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Novel membranes from conducting polymers
Journal of Membrane Science, 87 (1994) 23-34 Elsevier Science B.V., Amsterdam
Novel membranes
23
from conducting polymers*
J. Mansouri and R.P. Burford** School of Chemical Engineering and Industrial Chemistry, University of New South Wales, P.O. Box 1, Kensington, N.S. W. 2033 (Australia) (Beceived December 4,1992; accepted in revised form February 26,1993)
Abstract The first part of this paper presents the preparation and morphological studies of high surface area polypyrrole with fibrillar morphology. Mode of fibre growth within host membrane pores has been examined by ultra-microtome techniques. The rate of fibre growth in different solvents has been determined. The second part of this paper describes the formation of conducting composite membranes of polypyrrole with in-house prepared and commercial (“Biotran”) microporous polyamides. The conducting polymer was chemically polymerized using several techniques in association with the membranes in these studies. Surface and cross-sectional membrane morphology was studied by high-resolution field emission scanning electron microscopy, optical microscopy and transmission electron microscopy. Environmental stability of composite membranes has been examined by measuring the conductivity as a function of time. Key words: conducting polymers; membrane preparation and structure
1. Introduction
Polymer morphology must be controlled for many applications. In polymeric devices the chemical and molecular structure has to be appropriately tailored and the microstructure at the sub-micrometer level must also be optimized. A good example of such need for control we find for micro-separations. It is well understood that the performance (selectivity, durability, flux, rejection) of a composite membrane is dependent on its microstructure. For such membranes the microstructure consists of the structural characteristics of the single com*Paper presented at the Int. Membrane Science and Technology Conference, November 10-12, 1992, Sydney, Australia. **To whom correspondence should be addressed.
0376-7388/94/$07.00
ponent membranes (including pore size, shape and density and surface texture) together with the unique micro-structural characteristics of composite membranes, such as the dimensions of the various material domains, bonding between two domains and penetrations of one phase into another. For polymers where the formation of solid films can be achieved by precipitation or phase inversion from solution, many ways are available for membrane design. For most conducting polymers solubility is low and so restrictions exist for solvent casting. However, other strategies are possible to make textures with a tailored morphology at the submicrometer level. For example free-standing films of desirable thickness and/or porosity have been prepared and have been developed for gas-gas separations [ 1 ] and as ‘ion-sieves’
0 1994 Elsevier Science B.V. All rights reserved.
SSDZ0376-7388(93)E0034-H
24
[ 21. In the latter case, by appropriate selection of the dopant anion, electrodeposited polypyrrole films can be made to pass some ions several decades more efficient than others. Here it was shown that membrane porosity is closely related with ion affinity. Whilst there have been numerous descriptions of conducting polymer morphology, there are relatively few examples of highly ordered, high surface area products. We have previously described a range of structures including fibres and fractals [ 3 ] and Martin and coworkers have also illustrated some fibrillar forms [ 451. The high ionic and electronic conductivity [6] of conducting polymers with fibrillar structures render them potentially useful for membranerelated applications. In the first part of the present paper, the formation of densely packed polypyrrole fibres is described. A second approach is to combine conducting polymers with a second, conventional membrane to improve the performance. By analogy with the composite membranes of Cadotte et al. [ 71, an essentially continuous polypyrrole (or substituted analogue ) film has been deposited on a y-alumina or polycarbonate substrate to form reasonably durable permselective membranes for gas [ 81 and liquid separation [ 91. It is obvious that a broad spectrum of other approaches is possible, leading, for example, to membranes where the conducting polymer is uniformly or non-uniformly distributed, and where the pre-existing membrane constituent is retained or removed. The second part of this paper describes the formation of conductive composite membranes of polypyrrole with inhouse prepared polyamide and commercial polyamide. Polypyrrole was chosen because when associated with suitable anions (including ptoluene sulphonate, perchlorate and tetrafluoroborate), it retains adequate conductivity in various environments [ 10,111 compared with polymers with initially a higher conductivity such as polyacetylene. It is also easily polymer-
J. Mansouri,
R.P. Burford / J. Membrane
Sci. 87 (1994) 23-34
ized. On the other hand the ionic nature of doped conducting polypyrrole and its hydrogen bonding ability, make the composite of this polymer with conventional membranes more accessible to aqueous solution. The resulting membranes are thus expected to have an appropriate mechanical stability, higher selectivity for ions in aqueous solutions and possibly a higher fouling resistance. 2. Experimental Materials Pyrrole (Sigma Chemical Company) was distilled prior to use. Tetraethylammonium para-toluenesulphonate (Alfa/Aldrich Chemical Company), used as the dopant electrolyte to prepare fibrillar polypyrrole, was stored in a vacuum desiccator before use. Propylene carbonate (Sigma Chemical Company) and acetonitrile were used as received. Ferric chloride hexahydrate (an oxidant for the chemical polymerization of pyrrole), ethanol and hydrochloric acid were analytical grades and were used without further purification. Polyamide-6 in the form of a highly oriented, finely divided textile yarn (44 d tex/l3 filaments) was used for in-house prepared polyamide-6 membranes. Microporous polyamide membrane (from Biotrans), with an effective pore size of 0.22 pm and thickness of 144 p, was used as the substrate for the preparation of composite membranes. “Anotec” y-alumina membrane (pore density x 60%) with a nominal pore diameter of 0.2 pm and thickness of 65 ,um was used as the sacrificial host membrane for fibrillar polypyrrole formation. Fibrillur polypyrrok synthesis u-singa yalumina template A single compartment cell with a carbon plate as counter electrode and gold coated mem-
25
J. Mansouri, R.P. Burford /J. Membrane Sci. 87 (19%) 23-34
Fig. 1. Side view of anode assembly (Ref. [ 3 ] ) .
