12390
J. Phys. Chem. 1994, 98, 12390-12396
Parameters Influencing Zeolite Incorporation in PDMS Membranes Ivo F. J. Vankelecom, Else Scheppers, Robin Heus, and Jan B. Uytterhoeven* Centrum voor Oppervlaktechemie en Katalyse, Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, 3001 Leuven, Belgium
Received: April 26, 1994; In Final Form: July 11, 1994@
The incorporation of several types of zeolite in PDMS membranes is studied, by measuring the tensile strength, xylene sorption, and density of the membranes. The zeolite is shown to be involved in the cross-linking of the membrane. The interaction between the PDMS matrix and the zeolites results in reinforced membranes in the case of zeolite Y. The parameters influencing the dispersion of the zeolite in the membrane are investigated, as well as several aspects of the preparation method. Finally, the idea of cross-linking is applied to explain the results of watedethanol pervaporation.
Introduction Since the introduction of silicalite-filled PDMS (poly(dimethylsiloxane)) membranes by te Hennepe' and the commercial production of this membrane by the GlT Company, zeolite-filled membranes are frequently mentioned in the literature.'-'O Remarkable improvements on membrane performances due to incorporation of zeolites have been described. Several specific properties of the zeolites have already been brought to expression in composite membranes, such as their ion-exchange capacity,' adsorption capacity,12molecular sieving e f f e ~ t ,and ~ . ~hydrophobichydrophilic character.'-* In the first attempt to combine zeolites with a variety of organic polymers," Barrer and James demonstrated that adhesion problems occur at the zeolite-polymer interface, when using compression molding of intimate mixtures of a finely powdered polymer and the zeolite crystals. By dispersing the zeolite into a fluid silicone prepolymer, Paul and Kemp12 solved this problem. For the PDMS-silicalite system, an in-depth investigation on the membrane preparation9 resulted in a very successful adaptation of the preparation method, in which ultrafine crystals and prepolymerization of a highly diluted PDMS solution were essential. It enabled the authors to make ultrathin composite membranes with high zeolite loadings. Dispersion problems play a key role in the preparation of composite membranes, as was described already for related silica-siloxane systems. 13,14 By modeling results, te Hennepe' concluded that there was a marked influence of the silicalite filler on the properties of the polymer. The permeability of zeolite-loaded membranes was lower than expected. Physical cross-linking between the zeolite and the polymer or catalytic action of the zeolite on the polymer crosslinking reaction were given as possible explanations for this behavior. This decreased permeability was also observed in gas permeati~n.~ For a better understanding of the specific role played by zeolite fillers on the preparation, the physical properties, and the separation performance of the composite membranes, a study is made here on the influence of different zeolite characteristics on the membrane properties. This is essential for the correct interpretation of the membrane performance and for the optimization of the membrane preparation. PDMS was selected because it is the most frequently used polymer in composite membranes. Of all elastomers, it shows the greatest improvement of mechanical properties upon addition of inorganic fillers. @
Abstract published in Advance ACS Absrracts, October 15, 1994.
The influence of zeolite structure and crystal size on the properties of PDMS membranes is investigated. For most zeolites, a series of membranes with loadings varying between 0 and 55 wt % is made and characterized by measuring the tensile strength, density, and xylene sorption. Adhesion properties and dispersion of the zeolite in the polymer are studied by scanning electron microscopy (SEM). Combined with contact angle measurements, this technique reveals an anisotropy in the membrane. An ethanouwater mixture was chosen to test the cross-linking concept in pervaporation. Experimental Section Materials. Zeolite Pretreatment. The main characteristics of the zeolites used in this study are summarized in Table 1. The ZSM-5 series was supplied by the PQ Corp. and zeolite Y by Zeocat. The silicalite samples were kindly provided by Exxon and were calcined by heating at a rate of 3 "C/min to a final temperature of 550 "C, which was kept constant for 2 h more. The zeolites were stored in an exsiccator at a constant relative humidity of 80%. The water sorption at that relative humidity was determined experimentally for each zeolite. Polymers. The PDMS (RTV 615A en B, density 1.02 g/mL) was delivered by General Electric. Component A contains a prepolymer with vinyl groups, terminal at long PDMS chains. The cross-linker, containing several SiH groups per polymer chain, forms the much less viscous component B, together with platinum, acting as a catalyst in the hydrosilylation reaction. Methods. Membrane Preparation. Zeolite samples were weighed after equilibration at constant humidity, and the dry weight was calculated by subtracting the water content. Water was removed in a vacuum oven for a minimum of 1 h at 180 "C. Methyl isobutyl ketone (MIBK), dried over zeolite A, was added as solvent. A treatment of 1 h in an ultrasonic bath was applied to break the crystal aggregates and improve dispersion. The cross-linker was added to the zeolite suspension, and this mixture was further stirred for 2 h. Finally, the prepolymer was added, and mixing was continued for another hour. In order to obtain mixtures with approximately the same viscosity for all zeolite contents, more solvent was used for higher zeolite loadings. Air bubbles were removed from the suspensions by vacuum treatment. A layer of such suspension, with a thickness of 500 pm, was cast on a glass plate and placed for 30 min in the vacuum oven at 150 "C. The membrane was removed from the glass plate by immersion in warm water and was then air dried.
