- Email: [email protected]
Investigation of the gas-transport properties of polyaniline
journal of MEMBRANE SCIENCE JoumalofMembraneScience91
(1994) 1-12
Investigation of the gas-transport properties of polyaniline Susumu Kuwabata’,
Charles R. Martin*
Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA (Received September 23, 1993; accepted in revised form December 17, 1993)
Abstract Investigations of the rate and selectivity of gas transport in the electronically conductive polymer polyaniline are described. Both rate and selectivity were found to be dependent on the doping level. Permeability coefficients for all gases studied decreased with increasing doping level, and selectivity coefficients, in general, increased with doping level. The effects of other variables, including the nature of the counterion, temperature and duration of thermal processing of the film, were also explored. Both free-standing polyaniline films ( -2Qm thick) and thin film composite membranes based on polyaniline were studied. The thin film composites were prepared by coating the polyaniline film ( - 3+m thick) onto the surface of a microporous alumina support membrane. The freestanding and thin film composite membranes showed identical rates and selectivities of gas transport. The highest 02/Nz and C02/CH4 selectivity coefficients obtained were ffoz/NZ= 15 and (Ycoz/cH4= 55, respectively. Key words: Composite membranes; Gas separations; Membrane preparation and structure; Conductive polymers; Polyaniline
1. Introduction There is increasing interest in the idea of using synthetic membranes to do industrial separations [ l-5 1. This interest is fueled by the potential economic advantages inherent in membrane-separation technologies [l-5]. Furthermore, membrane-based separations can be viewed as an example of “green” (i.e., environmentally-friendly ) chemistry [ 6 ] in that membrane-based processes are potentially less energy intensive than other, more conventional, separations methods [ 3-5 1. However, as clearly pointed out in a recent report by the U.S. De‘Current address: Osaka University, Department of Applied Chemistry, Yamada-oka 2-1, Suita, Osaka 565, Japan. *Corresponding author.
partment of Energy, if membrane-based separations are to gain wide-spread use, membranes with higher permeabilities and higher transport selectivities will be required [ 41. Hence, there is a need to identify new types of membrane materials. We [ 7,8] and Anderson et al. [ 9, lo] have recently shown that electronically-conductive polymers [ 7- 111 are promising membrane materials for industrial gas separations. We investigated the conductive polymer poly (N-methylpyrrole) [ 7,8] and Anderson et al. studied polyaniline [ 9,10 1. Polyaniline appeared to be the most promising of these two conductive polymers in that the gas-transport selectivity data obtained by Anderson et al. were remarkable [ 9 1. Indeed, if these data are correct, polyaniline is one of the most exciting materials for potential
0376-7388/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SsDIO376-7388(94)00010-V
2
S. Kuwabata, CR. Martin /Journal ofMembrane Science 91 (I 994) l-12
applications in industrial gas separations to be discovered in many years. Because of the exceptional gas-transport data obtained in the previous investigation [ 93, we set out to explore the gas-transport properties of polyaniline for ourselves. Our approach, however, is different from that used by Anderson et al., who used thick ( N 0.1 mm) free-standing films of polyaniline to conduct their transport studies. We were interested in the concept of supported thin polymer films for membrane-based separations [ 7,8,12- I5 1. These “thin film composite membranes” consist of a thin film of the chemically selective polymer (e.g., the polyaniline) coated onto the surface of a microporous support membrane [ 7,8,12- 15 1. The advantage of this approach is that higher net fluxes across the composite can be obtained because the support is highly permeable and because the chemically-selective material is present as a thin film. We have investigated the gas-transport properties of polyaniline-based thin film composite membranes. We have not been able to reproduce the exceptional gas-transport selectivity obtained by Anderson et al. [ 91. We felt that this might be attributable to the fact that the polyaniline was present as a thin supported film rather than a thicker free-standing film. We have, therefore, also investigated the transport properties of free-standing polyaniline films. These membranes showed transport properties identical to those of our supported thin films; i.e., again, we were not able to reproduce the exceptional gastransport data obtained in the previous investigations [ 91. We describe the results of our investigations here.
membrane used to prepare the thin film composites was a microporous alumina filter (Anopore) [ 7,8,12- 15 1. This membrane contains cylindrical 200~nm diameter pores that run most of the way through the membrane’s thickness. These pores branch into -20~nm diameter pores at one membrane face. The polyaniline film was coated onto this branched-pore surface. 2.2. Polymer synthesis Polyaniline was synthesized according to the procedure of Huang et al. [ 161; Anderson et al. used the same procedure [ 93. A solution that was 1 M in aniline and 1 M in HCl was mixed with an equal volume of a solution that was 1 A4 in (NH4)&08 and 1 Min HCl. This results in oxidative polymerization of the aniline to produce the protically-doped (see below) emeraldine salt [ 16-l 91 form of polyaniline. The polymerization was allowed to proceed overnight. The resulting green precipitate was collected by filtration, rinsed with water and dried in vacua. The emeraldine salt was deprotonated by dispersing the insoluble powder in excess 1 M aqueous NH40H. This converted the emeraldine salt into the blue emeraldine base, which is also insoluble in water. The emeraldine base was then collected by filtration, rinsed with water and methanol and dried in vacua. One gram of the emeraldine base was then added to 100 ml of IV-methylpyrrolidone (NMP ). The resulting solution was stirred overnight and then filtered to remove the small amount of insoluble material. This solution was concentrated by partial evaporation of the solvent using a rotary evaporator. The stock solution obtained was 3.4% (w/w) in polyaniline.
2. Experimental 2.1. Materials
2.3. Preparation of composite membranes and free-standing polyaniline films
Aniline (Aldrich ) and N-methylpyrrolidone (Sigma) were vacuum distilled prior to use. Hexadecyltrichlorosilane (Hulls America), NH40H ( NH4 ) &OS ( Mallinckrodt ) and (Mallinckrodt ) were of reagent grade and were used without further purification. The support
The composite membranes were prepared by casting films of polyaniline (from the stock solution) onto the surface of the microporous alumina support membrane. Prior to film-casting, the membranes were treated with hexadecyltrichlorosilane [ 7,201. This treatment causes
S. Kuwabata, C.R. Martin /Journal of MembraneScience
hexadecyl groups to be covalently bound to all of the surfaces of the alumina membrane [ 7,201. As will be discussed in detail below, when polyaniline solution was applied to the surface of the as-received membrane, the solution simply flooded the pores of the membrane. In contrast, the polyaniline solution was excluded from the pores of the silane-treated membrane. As a result, vacuum evaporation of NMP from the surface of the silane-treated membrane resulted in a thin film of polyaniline coating the upper surface of this membrane. Films of 3 to 4 p thickness were used for these studies. Film thicknesses were determined using electron microscopy [ 7,12151. Free-standing polyaniline films were prepared by casting the polymer solution onto glass slides and evaporating the solvent in vacua [ 9 1. The polyaniline films were removed from the glass surface by immersion in water [ 91. Free-standing films of 25 to 30 pm thickness were used for these studies. Finally, both the supported and the free-standing films were thermally annealed after film formation. Mattes et al. recommend annealing at 125°C for 3 h [ lo]. We employed these (and various other) annealing temperatures and times. 2.4. Protic doping The NMP-soluble emeraldine base form of polyaniline was used to prepare both the supported and free-standing polyaniline films. This form of the polymer is nonionic and is an electronic insulator [ 16- 19 1. The base can, however, be protonated to yield the polycationic emeraldine salt: This is the electronically-conductive form of the polymer. This can be accomplished by immersion of the film in aqueous HCl [ 17- 19 1. The
91(1994) l-12
3
extent of the doping reaction shown in Eq. ( 1) can be varied from 0 (no protonated monomer units) to 50% (half of the monomer units protonated) by varying the concentration of HCl in the solution used to dope the polymer [ 17-191. Anderson et al. found that the gas-transport properties of polyaniline varied with the extent of doping [ 91; we obtained analogous results with the conductive polymer poly (N-methylpyrrole) [ 71. Hence, it is important to explore the effect of doping level on the gas-transport properties of such polymers. The free-standing polyaniline films were doped by immersion (for 24 h ) in aqueous solutions of HCl [ 17- 19 1. The doping level was controlled by varying the concentration of the HCl used [ 17- 19 1. After doping, the films were dried in vacua. The thin film composite membranes were doped by exposure to HCl vapor. (The tilmcoated side of the composite membrane was exposed to the HCl vapor. This approach was taken because immersion of the composite membrane into aqueous HCl caused the polyaniline film to detach from the microporous support.) The doping level was varied by varying the time of exposure to the HCl vapor. The doping level achieved for any exposure time was determined by measuring the electronic conductivity of the resulting film and relating conductivity to dop ing level [ 17- 19 ] (see below ) . Finally, the effect of repeated undoping and then redoping on the gas-transport properties of polyaniline were also explored [ 91. These studies were only done on the free-standing films. The as-cast polymer films were first fully-doped by immersion in 4 M HCl (see above). The fullydoped polymer was then undoped by immersion of the film (for 24 h) in 1.OMNH40H. The film was then redoped to a level of 36% by immersion 2nHCI
4
S. Kuwabata, CR. Martin /Journal ofMembraneScience
(for 24 h) in 0.0175 MHC1 [ 17-191. The film was then dried in vacua and the gas-transport properties were measured. The film was then undoped and then redoped (to the 36% level) and the gas-transport properties again measured. This undoping/redoping/measurement cycle was repeated up to five times. 2.5. Conductivity measurements
Conductivity data were used to determine the doping levels of the polyaniline films in the thin film composite membranes. The four-electrode assembly shown in Fig. 1 was employed. The Au band electrodes were prepared by sputtering Au through a mask onto a glass slide. Pt leads were attached to each electrode using silver epoxy. The Ag-epoxy was then coated with insulating epoxy. The Au band electrodes and the substrate glass slide were then coated with a thin polyaniline film of identical thickness to that used in the thin film composite membranes. This film was doped by exposure to HCl vapor as was done for the polyaniline films in the composite membranes. The underlying electrodes were then used to determine the conductivity of the film via the conventional four-probe method [ 2 11. MacDiarmid et al. have defined the relationship between conductivity and doping level for polyaniline [ 1719 ]. Hence, the conductivity data obtained could be used to determine the doping levels of the polyaniline films in the composite membranes.