Extended
Polymerization
~lsoporous -Gold -Adhesive
membrane
1
L Polypyrrole
coating
time
J
Etching
-1 Polypyrrole product
tape
Fig. 2. Schematic of fibrillar polypyrrole preparation.
branes as the working electrode (anode) was used for electropolymerization of pyrrole through membrane micropores. The anode assembly which is mainly the gold coated y-alumina membrane is the same as previously described [3] and is shown in Fig. 1. Electropolymerization was carried out under galvanostatic conditions, using a current limiting device, at room temperature. Typically a current density of 2 mA-cmm2 was employed. The polymerization solution contained pyrrole (0.3 M) and dopant (0.1 M). Distilled water, propylene carbonate and acetonitrile were each used as solvents. A small amount of distilled water (l-2vol.% ) was used as co-solvent [ 121 for non-aqueous polymerizations. The y-alumina was separated from polymer by immersing the composite in 1 M sodium hydroxide, typically for 0.5-1.0 hr. The resulting f’ibrillar polypyrrole was rinsed with distilled water and dried under ambient conditions. Figure 2 schematically represents the fibrillar polypyrrole formation.
Preparation of conductive composite membranes
Unlike the above electrodeposition, here pyrrole is chemically polymerized using FeC& as oxidizing agent. The polymerization solution typically contained pyrrole (1 M) and FeC& (2 M) in distilled water.
In-house prepared polyamide/polypyrrok Asymmetric nylon-6 membrane was prepared by the phase inversion method [ 131. The “dope” solution contained nylon-6 yarn (30 g ), hydrochloric acid (50 ml) as solvent, ethanol swelling agent (5 ml) and water (25 ml). In situ polymerization, in which either a 1 hr aged dope is cast in a pyrrole polymerization solution or is dipped as a preformed membrane in the polymerization solution for 30 min were successful techniques for preparation of composite membranes.
26
J. Mansouri,
R.P. Burford / J. Membrane
Sci. 87 (1994) 23-34
Fig. 3. Polypyrrole fibres formed in propylene carbonate polymerization solution.
Fig. 4. Fibres emerging from Anotec membrane with different cross-section shape.
Fig. 5. Upper section of composite membrane. (Non-hexagonal pores).
J. Mansouri, R.P. Burford /J. Membrane Sci. 87 (1994) 23-34
21
Fig. 6. One region of composite membrane at upper section (bexagonalpores). Fig. 7. Middle section of composite.
Composites of polypyrrole with commercial polyamide Two different procedures were used to incorporate polypyrrole in the conventional host membranes as follows: (1) The host membrane (about 5 cm x 5 cm square) was immersed in a 1 A4aqueous pyrrole (or 2 M aqueous FeCl, oxidant ) solution for 20 min. (2) The immersed host membrane was partially dried by blotting with filter paper, to remove excess superficial solution. (3) Another immersion in the corresponding oxidant (or pyrrole) solution for 20 min was performed. (4) The resulting composite membrane was washed with copious amounts of water to remove remaining monomer and oxidant and also loosely attached particles of polypyrrole. (5) The composite membranes were dried
35
0
20
40
Time
60
80
11 0
(min)
Fig. 8. Fibre length vs. time for different solvent. [M] = 0.3 mol-l-l, [M]/[O] =3, current density: 2 mA-cm-‘.
J. 1tiansouri, R.P. Burford /J. Membrane
28
Fig. 9. Top surface of asymmetric membrane.
in-house polyamide
Sci. 87 (1994) 23-34
Fig. 10. Top surface of membrane polypyrrole.
after coating
by
under ambient conditions for about 5-10 hr, depending on substrate type.
ultramicrotome, equipped with a power driven cutting edge, to overcome sample brittleness.
