CONDUCTIVE POLYMER MEMBRANES
J. SARRAZIN, M. PERSIN, M. CRETIN
Institut Européen des Membranes, UMR 5635 CNRS, ENSCM, UM2, Ecole Nationale Supérieure de Chimie de Montpellier, 8 rue de l'Ecole Normale, 34296 Montpellier Cedex 5, France. sarrazin / cit.enscm.fr
Since the electrochemical synthesis of a first flexible and stable polypyrrole (PPy) film of high conductivity by Diaz et al in 1979, many research works have been focused on conductive polymers (CP). A wide spectrum of applications was proposed, mainly related to the conductive properties of the material, but efforts towards membrane-based applications were, up to now, rather limited though efforts significantly increased in the 90's.
Preparation
Materials presenting electron or mixed ion/electron conductivity can be prepared by chemical or electrochemical polymerisation of some monomers (pyrroles, thiophenes, anilins...). Self-supported dense membranes or coating of porous membranes could be realised. Electrochemical synthesis is of special interest since it allows to vary many parameters and to get a precise monitoring of the preparation process by adjusting the electric signal, electric potential or current density, electrolysis duration, supporting electrolyte, solvent…
Properties
Properties of such electrochemically deposited conducting polymers can be followed by cyclic voltammetry (CV), quartz microbalance (QMB) and electrochemical impedance spectrometry (EIS). Actually, these techniques allow the investigation of the cation/anion doping/de-doping phenomena, which accompanies the oxidation state variations of the polymer. On the other hand, an appropriate choice of the monomer allows introducing some specific functionality in the membrane.
Applications
Some examples are presented of the involvement of CP membranes in different application domains. For instance, CP membranes can actually be employed in the constitution of sensors. Some results are also given in the field of separation techniques such as:
- the separation of some organic molecular species (methanol MeOH / methyl tertiobutyle ther MTBE,...) by pervaporation
-ion separation by dialysis through CP membranes functionalised by some complexing moieties.
WHY ARE PERMSELECTIVE POLYMERS LOW-PERMEABLE?
YU.YAMPOLSKII, A.ALENTIEV
A.V.Topchiev Institute of Petrochemical Synthesis, Membrane Center, 29 Leninsky Pr., 119991, Moscow, Russia
A trade-off trend in permeability (Pi) and selectivity (a ij=Pi/Pj) is maybe the most general observation made after examining gas permeation properties of several hundred glassy polymers, prospected membrane materials. Initially, as the information was accumulated, the cloud of the data points in the coordinate plane moved to the area of larger a i and Pi, however, in the middle of 90s this movement was virtually stopped. An important achievement of Robeson [1] was a suggestion of a concept of so-called upper bound that is the line above which no data points can be found. Now an impact of all the new polymers as membrane materials is usually tested by its position in the a ij versus Pi diagram, that is, below, in vicinity, or above the upper bound line. The parameters of the upper bound lines for different gas pairs correlate with molecular sizes of the penetrants. It implies that a position of the data points clouds is determined by detailed mechanism of diffusion and nano-structure of polymers.
Recently, it was shown that entirely different approaches can explain the position of the data points clouds and the upper bound lines. This can be made either on the basis of the transition state theory [2] or free volume model [3]. Therefore, successful models, which describe gas permeation properties of glassy polymers, should combine both approaches. Such models are known (see e.g. [4]), however their practical and wide application is impeded by absence of numerous parameters unknown for vast majority of polymers.
Recently, positron annihilation lifetime spectroscopy was applied systematically for determination of free volume size, size distribution and concentration of microcavities in polymers with widely varying gas permeability (e.g. in the range 1-104 Barrer). It was shown that the concentration N of free volume elements in various polymers is virtually constant and is about 1020 cm-3. It means that the variation of the fractional free volume is determined primarily by the changes of the volume of microcavities, which fall in the range Vf = 100-1000 A3. Using these volumes and a mean concentration N an estimation can be made of average distance l between neighboring free volume elements. It was shown that it is very close to the diffusion jump length of various gases in different polymers, which is known e.g. from computer simulation studies. In extra high free volume materials such as poly(trimethylsilyl propyne) the l values are as small as 5-7 A, whereas in conventional glassy polymers they are several times larger (15-25 A). It means that free volume sizes are responsible for the level of diffusivity and permeability, whereas selectivity of diffusion is determined primarily by the processes that proceed in the walls of free volume elements, that is inter-chain interaction and local small scale mobility of the groups should be responsible for permselectivity.
Several semi-empiric equations relating the diffusion jump length, activation energy of diffusion, and permeability coefficients with the fractional free volume, concentration of microcavities, and cohesion energy density were deduced and confirmed on the basis of the Database "Gas Permeation Parameters of Glassy Polymers".
