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Dr. Freeman wins the University CO-OP’s Hamilton Award

On March 28, 2007, Professor Freeman (center) accepted the University CO-OP’s Hamilton Award for the Best Research Paper published by a UT Faculty member in 2006. This honor, given to only one UT faculty member per year, was in recognition of the paper published by Dr. Freeman’s group in Science: Lin, H., E. van Wagner, B.D. Freeman, L.G. Toy, and R.P. Gupta, “Plasticization-Enhanced H2 Purification Using Polymeric Membranes,” Science, 311(5761), 639-642 (2006).

 Professor Freeman (center)
Professor Freeman (center)

ACS Annual Conference, San Francisco, CA. 10 – 14 September 2006

ACS 2006
San Francisco, CA, 10-14 September
Preprints of Attendees

Bryan McCloskey’s Abstract

Amphiphilic Graft Copolymers as Antifouling Coatings for Water Purification Membranes

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Hao Ju’s Abstract

Synthesis and Characterization of Surface Coated Ultrafiltration Membranes to Enhance Oil/Water Fouling Resistance

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Hobum Park’s Abstract

Water and Salt Transport Behavior through Hydrophilic-Hydrophobic Copolymer Membranes and Their Relations to Reverse Osmosis Membrane Performance

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Roy Raharjo’s Abstract

A Fundamental Study of Mixture Permeability, Solubility, and Diffusivity in Vapor Selective Polymers

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Scott Kelman’s Abstract

Enhancing the Chemical Resistance of High Fractional Free Volume Vapor-Selective Polymers

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Xiaoyan Wang’s Abstract

Effect of Free Volume on Gas Sorption in Polymeric Membranes

Xiao-Yan Wang, University of Texas at Austin
B. Freeman, University of Texas at Austin
I. C. Sanchez, University of Texas at Austin
A. J. Hill, CSIRO-CMIT, Australia

Poly(1-trimethylsilyl-1-propyne) (PTMSP), a solubility selective membrane, has the potential to be used in the natural gas separation. PTMSP ages rapidly as evidenced by the density increase with time; then the free volume decreases with time. Cavity size (free volume) distributions of PTMSP are calculated at different densities using an energetic based cavity-sizing algorithm. Free volume of PTMSP decreases with increasing densities. We also simulate solubility of CO2 and CH4 in PTMSP at different densities. Solubility shows no significant change as the density increases. This result is in contradictory with the prediction from the scale particle theory, but it is in agreement with experimental observation.

Key word: Solubility, free volume, molecular simulation, scale particle theory.

NAMS Annual Conference, Chicago, IL. 13 – 17 May 2006

North American Membrane Society Annual Meeting 2006
Chicago, IL, 13th – 17th May
Abstracts of Attendees

Brandon Rowe’s Abstract

Tracking Physical Aging of Thin Glassy Polymer Films by Ellipsometry

B. Rowe, University of Texas
D. Paul, University of Texas
B. Freeman, University of Texas

Current gas separation membranes are typically formed from glassy polymers because of their exceptional permeability-selectivity properties. Glassy polymers are non-equilibrium materials whose properties (e.g., density, permeability, etc.) spontaneously, but usually slowly, evolve over time towards an equilibrium state. This process is known as physical aging. Interestingly, the physical aging rate becomes orders of magnitude more rapid if the thickness of the film is decreased below about one micron[1]. This phenomenon is an intrinsically fascinating scientific issue, and understanding physical aging is valuable for the gas separation industry.

Commercially used membranes for gas separation have a thickness of approximately 0.1 microns or less and may continue to decrease as the technology for making these films develops. Films ranging from 400 nm to 1000 nm have recently been shown to have rapid and thickness dependent aging rates as determined by gas permeability measurements. In addition to studying the gas transport properties of polymer films, physical aging is being examined by tracking the change in density of polymer films over time using high precision ellipsometry. Ellipsometry is a sensitive optical technique for determining properties of surfaces and thin films by measuring the changes in the polarization state of light when it is reflected from a sample. The refractive index of a polymer sample, measured by ellipsometry, is related to the polymer density through the Lorentz-Lorenz equation. Studies show an increase of polymer density with time, which correlates with the reduction in size and/or concentration of free volume elements in the sample. The effect of humidity on ellipsometry measurements has been studied and a direct relationship between refractive index and relative humidity has been found. The influence of humidity on physical aging has also been investigated.

1. Huang, Y. and D.R. Paul, Physical aging of thin glassy polymer films monitored by gas permeability. Polymer, 2004. 45(25): p. 8377-8393.