0022-365419412098-12390$04.5010 0 1994 American Chemical Society
Zeolite Incorporation in PDMS Membranes
J. Phys. Chem., Vol. 98, No. 47, 1994 12391
TABLE 1: Main Characteristics of the Zeolites Used ZSM-5 2802
cation form
NH4 275b 0.4-0.8 MFI 1.76 410
3002
NH4
8020
NH4
silicalite 0.2 pm 2.0 p m NH4 NH4
borosilicate
3020
H
NH4
SiOJX203 240b 78b 336 11.82c 8006 size, pm 1-1.5 0.3-0.6 0.1-0.8 1.0-0.1 0.2 topology MFI MFI MFI MFI MFI density2' 1.76 1.76 1.76 1.76 1.76 int surface, m2/g 405 450 430 304 pore size, A straight channels 5.2 x 5.7 sinusoidal channels 5.3 x 5.6 All data were taken from the technical information given by the supplying companies. X = Al. X = B. The zeolite content is expressed in volume percent as (weight of zeolite/dze,ate)/(weightof zeolite/dze,lite weight of polymer/ dpolymer) in which d = density. Membrane Characterization. Membrane Sorption. Sorption of pure liquids in the membranes was investigated on 1.5 x 5-cm membrane strips at room temperature. In order to desorb the water from the polymer and the zeolite in particular, the membrane was pretreated at 150 "C under vacuum. The membranes were then immersed in the pure liquids for at least 1 h. The amount sorbed was determined by weight. The membrane surface was wiped dry, before weighing as quickly as possible, so as to minimize evaporation. This way, the error on these measurements was kept below 2%. Measurement of Membrane Strength. Membrane strips of 11 x 1.8 cm were tested in a Wolpert traction instrument. They were clenched and tom apart at a speed of 50 d m i n . To obtain good statistics, three different strips were tested for each membrane. The thickness of the membrane was measured by using a micrometer with an accuracy of 1 pm. Contact Angle. A drop of water was put on a piece of membrane, and the contact angle was measured automatically by using a light microscope (Kriiss G40). Both sides of the membrane were tested. Per droplet, 20 measurements were performed after adding water to the droplet each time. All membranes were pretreated with n-hexane, rinsed with water, and dried at 150 "C. Density. The method is based on Archimedes' law. First, a dried piece of membrane is weighed as such and then once more while immersed in an oil of known density. From the difference in weight and the oil density, the volume of the membrane is calculated. Dividing the membrane dry weight by this volume yields the density of the membrane. Scanning Electron Microscopy. SEM pictures were taken on a Jeol Microprobe 733 after gold coating. Pervaporation. The membrane is placed on a sintered stainless steel support and sealed with an O-ring, made from Teflon-coated silicone rubber. The 6 wt % ethanol feed is circulated at a temperature of 35 "C with a speed of 50 L/h. The permeate is trapped with liquid nitrogen, and the vacuum pressure is kept constant at about 2 mbar. Fluxes are normalized to a membrane thickness of 100 pm. For the top layers of the commercial membranes, 11 and 48 pm was taken for the unfilled and the zeolite-filled membranes, respectively, as calculated from SEM photos. Selectivity is expressed as
+
in which y refers to the concentration of the components in the permeate and x to the concentration in the feed. Analysis on GC was isothermal at 70 "C and isopropyl alcohol was used as the extemal standard. A GC, Type H p 5890 coupled to a HP 3392A integrator, was used with a WCOT fused silica column, having a CP-WAX-52 CB packing. The column specifications were 50-m length, 0.32-mm intemal diameter, 0.45-mm extemal
zeolite Y Na
350b 2.0
2.7b 0.5-2.0 FAU 1.27 672 7.4
MFI 1.76
p 1.20
$
? P
3
.-z L-
si
0.80 0.60
-e---
0.40
0.20
I
0.00 0.00
10.00
20.00
30.00
40.00
Zeolite content (~01%)
z 0
1.00
C
f 0.00 0.00
10.00
30.00
20.00
40.00
Zeolite content ( ~ 0 1 % )
1 .
