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AIChE Conference, Cincinnati, OH. 30 October – 4 November 2005

AIChE Annual Conference 2005
Cincinnati, OH, 30th October – 4th November
Abstracts of Attendees

Conor Braman’s Abstract

Polymerization Induced Phase Separation and its effects on water uptake, flux, and other properties of Crosslinked Poly(ethylene glycol)

C. Braman, University of Texas
B. Freeman, University of Texas
T. Kai, University of Texas
D. S. Kalika, University of Kentucky
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. The reason for this is twofold: (1) at higher water content the films exhibit both greater water uptake as well as higher water flux, and (2) many of the samples have undergone Polymerization Induced Phase Separation (PIPS), as evidenced by the opacity of the films after polymerization.

The impact of PIPS on the properties of the final films is quite dramatic. Despite having a higher apparent crosslinking density, films prepared with only PEGDA and water exhibit a higher water flux than those made with a mixture of PEGA, PEGDA, and water. The films comprised of only crosslinker and water also exhibit greater opacity, as measured by absorbance of visible light. Microscopy and MWCO experiments provide insight into the structural basis for this counter-intuitive phenomenon, i.e. higher water flux at higher crosslinking density. The formation of water pockets and channels in the nascent hydrogel during the polymerization process is believed to play a key role in these effects.

Ostuni, Emmanuelle. Holmlin, Erik. Takayama, Shuichi. and Whitesides, G.M. (2001). “A Survey of Structure-Property Relationships of Surfaces that Resist the Adsorption of Protein.” Langmuir (17): 5605-5620.

Ho, C.-C. Zydney, A. L. (1999). “Effect of membrane morphology on the initial rate of protein fouling during microfiltration.” Journal of Membrane Science (155): 261-275.

Scott Kelman’s Abstract

Crosslinking of High 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.

This study investigates the effect of crosslinking PTMSP on transport properties and physical aging. PTMSP films are crosslinked using bis azides, which have been shown to crosslink 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 correlation 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.

Nanoparticles such as fumed silica and titanium oxide were added to crosslinked PTMSP films and permeability of these films increased by up to 200% compared to crosslinked films with no nanoparticles. A systematic study of the effect of type, shape and amount of nanoparticles added to crosslinked PTMSP has been conducted.The crosslinked PTMSP N2, O2 and CH4 permeability stability is improved and films have been tested for up to 250 days. 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.Mixed gas experiments data show that crosslinked PTMSP displays enhanced mixed gas selectivities, similar to those in uncrosslinked PTMSP. The effect of vapor/gas composition, temperature and pressure on mixed gas permeation properties of crosslinked PTMSP has been studied, and the results from this study will be presented.

[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.

