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The unusual change of permeation rate in PDMS membranes filled with crystalline calixarene and its derivative
Journal of Membrane Science 279 (2006) 111–119
The unusual change of permeation rate in PDMS membranes filled with crystalline calixarene and its derivative Liang Liu a,b , Zhongyi Jiang a,b,∗ , Fusheng Pan a , Fubing Peng a , Hong Wu a a
b
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China State Key Laboratory of Functional Polymer Materials for Adsorption and Separation, Nankai University, Tianjin 300071, China Received 5 April 2005; received in revised form 14 November 2005; accepted 22 November 2005 Available online 6 January 2006
Abstract PDMS membranes filled with crystalline calixarene (CA) and calixarene derivative (CAD) were prepared for pervaporative removal of benzene from dilute aqueous solution. With the increase of CA and CAD content, the normalized permeation rate of the filled membranes did not exhibit monotonic change, but had minimum and maximum values instead. In order to explain this unusual phenomenon, swelling experiment, XRD and PALS measurement had been carried out. A model, which describes the mass transport in the CA-filled membranes by combining swelling and crystallinity of the membranes, was proposed and validated by CAD-filled membranes. Due to the higher hydrophobicity of CAD over CA, CAD-filled PDMS membranes exhibited higher separation factor than CA-filled PDMS membranes. In addition, both CA and CAD-filled PDMS membranes showed better permselectivity than control PDMS membranes. © 2005 Elsevier B.V. All rights reserved. Keywords: Pervaporation; Calixarene; Filled membrane; Crystallinity; Swelling; Normalized permeation flux
1. Introduction For the removal of volatile organic compounds (VOCs) from their dilute aqueous solutions, pervaporation has been proved to be an attractive and potentially cost-competitive approach. Among common organophilic polymer membrane materials, poly(dimethylsiloxane) (PDMS) has the superior performance attributed to its stronger affinity towards VOCs over water and very flexible chain structure [1–5]. However, there is still much room for the optimization and improvement of separation properties of existing PDMS-based membrane. As a simple and effective way, adding hydrophobic and organophilic filler such as carbon molecular sieves [6], silicalite [7–9], into PDMS matrix to increase the permselectivity has attracted considerable research interest. However, due to the rigid structure and unfavorable desorption characteristics, these fillers did not serve the purpose very well in most cases. Recently, the supermolecule calixarenes and their derivatives have received much attention for their role in bio-
∗
Corresponding author. Tel.: +86 22 2789 2143; fax: +86 22 2789 2143. E-mail address: [email protected] (Z. Jiang).
0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.11.041
logical, synthetic receptor models and separating agents. The flexible calixarene framework requires small energy change to accommodate guest molecules, which makes them exceptionally resourceful host molecules. It is well known that the -rich cavity in calixarene is suitable for the inclusion of neutral guest of complementary size including benzene, owing to supramolecular interaction. It can be thus supposed that the moderate supramolecular interaction, which lies between the physical adsorption and chemical adsorption, will potentially improve the pervaporation performance of PDMS membrane [10,11]. Uragami et al. [12] had investigated the effect of the addition of tert-butylcalix[4]arene (CA) to PMMA-b-PDMS membrane for the removal of benzene from dilute aqueous solution. The experimental results indicated that the addition of CA to PMMA-b-PDMS membrane increased separation factor from 1800 to 2300 and normalized permeation rate from 7 to 13 × 10−6 kg m/(m2 h). However, the inherent glassy characteristics of PMMA and the existence of microphase separation in PMMA-b-PDMS membrane seem not necessarily helpful for the permselectivity and permeability of membrane. In this paper, PDMS membranes filled with crystalline CA and its more hydrophobic derivative (CAD) are prepared for the
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(propoxy)calix[4]arene (CAD) were purchased from Nankai University, the molecular structures are shown in Fig. 1. All the other solvents and reagents obtained from commercial sources were of analytical grade and used as received. 2.2. Membrane preparation
Fig. 1. Molecular structure of CA and CAD.
pervaporative removal of benzene from dilute aqueous solution. A tentative model, which describes the mass transport in the CA-filled membranes considering swelling and crystallinity of the membranes, is presented and validated by CAD-filled membranes.
Both the CA-filled PDMS membranes (CA-PDMS membranes) and the CAD-filled PDMS membranes (CAD-PDMS membranes) were prepared by solution-casting mixtures of the fillers and PDMS/toluene solutions. The mixture was vigorously stirred to obtain a pseudo-homogenous solution and then the solution was cast onto the surface of organic glass plate. PDMS was crosslinked by tetraethylorthosilicate (TEOS) in the toluene solution at room temperature under the catalytic action of dibutyltin dilaurate. The weight ratio of PDMS:toluene:TEOS:dibutyltin dilaurate is 1:1:0.1:0.02. The solid membranes were formed after toluene evaporated completely. The membranes were further dried in vacuum for 24 h and swelled in the feed solutions for more than 24 h before use. 2.3. Pervaporation experiments
2. Experimental 2.1. Materials PDMS prepolymer (the viscosity was 5000 mPa s, the average molecular weight was about 40,000) was purchased from Beijing Huaer Co. Ltd. Both tert-butylcalix[4]arene (CA) and 5,11,17,23-tetrakis(tert-butyl)-25,26,27,28-tetrakis-
A schematic diagram of the pervaporation test unit is shown in Fig. 2. In the pervaporation experiment the air-side surface of all the membranes faced the feed side of the pervaporation cell and the effective membrane area was 25.6 × 10−4 m2 . For direct comparison of the performance of each membrane, the parameters during the experiments were controlled as follows: the feed concentration of benzene was about 0.14 wt%; the feed
Fig. 2. Schematic diagram of pervaporation experimental equipment. (1) Heater, (2) thermocouple, (3) feed container, (4) feed circulator, (5) flowmeter, (6) membrane cell, (7) cold trap, (8) liquid nitrogen, (9) desiccator, (10) buffer and (11) vacuum pump.
