Meetings & Events

Spring 1999 logo1999 MRS Spring Meeting & Exhibit

April 5-9, 1999 | San Francisco
Meeting Chairs: Katayun Barmak, James S. Speck, Raymond T. Tung, Paul D. Calvert



Symposium GG—Membranes

Chairs

              Dibakar Bhattacharyya              C. Jeffrey Brinker
                Dept of Chemical & Materials Engr        Dept of Ceramic Synth & Inorg Chem
                Univ of Kentucky                         Sandia National Labs
                Lexington, KY 40506                      Albuquerque, NM 87106
                606-257-2794                             505-272-7627

              David Needham
                Dept of Mech Engr & Matls Sci
                Duke Univ
                Hudson Hall
                Durham, NC 27708-0300
                919-660-5355

 

 * Invited paper

SESSION GG1: INORGANIC MEMBRANES
Chair: C. Jeffrey Brinker
Tuesday Morning, April 6, 1999
Salon 14 (M)
8:30 AM GG1.1
TEMPERATURE AND PRESSURE DEPENDENCE OF LIGHT GAS MIXTURE PERMEATION THROUGH SAPO-34 MEMBRANES. Joseph C. Poshusta, Eric A. Pape, Vu A. Tuan, John L. Falconer, Richard D. Noble , Univ of Colorado, Dept of Chemical Engineering, Boulder, CO.

Zeolite membranes are composed of inorganic microporous crystals that are attractive for gas separation because their pore sizes are uniform and typically less than 1 nm. Because zeolites are inorganic, they have excellent thermal stability and chemical resistance making them an ideal material as membranes for the separation of gas mixtures in a variety of applications. The silicoaluminophosphate analog of chabazite, SAPO-34, is an attractive membrane material for the separation of light gases, because the pore size of SAPO-34 crystals (0.4 to 0.45 nm) is close to the kinetic diameters of light gases. Continuous layers of SAPO-34 crystals were synthesized on the inside surface of porous alumina support tubes. Single gas and binary mixture permeances were measured as functions of temperature and pressure with pressure drop as the driving force for diffusion (i.e. no sweep gas was used). Single gas permeation through SAPO-34 membranes decreased with molecular size for H2, CO2, N2, CH4, and n-C4H10. The permeance of CO2 decreased as pressure increased with a constant pressure drop, and this behavior is consistent with the surface diffusion mechanism. Binary mixtures of CO2/CH4, N2/CH4, and H2/CH4 were separated using SAPO-34 membranes supported on porous alumina tubes. The room temperature CO2/CH4 composition based selectivity was 5.5 at 270 kPa feed pressure with a 138 kPa pressure drop. Below room temperature (229 K), the CO2/CH4 selectivity increased to 7.5. The room temperature selectivities of N2/CH4 and H2/CH4 were 1.8 and 3.8, respectively. Increasing temperature and pressure decreased the CO2/CH4 and N2/CH4 selectivities, but the H2/CH4 selectivity was constant up to 470 K, and decreased to 3.4 at 520 K. The temperature dependence of the CO2/CH4 and N2/CH4 selectivities is consistent with separation by competitive adsorption. Size selectivity controls the H2/CH4 separation, however, because high selectivity is observed at 520 K.

8:45 AM GG1.2
MICROPOROUS SILICA MEMBRANE FOR CO2 REMOVAL FROM NATURAL GAS. Chung-Yi Tsai 1, Siu-Yue Tam1, C. Jeffrey Brinker2, The University of New Mexico, NSF Center for Micro-Engineered Materials1,2, Sandia National Laboratories2, The Advanced Materials Laboratory, Albuquerque, NM.

For the removal of carbon dioxide from natural gas prior to delivery to a pipeline, membranes are more advantageous than conventional amine adsorption processing for use on offshore platforms, high CO2 content wells, small gas streams or environmentally sensitive areas. Unlike conventional polymeric membranes, porous silica membranes with higher permeance and selectivity are applicable at high wellhead pressures and do not suffer from CO2 plasticization, representing an alternative for CO2/CH4 separation. We processed ultramicroporous silica membranes with reproducible high permeances and high separation factors (3 x 10-4 cm3(STP)/s/cm2/cmHg and 160, respectively) exceeding that of the best-known membranes for separation of a 50/50 (v/v) CO2/CH4 gas mixture. The asymmetric membrane, composed of three major layers (ultramicropore layer, mesopore layer, and macropore layer) is further simulated by a two-dimensional mathematical model. This model, accounting for mass transfer in both the solid and the gas phases, is used to assess the influence of a number of operating parameters (temperature, pressure, and feed flow rates) for a tube-and-shell configuration, membrane separator. The model predictions compared to experimental results will be addressed in this presentation. This work was performed at Sandia National Laboratories and supported by the U.S. Department of Energy under Contract No. DE-AC04-94AL85000 and the Electric Power Research Institute.

9:00 AM *GG1.3
SYNTHESIS AND PERMEATION PROPERTIES OF CERAMIC-SUPPORTED ORIENTED MFI-TYPE MEMBRANES. Michael Tsapatsis , George Xomeritakis Department of Chemical Engineering, University of Massachusetts, Amherst, MA.

The technique of secondary (seeded) growth has been used to prepare highly intergrown and oriented MFI-type membranes with thickness in the range 5-30 micron on the surface of a-Al2O3 porous support disks. Initially the porous support surface is coated with colloidal silicalite-1 seed nanocrystals ( 100 nm in diameter) by dip coating the disks in a stable aqueous suspension of the nanocrystals. Subsequently the disks are brought in contact with a clear synthesis solution under hydrothermal conditions at various temperatures and times for the purpose of regrowing the seed nanocrystals to a continuous zeolite layer covering the entire support surface. The separation potential of the membranes was studied by single-component and binary permeation experiments using N2, SF6 and butane isomers. For the most successful preparations, the N2:SF6 single-component ratio was above Knudsen but rather modest (8-10) while the permeation flux of n-C4H10 and the n-C4H10:i-C4H10 ratio from 50/50 mixtures at 22°C was in the range 1.5-4.5x10-3 mol m-2 s-1 and 25-60, respectively. The n-C4H10:i-C4H10 ratio remained high up to about 100-140°C but decreased drastically to <10 at higher temperatures. The permeation results indicate that the membranes at hand exhibit shape-selective properties comparable to other in-situ grown MFI-type zeolite membranes reported in the literature.

9:30 AM GG1.4
DEFECT FREE MOLECULAR SIEVE COMPOSITE MEMBRANES. Tina M. Nenoff , Steven G. Thoma, Daniel Trudell, Alejandra V. Chavez, Phillip Pohl, Sandia National Labs, Albuquerque, NM.

Inorganic thin film membranes for catalysis and separations processes have been prepared by direct growth of molecular sieve crystals onto alumina supports and metal oxide disks. Molecular modeling methods have been employed to predict and design pore size and shapes for shape selectivity. Silica based systems employed a composite matrix of crystalline and amorphous coatings that create a defect free membrane. The choice of amorphous coating is highly dependent upon substrate, due to adhesion factors. The resulting membrantes consist of mono- to bi-layer, intergrown oriented crystals. Zinc based systems are synthesized under mild conditions which result in defect free membranes. Permeation results on the silica- and zinc-based membranes indicate molecular sieving capabilities for small gas molecules and organic molecules, respecively.

9:45 AM GG1.5
``ZEOLITE-LIKE'' SOL-GEL SILICA MEMBRANE. Sujit Naik 1, Chung-Yi Tsai1, C. Jeffrey Brinker1,2, Roger Assink2; 1The University of New Mexico, NSF Center for Micro-Engineered Materials and 2Sandia National Laboratories, Advanced Materials Laboratory, Albuquerque, NM.

A `molecular templating' technique was used for creating microporous amorphous silica with pore size comparable to that of zeolite ZSM-5. The tetrapropylammonium (TPA) cation, used as a structure-directing agent in ZSM-5 synthesis, was incorporated as a template within an inorganic sol-gel silica matrix and subsequently removed to create pores with molecular dimensions (pore size $\sim$ 5-6 $\AA$). This approach exploits the high drying stresses that develop in sol-gel silica to form organic-inorganic hybrid materials that upon calcination result in microporous silica. We describe the synthesis and characterization of microporous silica membranes for separation of isomers such as n-butane and isobutane. Microporous TPA-templated sol-gel silica was characterized using gas adsorption, TGA and XRD. 1H-29Si CP NMR and 2H NMR were used to determine the efficiency of templating in the organic-inorganic hybrid materials. The corresponding membranes are ultrathin, defect-free and have excellent processability. Single gas permeation experiments indicate that such membranes exhibit a molecular sieving behavior. The membranes show a high flux of 2$\times$10-4 cm3(STP)/s/cm2/cm-Hg for n-butane and ideal selectivity of 450 for N-2/SF-6 and 20 for n-butane/isobutane at 80$^{\circ}$C. Dual gas permeation experiments with a n-butane/isobutane mixture gave results consistent with the single gas measurements.