Fig. 1. Electrode assembly used to measure the conductivity of the polyaniline films.
91(1994) l-12
2.6. Gas-transport studies Gas-transport data were obtained using the standard single-gas permeation method [ 22-241. The sample to be studied (either the free-standing polyaniline film or the composite membrane) was sandwiched between a piece of filter paper and a piece of Al-foil tape (All-Foils, Inc. ). The Al-foil tape had a circular hole of area between 0.08 and 0.63 cm2 area (depending on thickness of the polyaniline film). This hole determined the area of membrane exposed to the permeant gas. The gas-transport cell used is described in ref. 24. It consists of an upper half-cell which is pressurized with the desired gas and a lower half-cell that contains a pressure transducer for monitoring gas flux across the membrane. Prior to gas-transport measurement, both the upper and lower half-cells were evacuated and the sample was allowed to outgas until a flat baseline was obtained from the pressure transducer in the lower half-cell. The desired gas was then introduced into the upper half-cell at a pressure of 50 psi ( 25 8.5 cmHg ) . The resulting pressure differential across the polyaniline film or composite membrane drives gas through the film/membrane and into the lower half cell. The corresponding pressure (P) versus time ( t ) transient in the lower half-cell was recorded on a strip-chart recorder. The slope of the linear part of the P-t transient provides the “permeance” (P’ ) of the gas across the membrane [ 121. P’ is a measure of the flux and typically has units of cm3 ( STP)/cm2 s cmHg; i.e., volume of gas transported across the membrane, per cm2 of membrane area, per set, per cmHg of pressure differential across the membrane. The permeability coefficient for the gas in a free-standing film was then determined by multiplying the permeance by the film thickness (in cm). The permeability coefficient is, in principle, the product of the diffusion coefficient (D) and the Henry’s law solubility coefficient (S) for the gas in the membrane [ 25 1. For example, the permeability coefficient for oxygen in a membrane (PO, ) can be expressed as follows [25]:
S. Kuwabata. C.R. Martin /Journal ofMembraneScience
PO,= DoJoz
(2)
The permeance of gas in a composite membrane (Pk ) is related to the permeance of the support (P$ ) and the permeance of the thin polyaniline film ( PF ) coating the support via l/P;,=l/P&+l/P;,
5
91(1994) l-12
same polyaniline film. For example, the ideal O,/ N2 selectivity coefficient, (YoZ/NZ, was calculated as follows [ 25 ] : aOz/Nz
(4)
= pO,/pN,
Obviously, the selectivity coefficient provides a measure of the selectivity of gas transport in the polyaniline film.
(3)
However, the Anopore support membranes used here have such high permeance that Eq. ( 3 ) simply reduces to Pk =Ff. Hence, the permeability coefficient for the gas in the polyaniline film covering the support membrane surface can be obtained by multiplying the permeance of the composite by the polyaniline film thickness [ 12 1. Finally, it is important to define the area used when calculating the permeability coefficient for a polyaniline film in a thin film composite membrane. Because the Anopore membranes are 57% porous [ 26 1, one might be tempted to use an area that is 57% of the geometric area as the true polyaniline film area. This assumes, however, that the gas flows linearly (like smoke from a smoke stack) from the pores at the membrane surface and does not access parts of the polyaniline film that are not directly above a pore. This is clearly an unrealistic assumption since there will always be a radial component to diffusion of the gas in the polyaniline film. Furthermore, because these films are rather thick and because the pores at the membrane surface are so closely spaced, the radial components will cause the diffusion layers created at each individual pore to merge. For this reason, we have used the geometric area of the membrane exposed to the gas in our calculation of the permeability coefftcient for the polyaniline films in our thin film composite membranes. That this is a valid assumption is proven by the fact that the permeability coefficients obtained for the polyaniline films in the composite membranes are identical to those obtained for the free-standing polyaniline films (where there is no ambiguity about Iilm area). Finally, ideal gas-transport selectivity coefficients were obtained by ratioing the permeability coefficients for two gases obtained from the
3. Results and discussion 3.1. EfSect of the alumina surface chemistry on the film-coating
process
In order to use the thin film composite membrane approach for studying gas transport in polymers, the surface of the support membrane must be coated with a contiguous, defect-free film (of known thickness) of the desired polymer. In our previous studies [ 7,8,12-l 5 ] this was accomplished by using interfacial polymerization methods to synthesize the polymer film at the surface of the support membrane. For the investigations described here, we chose to solution-cast the polyaniline film onto the surface of the support membrane. This approach was taken because we wanted to use polyaniline that was identical to that used in the previous investigations of gas transport in this material [ 91. However, as noted above, we found that when the polyaniline solution was applied to the surface of the as-received alumina support membrane, this solution simply flooded the pores in the membrane. As a result it was impossible to obtain a thin polyaniline film on the surface of the as-received membrane. The solvophobicity/philicity of the alumina membrane can be varied by using silane chemistry to attach functional groups to the surfaces of the membrane [ 7,201. When a drop of NMP is applied to the surface of an as-received alumina membrane, the NMP simply floods the pores in the membrane. This happens because NMP is a polar solvent (e.g., it is miscible with water) and the alumina surface is very hydrophilic. In contrast, we have found that when the surface of the Anopore membrane is derivatized
Fig. 2. Scanning electron micrograph of a cross section of a polyaniline thin film composite membrane.
