J. Phys. Chem. 1996, 100, 3753-3758
3753
Silylation To Improve Incorporation of Zeolites in Polyimide Films Ivo F. J. Vankelecom,* Sacha Van den broeck, Edouard Merckx, Hilde Geerts, Piet Grobet, and Jan B. Uytterhoeven Center for Surface Chemistry and Catalysis, Faculty of Agricultural and Applied Biological Sciences, Katholieke UniVersiteit LeuVen, Kardinaal Mercierlaan 92, 3001 LeuVen, Belgium ReceiVed: September 11, 1995; In Final Form: NoVember 1, 1995X
The silylation of borosilicate with APTS ((γ-aminopropyl)triethoxysilane) is studied as a tool to improve zeolite incorporation in PI (polyimide) films. Among other experimental parameters, a quantification of the zeolite outer surface silanols is performed in order to determine the most appropriate silylation conditions. Xylene sorptions, NMR (nuclear magnetic resonance) spectroscopy, and measurements of the specific surface of the zeolites were performed to characterize the silylation. Finally, the silylated zeolite was incorporated in PI films on which tensile strength, density, and xylene sorption were measured.
Introduction Gas separation and pervaporation are two relatively new membrane processes, separating gaseous or liquid mixtures, respectively, through a dense membrane. In 1988, zeolite filled PDMS (poly(dimethylsiloxane)) membranes were introduced in these processes for the first time.1 Even though these membranes performed very well in both pervaporation1-7 and gas separation,8,9 only a few authors have reported the incorporation of zeolites in rubbery polymers other9,10 than PDMS or in glassy polymers.11-13 Using the more rigid glassy polymers, the adhesion at the organic-inorganic interphase11 constitutes the main difficulty. For zeolite filled polyimides for instance, this bad adhesion leads to voids10 and low filler loadings.12 In this report, we study the possibility to improve adhesion by bridging the interface between inorganic materials and organic polymers with silylating agents. As a case study, zeolite (more in particular a borosilicate molecular sieve) filled polyimides (PI) were selected. Above the adhesion problems, the possible use of these composite polymers in membrane separation processes puts new restrictions on the system. Indeed, in order to have full advantage of the incorporated zeolite, the pores of the zeolite should still be completely accessible after silylation. An as complete as possible monolayer of the silylating agent would be desirable, in order to realize enough covalent bonds to improve adhesion. On the other hand, multilayer depositionscreating a new phase between the zeolite and the polymersshould be avoided, as it would change membrane properties drastically or even cause new voids. Most of the literature on silylation of inorganic fillers concerns silica, especially with APTS ((γ-aminopropyl)triethoxysilane),14-22 capable of linking fillers covalently to polyimides.23-25 Only a few zeolite silylations26-33 are described, but none of them focuses on incorporation in polymers. Our work aims at a minimal coverage of zeolite crystals with APTS to improve the incorporation of borosilicate in PI. It is described in literature23-25 how the polyamic acids (the precursors of PI) react with an APTS-silylated silica (Figure 1). The final heat treatment results in a ring closing reaction involving the APTS alkylamine, being more reactive than the aromatic amine of the polyamic acid.24 In analogy with this silica, a firm covalent bond between the polyimide and the silylated silanol groups at the borosilicate X
Abstract published in AdVance ACS Abstracts, January 15, 1996.