0.2 pm
-0.2pmfit
A
2.0pm
---- 2.0 pm fit
I
Figure 1. Influence of the zeolite crystal size on (a) sorption and (b) tensile strength. diameter, and 1.2-pm film thickness. The injector was at a temperature of 270 "C and the FID at 280 "C with a column head pressure of 62 Wa. Nitrogen was used as the carrier gas with a column flow of 1.6 mL/min and a split flow of 200 mL/ min. The purge vent was 3.5 mL/min, the oxygen flow 158 mL/min, the hydrogen 29.5 " i n , and the nitrogen make-up gas 19.5 mL/min. Results and Discussion
1. Crystal Size. As reported by Jia et al.,9 dispersion of the zeolite in the polymer is a delicate factor in the preparation of composite membranes. For the dispersion of solid particles in liquids, it is generally known that the particle size is an important parameter. Therefore, the influence of this parameter was studied on the dispersion of a zeolite in the PDMS polymer. Two silicalite samples with completely identical properties except for the size-a 10-fold difference-were incorporated in PDMS with several loadings. In Figure la, the sorption of p-xylene is plotted as a function of the zeolite content. In order to show the different tendency for both samples more clearly, a line is fitted through the experimental values. The exact values of the intercepts and the slopes, as well as the standard deviation on these numbers, are given in Table 4. It is clear that the zeolite particle size is indeed a significant parameter in the membrane preparation. The sorption capacity of a membrane
Vankelecom et al.
12392 J. Phys. Chem., Vol. 98, No. 47, 1994 is the sum of the sorption in the zeolite pores and in the polymer, possibly also in voids present in the polymer between badly dispersed zeolite particles or at the interface between polymer and zeolite. Membranes prepared with the small zeolite particles sorb more p-xylene (Figure la). Since the pore volume in the zeolite does not depend on particle size, it can be concluded that these small particles are not well dispersed. Dispersion is more difficult since van der Waals interparticle attraction forces dominate all other interactions in this relatively nonpolar solvent.14 As shown in Figure la, sorption measurements reflect a bad dispersion through an increased level of sorption. Tensile strength measurements can describe dispersion as well (Figure lb). As the interaction of silica with PDMS is attributed to the silanol groups at the outer surface,16-19small particles with a relatively large amount of surface silanols per weight unit are expected to increase the membrane strength the most. As shown in Figure lb, this is clearly not the case here. Only a bad zeolite dispersion can explain these results. Indeed, a nondispersed aggregate of zeolite particles forms a pinhole in the membrane which makes it break more easily. In this way, sorption and tensile strength may both provide information about dispersion. A bad dispersion will be reflected in an increased sorption and a decreased tensile strength. For the rest of the discussion, only one of the two measurements will be shown unless conflicting results were obtained. 2. Comments on the Preparation Method. The highest loading for a certain zeolite mentioned in this discussion does not imply that it was impossible to prepare membranes with higher loadings. For CBV-2802, for instance, 44 vol % is reported, whereas it was possible to prepare good membranes with loadings up to 57 vol %. However, membranes with very high zeolite loadings were brittle and could not be handled without a support layer, which made it impossible to use them for the tests applied in this work. The amount of solvent used to disperse the zeolites was increased proportionally to the zeolite content, by a factor optimized for each kind of zeolite. For high zeolite loadinghigh solvent content, vacuum treatment at 150 "C resulted in the boiling of the mixture and uneven membrane surfaces. Curing procedures in such cases included an evacuation step of 1 h at 50 "C before the final cure at 150 "C in vacuum. An appropriate pretreatment of the zeolite was found to be extremely important. Some zeolites are very hydrophilic and sorb water very fast. Equilibration at constant humidity is a procedure which ensures the prelevation of accurate amounts of zeolites in a way independent of the atmospheric humidity. However, prior to mixing with cross-linker or prepolymer, the zeolites must be carefully dried. Indeed, when the zeolites surface is covered with water molecules, the SiH groups of the polymer cross-linker may be hydrolyzed when added to the zeolite dispersion. A film of sorbed water surrounding a zeolite particle might prevent good contact between the zeolite particle and the hydrophobic polymer. Both effects would result in reduced strength of the membrane. The influence of sorbed water was studied in detail on a 20 vol % zeolite Y membrane. For this experiment, the zeolite was subjected to three different pretreatments and each time cooled to room temperature under vacuum before adding the solvent. As can be seen in Table 2, two treatments result in the same tensile strength: atmospheric drying at 300 "C (sample 300) and vacuum drying at 180 "C (sample 18Ov). Both pretreatments probably result in the same level of dehydration. A 180 "C treatment (sample 180) without vacuum leads to a decreased tensile strength, possibly due to the incomplete water removal.
