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non-aromatic hydrocarbon mixtures through plasma-grafted membranes
Journal of Membrane Science 154 (1999) 221±228
Pervaporation of aromatic/non-aromatic hydrocarbon mixtures through plasma-grafted membranes Hongyuan Wang, Kazuhiro Tanaka, Hidetoshi Kita, Ken-ichi Okamoto* Department of Advanced Material Science and Engineering, Faculty of Engineering, Yamaguchi University, Ube, Yamaguchi 755, Japan Received 19 February 1998; received in revised form 28 August 1998; accepted 11 September 1998
Abstract Using glycidyl methacrylate (GMA) as a grafting monomer and a porous high density polyethylene ®lm as a substrate, plasma-graft polymerized membranes were prepared by the different plasma treatment manners, namely, homogeneous bothside (HBS) and one side (OS) treatments. The poly(GMA)-grafted membranes displayed two different types of the feed composition dependence of permeation ¯ux and separation factor for pervaporation (PV) of benzene/cyclohexane (Bz/Cx) mixtures, depending on the plasma treatment manner and the graft yield. The membranes prepared by the HBS treatment and under mild polymerization conditions displayed the highest performance with a permeation ¯ux of 0.30±0.37 kg/m2 h and a separation factor of 19±22 at feed Bz of 60 wt% and 708C. The membranes exhibited high performance with excellent durability for PV of other aromatic/aliphatic hydrocarbon mixtures. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Composite membranes; Organic separations; Pervaporation; Plasma polymerized membranes; Poly(glycidyl methacrylate)
1. Introduction Membrane-based technology is currently regarded as a new frontier of chemical engineering and has been used for the puri®cation, concentration, and fractionation of ¯uid mixtures. Pervaporation (PV) is commonly considered to be a pro®table complement to distillation for the separation of azeotropic and closeboiling mixtures, which requires at present the use of energy-intensive processes [1]. In the membrane research ®eld, most of the polymeric materials have been found to be selective to water permeation, and *Corresponding author. Tel.: +81-0836-35-9961; fax: +81-083635-9965; e-mail: [email protected]
only a few have been found to be selective to the permeation of organic compounds [1]. However, the separation of organic±organic mixtures such as aromatic/non-aromatic hydrocarbon mixtures offers potentially the largest opportunity for energy and cost savings. So many efforts have been made to prepare PV membranes with good performance and good stability for separation of organic±organic mixtures [2±8]. Yamaguchi et al. [2,3] prepared membranes by a plasma-graft ®lling polymerization technique, where a grafted polymer offered the permselectivity and a porous substrate restrained swelling of the grafted polymer. These membranes showed good performance for separation of organic/organic and organic/water mixtures [2,3,9,10].
0376-7388/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0376-7388(98)00298-1
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H. Wang et al. / Journal of Membrane Science 154 (1999) 221±228
In our previous papers [4,5], we have reported on the preparation of composite membranes by plasmainduced graft polymerization of glycidyl methacrylate (GMA) onto porous substrates of high density polyethylene (PE) and on their morphology and PV properties. The grafted membranes with different morphological structures could be prepared by different plasma treatment manners (homogeneous both side (HBS) and one side (OS) treatments) combined with mild or intensive graft polymerization conditions. The membranes prepared by the HBS treatment and mild polymerization conditions were composed of middle, intermediate and surface parts. In the middle part, the graft polymerization hardly occurred and the porous structure of PE remained substantially unchanged. In the intermediate parts, the graft polymer chains grew up on the surface and in the inner amorphous region of micro®brils of PE, resulting in the ®lling of the pores, expansion of the amorphous layers and the loosening of the crystallites stacked in layers on both their sides. In the surface parts, more graft chains were formed and the micro®bril structure was completely split, and as a result, lost the ability to restrain the swelling of the graft chains. The morphology changed gradually from the intermediate parts through the surface sides. Under more intensive polymerization conditions, the intermediate parts became narrower, the morphology changed drastically from the intermediate to the surface parts, and the surface parts looked much more like the homo-polymer. In the case of the OS treatment, the grafted poly(GMA) was hardly formed on the untreated surface, but at the inner part of the untreated side the pores were rather ®lled with the graft polymer. The preliminary PV experiments indicated that the membranes prepared by the HBS treatment and the mild polymerization conditions displayed the much higher PV performance for benzene/cyclohexane mixtures. In this study, the PV properties of these GMAgrafted membranes were investigated for aromatic/ non-aromatic hydrocarbon mixtures in detail, compared with the poly(GMA) membranes fabricated on the PE substrate. 2. Experimental Two kinds of porous PE ®lms supplied by Tonen Chemical Co. Ltd. were used as substrates. They had