0022-3654/96/20100-3753$12.00/0
Figure 1. Reaction between surface bound APTS and polyamic acid (adapted from ref 24).
surface is believed to ensure a good adhesion in the system studied here. In contrast with the silylation of silica and of the inner surface of zeolites, the silylation of the zeolite outer surface is far more difficult to be monitored experimentally, since very low silylating agent concentrations are to be detected. Even elementary analysis and XPS (X-ray photoelectron spectroscopy) can hardly distinguish signals from noise and surely do not allow quantitative analysis.11 Together with NMR (nuclear magnetic resonance), indirect measurements were chosen in our experiments to characterize the silylation: short time sorption of p-xylene to see how silylation changed uptake, and equilibrium sorption to see whether inner pore silylation had reduced the available pore volume. Equilibrium as well as short time sorptions were also done for an equimolar mixture of xylene isomers, from which p-xylene selectivities could be calculated. Xylenes were chosen because the zeolite structure studied shows a very high shape selectivity for p-xylene.34,35 Moreover, xylenes are large molecules whose sorption is expected to change drastically when the zeolite pores are obstructed. Additional information about changes inside the zeolite pores was obtained from BET measurements. The quantification of the zeolite surface silanols was also investigated to estimate the concentration of silylating agent, which was preferred to be low in order to avoid multilayer deposition. To prove the enhanced adhesion in the composite membranes, density and tensile strength measurements36 were finally performed. © 1996 American Chemical Society
3754 J. Phys. Chem., Vol. 100, No. 9, 1996
Vankelecom et al.
TABLE 1: Main Characteristics of the Zeolites Used ZSM-5 cation form SiO2/X2O3 size (µm) internal surface (m2/g) a
CBV-2802
CBV-3002
borosilicate
NH4 275a 0.4-0.8 410
NH4 240a 1-1.5 405
H 11.82b 0.1-1.0 304
X ) Al. b X ) B.
Experimental Section Silylation of the Glassware. As low concentrations of silylating agent were applied, a foregoing silylation of all glassware was necessary. The glassware was dried at 100 °C for at least 15 min, after which it was exposed to a 0.1 vol % solution of trimethylchlorosilane (Merck, >99.5%) in toluene (Aldrich, 99+%, dried over zeolite A) for at least 12 h. The glassware was cleaned with acetone (Janssen Chimica, p.a.) and dried in the oven at 120 °C in order to remove all traces of solvent and nonreacted trimethylchlorosilane. Zeolites. The main characteristics of the borosilicate molecular sieve (Amoco) and two samples of ZSM-5 (PQ zeolites) are given in Table 1. All three zeolites have the MFI topology, which means that straight channels (5.2 × 5.7 Å) are perpendicular to sinusoidal channels (5.3 × 5.6 Å), forming a three dimensional network. The ZSM-5 samples contain aluminum, whereas the borosilicate contains boron. The zeolites were stored at a relative humidity (RH) of 80%. The ZSM-5 samples were only used in the determination of the number of outer surface hydroxyls to verify the validity of this method. They were not used in further silylation experiments. Determination of the Number of Hydroxyls on the Outer Surface of the Zeolite. A standard curve was made by measuring UV absorption of 2,4,6-collidine (Janssen Chimica, 99%) solutions at 260 nm (UV spectrophotometer Van der heyden, UV 321) with known concentrations in 2,2,4-trimethylpentane (Janssen Chimica, 99.5%). At this wavelength, the collidine was found to have the maximum extinction coefficient. For the sorption on the zeolite, 5 mL of a 0.0235 M solution of collidine in 2,2,4-trimethylpentane was added to 0.5 g of zeolite, predried under vacuum at 180 °C. After 2 h of sorption, the sample was centrifuged and the concentration of the supernatant was measured at 260 nm. The number of collidine molecules sorbed per gram of zeolite was then determined from this concentration. Using the zeolite density of 1.76 g/mL, the number of collidine reacted per zeolite volume was calculated. In order to express the results as a number of silanols per zeolite surface, the surface/volume ratio of the zeolite crystals was determined using the shape and size of the crystals, as obtained from SEM (scanning electron microscopy) pictures. Silylation of the Zeolite. A. General Procedure. As even the best coupling agent for a given composite may perform poorly if not applied properly,37 careful attention was paid in our experiments to the pretreatment of substrates14,15 and reagents,14-18,38,39 as well as to all other experimental condtions.18 A 1 g amount of zeolite (dry weight), 5 mL of toluene (Aldrich, 99+%, dried over zeolite A), and a magnetic stirrer were enclosed in a bottle and purged with N2. After the mixture was stirred for 30 min, 5 mL of a solution of 0.3 mmol (γaminopropyl)triethoxysilane (Fluka Chimica, 96%) was added per gram of dried zeolite. The bottle was closed again, purged with N2, and stirred for 1 h. After the reaction, the sample was filtered over a Buchner filter. The silylated zeolite was washed 3 times with dry toluene, after which it was dried overnight at 180 °C under vacuum before characterization or incorporation.