TABLE 2: Influence of the Pretreatment Temperature of the Zeolite on the Tensile Strength for a PDMS Membrane Containing 20 vol % of Zeolite Y pretreatment temp, "C tensile strength, N/mm2 180 180v 300
4.96 5.55 5.60
TABLE 3: Influence of the Order of Adding the PDMS Components on the Sorption in a PDMS Membrane Containing 20 vol % of Borosilicate order of addn" sorption, mL/g 1.091 1.226 1.132
C
P C + P
c = crosslinker added first, p = prepolymer added first, c both added at the same time.
+p =
TABLE 4: Standard Errors on the Slopes and Intercepts Using a 95% Confidence Interval zeolite intercept slope Sorption CBV-3002 1.139 f 0.061 -0.0169 f 0.0030 -0,0180 f 0.0018 CBV-8020 1.149 f 0.032 -0.0189 f 0.0027 CBV-2802 1.396 f 0.091 borosilicate 1.133 f 0.031 -0.0092 f 0.0024 zeolite Y 1.069 f 0.033 -0.0218 f 0.0014 silicalite 0.2 pm 1.070 f 0.057 -0.0101 f 0.0020 silicalite 2.0 pm 1.087 f 0.015 -0.0154 & 0.0006 Tensile Strength CBV-3002 5.379 f 0.393 -0.0667 f 0.0192 CBV-8020 CVB-2802
borosilicate zeolite Y
silicalite 0.2 pm silicalite 2.0pm
4.884 f 0.202 4.968 f 0.464 5.456 f 0.214 4.630 f 0.230 4.333 f 0.337 4.118 & 0.647
-0.0543 & 0.0113 -0.0545 f 0.0194 -0.0669 f 0.0213 0.0413 f 0.0106 -0.0214 f 0.0135 0.0240 f 0.0261
In Table 3, it is shown that the order of addition of components A and B is of importance. Membrane c was prepared as described in the general preparation. For membrane p, components A and B were added in reversed order, while they were added together for membrane c p. In the latter case, the mixture was stirred for 3 h. Since the sorption is lowest in the membrane that was prepared by adding the cross-linker (component B) first, it is assumed that the optimum interaction between the zeolite and the polymer is established that way. To explain this fact, we should take into account that component A, the prepolymer, makes very viscous solutions. A good dispersion of the zeolite into such solutions is difficult to obtain. Component B, the cross-linker, makes a rather thin solution in which a good dispersion is easily obtained. A more likely interpretation can be given on the basis of the chemical composition of both components. The cross-linker contains SiH functions. These may react with the surface silanols on the outer surface of the zeolite particles, especially in the presence of the Pt gatalyst and at the temperatures used for the membrane preparation. A real covalent link might thus be formed between the elastomer and the zeolite particles. On the other hand, the prepolymer carries vinyl groups. Those groups have no chemical affinity for the zeolite surface. When component A is added first, it will cover the zeolite surface and reduce the reaction of the zeolite with the cross-linker that is added later. 3. Comments on the Sorption Measurement. Although PDMS is a highly hydrophobic membrane, the immersion of the glass plate into the water for the membrane removal has an important influence on the membrane characteristics. It is
+
rbI-
Zeolite Incorporation in PDMS Membranes
3
-i 2 F
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.E
d
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1.40 1.20 1.00 0.80 0.60 0.40
.