CO2/C2H6 Separation Using Solubility Selective Membrane Materials

S. Kelman, University of Texas at Austin
H. Lin, University of Texas at Austin
B. Freeman, University of Texas at Austin

Carbon dioxide is an impurity which must be removed from natural gas streams in a process commonly known as natural gas conditioning. Ethane is a major component of natural gas streams, and it forms a minimum pressure azeotrope with carbon dioxide which hinders carbon dioxide separation [1]. At 293K, the carbon dioxide/ethane azeotrope has a carbon dioxide mole fraction of 0.7 [2]. Traditional techniques (e.g., chemical absorption using amine solutions and adsorption using molecular sieves), for breaking the carbon dioxide/ethane azeotrope are capital intensive and require complex process control [3]. This study investigates the effectiveness of membranes to break the carbon dioxide/ethane azeotrope. Membrane technology has been successfully implemented in natural gas processing facilities and has been shown to be effective in separating carbon dioxide from methane [1]. The membrane material selected to break the carbon dioxide/ethane azeotrope should have a high carbon dioxide/ethane selectivity and high carbon dioxide permeability. From previous work in our laboratory, crosslinked poly(ethylene oxide) [XLPEO] shows high carbon dioxide permeability and high carbon dioxide/ethane selectivity. The pure gas permeability and solubility of carbon dioxide and ethane in XLPEO has been measured at temperatures ranging from 253K to 308K and over a pressure range of 0 to 15 atmospheres. The permeability of carbon dioxide and ethane increase as temperature increases, while the gas solubility increases as temperature decreases. At 253K, the permeability of carbon dioxide increases strongly with increasing carbon dioxide partial pressure, since carbon dioxide strongly plasticizes XLPEO. These strong plasticizing effects are not observed at 253K for ethane. Mixed gas permeation experiments were conducted using a gas mixture containing 45.3 mol % carbon dioxide and the balance ethane. The mixed gas results show carbon dioxide strongly plasticizes XLPEO films at lower temperatures. Slight plasticization of the film is seen at 283K and 308K. Carbon dioxide plasticization decreases the mixed gas carbon dioxide/ethane selectivity, relative to pure gas values, because the ethane permeability is enhanced by the plasticization effects of carbon dioxide. The mixed gas selectivity increases as temperature decreases. At 253K and 10 atmospheres pressure the carbon dioxide permeability is 83 Barrers and the mixed gas carbon dioxide/ethane selectivity is 13. The pure gas and mixed gas carbon dioxide permeabilities and carbon dioxide/ethane selectivities were plotted on a Robeson plot and were found to be lie close to the upper bound. The performance of the membrane material was simulated using a computer model developed in our laboratory [4]. The mixed gas experimental conditions and results were used as parameters for the simulation. It was established that XLPEO is effective in breaking the carbon dioxide/ethane azeotrope. When the feed stream to the membrane module contains 45.3 mol% carbon dioxide and the balance ethane, 85% ethane recovery is achieved at an ethane purity of 78%, for a one pass separation at 253K.
of vapor/gas composition, temperature and pressure on mixed gas permeation properties of crosslinked PTMSP has been studied, and the results from this study will be presented.
[1] G. H. Gall and E. S. Sanders, Membrane Technology Removes Co2 from Liquid Ethane, Oil & Gas Journal, 100 (2002) 48-55.
[2] A. Fredenslund and J. Mollerup, Measurement and Prediction of Equilibrium Ratios for Ethane + Carbon Dioxide System, Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases, 70 (1974) 1653-60.
[3] S. Horstmann, K. Fischer, J. Gmehling and P. Kolar, Experimental Determination of the Critical Line for (Carbon Dioxide + Ethane) and Calculation of Various Thermodynamic Properties for (Carbon Dioxide + N-Alkane) Using the Psrk Model, Journal of Chemical Thermodynamics, 32 (2000) 451-464.
[4] D. T. Coker, B. D. Freeman and G. K. Fleming, Modeling Multicomponent Gas Separation Using Hollow-Fiber Membrane Contactors, AIChE Journal, 44 (1998) 1289-1302.

Scott Matteucci’s Abstract

Nanoparticle Filled Rubbery Polymer Membranes for CO2 Sequestration

S. Matteucci, The University of Texas at Austin
H. Lin, The University of Texas at Austin
B. Freeman, The University of Texas at Austin
V. Kusuma, The University of Texas at Austin
M. J. Yacaman, The University of Texas at Austin
Sumod Kalakkunnath, University of Kentucky
Douglass Kalika, University of Kentucky

Traditionally, the addition of impermeable particles to rubbery polymeric membranes reduces light gas and vapor permeability as particle loading increases. This phenomenon is well known for barrier materials, and there are numerous models, such as the one derived by Maxwell, that accurately predict the permeability loss of membranes filled with impermeable particles.[1]