the same average pore size of 0.03 mm with the thickness of 16 and 25 mm, and the porosity of 60% and 45%, respectively. The plasma-induced graft polymerization process has been described in detail elsewhere [4,5]. A porous substrate of 88 cm2 in size was inserted in a glass tube (30 cm in length and 4.5 cm in inner diameter) and treated for 120 s (HBS) or 60 s (OS) by Ar plasma (0.02 Pa and 10 w at 13.56 MHz). Then, without exposure to air, the substrate was immersed into a graft monomer solution degassed previously. Graft polymerization was carried out at 30±408C in a water bath, with shaking. After the polymerization, the grafted membranes were immersed in acetone overnight to remove the homopolymer and the unreacted monomer, and dried in an oven at 408C for 20 h. The membranes were weighed before and after the graft polymerization in the dry state, and the grafting yield was determined as weight of grafted polymer per unit area of porous substrate (mg/cm2). Poly(GMA) (PGMA) was prepared by a radical polymerization at 808C under nitrogen, using benzoyl peroxide (0.05 wt% to GMA) and dioxane, and puri®ed by reprecipitation from dioxane with methanol. PGMA membranes with thickness of 30±80 mm were prepared by casting a 5 wt% tetrahydrofuran (THF) solution of PGMA onto glass plates, followed by drying at 408C for 10 h and then in vacuum at 408C for 20 h. PGMA-PE membranes with thickness of 40± 80 mm were prepared by casting a 5 wt% THF solution of PGMA onto the porous PE substrate ®lms (25 mm in thickness), followed by drying at room temperature for 10 h and then in vacuum at 408C for 10 h. The PGMA-PE membranes thus prepared were transparent and could not be peeled off. PV was carried out at 40±788C by an ordinary method [6]. The effective membrane area was 19.6 cm2 and the total volume of the feed liquid was 100 ml. Permeate was collected in a cold trap in liquid nitrogen. Feed and permeate composition analysis was performed on a gas chromatography equipped with 3 m long column packed with DOP 30% Uniport R 60/80. The separation factor, , is de®ned as y 1 ÿ x=x 1 ÿ y;
(1)
where x and y denote the weight fraction (%) of aromatic component in the feed and permeate, respec-
H. Wang et al. / Journal of Membrane Science 154 (1999) 221±228
3. Results and discussion
tively. The GMA-grafted membranes were conditioned with benzene [5]; only the feed side of the membranes was kept in contact with benzene, and the permeate side was kept in a dry state in vacuum at 708C for 24 h. Both permeability and selectivity increased after the conditioning. PGMA membrane pieces of about 20 mg in weight were used in sorption experiments. Vacuum-dried membrane pieces were immersed in benzene (Bz), cyclohexane (Cx) and Bz/Cx mixtures. The closed containers were placed in a bath at 708C for 30 h to achieve the sorption equilibrium. Then the membrane pieces were taken out and carefully wiped with ®lter paper to remove the surface free solvents, and subsequently placed in a sample bottle containing a small amount of N-methyl pyrrolidone (NMP). The bottle was closed at once and then weighed on a micro balance. The composition of Bz and Cx in solventswollen membranes were determined by means of head space gas chromatography. The sorption amount is de®ned as S 100 Ws ÿ Wd =Wd ;
223
3.1. PV properties of the PGMA-grafted membranes for Bz/Cx system The Tg of grafted PGMA for membrane no. 2 in Table 1 was 758C and a little higher than that of a PGMA ®lm (708C). This may be due to higher polymerization degree of the grafted PGMA and/or its reduced segmental mobility in the pores. Under most of the PV experiments, the grafted PGMA was in the glassy state at least in the permeate side. The results of PV of Bz/Cx mixture are listed in Table 1. Fig. 1 shows the feed composition dependence of the permeation ¯ux and the separation factor. For every membrane, the permeation ¯ux of benzene qBz increased signi®cantly with an increase in benzene fraction in feed XBz. On the other hand, there were observed two different types of feed composition dependence of the permeation ¯ux of cyclohexane qCx and . As shown in Fig. 1(a), qCx increased by a factor of about 5 for membrane no. 3 with increasing XBz from 20 to 80 wt%, and decreased signi®cantly (type A). For membrane no. 5, qCx hardly changed with an increase in XBz, and at ®rst increased a little and then turned to decrease (type B). The value was much larger for membrane no. 3 than for no. 5 at XBz of 20 wt%, but it was only a little larger for no. 3 at XBz of 80 wt%. Among the membranes listed in Table 1, nos. 2±4 and 6 belong to type A, whereas nos. 1, 5, 7 and 8 belong to type B. It is noted that the membranes
(2)
where Wd and Ws denote the weight of dry and solventswollen membranes, respectively. Differential scanning calorimetry (DSC) was measured with a Seiko Instruments DSC-5200 at a heating and cooling rate of 10 K/min. Glass transition temperature Tg was determined from the onset point of a DSC signal on the ®rst heating.