10:30 AM GG1.6
IN-SITU COUNTER DIFFUSION CVD FOR PORE SIZE CONTROL OF INORGANIC POROUS MEMBRANE. Yasuyuki Egashira , Nobuhiko Mori, Yasushi Funakoshi and Korekazu Ueyama, Osaka Univ., Graduate School of Engineering Science, Division of Chemical Engineering, Osaka, JAPAN.

Combing a chemical vapor deposition (CVD) and catalytic reaction, a novel pore size control method for inorganic porous membrane, so called ${\it in-situ }$ counter diffusion CVD is proposed and its mechanism is modeled theoretically and availability examined experimentally. Basically, ${\it in-situ }$ counter diffusion CVD is one of the contour diffusion CVD, where two reactant gases, highly reactive each other, diffuse from both sides of the porous membrane and react inside the membrane, and high reactivity of the reactants results a thin deposition region. Counter diffusion CVD method has advantage of thin deposition region with low resistance for permeation, however, this method requires somewhat complicated reactor configuration, especially, separated feed of two reactants is necessarily. In ${\it in-situ }$ counter diffusion CVD, catalytic reaction is utilized to produce one of the reactant gas from source gases which do not react with the other reactant gas. On one side of the membrane covered to avoid gas diffusion and catalyst inserted between this cover and membrane. Then, source gases for the reactant diffuse from the other side of the membrane to produce the reactant gas over catalyst at covered side of the membrane, and this reactant back diffuses to react with the other reactant gas. In such configurations, counter diffusion CVD can achieved without feed separation. A reactor model of this ${\it in-situ }$ counter diffusion CVD method was developed and it describes relationship between reaction condition and deposition position. Availability of this CVD method was demonstrated experimentally applying to TiCl4/H2/CO2 reaction system. Using Cu based catalyst, H2O produced from H2 and CO2, then with TiCl4 to deposit TiO2 within porous media, such as Vycor Glass.

10:45 AM GG1.7
MOLECULAR SIEVE MEMBRANES VIA LASER ABLATION. Kenneth J. Balkus, Jr. , Ashley Scott, Trindad Munoz, University of Texas at Dallas, Department of Chemistry, Richardson, TX.

Pulsed laser ablation has been employed to deposit thin films of nanoporous metal oxides onto porous substrates. After a brief hydrothermal treatment highly crystalline continuous and some times oriented molecular sieve membranes are obtained. Recent results for the preparation of oriented silica molecular sieves UTD-1 and MCM-41 on stainless steel frits will be presented. The separation of linear paraffins and aromatics by a UTD-1 membrane shows enhanced permeation of the alkanes. Results using Ti-UTD-1 in a catalytic membrane reactor will also be presented.

11:00 AM GG1.8
MICROFABRICATED INORGANIC MEMBRANES FOR PROTEIN SEPARATION AND IMMUNOISOLATION. Tejal A. Desai , Dept. of Bioengineering, University of Illinois at Chicago, IL; Derek Hansford and Mauro Ferrari, University of California, Berkeley, CA.

The application of microfabrication technology to create precise separation and isolation membranes for biomedical applications is described. By utilizing fabrication techniques commonly employed in the microelectronics industry (MEMS), membranes were fabricated with well-controlled and uniform pore sizes, allowing the optimization of membrane parameters for cell immunoisolation and viral filtration. The membrane-forming process employs bulk and surface micromachining to define chambers within single crystalline silicon wafers that interface with the surrounding biological environment through polycrystalline silicon filter membranes with thicknesses less than ten microns. Pore sizes down to 18 nanometers have been attained through deposition and subsequent etching of sacrificial layers. Membranes were fabricated to present a high density of uniform pores to allow sufficient permeability to biomolecules such as oxygen, glucose, and insulin. The ability of the membranes to exclude the passage of larger proteins such as immunoglobulin G and albumin has also been characterized. The semipermeability of microfabricated membranes, their biocompatibility, ease in sterilization, along with their thermal and chemical stability, may provide an significant advantages for biomedical applicaions. Microfabrication technology may be applied to other materials of interest for the development of highly controlled membranes.

11:15 AM GG1.9
STEAM-RESISTANT $\gamma$-ALUMINA MEMBRANES. Arian Nijmeijer, Henk Kruidhof, Henk Verweij and Manon Timmerman-Oude Wolbers , Inorganic Materials Science, University of Twente, Enschede, NETHERLANDS.

For application in process industry membranes which are stable against aggressive environments are needed. State of the art mesoporous $\gamma$-alumina membranes suffer heavily from steam atmospheres. Phenomena like pore-growth and moreover blistering off of the mesoporous layer occur during steam-treatment. This makes these membranes unsuitable for many industrial applications. With the use of an anchoring agent this problem can be largely solved.

11:30 AM GG1.10
FABRICATION OF UF MEMBRANES BY LAMINATION TECHNIQUE USING NANOPARTICLE DERIVED CERAMIC FOILS. R. Nonninger , B. Walter, O. Binkle, H. Schmidt, Institut fuer Neue Materialien gem. GmbH, Saarbruecken, GERMANY.

A fabrication techniques of flat ceramic ultrafiltation membranes has been developed which is based on two steps, first a tape casting step to produce a porous thin film undependently of the substrate, followed by laminating the free-standing film on to the top of the porous ceramic substrate in order to fabricate an asymmetric ultrafiltration ceramic membrane module. The ultrathin ($\le$ 15 $\mu$m) tape casted ceramic films contained 16 wt.$\%$ binder and could be directly laminated on an already sintered porous Al2O3 support with a average pore size of 3 $\mu$m without using any intermediate layers. For the ceramic tape fabrication a tape casting process using a nanoscaled zirconia powder which particle size ranges between 20 and 50 nm has been developed. In order to obtain stable aqueous slurries with a solid content of 50 wt.$\%$ the surface of the ZrO2 powder had to be modified with a carboxylic acid in order to stabilize the zirconia particles electrostatically. A minimum of the viscosity (20 mPa$\cdot$s) and a minimum of the particle size distribution (d50: 55 nm) was achieved with 4 wt.$\%$ of the surface modifier. The laminated ZrO2 tapes were sintered 2 h at 1000$^{\circ}$C on the porous Al2O3 supports. In correlation of the investigated slurry properties a homogeneous sintered microstructure of the UF-membrane with a narrow pore size distribution (average diameter 30 nm) and a porosity of 54$\%$ were obtained. The 1-20 $\mu$m thick sintered zirconia layer is characterized by a defect free top layer. The develloped synthesis route is a usefull techniques to produce flat ceramic UF elements with ultrathin active membranes.

11:45 AM GG1.11 A CO2 GAS SENSOR OF La2O3 - LOADED SiO2-SnO2 MEMBRANE BY SOL-GEL TECHNIQUE. Nishengliang , Chen Yuquan, Luo Wei, Pan Min, Dept. of BME, Zhejiang University, Hangzhou, P.R.CHINA.

Detecting the concentration of CO2 at ppm level by a simple method is desirable not only in various industrial processes but also in environmental technology. The preparation of La2O3 - loaded SiO2 - SnO2 membrane materials, which starting materials are SnCl4 and tetraethyl orthosilicate (TEOS), used the sol-gel technique, is described. The CO2 sensitive properties of the SiO2 - SnO2 element, effected by the sol-gel processing and its different microstructures, La2O2 - loaded amount and the processing gas conditions, were investigated. The results show that the La2O3 - loaded SiO2 - SnO2 membrane gas sensor has quick response to ppm-levels CO2, and the sensitivity monotonically increases with the concentration of dry CO2 in range 300-3000 ppm. Here we have studied the sensitive property of the La2O3 - loaded SiO2 - SnO2 element by sol-gel technique in more detail.
 
 
 

SESSION GG2: POLYMER MEMBRANES
Chair: Dibakar Bhattacharyya
Tuesday Afternoon, April 6, 1999
Salon 14 (M)
1:30 PM *GG2.1
GAS SEPARATION MEMBRANE MATERIALS: NEW DEVELOPMENTS AND FUTURE NEEDS. W.J. Koros , Chemical Engineering Department, The University of Texas at Austin, TX.

The lack of fundamental materials science understanding often prevents the use of a membrane-based approach for solving important practical problems. Given this fact, the minimal interaction between the traditional membrane and materials science communities is surprising. One inhibiting factor that complicates effective interaction between the two communities is the diversity of potential applications of membrane technology. While this diversity makes membranes an exciting field, it also complicates the identification of which problems should be given priority attention by the materials science community. The membrane-based gas separation sub-area will be used to illustrate a recommended strategy for more active participation by the materials science community across the board in the membrane field. To narrow down the diverse array of areas for analysis, market size was used as a measure to select three important applications: air separation, natural gas separation and olefin-paraffin separations. Key challenges will be identified in each case, and constraints currently preventing unfettered approaches to meet these challenges will be discussed. Traditional organic polymer membranes, liquid membranes and even molecular sieving membranes are candidates for these applications. By better understanding the differences and similarities between these three applications, competitive technologies, and the basis for current constraints in membrane formation, avenues around these constraints will be considered. Materials science topical areas and approaches most likely to have a significant impact for each of the three applications will first be suggested. By also analyzing similarities in needs that transcend the specific applications, a strategy for promoting interactions between the membrane and materials science communities will be suggested. Such a strategy could be extended to encompass the full spectrum of gas, vapor and liquid separation applications and help advance the interests of both communities.