.
*.... .....a‘............... ----_
--I
.....
0.20 0.00 0.00
J. Phys. Chem., Vol. 98, No. 47, 1994 12393
.?. L
d 20.00
10.00
30.00
0.00
1 0
40.00
1
ZeoY ....... ZeoY fit
-Boro fit
I
A
3002
40
30
Zeolite content (vol%)
Zeolite content (vol%)
I
20
10
Boro
I
---- 3002
..............
fit
I
Figure 2. Influence of the type of zeolite on the membrane sorption.
expected that small quantities of water will sorb into the membrane, occupy part of the available sorption capacity in both the zeolite and the membrane polymer, and consequently reduce p-xylene sorption in both phases. The influence was studied on a pure PDMS membrane and a membrane containing 25 vol % zeolite CBV-2802. The membranes were weighed before and after the heat treatment. For the pure PDMS membrane, no significant weight loss was detected during the heat treatment, whereas the p-xylene sorption in the membrane was reduced by 3.6% after the treatment. For the zeolitecontaining membrane, an even higher loss of sorption capacity (8.8%) was detected, while the membrane weight loss after heat treatment was 1.5%. These results are surprising: since this zeolite sorbs 3.85 g of waterlg of zeolite, one would expect some sorbed water in the zeolite after the immersion of the membrane in the water. For a 25 vol % membrane, the amount of water sorbed in the zeolite accounts for 0.96 g/100 g of membrane. Removal of this water should result in a slightly increased p-xylene sorption. The observations can be explained as follows. After the cross-linking reaction of the polymer, the crosslinker component contains unreacted SiH groups that are hydrolyzed once the membrane is immersed in water. Upon heating, the resulting SiOH groups will condense into a siloxane bond with removal of water. This produces a more densely cross-linked membrane with a reduced p-xylene sorption. The effect is bigger for the zeolite-containing membrane, as the overall water sorption of this membrane is higher. 4. Type of Zeolite. Zeolite Y, silicalite, and borosilicate were selected to study the influence of the type of zeolite on the membrane incorporation (Figure 2 ) . For borosilicate, the line fitting was applied on the first six values only, since too large a deviation was observed at higher loadings. For silicalite and zeolite Y, a continuous decrease in sorption was found with increasing zeolite content. It may be concluded that the latter zeolites are dispersed very well, even at higher loadings. The borosilicate, on the other hand, has extremely small particles, and these are needle shaped. As proven in Figure 1, small crystals are indeed difficult to disperse. The decrease in sorption with higher loadings is not really surprising. The theoretical sorption capacity of a composite membrane can be calculated as the sum of the sorption in the zeolite and in the polymer, taking into account their respective weight fractions in the composite membrane. The zeolite was considered as being saturated, and the void pore volume was taken as the sorption capacity (Table 1). For the polymer, the experimental value for a pure PDMS membrane was taken. Two calculations were made: one in which the zeolite is considered
xh
h
0.60
e: 0.40
*s,e 0.20 d (1.00
I
t
I
20
10
0
30
40
Zeolite content (~01%) Exp.
-Fit
....... Theor.
----
Closed
Figure 3. Comparison between the experimental values (exp.), the fitted lines (fit), and the theoretical calculations for the sorption in membranes containing (a) CBV-3020 and (b) zeolite Y.