This way, the only water present in the system was the surface bound water of the borosilicate which was controlled by its pretreatment: 11.3 wt % when stored at 80% RH and less than 1 wt % when predried for 1 h at 180 °C. B. Nonsilylated Zeolites as Reference System. The toluene treatment was done in the same way as that for a normal silylation, but without adding silylating agent. C. Influence of Pretreatment Conditions. The influence of the pretreatment conditions of the zeolite was studied by comparing borosilicates equilibrated at 80% RH, with borosilicates that were submitted to a 1 h of heat pretreatment at 180 °C. D. Influence of the Posttreatment Conditions. On borosilicate, equilibrated at 80% RH before silylation, the influence of the posttreatment time (1 or 16 h at 180 °C vacuum) was investigated. E. Influence of the SolVent. Silylation in toluene was compared with silylation in chloroform (Janssen, p.a.). Characterization of the Silylated Borosilicate. A. Xylene Sorption. After drying, the silylated borosilicate was cooled to room temperature under vacuum and 0.5 g was weighed in a 7.5 mL vial. To this, 5 mL of a solution of p-xylene (Janssen Chimica, 97%) in 1,3,5-triisopropylbenzene (Janssen Chimica, 97%) or 5 mL of an equimolar solution of p-xylene, m-xylene (Janssen Chimica, 98%), and o-xylene (Janssen Chimica, 97%) in 1,3,5-triisopropylbenzene was added. In the experiments where equilibrium sorption was measured (after 2 days), the p-xylene concentration was 0.45 M, whereas 0.15 M of each xylene was present in the mixture. If sorption time was 2 min, the p-xylene concentration was 0.15 M and the xylene mixture contained 0.05 M of each compound. The samples were shaken continuously during sorption. After the sorption, the vials were centrifuged and 1 mL of the supernatant was transferred into another vial. An adapted amount of cyclohexane was then added as external standard: this amount was based on the presumed xylene excess. Finally, 0.3 µL was injected in the gas chromatograph (Hewlett-Packard GC, 5890 series II). In the case of sorption from a mixture, the selectivity for p-xylene was calculated as
Sp ) (sorption (mL/g) of p-xylene)/ (sorption (mL/g) of p-xylene + m-xylene + o-xylene) × 100 The following GC temperature (Chrompack, CP-Sil 5 CB column) was programmed: 8.5 min on 110 °C and then with a rate of 70 °C/min to the final temperature of 160 °C, which was kept constant for 7 min. The injector was at 270 °C and the FID (flame ionization detector) at 280 °C. B. Specific Surface of the Borosilicate. The specific surface was measured by means of an Ankersmit automatic surface analyzer, Model 4200, at a partial N2 pressure of 0.3. Before analysis, the borosilicate was dried at 180 °C under vacuum for at least 12 h. In the sample holder, 0.15 g of borosilicate was given a pretreatment of 180 °C under a He/N2 gas stream (He/N2 ) 0.7/0.3) for 90 min. The BET equation40 was used assuming multilayer sorption. C. Solid State 29Si NMR. 29Si-NMR spectroscopy was applied (Bruker, MSL 400) with a magnetic field strength of 9.4 T and a frequency of 79.46 MHz. Before doing the measurements, the samples were kept at a relative humidity of 100% by means of a saturated NH4Cl solution. The measurement was done under cross-polarization conditions with the sample spinning at a frequency of 4 kHz. The spectra were recorded in such a way that the peaks between -100 and -120 ppm, attributed to the Si atoms in the borosilicate, were of equal