2:00 PM GG2.2
NANOPOROUS GAS TRANSPORT IN SEMICRYSTALLINE BLOCK COPOLYMERS. Peter Kofinas , Peter L. Drzal, Sufi R. Ahmed, University of Maryland, Dept of Materials and Nuclear Engineering, College Park, MD.

The shear-induced orientation textures produced by plane strain compression of semicrystalline ethylene/ethylene-propylene (E/P and E/EP/E) diblock and triblock copolymers and their blends with amorphous EP homopolymers were investigated. Two dimensional small angle x-ray scattering (SAXS) was used to determine the domain spacing and lamellar orientation relative to the specimen boundaries. The SAXS experiments indicated that depending on the processing conditions orientation textures parallel, perpendicular and transverse to the plane of shear can be produced. Gas permeability coefficients were obtained for several gases at room temperature in these block copolymer systems and their blends with homopolymers. It was found that the gas transport properties of these semicrystalline systems were influenced by changes in processing conditions and solvent treatments to produce anisotropic gas transport properties and porous membranes. The separation properties of these polymer membranes was altered by changing the mechanism of gas transport. A selective solvent was used to remove the homopolymer and develop a uniform nanoporous structure in the oriented blend morphologies. Depending on the choice of solvent, various degrees of porosity could be induced in the polymeric membrane. The orientation texture produced from channel die processing was not disrupted by the introduction of the pores. It was shown that the induced porosity increased the permeability of the block copolymer membrane, while retaining some selectivity due to the confinement of the pores within the block copolymer self-assembled microphase separated morphology. The permeability results coupled with small angle x-ray scattering (SAXS) data, provide a direct connection between changes in microstructure to the observed changes in gas transport properties.

2:15 PM GG2.3
PERMEATION OF NOBLE GASES THROUGH POLYMER MEMBRANES STUDIED WITH A MASS SPECTROMETER. Holger Norenberg , G.D.W. Smith, G.A.D. Briggs, University of Oxford, Department of Materials, Oxford, UNITED KINGDOM; T. Miyamoto, Y. Tsukahara, Technical Research Institute, Toppan Printing Company, Sugito Takanodai, JAPAN.

Polymer membranes are used in a number of applications to prevent the permeation of certain gas species (barrier films) or to separate individual components of gas mixtures from each other. In order to improve these properties a much better understanding of the process of gas permeation through membranes is required. We have developed a new method of measuring gas permeation through thin polymer membranes. This method uses a mass spectrometer to measure the partial pressure of the permeated gas species, which originates from a confined volume of gas, as function of time [1]. The method works on very small amples (4 mm diameter) and is inherently calibration free. It can be used to study the permeation of individual components in a gas mixture. In a simple mathematical model an exponential decay of the partial pressure as function of time is predicted. The permeation of inert gases is useful for understanding mechanisms of gas transport through membranes in the absence of chemical interactions. Our experiments with light noble gases (He, Ne, Ar) confirm the predicted exponential behaviour. Experiments carried out with Krypton which has a much higher m/q ratio showed that only a fit with two exponential functions is satisfactory. We will show a number of examples with noble gases permeating through PET and OPP and discuss advantages and limitations of the method and the mathematical model. [1] Holger N{orenberg, T. Miyamoto, N. Fukugami Y. Tsukahara, G. D. W. Smith and G. A. D. Briggs, accepted for publication in Vacuum.

2:30 PM *GG2.4
BASIS OF PERMEABILITY/SELECTIVITY TRADEOFF RELATIONS IN POLYMERIC GAS SEPARATION MEMBRANES. Benny D. Freeman , NC State Univ, Dept of Chemical Engineering, Raleigh, NC.

Gas separation properties of polymer membrane materials follow distinct tradeoff relations: more permeable polymers are generally less selective and vice versa. Robeson (J. Membrane Sci. 62, p. 165, 1991) identified the best combinations of permeability and selectivity for important binary gas pairs pairs (O2/N2, CO2/CH4, H2/N2, etc.) and represented these permeability/selectivity combinations empirically as: $\alpha$=$\beta$*P$^(-\lambda)$, where P is the more permeable gas permeability, alpha is selectivity, and lamdba and beta are adjustable parameters. This presentation gives a fundamental theory for this observation. In the theory, lamdba depends only on gas size. beta depends on lamdba, gas condensability, and one adjustable parameter. Excellent agreement for these parameters with the published slope and intercept of the so-called upper bound permeability/selectivity tradeoff lines is demonstrated.

3:30 PM GG2.5
STRUCTURE-PROPERTY RELATIONSHIPS IN PROTON-CONDUCTING MEMBRANES: QUANTIFICATION OF CROSSLINKING AND CORRELATION WITH SWELLING AND OTHER PROPERTIES. Hans-Peter Brack , Guenther G. Scherer, Paul Scherrer Institute, Electrochemistry, Villigen PSI, SWITZERLAND; Daniel Fischer, Gustav Peter, Zuercher Hochschule Winterthur, Chemistry Department, Winterthur, SWITZERLAND.

We are preparing proton-conducting membranes by using the radiation-grafting method to introduce polystyrene-based grafted chains into fluoropolymer films. The polystyrene chains are sulfonated in a subsequent step to introduce proton conductivity. We reported earlier that introducing crosslinking into our radiation-grafted membranes by adding divinylbenzene (DVB) and/or triallylcyanurate into our grafting solutions leads to significant changes in membrane properties important for their application in Polymer Electrolyte Fuel Cells including: (1) swelling in water and resultant dimensional changes, (2) mechanical properties, (3) proton conductivity, (4) gas crossover, and (5) resistance to degradation. Unfortunately, the analysis of the DVB content of membranes is quite complex because technical grade DVB is typically a mixture of both meta- and para-DVB with the intermediates in the DVB production, 3- and 4-ethylvinylbenzene (EVB), still present in significant quantities. All four monomers, meta- and para-DVB and meta- and para-EVB, are radically polymerizable and thus capable of being incorporated during grafting. We report here on an infrared analysis of the content of both the para- and meta-isomers and styrene in our radiation-grafted films. This analysis indicates that the para isomers are preferentially introduced during grafting, even though the meta isomers are present in excess in the technical grade DVB used. More para-isomers are systematically introduced as the DVB content of the grafting mixture increases; however, the ratio of the para-/meta-isomers incorporated into the films increases with increasing DVB content in the grafting solution. Raman analysis of the grafted films indicates that in all cases very little of the DVB is actually fully reacted and thus functioning as crosslinks. Taken together with the swelling properties of these membranes, these results indicate that low levels of DVB incorporation reduce water swelling significantly and that further increases in DVB content do not lead to further reduction of swelling or hydration levels (H2O/-RSO3H).

3:45 PM GG2.6
THERMAL DEGRADATION OF RADIATION-GRAFTED FILMS AND MEMBRANES. Hans-Peter Brack , Gunther G. Scherer, Paul Scherrer Institute, Electrochemistry Dept, Villigen PSI, SWITZERLAND; Denise Ruegg, Heiner G. Bührer, Zurcher Hochschule Winterthur, Chemistry Dept, Winterthur, SWITZERLAND.

The thermal degradation under N2 has been investigated of some fluoropolymer films before and after: (1) gamma or electron beam irradiation, (2) subsequent grafting with styrene and, in some cases, (3) sulfonation of the grafted polystyrene component. The films investigated include: (a) polytetrafluoroethylene or PTFE, (b) poly(tetrafluoroethylene-co-hexafluoropropylene) or FEP, (c) polyvinylidenefluoride or PVDF, (d) poly(ethylene-alt-tetrafluoroethylene) or ETFE, and (e) polyethylene. The thermal degradation temperatures and activation energies for degradation decreased and the rates of weight loss increased as the fluorine content of the starting films decreased. Irradiation with a total dose of 20 kGy did not significantly affect the thermal degradation temperatures, rates of weight loss, or activation energies of the three film types studied: FEP, PVDF, and ETFE. The thermal degradation temperatures and activation energies of the grafted polystyrene component of the three grafted film types, FEP-g-PS, PVDF-g-PS, and ETFE-g-PS varied systematically as: PVDF < ETFE < FEP. It is proposed that the more highly fluorinated FEP matrix exerts a protecting effect on the grafted PS domains. For the grafted films based on a particular fluoropolymer type (FEP, ETFE, or PVDF), the activation energies were found to increase as the graft level (polystyrene content) increased. This increase in activation energy may result from longer grafted chain lengths and thus lower relative concentration of end groups. Thermal degradation is believed to begin at the end groups in many polymer systems. All of our films were irradiated with the same dose and grafted for the same period of time. Lower graft levels were obtained here by diluting the styrene used for grafting in toluene. Thus the grafted films based on the same fluoropolymer type are expected to have the same number of active radical sites and thus grafted PS chains, but they likely differ in the lengths of these chains.