L. Liu et al. / Journal of Membrane Science 279 (2006) 111–119
temperature was kept at 60 ◦ C; the feed flow rate was 120 L/h; the vacuum in the downstream side was about 1.3 kPa. The concentrations of the feed and the permeate were analyzed by gas chromatograph (HP 6890) equipped with flame ionization detector (FID) and PEG 20,000 filled column heated to 100 ◦ C. The separation factor of benzene/water mixture was calculated by Eq. (1) α=
Pb /Pw Fb /Fw
(1)
where Fb and Fw are the weight fractions of benzene and water in the feed solution and Pb and Pw are the weight fractions of benzene and water in the permeate, respectively. The normalized permeation rate of benzene (NPRb ) during pervaporation was determined from Eq. (2) NPRb =
lWbp tS
(2)
where l is the thickness of the membrane, Wbp is the weight of benzene in permeate, t is the permeation time and S is the effective membrane area.
2.8. XRD experiment To get the degree of crystallization of the membranes, X-ray diffraction (XRD) experiments were carried out using Rigaku D/max2500 diffractometer in the scan range of 3–35◦ (Cu K␣, 40 kV/100 mA).
3. Results and discussions 3.1. FTIR studies Figs. 3 and 4 show the FTIR spectra of CA and CAD. A characteristic strong and broad band exhibited at around 3200 cm−1 in Fig. 3, corresponds to O–H stretching vibration of the phenolic hydroxyl groups. From these two figures, it can be seen that propoxy group has replaced the phenolic hydroxyl group. The band appeared at around 2800–3000 cm−1 assigned to C–H stretching vibration. The intensity of the peak increased due to the more –CH3 and –CH2 groups of CAD. The peaks between 1100 and 1000 cm−1 are corresponding to C–O stretching vibrations of the derivative.
2.4. Infrared spectra measurements Infrared spectra of the CA and CAD were measured with an FT-IR Spectroscope (Nicolet-560, USA). Spectra were recorded in the range of 400–4000 cm−1 . 2.5. PALS experiment The positron annihilation lifetime spectroscopy (PALS) experiment (ORTEC, Advanced Measurement Technology, USA) was carried out using a fast–fast system with a resolution of 280 ps (under 22 Na window setting). 2.6. Density The density of membrane (ρm ) was obtained by weighing the samples of known area and thickness using electronic analytical balance (OHAUS Co., USA).
Fig. 3. IR spectrum of CA.
2.7. Swelling experiment After drying under a reduced pressure at room temperature, the membranes were weighed and immersed into feed solution with benzene concentration around 0.14 wt% and pure water in sealed conical flasks, respectively. When the equilibrium reached, the membranes were taken out of the flasks, wiped quickly with filter paper and weighed. The degree of swelling (Ds ) of the membranes is calculated from Eq. (3) Ds =
Ws − W d Wd
(3)
where Ws is the weight of the swollen membrane and Wd is the weight of the dried membrane.
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Fig. 4. IR spectrum of CAD.
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3.2. Effect of CA and CAD contents on benzene concentration in membranes Since the concentration of water in the feed solution is 99.86%, the water concentration in the membranes swelled in feed solution can be replaced by the water concentration in the membranes swelled in pure water. According to swelling experimental results, the benzene concentration in the membrane swelled in feed solution (Cb ) can be calculated using Eq. (4). And the results are shown in Fig. 5. Cb = Fig. 5. Effect of CA and CAD content on Cb .
Dsf − Dsw 1/ρm + (Dsf − Dsw )/ρb
(4)
where Dsf is Ds of the membranes swelled in feed solution, Dsw is Ds of the membranes swelled in pure water and ρb is the density of benzene. It can be seen that the swelling degree of
Fig. 6. XRD pattern of PDMS membranes with different CA and CAD content.
L. Liu et al. / Journal of Membrane Science 279 (2006) 111–119
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Table 1 p Crystal area, minicrystal area, total area, Dc and Dcf of the filled membranes with different CA and CAD contents p
Filler type
Filler content (wt%)
Crystal area
Minicrystal area
Total area
Dc (%)
Dcf (%)
– CA CA CA CA CA CA CAD CAD CAD CAD
0 1 2 3 5 7 10 1 2 3 5
0 16007 3352 3238 4875 6175 8895 2230 1926 1569 1875
12111 13859 6488 13083 13399 8262 14994 7868 7828 8205 8274
39203 50294 30122 34678 42403 32523 45519 23604 23322 23645 23395
30.89 27.56 21.54 37.73 31.60 25.40 32.94 33.33 33.56 34.70 35.37
0 31.83 11.13 9.34 11.50 18.99 19.54 9.45 8.26 6.64 8.01
membranes is not significant although slightly fluctuates with the filler content. 3.3. Effect of CA and CAD contents on crystallization degree of the membranes The degree of crystallization, Dc , can be estimated by the ratio of crystalline area to total area in the XRD patterns [13]. Fig. 6 presents the XRD patterns of the control membrane and filled membranes with different CA and CAD contents. The XRD pattern of the control membrane has only a broad peak between 10◦ and 15◦ , corresponding to the diffraction of the minicrystal of PDMS. The XRD patterns of the filled membranes with different CA and CAD contents show crystalline peaks with different intensity. The CA and CAD crystal areas, minicrystal p area, total area and the calculated Dc (degree of crystallization f for PDMS minicrystal) and Dc (degree of crystallization for crystalline fillers, CA and CAD) of each membrane are listed in Table 1. The existence of crystal in the membrane changes the chain packing of PDMS and thus the free volume of the membrane. The supporting data can be found in the next section. 3.4. Effect of CA and CAD contents on fractional free volume (fv ) of the filled membranes PALS is a direct method to calculate free volume in polymers [14,15]. A four-component spectrum will provide a rather good description on many situations [16]. Therefore, we choose fourcomponent spectrum to estimate free volume of PDMS and CAfilled PDMS membranes.
