Investigations of polymerizable multilayers as gas separation membranes

Journal of Membrane Science, 22 (1985) 187-197 Elsevier Science Publishers B . V., Amsterdam - Printed in The Netherlands

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INVESTIGATIONS OF POLYMERIZABLE MULTILAYERS AS GAS SEPARATION MEMBRANES*

O. ALBRECHT, A LASCHEWSKY and H . RINGSDORF Institut fur Organische Chemie der Johannes Gutenberg Universitat, J .J. Becher-Weg 18-20, Postfach 3980, D-6500 Mainz 1 (F . R. G.) (Received October 27, 1983, accepted February 27, 1984)

Summary Polymerizable Langmuir-Blodgett (LB) multilayers of several diacetylenic amphiphiles were investigated on gas-permeable, polymeric supports . Macroscopically homogeneous multilayer films of hexacosa-10,12-diynoic acid and pentacosa-10,12-diyne phosphonate could be built up on polypropylene and polytetrafluoroethylene materials . As shown by scanning electron microscopy, the microscopic homogeneity of the built-up multilayers was mainly controlled by the surface structure of the support material, whereas the chemical nature of the support and photopolymerization of the layers did not affect the high order of the films . Such LB multilayers deposited on porous support materials are able to reduce CH4 flow markedly when compared to uncoated materials . As monomeric and polymerized multilayers show the same gas permeability, the necessary stabilization of the multilayers by polymerization can be achieved without loss of quality of the coating . Thus, polymerizable LB composite membranes may be well suited for gas separation .

Introduction The investigations reported here were recently presented at the "Symposium on Synthetic Membranes in Science and Industry" in Tubingen, 1983 ; cf. also our recent publication in Macromolecules, entitled "Polymerizable Built up Multilayers on Polymer Supports" . Langmuir-Blodgett (LB) films [1,2] built up from monolayers were investigated for their potential use as gas separation membranes . These systems should be well suited for separation processes [3-7], because of their extreme thinness and their overall well-defined, microscopically homogeneous structure . The mechanical and chemical stability of the LB films is increased by the polymerization of reactive amphiphiles within the layers [8,9,12] . From a wide variety of polymerizable amphiphiles containing the acrylic, the methacrylic, the diene or the diyne group [7,10], we chose amphiphilic diacetylenes for the initial work ; diacetylenes polymerize under topochemical *Paper presented at the 4th Symposium on Synthetic Membranes in Science and Industry, Tubingen, F .R .G., September 6-9,1983 .

0376-7388/85/$03 .30 © 1985 Elsevier Science Publishers B .V .

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control only, to yield strongly coloured polymers [11] . This colour presents an easy way to provide information on, for example, the homogeneity of the covering of the support, the presence of macroscopic defects in the layers, and the progress of the polyreaction . The formation of LB multilayers on gaspermeable supports (e .g., polypropylene, polysulfone, polytetrafluorethylene, polyimide) was investigated for various amphiphiles [12] (Table 1) . Due to their favourable properties, built-up multilayers of diacetylene compounds have been investigated for various application possibilities [13-161 . TABLE 1 Diacetylene lipids used for LB composite membranes CH,-(CH, )12-C=-C-C=C-(CH, ),-COOH/Cd salt

1

CH3 -(CH2 )12-C=-C-C==C-(CH, ),-CH2 -O-PO S H,

2 anionic

CH3 -(CH, )11-C=-C-C==C-(CH2 ),-CH,-POSH,

3

CH3 -(CH2 )12-C-=C-C=-C-(CH2 ),-CO-O-CH,-CH 2

N+/ CH3 4 cationic

CH 3 -(CH2 )12-C=-C-C=-C-(CH2 )s -CO-O-CH2 -CH,

CH 3

CH 3 -(CH 2 )12-C=C-C=--C-(CH2 )s-COO-CH2 CH3 -(CH2 )12 -C=C-C=C-(CH 2 ) s - COO-CH t (CH 3 ) 3 N+-CH2 -CH2 -O-P - 02 -O~H2