Recently, nonporous metal oxide nanoparticles (primary particle diameter as low as 2.5 nm) have been dispersed in rubbery polymer to make membranes that have over an order of magnitude higher light gas (i.e., CO2, N2, O2, H2) permeability with little or no change in selectivity relative to the neat polymer, which runs counter to traditional filler rubbery polymers. For example, the CO2 permeability was 1100 barrers in filled 1,2-butadiene as compared to 52 barrers for the unfilled polymer. For both materials, the CO2/N2 selectivity was 14, at 35 oC and 3.4 atm. Nanoparticle filled poly(ethylene oxide) membranes reached permeabilities as high as 1700 barrer while maintaining a CO2/N2 selectivity of 25, at 35 oC and 3.4 atm. The degree of permeability enhancement is particle loading dependant, with maximum particle loading over 50 weight percent for some materials. Nanocomposites have been prepared with different polymer matrices (e.g., polar, non-polar, and crosslinked rubbery polymers) and different particle surface chemistries (e.g., MgO, SiO2, TiO2, etc.). These materials have been characterized using light gas sorption and permeation to monitor gas transport properties as well as SEM and TEM to characterize particle distribution within the polymer matrix.

Furthermore, nanocomposites with rubbery matrices often exhibit significantly improved gas transport behavior at low temperatures. Both light gas permeability and selectivity increases substantially with decreasing temperature. However, in some of these materials the gas transport enhancements are limited by the onset of nanoparticle-induced polymer crystallization, as characterized by permeation and DSC experiments.

[1] R. M. Barrer, J. A. Barrie and M. G. Rogers, Heterogenous Membranes: Diffusion in Filled Rubber, Journal of Polymer Science, Part A: Polymer Chemistry, 1 (1963) 2565-2586

Permeability Enhancement in Nanoparticle Filled Polymeric Membranes

S. Matteucci, The University of Texas at Austin
H. Lin, The University of Texas at Austin
B. Freeman, The University of Texas at Austin
V. Kusuma, The University of Texas at Austin
M. J. Yacaman, The University of Texas at Austin
Sumod Kalakkunnath, University of Kentucky
Douglass Kalika, University of Kentucky
A. Hill, CSIRO, Manufacturing & Infrastructure Technology

Traditionally, the addition of impermeable particles to rubbery polymeric membranes reduces light gas and vapor permeability as particle loading increases. This phenomenon is well known for barrier materials, and there are numerous models, such as one derived by Maxwell, that accurately predict permeability decrease in membranes filled with impermeable particles.[1]

Recently, nanoparticle filled polymers have been prepared that have over an order of magnitude higher light gas (i.e., CO2, N2, O2, H2) permeability with little or no change in selectivity relative to that of the unfilled polymer, which runs counter to traditional filled polymers. This phenomena has been observed in a broad range of polymeric materials, from high free volume stiff-chain polyacetylenes and crosslinked poly(ethylene oxide) to commodity materials such as 1,2-polybutadiene and poly(ethylene-co-1-octene). The degree of permeability enhancement is polymer and particle loading dependent, and our studies include a wide range of polymer and particle chemistries, including situations where the polymer and particle can react. Moreover, nanocomposite light gas permeability and selectivity are highly dependent on nanoparticle surface chemistry. The nanoparticles are nonporous and are primarily from the metal oxide family (MgO, SiO2, TiO2, etc.). Some of the particles are available as small as 2.5 nm primary particle diameter. These nanocomposites have been characterized using light gas sorption and permeation to monitor gas transport properties as well as SEM and TEM to characterize particle distribution within the polymer matrix.

Using appropriate nanoparticle and polymer combinations permits preparation of nanocomposite membranes that are over 90 weight percent nanoparticles. In such instances, it has been necessary to investigate fractional free volume and membrane void space behavior to characterize the structure of materials with such extremely high nanoparticle loadings.

[1] R. M. Barrer, J. A. Barrie and M. G. Rogers, Heterogenous Membranes: Diffusion in Filled Rubber, Journal of Polymer Science, Part A: Polymer Chemistry, 1 (1963) 2565-2586.