The free volume is assumed as a spherical potential well surrounded by an electron layer of thickness r, and the following expression was employed to relate ortho-positronium (o-Ps) pick-off lifetime, τ 3 and τ 4 , and radius of free volume holes, r [17,18] −1 1 r 1 2πr τ= (5) 1− + sin 2 r + r 2π r + r where r is the electron layer thickness with an estimated value 0.1656 nm. In addition, the fractional free volume fv can be calculated from empirical relationship [17,18] fv = CVF I
(6)
This equation can be also expressed as VF I =
fv C
and
VF =
4π 3 r 3
(7)
where VF and I are free volume of the sphere and intensity of o-Ps, respectively, and C is a empirical constant. Thus, fv of the membranes can be represented using the values of VF3 I3 + VF4 I4 . The results of PALS experiment and the calculated values of VF3 I3 + VF4 I4 are listed in Table 2. 3.5. Effect of CA and CAD contents on NPRb of the filled membranes The effects of CA and CAD contents on NPRb are shown in Fig. 7 by the solid lines. For CA-PDMS membranes, there exist a valley value of NPRb at 1 wt% CA content and a peak
Table 2 Free volume parameters of filled membranes with different CA content CA content (wt%)
τ 3 (ns)
I3 (%)
τ 4 (ns)
I4 (%)
r3 (nm)
r4 (nm)
VF3 I3 + VF4 I4
0 1 2 3 5 7 10
2.8669 2.0498 1.5776 2.2305 2.1551 1.7713 2.099
15.7348 7.7547 5.1749 9.6835 7.279 7.0631 10.7566
14.2949 3.8543 3.471 3.8321 3.6577 3.5864 3.9925
1.5549 8.6763 14.2157 13.5058 12.2631 11.6889 10.6685
0.3537 0.2894 0.2425 0.3051 0.2986 0.2629 0.2937
0.7636 0.4154 0.393 0.4141 0.4041 0.3999 0.4231
5.8164 3.3924 3.9235 5.1692 4.2014 3.6688 4.5262
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3.6. A proposed model for description of NPRb of PDMS membranes filled with CA It is interesting to notice that the membrane with 1 wt% CA content shows maximum Dcf than the other filled membranes with higher CA content, whereas the membrane with 3 wt% CA content shows the minimum Dcf . Based on the experimental results of swelling experiment and XRD measurement, a tentative model [19] is proposed to fit the NPRb of the PDMS membranes filled with crystalline CA fp p γDc fp p Cb NPRb = NPRb p (8) p Cb γDc + Dcf Fig. 7. Effect of the CA and CAD content on NPRb .
value of NPRb at 3 wt% CA content. For CAD-PDMS membranes, NPRb varies in a narrow range between 14.5 × 10−6 and 17.6 × 10−6 kg m/(m2 h). The relation between fv and NPRb of the membranes is shown in Fig. 8. As can be seen, the values of VF3 I3 + VF4 I4 showed the same changing trend as NPRb did, coincidently. This phenomenon is tentatively explained as follows: for noncrystalline, soluble and benzene preferential-adsorption filler, the adsorption or solution amount of benzene in the membranes will increase with the increase of filler amount and the swelling degree of the filler membranes will increase accordingly. However, the crystalline characteristics of fillers will be of some resistance to membrane swelling and meanwhile occupy part of the permeation space for benzene crossing through. The limited solubility of filler in the toluene makes the situation more complicated. The synergism of these factors causes the membranes to exhibit usual permeation behavior which deserves carrying out some theoretical analysis and calculation.
where the superscripts fp, p and f represent the filled polymer, the control polymer and the filler, respectively. The detailed description about this model was presented in Appendix A. Due to the existence of amorphous part in the PDMS, the diffraction peak becomes broader and the crystalline area of PDMS in the XRD patterns is often overestimated. In order to eliminate this effect, the correction factor γ is introduced and the fitting value of γ is 0.3308 according to the experiment data and Eq. (8). The fitting results are shown in Fig. 7 by the broken line, and the correlation coefficient R2 is 0.9473. 3.7. Validation of the proposed model using CAD-filled PDMS membranes Given γ is 0.3308, the NPRb of PDMS membranes with different CAD content can be calculated using the above proposed model, based on the experiment data of swelling experiment and XRD measurement. As shown in Fig. 8, the values of NPRb obtaining from Eq. (8) (the broken line) are close to the values of NPRb obtaining from pervaporation experiment (the solid line) and the relative errors are all within 17%. The good agreement between the calculated results and experimental results validates that Eq. (8) is appropriate for describing NPRb of the filled PDMS membranes containing crystalline filler. For other polymer membranes, the correction factor γ may have different values. 3.8. Effect of CA and CAD contents on the separation factor of the filled membranes
Fig. 8. Relationship between NPRb and VF3 I3 + VF4 I4 for membranes with different CA content.