Hobum Park’s Abstract

Novel Sulfonated Poly(arylene ether) Copolymer Membranes and Their Use in Reverse Osmosis: Fundamental Water and Salt Transport Study, Chlorine Stability and Anti-Fouling Characteristics

H. Park, University of Texas at Austin
B. Freeman, University of Texas at Austin
J.E. McGrath, Virginia Tech University
Z. Zhang, Virginia Tech University
G. Fan, Virginia Tech University
M. Sankir, Virginia Tech University
A. Roy, Virginia Tech University
H.S. Lee, Virginia Tech University
A Badami, Virginia Tech University

Recently, the interest in preparing new RO materials having excellent chlorine tolerance, similar or higher water flux and rejection and markedly reduced fouling relative to the conventional RO membranes is increasing. In the late 1980s, a few sulfonated polymers were reported to have promising RO properties and excellent chlorine tolerance, but the lack of control of the sulfonation process made the preparation of reproducible the materials challenging1. Recently, a systematic series of sulfonated poly(arylene ether) copolymer membranes developed at Virginia Tech have been prepared by directly copolymerizing a disulfonated monomer into the polymer backbone2. The present study focuses on exploring fundamental structure-property relationships in these materials. These polymers have excellent chlorine tolerance, low fouling in oily or protein-containing waters, and high water flux with moderate salt rejection. A fundamental study of salt diffusivity, solubility and permeability in these sulfonated copolymer membranes is providing basic information useful for molecular engineering these materials towards better property profiles.

1. R.J. Petersen, “Composite Reverse Osmosis and Nanofiltration Membranes” J. Membrane Sci. (1993), 83, 81-150. 2. M.A. Hickner, H. Ghassemi, Y.S. Kim, B. Einsla and J.E. McGrath, “Alternative Polymer Systems for Proton Exchange Membranes”, Chem. Rev. (2004), 104, 4587-4611.

Roy Raharjo’s Abstract

A Fundamental Study of Mixture Permeability, Solubility, and Diffusivity in Vapor Selective Polymers

R. Raharjo, University of Texas at Austin
B. Freeman, University of Texas at Austin
E. Sanders, Air Liquide, Newport, DE

Membrane separation technology has recently emerged as a potential alternative technique to remove higher hydrocarbons (C3+) from natural gas1. The material that could be suitable, and, in fact, is currently being considered, for such separation is silicone rubber or poly(dimethylsiloxane) (PDMS). Aside from its high gas permeabilities (i.e., highest among rubbery polymers), this polymer is more permeable to the larger, more condensable higher hydrocarbon than to smaller, less condensable permanent gases, such as methane. As a result, methane, the major constituent in natural gas, can be kept at high pressure, which eliminates the cost of recompression that would be incurred if a methane-selective membrane were used for this separation.

The gas sorption and transport properties in PDMS have been studied in the past. Most of the references in the literature, however, report only its pure gas properties; only a few actually investigate its mixture properties. To accurately estimate the membrane separation performance, the use of mixtures in the study is required. This paper presents the n C4H10/CH4 mixed gas permeability, solubility, and diffusivity in PDMS at various temperatures from -20oC to 50oC. The dilation isotherms of the mixture are also reported to complement the mixture sorption data.

The CH4 permeability, solubility, and diffusivity increase as the n C4H10 fugacity in the mixture increases. The n C4H10 sorption and transport properties in mixtures are unaffected by the presence of CH4 and are similar to those in pure gas conditions. The overall n C4H10/CH4 selectivity increases as n C4H10 activity in the mixture increases and temperature decreases. This report represents the first combined presentation of gas mixture permeability, solubility, and diffusivity in PDMS, which is the most widely used vapor separation polymer.

Another polymer whose properties will be discussed is poly(1-trimethylsilyl-1-propyne) (PTMSP), an ultra-high free volume, solubility selective, glassy polymer. These studies will help illustrate the fundamental mechanism of the competitive permeation, sorption, and diffusion in glassy polymers.

Xiaoyan Wang’s Abstract

Physical aging in poly[1-(trimethylsilyl)-1-propyne] (PTMSP)membranes: Effects on free volume and gas permeability

X. Wang, University of Texas at Austin
R. Raharjo, University of Texas at Austin
B. Freeman, University of Texas at Austin
I. Sanchez, University of Texas at Austin

Poly(1-trimethylsilyl-1-propyne) (PTMSP), the most permeable polymer known, is a fast aging material. The permeability of PTMSP to gases and vapors decreases dramatically with physical aging. Cavity size (free volume) distributions were calculated in as-cast and aged PTMSP using an energetic based cavity-sizing algorithm. The large cavities found in as-cast PTMSP disappear in aged PTMSP, which is consistent with the positron annihilation lifetime spectroscopy (PALS) measurements. We also characterized the connectivity of cavities both in as-cast and aged PTMSP membranes. Cavities are more connected in as-cast PTMSP than in aged PTMSP. The average cavity sizes calculated from computer simulation are in good agreement with PALS measurements. The transport and sorption properties of gases in as- cast and aged PTMSP are also measured by molecular simulation. Computer simulations showed the decrease of permeability and the increase of permeability selectivity in PTMSP membranes with physical aging, which agrees withe experimental observations. The reduction in gas permeability with physical aging results mainly from the decrease of diffusion coefficients. Solubility coefficients show no significant changes with physical aging.

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.