with hexadecyltrichlorosilane, NMP beads up on the membrane surface and the pores do not become flooded. As a result, when a solution of polyaniline in NMP is applied across the surface of the derivatized membrane, the polymer cannot enter the pores. Upon removal of the solvent, a thin film of polyaniline that coats the surface of the membrane is obtained. A scanning electron micrograph of a cross section of such a polyaniline/alumina composite membrane is shown in Fig. 2. As can be seen in this micrograph, a contiguous and uniform thin film coats the surface of the membrane. This film was obtained by applying 2.8 mg crnm2 of polyaniline (in NMP solution) to the membrane surface. This film is 3.0 pm in thickness. 3.2. Obtaining defect-free films If even a minute number of microscopic defects are present in a polymer film, gas will be transported across the film via Knudsen diffusion in these defects [ 2,12 1. This, of course, makes it impossible to investigate the gas-transport properties of the polymer since the gas uses the defects to bypass the polymer. It is important, therefore, to be sure that there are essentially no defects in the polymer film to be inves-
tigated. Investigations of the rate and selectivity of gas transport can be used to probe for defects in thin polymer films [ 2,12- 15 1. If Knudsen diffusion is the dominate transport mechanism, the selectivity coefficient obtained for two gases [i.e., Eq. (4) ] will adopt the characteristic Knudsen-diffusion value [ 2,12 1. This Knudsen-diffusion selectivity coefficient, (Y&,~~, is given by K aOz/Nl=
(MW2/MWl
)1’2
(5)
where MW, and MI%‘, are the molecular weights of the two gases. For O2 and N2 this selectivity coefficient is (Y&/NZ= 0.93 [ 121. In contrast, if the film is defect free, the selectivity coefficient will assume a value that reflects the diffusivities and solubilities of the gases in the polymer [see Eq. (2 ) 1. Furthermore, this true polymer-transport selectivity coefficient is typically much larger than the Knudsen-diffusion selectivity coefficient. For example, Anderson et al. obtained ao~/N* =9.0 to 9.5 for defect-free, undoped polyaniline films [ 9 1. Hence, the magnitude of the selectivity coefficient can be used to probe for defects in polymer films [ 12 1. Fig. 3A shows a plot of the 02/N2 selectivity coefficient versus film thickness for free-stand-
S. Kuwabata, CR. Martin / JournalofMembraneScience
undoped polyaniline films. When the film is thin (e.g., 10 pm), ooz/Nz is essentially at the Knudsen-diffusion value (0.93 ), indicating that these thin films are defective. However, the selectivity coefficient increases with film thickness and ultimately levels at thicknesses greater than 20~ (Fig. 3A). The maximum selectivity coefficient value achieved, (Yoz/Nz =9.2, is identical to the selectivity coefficient obtained by Anderson et al. [ 91 for tmdoped polyaniline. These data show that free-standing films with thicknesses greater than 20 ,MIIare essentially defect free. Fig. 3B shows the corresponding permeability coefficient data as a function of film thickness. As would be expected, the permeability coefficients decrease with film thickness and then level at thicknesses greater than 20 ,um. The films used in these investigations had thicknesses between 20 and 30 pm. Analogous selectivity (Fig. 4A) and permea-
I
91(1994) 1-12
bility coefficients (Fig. 4B) versus film thickness data were obtained for the composite membranes. (The film thickness used for the x-axis in Fig. 4 is the thickness of the polyaniline layer coating the alumina support membrane surface. ) The data in Fig. 4 show the same trends as was observed for the free-standing films (Fig. 3 ) ; however, defect-free films that are an order of magnitude thinner can be obtained. This is one of the advantages of using the composite membrane approach for studying gas transport in polymers [ 12 1. This is important because the duration of a gas-transport experiment is dependent on the thickness of the film under investigation; i.e., it takes longer to acquire the requisite gas-transport data for thicker films. Because thinner defect-free films can be obtained via the composite membrane approach (compare Figs. 3 and 4), the desired gas-transport data can be
II/:
0’
0 5
10
15
20
Film thickness
25
30
(pm)
Fig. 3. Effect of polyaniline film thickness on the Oz/Nz selectivity coeffkient (A) and the O2 and N2 permeability coeffkients (B ) . Free-standing films.
1
’
’ 2
’
’
’
3
Film thxkness
’
’
4 (pm)
Fig. 4. Effect of polyaniline film thickness on the 02/N2 selectivity coeffkient (A) and the O2 and N2 permeability coeffkients (B ) . Thin film composite membranes.
S. Kuwabata, C.R. Martin /Journal
8
ofMembrane
Science 91(1994)
I-12
obtained in shorter times when composite membranes are used. In order for the polyaniline film to be defect free, the film must be thick enough to bridge the pores at the surface of the alumina membrane [ 12 1. It is somewhat surprising that films greater than 3 p in thickness are required to bridge the pores at the surface of this membrane (Fig. 4 ) . This is surprising because these pores are ostensibly 20 nm in diameter, and one would think that a film that is substantially thinner than 3 pm would be able to bridge such nanoscopic pores. We obtained analogous results in a previous investigation of this type [ 141. Careful electron microscopic analysis revealed defects, at the surface of the Anopore membrane, that were orders of magnitude larger than the 20-nm diameter pores [ 141. Films of 3-pm thickness are required to bridge over these large defects [ 141. 3.3. Conductivity versus doping level in thin polyaniline films The polyaniline films in the thin film composite membranes were doped by exposure to HCl vapor. The doping level was controlled by varying the exposure time. Conductivity measurements (Fig. 1) were used to determine the doping level. Fig. 5 shows a plot of conductivity versus duration of exposure of the film to the HCl 10'
I
I
I
I
100 --
E lo-’ .: lo-* h
‘k=
2 p 10~3
s
1 o.4
1o-5
I
0
100
I
200 Time
I
300
I
400
500
(set)
Fig. 5. Relationship between conductivity of a polyaniline film (3.0~ym thickness) and exposure time of the film to HCl vapor.
001
0
10
20 Doping
30 level
40
50
(%)
Fig. 6. Effect of polyaniline doping level on permeability coefficients for the indicated gases. Thin film composite membranes, Cl--doped.
vapor. The conductivity increases with time of exposure and ultimately levels at exposure times greater than 400 s. A maximum conductivity of 1.3 S cm- I was achieved; this is comparable to the value obtained by MacDiarmid et al. for ftilydoped polyaniline [ 17- 19 1. MacDiarmid et al. have elucidated the relationship between conductivity and doping level in polyaniline [ 17- 191. If we define the highest conductivity achievable as a,,,,, films having doping levels of 13, 22, 3 1, and 38% have conductivities of 0.00 14, 0.17, 0.37, and 0.55 o,,, respectively [ 17- 19 1. Application of these data to Fig. 5 shows that exposure times of 5, 105, 148, and 2 15 s produced doping levels of 13,22, 3 1, and 38%, respectively. Films with these doping levels were used to explore the effect of doping level on gas transport in polyaniline.
S. Kuwabata, C.R. Martin /JournalofMembraneScience
91(1994) l-12
, 60
I
,
/
,
1
-
a3,,/CH<
-
.-r
z
$ ul
I
I
,
I
I
,
/1. I
I
0
10
I
I
20
Doping
I
30 level
,
40
50
(%)
Fig. 7. Effect of polyaniline doping level on the indicated selectivity coefficients. Membranes as per Fig. 6.
Table 1 Effect of curing time and temperature on selectivity coefficients for polyaniline films Type of film
Free-standing’
‘Doped to 36%. bDoped to 38%.
Selectivity coeffkient
Curing temp. (“C)
Cluing time (h)
WNz
CO&X,
90 125 140 125 125
3 3 3 2 4
15.0 14.7 14.9 15.2 14.8
53.4 51.9 51.9 53.8 50.7
60 90 125 140 125 125 125
0 3 3 3 3 1 2 4
15.2 14.9 14.9 15.3 14.3 15.1 15.1 14.8
55.3 52.5 51.5 54.0 50.1 54.2 54.8 53.0
Compositeb
A
0’
A
O,/ N
20
I
I
I
I
I
I
0
1
2
3
4
5
I
Number of doping and undoping
Fig. 8. Effect of repeated doping and undoping of polyaniline films on the CO&H., and 02/N2 selectivity coeffkients.
3.4. Eficct of doping level on permeability and selectivity of gas transport
Fig. 6 shows the effect of doping level on the magnitude of the permeability coefficients for various gases in polyaniline. The permeability coefficients for all gases decrease with doping level. It is well known that introducing ions into a polymer [see Eq. ( 1) 1, typically decreases the free volume in the polymer phase [ 27 1. This decrease in free volume is undoubtedly responsible for the decrease in permeability coefficient with doping level seen in Fig. 6. It is well known that an inverse relationship exists between permeability and selectivity of gas transport in polymers [ 4,7,12,25 1. That is, polymers that show low permeability coefficients typically also show high gas-transport selectivity coefftcients. If this general trend holds true for polyaniline, the data in Fig. 6 would suggest that gas-transport selectivity should increase with doping level. Fig. 7 shows that this is, in general, true; however, selectivity coefficients appear to level at doping levels above 38%. This leveling of the selectivity coefficient undoubtedly arises from a similar (although less pronounced) leveling of the permeability coefficient with doping level (last two points in Fig. 6 ). The data in Figs. 6 and 7 were obtained from polyaniline-based thin film composite mem-
10
S. Kuwabata, C.R. Martin /Journal of Membrane Science 91 (I 994) l-12
Table 2 Effect of counterion on permeability and selectivity coeffkients Type of film and counterion
Cl-doped composite’ Cl-doped free standingb NO: doped composite’ SO:- -doped free-standingb PTS-doped free standingb*”
Permeability coefficient (barrers)
in polyaniline Selectivity coefficient 02/N2
N2
02
0.0112 0.0096 0.0087 0.0089 0.0067
0.164 0.142 0.129 0.119 0.100
14.8 14.7 14.8 13.4 14.9
Permeability coefficient (barrers)
Selectivity coefficient C02KH4
Cb
(332
0.0092 0.0089 0.0073 0.0082 0.0073
0.505 0.462 0.401 0.426 0.401
54.9 51.9 54.9 52.0 54.9
“Doping level, 38%. bDoping level, 36%. “PTS, para-toluenesulfonate.
