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Gas transfer in supported Langmuir-Blodgett films of polymeric lipids
241
Thin Solid Films, 180 (1989) 241-248
GAS TRANSFER IN SUPPORTED LANGMUIR-BLODGETT FILMS OF POLYMERIC LIPIDS PIETER STROEVE, MANUEL A. N. COELHO, SHENGXIONG DONG AND PAUL LAM
Organized Research Programon Polymeric UltrathinFilm Systems, Department of Chemical Engineering, University of California, Davis, CA 95616 (U.S.A.) LAWRENCE B. COLEMAN AND THOMAS G. FISKE
Organised Research Program on Polymeric Ultrathin Film Systems, Department of Physics, University of California, Davis, CA 95616 (U.S.A.) HELMUT RINGSDORF AND JURGEN SCHNEIDER
lnstitut fur Organische Chemic, Universitat Mainz, Mainz D-6500 (F.R.G) (Received April 25, 1989; accepted June 14, 1989)
Asymmetric membranes were fabricated by depositing Langmuir-Blodgett (LB) films of polymeric lipids on porous supports. Three polymeric lipids were used for deposition. Up to 50 Y-type layers were deposited on one side of the porous supports of polypropylene (Celgard) and polytetrafluoroethylene (GORE-TEX) membranes. A marked decrease in gas transfer with increasing number of LB layers was observed for the asymmetric membranes fabricated with the porous polypropylene. Gas transfer in the asymmetric membranes using porous polytetrafluoroethylene remained unchanged with the number of polymeric layers. The LB multilayers on polypropylene showed no evidence for pores or cracks as determined by scanning electron microscopy (at a magnification of 40000x), but the LB multilayers on polytetrafluoroethylenerevealed large cracks. The gas permeabilities of nitrogen, methane and carbon dioxide in the polypropylene-based asymmetric membranes were a function of the molecular weight of the gas for two of the three polymers which suggests that micropores are present in the LB films. Heat annealing decreased the gas permeabilities from 0% to 40%, but only one polymer showed a change in selectivity.
1. INTRODUCTION
The deposition of Langmuir-Blodgett (LB) polymeric layers on porous substrates can be used to create asymmetric membranes 1. Asymmetric membranes with ultrathin polymeric skins at one side of the membrane have applications for separation processes in reverse osmosis, gas cleaning and dialysis2~. The fabrication of asymmetric membranes with polymeric LB multilayers was previously achieved by polymerizing reactive amphiphiles in LB films after deposition 1. However, polymerization can cause cracks in LB films 5. In this work, preformed polymeric lipids were used for the LB multilayers. The polymers contained main chain spacers and hydrocarbon side groups 6'7. These 0040-6090/89/$3.50
© ElsevierSequoia/Printedin The Netherlands
242
P. STROEVEet al.
polymers were deposited on porous polypropylene (Celgard 2400) and polytetrafluoroethylene (GORE-TEX). The gas permeabilities of nitrogen, methane and carbon dioxide in the asymmetric membranes were determined as a function of the number of multilayers. Scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and Fourier transform IR (FTIR) spectroscopy were used to characterize the LB layers. 2. EXPERIMENTAL DETAILS The structures of the polymeric lipids PE-34, CO-10.5 and 2,terpoly 1:5:5 are given in Fig. 1. The general methods of synthesis have been described previously 7'8. CHZ'- ( CHZ) 14--'-CH i - 0 --i~ H2
F-
cx~-cc"~-cxi-°-~x
o
o
CH2- O - P! - 0 - (CFL.CH.,O) a. ~c 4- C - C! - CH.o 0~oo
PE-34 I
0 0 0 CH2 CHz'- (CH~,~" CH.x II II III . . . . "N-C-(CH~I~-C--O-(CH~H~O-C-¢--CHz
c~ccx~c.~
HOOC '
CH
! CO- I 0.5
x~,c,°\ H~s?CIe
?
/N-C-(CH
?
? ..~
2 )~'- C - O - - ( C H 2 ) _ ' m O - C - -
z
0
C - CH s
I CH2
,
CH 2 HO - - ( C H 2 ) 2 . , - 0 - C - -
i~-H
CHz
_2,Z.c.X:l~l~ Fig. 1. Polymeric lipids used for LB multilayers.