as being available for xylene sorption (indicated as “Theor.” on Figure 3), and another in which no sorption in the zeolite is assumed (“Closed” on Figure 3). In Figure 3, these calculated values are compared with the experimental sorptions for silicalite and zeolite Y loaded membranes. In the case of silicalite, the fitted line shows a slightly steeper slope than both theoretical lines. This means that the sorption in the composite membrane is restricted compared to what could be expected theoretically. Even the blockage of all zeolite pores cannot explain this observation. The reduced sorption capacity reflects a cross-linking action of the zeolite on the membrane polymer, as suggested before.’ Whether this is a chemical or a physical cross-linking cannot be concluded out of these experiments. Considering the reactivity of silanol groups toward the SiH groups of RTV-615 B and also the effect of the order of mixing, we believe that chemical cross-linking is present to some extent. The calculated lines for zeolite Y loaded membranes are less steep than those for the silicalite-loaded membranes. This is due to the lower density of zeolite Y. It is clear that the reduction of sorption is larger for membranes with zeolite Y than for silicalite-filled membranes. These results are schematically overviewed in Figure 4a. In this figure, the slopes of the experimental sorption curves for the various zeolites tested are converted into a membrane sorption loss (mL,lg) per 10 vol % of zeolite incorporated. A high value means a very strong decrease of the sorption with increasing zeolite loading. In the figure, two horizontal lines are drawn, representing the two theoretical calculations. The upper value corresponds to the slope of the theoretical sorption, calculated on the assumption that the zeolite pores are completely blocked. The lower line represents the situation where the pores are completely open. Looking at this figure, it is clear that the behavior of the borosilicate-loaded membranes cannot be explained unless we accept that the dispersion is exceptionally bad.
Vankelecom et al.
12394 J. Phys. Chem., Vol. 98, No. 47, 1994 h
W
-0*25 -0.20
I
0
-0.15 L
-
-0.10
VI
*
-0.05
. I
e 5:
0.00
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0.20
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2
0.00
-0.20
L
& -0.40 a3 M 2 -0.60 f
u
-0.80
Ql 7;
n
PDMSA B C D E F Figure 5. Comparison between the experimental and the theoretically calculated densities (A = CBV-2802, 33 vol %; B = CBV-3002, 35 V O ~%; C = CBV-8020, 31 V O ~%; D = CBV-3020, 31 V O ~%; E = Boro, 8 vol %; F = zeolite Y,34 vol %).
L
Figure 4. Comparison between (a) the experimental and the theoretically calculated sorption losses and between (b) the changes in tensile strength per 10 vol % of zeolite incorporated in PDMS.
Zeolite Y shows a remarkable deviation from the theoretical values. This can be explained by assuming a partial invasion of the polymer chains into the zeolite framework, just as proposed by Al-ghamdi and Mark.20 If this assumption is correct, a drastic increase in the tensile strength of a zeolite Y containing membrane should be observed, which is indeed the case. In Figure 4b, the slopes of the curves plotting tensile strength as a function of zeolite content are recalculated to a change in tensile strength per 10 vol % of zeolite, in the same way as for the sorption measurements mentioned before. A negative value in Figure 4b means a decrease of the tensile strength with increasing zeolite content. Clearly, all the ZSM-5 zeolites negatively influence the mechanical properties of the membrane. It is striking that borosilicate has no abnormal behavior on this graph. Probably, the aggregates of the very small borosilicate particles are still small, with little effect on the tensile strength. On the other hand, the voids inside the aggregates may be large enough to increase the sorption capacity of the membrane above the expected values. Zeolite Y clearly shows a distinct behavior: it is the only zeolite which produces a significant increase in tensile strength of the membranes. This is indeed the effect expected from a cross-linker. In the case of zeolite Y, the physical cross-linking is more important than the chemical cross-linking. 5. Density Measurements. The influence of zeolite incorporation on membrane properties is also reflected in density measurements. Figure 5 shows the measured densities compared to the densities that were calculated out of the literature data for the zeolite2' and polymer2*having no interaction at all with each other. For each zeolite, a membrane loading was chosen for which sorption and tensile strength indicated a defectfree membrane. In all cases, the theoretical value is much lower than the experimental one, and the difference increases as the zeolite content of the membrane increases. This clearly shows that a strong interaction exists between the zeolite and the polymer, being once again more pronounced for zeolite Y. 6. Membrane Anisotropy. SEM pictures reveal the appearance of anisotropy in a zeolite-filled PDMS membrane. A 30 vol % zeolite Y membrane was studied by SEM, and pictures
Figure 6. SEM pictures of the cross section (a (left) = top side of the membrane; b (right) = bottom side) of a PDMS membrane containing 30 vol % of zeolite Y.