4:00 PM GG2.7
PSEUDOCROWN ETHERS AS FIXED SITE CARRIERS IN FACILITATED TRANSPORT MEMBRANES. Brian J. Elliott , W. Brinson Willis and Christopher N. Bowman, University of Colorado, Dept. of Chemical Engineering, Boulder, CO.

Polymeric membranes were synthesized by modifying porous polyethylene membrane supports for separations of metal ions in aqueous sources. A polymerization synthesis technique was used to form pseudocrown ethers (polymeric networks that contain crown ether-like moieties) in the pores of the membrane support. The pseudocrown ethers were formed in situ during free radical photopolymerization of poly(ethylene glycol) diacrylates with the use of templating ions. The templating ions were used to induce the poly(ethylene glycol) diacrylate monomers into a circular conformation prior to the photopolymerization, thus bringing the two reactive endgroups into close proximity. The degree of pseudocrown ether formation was found to be a function of the metal ion / monomer complex geometry, strength and abundance, and this pseudocrown ether formation occurred with a reduction in the amount of autoacceleration as compared to the bulk polymerization (i.e. non-templated). This reduction in autoacceleration was due to the decreased crosslinking in the pseudocrown ether network relative to the non-templated network. The possible crown structures obtained from the monomeric poly(ethylene glycol 200) diacrylate were 21-pseudocrown-6 and/or 18-pseudocrown-5. Pseudocrown ether membranes used in facilitated transport separations were selective for potassium over sodium which is consistent with results obtained from membranes that contain dibenzo-18-crown-6 as a fixed site carrier. By maximizing the concentration of pseudocrown ethers a selectivity for potassium over neodymium of 3,700 was obtained with a flux of potassium several orders of magnitude higher than typical fixed site carrier membranes. Pseudocrown ether membranes that are synthesized from poly(ethylene glycol) diacrylate analogs that contain nitrogen or sulfur in replace of some or all of the oxygen atoms in the ethylene glycol chain should exhibit selectivity for certain transition metals which are of greater interest with regard to pollution prevention and purification of water.

4:15 PM GG2.8
Transferred to GG6.3

4:30 PM GG2.9
ELECTROCHEMICALLY PRODUCED MEMBRANES FOR ULTRAFILTRATION AND REVERSE-OSMOSIS. Lidia Kolzunova , Inst of Chemistry, Far East Dept, Russian Academy of Sciences, Vladivostok, RUSSIA.

Numerous problems of water purification and organic and biological moleculs separation can be solved by ultrafiltration and reverse-osmosis and need diverse membranes and various formation methods. Progressive, one-stage (2-30 min) and cheap technology of polymer permselective membrane production is formation of porous films by electrochemically initiated polymerization of aqueous vinyl monomers (Pat. RF 1560280). The method allows to control copolymer composition, macromolecule cross-linking, microstructure development, pore configuration and size, membrane thickness by change of electrolysis conditions and bath compounds. Membranes have high polymer matrix homogeneity in consequence of monomer molecules orientation in electric field perpendicularly to electrode surface and synthesis of polymer with narrow molecular mass distribution.
Formation conditions of films and composite membranes and modules were determined and influence of them on structure, properties, penetration and selectivity were researched. The dense film structures are formed by -(1.16-1.20) V. These membranes have low penetration and high selectivity (99.9%). Pore size and framework friability of polymer films increase with growth of electropolymerization velocity (>-1.3 V). Membranes are formed by potentiostatic electrolysis have higher penetration than produced by galvanostatically. Polymer cross-linking by bifunction monomers allows to form hard polymer structures with both low friability and small swelling. Pore deformation of these membranes by pressure is insignificant. Membranes are stable during exploitation and possess high filtration velocity.
The properties and exploitation characteristics of ultrafilter and reverse-osmotic membranes are studied. All membranes have small area of hysteresis loop that testify high elasticity, hardness and deformation stability of membrane structure. Membranes are stable in acids and alkalis (pH 1-14). Using of formaldehyde not only as copolymerizing and cross-linking but anticeptic component permits to prevent microbiological membrane desintegration. These membranes can be used for cleaning of aggressive solutions and biological mediums.

4:45 PM GG2.10
POLYMER MEMBRANES AS SENSITIVE ELEMENTS OF IMMUNOSENSORS. Tetyana Panasyuk, Sergiy Piletisky, Ganna Eliska, Inst of Molecular Biology and Genetics, Dept of Translational Mechanisms of Genetic Information, Kyiv, UKRAINE; Rinat Nigmatullin, Inst of Colloid and Water Chemistry, Dept of Physical Chem of Membranes, Kyiv, UKRAINE; Mikhaylo Bryk , National Univ `Kyiv-Mohyla Academia', Dept of Chemistry, Kyiv, UKRAINE.

Biosensors are analytical devices that respond selectively to analytes in an appropriate sample and convert the results into an electrical or other signals via a combination of a biological recognition system and a physico-chemical transducer. Data presented in this report indicates that bioaffinity membranes could be responsible for both molecular recognition and signal generation. The immunosensors discussed use porous polymeric membranes as a support for the antibody and at the same time as an active part of the transducer, such a sensor system with a bioaffinity polymeric membrane (with immobilized Ab) is able to change its ion permeability during immunochemical reaction.
The methods of obtaining of bioaffinity membranes based on acrylonitrile-co-acrylic acid and polyvinylchloride have been developed. The influence of porous structure on effectiveness of immunochemical reaction at bioaffinity membranes have been studied.
Using of hydrophobic polyvinylchloride membranes allow to prepare membranes with a hydrophilic outer surface and hydrophobic pores or partially hydrophilised pores through the width of the membrane thickness. It is essential to localize the antigen-antibody (Ag-Ab) reaction in pores so that surface immobilisation is avoided. In such a way, even small quantities of Ag in solution could influence membrane permeability which would allow the improvement of the sensor response. The modification of polyvinylchloride membranes by non-ionogenic detergent has been studied. The effectiveness immunoreagent sorption by PVC membranes is time dependent on treatment in detergent solution. The conditions of partial hydrophilization of PVC membrane for best immunoreagent sorption have been established.
The conductometric system was used for sensor response determination. Response of immunosensors correlates with effectiveness of immunochemical reaction. Both immusensor based on bioaffine acrylonitrile-co-acrylic acid membranes and polyvinylchloride membranes exhibit higher sensitivity to rabbit IgG (Ag) (5 ng/ml) and a higher rate compared with the ELISA-method (the response time is 5-6 min).
 

SESSION GG3: BIOMEMBRANES
Chair: David Needham
Wednesday Morning, April 7, 1999
Salon 14 (M)
8:30 AM GG3.1
THE MECHANOCHEMISTRY OF LIPID VESICLES EXAMINED BY MICROPIPET MANIPULATION TECHNIQUES. David Needham , Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC.

The lipid bilayer membrane is a truly remarkable engineering material, - it surrounds every cell on the planet providing a mechanical, chemical, and electrical barrier for the cell. It also acts as a 2 dimensional solvent for the protein components of the cell membrane. It is however only 5 nm thick, and, with an area compressibility that is equivalent to bulk compressibilities between ordinary liquids and gases, and a bending stiffness of only a few kT, the lipid bilayer is one of the thinnest and softest materials known. As a consequence, it is both fragile and inherently difficult to resolve optically. Because of these physical limitations, direct measurements of the full range of material and interactive properties of lipid membranes have only been possible by the development of sensitive micropipet manipulation techniques and the creation of appropriate preparative procedures that produce large (20 to 30 micron) single-walled lipid vesicles that can be seen in the optical microscope.
The glass micropipet provides a unique way of applying well defined stresses to a vesicle capsule whilst at the same time acting as a sensitive transducer of vesicle membrane area and volume change. Using a suction pipet, a single lipid vesicle can be manipulated, and several mechanochemical experiments can be performed that characterize: membrane area expansion, tensile failure and bending; adsorption, uptake and desorption of various membrane-soluble components; membrane permeability coefficient and pore formation; thermal bilayer transitions; membrane yield shear and shear viscosity for gel phase bilayers; intermembrane adhesion energy that results from the cumulation of several attractive and repulsive colloidal potentials.
This overview will focus on the various micropipet methods that have been specifically developed since 1980 to study the mechanochemical features of lipid bilayer vesicles (Needham, D. and D. V. Zhelev. Vesicles., eds. Rosoff, Marcel Dekker, New York and Basel, 1996 p.373-444). The information gained from such studies not only characterizes the membrane and its intermembrane interactions from a fundamental materials science perspective, it also provides essential materials property data that are required for the successful design and deployment of lipid vesicle capsules in applications such as drug delivery.