243
G A S T R A N S P O R T IN P O L Y M E R I C LB FILMS
The porous hydrophobic supports were Celgard 2400 and G O R E - T E X membranes. The G O R E - T E X membranes had a nominal pore size of 0.02 pm. Asymmetric membranes were fabricated by coating only one side of the porous supports with LB layers. Coating was achieved by clamping two membranes back to back in an aluminum ring. The polymeric lipids PE-34, CO-10.5 and 2,terpoly 1:5:5 were deposited at surface pressures of 30 mN m - 1, 40 mN m - 1 and 40 mN m - 1 respectively. The surface pressure vs. specific area for two of the polymeric lipids has been reported previously 7"s. The dipping speed was 0.5 cm min- ~ for the downstroke, and 0.3 cm min- ~ for the upstroke. A waiting period of 5 min was used between the upstroke and downstrokes. Y-type deposition was observed for all polymers. The transfer ratio was between 0.8 and 1 for the downstroke and between 1 and 1.2 for the upstroke. Gas permeabilities were obtained at 25°C in a Skirrow-Barrer-type permeation apparatus 9. The maximum initial pressure in the upstream chamber was 20 Torr and the initial pressure in the downstream chamber was less than 0.1 Torr. The pressure change in the downstream chamber was recorded with a differential pressure transducer (MKS Baratron). 3.
RESULTS A N D D I S C U S S I O N
The experimental results on gas permeation through the uncoated Celgard and the LB-coated Celgard (asymmetric) membranes are reported in Table I. For the uncoated Celgard the characteristic pore size 1° is 40nm and, at the absolute pressures used in this study (20Torr or less), gas transfer across the membrane occurs by Knudsen diffusion inside the pores. The gas permeability P divided by the membrane thickness L varies approximately as the square root of the molecular weight of the gas. For the uncoated membranes, L is 25 tam. Because of the very high permeability of the uncoated membrane, the LB multilayers represent more than 90% of the total mass transfer resistance in the asymmetric membranes. Thus the parameter P/L is effectively a property of the LB layer of the asymmetric membrane, and L is the thickness of the LB multilayer. The approximate thickness of each monolayer in the LB film is 2.5 nm. The parameter P/L decreases with increasing TABLE I GAS PERMEABILITIES FOR COMPOSITE MEMBRANES OF POLYMERIC LIPIDS P E - 3 4 AND C O - I 0 . 5 ON CELGARD 2400 A t 25 '~C
Lipid
Number o f
( P / L ) x 10 s ( c m 3 ( S T P ) s - l c m - 2 T o r r - 1)
layers
PE-34 PE-34 CO-10.5 C O - 10.5 CO-10.5
0 10 50 10 20 30
CH,
N2
CO 2
196 15.4 3.6 23.6 9.0 3.4
159 12.0 2.4 21.5 7. I 2.3
126 9.4 2.2 15.7 6.2 2.2
244
P. STROEVEet al.
number of LB layers. Curiously, the P/L ratios lor nitrogen, carbon dioxide and methane vary approximately as the square root of the molecular weight of the gases, which indicates the possibility of micropores in the membranes and the presence of Knudsen diffusion at these gas pressures• However, SEM photographs at magnifications of about 40 000x did not reveal any cracks or pores for polymers deposited on Celgard 2400. An example of a micrograph for Celgard coated by CO-10.5 is shown in Fig. 2. However, the resolution is not sufficient to reveal pores smaller than 15 nm.
Fig. 2. SEM micrograph of Celgard coated with 30 multilayers of CO-10.5. (Magnification, 23 270×.)
m Z
[]
[]
,'o
2'o
[]
.01
.001 .J
E Q
.0001
3'o
40
Numb~ ~ Idli ~ Fig. 3. Gas permeability values of nitrogen through PE-34 on GORE-TEX (O) or Celgard 2400 (@) membranes.
GAS TRANSPORT IN POLYMERIC LB FILMS
245
A comparison of the gas permeabilities divided by the thicknesses of the controlhng resistance of PE-34, for either GORE-TEX or Celgard, is shown in Fig. 3. Whereas LB deposition of PE-34 on Celgard causes a significant decrease in the P/L ratio, the G O R E - T E X membranes remain unaffected. SEM studies of the LB films on the G O R E - T E X membranes revealed the presence of large cracks after a diffusion experiment, as shown in Fig. 4. For the polymeric LB films on GORETEX, SEM photographs revealed some defects, such as dimples and pinholes, in the LB films. Cracks were not present in the membranes before they were mounted in the diffusion cell. Because of the presence of the large cracks the LB film did not cause any significant effect on the mass transfer rate and, therefore, the controlling resistance was still the porous support. These results were found for all polymers deposited on GORE-TEX. Presumably stresses in the LB layers, due to the mounting of the asymmetric membranes in the diffusion cell, caused the cracks.