Nanoparticle-Induced Desilylation of Substituted Acetylene Polymers to Prepare Gas Separation Membranes with Exceptional Chemical Resistance

S. Matteucci, The University of Texas at Austin
R. Raharjo, The University of Texas at Austin
B. Freeman, The University of Texas at Austin
T. Sakaguchi, Kyoto University
T. Masuda, Kyoto University

Many membrane applications require separation of organic vapors from permanent gases.[1] Such separations include the purification of natural gas and hydrogen recovery in refineries. Potential membrane candidate materials include substituted polyacetylenes, which have permeation and selectivity properties that are desirable for organic vapor removal from permanent gases. For instance, the mixed gas n-butane/CH4 selectivity is 48 and the pure n-butane permeability is 80,000 barrer in poly(1-trimethylsilyl-1-propyne) (PTMSP).[2] Moreover, both n-butane/CH4 selectivity and n-butane permeability increase when surface treated fumed silica nanoparticles have been dispersed in the polymer.[2] The utility of substituted polyacetylenes for these applications is limited by poor membrane chemical stability towards organic liquids and vapors. PTMSP, poly[1-phenyl-2-[p-(trimethylsilyl)phenyl]acetylene] (PTMSDPA), poly(4-methyl-2-pentyne) and poly(methylacetylene) readily dissolve in industrially relevant organic components (toluene, hexane, etc.) that could be present in industrial feed streams to the membrane.[3-6] However, polyacetylenes such as poly(acetylene) and poly(diphenylacetylene) (PDPA) are insoluble in most organic solvents.[6,7] Currently, the only reported method for making PDPA is to desilylate PTMSDPA using trifluoroacetic acid, so these chemically stable materials cannot be prepared as membranes via conventional processing protocols. Although the resulting material is chemically stable, the permeability and n-butane/permanent gas selectivity decrease significantly.[8] We discovered a method for preparing partially desilylated polyacetylene nanocomposites. Basic nanoparticles (e.g., MgO), when dispersed in polymers such as PTMSDPA, remove trimethylsilyl groups from the polymer backbone. Small molecule compounds were also used to demonstrate the desilylation reaction. Then, the polyacetylenes were partially desilylated using nanoparticles. When possible, the products of the reaction were characterized using XPS, FTIR, and NMR. Gas transport properties were characterized. Interestingly, nanoparticle-desilylated polymers are insoluble in common hydrocarbon solvents, and they have higher gas permeability than the polymers before desilylation. This discovery permits the preparation of high permeability, high selectivity, chemically stable, reverse-selective membranes.

[1] R. W. Baker, Membrane Technology and Applications, McGraw-Hill, New York, 2000.

[2] T. C. Merkel, Z. He, I. Pinnau, B. D. Freeman, A. J. Hill and P. Meakin, Effect of Nanoparticles on Gas Sorption and Transport in Poly(1-Trimethylsilyl-1-Propyne), Macromolecules, 36 (2003) 8406-8414.

[3] T. Masuda, E. Isobe and T. Higashimura, Polymerization of 1-(Trimethyl)-1-Propyne by Halides of Niobium(V) and Tantalum(V) and Polymer Properties, Macromolecules, 18 (1985) 841-845.

[4] K. Tsuchihara, T. Masuda and T. Higashimura, Polymerization of Silicon-Containing Diphenylacetylenes and High Gas Permeability of the Product Polymers, Macromolecules, 25 (1992) 5816-5820.

[5] T. C. Merkel, B. D. Freeman, R. J. Spontak, Z. He, I. Pinnau, P. Meakin and A. J. Hill, Sorption, Transport, and Structural Evidence for Enhanced Free Volume in Poly(4-Methyl-2-Pentyne)/ Fumed Silica Nanocomposite Membranes, Chemical Materials, 15 (2003) 109-123.

[6] J. C. W. Chien, G. E. Wnek, F. E. Karasz and J. A. Hirsch, Electrically Conducting Acetylene-Methylacetylene Copolymers. Synthesis and Properties, Macromolecules, 14 (1981) 479-485.