Table 1 Characterization and PV separation results of PGMA-grafted membranesa No.
1 2 3 4 5 6 7 8 a
Graft yield (mg/cm2)
0.7 1.7 1.9 2.4 2.2 3.0 2.0 1.7
Polymerization conditionsb
Thickness (mm)c
Conc. (vol%)
Temp. (8C)
Time (min)
3 3 3 3 15 15 3 5
30 30 30 30 30 40 30 40
35 33 35 60 300 170 155 20
17 26 (29) (32) (26) 26 26 37
PV resultsd Q (kg/m2 h)
(dimensionless)
0.46 0.37 0.30 0.35 0.18 0.06 0.23 0.36
13.5 19 22 22 13.3 23 14.3 13.7
Thickness of PE: nos. 1±7: 16 mm; no. 8: 25 mm. Plasma treatment manner: nos. 1±6: HBS; nos. 7 and 8: OS. Shaking speed: nos. 1 and 4: slow, 80 rpm; nos. 2 and 3, 5±8: middle, 120 rpm. c Measured with TEM or (SEM). d At XBz60 wt% and 708C. For nos. 7 and 8, the untreated surface was in contact with the feed solution. b
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Fig. 1. Feed composition dependence of qBz, qCx and to Bz/Cx mixture at 708C for the PGMA-grafted membranes: (a) no. 3 (*/*) and no. 5 (~/~); (b) no. 1 (~); no. 2 (&); no. 6 (5/!); no. 8 (*/*).
prepared by the HBS treatment and with the larger grafting yield displayed a behavior of type A, whereas the membranes either prepared by the HBS treatment and with a smaller grafting yield or prepared by the OS treatment behaved as type B. The behavior of type A is usually observed for polymeric membranes [6,8], and is explained by the increased swelling of the membranes with an increase in XBz. On the other hand, the behavior of type B might be attributed to the presence of some defects in the membranes, which act as pinholes in the unswollen state rather than in the swollen state. Channels of the pores incompletely ®lled with the graft polymer chains are considered as such defects. Fig. 2 shows the temperature dependence of qBz, qCx, and at XBz of 60 wt%. The activation energies of qBz and qCx, EBz and ECx are summarized in Table 2. Both EBz and ECx were much larger for membrane no. 6 than for the other membranes. For membrane nos. 6 and 3, EBz was smaller than ECx, and increased signi®cantly with a decrease in
Table 2 Activation energies of permeation flux E of PGMA-grafted membranes at xBz of 60 wt% Membrane no.
EBz (kJ/mol)
ECx (kJ/mol)
1 3 5 6 8
55 50 53 65 59
57 65 62 90 61
temperature. On the other hand, for membrane nos. 1 and 8, hardly depended on temperature. It is noted that these membranes had also weak dependence of on feed composition. This suggests that no increase in with decreasing temperature might be attributed to the similar reason, namely, the presence of some defects. The in¯uence of defects may be larger at lower temperature. The results mentioned above lead to the following conclusions concerning the relationship between the
H. Wang et al. / Journal of Membrane Science 154 (1999) 221±228
225
Fig. 2. Temperature dependence of qBz, qCx and to Bz/Cx mixture at xBz of 60 wt% for the PGMA-grafted membranes: no. 1 (*/*); no. 3(&/&); no. 5(~/~); no. 6 (5/!).