Fig. 4. SEM micrograph of GORE-TEX coated with 30 layers of lipid PE-34. (Magnification, 445x.)
Figure 5 shows the dependence of P/L on the upstream pressure of nitrogen and the number of CO-10.5 multilayers on Celgard. Results are shown for 10, 20 and 30 LB layers of CO-10.5 on Celgard. The P/L ratio is independent of the gas pressure in the upstream chamber, indicating no change in the physical properties of the LB layers due to the pressure. Table II shows the effect of heat annealing on the gas permeability for the polymeric lipid PE-34 on the porous polypropylene membrane. The asymmetric membranes were heated for 1 h at either 60 or 80°C. The gas permeation measurements were made at 25 °C. Recent studies on the effect of heating on the molecular orientation of polymeric lipids suggest that for PE-34 and CO 10.5 the lipidic side chains maintain some order if heated to the melting point t t. From DSC and FTIR measurements the melting point is 54.5 °C for PE-34 and 44.5 °C for CO10.51 x. Below the melting point, heating of the asymmetric membranes did not alter the gas permeabilities. For the polymers deposited on G O R E - T E X heat annealing at 40-80 °C caused the defects (dimples and pinholes) in the LB films to disappear.
246
P.
STROEVEet al.
0.0O3.
A D Z
E
•
lOl~/ers
•
2o ~.y=,=
•
30 I~='s
o
E u o" I
0.002.
A lb. I,,,
0.001
0.000
m
!
o
lO
20
Upstmem Prmmum (mmHg)
Fig. 5. E x p e r i m e n t a l values of the gas p e r m e a b i l i t y for nitrogen as function of the u p s t r e a m pressure in CO-10.5 on Celgard 2400 m e m b r a n e s : • , ten layers; A , 20 layers; m, 30 layers.
T A B L E 11 EFFECT OF HEAT ANNEALING ON THE GAS PERMEABILITY FOR THE POLYMERIC LIPID PE-34 ON CELGARD 2400 AT 25 C
Annealing temperature
Number ~[" LB layers
c) 60 60 80
[0 50 50
( P/L) × IO s (cm3(STp)s l c m - Z T o r r - 1 ) CH4
N2
CO 2
13.5 3. I 2.7
10.1 2.3 1.5
8.2 2.2 1.4
Heating above the melting point causes some disordering of the lipidic side chains of the LB polymers 11. Heat annealing to 60 or 80 °C reduced the gas permeabilities divided by the LB thickness by 0 % 4 0 % . Heating did not alter the selectivity for nitrogen, methane or carbon dioxide permeation in PE-34 or CO 10.5, suggesting that in these films the micropores did not seal with annealing. Heating to 100 °C caused large holes to form in all the LB films studied here. Heat annealing at 60°C of 2,terpoly 1:5:5 on Celgard 2400 did show a significant effect on the permeability as indicated in Fig. 6. The selectivity of methane over carbon dioxide was decreased from 1.4 to 0.68 when compared with asymmetric membranes which were not annealed. Gas permeabilities for nitrogen, methane and carbon dioxide were also a factor of 4 lower than those found for PE34 and CO-10.5 on Celgard 2400. These results suggest that 2,terpoly 1:5:5 LB
247
GAS TRANSPORT IN POLYMERIC LB FILMS
multilayers on Celgard produced asymmetric membranes with few micropores. The increased permeability of carbon dioxide after annealing may be due to the presence of the tertiary amines in the 2,terpoly 1:5:5 LB film. 8
[]
7" E o. E ? u"
8-
044
O
N,?.
•
CG2
S-
=. u w
4-
a. I-
W v
g
w
E
•
3" 2-
.J
O
0
20
30 '
4'0
5',
60 '
70
heat T(C)
Fig. 6. Effect of heating on the gas permeability of ten layers of 2,terpoly 1:5:5 on Celgard 2400: O, nitrogen; Fq, methane; A, carbon dioxide.