5 zwitterionic

Results Polymerizable lipids and support materials used

Of the investigated supports, several polypropylene (PP1, PP2, PP3) and polytetrafluoroethylene (PTFE 1, PTFE 2) materials proved to be best for multilayer deposition (Table 2) . These, when cleaned thoroughly, could be coated easily . With all five diacetylene lipids we were able to build multilayers on the supports mentioned above . Best results were obtained with the cadmium salt of carboxylic acid 1 and phosphonate 3, and therefore detailed studies were performed with these compounds . TABLE 2 Hydrophobic support materials for multilayer deposition Support

Trade name

Origin

PP1 PP2 PP3 PTFE1 PTFE2

Special fabrication Celgard 2400 Celanese (U.S.A.) Trespaphan PED6 Kalle (F .R.G.) Teflon foil 0 .2 mm Huth (F .R.G .) Goretex BF31T2 Gore (U .S.A.)

Material Polypropylene Polypropylene Polypropylene Polytetrafluoroethylene Polytetrafluoroethylene



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Spreading behaviour The spreading behaviour of the lipids 1 and 3 is characterized by the surface pressure vs . area diagrams (Fig. 1) . Under the conditions chosen, both lipids meet the basic requirement for extended LB experiments, the formation of a long-term stable monolayer in the solid analogous phase . To increase the collapse surface pressure and to guarantee the long-term stability of the monolayer, in case of lipid 1 a subphase containing 1 g CdCl 2 • H 2 O per litre was used (171 .

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Monolayer deposition The surfaces of the cleaned support materials are strongly hydrophobic . Under the conditions chosen, a monolayer deposition takes place for both lipids at each downward and upward trip (Y-deposition) .

Polymerization in the multilayers The topochemical polymerization of diacetylenes by UV light in the solid state, in monolayers, in multilayers and in liposomes are well known [11, 18-201 . The conjugated double and triple bonds of the polymer backbone absorb strongly in the visible region [111 . The polymerization of 1 and 3 in the multilayers was recorded by Vis spectroscopy as a function of UV irradiation time . Figure 3 shows the changes in the spectra of multilayers of 3 with time of irradiation on the transparent PP3 support .



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On irradiation of up to 3 min an absorption peak at 640 nm is formed (blue form) . Further irradiation produces a new absorption maximum at 540 nm and a second new peak at 495 nm (red form), whereas the 640 nm peak slowly disappears . The shift in the absorption maximum is related to a conformational change of the polymer backbone [21] . Prolonged exposure to UV light decreases the overall absorption due to degradation processes [21,22] The Vis spectra of multilayers of 1 [17] show nearly the same form and UV irradiation dependence as films of 3 . In the past, similar spectra of polymeric diacetylenes have been published and discussed [20-23] .

b Fig . 4 . SEM micrographs of (a) uncoated PP2, magnification 10,000 X ; ( b) PP2 coated with 16 layers of the cadmium salt of 1, blue form of the polymer (3 min irradiation), magnification 5,000 X .

a Fig . 5 . SEM micrographs of (a) uncoated PP3, magnification 10,000 X ; ( b) PP3, coated with 16 layers of the cadmium salt of 1, blue form of the polymer (3 min irradiation), magnification 5,000 X .

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Scanning electron microscopy (SEM) Excepting the border region of the coated area, the colour of the polymerized multilayers of 1 and 3 indicates, macroscopically, the homogeneity and the nearly defect-free coating of all the five supports . However, SEM revealed important differences, based on the different surface structure of the supports as shown for multilayers of 1 in Figs . 4-8. The naked PP1 (Fig . 6a) is characterized by a rough surface with two types of holes. Domains with holes of ca . 0 .4 gm diameter are separated by zones of holes of ca . 1 .5 gm diameter . In contrast, PP2 (Fig . 4a) has a regular surface with uniform holes of ca . 0 .2 X 0 .05 gm, whereas PP3 (Fig. 5a) is smooth and devoid of pores . PTFE 1 is a non-porous material with a fissured surface (Fig . 7a) ; PTFE 2 shows a smooth, plain surface with pores of ca . 0 .2 gm diameter .