Silylation To Improve Incorporation of Zeolites size in every spectrum. In that way, the borosilicate concentration was assumed to be the same in every spectrum, enabling a more correct interpretation of the intensities of the peaks between -49 and -58 ppm. Incorporation of Silylated Borosilicates in PI. The incorporation of borosilicates in PI and the characterization of the composite membranes were done in the same way as described before for the non-silylated borosilicates.12 After drying, the silylated borosilicates were dispersed in NMP (N-methyl-2pyrrolidone; Aldrich, 99.9%) during 1 h in an ultrasonic bath, as a 15 wt % dispersion. Meanwhile, the 15 wt % polyamic acid solution was prepared by adding 2.631 g ODA (4,4′oxydianiline; Janssen Chimica, 98%) to 30 mL of NMP. This was stirred for 15 min at room temperature, after which an equimolecular amount of 2.838 g of PMDA (pyromelletic dianhydride, Janssen Chimica, 99%) was added at 0 °C. Stirring was continued for several hours during which solution temperature slowly increased as a consequence of the exothermic polymerization reaction. It was after this stage that the borosilicate suspension was added. The stirring was continued for another 2 h, after which the air, entrapped in the solution during mixing, was removed by applying vacuum. The mixture was then cast on a glass plate with a thickness of 250 µm. Removal of the solvent and curing took place in a programmed vacuum oven. After 1 h at 25 °C, the temperature was raised at a rate of 10 °C/h to 70 °C, which was maintained for 6 h. After a further increase of 30 °C/h, the final temperature of 220 °C was reached. It was kept constant for 3 h. All reagents were used as such, except for the PI monomers that were purified by vacuum sublimation at 60 °C. The borosilicate content is expressed in volume percent as (weight of borosilicate/ dborosilicate)/(weight of borosilicate/dborosilicate + weight of polymer/ dpolymer) in which d ) density (dpolymer ) 1.4050 mL/g).12 Results and Discussion 1. Number of External Surface Hydroxyls on the Zeolite. Given the importance of a monolayer deposition, the knowledge of the number of external surface silanols per gram of zeolite was needed first for dosing the amount of silylating agent for reaction. This number was determined by sorbing collidinesa strong basesfrom 2,2,4-trimethylpentane solutions, both molecules being too large to enter the zeolite pores. This was a requirement for the base to limit the interaction to the external silanols, but also for the solvent. Indeed, correct concentration calculations require a constant solvent volume throughout the reaction. UV spectrophotometry is one of the few methods suitable for this quantification. Many other characterization techniques were either insufficiently sensitive or could not discriminate the few silanols at the external surface from the large number of bulk hydroxyls of the zeolite. Furthermore, the base needed a strong UV absorption, since only very small amounts of silanols are present on the outer surface. The solvent did not interfere with the base absorption on the wave length used. In order to determine the time needed to come to equilibrium sorption, the amount of collidine in solution was determined after 30, 60, 90, and 120 min for borosilicate. Sorption equilibrium was found to be reached in less than 30 min, and a value of 0.056 ( 0.005 mmol/g borosilicate was measured. The amounts of collidine sorbed on borosilicate, as well as on the two different ZSM-5 samples, are shown in Table 2, recalculated to a number of surface hydroxyls per 100 Å2 of outer zeolite surface. The recalculation gains insight in the spacial distribution of the silanols over the zeolite surface. It gives an idea about the space one silylating agent has to react
J. Phys. Chem., Vol. 100, No. 9, 1996 3755 TABLE 2: Amont of Collidine Sorbed and the Number of Hydroxyl Groups per 100 Å2 of External Zeolite Surface for the Three Zeolites Investigated zeolite borosilicate CBV-2802 CBV-3002