The mass transfer results for CO-10.5 and PE-34 on Celgard 2400 are disappointing. From SEM photographs the polymers showed good coating of the porous substrates up to the highest magnification (about 40000x) employed in these studies. The possible presence of micropores suggests that superior LB films are needed to seal the porous substrates completely. The new polymers with fluorocarbon side chains, synthesized at the University of Mainz ~2, appear to be good candidates for asymmetric membrane fabrication. The polymers maintain order in the LB films after heating to 180°C x2, and thus the polymers may maintain structural integrity during an annealing process for asymmetric membranes• 4. CONCLUSIONS
Gas permeability measurements of nitrogen, methane and carbon dioxide in asymmetric membranes composed of polymeric LB layers on porous support suggest that the asymmetric membranes may contain micropores through which the gas molecules diffuse preferentially. Presumably Knudsen diffusion takes place in the pores. Only for the 2,terpoly 1:5:5 LB multilayers did annealing give a change in the gas selectivity, suggesting that the chemical nature of this LB film influences the gas permeation rate.
248
P. STROEVEet al.
ACKNOWLEDGMENTS F u n d i n g for this research was provided in part by the N a t i o n a l Science F o u n d a t i o n (CBT Division). O n e o f the a u t h o r s ( M . A . N . C . ) would like to t h a n k F u n d a q ~ o Calouste G u l b e n k i a n for financial support. The a u t h o r s w o u l d like to t h a n k Mr. Christian Erdelen o f Universitat M a i n z for the sample o f 2 , t e r p o l y 1 : 5: 5. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
O. Albrecht, A. Laschewskyand H. Ringsdorf, J. Membr. Sci., 22 (1985) 187. P. Meares (ed.), Membrane Separation Processes, Elsevier,Amsterdam, 1976. H.K. Lonsdale, J. Membr. Sci., 10 (1982) 81. D.R. Lloyd (ed.), Materials Science of Synthetic Membranes, in Am. Chem. Soc. Syrup. Ser., 269 (1985). J.P. Rabe, J. F. Rabolt, C. A. Brown and J. D. Swalen, Thin Solid Films, 133 (1985) 153. H. Ringsdorf, G. Schmidt and J. Schneider, Thin Solid Films, 152 (1987) 207. A. Laschewsky, H. Ringsdorf, G. Schmidt and J. Schneider,J. Am. Chem. Soc., 109 (1987) 788. M.B. Biddle, J. B. Lando, H. Ringsdorf, G. Schmidt and J. Schneider, Colloid Polym. Sci., 266 (1988) 806. G. Skirrow and R. M. Barrer, J. Polym. Sci., 3 (1948) 549. K.R. Krovvidi, A. Muscat, P. Stroeve and E. Ruckenstein, J. Colloidlnterface Sci., 100 (1984) 497. L.B. Coleman, T. G. Fiske, P. Stroeve, M. A. N. Coelho and S. Dong, Thin Solid Films, 178 (1989) 227. J. Schneider, C. Erdelen, H. Ringsdorf and J. F. Rabolt, Macromolecules, 22 (1989) 3475.
Thin Solid Films, 180 (1989) 241-248
GAS TRANSFER IN SUPPORTED LANGMUIR-BLODGETT FILMS OF POLYMERIC LIPIDS PIETER STROEVE, MANUEL A. N. COELHO, SHENGXIONG DONG AND PAUL LAM
Organized Research Programon Polymeric UltrathinFilm Systems, Department of Chemical Engineering, University of California, Davis, CA 95616 (U.S.A.) LAWRENCE B. COLEMAN AND THOMAS G. FISKE
Organised Research Program on Polymeric Ultrathin Film Systems, Department of Physics, University of California, Davis, CA 95616 (U.S.A.) HELMUT RINGSDORF AND JURGEN SCHNEIDER
lnstitut fur Organische Chemic, Universitat Mainz, Mainz D-6500 (F.R.G) (Received April 25, 1989; accepted June 14, 1989)
Asymmetric membranes were fabricated by depositing Langmuir-Blodgett (LB) films of polymeric lipids on porous supports. Three polymeric lipids were used for deposition. Up to 50 Y-type layers were deposited on one side of the porous supports of polypropylene (Celgard) and polytetrafluoroethylene (GORE-TEX) membranes. A marked decrease in gas transfer with increasing number of LB layers was observed for the asymmetric membranes fabricated with the porous polypropylene. Gas transfer in the asymmetric membranes using porous polytetrafluoroethylene remained unchanged with the number of polymeric layers. The LB multilayers on polypropylene showed no evidence for pores or cracks as determined by scanning electron microscopy (at a magnification of 40000x), but the LB multilayers on polytetrafluoroethylenerevealed large cracks. The gas permeabilities of nitrogen, methane and carbon dioxide in the polypropylene-based asymmetric membranes were a function of the molecular weight of the gas for two of the three polymers which suggests that micropores are present in the LB films. Heat annealing decreased the gas permeabilities from 0% to 40%, but only one polymer showed a change in selectivity.