Fig. 6. SEM micrographs of (a) uncoated PP1, magnification 500 x ; ( b) PP1, coated with 16 layers of the cadmium salt of 1, blue form of the polymer (3 min irradiation), magnification 250 X , (c) detail of (b), magnification 5,000 x .

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Fig. 7 . SEM micrographs of (a) uncoated PTFE1, magnification 500 X , ( b) PTFE1, coated with 12 layers of the cadmium salt of 1, blue form of the polymer (3 min irradiation, magnification 250 X .

a Fig. 8 . SEM micrographs of (a) uncoated PTFE 2, magnification 10,000 X , ( b) PTFE2 coated with 12 layers of the cadmium salt of 1, blue form of the polymer (3 min irradiation), magnification 5,000 X .

After the deposition of several monolayers of the cadmium salt of 1, the modified surfaces of PP1, PP2 and PP3 still show significant differences . This is demonstrated by the comparison of the five sets of micrographs in Figs. 4-8 . Comparing the coated polypropylenes, the layers on PP2 (Fig . 4) and PP3 (Fig . 5) exhibit smooth and, up to a magnification of 10,000, defectfree surfaces, although the surface textures are different due to the different

194

supports . In the layers deposited on PP1 (Fig. 6), however, rift systems can be observed along with additional uncovered spots, both of which reach down to the surface of the support (Figs . 6b and c) . The comparison of these micrographs with the surface of PP1 (Fig . 6a) strongly suggest that the cracks stretch above the zones of the large holes . At high magnification, the crack can be seen to be a complete fracture of the built-up multilayer (Figs . 6c and d) . The rough surface of the support material can be seen clearly . In contrast, Fig . 4b of coated PP2 shows that small pores can be bridged by the multilayer, if the support surface is smooth enough . Further comparison with the micrographs of coated PP3, Fig . 5b, suggests that the smoother and more regular the surface of the support material is (Figs . 4a-6a), the more perfect is the modified surface after multilayer deposition . The highest magnifications of the three coated polypropylenes, Figs . 4b, 5b and 6c, show that all the minor deformations in the surface of these supports are perfectly covered . In agreement with the SEM micrographs of the coated polypropylenes, the surface structure of the coated polytetrafluoroethylenes also depends on the surface structure of the support . The rough surface of PTFE1 cannot be coated perrectly (Fig . 7) . On the plain, smooth surface of PTFE2, in contrast, a perfect multilayer is built up, covering the pores completely (Fig . 8) . Considering the similar results for PP and PTFE supports, the chemical nature of the support material seems to be of minor importance for the perfection of the deposited layers . The polymerization of diacetylene multilayers leads to a structural change of the layers, which may result in a shrinkage of the coating [13] . Therefore, the influence of the polymerization on the lipid films was investigated too . On all supports, the monomeric and polymerized multilayers of 1 and 3 in the blue (3 min irradiated) and the red form of the polymer (20 min irradiated) did not exhibit significant differences . The presence or absence, the shape and the size of defects in the multilayers were not affected by the photopolymerization . Bilayers, however, show a markedly increased number of defects when polymerized to the red polymer form [12] .