[7] A. Niki, T. Masuda and T. Higashimura, Effects of Organometallic Cocatalysts on the Polymerization of Distributed Acetylenes by Tantalum Chloride and Niobium Clhoride, Journal of Polymer Science Part A: Polymer Chemistry, 25 (1987) 1553-1562.

[8] M. Teraguchi and T. Masuda, Poly(Diphenylacetylene) Membranes with High Gas Permeability and Remarkable Chiral Memory, Macromolecules, 35 (2002) 1149-1151.

Latest News from the Freeman Group August 2005

Scott Kelman and Scott Matteucci were both awarded prizes for their poster presentations at the North American Membrane Society in Providence, Rhode Island.

Roy Raharjo has returned from Kyoto University in Japan. He gave a presentation to the group about what he learnt during his time in Professor Toshio Masuda’s laboratories.

Alyson Sagle won a Ford Graduate Student Travel Award from the ACS PMSE division. She intends to use this to attending the upcoming ACS meeting in WAshington, DC. in August.

NAMS Conference, Providence, RI. 11 – 15 June 2005

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.

Scott Matteucci’s Abstract

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.

Haiqing Lin’s Abstract

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.

Latest News from the Freeman Group – May 2005

May 2005

Lauren Greenlee and Bryan McCloskey were both recipients of the National Science Foundation Graduate Research Fellowship. These fellowships are awarded to the top 1000 first or second year graduate students in all fields of science and engineering. Keith Ashcraft and Alyson Sagle achieved honorable mention in these awards. Our congratultions goes out to all of them. It is real honor to have such a number of people recognized.

Scott Kelman and Scott Matteucci were both awarded $1000 travel awards from the North American Membrane Society. They intend to use these awards to attend the NAMS 2005 annual meeting in Providence, Rhode Island.

Roy Raharjo is traveling to Kyoto University in Japan in June to spend 1 month in Professor Toshio Masuda’s laboratories. He will be learning about the synthesis of disubstituted polyacetylenes.

AIChE Conference, Austin, TX. 7 – 12 November 2004

AIChE Annual Conference 2004
Austin, Tx, 7th – 12th November
Abstracts of Attendees

Conor Braman’s Abstract

Water Transport and Fouling Properties of Crosslinked Poly(ethylene glycol)

C. Braman, University of Texas
B. Freeman, University of Texas
Teruhiko Kai, University of Texas
Frank Onorato, Pall Corporation
Rich Salinaro, Pall Corporation
Douglass S. Kalika, University of Kentucky
Sumod Kalakkunnath, University of Kentucky

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. Water uptake of the free-standing films increased by an order of magnitude as water concentration in the polymerization mixture increased.

Water vapor sorption in PEGDA films was conducted to examine the thermodynamics of water uptake and water diffusion in these materials. The water diffusion coefficient decreased significantly (by approximately one order of magnitude or more in some cases) as water activity increased. PEGDA films 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.

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.

Scott Kelman’s Abstract

Crosslinking PTMSP using Bis Azides and the Effects of Permeation Properties and Chemical Stability

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.

TThe 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 solubility of crosslinked PTMSP was similar to that of uncrosslinked PTMSP. 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 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 that of pure PTMSP. Soaking the crosslinked membrane in methanol and adding nanoparticles such as fumed silica were successfully 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.

Haiqing Lin’s Abstract

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
Sumod Kalakkunnath, University of Kentucky
Douglass S. Kalika, University of Kentucky

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 should 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 will present the 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 thus 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 measurements, positron annihilation lifetime spectroscopy (PALS), and glass transition temperature.