The effects of CA and CAD contents on the separation factor of the membranes are shown in Fig. 9. For the separation factor of there exist a valley value at 1 wt% CA content and a peak value at 3 wt% CA content. This phenomenon may be ascribed to the following two different factors: the preferential affinity of CA towards benzene causes the approximate monotonic change of Cb with CA content; on the other hand, Dc exhibits the non-monotonic change with CA content. For CAD-f-PDMS membranes, the separation factor increases monotonously as the CAD content increases and is much higher than that of CA-f-PDMS membranes. This can be explained tentatively as follows: due to the relatively weak polarity of the propoxy group in CAD compared to the hydroxyl group
L. Liu et al. / Journal of Membrane Science 279 (2006) 111–119
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Table 3 Comparison of the pervaporation performance of different membranes Membrane material
Temperature (◦ C)
Benzene concentration (wt%)
Benzene flux (g/(m2 h))
Separation factor
Resource
PVDF PDMSDVS PDMSDMMA PDMSDVB PDMSEGDM PFA-g-PDMS PMMA-g-PDMS PEMA-g-PDMS CA-filled PDMS-g-PMMA CA-filled PDMS-b-PMMA Silicone rubber composite membrane PDMS membrane CA-f-PDMS membrane CAD-f-PDMS membrane
25 40 40 40 40 40 40 40 40 40 40 60 60 60
0.08 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.13 0.14 0.14 0.14
43 70.9 51.4 45.5 49.6 21.6 12 9.3 23 43 75 129 116 63
250 2886 1853 3099 2011 4600 3700 2400 1800 2300 629 3275 5604 8029
[20] [10] [10] [10] [10] [21] [21] [11] [22] [12] [23] This study This study This study
NPRb and fractional free volume of the filled membrane indicated that the addition of crystalline filler influenced the chain packing of PDMS and thus the diffusion characteristics of the filled membrane. Due to the higher hydrophobicity of CAD over CA, CAD-filled PDMS membranes exhibited higher separation factor than that of CA-filled PDMS membranes. Furthermore, both CA and CAD-filled PDMS membranes exhibited higher separation factor than that of control PDMS membranes. Acknowledgement Fig. 9. Effect of CA and CAD content on the separation factor.
in CA, CAD molecules show higher hydrophobicity than that of CA molecules. According to solubility parameter theory, CAD-f-PDMS membranes with the closer solubility parameter to benzene should exhibit higher selectivity than CA-f-PDMS membranes. The comparison of pervaporation performance of our work and literature work is given in Table 3. As can be seen, the membranes in this study show better performance than those in other studies due to the moderate interaction between calixarene molecule and benzene as well as the appropriate interaction between calixarene and PDMS.
We extremely appreciate the financial support from the CrossCentury Talent Raising Program of Ministry of Education of China, the Sinopec Research Program (No. X500003), the key project of Ministry of Education of China. Appendix A According to Fick’s law, the permeation rate (PRi ) (kg/(m2 h)) of component i in a polymer membrane can be defined as PRi = −Di
dCi dl
(A.1)
or Cif − Cip l
4. Conclusions
PRi = Di
PDMS membranes filled with crystalline supramolecule CA and CAD were prepared for the pervaporative removal of benzene from dilute aqueous solution. Due to the synergism of swelling and crystallization, NPRb of the membranes existed a maximum value and a minimum value with the increase of CA content. Combining the experimental results of swelling experiment and XRD measurement, a model is proposed to describe NPRb of the CA-filled membranes and validated by CAD-filled membranes. The same changing trends between
where Di is the diffusion coefficient which depends strongly on the properties of components and the microstructure of the membrane; l is the thickness of the membrane; Cif , Cip are the component concentrations in the feed side and permeate side of the membrane. Because of the extremely low pressure at the downstream, Cip is usually considered to be zero. To eliminate the effect of the membrane thickness, the normalized permeation rate NPR, the product of l and PR, is adopted. Then, Eq. (A.2) may be
(A.2)
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L. Liu et al. / Journal of Membrane Science 279 (2006) 111–119
expressed in a simplified form. NPRi = Ci Di
(A.3)
where Ci is the component concentration in the feed side of the membrane. To describe the permeability of the component in heterogeneous polymer systems which is represented by NPR in this paper, several models have been derived. Among them Maxwell model, which was proposed for analyzing the specific resistance of a continuum filled with dilute suspension of spheres, was frequently adopted. −1 fp NPRi β+2 NPRfi = 1 + 3φ β= (A.4) − φ f f p p β−1 NPRi NPRi where the superscripts fp, p and f represent the filled polymer, the polymer and the filler, respectively, and φf is the volume fraction of the filler. When the filler is impermeable, NPRfi = 0 and Eq. (A.4) can be simplified: 1 − φf fp p (A.5) NPRi = NPRi 1 + φ2f Eq. (A.5) has been used successfully to describe the effect of filler content on permeability in a variety of filled polymers fp [24,25]. When Eqs. (A.3) and (A.5) are combined, NPRi can be expressed by Eq. (A.6): 1 − φf fp fp fp p p (A.6) NPRi = Ci Di = Ci Di 1 + φ2f fp
p
It is often assumed that in Eq. (A.6) Ci is equal to Ci . In fact, when the filler has a considerable adsorption capacity towards fp p the permeating components, Ci is usually higher than Ci and Eq. (A.6) needs to be modified as Eq. (A.7): p 1 − φf 1 − φf fp fp p fp Ci p = C i p Di NPRi = Ci Di Ci 1 + φ2f 1 + φ2f fp 1 − φf Ci p (A.7) = p × NPRi Ci 1 + φ2f Furthermore, when the filler is of crystallinity, the diffusion behavior of the permeating components in the membrane will alter more or less. In order to reflect the effect of crystallization, the parameter Dc , which denotes the degree of crystallization, is introduced and the factor 1−φφff in Eq. (A.7) is substituted 1+ 2 p p γDc by , where Dc is the Dc value of the polymer deterp γDc +Dcf mined by the ratio of the minicrystal area to the total area in the XRD pattern and Dcf is the Dc value of the filler in the polymer determined by the ratio of the filler’s crystal area to the total area in the XRD patterns, as shown in Fig. 6. A correction factor γ is introduced to eliminate the disturbance from the amorphous part of polymer. Considering the above-mentioned factors, a model is tentatively proposed in order to fit the permeability of the component
in the polymer filled with crystal adsorbent more accurately (Eq. (A.8)): fp NPRi
=
fp pC NPRi ip Ci
p
γDc p γDc + Dcf
.