8:45 AM GG3.2
FOLDING AND RESPREADING IN LUNG SURFACTANT - INFLUENCE OF CHAIN LENGTH OF LIPIDS AND PRESENCE OF PROTEINS. Karlheinz Graf , Anja von Nahmen, Joseph A. Zasadzinski, Dept of Chemical Engineering, University of California, Santa Barbara, CA; Gerhard Schwarz, Dept of Biophysical Chemistry, Biocenter of the University, Basel, SWITZERLAND; Alan J. Waring, Dept of Pediatrics, Drew University-King Medical Center and University of California, Los Angeles, CA.

Lung surfactant, a mixture of saturated lecithins, phospatidylglycerols, fatty acids, and four specific proteins plays an important role in the ability of higher developed, living organisms to breathe. During volume change in the aveolar interface the surfactant maintains aveolar stability by lowering the surface tension during expiration [1]. Langmuir isotherms and fluorescence microscopy show the corresponding structural changes within a monolayer of the lung surfactant as the films are compressed [2]. It has been shown that processes like bilayer formation, folding and respreading of lipids and protein are essential to viable lung surfactant during compression and expansion. These processes are mainly influenced by the presence of charged lipid compounds and lung surfactant proteins.
We present investigations on the chain length of the lipid compounds which is known to affect the stiffness of the film and therefore might affect the collapse behaviour of the membrane. Here we concentrate on a model lipid mixture with the proteins SP-B and SP-C on a buffered subphase at pH $\approx$ 6.9 to mimic the biological system.
The proteins are responsible for all processes to occur. In order to explain this general behaviour on the base of theoretical models, it is a crucial point to know how much of the proteins actually is in the monolayer. This can be achieved by studying the partitioning of proteins between monolayer and subphase by a simple thermodynamic study [3].
References
[1] Ka Yee Lee, Michael M. Lipp, Joseph A. Zasadzinski, Alan J. Waring, Coll. Surf. A, 128 (1997) 255-242.
[2] Michael M. Lipp, Ka Yee Lee, Alan J. Waring, Joseph A. Zasadzinski, Biophys. J., 72 (1997) 2783-2804.
[3] Gerhard Schwarz, Susanne E. Taylor, Langmuir, 11 (1995) 4341-4346.

9:00 AM GG3.3
VESOSOMES - A NEW DRUG DELIVERY SYSTEM. Edward Kisak , Bret Coldren, Michael Kennedy, Dirk Trommeshauser, Joseph A. Zasadzinski, Univeristy of California at Santa Barbara, Dept of Chemical Engineering, Santa Barbara, CA.

Vesosomes consist of a sized aggregate of unilamellar vesicles attached to each other due to the interaction between ligand - receptor combinations. These aggregates are encapsulated by a second bilayer again using the ligand-receptor interaction. The aggregated vesicles can be of similar or varied membrane and interior composition. The vesosomes can incorporate a variety of water soluble or lipid soluble drugs within the interior vesicles, or the exterior capsule, or both. We studied the effects of using different ligand-receptor combinations and ratios to rapidly prepare vesicle aggregates of a fixed sized. The stability of these aggregates was also tested. Research into a variety of methods of aggregate encapsulation has also been carried out.

9:15 AM GG3.4
THE PRODUCTION OF LIPID VESICLES BY EXTRUSION THROUGH POLYCARBONATE MEMBRANES. P. Patty, C. Asman, D.G. Hunter and B.J. Frisken , Burnaby, CANADA.

The production of lipid vesicles is an important aspect of the application of vesicle systems to drug delivery technologies. One popular production method involves pushing or extruding a lipid suspension through the cylindrical pores of polycarbonate membranes1. However, the actual mechanism by which the polydisperse, multilamellar lipid suspension breaks up into a relatively monodisperse population of vesicles is not well understood. We have characterized vesicles produced under different extrusion parameters and from different lipids. We find that vesicles are only produced above a threshold extrusion pressure and that the lysis tension of the membrane can be estimated from this minimum pressure. We also find that the final size of the vesicles decreases as the applied pressure increases. We have shown that the final size of the vesicles produced depends directly on the pressure applied, rather than the flowrate or shear that result from it.
1M.J. Hope, M.B. Bally, G. Webb, and P.R. Cullis, Biochim. Biophys. Acta. 812, 55-65 (1985).

9:30 AM GG3.5
ELASTIC PROPERTIES AND MICROSTRUCTURES OF CATANIONIC SURFACTANTS. Hee-Tae Jung , J.A. Zasadzinski, Department of Chemical Engineering and Materials, University of California, Santa Barbara, CA; Dan Iampietro, E.W. Kaler, Department of Chemical Engineering, University of Delaware, Newark, DE.

Spontaneous equilibrium vesicles can be prepared from aqueous mixtures of single tailed cationic and anionic surfactants. Bending elastic constants of mixtures of two surfactants were determined by cryo TEM of vesicle phases followed by image analysis in order to determine the vesicle size distribution. The experimental results are in a good agreement with the thermodynamic theory of aggregation. We compare the bending constants and microstructures of the fluorinated mixtures with the hydrogenated mixture. The hydrogenated vesicles are about kT, the longer fluorinated is about 10 kT. The replacement of long chain fluorinated molecules by short ones dramatically reduce bending modulus from  10kT to  kT. The microstructure and morphology of the surfactants are also discussed.

9:45 AM GG3.6
REVERSIBLE COLLAPSE PROCESSES IN MIXED LIPID-PROTEIN-FILMS. Anja von Nahmen , Dawn Takamoto, Karlheinz Graf, Junqi Ding, Joseph Zasadzinski, University of California at Santa Barbara, Dept. of Chemical Engineering, Santa Barbara, CA; Ka Yee C. Lee, University of Chicago, Dept. of Chemistry, Chicago, IL; Alan Waring, University of California at Los Angeles, CA; MLK, Drew University Medical Center and Perinatal Labs, Los Angeles, CA.

A complex mixture of lipids and proteins - commonly known as lung surfactant - lines the alveolar air-water-interface. It reduces the surface tension of the interface and thereby the work of breathing. A low surface tension is generally assigned to saturated and rigid lipids. To insure reduction of the surface tension during inhalation lung surfactant needs sufficient respreading kinetics. Unsaturated and fluid lipids are essential to ensure a good respreadability, but do not form stable monolayer at low surface tension. Recent studies suggest that the key to ensure both - low surface tension and good respreadability - is a reversible transition from a two-dimensional monomolecular layer to a partionally three-dimensional system. The nature of this collapse process strongly depends on film composition. In this study we investigate complex model systems of saturated, unsaturated, and charged lipids that also contain surfactant proteins B and C. Using Langmuir technique in combination with fluorescence microscopy as well as atomic force microscopy allows for detailed investigation of the phase behavior and morphology of the model systems. We find that addition of surfactant proteins makes the collapse process more reversible. Two different collapse mechanisms can be observed: the formation of stacked protein-rich lipid bilayers and a long range buckling of the film. We assign the observed morphology to single compounds or specific lipid-protein or lipid-lipid interactions.

10:30 AM *GG3.7
MICROMECHANICS OF A RELATIVELY SIMPLE AND NATURAL BIOMEMBRANE COMPOSITE. D. Discher , J. Lee, D. Wong, C. Picart, University of Pennsylvania, Philadelphia, PA.

In a material sense, biomembranes are interconnecting lamellar composites with fluid as well as solid like characteristics. The fluidity primarily arises from the lipid bilayer, but on the cell's inner face one often finds a sparse, sometimes ordered network of proteins. The red blood cell provides a simple yet classic example of this. Short (relative to a persistence length, Lp) actin filaments are intertriangulated by non-covalent association with the protein spectrin which, in turn, has a contour length Lc $\gg$ Lp. We have simulated coarse-grained versions of such a polymeric structure at multiple scales by Metropolis Monte Carlo and in both nano- and meso-scale ensembles. The theoretical results demonstrate several features which we have partially confirmed by direct experiment. First, the statistical mechanics of this soft, wet network is borne out by constrained fluctuations of attached nanoparticles that, by equipartition, also yield a very simple and local estimate for network elasticity. Second, despite such thermal motion, micropatterned photobleaching shows the network can sustain very large strains with material stretching at least of order $\sim$200%; further, the network's in-plane compressibility appears much softer than that of the overlying lipid bilayer. Third, in such large deformation, fluorescence polarization shows that the actin nanofilaments tend to stably reorient in the maximum strain direction, consistent with an ordered but distorted microstructure. Thus, in comparison with the fluid lipid bilayer, the network is softer in many respects, but still maintains a solid-like character.

11:00 AM GG3.8
POLYMOSOMES: NOVEL CAPSULAR STATES OF DI-BLOCK COPOLYMERS. D. Discher , B. Discher, J. Lee, Y. Won*, D. Ege, F. Bates*, D. Hammer, University of Pennsylvania, Philadelphia, PA; *University of Minnesota, MN.