branes. Analogous data were obtained from freestanding films that were doped to 36 and 50%. The free-standing film doped to 36% showed O,/ Nz and C02/CH4 selectivity coefficients of (Xoz/NZ= 14.9 and (YCo2/cH.,= 54.3. The freestanding film doped to 50% showed oo2/N2 = 15.2 = 54.6. These selectivity coeffiand k02/CH4 cients are indistinguishable from the corresponding selectivity coefficients obtained for the thin film composite membranes (Fig. 7 ) . This is an important result because we were concerned that the presence of the alumina support might somehow alter the gas transport properties of the polyaniline film. These data show that this is not the case. 3.5. EfSect of other experimental parameters on the gas-transport properties ofpolyaniline Mattes et al. suggest that polyaniline shows its highest gas-transport selectivity after heat-curing at 125 “C [ lo]. All of the data discussed above were obtained for films cured in this way. In addition, we have systematically investigated the effect of curing time and temperature on the selectivity of gas transport in polyaniline. Both free-standing and thin film composite membranes were studied. The results obtained are shown in Table 1. Curing temperatures ranging from room temperature to 140°C were investigated as were curing times ranging from 0 to 4 h. As indicated in Table 1, selectivity coefficients
were found to be independent of curing time and temperature. In addition, these data reinforce the point that there is no difference between the gastransport properties of the free-standing and composite membranes. Anderson et al. all suggest that repeated doping and undoping of the polyaniline film improves gas-transport selectivity [ 9 1. We have also investigated this point. The data obtained are shown in Fig. 8. These data show that, in our hands, repeated doping and undoping does not affect gas-transport selectivity in polyaniline. Finally, we have also explored the effect of varying the counterion present in the polyaniline film on the gas-transport properties of the fully doped polymer. The data obtained are shown in Table 2. The indicated counterion was incorporated into the film by exposing the undoped polymer to either the vapor or a solution of the appropriate acid. While the identity of the counterion has some effect on the rate of gas transport, the effect of counterion on selectivity is minimal (Table 2 ) .
4. Conclusions We have explored the effects of doping level, curing time and temperature, and various other experimental parameters on rate and selectivity of gas transport in polyaniline. Both free-standing and thin film composite membranes were in-
S. Kuwabata, C.R. Martin /Journal of Membrane Science 91(1994) I-12
vestigated. (Identical results were obtained for both types of samples. ) We have found that only doping level has a major effect on the rate and selectivity of gas transport in this polymer. The highest selectivity coefficients that we have obtained ( &&/NZ= 15 and (YcoZ/cH4 = 55 ) are significantly smaller than the selectivity coefficients obtained by Anderson et al. ( (YoZ,/NZ = 30 =336). We cannot, at this time, and %0+2H, explain this discrepancy. It is important to compare the gas-transport data obtained here for polyaniline with data for other polymers that are of potential interest for gas separations. Robeson has recently compiled gas-transport permeability and selectivity data for an enormous number of polymeric materials [ 25 1. Robeson has plotted these data as log cxfor various gas pairs versus the log of the permeability coefficient for the more permeable gas of the pair [ e.g.9 hi! ( a02/N2 ) versus log ( PO, ) 1. This analysis revealed an upper-bound relationship for all of the gas pairs [25]. This upper bound is, figuratively speaking, a “line in the sand” that describes the transport properties of the best polymeric materials, for gas separations, known today. If a new polymeric material is to be judged as promising for membrane-based gas separations, its combination of selectivity and permeability coefficients must lie above this upper-bound line. Using this criteria, polyaniline must be regarded as a promising material for 02/Nz separations because its combination of c&/N2 = 15 and Po2= 0.16barrers (see Table 2 ) places it above Robeson’s upper bound [ 25 1. In contrast, for C02/CH4, polyaniline’s selectivity/permeability combination is well below the upper bound [25]. Polyaniline is not, therefore, a promising material for separation of COZ from CH4. Finally, if polyaniline is to be used as a membrane material in a practical gas-separations module, it is clear that some type of composite membrane, in which the polyaniline is present as an ultrathin (i.e., less than lOO-nm thick) film [ 7,12-l 51, will be required. It seems unlikely that the solution-casting approach used here will provide defect-free films of sufficient thinness. We have described an interfacial polymerization
11
method that might provide defect-free films that are sufficiently thin [ 71. Furthermore, a better support material (preferably a hollow-fiber support) will also be needed. We are currently addressing these important practical problems. Acknowledgments This work was supported by the Dow Chemical Company. References [ 1 ]P.H.
Abelson, Synthetic membranes, Science, 224 (1989) 1421. [ 21 J.M.S. Henis and M.K. Tripodi, The developing technology of gas separating membranes, Science, 220 (1983) 11. [ 31 J. Haggin, Membrane technology has achieved success, yet lags potential, Chem. Eng. News, 68 (Oct. 1) ( 1990)
[ 4 ] &.
Department of Energy Report No. DE90-0 1170 Membrane separations systems - A research needs as: sessment, U.S. DOE, Washington, DC, April 1990. [ 51R.W. Spillman, Economics of gas separations membranes, Chem. Eng. Prog., 85 ( 1989) 41. [ 61 A.M. Thayer, Pollution reduction, Chem. Eng. News, 70 (Nov. 16) (1992) 22. [ 71 W. Liang and C.R. Martin, Gas transport in electronically conductive polymers, Chem. Materials, 3 ( 199 1) 390. [ 8 ]C.R. Martin, W. Liang, V. Menon, R. Parthasarathy and A. Parthasarathy, Electronically conductive polymers as chemically-selective layers in membrane-based separations, Synth. Met., 55-57 (1993) 3766. [ 91 M.R. Anderson, B.R. Mattes, H. Reiss, and R.B. Kaner, Conjugated polymer films for gas separations, Science, 252 (1991) 1412. [ lo] B.R. Mattes, M.R. Anderson, J.A. Conklin, H. Reiss and R.B. Kaner, Morphological modification of polyaniline films for separation of gases, Synth. Met, 57 (1993) 3655. [ 11]T.A. Skotheim (Ed), Handbook of Conductive Polymers, Marcel Dekker, New York, 1986. [ 12]C. Liu and C.R. Martin, Composite membranes from photochemical synthesis of ultrathin films, Nature, 352 (1991) 50. [ 13 ] C. Liu, M.W. Espenscheid, W.-J. Chen and C.R. Martin, Electrochemical synthesis of ultrathin film composite membranes, J. Am.. Chem. Sot., 112 ( 1990) 2458. [ 14]C. Liu, W.-J. Chen and C.R. Martin, Electrochemical synthesis of ultrathin film composite membranes, J. Membrane Sci., 65 ( 1992) 113.
12
S. Kuwabata, C.R. Martin /Journal of Membrane Science 91(1994) l-12
[ 15 ] B. Ballarin, C.J. Brumlik, D.R. Lawson, W. Liang, L.S. Van Dyke and C.R. Martin, Chemical sensors based on ultrathin film composite membranes - a new concept in sensor design. Anal. Chem., 64 ( 1992) 2647. [ 16JW.S. Huang, B.D. Humphrey and A.G. MacDiarmid, Polyaniline, a novel conducting polymer. Morphology and chemistry of its oxidation and reduction in aqueous electrolytes, J. Chem. Sot., Faraday Trans. 1,82 ( 1986) 2385, [ 17 ] J.C. Chiang and A.G. MacDiarmid, Polyaniline: protonic doping of the emeraldine form to the metallic regime, Synth. Met., 13 (1986) 193. [ 18lJ.M. Ginder, A.J. Richter, A.G. MacDiarmid, and A.J. Epstein, Insulator-to-metal transition in polyaniline, Solid State Commun., 63 (1987) 97. [ 19]F. Zuo, M. Angelopoulos, A.G. MacDiarmid and A.J. Epstein, Transport studies of protonated emeraldine polymer: a granular polymeric metal system, J. Phys. Rev. B, 36 (1987) 3475. [2O]C.J. Brumlik and C.R. Martin, Template synthesis of metal microtubules, J. Am.. Chem. Sot., 113 (I 991) 3174. [21 ]H. Kim and H.A. Laitinen, Composition and conductivity of tin oxide films prepared by pyrohydrolytic decomposition of tin(lV) compounds, J. Am. Ceram. Sot., 58 (1975) 23.
[22]W.J. Koros, A.D. Chan and D.R. Paul, Sorption and transport of various gases in polycarbonate, J. Membrane Sci., 2 (1977) 165. [ 23lA.C. Puleo, N. Murganandum and D.R. Paul, Gas sorption and transport in substituted polystyrenes, J. Polym. Sci. Part B: Polym. Phys. Ed., 27 (1989) 2385. [ 241 C. Liu, Development of High-Performance Composite Membranes for Electrochemical Processes and Gas Separations, Ph.D. Dissertation, Texas A&M University, 1991. [ 25 ] L.M. Robeson, Correlation of separation factor versus permeability for polymeric membranes, J. Membrane Sci., 62 (1991) 165. [ 26 ] C.A. Foss, Jr., M.J. Tiemey and C.R. Martin, Template synthesis of infrared-transparent metal microcylinders: comparison of optical properties with the predictions of effective medium theory, J. Phys. Chem., 96 ( 1992) 9001. [ 27lV.D. Mattera, Jr. and W.M. Risen, Jr., Composition dependence of glass transition temperature of sulfonatedpolystyrene ionomers, J. Polym. Sci. Part B: Polym. Phys., 24 (1986) 753.