(A.8)
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The unusual change of permeation rate in PDMS membranes filled with crystalline calixarene and its derivative Liang Liu a,b , Zhongyi Jiang a,b,∗ , Fusheng Pan a , Fubing Peng a , Hong Wu a a
b
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China State Key Laboratory of Functional Polymer Materials for Adsorption and Separation, Nankai University, Tianjin 300071, China Received 5 April 2005; received in revised form 14 November 2005; accepted 22 November 2005 Available online 6 January 2006
Abstract PDMS membranes filled with crystalline calixarene (CA) and calixarene derivative (CAD) were prepared for pervaporative removal of benzene from dilute aqueous solution. With the increase of CA and CAD content, the normalized permeation rate of the filled membranes did not exhibit monotonic change, but had minimum and maximum values instead. In order to explain this unusual phenomenon, swelling experiment, XRD and PALS measurement had been carried out. A model, which describes the mass transport in the CA-filled membranes by combining swelling and crystallinity of the membranes, was proposed and validated by CAD-filled membranes. Due to the higher hydrophobicity of CAD over CA, CAD-filled PDMS membranes exhibited higher separation factor than CA-filled PDMS membranes. In addition, both CA and CAD-filled PDMS membranes showed better permselectivity than control PDMS membranes. © 2005 Elsevier B.V. All rights reserved. Keywords: Pervaporation; Calixarene; Filled membrane; Crystallinity; Swelling; Normalized permeation flux
1. Introduction For the removal of volatile organic compounds (VOCs) from their dilute aqueous solutions, pervaporation has been proved to be an attractive and potentially cost-competitive approach. Among common organophilic polymer membrane materials, poly(dimethylsiloxane) (PDMS) has the superior performance attributed to its stronger affinity towards VOCs over water and very flexible chain structure [1–5]. However, there is still much room for the optimization and improvement of separation properties of existing PDMS-based membrane. As a simple and effective way, adding hydrophobic and organophilic filler such as carbon molecular sieves [6], silicalite [7–9], into PDMS matrix to increase the permselectivity has attracted considerable research interest. However, due to the rigid structure and unfavorable desorption characteristics, these fillers did not serve the purpose very well in most cases. Recently, the supermolecule calixarenes and their derivatives have received much attention for their role in bio-
∗
Corresponding author. Tel.: +86 22 2789 2143; fax: +86 22 2789 2143. E-mail address: [email protected] (Z. Jiang).
0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.11.041
logical, synthetic receptor models and separating agents. The flexible calixarene framework requires small energy change to accommodate guest molecules, which makes them exceptionally resourceful host molecules. It is well known that the -rich cavity in calixarene is suitable for the inclusion of neutral guest of complementary size including benzene, owing to supramolecular interaction. It can be thus supposed that the moderate supramolecular interaction, which lies between the physical adsorption and chemical adsorption, will potentially improve the pervaporation performance of PDMS membrane [10,11]. Uragami et al. [12] had investigated the effect of the addition of tert-butylcalix[4]arene (CA) to PMMA-b-PDMS membrane for the removal of benzene from dilute aqueous solution. The experimental results indicated that the addition of CA to PMMA-b-PDMS membrane increased separation factor from 1800 to 2300 and normalized permeation rate from 7 to 13 × 10−6 kg m/(m2 h). However, the inherent glassy characteristics of PMMA and the existence of microphase separation in PMMA-b-PDMS membrane seem not necessarily helpful for the permselectivity and permeability of membrane. In this paper, PDMS membranes filled with crystalline CA and its more hydrophobic derivative (CAD) are prepared for the
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(propoxy)calix[4]arene (CAD) were purchased from Nankai University, the molecular structures are shown in Fig. 1. All the other solvents and reagents obtained from commercial sources were of analytical grade and used as received. 2.2. Membrane preparation
Fig. 1. Molecular structure of CA and CAD.
pervaporative removal of benzene from dilute aqueous solution. A tentative model, which describes the mass transport in the CA-filled membranes considering swelling and crystallinity of the membranes, is presented and validated by CAD-filled membranes.
Both the CA-filled PDMS membranes (CA-PDMS membranes) and the CAD-filled PDMS membranes (CAD-PDMS membranes) were prepared by solution-casting mixtures of the fillers and PDMS/toluene solutions. The mixture was vigorously stirred to obtain a pseudo-homogenous solution and then the solution was cast onto the surface of organic glass plate. PDMS was crosslinked by tetraethylorthosilicate (TEOS) in the toluene solution at room temperature under the catalytic action of dibutyltin dilaurate. The weight ratio of PDMS:toluene:TEOS:dibutyltin dilaurate is 1:1:0.1:0.02. The solid membranes were formed after toluene evaporated completely. The membranes were further dried in vacuum for 24 h and swelled in the feed solutions for more than 24 h before use. 2.3. Pervaporation experiments
2. Experimental 2.1. Materials PDMS prepolymer (the viscosity was 5000 mPa s, the average molecular weight was about 40,000) was purchased from Beijing Huaer Co. Ltd. Both tert-butylcalix[4]arene (CA) and 5,11,17,23-tetrakis(tert-butyl)-25,26,27,28-tetrakis-
A schematic diagram of the pervaporation test unit is shown in Fig. 2. In the pervaporation experiment the air-side surface of all the membranes faced the feed side of the pervaporation cell and the effective membrane area was 25.6 × 10−4 m2 . For direct comparison of the performance of each membrane, the parameters during the experiments were controlled as follows: the feed concentration of benzene was about 0.14 wt%; the feed
Fig. 2. Schematic diagram of pervaporation experimental equipment. (1) Heater, (2) thermocouple, (3) feed container, (4) feed circulator, (5) flowmeter, (6) membrane cell, (7) cold trap, (8) liquid nitrogen, (9) desiccator, (10) buffer and (11) vacuum pump.