1. L.M.Robeson, J. Membr. Sci., 62, 165 (1991)
2. B.D.Freeman, Macromolecules, 32, 375 (1999).
3. A.Alentiev, Yu.Yampolskii, J. Membr. Sci., 165, 201 (2000).
4. J.S.Vrentas, J.L.Duda, J. Polym., Sci., Polym. Phys. Ed., 15, 403, 417 (1977).
Gas Separation Membranes: Needs for Combined Materials Science and Processing Approaches
William J. Koros
Department of Chemical Engineering, University of Texas at Austin, Austin, TX
Summary: Huge markets would exist for high volume gas separation membranes if more robust and higher selectivity membranes were economically available. Many of these markets include totally new paradigms, such as fuel cell driven vehicles and membrane reactors for hydrocarbon production. Others involve displacing entrenched large scale separations processes with more advanced versions of first generation "conventional" membranes. Existing materials and formation processes cannot exploit most of these opportunities, so basic research is needed. This research must occur with an awareness that competition to displace highly optimized conventional technologies such as absorption, cryogenic distillation and adsorption must consider economic as well as technical efficacy. Next generation membrane processes should, therefore, maintain attractive economics associated with current polymer-based membranes, while greatly extending performance properties. Several "contender" strategies based largely on polymers and specialized polymer processing approaches for achieving this ambitious goal will be considered in more detail.
Polymer electrolyte membranes for fuel cells
Carmen Manea and Marcel Mulder
University of Twente, Faculty of Chemical Technology, PO Box 217, 7500 AE Enschede, Netherlands
The interest in polymer electrolyte fuel cells (PEFC) has received increasing attention in the last decade. This is especially due to the interest in transportation where the PEFC is used as a primary power source in electrical driven cars and autobuses. However there are other applications such as low power mobile electronics (laptops, mobile phones, audio systems) where PEFCs will replace batteries. Finally, a third interesting application are stationary decentralised power supply systems.
The PEFC employs a polymeric membrane as electrolyte which is an electronic insulator but an excellent proton conductor. Most of the materials used today are based on a fluorocarbon polymer backbone to which a sulfonic group is attached to give the material the proton conductive properties. However, other materials may be employed as well based on the chemical, thermal and conductive properties. In order to compare various materials two classifications will be made based on the type of fuel, i.e. hydrogen or methanol, and the application temperature (low temperature T < 90 C and high temperature 90 < T < 200). In this way a matrix has been obtained with four classes. Four each class the required properties will be identified and this will be related to polymeric material properties. For instance when methanol is used as a fuel in a direct methanol fuel cell, the permeability of methanol across the membrane becomes very important and should be as low as possible. Despite their excellent material properties perfluoro-based polymers such as Nafion are only suitable in one class, being the low temperature applications in hydrogen fuel cells. This implies that for the other classes new materials are required with conductive properties comparable to Nafion. Various characterisation methods will be discussed such as ion exchange capacity, degree of swelling in methanol/water mixtures, electrical resistance, perm-selectivity, methanol and water permeability, methanol and water sorption isotherms, chemical and thermal stability tests and i-V characteristics and it will be indicated which method is specific for a certain class. Besides polymer electrolytes (ion-exchange materials) non-ionic polymers are attractive as candidates for specific applications. Finally, a new class of materials will be discussed with proton donor-proton acceptor properties.
CONTROLLED FLUX BEHAVIOUR OF MEMBRANE FRACTIONATION
J. HOWELL, T ARNOT, D. HATZIANTONIOU, H. CHUA AND R. FIELD
Dept. of Chem. Eng., University of Bath, Bath BA2 7AY cesjah / bath.ac.uk
This paper will review the use of controlled flux operation in
micro and ultrafiltration. The concept of critical fluxes will be
discussed and the major influences defined. Modern applications
of controlled flux operation will be mentioned and the use of
intermittent operation of a microfiltration membrane plant
demonstrated. Intermittent operation stops permeate flow allowing
detachment of fouling layers under the shear applied by air
bubbles. In this intermittent operation mode it is possible to
operate waste water treatment plants under either high or low
flux operation simply by changing the air flow. The flux can be
changed by a factor of three repeatedly up and down in sustained
operation.
With macromolecular fractionation controlled flux is necessary
to maximise fractionation. 
Controlled pressure leads to progressive fouling and
loss of separation factor. Even under controlled flux operation
with a complex polysaccharide mixture it is shown that separation
decreases sharply as flux is increased. This is because whilst
the high molecular weight components suffer decreasing rejection
the lower molecular weight components are increasingly rejected.