(1994) 1-12
Investigation of the gas-transport properties of polyaniline Susumu Kuwabata’,
Charles R. Martin*
Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA (Received September 23, 1993; accepted in revised form December 17, 1993)
Abstract Investigations of the rate and selectivity of gas transport in the electronically conductive polymer polyaniline are described. Both rate and selectivity were found to be dependent on the doping level. Permeability coefficients for all gases studied decreased with increasing doping level, and selectivity coefficients, in general, increased with doping level. The effects of other variables, including the nature of the counterion, temperature and duration of thermal processing of the film, were also explored. Both free-standing polyaniline films ( -2Qm thick) and thin film composite membranes based on polyaniline were studied. The thin film composites were prepared by coating the polyaniline film ( - 3+m thick) onto the surface of a microporous alumina support membrane. The freestanding and thin film composite membranes showed identical rates and selectivities of gas transport. The highest 02/Nz and C02/CH4 selectivity coefficients obtained were ffoz/NZ= 15 and (Ycoz/cH4= 55, respectively. Key words: Composite membranes; Gas separations; Membrane preparation and structure; Conductive polymers; Polyaniline
1. Introduction There is increasing interest in the idea of using synthetic membranes to do industrial separations [ l-5 1. This interest is fueled by the potential economic advantages inherent in membrane-separation technologies [l-5]. Furthermore, membrane-based separations can be viewed as an example of “green” (i.e., environmentally-friendly ) chemistry [ 6 ] in that membrane-based processes are potentially less energy intensive than other, more conventional, separations methods [ 3-5 1. However, as clearly pointed out in a recent report by the U.S. De‘Current address: Osaka University, Department of Applied Chemistry, Yamada-oka 2-1, Suita, Osaka 565, Japan. *Corresponding author.
partment of Energy, if membrane-based separations are to gain wide-spread use, membranes with higher permeabilities and higher transport selectivities will be required [ 41. Hence, there is a need to identify new types of membrane materials. We [ 7,8] and Anderson et al. [ 9, lo] have recently shown that electronically-conductive polymers [ 7- 111 are promising membrane materials for industrial gas separations. We investigated the conductive polymer poly (N-methylpyrrole) [ 7,8] and Anderson et al. studied polyaniline [ 9,10 1. Polyaniline appeared to be the most promising of these two conductive polymers in that the gas-transport selectivity data obtained by Anderson et al. were remarkable [ 9 1. Indeed, if these data are correct, polyaniline is one of the most exciting materials for potential
0376-7388/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SsDIO376-7388(94)00010-V
2
S. Kuwabata, CR. Martin /Journal ofMembrane Science 91 (I 994) l-12
applications in industrial gas separations to be discovered in many years. Because of the exceptional gas-transport data obtained in the previous investigation [ 93, we set out to explore the gas-transport properties of polyaniline for ourselves. Our approach, however, is different from that used by Anderson et al., who used thick ( N 0.1 mm) free-standing films of polyaniline to conduct their transport studies. We were interested in the concept of supported thin polymer films for membrane-based separations [ 7,8,12- I5 1. These “thin film composite membranes” consist of a thin film of the chemically selective polymer (e.g., the polyaniline) coated onto the surface of a microporous support membrane [ 7,8,12- 15 1. The advantage of this approach is that higher net fluxes across the composite can be obtained because the support is highly permeable and because the chemically-selective material is present as a thin film. We have investigated the gas-transport properties of polyaniline-based thin film composite membranes. We have not been able to reproduce the exceptional gas-transport selectivity obtained by Anderson et al. [ 91. We felt that this might be attributable to the fact that the polyaniline was present as a thin supported film rather than a thicker free-standing film. We have, therefore, also investigated the transport properties of free-standing polyaniline films. These membranes showed transport properties identical to those of our supported thin films; i.e., again, we were not able to reproduce the exceptional gastransport data obtained in the previous investigations [ 91. We describe the results of our investigations here.
membrane used to prepare the thin film composites was a microporous alumina filter (Anopore) [ 7,8,12- 15 1. This membrane contains cylindrical 200~nm diameter pores that run most of the way through the membrane’s thickness. These pores branch into -20~nm diameter pores at one membrane face. The polyaniline film was coated onto this branched-pore surface. 2.2. Polymer synthesis Polyaniline was synthesized according to the procedure of Huang et al. [ 161; Anderson et al. used the same procedure [ 93. A solution that was 1 M in aniline and 1 M in HCl was mixed with an equal volume of a solution that was 1 A4 in (NH4)&08 and 1 Min HCl. This results in oxidative polymerization of the aniline to produce the protically-doped (see below) emeraldine salt [ 16-l 91 form of polyaniline. The polymerization was allowed to proceed overnight. The resulting green precipitate was collected by filtration, rinsed with water and dried in vacua. The emeraldine salt was deprotonated by dispersing the insoluble powder in excess 1 M aqueous NH40H. This converted the emeraldine salt into the blue emeraldine base, which is also insoluble in water. The emeraldine base was then collected by filtration, rinsed with water and methanol and dried in vacua. One gram of the emeraldine base was then added to 100 ml of IV-methylpyrrolidone (NMP ). The resulting solution was stirred overnight and then filtered to remove the small amount of insoluble material. This solution was concentrated by partial evaporation of the solvent using a rotary evaporator. The stock solution obtained was 3.4% (w/w) in polyaniline.
2. Experimental 2.1. Materials
2.3. Preparation of composite membranes and free-standing polyaniline films
Aniline (Aldrich ) and N-methylpyrrolidone (Sigma) were vacuum distilled prior to use. Hexadecyltrichlorosilane (Hulls America), NH40H ( NH4 ) &OS ( Mallinckrodt ) and (Mallinckrodt ) were of reagent grade and were used without further purification. The support
The composite membranes were prepared by casting films of polyaniline (from the stock solution) onto the surface of the microporous alumina support membrane. Prior to film-casting, the membranes were treated with hexadecyltrichlorosilane [ 7,201. This treatment causes
S. Kuwabata, C.R. Martin /Journal of MembraneScience
hexadecyl groups to be covalently bound to all of the surfaces of the alumina membrane [ 7,201. As will be discussed in detail below, when polyaniline solution was applied to the surface of the as-received membrane, the solution simply flooded the pores of the membrane. In contrast, the polyaniline solution was excluded from the pores of the silane-treated membrane. As a result, vacuum evaporation of NMP from the surface of the silane-treated membrane resulted in a thin film of polyaniline coating the upper surface of this membrane. Films of 3 to 4 p thickness were used for these studies. Film thicknesses were determined using electron microscopy [ 7,12151. Free-standing polyaniline films were prepared by casting the polymer solution onto glass slides and evaporating the solvent in vacua [ 9 1. The polyaniline films were removed from the glass surface by immersion in water [ 91. Free-standing films of 25 to 30 pm thickness were used for these studies. Finally, both the supported and the free-standing films were thermally annealed after film formation. Mattes et al. recommend annealing at 125°C for 3 h [ lo]. We employed these (and various other) annealing temperatures and times. 2.4. Protic doping The NMP-soluble emeraldine base form of polyaniline was used to prepare both the supported and free-standing polyaniline films. This form of the polymer is nonionic and is an electronic insulator [ 16- 19 1. The base can, however, be protonated to yield the polycationic emeraldine salt: This is the electronically-conductive form of the polymer. This can be accomplished by immersion of the film in aqueous HCl [ 17- 19 1. The
91(1994) l-12
3
extent of the doping reaction shown in Eq. ( 1) can be varied from 0 (no protonated monomer units) to 50% (half of the monomer units protonated) by varying the concentration of HCl in the solution used to dope the polymer [ 17-191. Anderson et al. found that the gas-transport properties of polyaniline varied with the extent of doping [ 91; we obtained analogous results with the conductive polymer poly (N-methylpyrrole) [ 71. Hence, it is important to explore the effect of doping level on the gas-transport properties of such polymers. The free-standing polyaniline films were doped by immersion (for 24 h ) in aqueous solutions of HCl [ 17- 19 1. The doping level was controlled by varying the concentration of the HCl used [ 17- 19 1. After doping, the films were dried in vacua. The thin film composite membranes were doped by exposure to HCl vapor. (The tilmcoated side of the composite membrane was exposed to the HCl vapor. This approach was taken because immersion of the composite membrane into aqueous HCl caused the polyaniline film to detach from the microporous support.) The doping level was varied by varying the time of exposure to the HCl vapor. The doping level achieved for any exposure time was determined by measuring the electronic conductivity of the resulting film and relating conductivity to dop ing level [ 17- 19 ] (see below ) . Finally, the effect of repeated undoping and then redoping on the gas-transport properties of polyaniline were also explored [ 91. These studies were only done on the free-standing films. The as-cast polymer films were first fully-doped by immersion in 4 M HCl (see above). The fullydoped polymer was then undoped by immersion of the film (for 24 h) in 1.OMNH40H. The film was then redoped to a level of 36% by immersion 2nHCI
4
S. Kuwabata, CR. Martin /Journal ofMembraneScience
(for 24 h) in 0.0175 MHC1 [ 17-191. The film was then dried in vacua and the gas-transport properties were measured. The film was then undoped and then redoped (to the 36% level) and the gas-transport properties again measured. This undoping/redoping/measurement cycle was repeated up to five times. 2.5. Conductivity measurements
Conductivity data were used to determine the doping levels of the polyaniline films in the thin film composite membranes. The four-electrode assembly shown in Fig. 1 was employed. The Au band electrodes were prepared by sputtering Au through a mask onto a glass slide. Pt leads were attached to each electrode using silver epoxy. The Ag-epoxy was then coated with insulating epoxy. The Au band electrodes and the substrate glass slide were then coated with a thin polyaniline film of identical thickness to that used in the thin film composite membranes. This film was doped by exposure to HCl vapor as was done for the polyaniline films in the composite membranes. The underlying electrodes were then used to determine the conductivity of the film via the conventional four-probe method [ 2 11. MacDiarmid et al. have defined the relationship between conductivity and doping level for polyaniline [ 1719 ]. Hence, the conductivity data obtained could be used to determine the doping levels of the polyaniline films in the composite membranes.