L. Liu et al. / Journal of Membrane Science 279 (2006) 111–119
temperature was kept at 60 ◦ C; the feed flow rate was 120 L/h; the vacuum in the downstream side was about 1.3 kPa. The concentrations of the feed and the permeate were analyzed by gas chromatograph (HP 6890) equipped with flame ionization detector (FID) and PEG 20,000 filled column heated to 100 ◦ C. The separation factor of benzene/water mixture was calculated by Eq. (1) α=
Pb /Pw Fb /Fw
(1)
where Fb and Fw are the weight fractions of benzene and water in the feed solution and Pb and Pw are the weight fractions of benzene and water in the permeate, respectively. The normalized permeation rate of benzene (NPRb ) during pervaporation was determined from Eq. (2) NPRb =
lWbp tS
(2)
where l is the thickness of the membrane, Wbp is the weight of benzene in permeate, t is the permeation time and S is the effective membrane area.
2.8. XRD experiment To get the degree of crystallization of the membranes, X-ray diffraction (XRD) experiments were carried out using Rigaku D/max2500 diffractometer in the scan range of 3–35◦ (Cu K␣, 40 kV/100 mA).
3. Results and discussions 3.1. FTIR studies Figs. 3 and 4 show the FTIR spectra of CA and CAD. A characteristic strong and broad band exhibited at around 3200 cm−1 in Fig. 3, corresponds to O–H stretching vibration of the phenolic hydroxyl groups. From these two figures, it can be seen that propoxy group has replaced the phenolic hydroxyl group. The band appeared at around 2800–3000 cm−1 assigned to C–H stretching vibration. The intensity of the peak increased due to the more –CH3 and –CH2 groups of CAD. The peaks between 1100 and 1000 cm−1 are corresponding to C–O stretching vibrations of the derivative.
2.4. Infrared spectra measurements Infrared spectra of the CA and CAD were measured with an FT-IR Spectroscope (Nicolet-560, USA). Spectra were recorded in the range of 400–4000 cm−1 . 2.5. PALS experiment The positron annihilation lifetime spectroscopy (PALS) experiment (ORTEC, Advanced Measurement Technology, USA) was carried out using a fast–fast system with a resolution of 280 ps (under 22 Na window setting). 2.6. Density The density of membrane (ρm ) was obtained by weighing the samples of known area and thickness using electronic analytical balance (OHAUS Co., USA).
Fig. 3. IR spectrum of CA.
2.7. Swelling experiment After drying under a reduced pressure at room temperature, the membranes were weighed and immersed into feed solution with benzene concentration around 0.14 wt% and pure water in sealed conical flasks, respectively. When the equilibrium reached, the membranes were taken out of the flasks, wiped quickly with filter paper and weighed. The degree of swelling (Ds ) of the membranes is calculated from Eq. (3) Ds =
Ws − W d Wd
(3)
where Ws is the weight of the swollen membrane and Wd is the weight of the dried membrane.
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Fig. 4. IR spectrum of CAD.
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3.2. Effect of CA and CAD contents on benzene concentration in membranes Since the concentration of water in the feed solution is 99.86%, the water concentration in the membranes swelled in feed solution can be replaced by the water concentration in the membranes swelled in pure water. According to swelling experimental results, the benzene concentration in the membrane swelled in feed solution (Cb ) can be calculated using Eq. (4). And the results are shown in Fig. 5. Cb = Fig. 5. Effect of CA and CAD content on Cb .
Dsf − Dsw 1/ρm + (Dsf − Dsw )/ρb
(4)
where Dsf is Ds of the membranes swelled in feed solution, Dsw is Ds of the membranes swelled in pure water and ρb is the density of benzene. It can be seen that the swelling degree of
Fig. 6. XRD pattern of PDMS membranes with different CA and CAD content.
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Table 1 p Crystal area, minicrystal area, total area, Dc and Dcf of the filled membranes with different CA and CAD contents p
Filler type
Filler content (wt%)
Crystal area
Minicrystal area
Total area
Dc (%)
Dcf (%)
– CA CA CA CA CA CA CAD CAD CAD CAD
0 1 2 3 5 7 10 1 2 3 5
0 16007 3352 3238 4875 6175 8895 2230 1926 1569 1875
12111 13859 6488 13083 13399 8262 14994 7868 7828 8205 8274
39203 50294 30122 34678 42403 32523 45519 23604 23322 23645 23395
30.89 27.56 21.54 37.73 31.60 25.40 32.94 33.33 33.56 34.70 35.37
0 31.83 11.13 9.34 11.50 18.99 19.54 9.45 8.26 6.64 8.01
membranes is not significant although slightly fluctuates with the filler content. 3.3. Effect of CA and CAD contents on crystallization degree of the membranes The degree of crystallization, Dc , can be estimated by the ratio of crystalline area to total area in the XRD patterns [13]. Fig. 6 presents the XRD patterns of the control membrane and filled membranes with different CA and CAD contents. The XRD pattern of the control membrane has only a broad peak between 10◦ and 15◦ , corresponding to the diffraction of the minicrystal of PDMS. The XRD patterns of the filled membranes with different CA and CAD contents show crystalline peaks with different intensity. The CA and CAD crystal areas, minicrystal p area, total area and the calculated Dc (degree of crystallization f for PDMS minicrystal) and Dc (degree of crystallization for crystalline fillers, CA and CAD) of each membrane are listed in Table 1. The existence of crystal in the membrane changes the chain packing of PDMS and thus the free volume of the membrane. The supporting data can be found in the next section. 3.4. Effect of CA and CAD contents on fractional free volume (fv ) of the filled membranes PALS is a direct method to calculate free volume in polymers [14,15]. A four-component spectrum will provide a rather good description on many situations [16]. Therefore, we choose fourcomponent spectrum to estimate free volume of PDMS and CAfilled PDMS membranes.