Gas permeation measurements Preliminary investigations of the flow of CH 4 through the polymerizable LB composite membranes were made on porous support materials . The lipid/ support systems 1/PP1, 1/PP2, 1/PTFE2 and 3/PP2 were used . Membranes built up on PP1 did not reduce the gas flow [17] . However, the LB composite membranes 1/PP2, 1/PTFE2 and 3/PP2 reduced the gas flow markedly . This is in agreement with the SEM micrographs of the coated support materials (Figs . 4, 5 and 7) . The cracks visible in the layers deposited on PP1 seem to act as channels for the gas stream . The system 1 /PP2 gave best results : by 18 deposited layers, the gas flow is reduced thirty-fold compared to uncoated PP2 . As seen in Fig. 9, the flow rate of CH 4 shows a sharp decrease with the increase in the number of layers deposited, a proof for the high quality of the coating .



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Figure 9 demonstrates also that the polymerization of the multilayers (at least for more than 6 layers deposited) causes no changes in the flow rate, i .e ., the stabilization of the lipid films, a crucial point for any application, can be achieved by polymerization without loss of quality of the coating . The increased stability against mechanical stress and organic solvents of the polymerized multilayers could be shown by SEM surface investigations [12] .

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Although a coating of high quality is obtained, the magnitude of the flow reduction and some small defects visible in the SEM (even if very rare) suggest that the main gas flow still passes through minor defects . The "true" flow rates are assumed to be much smaller than those determined . This will be checked by experiments using gases of different polarity, which, in case of a perfect multilayer, should show different flow rates . Conclusions Polymerizable multilayers on porous supports have been shown to reduce the gas flow of CH 4 markedly . The necessary high quality of extended coated areas may be achieved with the LB technique . It could be shown that the stabilization of these multilayers by polymerization, which is indispensable for any application, does not affect the quality of the lipid films . Thus, such polymerizable LB composite membranes may be well suited for gas separation .

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The next step in the investigations on this new type of membrane will be the elimination of the minor defects assumed in the coating . This has to be followed by a proof of separation of a gas mixture by this type of membrane . Experimental Materials The synthesis of hexacosa-10,12-diyonic acid [24] and pentacosa-10,12-diyne phosphonate [25] is described elsewhere . The cadmium chloride was pure grade (Merck) . The water was distilled and purified by a Millipore water purification system (Milli Q, Millipore Corp .) Methods The monolayers of 1 were spread on subphases containing 1 g CdCl 2 • H2 O per litre from hexane solutions of about 0 .5 mg/ml . The monolayers of 3 were spread on water from 9 :1 hexane :ethanol solutions of about 0 .1 mg/ml. The surface pressure area diagrams were recorded by a computer-controlled film balance [26] . For monolayer deposition, a commercial film balance was used . To achieve a coating exclusively on one side of the support, and to reinforce the flexible support, it was attached to a teflon disc of 50 mm diameter . A teflon ring of 45 mm inner diameter was used to fasten the polymeric support to the disc . This way, the central areas of 45 mm diameter of the supports were coated . In the cleaning procedure PP1, PP2, PTFE1 and PTFE2 were washed twice with p .a . grade ether ; PP3 was washed twice with p .a . grade acetone. The teflon holder was cleaned in the same way as the support . The assembled holder and support were rinsed several times with water . Monolayers of 1 were deposited at a surface pressure of 25 mN/m 2 with a dipping speed of 5 cm/min downwards and 2 cm/min upwards at 20 .2 ± 0 .2° C . Monolayers of 3 were deposited at a surface pressure of 31 mN/m 2 , with a dipping speed of 5 cm/min downwards and 0 .5 cm/min upwards, with an interval of 3 min between two subsequent dips . The layers were polymerized in air by a pen ray UV lamp (Hamamatsu Corp ., Model No . 937-002) with an intensity of 0 .5 mW-cm -" 2 . The UV spectra were recorded on a Beckmann Model 25 spectrometer . Scanning electron micrographs were taken by a Cambridge Mark II A electron microscope . The samples were sputtered with gold . The equipment for the gas permeation measurements was of a standard type, and is described elsewhere [27] . The flow rates were determined barometrically at a pressure differential of 1 .5 bar, using only the central area of 30 mm diameter of the coated and uncoated polypropylene supports .

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