mmol of collidine/(g of zeolite) no. of silanols/100 Å2 0.056 0.029 0.020
2.9 3.1 4.4
TABLE 3: Sorption and Selectivity for Borosilicate before and after Toluene Treatment sorption (µL/g) treatment none toluene
sorption time
p-xylene
xylenes
Sp (%)
2 min equilibrium 2 min equilibrium
83 141 64 114
69 123 56 108
83 62 94 73
and how a second silylating agent will be hindered in reaction with an adjacent silanol. In agreement with SEM observations, the borosilicate crystals were considered as needle shaped and the ZSM-5 samples as spheres. The numbers given in Table 2 correspond very well to literature values (1-5.5 OH/nm2) obtained with IR for silica.41 A possible disadvantage of this indirect measurement of the number of surface silanols could be that collidine might be so large, that it is sterically impossible to have one molecule of collidine sorbed on every silanol, due to the hindrance of the neighboring collidine. Therefore, the number calculated was interpreted as a minimal value. 2. Nonsilylated Borosilicate as Reference System. As the borosilicate undergoes several treatments before and during the silylation, it should be investigated first how these treatments influence the sorption and the BET results obtained on the silylated samples. For untreated borosilicate, the p-xylene sorption and the sorption of the xylene mixture are not yet at equilibrium after 2 min (Table 3). The values for the xylene mixture are lower than for p-xylene. Indeed, m-xylene and o-xylene, having a larger kinetic diameter, sorb less fast than the pure p-xylene, and their presence in the borosilicate pores also slows down the p-xylene uptake. At equilibrium, the presence of m- and o-xylene disturbs the packing of the p-xylene molecules, leading to a reduced global sorption capacity.34 It should be noticed that the p-xylene selectivity is decreasing with time. The prolonged sorption time compensates the slower diffusion of m-xylene and o-xylene relative to the fast sorbing p-xylene. A BET value of 319 m2/g was found, agreeing fairly well with the one given by the company (304 m2/g). For borosilicate treated with toluene, the sorption after 2 min is smaller and the selectivity larger than in the case of nontreated borosilicate. Since a toluene peak appeared on the chromatograms of the xylene sorptions, toluene did not seem to be evacuated from the borosilicate pores completely, in spite of the 180 °C treatment under vacuum for 1 night. The acidic hydroxyls present in the borosilicate probably interact strongly with the π-electrons of the benzene ring of toluene,35 rendering desorption difficult. Surprisingly, the BET value was decreased from 319 to 249 m2/g after the toluene treatment. It would be unrealistic to ascribe this to the sorbed toluene. It is more likely that a borosilicate fraction disappeared through the Buchner filter during the filtration of the borosilicate, as will be confirmed in the following paragraphs. 3. Silylation with APTS. Knowing the number of surface silanol groups and having the characteristics of the appropriate reference system, silylation could be performed and characterized. A possible solvent effect and the sorption characteristics of the silylated borosilicate were examined. As it is important
3756 J. Phys. Chem., Vol. 100, No. 9, 1996
Vankelecom et al. TABLE 5: Equilibrium Xylene Sorptions for APTS-Silylated Borosilicate with Different Pretreatment Temperature sorption (µL/g) temp (°C)
p-xylene
xylenes
25 180
107 135
60 100
TABLE 6: Xylene Sorptions and Selectivities for APTS-Silylated Borosilicate as a Function of the Sorption TIme and the Posttreatment Time sorption (%) Figure 2. 29Si solid state NMR spectra of APTS-silylated borosilicate with a pretreatment of 180 °C (A) and at 80% RH (B).