The free volume is assumed as a spherical potential well surrounded by an electron layer of thickness r, and the following expression was employed to relate ortho-positronium (o-Ps) pick-off lifetime, τ 3 and τ 4 , and radius of free volume holes, r [17,18] −1 1 r 1 2πr τ= (5) 1− + sin 2 r + r 2π r + r where r is the electron layer thickness with an estimated value 0.1656 nm. In addition, the fractional free volume fv can be calculated from empirical relationship [17,18] fv = CVF I
(6)
This equation can be also expressed as VF I =
fv C
and
VF =
4π 3 r 3
(7)
where VF and I are free volume of the sphere and intensity of o-Ps, respectively, and C is a empirical constant. Thus, fv of the membranes can be represented using the values of VF3 I3 + VF4 I4 . The results of PALS experiment and the calculated values of VF3 I3 + VF4 I4 are listed in Table 2. 3.5. Effect of CA and CAD contents on NPRb of the filled membranes The effects of CA and CAD contents on NPRb are shown in Fig. 7 by the solid lines. For CA-PDMS membranes, there exist a valley value of NPRb at 1 wt% CA content and a peak
Table 2 Free volume parameters of filled membranes with different CA content CA content (wt%)
τ 3 (ns)
I3 (%)
τ 4 (ns)
I4 (%)
r3 (nm)
r4 (nm)
VF3 I3 + VF4 I4
0 1 2 3 5 7 10
2.8669 2.0498 1.5776 2.2305 2.1551 1.7713 2.099
15.7348 7.7547 5.1749 9.6835 7.279 7.0631 10.7566
14.2949 3.8543 3.471 3.8321 3.6577 3.5864 3.9925
1.5549 8.6763 14.2157 13.5058 12.2631 11.6889 10.6685
0.3537 0.2894 0.2425 0.3051 0.2986 0.2629 0.2937
0.7636 0.4154 0.393 0.4141 0.4041 0.3999 0.4231
5.8164 3.3924 3.9235 5.1692 4.2014 3.6688 4.5262
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3.6. A proposed model for description of NPRb of PDMS membranes filled with CA It is interesting to notice that the membrane with 1 wt% CA content shows maximum Dcf than the other filled membranes with higher CA content, whereas the membrane with 3 wt% CA content shows the minimum Dcf . Based on the experimental results of swelling experiment and XRD measurement, a tentative model [19] is proposed to fit the NPRb of the PDMS membranes filled with crystalline CA fp p γDc fp p Cb NPRb = NPRb p (8) p Cb γDc + Dcf Fig. 7. Effect of the CA and CAD content on NPRb .
value of NPRb at 3 wt% CA content. For CAD-PDMS membranes, NPRb varies in a narrow range between 14.5 × 10−6 and 17.6 × 10−6 kg m/(m2 h). The relation between fv and NPRb of the membranes is shown in Fig. 8. As can be seen, the values of VF3 I3 + VF4 I4 showed the same changing trend as NPRb did, coincidently. This phenomenon is tentatively explained as follows: for noncrystalline, soluble and benzene preferential-adsorption filler, the adsorption or solution amount of benzene in the membranes will increase with the increase of filler amount and the swelling degree of the filler membranes will increase accordingly. However, the crystalline characteristics of fillers will be of some resistance to membrane swelling and meanwhile occupy part of the permeation space for benzene crossing through. The limited solubility of filler in the toluene makes the situation more complicated. The synergism of these factors causes the membranes to exhibit usual permeation behavior which deserves carrying out some theoretical analysis and calculation.
where the superscripts fp, p and f represent the filled polymer, the control polymer and the filler, respectively. The detailed description about this model was presented in Appendix A. Due to the existence of amorphous part in the PDMS, the diffraction peak becomes broader and the crystalline area of PDMS in the XRD patterns is often overestimated. In order to eliminate this effect, the correction factor γ is introduced and the fitting value of γ is 0.3308 according to the experiment data and Eq. (8). The fitting results are shown in Fig. 7 by the broken line, and the correlation coefficient R2 is 0.9473. 3.7. Validation of the proposed model using CAD-filled PDMS membranes Given γ is 0.3308, the NPRb of PDMS membranes with different CAD content can be calculated using the above proposed model, based on the experiment data of swelling experiment and XRD measurement. As shown in Fig. 8, the values of NPRb obtaining from Eq. (8) (the broken line) are close to the values of NPRb obtaining from pervaporation experiment (the solid line) and the relative errors are all within 17%. The good agreement between the calculated results and experimental results validates that Eq. (8) is appropriate for describing NPRb of the filled PDMS membranes containing crystalline filler. For other polymer membranes, the correction factor γ may have different values. 3.8. Effect of CA and CAD contents on the separation factor of the filled membranes
Fig. 8. Relationship between NPRb and VF3 I3 + VF4 I4 for membranes with different CA content.