1. INTRODUCTION
The deposition of Langmuir-Blodgett (LB) polymeric layers on porous substrates can be used to create asymmetric membranes 1. Asymmetric membranes with ultrathin polymeric skins at one side of the membrane have applications for separation processes in reverse osmosis, gas cleaning and dialysis2~. The fabrication of asymmetric membranes with polymeric LB multilayers was previously achieved by polymerizing reactive amphiphiles in LB films after deposition 1. However, polymerization can cause cracks in LB films 5. In this work, preformed polymeric lipids were used for the LB multilayers. The polymers contained main chain spacers and hydrocarbon side groups 6'7. These 0040-6090/89/$3.50
© ElsevierSequoia/Printedin The Netherlands
242
P. STROEVEet al.
polymers were deposited on porous polypropylene (Celgard 2400) and polytetrafluoroethylene (GORE-TEX). The gas permeabilities of nitrogen, methane and carbon dioxide in the asymmetric membranes were determined as a function of the number of multilayers. Scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and Fourier transform IR (FTIR) spectroscopy were used to characterize the LB layers. 2. EXPERIMENTAL DETAILS The structures of the polymeric lipids PE-34, CO-10.5 and 2,terpoly 1:5:5 are given in Fig. 1. The general methods of synthesis have been described previously 7'8. CHZ'- ( CHZ) 14--'-CH i - 0 --i~ H2
F-
cx~-cc"~-cxi-°-~x
o
o
CH2- O - P! - 0 - (CFL.CH.,O) a. ~c 4- C - C! - CH.o 0~oo
PE-34 I
0 0 0 CH2 CHz'- (CH~,~" CH.x II II III . . . . "N-C-(CH~I~-C--O-(CH~H~O-C-¢--CHz
c~ccx~c.~
HOOC '
CH
! CO- I 0.5
x~,c,°\ H~s?CIe
?
/N-C-(CH
?
? ..~
2 )~'- C - O - - ( C H 2 ) _ ' m O - C - -
z
0
C - CH s
I CH2
,
CH 2 HO - - ( C H 2 ) 2 . , - 0 - C - -
i~-H
CHz
_2,Z.c.X:l~l~ Fig. 1. Polymeric lipids used for LB multilayers.
243
G A S T R A N S P O R T IN P O L Y M E R I C LB FILMS
The porous hydrophobic supports were Celgard 2400 and G O R E - T E X membranes. The G O R E - T E X membranes had a nominal pore size of 0.02 pm. Asymmetric membranes were fabricated by coating only one side of the porous supports with LB layers. Coating was achieved by clamping two membranes back to back in an aluminum ring. The polymeric lipids PE-34, CO-10.5 and 2,terpoly 1:5:5 were deposited at surface pressures of 30 mN m - 1, 40 mN m - 1 and 40 mN m - 1 respectively. The surface pressure vs. specific area for two of the polymeric lipids has been reported previously 7"s. The dipping speed was 0.5 cm min- ~ for the downstroke, and 0.3 cm min- ~ for the upstroke. A waiting period of 5 min was used between the upstroke and downstrokes. Y-type deposition was observed for all polymers. The transfer ratio was between 0.8 and 1 for the downstroke and between 1 and 1.2 for the upstroke. Gas permeabilities were obtained at 25°C in a Skirrow-Barrer-type permeation apparatus 9. The maximum initial pressure in the upstream chamber was 20 Torr and the initial pressure in the downstream chamber was less than 0.1 Torr. The pressure change in the downstream chamber was recorded with a differential pressure transducer (MKS Baratron). 3.