Scott Matteucci’s Abstract

Interaction of Basic Nanoparticles with Polyacetylenes and Their Effect on Gas Transport Properties

S. Matteucci, The University of Texas at Austin
B. Freeman, University of Texas at Austin

Due to growth in use of H2 for refining and fuel cell applications, there is growing interest in finding economically and industrially feasible methods of purifying H2. Currently H2 is produced from steam reforming of hydrocarbons, which produces byproducts such as CO2, H2O, and CO. Relative to current technologies for purifying H2, membranes are compact, modular, and can have low capital costs [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 pressure, there is significant interest in membranes that could remove minor components (e.g. CO2) and maintain H2 at high pressure. High free volume, stiff-chain, 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 is to use nanoparticles to selectively improve the solubility of CO2 in nanocomposite membranes, thereby increasing CO2 / H2 selectivity. 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 than those previously reported pure polymer or PTMSP/FS composites [2] (reaching CO2 permeability of 55,000 Barrers). Both polymer-particle and gas-particle interactions contribute to the altered transport properties. Increasing particle loading increases gas sorption in the nanocomposites, but there is a threshold concentration of particles below which the sorption enhancement is not observed. FTIR and XPS reveal a chemical reaction between the basic nanoparticles and PTSMP, which may account for the threshold. To determine the effects of the reaction on the dispersion of particles, AFM tapping mode experiments have been conducted. AFM phase profiles are qualitatively consistent with sorption and permeation trends observed in the nanocomposite membranes.

[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.

Roy Raharjo’s Abstract

The Effect of DEsilylation on Gas Sorption and Transport Properties in Poly[1-phenyl-2-[p-(trimethylsilyl)phenyl]acetylene] (PTMSDPA)

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
X. Wang, University of Texas at Austin
I. Sanchez, University of Texas at Austin

Poly[1-phenyl-2-[p-(trimethylsilyl)phenyl]acetylene] (PTMSDPA) is a highly permeable, glassy, disubstituted acetylene-based polymer. Like poly(1-trimethylsilyl-1-propyne) (PTMSP), this polymer is permeable to larger, more condensable hydrocarbons than to smaller, less condensable permanent gases. Desilylation was performed on this polymer to improve its chemical resistance. The resulting polymer, poly(diphenylacetylene) (PDPA), has a lower fractional free volume (0.23) and is insoluble in most common organic solvents (i.e., toluene, chloroform, hexane). The pure gas permeation and sorption properties of various light gases and hydrocarbons in PTMSDPA and PDPA at 35oC are reported and compared to those in other disubstituted polyacetylenes. A significant decrease in the gas permeability values, most likely due to the decrease in the FFV, is observed after the desilylation. For example, the permeability of nitrogen is reduced almost 50%, from 640 to 280 Barrer. The permeability of n-butane is reduced even more, from 16000 to “only” 2400 Barrer. The effect of desilylation on the gas permeability coefficient, solubility coefficient, and diffusion coefficient in the polymer is further discussed. In addition, the result of the aging study of the two polymers, PTMSDPA and PDPA, is also reported.

ACS Conference, Philadelphia, PA. 22 – 26 August 2004

ACS 2004 Conference
Philadelphia, Pa, 22 – 26th August
Abstracts of Attendees

Haiqing Lin’s Abstract

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

Polymeric membranes are used in many applications, including gas separations, due to inherently low energy requirements for molecular scale separations. Hydrogen, a potential future energy source, is usually 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 of reverse selectivity is opposite to that exhibited by the vast majority of polymers. Here, polar, rubbery, branched hydrogels photopolymerized from poly(ethylene glycol) diacrylate (PEGDA) and poly(ethylene glycol) methyl ether acrylate (PEGMEA) are shown to have outstanding performance for acid/polar gas removal from H2. Surprisingly, introduction of methyl ether chain ends (i.e., PEGMEA) markedly improves CO2/H2 selectivity from 19 to 40 and CO2 permeability from 6.6 to 52 Barrers at -20oC; these materials exhibit the best separation performance reported to date for solid non-facilitated transport membranes.