130 T ovaJ 125 120 om E 115 Q Y 110 0 Q Y E 105 100 95 n
Bottom
fl
d
A B C D E F A B C D E F Figure 7. Comparison between the contact angles measured on both membrane sides (A-F = same as in Figure 5). The horizontal lines represent the contact angles found for the pure PDMS membrane.
were taken from the membrane cross section (Figure 6). Macroscopically, the anisotropy was already visible: the membrane side in contact with the glass plate during the curing (bottom side of the membrane) was shiny, while the upper side of the membrane had a matt appearance. SEM shows that the upper side of the membrane is very uneven, with zeolite particles pointing out of the surface but still covered with polymer. The bottom side of the membrane has a very even and smooth outline with no zeolite particles pointing through the polymer. The cross section confirms the good incorporation of the zeolite in the polymer material. Some voids appear in the membrane, especially near the matt side of the membrane. A sedimentation of the zeolites in the mixture at the very beginning of the curing procedure explains this phenomenon. Anisotropy of the membrane is also shown by measuring the contact angle between the polymer surface and a droplet of water (Figure 7), giving an idea of the hydrophobic character of the membrane surface. As glass and air have a different hydrophobicity, this slight anisotropy of the pure PDMS was expected. Upon adding zeolite, all zeolite-filled membranes show an
Zeolite Incorporation in PDMS Membranes
J. Phys. Chem., Vol. 98, No. 47, 1994 12395
0.06 L
E v
.-E g r2
0.05 0.04 0.03 0.02 0.01 0
PDMS
Si1
Figure 8. Sorption (mL/g) of water and ethanol in an unfilled PDMS membrane and a membrane containing 20 vol % of CBV-2802 (indicated as sil). I
PDMS
Si1
I
GFT
1
1
I
I
GFTsil
I
-T z
c
'"
-
Figure 9. Water, alcohol, and total flux in the pervaporation of 6 wt % ethanol solution at 35 "C. increased contact angle for the upper membrane side. For the bottom side, all composite membranes show a more hydrophilic character than the pure PDMS, indicating a sedimentation of the zeolites-being more hydrophilic than the polymer. It should be taken into consideration that the uneven membrane surface present at the top side might make an accurate measurement of the contact angle impossible. 7. Pervaporation Results. Prepared 20 vol % silicalite (CBV-2802) membranes were tested in the pervaporation of an ethanoywater mixture. To gain more insight into the influence of the zeolite on the membrane process, sorption of the pure liquids in the membranes was investigated first, as shown in Figure 8. As-in contrast to the xylene sorption mentioned above-the zeolite sorbs more ethanol than the PDMS,2 addition of zeolite should increase the overall sorption of the composite material. The reduced sorption for the alcohol in the composite membrane can be explained as a consequence of the crosslinking action of the zeolite on the polymer. On the other hand, water sorption increases since 3.85 wt % water sorption is determined for silicalite in the 80% relative humidity exsiccator and only 0.02 wt % in PDMS from the pure liquids. It clearly proves the less hydrophobic character for the silicalite than for PDMS, in agreement with the above-mentioned contact angles. Even though sorption measurements are performed on the pure liquids and pervaporation on mixtures, these experiments prove that the water affinity is higher for the composite membrane than for pure PDMS. Ethanol is still preferentially sorbed, however. The pervaporation results in Figure 9 for the self-prepared zeolite-filled membrane can be explained by considering the cross-linking reaction between the PDMS-polymer and the silicalite crystals. Compared with unfilled PDMS, the alcohol flux increases, the water flux decreases, and, consequently, selectivity increases when silicalite is added. This is in contrast with what could be expected from the sorption results: as water diffuses faster than ethanol in both the polymer and the zeolite, the increased water sorption should be reflected in an increased water flux. Clearly, the cross-linking of the zeolite reduces the possibility for the membrane to swell, leading to an increased