Lipid-based vesicles find application in various products ranging from pharmaceuticals where they are used in drug delivery, to cosmetics, to paints and coatings. The amphiphillic character of lipids has been synthetically mimicked in the last several years with di-block copolymers, some of which are well known to form lamellar phases in water. Using polyethylethylene/polyethyleneoxide (PEE-PEO; 3.9 kD with 10% polydispersity), we have been able to form soft, unilamellar vesicular capsules, or `polymosomes'. We have studied these vesicles by micromanipulation and found some unique properties. Polymosome membranes reproducibly have a surface area compressibility and a bending modulus which approximate those of lipid membranes, and yet vesicle pressurization shows that the polymosome wall is far tougher, with areal strains at failure estimated to be almost 20% in comparison to the $\sim$few% for lipids. The polymosomes also appear to be an order of magnitude less permeable to water than liposomes, but osmotic adjustment of polymosome volume is still possible and generates a range of vesicle shapes that compare well with phase diagrams of lipid vesicle morphology. Such behavior is, in theory, predicated on a bilayer structure and a lack of molecular flip-flop between layers. These and additional results suggest that capsular states of diblock copolymers may, in some cases, provide performance improvement over lipid capsules.

11:15 AM GG3.9
MASS-ELECTRO ANALYSIS OF LIPID MEMBRANES FOR CHINESE WINE TASTING SENSING MODEL. Chen Yuquan, Pan Min , Dept of BME, Zhejiang Univ, Hangzhou, P.R.CHINA.

The Lipid membrane is ubiquitous in biology. Olfaction is one of the important senses of human being. Lipids are important not only in life processes but also in making membranes for artificial olfaction and taste sensors. Recently mass and electrical property of lipids is investigated separately using PYE electrical chip and IDT (interdigital electrodes) or potential measurement etc to analysis the relationship between lipid membrane structure and the sensing property. This paper describes a new compound mass-electron analysis method of lipid model. A novel compound sensor with the lipid membrane was developed; it can get the mass responses and electrical responses separately and simultaneously with one lipid membrane. The results of analysis and experiment will be employed to optimize tasting sensor array design for Chinese wine measurement. Different lipid membranes, such as polyvinyl chloride and dioctyl phenylphosphonate(DOP) or trioctyl methyl ammonium chloride(TOMA) or the compound lipid material with different ratio of DOP and TOMA or the cholesterol oleic acid, prepare processing and their deposition method and membrane thickness and bass chip preprocessing are also studied. As a system, later 8 kinds Chinese wine and their grades with a lipid sensor array with 7 kinds lipid membranes were measured by an advanced dynamic regression technique, it shows the precise is better than the normal method.

11:30 AM GG3.10
VARIABLE TEMPERATURE FLUID STAGE FOR AFM. Srinivas Manne, Richard K. Workman , University of Arizona, Dept of Physics, Tucson, AZ.

The design of a simple variable temperature fluid stage for an Atomic Force Microscope is presented. The stage is based on a thermoelectric heating/cooling element which allows control of sample and fluid temperature from -4C to 130C. This allows the study of phase transitions and other temperature dependent phenomena. While this fliud cell is useful for imaging in gas or fluids, we will present results on micelle phase transistions at the solid liquid interface.

11:45 AM GG3.11 INTERACTIONS BETWEEN POLY(2-ETHYLACRYLIC ACID) AND LIPID BILAYER MEMBRANES: EFFECTS OF CHOLESTEROL AND GRAFTED POLY(ETHYLENE GLYCOL). David Needham, Jeff Mills and Gary Eichenbaum, Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC.

The inclusion of cholesterol in lipid bilayer membranes causes a condensation of the interface and a dramatic decrease in elastic area compressibility of the bilayer as well as its permeability to water. The presence of PEG grafted to the bilayer interface, via the incorporation of PEG-lipids into the bilayer, can inhibit the close approach of globular macromolecules and micelles. It is of interest then to determine if these two different kinds of barriers can hinder the approach and interaction of a water-soluble polymer that can be triggered to undergo a conformational transition due to a lowering of solution pH making it less soluble in aqueous media and more soluble in lipid bilayers, which it can then permeabilize. We have studied the exchange of the protonatable polymer, poly (2-ethylacrylic acid) (PEAA), with vesicle membranes containing cholesterol from 0 to 60 mol% or PEG2000-lipid (5 mol%). The release of an entrapped dye from 100 nm extruded liposomes was used as an assay for membrane perturbation by the polymer as a function of pH. The inclusion of cholesterol was found to reduce the pH at which the polymer caused release of internal contents from the lipid vesicles, and the degree of polymer protonation (i.e., hydrophobicity) correlated well with the increase in elastic expansion modulus of the vesicle. The results are discussed in terms of a balance between polymer solubility and membrane expansion. With respect to the PEG barrier, the presence of 5 mol% PEG2000, which represents full surface coverage, did not prevent PEAA from inducing contents release, demonstrating that highly hydrated polymeric layers are not effective barriers for other water soluble polymers, and may point to some association between the two polymers.
 
 

SESSION GG4: HYBRID MEMBRANES
Chair: Benny D. Freeman
Wednesday Afternoon, April 7, 1999
Salon 14 (M)
1:30 PM GG4.1
CHALLENGING PERVAPORATION SEPARATIONS USING POLYMERIC AND ZEOLITE MEMBRANES. Dhaval S. Shah , Dibakar Bhattacharyya, University of Kentucky, Dept. of Chemical and Materials Engineering, Lexington, KY; Ajit Ghorpade, SmithKline Beecham Corporation, King of Prussia, PA.

The separation and recovery of solvents is of major interest to many industries ranging from pharmaceuticals to electronic materials. Pervaporation is a membrane-based process used for the separation of solvents. Based on material properties, pervaporation membranes can be mainly classified into two types; polymeric and ceramic membranes. This research concentrates on separation studies of various solvent systems using these membranes. In polymeric membranes, depending on the nature of the polymer, a particular species will preferentially permeate through the membrane. Experiments were performed with alcohol-water mixtures in the presence and absence of inorganic salts using hydrophilic PVA-based polymeric membranes. The flux for ethanol-water mixture increased from 0.05 to 1.2 kg/m2/hr as the feed water concentration was increased from 3 to 70 wt % at 333 K. The importance of plasticization and coupling effects, in alcohol-water separation using PVA membranes, will be discussed. Silicone-based organophilic polymeric membranes were also used to study the separation of various solvent mixtures. High selectivities (100-200) were obtained for ethyl acetate-water and tetrahydrofuran-water mixtures because of the strong affinity of these compounds for the polydimethylsiloxane (PDMS) active layer of the membrane. The main drawbacks of polymeric membranes are their low resistance to solvents like dimethylformamide (DMF) and dimethylsulfoxide (DMSO) and low selectivities in dilute solutions. Inorganic membranes help overcome this limitation of polymeric membranes. Zeolite membranes were used to study the separation characteristics of several solvent-water mixtures. Zeolites are crystalline, hydrated aluminosilicates of group I and group II elements which have a very narrow pore size distribution. A partial/total exclusion of a particular species and a high sorption of aqueous species may be achieved in zeolite membranes leading to high selectivities. For 30 wt % ethanol solution, zeolite membrane selectivity (>1000) was found to be 2 orders of magnitude higher than that for the PVA membrane ($\sim$7).

1:45 PM GG4.2
HYBRID POLYIMIDE-SILICA MEMBRANES FOR GAS SEPARATION. Eva Marand , Chris J. Cornelius, Nicolas G. Vidonic, Virginia Polytechnic Institute and State University, Department of Chemical Engineering, Blacksburg, VA.

Hybrid polyimide-silica membranes were fabricated using a functionalized hexafluorinated polyimide and alkoxysilane precursors, reacted under sol-gel conditions, forming an interpenetrating network of silica within a polymer matrix. The covalent linkages between these two components drive mixing on a molecular level, leading to a highly homogeneous system. Gas permeability results show a maximum in permeability as a function of silica content at about 10 weight percent silica, even though the membrane density continuously increases with silica content. Simultaneous increase in gas selectivities with silica content suggests that the packing of the polyimide chains is indeed affected by the presence of the silica phase. The presence of the inorganic component constrains the polymer chains, decreasing their conformational entropy, which leads to more open chain packing. However, the packing arrangements must be quite uniform since the selectivity also increases. At high silica concentrations the inorganic component starts to phase separate, acting more as a filler, lowering the overall permeability. This behavior has been examined for a number of different alkoxysilane precursors including tetraethoxysilane, dimethyldiethoxy silane and phenyltriethoxysilane, which give rise to different cross-link densities and different morphologies due to molecular compatibility differences between the components and different reaction rates under sol-gel conditions. The permeability results are also correlated with the dynamic mechanical response, results of wide-angle X-ray diffraction studies and electron microscopy images for these materials.