TABLE 4: Sorptions after 2 min and BET Values of Borosilicate Silylated in Different Solvents (Pretreatment at 80 °C) sorption (%) p-xylene xylenes toluene chloroform
80 100
86 100
Sp (%)
BET value (m2/g)
95 76
230 248
to have only one, well-bonded silicone layer on the borosilicate, it was also investigated whether the influences found by Caravajal et al.14 for the silylation of silica hold for borosilicates as well. These authors found that a small amount of surface bound water (obtained by pretreating silica at 200 °C) was essential to have silylation but that more water on the silica resulted in multilayered APTS. A. Influence of the SolVent. When compared to chloroform (sorption equaled to 100%), the sorptions in Table 4 reveal a slower diffusion of the xylenes in the case of a borosilicate silylated in toluene. Clearly, chloroform is more easily removed from the borosilicate pores during the posttreatment, because of its higher volatility and the absence of π-electrons. This is confirmed when the selectivity for p-xylene is considered: a decreased p-xylene selectivity is observed when the borosilicate is silylated in chloroform. For both solvents, the same BET values for the silylated borosilicates are obtained. It proves that the low BET value of 249 m2/g (found for the reference system after being contacted with toluene) is indeed caused by a loss of borosilicate particles during the filtration through the Buchner filter. B. Influence of the Pretreatment Temperature. The results of the 29Si solid state NMR are presented in Figure 2 for two different pretreatments. Symmetric with regard to the intense peak (ascribed to the Si from the borosilicate) to which they belong, spinning side bands can be seen. This makes the correct interpretation of the spectra more difficult, since the side bands are situated between -49 and -58 ppm, exactly the region in which the APTS signals appear. The peaks have been assigned by Caravajal et al.:14 -49 ppm corresponds with a Si in the monodendate form and -58 and -66 ppm with the bi- and tridentate forms, respectively. The intensity of the side bands has to be subtracted from the intensity of these bands ascribed to the APTS Si. Figure 2 shows that the more water sorbed on the borosilicate, the more intense the APTS Si peak. The borosilicate with a pretreatment of 180 °C (A) gives hardly any APTS signal. However, this does not mean no reaction occurred at all. Indeed, the reactive outer surface of the borosilicate is about 2 orders of magnitude smaller than that of the silicas reported in literature. Consequently, such a limited amount of APTS molecules may fall beyond the NMR-detection level. For the same reason, it is impossible here to distinguish between the three different peaks occurring between -49 and -66 ppm.
sorption time: 5 min 1h 16 h equilibrium sorption 1h 16 h
p-xylene
xylenes
Sp (%)
65 100
24 100
67 46
95 100
84 100
56 56
Due to the small reactive surface, only a limited amount of each APTS form (mono-, bi-, or tridentate form) is present, and no information can be obtained about the exact form in which the APTS is present on the borosilicate surface. The only conclusion that can be drawn here is that the presence of lots of water at the borosilicate surface induces an uncontrolled APTS reaction, leading to multilayer coverage, in agreement with literature data for silica.14 As Caravajal et al.14 proved the APTS reaction to take place on silica pretreated at 200 °C, it can be assumed reasonably that the borosilicate considered heressorbing water even stronger than silicasstill contains enough surface water to make the APTS hydrolyze and react subsequently. Confirmation of these results is found in xylene sorptions. Table 5 reveals that a high pretreatment temperature for silylation results in the highest borosilicate sorption capacity. With low pretreatment temperatures, sorption is reduced due to the presence of an APTS multilayer around the borosilicate. C. Influence of the Posttreatment Time. The xylene sorptions measured on silylated borosilicates posttreated at 180 °C under vacuum during 16 h are equaled to 100% in Table 6. The samples with a short posttreatment clearly show a reduced sorption. This is explained by the incomplete removal of toluene after only 1 h. When xylene molecules sorb in that case, the back-diffusion of the toluene hinders and slows down the xylene uptake. Due to the larger kinetic diameter of o-xylene and m-xylene, the effect is bigger on the sorption of the xylene mixture than on the sorption of pure p-xylene. The figures in Table 6 also show that the effects on sorption are smaller as the sorption time increases. Indeed, the slow back-diffusion of toluene will then be more complete, and the xylene equilibrium sorption can be approached more closely. 