solubility for these compounds. Completely fluorinated polymers can exhibit low solubility for higher hydrocarbons. In this presentation, the experimentally-determined pure and mixed gas transport properties of these materials will be presented. Also, their potential as plasticization-resistant membranes will be discussed based on CO2-CH4 mixed-gas permeation results up to industrially-significant CO2 partial pressures. For example, Hyflon AD 80, which is a copolymer containing 80 mole% 2,2,4-trifluoro-5-trifluoromethoxy-1,3- dioxole (TTD) and 20 mole% tetrafluoroethylene (TFE), exhibits very high CO2 permeability (of the order of 200 Barrers) and reasonable CO2/CH4 selectivity (in the range of 9 to 12) for feed streams up to 800 psia containing 20% CO2.
(1) Bos, A.; Punt, I.; Strathmann, H.; Wessling, M. AIChE J. 2001, 47, 1088- 1093. (2) Bos, A.; Punt, I. G. M.; Wessling, M.; Strathmann, H. J. Polym. Sci. Part B. Polym. Phys. 1998, 36, 1547-1556. (3) Bos, A.; Punt, I. G. M.; Wessling, M.; Strathmann, H. Separation and Purification Technology 1998, 14, 27- 39. (4) Staudt-Bickel, C.; Koros, W. J. J. Membrane Sci. 1999, 155, 145-154. (5) Wind, J. D.; Staudt-Bickel, C.; Paul, D. R.; Koros, W. J. Industrial and Engineering Chemistry Research 2002, 41, 6139-6148. (6) Wind, J. D.; Staudt-Bickel, C.; Paul, D. R.; Koros, W. J. Macromolecules 2003, 36, 1882-1888.
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Roy Raharjo’s Abstract
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The Effect of Substituent Size and Shape on Gas Transport Properties in Substituted Polyacetylenes
R. Raharjo, University of Texas at Austin
H. Lee, University of Texas at Austin
B. Freeman, University of Texas at Austin
T. Sakaguchi, Kyoto University
T. Masuda, Kyoto University
Poly[1-phenyl-2-[p-(trimethylsilyl) phenyl]acetylene] (PTMSDPA) is a highly permeable, glassy, substituted acetylene polymer. Like poly(1- trimethylsilyl-1-propyne) (PTMSP), this polymer is more permeable to larger, more condensable hydrocarbons than to smaller, less condensable permanent gases. Therefore, it may be useful for separations such as the removal of higher hydrocarbons from natural gas or hydrogen streams. Although PTMSDPA has outstanding properties for such separations, PTMSDPA is soluble in common hydrocarbons and, therefore, would not be suitable for use as a commercial membrane in such applications. However, desilylation of this polymer renders it completely insoluble, thereby strongly improving its chemical resistance. The resulting polymer, poly(diphenylacetylene) (PDPA), has a fractional free volume (FFV) of 0.23, slightly lower than that in PTMSDPA (0.26). The pure gas permeation properties of various light gases and hydrocarbons in PTMSDPA and PDPA at 35oC are reported and compared to those in other substituted polyacetylenes. A significant decrease in gas permeability values, most likely due to the decrease in the FFV, is observed after desilylation. For example, the permeability of nitrogen is reduced by almost 50%, from 640 to 280 Barrers. The permeability of n- butane is reduced even more, from 16000 to 2400 Barrers. The permeability measurement was done at 35oC with an upstream pressure of 50 psig (4.46 bar) for all permanent gases, methane, ethane, and propane and 8 psig (1.56 bar) for n-butane. The downstream pressure was maintained at atmospheric.
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