RESULTS A N D D I S C U S S I O N
The experimental results on gas permeation through the uncoated Celgard and the LB-coated Celgard (asymmetric) membranes are reported in Table I. For the uncoated Celgard the characteristic pore size 1° is 40nm and, at the absolute pressures used in this study (20Torr or less), gas transfer across the membrane occurs by Knudsen diffusion inside the pores. The gas permeability P divided by the membrane thickness L varies approximately as the square root of the molecular weight of the gas. For the uncoated membranes, L is 25 tam. Because of the very high permeability of the uncoated membrane, the LB multilayers represent more than 90% of the total mass transfer resistance in the asymmetric membranes. Thus the parameter P/L is effectively a property of the LB layer of the asymmetric membrane, and L is the thickness of the LB multilayer. The approximate thickness of each monolayer in the LB film is 2.5 nm. The parameter P/L decreases with increasing TABLE I GAS PERMEABILITIES FOR COMPOSITE MEMBRANES OF POLYMERIC LIPIDS P E - 3 4 AND C O - I 0 . 5 ON CELGARD 2400 A t 25 '~C
Lipid
Number o f
( P / L ) x 10 s ( c m 3 ( S T P ) s - l c m - 2 T o r r - 1)
layers
PE-34 PE-34 CO-10.5 C O - 10.5 CO-10.5
0 10 50 10 20 30
CH,
N2
CO 2
196 15.4 3.6 23.6 9.0 3.4
159 12.0 2.4 21.5 7. I 2.3
126 9.4 2.2 15.7 6.2 2.2
244
P. STROEVEet al.
number of LB layers. Curiously, the P/L ratios lor nitrogen, carbon dioxide and methane vary approximately as the square root of the molecular weight of the gases, which indicates the possibility of micropores in the membranes and the presence of Knudsen diffusion at these gas pressures• However, SEM photographs at magnifications of about 40 000x did not reveal any cracks or pores for polymers deposited on Celgard 2400. An example of a micrograph for Celgard coated by CO-10.5 is shown in Fig. 2. However, the resolution is not sufficient to reveal pores smaller than 15 nm.
Fig. 2. SEM micrograph of Celgard coated with 30 multilayers of CO-10.5. (Magnification, 23 270×.)
m Z
[]
[]
,'o
2'o
[]
.01
.001 .J
E Q
.0001
3'o
40
Numb~ ~ Idli ~ Fig. 3. Gas permeability values of nitrogen through PE-34 on GORE-TEX (O) or Celgard 2400 (@) membranes.
GAS TRANSPORT IN POLYMERIC LB FILMS
245
A comparison of the gas permeabilities divided by the thicknesses of the controlhng resistance of PE-34, for either GORE-TEX or Celgard, is shown in Fig. 3. Whereas LB deposition of PE-34 on Celgard causes a significant decrease in the P/L ratio, the G O R E - T E X membranes remain unaffected. SEM studies of the LB films on the G O R E - T E X membranes revealed the presence of large cracks after a diffusion experiment, as shown in Fig. 4. For the polymeric LB films on GORETEX, SEM photographs revealed some defects, such as dimples and pinholes, in the LB films. Cracks were not present in the membranes before they were mounted in the diffusion cell. Because of the presence of the large cracks the LB film did not cause any significant effect on the mass transfer rate and, therefore, the controlling resistance was still the porous support. These results were found for all polymers deposited on GORE-TEX. Presumably stresses in the LB layers, due to the mounting of the asymmetric membranes in the diffusion cell, caused the cracks.
Fig. 4. SEM micrograph of GORE-TEX coated with 30 layers of lipid PE-34. (Magnification, 445x.)
Figure 5 shows the dependence of P/L on the upstream pressure of nitrogen and the number of CO-10.5 multilayers on Celgard. Results are shown for 10, 20 and 30 LB layers of CO-10.5 on Celgard. The P/L ratio is independent of the gas pressure in the upstream chamber, indicating no change in the physical properties of the LB layers due to the pressure. Table II shows the effect of heat annealing on the gas permeability for the polymeric lipid PE-34 on the porous polypropylene membrane. The asymmetric membranes were heated for 1 h at either 60 or 80°C. The gas permeation measurements were made at 25 °C. Recent studies on the effect of heating on the molecular orientation of polymeric lipids suggest that for PE-34 and CO 10.5 the lipidic side chains maintain some order if heated to the melting point t t. From DSC and FTIR measurements the melting point is 54.5 °C for PE-34 and 44.5 °C for CO10.51 x. Below the melting point, heating of the asymmetric membranes did not alter the gas permeabilities. For the polymers deposited on G O R E - T E X heat annealing at 40-80 °C caused the defects (dimples and pinholes) in the LB films to disappear.