selectivity in the polymer phase for the preferentially sorbed component,23 ethanol in this case. The changed polymer properties in the vicinity of the zeolite particles might also provide an explanation for the lack of consistency shown by transport where the same polymer properties are assumed throughout the whole membrane, irrespective of the zeolite presence. As a more detailed investigation of the role of the zeolite in composite PDMS membranes on pervaporation will follow the comparison with other systems will be restricted here to two commercially available membranes (GFI' Co.), one silicalite filled and another unfilled. Compared with the self-prepared PDMS films, much lower fluxes are found for both water and ethanol. This can be ascribed to the attachment of the GFT top layer to a support, which reduces the swelling of the membrane and consequently the fluxes. The incorporation of silicalite improves membrane performance far less for the commercial membranes, even though 50 vol % of zeolite is added here in comparison with the 20 vol % for the self-prepared membrane: selectivity changes from 6.7 to 6.8 when silicalite is added, in contrast with the 200% increase for the self-prepared membranes (selectivities of 5.1 and 10.2). The chemical nature of the PDMS can provide an explanation for this phenomenon. Indeed, the GFI' PDMS chains carry terminal acrylate groups that cross-link after electron beam irradiation. This inhibits the PDMS chains to react with the zeolite surface. It reduces the interactions and, consequently, the incorporation. It means that no drastic changes happen in the polymer material when zeolite is added. Indeed, Bartels-Caspers et aL2 found strong agreement between the experimentally determined sorption isotherm of ethanol in the zeolite-filled silicone rubber and the isotherm obtained by adding up the amount of solvent sorbed in ZSM-5 and in the pure silicone rubber with the same acrylate groups. In other words, the sorption of ZMS-5 and this kind of silicone rubber was not affected by each other. The silicalitefilled GFT membrane (GFTsil) has the highest flux of both, due to an increase in both water and alcohol flux. The increased water flux could be caused-just as in our systems-by a more hydrophilic character of the zeolite compared with the PDMS. However, this is not logical for silicalite, incorporated in a polymer carrying many polar acrylate groups. More probably, a poor adhesion is established between the acrylate groups and the zeolite. This can also explain why selectivity did not change when adding the ethanol-selective zeolite. Apart from inhibiting chemical cross-linking, the acrylate group probably reduces the physical interactions as well. It proves once more how important a strong interaction is between the zeolite and the polymer to obtain significantly beneficial results from composite membranes. Conclusions The incorporation of zeolites in PDMS induces important changes in the polymer. A good dispersion of the zeolite is crucial but extremely difficult to obtain, especially when small particles are involved. All zeolites show strong interactions with the polymer. Apart from physical cross-linking attributed to van der Waals interactions, chemical cross-linking is proposed involving a reaction of the surface hydroxyls on the zeolite with the polymer. In the case of zeolite Y, the physical crosslinking is dominant, ascribed to the invasion of the zeolite pores by the PDMS chains. The preparation method used results in slightly anisotropic membranes. The developed concept of zeolite cross-linking is proven to form a good base to explain pervaporation results, in which strong filler-polymer interactions were found to be essential to improve separations with composite membranes.
12396 J. Phys. Chem., Vol. 98, No. 47, 1994 Acknowledgment. We are grateful to the Belgian Govemment for the support in the frame of a IUAP-PA1 grant on Supramolecular Catalysis. I.F.J. acknowledges a fellowship as Research Assistant from the Belgian National Fund of Scientific Research (N.F.W.O.). We thank the VITO (Mol, Belgium) (for doing the contact angle measurements. References and Notes (1) te Hennepe, H. J. C.; Bargeman, D.; Mulder, M. H. V.; Smolders, C. A. J. Membr. Sei. 1987, 35, 39. (2) Bartels-Caspers, C.; Tusel-Langer, E.; Lichtenthaler, R. N. J . Membr. Sei. 1992, 70, 75. (3) Dotremont, C.; Goethaert, S.; Vandecasteele, C. Desalination 1993, 91, 177. (4) Duval, J.-M.; Folkers, B.; Mulder, M. H. V.; Desgrandchamps, G . ; Smolders, C. A. J . Membr. Sci. 1993, 80, 189. ( 5 ) Lamer, T.; Voilley, A,; Beaumelle, D.; Marin, M. Ricents Prog. Genie Procidis 1992, 6, 21, 419. (6) Goethaert, S . ; Dotremont, C.; Kuijpers, M.; Michiels, M.; Vandecasteele, C. J. Membr. Sei. 1993, 78, 135. (7) Goldman, M.; Fraenkel, D.; Levin, G. J . Appl. Polym. Sei. 1989, 37, 1791. ( 8 ) Jia, M.-D.; Peinemann, K.-V.; Behling, R.-D. J . Membr. Sci. 1991, 57, 289. (9) Jia, M.-D.; Peinemann, K.-V.; Behling, R.-D. J . Membr. Sei. 1992, 73, 119.
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