Microscopy
Stability
Morphologies of gold coated samples were studied with a Cambridge Stereoscan S360 SEM using an accelerating voltage of 25 kV. Higher resolution micrographs were obtained using a Hitachi S900 field emission scanning electron microscope at 2 and 10 kV. Samples were coated with ca. 2-4 nm of platinum or chromium for these studies. Microtomed sections from embedded samples in Spurr resin were prepared by using a Reichart Jung “Ultracut” microtome for observation in an Hitachi 7000 TEM. Sectioning of y-aluminalppy composite samples was done using a RMC MT-7
The environmental stability of the composite membranes was determined by measuring the electrical conductivity by the standard fourpoint probe method [4] at room temperature, at various ageing times. 3. Results and discussion Fibrillar polypyrrole The polypyrroles have dense fibrillar morphologies for each of the solvents investigated. Figure 3 shows a micrograph of polypyrrole
J. Mansouri, R.P. Burford /J. Membrane Sci. 87 (1994) 23-34
Fig. 11. Bottom surface of host membrane.
Fig. 12. Bottom surface of composite membrane.
prepared in propylene carbonate. It is essentially the same for water and for acetonitrile. Polypyrrole fibres grow through the pores of the host membrane. The presence of fibres with different cross-section shapes (such as circular, trilobate and hexagonal) (Fig. 4 and elsewhere as described below) can be attributed to non-uniformity of membrane structure as is evident in sectioned composite samples (Figs.
in this surface adsorption phenomenon. Growth rate studies were done for different solvents. Fibre length was measured directly from the corresponding micrographs obtained for samples prepared at different polymerization times ranging from 5 to 100 min. The plot of fibre length versus polymerization time is shown in Fig. 8, from which it appears that growth for each solvent shows a similar trend, with a nominal growth rate of 0.15-0.3 p/min.
5,6). Some contraction of polypyrrole during polymerization has occurred, due to the density difference between monomer and polymer (p,/ p,, N 0.71). This is shown in Fig. 7. The existance of some tubes (Fig. 6) suggest that polypyrrole at first deposits on the surface of the pore walls. They may then grow radially to fill each pore and ultimately form solid fibres. Some kind of binding between the growing polymer and the pore walls might be involved
Conductive composite membranes Polypyrrole/in-house prepared polyamide-6 membrane The top surface of the host (i.e. untreated) and composite membranes is shown in Figs. 9 and 10 respectively. The original pores have disappeared in the composite surface (Fig. 10)
30
J. Mansouri,
R.P. Burford / J. Membrane
Sci. 87 (1994) 23-34
Fig. 13. Cross-section of composite membrane. Thin polypyrrole film can be seen at the top surface, marked with an arrow.
Fig. 14. Incorporation of ppy into cell walls.
PA
Fig. 17. Surface morphology of composite membrane (M/ 0).
confirming that polypyrrole has completely covered the surface. However, the bottom surface of the host has very large pores, and now the polypyrrole coating the surface, with the pores only marginally affected. Thus in the higher magnification micrographs (Figs. 11 and
12 ) the original gross morphology is similar, but the surface texture has been altered. So the cell walls are modified from a semi-fibrous aggregate to a more granular texture, with pore diameters remaining in the l-2 p range. Transmission electron and optical micros-
Fig. 16. Surface morphology of host commercial membrane.
J. Mansouri,
32
R.P. Burford / J. Membrane
Sci. 87 (1994) 23-34
copy of membrane sections reveal more information on the mode of incorporation of polypyrrole in host membranes. A thin, uniform, continuous layer (skin) of polypyrrole at the membrane top surface is indicated by the arrow in the optical micrograph of the composite membrane (Fig. 13). TEM micrographs of composite membranes suggest that polypyrrole has penetrated within the polyamide cell walls (Fig. 14). Figure 15 is a TEM montage of the overall composite membrane section and it appears that the polypyrrole is uniformly distributed. Although the dark material might appear to form a web in thin sections, the complete membrane contains a continuum of polypyrrole, as indicated by the continuous black film obtained as a residue after polyamide dissolution.
Fig. 18. Surface morphology of composite membrane M).
(O/
H-PAIPPY
0’ 0
400
800
1,200
I.600
Time (hours) Fig. 19. Conductivity of polypyrrole impregnated PA membranes vs. exposure time to air, 25°C.