The effects of CA and CAD contents on the separation factor of the membranes are shown in Fig. 9. For the separation factor of there exist a valley value at 1 wt% CA content and a peak value at 3 wt% CA content. This phenomenon may be ascribed to the following two different factors: the preferential affinity of CA towards benzene causes the approximate monotonic change of Cb with CA content; on the other hand, Dc exhibits the non-monotonic change with CA content. For CAD-f-PDMS membranes, the separation factor increases monotonously as the CAD content increases and is much higher than that of CA-f-PDMS membranes. This can be explained tentatively as follows: due to the relatively weak polarity of the propoxy group in CAD compared to the hydroxyl group
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Table 3 Comparison of the pervaporation performance of different membranes Membrane material
Temperature (◦ C)
Benzene concentration (wt%)
Benzene flux (g/(m2 h))
Separation factor
Resource
PVDF PDMSDVS PDMSDMMA PDMSDVB PDMSEGDM PFA-g-PDMS PMMA-g-PDMS PEMA-g-PDMS CA-filled PDMS-g-PMMA CA-filled PDMS-b-PMMA Silicone rubber composite membrane PDMS membrane CA-f-PDMS membrane CAD-f-PDMS membrane
25 40 40 40 40 40 40 40 40 40 40 60 60 60
0.08 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.13 0.14 0.14 0.14
43 70.9 51.4 45.5 49.6 21.6 12 9.3 23 43 75 129 116 63
250 2886 1853 3099 2011 4600 3700 2400 1800 2300 629 3275 5604 8029
[20] [10] [10] [10] [10] [21] [21] [11] [22] [12] [23] This study This study This study
NPRb and fractional free volume of the filled membrane indicated that the addition of crystalline filler influenced the chain packing of PDMS and thus the diffusion characteristics of the filled membrane. Due to the higher hydrophobicity of CAD over CA, CAD-filled PDMS membranes exhibited higher separation factor than that of CA-filled PDMS membranes. Furthermore, both CA and CAD-filled PDMS membranes exhibited higher separation factor than that of control PDMS membranes. Acknowledgement Fig. 9. Effect of CA and CAD content on the separation factor.
in CA, CAD molecules show higher hydrophobicity than that of CA molecules. According to solubility parameter theory, CAD-f-PDMS membranes with the closer solubility parameter to benzene should exhibit higher selectivity than CA-f-PDMS membranes. The comparison of pervaporation performance of our work and literature work is given in Table 3. As can be seen, the membranes in this study show better performance than those in other studies due to the moderate interaction between calixarene molecule and benzene as well as the appropriate interaction between calixarene and PDMS.
We extremely appreciate the financial support from the CrossCentury Talent Raising Program of Ministry of Education of China, the Sinopec Research Program (No. X500003), the key project of Ministry of Education of China. Appendix A According to Fick’s law, the permeation rate (PRi ) (kg/(m2 h)) of component i in a polymer membrane can be defined as PRi = −Di
dCi dl
(A.1)
or Cif − Cip l
4. Conclusions
PRi = Di
PDMS membranes filled with crystalline supramolecule CA and CAD were prepared for the pervaporative removal of benzene from dilute aqueous solution. Due to the synergism of swelling and crystallization, NPRb of the membranes existed a maximum value and a minimum value with the increase of CA content. Combining the experimental results of swelling experiment and XRD measurement, a model is proposed to describe NPRb of the CA-filled membranes and validated by CAD-filled membranes. The same changing trends between
where Di is the diffusion coefficient which depends strongly on the properties of components and the microstructure of the membrane; l is the thickness of the membrane; Cif , Cip are the component concentrations in the feed side and permeate side of the membrane. Because of the extremely low pressure at the downstream, Cip is usually considered to be zero. To eliminate the effect of the membrane thickness, the normalized permeation rate NPR, the product of l and PR, is adopted. Then, Eq. (A.2) may be
(A.2)
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expressed in a simplified form. NPRi = Ci Di
(A.3)
where Ci is the component concentration in the feed side of the membrane. To describe the permeability of the component in heterogeneous polymer systems which is represented by NPR in this paper, several models have been derived. Among them Maxwell model, which was proposed for analyzing the specific resistance of a continuum filled with dilute suspension of spheres, was frequently adopted. −1 fp NPRi β+2 NPRfi = 1 + 3φ β= (A.4) − φ f f p p β−1 NPRi NPRi where the superscripts fp, p and f represent the filled polymer, the polymer and the filler, respectively, and φf is the volume fraction of the filler. When the filler is impermeable, NPRfi = 0 and Eq. (A.4) can be simplified: 1 − φf fp p (A.5) NPRi = NPRi 1 + φ2f Eq. (A.5) has been used successfully to describe the effect of filler content on permeability in a variety of filled polymers fp [24,25]. When Eqs. (A.3) and (A.5) are combined, NPRi can be expressed by Eq. (A.6): 1 − φf fp fp fp p p (A.6) NPRi = Ci Di = Ci Di 1 + φ2f fp
p
It is often assumed that in Eq. (A.6) Ci is equal to Ci . In fact, when the filler has a considerable adsorption capacity towards fp p the permeating components, Ci is usually higher than Ci and Eq. (A.6) needs to be modified as Eq. (A.7): p 1 − φf 1 − φf fp fp p fp Ci p = C i p Di NPRi = Ci Di Ci 1 + φ2f 1 + φ2f fp 1 − φf Ci p (A.7) = p × NPRi Ci 1 + φ2f Furthermore, when the filler is of crystallinity, the diffusion behavior of the permeating components in the membrane will alter more or less. In order to reflect the effect of crystallization, the parameter Dc , which denotes the degree of crystallization, is introduced and the factor 1−φφff in Eq. (A.7) is substituted 1+ 2 p p γDc by , where Dc is the Dc value of the polymer deterp γDc +Dcf mined by the ratio of the minicrystal area to the total area in the XRD pattern and Dcf is the Dc value of the filler in the polymer determined by the ratio of the filler’s crystal area to the total area in the XRD patterns, as shown in Fig. 6. A correction factor γ is introduced to eliminate the disturbance from the amorphous part of polymer. Considering the above-mentioned factors, a model is tentatively proposed in order to fit the permeability of the component
in the polymer filled with crystal adsorbent more accurately (Eq. (A.8)): fp NPRi
=
fp pC NPRi ip Ci
p
γDc p γDc + Dcf
.
(A.8)
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