Gordon Research Conference, New London, NH. 1-6 August 2004

Gordon Research Conference 2004
New London, NH, 1 – 6th August 2004
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.

For the freestanding films, data on water flux and fouling resistant properties will be presented. Polymerization induced phase separation (PIPS) and its relevance to polymer transport properties will be discussed as well.

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 ideally 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. This strategy is still in the development stage, and the challenges therein will be discussed. Preliminary 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.

Hao Ju’s Abstract

Membrane Performance Modification via Mechanical Stretching

B. McCloskey, University of Texas at Austin
H. Ju, University of Texas at Austin
B. Freeman, University of Texas at Austin
D. Lloyd, University of Texas at Austin
D. Lawler, University of Texas at Austin
L. Worrel, University of Texas at Austin
J. Moorhouse, University of Texas at Austin
Y. Wu, University of Texas at Austin

One of the primary concerns in the application of microfiltration and ultrafiltration membranes is membrane fouling. There are numerous membrane properties (and foulant properties) that have an affect on fouling mechanisms and subsequently membrane performance. The literature has suggested that a membrane’s pore aspect ratio (defined as the ratio between the pore’s major and minor axis) has an affect on certain bacteria attachment to poly(ether sulfone) (PES) membranes. Therefore, by physically controlling the aspect ratio of membrane pores (i.e. through a membrane stretching process), the fouling characteristics of the membrane can be modified. The purpose of the research presented here is to look at the effect that membrane pore geometry has on membrane performance (most notably flux and rejection) when filtering solutions containing various foulants.

In this study, ~0.2 mm mean pore diameter phase inversion PES membranes were used to purify three solutions in dead-end and crossflow filtration. One can see from the data that uniaxially stretching these membranes to 150% of their original length increases pure water flux. Stretching also increases flux and reduces flux decline, but has little effect on rejection for feed solutions containing 1g/L BSA. Stretching has little effect on flux, flux decline, and rejection for feed solutions containing oil-water emulsions at 1500 ppm. After small amounts of time, stretching reduces flux and rejection for the microsphere solution, but the flux and rejection for stretched and unstretched samples converge to a single value for longer operating times.

Bryan McCloskey’s Abstract

Membrane Performance Modification via Mechanical Stretching

B. McCloskey, University of Texas at Austin
H. Ju, University of Texas at Austin
B. Freeman, University of Texas at Austin
D. Lloyd, University of Texas at Austin
D. Lawler, University of Texas at Austin
L. Worrel, University of Texas at Austin
J. Moorhouse, University of Texas at Austin
Y. Wu, University of Texas at Austin

One of the primary concerns in the application of microfiltration and ultrafiltration membranes is membrane fouling. There are numerous membrane properties (and foulant properties) that have an affect on fouling mechanisms and subsequently membrane performance. The literature has suggested that a membrane’s pore aspect ratio (defined as the ratio between the pore’s major and minor axis) has an affect on certain bacteria attachment to poly(ether sulfone) (PES) membranes. Therefore, by physically controlling the aspect ratio of membrane pores (i.e. through a membrane stretching process), the fouling characteristics of the membrane can be modified. The purpose of the research presented here is to look at the effect that membrane pore geometry has on membrane performance (most notably flux and rejection) when filtering solutions containing various foulants.

In this study, ~0.2 mm mean pore diameter phase inversion PES membranes were used to purify three solutions in dead-end and crossflow filtration. One can see from the data that uniaxially stretching these membranes to 150% of their original length increases pure water flux. Stretching also increases flux and reduces flux decline, but has little effect on rejection for feed solutions containing 1g/L BSA. Stretching has little effect on flux, flux decline, and rejection for feed solutions containing oil-water emulsions at 1500 ppm. After small amounts of time, stretching reduces flux and rejection for the microsphere solution, but the flux and rejection for stretched and unstretched samples converge to a single value for longer operating times.