PV performance and the preparation conditions of the PGMA-grafted membranes. The membranes prepared by the HBS treatment and the mild graft polymerization conditions and having a grafting yield of about 2 mg/cm2, have the best PV performance; that is, total permeation ¯ux Q of 0.30±0.37 kg/m2 h and of 19± 22 for the membrane nos. 2±4 at XBz of 60 wt% and 708C. The membranes prepared by the HBS treatment and the intensive polymerization conditions have a lower permeation ¯ux due to the much less permeable intermediate parts. The membranes prepared by either the HBS treatment and the intensive polymerization conditions or the OS treatment have the lower separation factor even for a grafting yield of 2 mg/cm2. This is probably due to the presence of defects, which act as more serious pinholes in the less swollen state at the lower XBz and/or at the lower temperature. The membrane no. 3 was subjected to a durability test. During the durability test of about seven months, the membrane was always contacted with feed mixtures in a PV apparatus and PV experiments were
Fig. 3. Durability of the PGMA-grafted membrane no. 3 for PV of Bz/Cx mixture.
intermittently carried out for 500 h in total. The results of durability experiment are shown in Fig. 3. Both of qBz and qCx increased with the experimental time up to 20 days (PV running time up to 100 h). Then qBz, qCx and were almost kept constant. The PGMA-grafted membrane showed good stability for PV of Bz/Cx mixtures. 3.2. PV properties of PGMA and PGMA-PE membranes for Bz/Cx system PGMA swelled extremely in benzene at higher temperatures and a PGMA membrane with thickness of 70 mm was used for the PV measurements, of which the results are listed in Table 3. The membrane seemed to be capable of PV measurements for Bz/ Cx mixtures but could not tolerate pure benzene
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Table 3 PV separation results of PGMA membrane (70 mm in thickness) at 708C
Table 4 Sorption amounts and sorption, diffusion and PV separation factors in PGMA-PE membranes at 708C
PV run. Feed composition (wt%)
Permeation flux Separation factor Q (kg/m2 h) (dimensionless)
Feed comp. xBz
1 2 3 4 5
0.04 0.04 0.18 0.79 0.10
60 60 80 100 60
15.0 12.7 8.5 ± 2.0
permeation, and lost its selectivity after the measurement for one week (PV run 5 in Table 3). PV of Bz/Cx mixtures (XBz20±80 wt%) through a PGMA-PE membrane was carried out with the back side (PE side) in contact with the feed solutions, and the results are shown in Fig. 4. The feed composition dependence of qCx and belong to type A, but both of qBz and qCx increased more with an increase in XBz, compared with the plasma-grafted membrane no. 3, indicating much larger increases in the swelling. The values of the PGMA-PE membrane were similar to those of the PGMA membrane, but smaller than those of no. 3, indicating that the swelling-restraint effect of the PE substrate is much higher for the grafting than for the impregnation.
Sorption amounts
Separation factors
SBz SCx S (g/100 g dry polymer) 0 40 60 80 100
21 27 32 130
0.4 5.5 4.8 3.0
5.4 3.7 2.7
D
5.3 3.8 3.2
28 15 8.6
The sorption amounts of Bz/Cx mixtures in PGMA membranes are shown in Table 4. The sorption separation factor S was calculated using Eq. (1) and is listed in Table 4. S decreased with an increase in XBz. Assuming that the swelling state of the PGMA-PE membrane at the feed side in the PVexperiments is not seriously different from that in the sorption experiments, the diffusion separation factor D was calculated using the values of PV for the PGMA-PE membrane. As shown in Table 4, S and D were almost the same, indicating half-and-half contribution of the sorption and diffusion processes to the PV separation of Bz/Cx mixtures. Poly(ethylene oxide imide) segmented copolymer (PEO-PI) membranes have been reported to have high permeation ¯uxes with rather low separation factors for PV of Bz/Cx mixtures. For example, the PEO-PI membrane with poly(ethylene oxide) (PEO) segment of 48 wt%, of which degree of polymerization was 23 and Tg was ÿ508C, displayed speci®c permeation ¯ux of 13 kg mm/(m2 h) and of 7.0 at XBz of 60 wt% and 508C [6]. In this case, the value was divided into S and D values of 3.2 and 2.2, respectively. Compared with the PEO-PI membrane, the higher of the PGMA-PE membrane is mainly due to the higher D. This is because the permeation matrix is rubbery for the former but glassy for the latter. The lower values reported for the poly(methyl acrylate)-grafted membranes [2,3,5] may be attributed to the similar reason. 3.3. PV of the PGMA-grafted membranes for other organic systems
Fig. 4. Feed composition dependence of qBz, qCx and for Bz/Cx mixture at 708C for the PGMA-PE membrane.