Surface morphology of commercial polyamidelpolypyrrole composite membranes The surface morphology of these composite membranes in terms of roughness and globular structure is similar to composites of ppy/inhouse prepared polyamide membranes. The originally smooth membranes surface has been roughened upon coating, as shown for control and coated membranes in Figs. 16-18. Composite membranes are either designated M/O (host membrane dipped into monomer first, then oxidant) or O/M (the reverse sequence ) . When Figs. 17 and 18, micrographs of composites prepared by two different procedures, are compared, the following points can be made: (1) For M/O composites, polypyrrole particles which are mostly in the form of agglomerates, are attached to the outer membrane surface and there are some particles trapped inside the pores. However, for O/M composites, particles have dispersed over both the outer and inner surface of the host membrane. (2 ) The surface texture of O/M composites is rougher than for M/O composites. (3) The original microporous morphology of
33
J. Mansouri, R.P. Burford /J. Membrane Sci. 87 (1994) 23-34
the host membrane was unchanged after coating in both composites. Conductivity stability The environmental stability of conductive composite membrane was determined by measuring the conductivity for different times of exposure at ambient conditions, and is shown in Fig. 19. It can be seen that in-house prepared membranes exhibited a higher conductivity than commercial polyamide membrane. This can be mainly related to the different morphological aspects of these two membranes. The average pore size in the in-house prepared polyamide membrane is larger than that in the commercial one. This gives a smaller surface area within the membrane so that thicker coatings occur for a given mass of polypyrrole formed. The thicker conducting polymer layer at a unit surface area results in a higher conductivity for this membrane. After samples had been aged for up to 60 days at 25 ‘C and low humidity, the conductivity was remeasured. Whilst a relatively modest decline ( x 10%) was observed, recent preliminary results suggest that a significant loss in conductivity may occur in the continuous presence of water. Substantial embrittlement compared with the host has also to be avoided. The mechanical behaviour and other performance indicators of these composite membranes are currently investigated in our laboratory. 4. Conclusion Fibrillar polypyrrole Variations in fibre cross-section shape are mainly due to heterogeneities in host membrane structure, as has been shown in micrographs of sections of the composite. The presence of hollow tibres suggests that polypyrrole,
initially, deposits on the surface of the pore walls. Due to density differences between the monomer and the polymer there is contraction during fibre formation, leading to alteration in fibre shape. Fibre growth kinetics are of a similar form for the three solvents studied, with acetonitrile giving the highest rate. Conductive composite membrane Irrespective of membrane type, thin uniform coatings of conducting polymer can be prepared. TEM and SEM photos of asymmetric in-house prepared polyamide membranes show that a thin layer of conducting polymer has completely covered the membrane top surface. Composites of commercial polyamide membranes, prepared by two different methods, show a different morphology in terms of surface smoothness and aggregation of polypyrrole particles. The conductivity of the composites is dependent on the surface morphology of the host membranes. Acknowledgement The support provided by the Australian Grants Committee is gratefully acknowledged. Assistance in advanced electron microscopy techniques was provided by M.R. Dickson and P.B. Marks. References M.R. Anderson, B.R. Mattes, H. Keiss and R.B. Kaner, Gas separation membranes from conjugatedpolymer films, Syn. Met., 41-43 (1991) 1151. E. Beelen, J. Riga and J.J. Verb&, Electrochemical doping of polypyrrole: XPS study, Syn. Met., 41-43 (1991) 449. S.N. Atchison, R.P. Burford, T.A. Darragh and T. Tongtam, Morphology of high surface area polypyrrole structures, Polym. Int., 26 (1991) 261.
J. Mansouri, R.P. Burford / J. Membrane
34 4
5 6
Z. Cai and C.R. Martin, Electronically conductive polymer fibres with mesoscopic diameters shown enhanced electronic conductivities, J. Am. Chem. Sot., 111 (1989) 4138. C.R. Martin, Template synthesis of polymeric and metal microtubules, Adv. Mater., 3 (1991) 457. L.S. Van Dyke and C.R. Martin, Fibrillar electronitally conductive polymers show enhanced rates of charge transport, Syn. Met., 36 (1990) 275. J.E. Cadotte, R.S. King, R.J. Majerle and R.J. Petersen, Interfacial synthesis in the preparation of reverse osmosis membrane, Maromol. Sci. Chem., Al5 (1981)
10
727.
13
W. Liang and C.R. Martin, Gas transport in electronically conductive polymers, Chem. Mater., 3 (1991) 390. D.L. Feldheim and M. Elliot, Switchable gate membranes. Conducting polymer films for the selective transport of neutral solution species, J. Membrane Sci., 70 (1992) 9.
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12
14
Sci. 87 (1994) 23-34
V.T. Traung, B.C. Ennis, T. Turner and C.M. Jenden, Thermal stability of polypyrroles, Polym. Int., 27 (1992) 187. N.C. Billingham and P.D. Calve& Electrically conductingpolymers - A polymer science viewpoint, Adv. Polym. Sci., 90 (1989) 1. D. Bloor, R.D. Hercliffe, G.C. Galiotis and R.J. Young, The mechanical properties of polypyrroles plates, in: H. Kuzmany, M. Mehring and S. Roth (Eds.), Electronic Properties of Polymers and Related Compounds, Springer Series of Solid State Sciences, No. 63, Springer, Berlin, 1989, p. 179. R.M. McDongh, C.J.D. Fell and A.G. Fane, Characteristics of membranes formed by acid dissolution of polyamides, J. Membrane Sci., 31 (1987) 321. F.M. Smits, Measurement of sheet resistivities with the four-point probe, Bell Syst. Tech. J., 37 (1958) 711.