4. Incorporation of Silylated Borosilicates in PI. A. Effect on Tensile Strength and Density. The borosilicates used for incorporation were silylated in chloroform. Indeed, the preparation of PI membranes loaded with borosilicates silylated in toluene was found to be impossible, as holes appeared in the cured PI films. These holes were attributed to the evaporation of toluene, liberated from the borosilicate pores during the imidization at high temperature under vacuum. In Figure 3, the effect of the APTS silylation is shown on polyimide membranes for an increasing amount of borosilicate incorporated. Silylated borosilicates with the 180 °C pretreatment were chosen for this incorporation, since the multilayer found for the 80% RH pretreatment was undesired for the membrane application. The tensile strength (Figure 3A) of every membrane with silylated borosilicate is higher than for the corresponding
Silylation To Improve Incorporation of Zeolites
J. Phys. Chem., Vol. 100, No. 9, 1996 3757 Conclusions
Figure 3. Comparison between the tensile strengths (A) and the densities (B) of APTS-silylated and non-silylated borosilicate, incorporated in polyimide at several loadings.
In order to improve the incorporation of zeolites in PI, the silylation of borosilicate molecular sieve with APTS was studied. UV absorption of collidine was found to be sensitive enough to estimate the number of hydroxyls present at the outer zeolite surface. The same important role of surface bound water as for silicas was found here for borosilicate silylation. However, due to the small outer surface of the borosilicates, NMR could not determine the exact APTS form present on the borosilicate. Toluene was found to be a good solvent for silylation, but it was removed very slowly and incomplete from the borosilicate pores after reaction. Therefore, silylation in chloroform was preferred in view of the subsequent incorporation of the silylated borosilicates in polyimide membranes. Density and tensile strength measurements on these composite membranes proved indeed a better incorporation of borosilicates after silylation with APTS without changing sorption. It makes these newly developed composite PI membranes promising tools in future membrane processes. Surely, the reported concept is not limited to this one PI-borosilicate system, but can be expanded to many other composite polymer systems, for as long as the appropriate silylating agent is chosen in the right experimental conditions to link the filler to the polymer. Acknowledgment. We are grateful to the Belgian Government for the support in the frame of a IUAP-PAI grant on Supramolecular Catalysis. I.F.J.V. acknowledges a fellowship as Post Doctoral Research Fellow from the Catholic University of Leuven, Belgium. References and Notes
Figure 4. Sorption as a function of time in polyimide membranes, containing silylated (APTS) or unsilylated (Boro) borosilicate (membranes with a 20 wt % loading were used for p-xylene sorption and 10 wt % for xylene sorption).
membrane with non-silylated borosilicate. This proves that indeed a real covalent bond is formed between the silylated borosilicate and the polyimide. These results are confirmed when comparing the densities (Figure 3B) of polyimides with silylated and unsilylated borosilicates in different loading. It is clear that the silylated borosilicates make the membrane density approach more the theoretical density,12 indicating a better incorporation of the borosilicate. These results also provide evidence for the fact that silylation had occurred, even if no clear APTS signals could be observed in the NMR spectra of such a pretreated borosilicate sample. B. Sorption. Some polyimide membranes containing silylated borosilicates were immersed in xylenes in order to investigate the influence of the silylation on the sorption rate and sorption capacity of the membranes. For the first measurements after 1 week as well as for the equilibrium measurement after 5 weeks, no differences with the reference membranes could be observed for either the xylene mixture or the pure p-xylene (Figure 4). It means that the silylation improves the incorporation of the borosilicate in the polymer (Figure 3), without altering the sorption properties of the composite membranes, an essential requirement for their use in pervaporation or gas separation.
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