2:00 PM GG4.3
MONOLAYERS OF AMPHIPHILIC FULLERENE DERIVATIVES CONTAINING AN AZOBENZENE CHROMOPHORE. Kei Oishi , Masahito Sano, Seiji Shinkai, Chemotransfiguration Project, JST, Fukuoka, JAPAN.

One of the problems of producing stable monolayers of fullerene [60] at the air-water interface was the tendency of fullerene to form 3D aggregates. Here, we have synthesized an amphiphilic fullerene derivative giving Langmuir-Blodgett films with monolayer domains of uniform thickness extending over tens of microns. The derivative has an azobenzene chromophore and an ammonium head group. It gives an expanded surface pressure-area isotherm with a limiting area of 0.95 nm2/molecule. This area compares well with the limiting area of an ideal monolayer. The UV-VIS reflection spectroscopy on the water surface shows no specific exciton peaks. These results indicate a significant hydration of the azobenzene segment and suggest a model of dangling monolayers that all parts of the molecule other than the fullerene head are submerged into the water phase.

2:15 PM GG4.4
PROPERTIES OF COMPOSITE MEMBRANES PREPARED FROM PVDF AND INORGANIC PARTICLES. Aldo Bottino, Gustavo Capannelli, Valentina D'Asti, Paolo Piaggio , Genova Univ, Genova, ITALY.

The use of composite materials can play an important role for the preparation of novel membranes with controlled properties which can be widely used in the traditional separation processes (e.g. gas separation and ultrafiltration) as well as in emerging and innovative applications, like catalytic membrane reactors and devices. The interest of this preparative route derives from the well known point that polymeric-inorganic composites can combine in a favourable manner the properties of the separate organic and inorganic materials or can even exhibit new properties that are not found in the separate one-phase materials. On this basis it would be therefore possible to produce membranes with chemical and physical properties tailored for a given application. In order to verify this statement we have studied the properties of asymmetric membranes prepared by the phase inversion technique from blends of polyvinylidene fluoride (PVDF), i.e. a polymer widely used in membrane science and technology, and inorganic particles like silica and zeolites. The results obtained for these composite membranes will be shown with regard to their permeability, porosity, hydrophilicity and compared to those observed for the corresponding PVDF membranes. Moreover the insertion of a catalist into some of these membranes has been investigated and first results of their catalitic activity in some standard reactions will be shown. The effect induced on the membrane morphology by different preparative conditions have been analysed by vibrational spectroscopy and electron microscopy. In this way the role played in the formation of the asymmetric structure of the membrane by the type of solvent, the nature of the inorganic particles, the temperature and the air exposure time, has been investigated. In particular the analysis of the FTIR-ATR and PAS spectra allowed an effective characterisation of both surfaces of the membranes.

2:30 PM GG4.5
ORGANIC-INORGANIC MEMBRANES PREPARED FROM POLYETHERDIAMINE AND EPOXY-CONTAINING INORGANIC PRECURSORS. S.P. Nunes , K.V. Peinemann, E. Rikowski, GKSS-Research Center, Geesthacht, GERMANY; M.L. Sforca, I.V.P. Yoshida, University of Campinas, BRAZIL.

Dense membranes with organic and inorganic polymer segments were prepared by reacting polymeric diamines contaning ethylene glycol and propylene glycol blocks with epoxy modified inorganic precursors. Polymeric diamines with molecular weight 900 and 2000 g/mol were used as organic segments. For the first part of this work the inorganic precursor was glycidoxypropyl trimethoxy silane (GPTMS) or mixtures of GPTMS and tetraethoxysilane (TEOS). The hybrid was prepared by mixing different proportions of epoxysilane and diamine in tetrahydrofurane, followed by hydrolysis and condensation of the alkoxy groups by dropping 0.15 M HCl into the solution and stirring during about 20 hours. Dense films, after drying at 80 C, under vacuum, were characterized by nuclear magnetic resonance and dynamic mechanical analysis. The rigidity of the films increased with increasing epoxy content and also with addition of TEOS. Since the hybrid polymer contains long polyether segments, the films are quite hydrophilic. The water absorption was measured as a function of time to give values of water diffusivity (D) and solubility (S). The water solubility in the films prepared from diamine with molecular weight 900 g/mol was about twice that of films prepared from diamines with molecular weight 2000 g/mol. Increasing the TEOS content, the water diffusivity decreased, but the water solubility increased. Composite membranes were prepared by coating an asymmetric porous poly(vinylidene fluoride) support with a dense layer of the organic-inorganic hybrids. The membranes were then tested for nanofiltration. Membranes with cut off 2300 g/mol and water flow 4.4 l/h m2 bar were obtained from amines with longer polyether segments; the cut off could be decreased to 860 g/mol by addition of TEOS, keeping the water flow around 2.5 l/h m2 bar. The dense membranes were tested for gas permeation. The presence of amine groups as well as polyether segments contributes to high CO2 permeabilities (up to 125 Barrer), relative to other gases. Membranes with carbon dioxide/nitrogen selectivity up to 85 were obtained. Just few polymeric materials have been reported in the literature with such high values both for permeability and carbon dioxide/nitrogen selectivity. As a second epoxy modified inorganic precursor, polyhedral silsesquioxanes with cube-octameric R8 Si8 O12 (R = epoxy) were synthesized, dissolved in tetrahydrofurane and further reacted with the same diamines used above to form dense membranes for gas permeation. Beside their evaluation as membranes, the films were also characterized by electron microscopy and thermal analysis.

2:45 PM GG4.6
FABRICATION, CHARACTERIZATION AND UTILIZATION OF NEW TYPES OF MEMBRANES. Younan Xia , Byron Gates, Univ of Washington, Seattle, WA.

We have successfully fabricated two new types of membranes: 2-D membranes with tunable pore sizes and 3-D membranes with highly ordered porous structures. Both membranes were prepared by molding against appropriate template structures. We are going to present fabrication procedures and characterization results, as well as several schemes for the funtionalzation and utilization of these membranes.
 

SESSION GG5: BILAYER/MULTILAYER MEMBRANES AND SENSORS
Chair: Dennis E. Discher
Wednesday Afternoon, April 7, 1999
Salon 14 (M)
3:30 PM GG5.1
OIL-SWOLLEN BILAYERS BY A MYELIN TRANSMEMBRANE PROTEIN AND BY SYNTHETIC PEPTIDES. N. Taulier, C. Nicot, M. Waks, LIP, CNRS 7623, Universitè Paris VI, Paris, FRANCE; R. Ober, LPMC, CNRS 792, Collège de France, Gif-sur-Yvette, FRANCE; T. Gulik-Krzywicki, CGM, CNRS 2420, Gif-sur-Yvette, FRANCE; R.S. Hodges, P. Semchuk, PENCE Medical Center, University of Alberta, CANADA; and W. Urbach , ENS, LPS, CNRS, Paris, FRANCE.

We have explored the interaction of the Myelin Proteolipid with a nonionic lamellar (L$\alpha$) phase. In contrast to water-soluble proteins, the spontaneous insertion of the 4 $\alpha$-helix transmembrane protein induces a progressive expulsion of oil and a simultaneous squeezing of the interlamellar space. The interlamellar distance is only function of the protein-to-surfactant molar ratio (up to a maximum) and scales as C-0.5, where C is the protein surface density. Above this limiting value, the mean interlamellar distance is optimal and leads to a plateau. We have investigated the mechanism of this unusual phenomenon with a series of single $\alpha$-helical peptides of increasing length (from 8 to 16 hydrophobic amino-acids). It depends among others on the ratio of the peptide/surfactant length and peptide hydrophobicity. The results are presented and discussed in terms of the Helfrich theory applied to bilayers decorated with penetrating molecular inclusions, stabilized by entropic forces. Since hydrophobic forces alone play a crucial role in the insertion of transmembrane peptide chains into bilayers, the aim of this work is to provide a simple model for the ``zipper like'' function attributed to the myelin transmembrane protein in maintaining the multilamellar, compact architecture of myelin. On freeze-fracture electron micrographs, where proteins appear as membrane inclusions, we have determined the radial distribution function of inclusions: g(r). Short-range correlation observed in the transmembrane protein position is characteristic of two-dimensional interacting particles in fluid state. Analysis of g(r) allows the determination of interprotein forces.

3:45 PM GG5.2
PYROLYZED RESISTS FOR BIO-MEMS APPLICATIONS. Marc Madou , Ohio State University, Department of Materials Science and Engineering and Department of Chemistry, Columbus, OH.