Rajeev Prabhakar’s Abstract

Not Available

Alyson Sagle’s Abstract

Characterization of Commercial Reverse Osmosis Membranes

A. Sagle, University of Texas at Austin
B. Freeman, University of Texas at Austin

Reverse osmosis (RO) membranes have been used for several decades to purify water. Most commercial RO membranes today are thin film composites of crosslinked polyamides on a polysulfone support. The membranes used in this work, GE Osmonics AG and AK RO membranes, are thin film composites of this nature. Work was done to characterize the performance of these materials. As a starting point, water flux measurements in dead-end cells were done with phosphate buffered water of pH 7.4. Fouling by oil was also investigated in dead-end cells by measuring the flux and rejection from an oil/water emulsion. It was found that the surfactant used to create the emulsion was also contributing to membrane fouling. Further oil/water fouling studies will be conducted with a different surfactant. Protein fouling and rejection was also tested in both dead-end and crossflow configurations. The RO membranes showed excellent rejection of protein with a nominal decrease in flux in a crossflow configuration.

In addition to fouling studies, chemical stability of the membranes was also tested. Polyamide membranes have shown that exposure to even small levels of chlorine greatly reduces their performance. Chlorine is often used in water treatment processes as a disinfectant or for membrane cleaning. Studies were done to examine the chemistry behind the degradation and the effects of the amount of chlorine exposure on the membrane performance. FTIR analysis showed that the polyamide underwent ring chlorination as the literature suggests. This basic work characterizing commercial membranes lays a foundation for future research into the modification of membranes to reduce fouling. Possibilities being considered for modification include grafting molecular branches onto the membrane surface. A second method could be applying a hydrophilic thin coating to the surface. This can be done via several methods (i.e. hydrogen abstraction, interfacial reactions, or redox reactions). A wide range of materials are available for use in modifying membrane surfaces, however factors such as mechanical strength, hydrophilicity, and functionality all must be considered.

Yuan-Hsuan Wu’s Abstract

Membrane Performance Modification via Mechanical Stretching

B. McCloskey, University of Texas at Austin
H. Ju, University of Texas at Austin
B. Freeman, University of Texas at Austin
D. Lloyd, University of Texas at Austin
D. Lawler, University of Texas at Austin
L. Worrel, University of Texas at Austin
J. Moorhouse, University of Texas at Austin
Y. Wu, University of Texas at Austin

One of the primary concerns in the application of microfiltration and ultrafiltration membranes is membrane fouling. There are numerous membrane properties (and foulant properties) that have an affect on fouling mechanisms and subsequently membrane performance. The literature has suggested that a membrane’s pore aspect ratio (defined as the ratio between the pore’s major and minor axis) has an affect on certain bacteria attachment to poly(ether sulfone) (PES) membranes. Therefore, by physically controlling the aspect ratio of membrane pores (i.e. through a membrane stretching process), the fouling characteristics of the membrane can be modified. The purpose of the research presented here is to look at the effect that membrane pore geometry has on membrane performance (most notably flux and rejection) when filtering solutions containing various foulants.

In this study, ~0.2 mm mean pore diameter phase inversion PES membranes were used to purify three solutions in dead-end and crossflow filtration. One can see from the data that uniaxially stretching these membranes to 150% of their original length increases pure water flux. Stretching also increases flux and reduces flux decline, but has little effect on rejection for feed solutions containing 1g/L BSA. Stretching has little effect on flux, flux decline, and rejection for feed solutions containing oil-water emulsions at 1500 ppm. After small amounts of time, stretching reduces flux and rejection for the microsphere solution, but the flux and rejection for stretched and unstretched samples converge to a single value for longer operating times.

NAMS Conference, Honolulu, HI. 26 – 30 June 2004

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.

Scott Kelman’s Abstract

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.

Haiqing Lin’s Abstract

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.

Scott Matteucci’s Abstract

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.

Rajeev Prabhakar’s Abstract

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.

Roy Raharjo’s Abstract

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.