MEMBRANE HYBRID SYSTEMS: IDEA, MECHANISMS AND PERFORMANCES
R. WÓDZKI
Faculty of Chemistry, Nicolaus Copernicus University, 87-100 Toruń, Poland
wodzki / chem.uni.torun.pl
Two synthetic membranes of different physical and chemical nature, e.g. a liquid membrane (LM) combined in a series with an ion-exchange polymer membrane (IEM) form a simplest hybrid membrane system. The effective operation of such membrane assembly is conditioned by the consistency of transport mechanisms coupled by interfacial phenomena, e.g. ion-exchange dialysis (IEM) and pertraction mediated by an ionic carrier (LM), or pervaporation and pertraction - both based on a solution-diffusion mechanism. The system composed of IEM and LM is equivalent to the cell envelope of various bacteria architecture of which was treated as a pattern for constructing some new biomimetic membrane systems. For practical purposes the multimembrane hybrid system (MHS) composed of two-ion exchange membranes separated by a liquid membrane can be applied as a new membrane device combining the properties of the functionalized polymer (stably working) and liquid membranes (high selectivity contribution). The MHS was further applied as the basic unit for constructing a number of more complex integrated membrane systems. The system DD-MHS was constructed as a combination of the Donan dialysis and the MHS transport phenomena. The MHS-PV system was constructed by coupling the simultaneous pertraction and pervaporation process of a liquid membrane phase. The both systems clearly improve the performances of a liquid membrane as a most labile component of the system, and result in high selectivity and long time of operation under different conditions. The final system was composed by integrating DD-MHS and MHS-PV into a new membrane system DD-MHS[FLM-PV] exploiting the properties of IEM in Donnan dialysis (preconcentration) and ion-exchange dialysis (interfacial ion-exchange), the properties of flowing liquid membrane (FLM) containing a specific carrier (selective pertraction of cations) and the properties of a pervaporation membrane (water removal from organic phase). The systems were applied for the recovery of target cations from a number of synthetic and industrial electroplating rinse solutions or carboxylic acids from fermentation broths.
Poly (ether imide) membranes formation by water vapor induced phase inversion
Deratani A.3, Menut P.1,
Ripoche a., Caquineau H.2, Dupuy C.1
1: Lab. de Génie des Procédés, Université Montpellier II, bat
15 cc 024, 34095 Montpellier cedex 5, (France),
menut / crit.univ-montp2.fr
2: Lab. de Génie Electrique de Toulouse Université Paul Sabatier, 31062 Toulouse cedex 4 (France),
3: Lab. des Matériaux et Procédés Membranaires, IEM cc47, Université Montpellier II, 34095 Montpellier cedex 5 (France), deratani / iemm.univ-montp2.fr
Polymeric membranes have been developed in recent years for numerous industrial applications. The properties of a polymer film depend, in a large part, on its microstructure, which is controlled by the elaboration process. The first step consists of the casting or spinning of a homogeneous polymer solution. Thereafter, depending on the process conditions, various membrane materials featured by completely different morphology can be prepared that is, for example, a dense homogeneous film by dryness of the polymer solution and an asymmetric membrane with a greater or lesser porous structure by immersion in a coagulation bath. In the latter case, diffusion of non-solvent in and of solvent out the formed layer induces a phase separation yielding the formation of pores. This paper reports on an intermediate process which consists in supplying the non-solvent from the vapour phase, so-called "non-solvent vapour induced phase separation". The milder conditions of phase inversion taking place in this technique results in an easier possibility of understanding how the microstructure is formed which can afford a better control of the properties of the membranes obtained.
Poly(ether imide) was chosen as a model material because of the large range of polyimide film applications. The phase inversion was initiated from N-methyl pyrrolidone (NMP) solutions using water vapour as the non-solvent. After casting, the polymer liquid film was dried under a gas stream at various relative humidity. Water induces in the film a phase separation, via nucleation and growth of a polymer-lean phase dispersed in a polymer-rich phase. The polymer-poor phase droplets slowly grow and eventually coalesce until polymer rich-phase forming the foam wall structure reaches the gelification concentration. The membrane materials obtained can present a microcellular structure, composed of cells of various shape and size. The influence of such parameters as temperature, relative humidity or gas flux were investigated in a reactor especially designed for the purpose.
The results show that the morphology of the film produced (distribution of cell size and shape, cell wall thickness and inter-cell connectivity) can be varied depending on the elaboration process conditions applied. Formation of the structure and influence of the parameters studied will be discussed in terms of thermodynamic (ternary diagram) and kinetic aspects of the phase separation. On this basis, we propose a phenomenological model to explain what happens during the film formation.
Membrane Gas Separation Opportunities in the Control of Greenhouse Effect
Pushpinder S. Puri
Air Products and Chemicals, Inc., Allentown, Pennsylvania, U.S.A.
The sun's radiation which reach the earth's surface result in the heating of the ground surface, melting of the ice and snow, evaporation of water and plant photosynthesis. The earth's hot surface, in turn, acts as a radiator of energy in the infrared band. A majority of the outgoing infrared radiation is absorbed by a few naturally occurring atmospheric gases, such as carbon dioxide, water vapor, methane, ozone, nitrous oxide, chlorofluorocarbon (CFC's), halons, etc., known as greenhouse gases. As the concentration of these gases increases in the earth's atmosphere, the retention of radiated energy also increases, resulting in the warming of the earth's surface. This paper describes various opportunities offered by membranes to capture the greenhouse gases. Of the various gases listed above, CO2, because of its high concentrations in the atmosphere, is responsible for half of the human contribution to global warming. Therefore, use of membrane technology to control greenhouse effect is illustrated by the technology used to capture carbon dioxide. The membrane technology described here, however, is applicable to other greenhouse gases as well.