Novel membranes
23
from conducting polymers*
J. Mansouri and R.P. Burford** School of Chemical Engineering and Industrial Chemistry, University of New South Wales, P.O. Box 1, Kensington, N.S. W. 2033 (Australia) (Beceived December 4,1992; accepted in revised form February 26,1993)
Abstract The first part of this paper presents the preparation and morphological studies of high surface area polypyrrole with fibrillar morphology. Mode of fibre growth within host membrane pores has been examined by ultra-microtome techniques. The rate of fibre growth in different solvents has been determined. The second part of this paper describes the formation of conducting composite membranes of polypyrrole with in-house prepared and commercial (“Biotran”) microporous polyamides. The conducting polymer was chemically polymerized using several techniques in association with the membranes in these studies. Surface and cross-sectional membrane morphology was studied by high-resolution field emission scanning electron microscopy, optical microscopy and transmission electron microscopy. Environmental stability of composite membranes has been examined by measuring the conductivity as a function of time. Key words: conducting polymers; membrane preparation and structure
1. Introduction
Polymer morphology must be controlled for many applications. In polymeric devices the chemical and molecular structure has to be appropriately tailored and the microstructure at the sub-micrometer level must also be optimized. A good example of such need for control we find for micro-separations. It is well understood that the performance (selectivity, durability, flux, rejection) of a composite membrane is dependent on its microstructure. For such membranes the microstructure consists of the structural characteristics of the single com*Paper presented at the Int. Membrane Science and Technology Conference, November 10-12, 1992, Sydney, Australia. **To whom correspondence should be addressed.
0376-7388/94/$07.00
ponent membranes (including pore size, shape and density and surface texture) together with the unique micro-structural characteristics of composite membranes, such as the dimensions of the various material domains, bonding between two domains and penetrations of one phase into another. For polymers where the formation of solid films can be achieved by precipitation or phase inversion from solution, many ways are available for membrane design. For most conducting polymers solubility is low and so restrictions exist for solvent casting. However, other strategies are possible to make textures with a tailored morphology at the submicrometer level. For example free-standing films of desirable thickness and/or porosity have been prepared and have been developed for gas-gas separations [ 1 ] and as ‘ion-sieves’
0 1994 Elsevier Science B.V. All rights reserved.
SSDZ0376-7388(93)E0034-H
24
[ 21. In the latter case, by appropriate selection of the dopant anion, electrodeposited polypyrrole films can be made to pass some ions several decades more efficient than others. Here it was shown that membrane porosity is closely related with ion affinity. Whilst there have been numerous descriptions of conducting polymer morphology, there are relatively few examples of highly ordered, high surface area products. We have previously described a range of structures including fibres and fractals [ 3 ] and Martin and coworkers have also illustrated some fibrillar forms [ 451. The high ionic and electronic conductivity [6] of conducting polymers with fibrillar structures render them potentially useful for membranerelated applications. In the first part of the present paper, the formation of densely packed polypyrrole fibres is described. A second approach is to combine conducting polymers with a second, conventional membrane to improve the performance. By analogy with the composite membranes of Cadotte et al. [ 71, an essentially continuous polypyrrole (or substituted analogue ) film has been deposited on a y-alumina or polycarbonate substrate to form reasonably durable permselective membranes for gas [ 81 and liquid separation [ 91. It is obvious that a broad spectrum of other approaches is possible, leading, for example, to membranes where the conducting polymer is uniformly or non-uniformly distributed, and where the pre-existing membrane constituent is retained or removed. The second part of this paper describes the formation of conductive composite membranes of polypyrrole with inhouse prepared polyamide and commercial polyamide. Polypyrrole was chosen because when associated with suitable anions (including ptoluene sulphonate, perchlorate and tetrafluoroborate), it retains adequate conductivity in various environments [ 10,111 compared with polymers with initially a higher conductivity such as polyacetylene. It is also easily polymer-
J. Mansouri,
R.P. Burford / J. Membrane
Sci. 87 (1994) 23-34
ized. On the other hand the ionic nature of doped conducting polypyrrole and its hydrogen bonding ability, make the composite of this polymer with conventional membranes more accessible to aqueous solution. The resulting membranes are thus expected to have an appropriate mechanical stability, higher selectivity for ions in aqueous solutions and possibly a higher fouling resistance. 2. Experimental Materials Pyrrole (Sigma Chemical Company) was distilled prior to use. Tetraethylammonium para-toluenesulphonate (Alfa/Aldrich Chemical Company), used as the dopant electrolyte to prepare fibrillar polypyrrole, was stored in a vacuum desiccator before use. Propylene carbonate (Sigma Chemical Company) and acetonitrile were used as received. Ferric chloride hexahydrate (an oxidant for the chemical polymerization of pyrrole), ethanol and hydrochloric acid were analytical grades and were used without further purification. Polyamide-6 in the form of a highly oriented, finely divided textile yarn (44 d tex/l3 filaments) was used for in-house prepared polyamide-6 membranes. Microporous polyamide membrane (from Biotrans), with an effective pore size of 0.22 pm and thickness of 144 p, was used as the substrate for the preparation of composite membranes. “Anotec” y-alumina membrane (pore density x 60%) with a nominal pore diameter of 0.2 pm and thickness of 65 ,um was used as the sacrificial host membrane for fibrillar polypyrrole formation. Fibrillur polypyrrok synthesis u-singa yalumina template A single compartment cell with a carbon plate as counter electrode and gold coated mem-
25
J. Mansouri, R.P. Burford /J. Membrane Sci. 87 (19%) 23-34
Fig. 1. Side view of anode assembly (Ref. [ 3 ] ) .
