NAMS 2005 Conference
Providence, Rhode Island 11 – 15th June
Abstracts of Attendees
Scott Kelman’s Abstract
Crosslinking and Stabilization of High Fractional Free Volume Polymers for the Separation of Organic Vapors from Permanent Gases
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 at 25oC is 35, which is the highest value ever reported for this gas pair [2]. Such properties make PTMSP an interesting material for vapor/gas separations. However, gas permeabilities in PTMSP are sensitive to processing history and time [3]. PTMSP undergoes significant physical aging, which is the gradual relaxation of non-equilibrium excess free volume in glassy polymers. PTMSP is also soluble in many organic compounds, leading to potential dissolution of the membrane in process streams where its separation properties are of greatest interest. These phenomena compromise the practical utility of PTMSP.
The main intent of this study is to investigate the effect of crosslinking PTMSP on transport properties and physical aging. PTMSP films are crosslinked using bis azides, which have been shown to be successful in crosslinking PTMSP [4]. The crosslinking chemistry is discussed and the extent of crosslinking is correlated with the transport properties of this polymer When PTMSP is crosslinked, it becomes insoluble in common PTMSP solvents such as toluene, cyclohexane and tetrahydrafuran. Thus, there is a significant increase in the chemical stability due to crosslinking. The initial permeability of PTMSP decreased with increasing crosslinking due to the loss in fractional free volume (FFV) upon crosslinking. The O2/N2 selectivity increased, as the FFV decreased, showing that crosslinked PTMSP is more size selective than uncrosslinked PTMSP. A strong relationship between permeability and 1/FFV was found. The sorption properties of PTMSP were unaffected by crosslinking, so the decrease in permeability was due to a decrease in diffusion coefficients.
The crosslinked PTMSP N2, O2 and CH4 permeabilities were stable for 100 days, which has been the limit of our tests to date. The increased stability may be due to the crosslinks constraining the PTMSP chains and not allowing them to relax the excess, non equilibrium FFV that is inherent in PTMSP. Over the same time scale, n-Butane permeability increased by 20%. This result is interesting and could be due to n-butane conditioning the membrane. Further research is required to fully understand the time dependence of permeation properties of crosslinked PTMSP. Initial mixed gas data show that crosslinked PTMSP displays enhanced mixed gas selectivities, similar to those in uncrosslinked PTMSP. Further research to investigate the effect of vapor/gas composition, temperature and pressure on mixed gas permeation properties of crosslinked PTMSP is being performed and will be described in this presentation.
[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.[/tab]
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Scott Matteucci’s Abstract
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Gas Transport Properties of Nanoparticle Filled Rubbery Polymers
S. Matteucci, The University of Texas at Austin
H. Lin, 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].
Traditionally, the addition of impermeable particles to rubbery polymeric membranes results in a reduction in permeability as particle loading increases. Recently, we have prepared nanoparticle filled rubbery polymers that have up to 4 times higher light gas (i.e., CO2, N2, O2, H2) permeability with little to no change in light gas/light gas (CO2/H2, O2/N2) selectivity relative to the neat polymer. As an example filled crosslinked poly(ethylene oxide) has a CO2 permeability of 1700 barrer and CO2/H2 selectivity of 5. The degree of permeability enhancement is polymer and particle loading dependent and our studies include polar, non-polar, and crosslinked rubbery polymers with increasing loadings of nanoparticles. These materials have been characterized using light gas sorption and permeation to monitor gas transport properties as well as AFM to characterize particle distribution within the polymer matrix.
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Haiqing Lin’s Abstract
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A New Direction for the Design of Polymer Membrane Materials for CO2/H2 and CO2/CH4 Separations: Crosslinked Rubbery Polymers
Haiqing Lin, Scott Kelman, Benny D. Freeman, Lora G. Toy, Raghubir P. Gupta, Sumod Kalakkunnath and Douglass Kalika
Carbon dioxide is an impurity that must be removed from mixtures with light gases such as CH4 and H2, and the scale of these separations can be enormous. It is highly desirable to selectively remove CO2, thereby maintaining the light gas at or near feed pressure (in the case of H2 and CH4) to avoid expensive recompression of the desired light gas product. We present results of studies aimed at separating molecules based on the relative solubility of the penetrants in the membrane. Based on an extensive survey of interactions between polar moieties in polymers and CO2, polar ether units in ethylene oxide were identified as promising candidates for preparing materials with high acid gas permeability and high selectivity for larger CO2 over smaller H2. Specifically, we have prepared a series of crosslinked rubbery copolymers by photopolymerizing or thermally polymerizing mixtures of poly(ethylene glycol) diacrylate (PEGDA: CH2=CHCOO(CH2CH2O)nOCCH=CH2, n = 14) and poly(ethylene glycol) methyl ether acrylate (PEGMEA: CH2=CHCO(OCH2CH2)nOCH3, n = 8.5).
These materials exhibit the best CO2/H2 separation performance reported to date for solid non-facilitated transport membranes, and the separation properties can be improved by decreasing temperature. For example, when a copolymer containing 30 wt% PEGDA and 70 wt% PEGMEA was tested using a mixture containing 80% CO2 and 20% H2 at 21 atm, CO2 permeability and CO2/H2 mixture gas selectivity at 35C are 440 Barrers and 9.4, respectively, and they are 410 Barrers and 31 at -20C, respectively. Interestingly, as CO2 partial pressure increases from 3.5 to 17 atm at -20C, CO2 permeability increases by almost one order of magnitude, from 45 to 410, and mixed gas CO2/H2 selectivity increases by about 25%, from 25 to 31.
These rubbery materials also exhibit excellent CO2/CH4 separation properties. Unlike conventional glassy polymers used for this application, such as polyimides, plasticization has a minimal effect on CO2/CH4 selectivity in these materials. For a gas feed containing 80% CO2 and 20% CH4, CO2/CH4 selectivity in a copolymer containing 30 wt.% PEGDA and 70 wt% PEGMEA decreases slightly from 14 to 12 at 35C while it decreases from 51 to 32 as CO2 partial pressure increases from 3.6 to 17 atm.
In summary, pure gas permeation, sorption and diffusion data are presented for a series of these materials of systematically varying crosslinker/monomer content. The effect of temperature on these transport properties is explored. Mixed gas permeation data are reported fro CO2/H2, CO2/CH4, and CO2/C2H6 mixtures as a function of gas composition, temperature, and pressure. The experimental data are interpreted in terms of conventional models for sorption, diffusion and permeation.
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