246
P.
STROEVEet al.
0.0O3.
A D Z
E
•
lOl~/ers
•
2o ~.y=,=
•
30 I~='s
o
E u o" I
0.002.
A lb. I,,,
0.001
0.000
m
!
o
lO
20
Upstmem Prmmum (mmHg)
Fig. 5. E x p e r i m e n t a l values of the gas p e r m e a b i l i t y for nitrogen as function of the u p s t r e a m pressure in CO-10.5 on Celgard 2400 m e m b r a n e s : • , ten layers; A , 20 layers; m, 30 layers.
T A B L E 11 EFFECT OF HEAT ANNEALING ON THE GAS PERMEABILITY FOR THE POLYMERIC LIPID PE-34 ON CELGARD 2400 AT 25 C
Annealing temperature
Number ~[" LB layers
c) 60 60 80
[0 50 50
( P/L) × IO s (cm3(STp)s l c m - Z T o r r - 1 ) CH4
N2
CO 2
13.5 3. I 2.7
10.1 2.3 1.5
8.2 2.2 1.4
Heating above the melting point causes some disordering of the lipidic side chains of the LB polymers 11. Heat annealing to 60 or 80 °C reduced the gas permeabilities divided by the LB thickness by 0 % 4 0 % . Heating did not alter the selectivity for nitrogen, methane or carbon dioxide permeation in PE-34 or CO 10.5, suggesting that in these films the micropores did not seal with annealing. Heating to 100 °C caused large holes to form in all the LB films studied here. Heat annealing at 60°C of 2,terpoly 1:5:5 on Celgard 2400 did show a significant effect on the permeability as indicated in Fig. 6. The selectivity of methane over carbon dioxide was decreased from 1.4 to 0.68 when compared with asymmetric membranes which were not annealed. Gas permeabilities for nitrogen, methane and carbon dioxide were also a factor of 4 lower than those found for PE34 and CO-10.5 on Celgard 2400. These results suggest that 2,terpoly 1:5:5 LB
247
GAS TRANSPORT IN POLYMERIC LB FILMS
multilayers on Celgard produced asymmetric membranes with few micropores. The increased permeability of carbon dioxide after annealing may be due to the presence of the tertiary amines in the 2,terpoly 1:5:5 LB film. 8
[]
7" E o. E ? u"
8-
044
O
N,?.
•
CG2
S-
=. u w
4-
a. I-
W v
g
w
E
•
3" 2-
.J
O
0
20
30 '
4'0
5',
60 '
70
heat T(C)
Fig. 6. Effect of heating on the gas permeability of ten layers of 2,terpoly 1:5:5 on Celgard 2400: O, nitrogen; Fq, methane; A, carbon dioxide.
The mass transfer results for CO-10.5 and PE-34 on Celgard 2400 are disappointing. From SEM photographs the polymers showed good coating of the porous substrates up to the highest magnification (about 40000x) employed in these studies. The possible presence of micropores suggests that superior LB films are needed to seal the porous substrates completely. The new polymers with fluorocarbon side chains, synthesized at the University of Mainz ~2, appear to be good candidates for asymmetric membrane fabrication. The polymers maintain order in the LB films after heating to 180°C x2, and thus the polymers may maintain structural integrity during an annealing process for asymmetric membranes• 4. CONCLUSIONS
Gas permeability measurements of nitrogen, methane and carbon dioxide in asymmetric membranes composed of polymeric LB layers on porous support suggest that the asymmetric membranes may contain micropores through which the gas molecules diffuse preferentially. Presumably Knudsen diffusion takes place in the pores. Only for the 2,terpoly 1:5:5 LB multilayers did annealing give a change in the gas selectivity, suggesting that the chemical nature of this LB film influences the gas permeation rate.
248
P. STROEVEet al.
ACKNOWLEDGMENTS F u n d i n g for this research was provided in part by the N a t i o n a l Science F o u n d a t i o n (CBT Division). O n e o f the a u t h o r s ( M . A . N . C . ) would like to t h a n k F u n d a q ~ o Calouste G u l b e n k i a n for financial support. The a u t h o r s w o u l d like to t h a n k Mr. Christian Erdelen o f Universitat M a i n z for the sample o f 2 , t e r p o l y 1 : 5: 5. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
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