Extended
Polymerization
~lsoporous -Gold -Adhesive
membrane
1
L Polypyrrole
coating
time
J
Etching
-1 Polypyrrole product
tape
Fig. 2. Schematic of fibrillar polypyrrole preparation.
branes as the working electrode (anode) was used for electropolymerization of pyrrole through membrane micropores. The anode assembly which is mainly the gold coated y-alumina membrane is the same as previously described [3] and is shown in Fig. 1. Electropolymerization was carried out under galvanostatic conditions, using a current limiting device, at room temperature. Typically a current density of 2 mA-cmm2 was employed. The polymerization solution contained pyrrole (0.3 M) and dopant (0.1 M). Distilled water, propylene carbonate and acetonitrile were each used as solvents. A small amount of distilled water (l-2vol.% ) was used as co-solvent [ 121 for non-aqueous polymerizations. The y-alumina was separated from polymer by immersing the composite in 1 M sodium hydroxide, typically for 0.5-1.0 hr. The resulting f’ibrillar polypyrrole was rinsed with distilled water and dried under ambient conditions. Figure 2 schematically represents the fibrillar polypyrrole formation.
Preparation of conductive composite membranes
Unlike the above electrodeposition, here pyrrole is chemically polymerized using FeC& as oxidizing agent. The polymerization solution typically contained pyrrole (1 M) and FeC& (2 M) in distilled water.
In-house prepared polyamide/polypyrrok Asymmetric nylon-6 membrane was prepared by the phase inversion method [ 131. The “dope” solution contained nylon-6 yarn (30 g ), hydrochloric acid (50 ml) as solvent, ethanol swelling agent (5 ml) and water (25 ml). In situ polymerization, in which either a 1 hr aged dope is cast in a pyrrole polymerization solution or is dipped as a preformed membrane in the polymerization solution for 30 min were successful techniques for preparation of composite membranes.
26
J. Mansouri,
R.P. Burford / J. Membrane
Sci. 87 (1994) 23-34
Fig. 3. Polypyrrole fibres formed in propylene carbonate polymerization solution.
Fig. 4. Fibres emerging from Anotec membrane with different cross-section shape.
Fig. 5. Upper section of composite membrane. (Non-hexagonal pores).
J. Mansouri, R.P. Burford /J. Membrane Sci. 87 (1994) 23-34
21
Fig. 6. One region of composite membrane at upper section (bexagonalpores). Fig. 7. Middle section of composite.
Composites of polypyrrole with commercial polyamide Two different procedures were used to incorporate polypyrrole in the conventional host membranes as follows: (1) The host membrane (about 5 cm x 5 cm square) was immersed in a 1 A4aqueous pyrrole (or 2 M aqueous FeCl, oxidant ) solution for 20 min. (2) The immersed host membrane was partially dried by blotting with filter paper, to remove excess superficial solution. (3) Another immersion in the corresponding oxidant (or pyrrole) solution for 20 min was performed. (4) The resulting composite membrane was washed with copious amounts of water to remove remaining monomer and oxidant and also loosely attached particles of polypyrrole. (5) The composite membranes were dried
35
0
20
40
Time
60
80
11 0
(min)
Fig. 8. Fibre length vs. time for different solvent. [M] = 0.3 mol-l-l, [M]/[O] =3, current density: 2 mA-cm-‘.
J. 1tiansouri, R.P. Burford /J. Membrane
28
Fig. 9. Top surface of asymmetric membrane.
in-house polyamide
Sci. 87 (1994) 23-34
Fig. 10. Top surface of membrane polypyrrole.
after coating
by
under ambient conditions for about 5-10 hr, depending on substrate type.
ultramicrotome, equipped with a power driven cutting edge, to overcome sample brittleness.