Fig. 1. Electrode assembly used to measure the conductivity of the polyaniline films.
91(1994) l-12
2.6. Gas-transport studies Gas-transport data were obtained using the standard single-gas permeation method [ 22-241. The sample to be studied (either the free-standing polyaniline film or the composite membrane) was sandwiched between a piece of filter paper and a piece of Al-foil tape (All-Foils, Inc. ). The Al-foil tape had a circular hole of area between 0.08 and 0.63 cm2 area (depending on thickness of the polyaniline film). This hole determined the area of membrane exposed to the permeant gas. The gas-transport cell used is described in ref. 24. It consists of an upper half-cell which is pressurized with the desired gas and a lower half-cell that contains a pressure transducer for monitoring gas flux across the membrane. Prior to gas-transport measurement, both the upper and lower half-cells were evacuated and the sample was allowed to outgas until a flat baseline was obtained from the pressure transducer in the lower half-cell. The desired gas was then introduced into the upper half-cell at a pressure of 50 psi ( 25 8.5 cmHg ) . The resulting pressure differential across the polyaniline film or composite membrane drives gas through the film/membrane and into the lower half cell. The corresponding pressure (P) versus time ( t ) transient in the lower half-cell was recorded on a strip-chart recorder. The slope of the linear part of the P-t transient provides the “permeance” (P’ ) of the gas across the membrane [ 121. P’ is a measure of the flux and typically has units of cm3 ( STP)/cm2 s cmHg; i.e., volume of gas transported across the membrane, per cm2 of membrane area, per set, per cmHg of pressure differential across the membrane. The permeability coefficient for the gas in a free-standing film was then determined by multiplying the permeance by the film thickness (in cm). The permeability coefficient is, in principle, the product of the diffusion coefficient (D) and the Henry’s law solubility coefficient (S) for the gas in the membrane [ 25 1. For example, the permeability coefficient for oxygen in a membrane (PO, ) can be expressed as follows [25]:
S. Kuwabata. C.R. Martin /Journal ofMembraneScience
PO,= DoJoz
(2)
The permeance of gas in a composite membrane (Pk ) is related to the permeance of the support (P$ ) and the permeance of the thin polyaniline film ( PF ) coating the support via l/P;,=l/P&+l/P;,
5
91(1994) l-12
same polyaniline film. For example, the ideal O,/ N2 selectivity coefficient, (YoZ/NZ, was calculated as follows [ 25 ] : aOz/Nz
(4)
= pO,/pN,
Obviously, the selectivity coefficient provides a measure of the selectivity of gas transport in the polyaniline film.
(3)
However, the Anopore support membranes used here have such high permeance that Eq. ( 3 ) simply reduces to Pk =Ff. Hence, the permeability coefficient for the gas in the polyaniline film covering the support membrane surface can be obtained by multiplying the permeance of the composite by the polyaniline film thickness [ 12 1. Finally, it is important to define the area used when calculating the permeability coefficient for a polyaniline film in a thin film composite membrane. Because the Anopore membranes are 57% porous [ 26 1, one might be tempted to use an area that is 57% of the geometric area as the true polyaniline film area. This assumes, however, that the gas flows linearly (like smoke from a smoke stack) from the pores at the membrane surface and does not access parts of the polyaniline film that are not directly above a pore. This is clearly an unrealistic assumption since there will always be a radial component to diffusion of the gas in the polyaniline film. Furthermore, because these films are rather thick and because the pores at the membrane surface are so closely spaced, the radial components will cause the diffusion layers created at each individual pore to merge. For this reason, we have used the geometric area of the membrane exposed to the gas in our calculation of the permeability coefftcient for the polyaniline films in our thin film composite membranes. That this is a valid assumption is proven by the fact that the permeability coefficients obtained for the polyaniline films in the composite membranes are identical to those obtained for the free-standing polyaniline films (where there is no ambiguity about Iilm area). Finally, ideal gas-transport selectivity coefficients were obtained by ratioing the permeability coefficients for two gases obtained from the
3. Results and discussion 3.1. EfSect of the alumina surface chemistry on the film-coating
process
In order to use the thin film composite membrane approach for studying gas transport in polymers, the surface of the support membrane must be coated with a contiguous, defect-free film (of known thickness) of the desired polymer. In our previous studies [ 7,8,12-l 5 ] this was accomplished by using interfacial polymerization methods to synthesize the polymer film at the surface of the support membrane. For the investigations described here, we chose to solution-cast the polyaniline film onto the surface of the support membrane. This approach was taken because we wanted to use polyaniline that was identical to that used in the previous investigations of gas transport in this material [ 91. However, as noted above, we found that when the polyaniline solution was applied to the surface of the as-received alumina support membrane, this solution simply flooded the pores in the membrane. As a result it was impossible to obtain a thin polyaniline film on the surface of the as-received membrane. The solvophobicity/philicity of the alumina membrane can be varied by using silane chemistry to attach functional groups to the surfaces of the membrane [ 7,201. When a drop of NMP is applied to the surface of an as-received alumina membrane, the NMP simply floods the pores in the membrane. This happens because NMP is a polar solvent (e.g., it is miscible with water) and the alumina surface is very hydrophilic. In contrast, we have found that when the surface of the Anopore membrane is derivatized
Fig. 2. Scanning electron micrograph of a cross section of a polyaniline thin film composite membrane.
with hexadecyltrichlorosilane, NMP beads up on the membrane surface and the pores do not become flooded. As a result, when a solution of polyaniline in NMP is applied across the surface of the derivatized membrane, the polymer cannot enter the pores. Upon removal of the solvent, a thin film of polyaniline that coats the surface of the membrane is obtained. A scanning electron micrograph of a cross section of such a polyaniline/alumina composite membrane is shown in Fig. 2. As can be seen in this micrograph, a contiguous and uniform thin film coats the surface of the membrane. This film was obtained by applying 2.8 mg crnm2 of polyaniline (in NMP solution) to the membrane surface. This film is 3.0 pm in thickness. 3.2. Obtaining defect-free films If even a minute number of microscopic defects are present in a polymer film, gas will be transported across the film via Knudsen diffusion in these defects [ 2,12 1. This, of course, makes it impossible to investigate the gas-transport properties of the polymer since the gas uses the defects to bypass the polymer. It is important, therefore, to be sure that there are essentially no defects in the polymer film to be inves-
tigated. Investigations of the rate and selectivity of gas transport can be used to probe for defects in thin polymer films [ 2,12- 15 1. If Knudsen diffusion is the dominate transport mechanism, the selectivity coefficient obtained for two gases [i.e., Eq. (4) ] will adopt the characteristic Knudsen-diffusion value [ 2,12 1. This Knudsen-diffusion selectivity coefficient, (Y&,~~, is given by K aOz/Nl=
(MW2/MWl
)1’2
(5)
where MW, and MI%‘, are the molecular weights of the two gases. For O2 and N2 this selectivity coefficient is (Y&/NZ= 0.93 [ 121. In contrast, if the film is defect free, the selectivity coefficient will assume a value that reflects the diffusivities and solubilities of the gases in the polymer [see Eq. (2 ) 1. Furthermore, this true polymer-transport selectivity coefficient is typically much larger than the Knudsen-diffusion selectivity coefficient. For example, Anderson et al. obtained ao~/N* =9.0 to 9.5 for defect-free, undoped polyaniline films [ 9 1. Hence, the magnitude of the selectivity coefficient can be used to probe for defects in polymer films [ 12 1. Fig. 3A shows a plot of the 02/N2 selectivity coefficient versus film thickness for free-stand-
S. Kuwabata, CR. Martin / JournalofMembraneScience
undoped polyaniline films. When the film is thin (e.g., 10 pm), ooz/Nz is essentially at the Knudsen-diffusion value (0.93 ), indicating that these thin films are defective. However, the selectivity coefficient increases with film thickness and ultimately levels at thicknesses greater than 20~ (Fig. 3A). The maximum selectivity coefficient value achieved, (Yoz/Nz =9.2, is identical to the selectivity coefficient obtained by Anderson et al. [ 91 for tmdoped polyaniline. These data show that free-standing films with thicknesses greater than 20 ,MIIare essentially defect free. Fig. 3B shows the corresponding permeability coefficient data as a function of film thickness. As would be expected, the permeability coefficients decrease with film thickness and then level at thicknesses greater than 20 ,um. The films used in these investigations had thicknesses between 20 and 30 pm. Analogous selectivity (Fig. 4A) and permea-
I
91(1994) 1-12
bility coefficients (Fig. 4B) versus film thickness data were obtained for the composite membranes. (The film thickness used for the x-axis in Fig. 4 is the thickness of the polyaniline layer coating the alumina support membrane surface. ) The data in Fig. 4 show the same trends as was observed for the free-standing films (Fig. 3 ) ; however, defect-free films that are an order of magnitude thinner can be obtained. This is one of the advantages of using the composite membrane approach for studying gas transport in polymers [ 12 1. This is important because the duration of a gas-transport experiment is dependent on the thickness of the film under investigation; i.e., it takes longer to acquire the requisite gas-transport data for thicker films. Because thinner defect-free films can be obtained via the composite membrane approach (compare Figs. 3 and 4), the desired gas-transport data can be
II/:
0’
0 5
10
15
20
Film thickness
25
30
(pm)
Fig. 3. Effect of polyaniline film thickness on the Oz/Nz selectivity coeffkient (A) and the O2 and N2 permeability coeffkients (B ) . Free-standing films.