Photopatterned resists pyrolyzed at different temperatures and in different ambient gases can be used as a carbonaceous material for micro electro mechanical systems (MEMS).The carbon films were characterized by several analytical techniques, viz. profilometry, thermogravimetric analysis, four-point probe measurements, SEM, TEM, AFM, XPS and Raman spectroscopy. In addition cyclic voltammetry was performed on the carbon film electrodes, and the carbon films have been compared to glassy carbon for their electrochemical behavior.The carbon films prepared at the higher temperatures showed good electrochemical reversibility similar to that of glassy carbon. The carbon films thus produced have potential applications in MEMS, and as electrodes in batteries, capacitors, sensors, etc. Two specific applications of this new material will be discussed (I) complex shaped electrodes for specific electrochemical sensors such as glucose sensors, (II) artificial muscle with C-MEMS as valve seats. In the first application it is shown that carbon surface modifications enable one to favor the desired reactions (e.g. with glucose mediators) over interfering reactions (e.g. reactions with reducing compounds such as ascorbic acid). In the second application it will be demonstrated how polymeric artificial muscle (a blend of hydrogel with redox polymer) can be grown electrochemically on a C-MEMS valve seat and how this artificial muscle can be actuated electrochemically and chemically to open and close the valve.

4:00 PM GG5.3
ORDERED ASSEMBLY OF PROTEINS ON SURFACES. Leonidas G. Bachas , J. Wang, W. Huang, Department of Chemistry, University of Kentucky, Lexington, KY; S. Viswanath, J. Liu, D. Bhattacharyya, Department of Chemical & Materials Engineering, University of Kentucky, Lexington, KY.

Membrane materials with immobilized enzymes play a central role in the development of biosensors, bioreactors and diagnostics. The functional groups on the protein, that serve as sites for the attachment, are generally distributed throughout the protein structure. Often this results in a random orientation of protein molecules on the surface. In this presentation, several approaches that facilitate the ordered assembly of enzymes on a variety of materials, including membranes, will be described. Biotinylation reactions performed under controlled conditions were used to immobilize enzymes in a layer-by-layer fashion onto surfaces. For other enzymes, a specific attachment site was introduced by gene fusion and site-directed mutagenesis. These strategies were applied to achieve site-specific immobilization of several enzymes including alkaline phosphatase, $\beta$-galactosidase, and subtilisin. The site-specific immobilization led to orientation of the enzyme molecules on the surface of materials and to a higher activity compared to the conventional immobilization methods.

4:15 PM GG5.4
CALMODULIN-MEDIATED REVERSIBLE IMMOBILIZATION OF ENZYMES. Vesna Schauer-Vukasinovic, Greta Schrift, Sylvia Daunert , University of Kentucky, Department of Chemistry, Lexington, KY.

Reversible immobilization of $\beta$-lactamase and glutathione S-transferase (GST) was performed using a system based on the binding of calmodulin (CaM) to a modified surface containing phenothiazine. Specifically, a fusion protein between the C-terminus of CaM and the N-terminus of $\beta$-lactamase was prepared, and the fusion protein was immobilized in the presence of Ca2+ through the CaM part of the molecule to silica particles covalently modified with phenothiazine. The immobilized fusion protein retains the $\beta$-lactamase enzymatic activity, as well as the ability of CaM to bind to phenothiazine. The activity of the immobilized enzyme was determined using a continuous flow method. The reversibility and reproducibility of the fusion protein loading on the solid surface was evaluated by measuring the rate of reaction of $\beta$-lactamase after each consecutive loading of the enzyme onto the particles. The versatility of this immobilization approach was studied by reloading the column with a different enzyme, namely glutathione S-transferase, which was fused to the N-terminus of CaM. The activity of GST showed that GST immobilized through the CaM part of the fusion protein is catalytically active for over one month. This approach may find useful applications in the fields of biosensors and diagnostics.

4:30 PM GG5.5
COEXISTENCE OF BUCKLED AND FLAT MONOLAYERS: MORE EVIDENCES FOR SURFACTANT PROTEINS TO KEEP MONOLAYERS STABLE. Junqi Ding 1, Anja von Nahmen1, Alan Waring2, Joseph Zasadzinski1*, Robert Notter3, 1Department of Chemical Engineering, University of California, Santa Barbara, CA; 2MLK, Drew University Medical Center and Perinatal Labs, Harbor, UCLA, CA; 3Department of Pediatrics and Environmental Medicine, University of Rochester, Rochester, NY.

Characteristic features of lung surfactants are the ability to sustain low surface tension and good respreadability. Both are limited by a two to three dimensional instability called collapse. In this study, we use Brewster-Angle and Fluorescence Microscopy with Langmuir isotherm to study collapse behaviors of Survanta$^{\rm TM}$ and Curosurf$^{\rm TM}$ (natural lung surfactant extracts). We supplement synthetic surfactant protein B to the natural extracts which improves surfactant performance in animal models. On compression to low surface tension, a large amplitude buckling occurs, especially with the additional synthetic protein B. The elongated structures remain attached to the interface and are reversibly reincorporated into the monolayer upon expansion. These collapse behaviors keep the monolayer stable at low surface tension and make the surfactants respread easily. The buckling structures are identical to those seen in model lipid and SP-B protein mixtures.

4:45 PM GG5.6
Abstract Withdrawn.
 

SESSION GG6: POSTER SESSION:
MEMBRANES
Chair: C. Jeffrey Brinker
Wednesday Evening, April 7, 1999
8:00 P.M.
Salon 7 (M)
GG6.1
Abstract Withdrawn.

GG6.2
DEVELOPMENT OF ``NEW LIPID'' BY THE AGGREGATION OF SYNTHETIC 10-ALKYL ISOALLOXAZINES IN AQUEOUS MEDIA. Shveta Chaudhary , Abha Awasthi, S.M.S. Chauhan, Department of Chemistry, University of Delhi, Delhi, INDIA.

Bioorganic approaches to design and construction of synthetic cells have been focus of much current interest. The barrier that defines the interior and exterior of a living cell is a cell membrane. Lipids being predominent constituent of biological membranes, their important functions are generally derived from their propensity to self organize. Such self assembly due to amphiphilic nature of lipids provide internal mesophases (smectic) with controlled sizes, shapes and microenviroments. Most of synthetic amphiphilic molecules organize in the aqueous as well as organic solvents depending upon their structure and polarity. The 10- substituted isoalloxazines, FMN and FAD are present as prosthetic group in various flavoenzymes and flavoprotein. The 10-substituted isoalloxazine is amphiphilic in nature. The substitution of different alkyl groups at position 10 of isoalloxazine ring increases the amphiphilicity of flavin nucleus. The sonic dispersal of isoalloxazines of different chain length at 10 position in aqueous media (5 $\%$ methanol) formed vesicles of varying sizes. The 10-alkyl isoalloxazine were synthesized by cyclocondensation of o-chloro nitrobenzene with different amines followed by catalytic hydrogenation and treatment with alloxan.The vesicle formation by the aggregation of isoalloxazine are supported by different techniques like transmission electron microscopy, quasielastic light scattering etc.. Thus, the synthetic 10-alkyl isoalloxazines are behaving as an ideal amphiphiles or new lipid.

GG6.3 PRELIMINARY RESULTS ON IN VITRO AND IN VIVO BIOCOMPATIBILITY OF CHITOSAN-XANTHAN POLYIONIC COMPLEX. Fatiha Chellat, Maryam Tabrizian , Ecole Polytechnique, Biomedical Engineering Inst, Montreal, Qc, CANADA; Severian Dumitriu, Estaban Chornet, Sherbrooke Univ, Dept of Chemical Engineering, Sherbrooke, Qc, CANADA; Charles Hilaire Rivard, Ste Justine Hospital, Montreal, Qc, CANADA; L'hocine Yahia, Ecole Polytechnique, Biomedical Engineering Inst, Montreal, Qc, CANADA.

Biodegradable polymers are among the most widely used materials in pharmaceutical sciences for drug delivery systems. Chitosan and xanthan have been extensively studied for this purpose. Here, we focused on Chitosan-Xanthan microspheres hydrogel-based obtained after chitosan and xanthan complexation. The polycationic propeties of chitosan allows the interaction with xanthan gum having polyanionic properties resulting in a polyelectrolyte complex. In this study we report on the in vitro and in vivo biocompatibility of the complex. For in vitro studies, we used L-929 and J-774 cell lines, respectively fibroblastic and macrophages cells. For the first one, we studied the influence of chitosan-xanthan microspheres on cell viability using MTT test. The assessment of cytokines {IL-1b and TNF-a} and nitric oxide secretion was performed on J-774 cell line. For these studies, several concentrations of microspheres and their degradation products were used. For in vivo studies, tablets with 1 cm in diameter and 1 mm in thickeness were implanted subcutaneously in the back of male Wistar rats at the interscapular level. The results showed no cytotoxic effects of the microspheres on L-929 cells. An increase in nitric oxide formation was observed when the concentration of microspheres increased. TNF-a production increased significantly after macrophages incubation with the macrospheres. A maximum cytokine production is reached for the concentration of 1 mg/ml of microspheres. The degradation products affected the cytokines secretion as a function of microspheres incubation time in culture medium. In vivo studies showed a good biocompatibilty of chitosan-xanthan microspheres. They also indicated that the degradation of the particles and their fragmentation result in further normal foreign body reactions.