Microscopy
Stability
Morphologies of gold coated samples were studied with a Cambridge Stereoscan S360 SEM using an accelerating voltage of 25 kV. Higher resolution micrographs were obtained using a Hitachi S900 field emission scanning electron microscope at 2 and 10 kV. Samples were coated with ca. 2-4 nm of platinum or chromium for these studies. Microtomed sections from embedded samples in Spurr resin were prepared by using a Reichart Jung “Ultracut” microtome for observation in an Hitachi 7000 TEM. Sectioning of y-aluminalppy composite samples was done using a RMC MT-7
The environmental stability of the composite membranes was determined by measuring the electrical conductivity by the standard fourpoint probe method [4] at room temperature, at various ageing times. 3. Results and discussion Fibrillar polypyrrole The polypyrroles have dense fibrillar morphologies for each of the solvents investigated. Figure 3 shows a micrograph of polypyrrole
J. Mansouri, R.P. Burford /J. Membrane Sci. 87 (1994) 23-34
Fig. 11. Bottom surface of host membrane.
Fig. 12. Bottom surface of composite membrane.
prepared in propylene carbonate. It is essentially the same for water and for acetonitrile. Polypyrrole fibres grow through the pores of the host membrane. The presence of fibres with different cross-section shapes (such as circular, trilobate and hexagonal) (Fig. 4 and elsewhere as described below) can be attributed to non-uniformity of membrane structure as is evident in sectioned composite samples (Figs.
in this surface adsorption phenomenon. Growth rate studies were done for different solvents. Fibre length was measured directly from the corresponding micrographs obtained for samples prepared at different polymerization times ranging from 5 to 100 min. The plot of fibre length versus polymerization time is shown in Fig. 8, from which it appears that growth for each solvent shows a similar trend, with a nominal growth rate of 0.15-0.3 p/min.
5,6). Some contraction of polypyrrole during polymerization has occurred, due to the density difference between monomer and polymer (p,/ p,, N 0.71). This is shown in Fig. 7. The existance of some tubes (Fig. 6) suggest that polypyrrole at first deposits on the surface of the pore walls. They may then grow radially to fill each pore and ultimately form solid fibres. Some kind of binding between the growing polymer and the pore walls might be involved
Conductive composite membranes Polypyrrole/in-house prepared polyamide-6 membrane The top surface of the host (i.e. untreated) and composite membranes is shown in Figs. 9 and 10 respectively. The original pores have disappeared in the composite surface (Fig. 10)
30
J. Mansouri,
R.P. Burford / J. Membrane
Sci. 87 (1994) 23-34
Fig. 13. Cross-section of composite membrane. Thin polypyrrole film can be seen at the top surface, marked with an arrow.
Fig. 14. Incorporation of ppy into cell walls.
PA
Fig. 17. Surface morphology of composite membrane (M/ 0).
confirming that polypyrrole has completely covered the surface. However, the bottom surface of the host has very large pores, and now the polypyrrole coating the surface, with the pores only marginally affected. Thus in the higher magnification micrographs (Figs. 11 and
12 ) the original gross morphology is similar, but the surface texture has been altered. So the cell walls are modified from a semi-fibrous aggregate to a more granular texture, with pore diameters remaining in the l-2 p range. Transmission electron and optical micros-
Fig. 16. Surface morphology of host commercial membrane.
J. Mansouri,
32
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copy of membrane sections reveal more information on the mode of incorporation of polypyrrole in host membranes. A thin, uniform, continuous layer (skin) of polypyrrole at the membrane top surface is indicated by the arrow in the optical micrograph of the composite membrane (Fig. 13). TEM micrographs of composite membranes suggest that polypyrrole has penetrated within the polyamide cell walls (Fig. 14). Figure 15 is a TEM montage of the overall composite membrane section and it appears that the polypyrrole is uniformly distributed. Although the dark material might appear to form a web in thin sections, the complete membrane contains a continuum of polypyrrole, as indicated by the continuous black film obtained as a residue after polyamide dissolution.
Fig. 18. Surface morphology of composite membrane M).
(O/
H-PAIPPY
0’ 0
400
800
1,200
I.600
Time (hours) Fig. 19. Conductivity of polypyrrole impregnated PA membranes vs. exposure time to air, 25°C.