1
’
’ 2
’
’
’
3
Film thxkness
’
’
4 (pm)
Fig. 4. Effect of polyaniline film thickness on the 02/N2 selectivity coeffkient (A) and the O2 and N2 permeability coeffkients (B ) . Thin film composite membranes.
S. Kuwabata, C.R. Martin /Journal
8
ofMembrane
Science 91(1994)
I-12
obtained in shorter times when composite membranes are used. In order for the polyaniline film to be defect free, the film must be thick enough to bridge the pores at the surface of the alumina membrane [ 12 1. It is somewhat surprising that films greater than 3 p in thickness are required to bridge the pores at the surface of this membrane (Fig. 4 ) . This is surprising because these pores are ostensibly 20 nm in diameter, and one would think that a film that is substantially thinner than 3 pm would be able to bridge such nanoscopic pores. We obtained analogous results in a previous investigation of this type [ 141. Careful electron microscopic analysis revealed defects, at the surface of the Anopore membrane, that were orders of magnitude larger than the 20-nm diameter pores [ 141. Films of 3-pm thickness are required to bridge over these large defects [ 141. 3.3. Conductivity versus doping level in thin polyaniline films The polyaniline films in the thin film composite membranes were doped by exposure to HCl vapor. The doping level was controlled by varying the exposure time. Conductivity measurements (Fig. 1) were used to determine the doping level. Fig. 5 shows a plot of conductivity versus duration of exposure of the film to the HCl 10'
I
I
I
I
100 --
E lo-’ .: lo-* h
‘k=
2 p 10~3
s
1 o.4
1o-5
I
0
100
I
200 Time
I
300
I
400
500
(set)
Fig. 5. Relationship between conductivity of a polyaniline film (3.0~ym thickness) and exposure time of the film to HCl vapor.
001
0
10
20 Doping
30 level
40
50
(%)
Fig. 6. Effect of polyaniline doping level on permeability coefficients for the indicated gases. Thin film composite membranes, Cl--doped.
vapor. The conductivity increases with time of exposure and ultimately levels at exposure times greater than 400 s. A maximum conductivity of 1.3 S cm- I was achieved; this is comparable to the value obtained by MacDiarmid et al. for ftilydoped polyaniline [ 17- 19 1. MacDiarmid et al. have elucidated the relationship between conductivity and doping level in polyaniline [ 17- 191. If we define the highest conductivity achievable as a,,,,, films having doping levels of 13, 22, 3 1, and 38% have conductivities of 0.00 14, 0.17, 0.37, and 0.55 o,,, respectively [ 17- 19 1. Application of these data to Fig. 5 shows that exposure times of 5, 105, 148, and 2 15 s produced doping levels of 13,22, 3 1, and 38%, respectively. Films with these doping levels were used to explore the effect of doping level on gas transport in polyaniline.
S. Kuwabata, C.R. Martin /JournalofMembraneScience
91(1994) l-12
, 60
I
,
/
,
1
-
a3,,/CH<
-
.-r
z
$ ul
I
I
,
I
I
,
/1. I
I
0
10
I
I
20
Doping
I
30 level
,
40
50
(%)
Fig. 7. Effect of polyaniline doping level on the indicated selectivity coefficients. Membranes as per Fig. 6.
Table 1 Effect of curing time and temperature on selectivity coefficients for polyaniline films Type of film
Free-standing’
‘Doped to 36%. bDoped to 38%.
Selectivity coeffkient
Curing temp. (“C)
Cluing time (h)
WNz
CO&X,
90 125 140 125 125
3 3 3 2 4
15.0 14.7 14.9 15.2 14.8
53.4 51.9 51.9 53.8 50.7
60 90 125 140 125 125 125
0 3 3 3 3 1 2 4
15.2 14.9 14.9 15.3 14.3 15.1 15.1 14.8
55.3 52.5 51.5 54.0 50.1 54.2 54.8 53.0
Compositeb
A
0’
A
O,/ N
20
I
I
I
I
I
I
0
1
2
3
4
5
I
Number of doping and undoping
Fig. 8. Effect of repeated doping and undoping of polyaniline films on the CO&H., and 02/N2 selectivity coeffkients.
3.4. Eficct of doping level on permeability and selectivity of gas transport
Fig. 6 shows the effect of doping level on the magnitude of the permeability coefficients for various gases in polyaniline. The permeability coefficients for all gases decrease with doping level. It is well known that introducing ions into a polymer [see Eq. ( 1) 1, typically decreases the free volume in the polymer phase [ 27 1. This decrease in free volume is undoubtedly responsible for the decrease in permeability coefficient with doping level seen in Fig. 6. It is well known that an inverse relationship exists between permeability and selectivity of gas transport in polymers [ 4,7,12,25 1. That is, polymers that show low permeability coefficients typically also show high gas-transport selectivity coefftcients. If this general trend holds true for polyaniline, the data in Fig. 6 would suggest that gas-transport selectivity should increase with doping level. Fig. 7 shows that this is, in general, true; however, selectivity coefficients appear to level at doping levels above 38%. This leveling of the selectivity coefficient undoubtedly arises from a similar (although less pronounced) leveling of the permeability coefficient with doping level (last two points in Fig. 6 ). The data in Figs. 6 and 7 were obtained from polyaniline-based thin film composite mem-
10
S. Kuwabata, C.R. Martin /Journal of Membrane Science 91 (I 994) l-12
Table 2 Effect of counterion on permeability and selectivity coeffkients Type of film and counterion
Cl-doped composite’ Cl-doped free standingb NO: doped composite’ SO:- -doped free-standingb PTS-doped free standingb*”
Permeability coefficient (barrers)
in polyaniline Selectivity coefficient 02/N2
N2
02
0.0112 0.0096 0.0087 0.0089 0.0067
0.164 0.142 0.129 0.119 0.100
14.8 14.7 14.8 13.4 14.9
Permeability coefficient (barrers)
Selectivity coefficient C02KH4
Cb
(332
0.0092 0.0089 0.0073 0.0082 0.0073
0.505 0.462 0.401 0.426 0.401
54.9 51.9 54.9 52.0 54.9
“Doping level, 38%. bDoping level, 36%. “PTS, para-toluenesulfonate.