Surface morphology of commercial polyamidelpolypyrrole composite membranes The surface morphology of these composite membranes in terms of roughness and globular structure is similar to composites of ppy/inhouse prepared polyamide membranes. The originally smooth membranes surface has been roughened upon coating, as shown for control and coated membranes in Figs. 16-18. Composite membranes are either designated M/O (host membrane dipped into monomer first, then oxidant) or O/M (the reverse sequence ) . When Figs. 17 and 18, micrographs of composites prepared by two different procedures, are compared, the following points can be made: (1) For M/O composites, polypyrrole particles which are mostly in the form of agglomerates, are attached to the outer membrane surface and there are some particles trapped inside the pores. However, for O/M composites, particles have dispersed over both the outer and inner surface of the host membrane. (2 ) The surface texture of O/M composites is rougher than for M/O composites. (3) The original microporous morphology of
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J. Mansouri, R.P. Burford /J. Membrane Sci. 87 (1994) 23-34
the host membrane was unchanged after coating in both composites. Conductivity stability The environmental stability of conductive composite membrane was determined by measuring the conductivity for different times of exposure at ambient conditions, and is shown in Fig. 19. It can be seen that in-house prepared membranes exhibited a higher conductivity than commercial polyamide membrane. This can be mainly related to the different morphological aspects of these two membranes. The average pore size in the in-house prepared polyamide membrane is larger than that in the commercial one. This gives a smaller surface area within the membrane so that thicker coatings occur for a given mass of polypyrrole formed. The thicker conducting polymer layer at a unit surface area results in a higher conductivity for this membrane. After samples had been aged for up to 60 days at 25 ‘C and low humidity, the conductivity was remeasured. Whilst a relatively modest decline ( x 10%) was observed, recent preliminary results suggest that a significant loss in conductivity may occur in the continuous presence of water. Substantial embrittlement compared with the host has also to be avoided. The mechanical behaviour and other performance indicators of these composite membranes are currently investigated in our laboratory. 4. Conclusion Fibrillar polypyrrole Variations in fibre cross-section shape are mainly due to heterogeneities in host membrane structure, as has been shown in micrographs of sections of the composite. The presence of hollow tibres suggests that polypyrrole,
initially, deposits on the surface of the pore walls. Due to density differences between the monomer and the polymer there is contraction during fibre formation, leading to alteration in fibre shape. Fibre growth kinetics are of a similar form for the three solvents studied, with acetonitrile giving the highest rate. Conductive composite membrane Irrespective of membrane type, thin uniform coatings of conducting polymer can be prepared. TEM and SEM photos of asymmetric in-house prepared polyamide membranes show that a thin layer of conducting polymer has completely covered the membrane top surface. Composites of commercial polyamide membranes, prepared by two different methods, show a different morphology in terms of surface smoothness and aggregation of polypyrrole particles. The conductivity of the composites is dependent on the surface morphology of the host membranes. Acknowledgement The support provided by the Australian Grants Committee is gratefully acknowledged. Assistance in advanced electron microscopy techniques was provided by M.R. Dickson and P.B. Marks. References M.R. Anderson, B.R. Mattes, H. Keiss and R.B. Kaner, Gas separation membranes from conjugatedpolymer films, Syn. Met., 41-43 (1991) 1151. E. Beelen, J. Riga and J.J. Verb&, Electrochemical doping of polypyrrole: XPS study, Syn. Met., 41-43 (1991) 449. S.N. Atchison, R.P. Burford, T.A. Darragh and T. Tongtam, Morphology of high surface area polypyrrole structures, Polym. Int., 26 (1991) 261.
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Z. Cai and C.R. Martin, Electronically conductive polymer fibres with mesoscopic diameters shown enhanced electronic conductivities, J. Am. Chem. Sot., 111 (1989) 4138. C.R. Martin, Template synthesis of polymeric and metal microtubules, Adv. Mater., 3 (1991) 457. L.S. Van Dyke and C.R. Martin, Fibrillar electronitally conductive polymers show enhanced rates of charge transport, Syn. Met., 36 (1990) 275. J.E. Cadotte, R.S. King, R.J. Majerle and R.J. Petersen, Interfacial synthesis in the preparation of reverse osmosis membrane, Maromol. Sci. Chem., Al5 (1981)
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W. Liang and C.R. Martin, Gas transport in electronically conductive polymers, Chem. Mater., 3 (1991) 390. D.L. Feldheim and M. Elliot, Switchable gate membranes. Conducting polymer films for the selective transport of neutral solution species, J. Membrane Sci., 70 (1992) 9.
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V.T. Traung, B.C. Ennis, T. Turner and C.M. Jenden, Thermal stability of polypyrroles, Polym. Int., 27 (1992) 187. N.C. Billingham and P.D. Calve& Electrically conductingpolymers - A polymer science viewpoint, Adv. Polym. Sci., 90 (1989) 1. D. Bloor, R.D. Hercliffe, G.C. Galiotis and R.J. Young, The mechanical properties of polypyrroles plates, in: H. Kuzmany, M. Mehring and S. Roth (Eds.), Electronic Properties of Polymers and Related Compounds, Springer Series of Solid State Sciences, No. 63, Springer, Berlin, 1989, p. 179. R.M. McDongh, C.J.D. Fell and A.G. Fane, Characteristics of membranes formed by acid dissolution of polyamides, J. Membrane Sci., 31 (1987) 321. F.M. Smits, Measurement of sheet resistivities with the four-point probe, Bell Syst. Tech. J., 37 (1958) 711.