branes. Analogous data were obtained from freestanding films that were doped to 36 and 50%. The free-standing film doped to 36% showed O,/ Nz and C02/CH4 selectivity coefficients of (Xoz/NZ= 14.9 and (YCo2/cH.,= 54.3. The freestanding film doped to 50% showed oo2/N2 = 15.2 = 54.6. These selectivity coeffiand k02/CH4 cients are indistinguishable from the corresponding selectivity coefficients obtained for the thin film composite membranes (Fig. 7 ) . This is an important result because we were concerned that the presence of the alumina support might somehow alter the gas transport properties of the polyaniline film. These data show that this is not the case. 3.5. EfSect of other experimental parameters on the gas-transport properties ofpolyaniline Mattes et al. suggest that polyaniline shows its highest gas-transport selectivity after heat-curing at 125 “C [ lo]. All of the data discussed above were obtained for films cured in this way. In addition, we have systematically investigated the effect of curing time and temperature on the selectivity of gas transport in polyaniline. Both free-standing and thin film composite membranes were studied. The results obtained are shown in Table 1. Curing temperatures ranging from room temperature to 140°C were investigated as were curing times ranging from 0 to 4 h. As indicated in Table 1, selectivity coefficients
were found to be independent of curing time and temperature. In addition, these data reinforce the point that there is no difference between the gastransport properties of the free-standing and composite membranes. Anderson et al. all suggest that repeated doping and undoping of the polyaniline film improves gas-transport selectivity [ 9 1. We have also investigated this point. The data obtained are shown in Fig. 8. These data show that, in our hands, repeated doping and undoping does not affect gas-transport selectivity in polyaniline. Finally, we have also explored the effect of varying the counterion present in the polyaniline film on the gas-transport properties of the fully doped polymer. The data obtained are shown in Table 2. The indicated counterion was incorporated into the film by exposing the undoped polymer to either the vapor or a solution of the appropriate acid. While the identity of the counterion has some effect on the rate of gas transport, the effect of counterion on selectivity is minimal (Table 2 ) .
4. Conclusions We have explored the effects of doping level, curing time and temperature, and various other experimental parameters on rate and selectivity of gas transport in polyaniline. Both free-standing and thin film composite membranes were in-
S. Kuwabata, C.R. Martin /Journal of Membrane Science 91(1994) I-12
vestigated. (Identical results were obtained for both types of samples. ) We have found that only doping level has a major effect on the rate and selectivity of gas transport in this polymer. The highest selectivity coefficients that we have obtained ( &&/NZ= 15 and (YcoZ/cH4 = 55 ) are significantly smaller than the selectivity coefficients obtained by Anderson et al. ( (YoZ,/NZ = 30 =336). We cannot, at this time, and %0+2H, explain this discrepancy. It is important to compare the gas-transport data obtained here for polyaniline with data for other polymers that are of potential interest for gas separations. Robeson has recently compiled gas-transport permeability and selectivity data for an enormous number of polymeric materials [ 25 1. Robeson has plotted these data as log cxfor various gas pairs versus the log of the permeability coefficient for the more permeable gas of the pair [ e.g.9 hi! ( a02/N2 ) versus log ( PO, ) 1. This analysis revealed an upper-bound relationship for all of the gas pairs [25]. This upper bound is, figuratively speaking, a “line in the sand” that describes the transport properties of the best polymeric materials, for gas separations, known today. If a new polymeric material is to be judged as promising for membrane-based gas separations, its combination of selectivity and permeability coefficients must lie above this upper-bound line. Using this criteria, polyaniline must be regarded as a promising material for 02/Nz separations because its combination of c&/N2 = 15 and Po2= 0.16barrers (see Table 2 ) places it above Robeson’s upper bound [ 25 1. In contrast, for C02/CH4, polyaniline’s selectivity/permeability combination is well below the upper bound [25]. Polyaniline is not, therefore, a promising material for separation of COZ from CH4. Finally, if polyaniline is to be used as a membrane material in a practical gas-separations module, it is clear that some type of composite membrane, in which the polyaniline is present as an ultrathin (i.e., less than lOO-nm thick) film [ 7,12-l 51, will be required. It seems unlikely that the solution-casting approach used here will provide defect-free films of sufficient thinness. We have described an interfacial polymerization
11
method that might provide defect-free films that are sufficiently thin [ 71. Furthermore, a better support material (preferably a hollow-fiber support) will also be needed. We are currently addressing these important practical problems. Acknowledgments This work was supported by the Dow Chemical Company. References [ 1 ]P.H.
Abelson, Synthetic membranes, Science, 224 (1989) 1421. [ 21 J.M.S. Henis and M.K. Tripodi, The developing technology of gas separating membranes, Science, 220 (1983) 11. [ 31 J. Haggin, Membrane technology has achieved success, yet lags potential, Chem. Eng. News, 68 (Oct. 1) ( 1990)
[ 4 ] &.
Department of Energy Report No. DE90-0 1170 Membrane separations systems - A research needs as: sessment, U.S. DOE, Washington, DC, April 1990. [ 51R.W. Spillman, Economics of gas separations membranes, Chem. Eng. Prog., 85 ( 1989) 41. [ 61 A.M. Thayer, Pollution reduction, Chem. Eng. News, 70 (Nov. 16) (1992) 22. [ 71 W. Liang and C.R. Martin, Gas transport in electronically conductive polymers, Chem. Materials, 3 ( 199 1) 390. [ 8 ]C.R. Martin, W. Liang, V. Menon, R. Parthasarathy and A. Parthasarathy, Electronically conductive polymers as chemically-selective layers in membrane-based separations, Synth. Met., 55-57 (1993) 3766. [ 91 M.R. Anderson, B.R. Mattes, H. Reiss, and R.B. Kaner, Conjugated polymer films for gas separations, Science, 252 (1991) 1412. [ lo] B.R. Mattes, M.R. Anderson, J.A. Conklin, H. Reiss and R.B. Kaner, Morphological modification of polyaniline films for separation of gases, Synth. Met, 57 (1993) 3655. [ 11]T.A. Skotheim (Ed), Handbook of Conductive Polymers, Marcel Dekker, New York, 1986. [ 12]C. Liu and C.R. Martin, Composite membranes from photochemical synthesis of ultrathin films, Nature, 352 (1991) 50. [ 13 ] C. Liu, M.W. Espenscheid, W.-J. Chen and C.R. Martin, Electrochemical synthesis of ultrathin film composite membranes, J. Am.. Chem. Sot., 112 ( 1990) 2458. [ 14]C. Liu, W.-J. Chen and C.R. Martin, Electrochemical synthesis of ultrathin film composite membranes, J. Membrane Sci., 65 ( 1992) 113.
12
S. Kuwabata, C.R. Martin /Journal of Membrane Science 91(1994) l-12
[ 15 ] B. Ballarin, C.J. Brumlik, D.R. Lawson, W. Liang, L.S. Van Dyke and C.R. Martin, Chemical sensors based on ultrathin film composite membranes - a new concept in sensor design. Anal. Chem., 64 ( 1992) 2647. [ 16JW.S. Huang, B.D. Humphrey and A.G. MacDiarmid, Polyaniline, a novel conducting polymer. Morphology and chemistry of its oxidation and reduction in aqueous electrolytes, J. Chem. Sot., Faraday Trans. 1,82 ( 1986) 2385, [ 17 ] J.C. Chiang and A.G. MacDiarmid, Polyaniline: protonic doping of the emeraldine form to the metallic regime, Synth. Met., 13 (1986) 193. [ 18lJ.M. Ginder, A.J. Richter, A.G. MacDiarmid, and A.J. Epstein, Insulator-to-metal transition in polyaniline, Solid State Commun., 63 (1987) 97. [ 19]F. Zuo, M. Angelopoulos, A.G. MacDiarmid and A.J. Epstein, Transport studies of protonated emeraldine polymer: a granular polymeric metal system, J. Phys. Rev. B, 36 (1987) 3475. [2O]C.J. Brumlik and C.R. Martin, Template synthesis of metal microtubules, J. Am.. Chem. Sot., 113 (I 991) 3174. [21 ]H. Kim and H.A. Laitinen, Composition and conductivity of tin oxide films prepared by pyrohydrolytic decomposition of tin(lV) compounds, J. Am. Ceram. Sot., 58 (1975) 23.
[22]W.J. Koros, A.D. Chan and D.R. Paul, Sorption and transport of various gases in polycarbonate, J. Membrane Sci., 2 (1977) 165. [ 23lA.C. Puleo, N. Murganandum and D.R. Paul, Gas sorption and transport in substituted polystyrenes, J. Polym. Sci. Part B: Polym. Phys. Ed., 27 (1989) 2385. [ 241 C. Liu, Development of High-Performance Composite Membranes for Electrochemical Processes and Gas Separations, Ph.D. Dissertation, Texas A&M University, 1991. [ 25 ] L.M. Robeson, Correlation of separation factor versus permeability for polymeric membranes, J. Membrane Sci., 62 (1991) 165. [ 26 ] C.A. Foss, Jr., M.J. Tiemey and C.R. Martin, Template synthesis of infrared-transparent metal microcylinders: comparison of optical properties with the predictions of effective medium theory, J. Phys. Chem., 96 ( 1992) 9001. [ 27lV.D. Mattera, Jr. and W.M. Risen, Jr., Composition dependence of glass transition temperature of sulfonatedpolystyrene ionomers, J. Polym. Sci. Part B: Polym. Phys., 24 (1986) 753.