Table 5 lists PV properties of the plasma-grafted membrane no. 3 for the binary systems of Bz/n-
H. Wang et al. / Journal of Membrane Science 154 (1999) 221±228 Table 5 PV results of aromatic/non-aromatic hydrocarbon mixtures for the PGMA-grafted membrane no. 3 Feed solutions
Bz/Cx Bz/n-Hx Tol/n-Oct Tol/i-Oct
PV resultsa Q (kg/m2 h)
0.30 0.22 0.12 0.09
22 12 20 210
a
PV conditions: at the aromatics feed composition of 60 wt% and 708C.
227
sorption amount was 57 g/100 g-dry-polymer. For nHx, n-Oct, and i-Oct, the sorption amounts were too low to be determined (less than 0.4 g/100 g-dry-polymer). The values were in the order of Tol/i-Oct>>Tol/n-Oct>Bz/Cx>Bz/n-Hx. The PGMA-grafted membrane showed a very high value of 210 for Tol/i-Oct mixture with a rather low Q value at XTol of 60 wt%. It also showed higher and lower Q for the Tol/n-Oct system than for the Bz/Cx system. This phenomenon is different from the case of sulfonylcontaining polyimide membranes [8]. This is due to the lower swelling state of the membrane in the former system. The Q values were larger for Bz/Cx and Bz/nHx systems. 4. Conclusions 1. The PGMA-grafted membranes with a graft yield of about 2 mg/cm2 prepared by the HBS treatment and the mild graft polymerization conditions displayed the much higher PV performance than the membranes prepared by either the HBS treatment and the intensive polymerization conditions or the OS treatment, e.g., Q of 0.30±0.37 kg/ m2 h and of 19±22 at XBz of 60 wt% and 708C for Bz/Cx mixture. 2. Compared with the PGMA-PE membrane, the PGMA-grafted membranes showed higher PV performance with much better durability. 3. The contribution of the sorption and diffusion processes to the PV separation of Bz/Cx mixture was half-and-half for the PGMA-PE membrane. 4. For the PGMA-grafted membrane, was in the order of Tol/i-Oct Tol/n-Oct>Bz/Cx>Bz/n-Hx, and Q was larger for the benzene system.
Fig. 5. Feed composition dependence of q and for Bz/Cx, Bz/nHx, Tol/n-Oct and Tol/i-Oct mixtures at the feed composition of 60 wt% aromatics and 708C for the PGMA-grafted membrane no. 3.
hexane(n-Hx), toluene(Tol)/n-octane(n-Oct), and Tol/ iso-octane(i-Oct) at 708C and feed composition of the aromatic component of 60 wt%. Fig. 5 shows the feed composition dependence of q and . For toluene, the
Acknowledgements The authors thank Tonen Chemical for supplying the porous high-density polyethylene ®lms. This work was supported partly by a grant-in-aid for Developmental Scienti®c Research (no. 08455367) from the ministry of Education, Science, Sports, and Culture of Japan, and also by Petroleum Energy Center and Japan Chemical Innovation Institute at the R&D project for membrane for petroleum component separation.
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References [1] X. Feng, R.Y.M. Huang, Liquid separation by membrane pervaporation: A review, Ind. Eng. Chem. Res. 36 (1997) 1048±1066. [2] T. Yamaguchi, S. Nakao, S. Kimura, Plasma-graft filling polymerization: preparation of a new type of pervaporation membrane for organic liquid mixtures, Macromolecules 24 (1991) 5522±5527. [3] T. Yamaguchi, S. Nakao, S. Kimura, Solubility and pervaporation properties of the filling-polymerized membrane prepared by plasma-graft polymerization for pervaporation of organic±liquid mixtures, Ind. Eng. Chem. Res. 31 (1992) 1914±1919. [4] H. Wang, X. Lin, K. Tanaka, H. Kita, K. Okamoto, Morphology of graft polymerized membranes of alkyl methacrylate and their pervaporation properties, J. Photopolym. Sci. Technol. 10 (1997) 119±122. [5] H. Wang, X. Lin, K. Tanaka, H. Kita, K. Okamoto, Preparation of plasma-grafting polymer membranes and their
[6]
[7] [8]
[9]
[10]
pervaporation properties to benzene/cyclohexane mixtures, J. Polym. Sci., 36 (1998) 2247±2259. N. Tanihara, N. Umeo, T. Kawabata, K. Tanaka, H. Kita, K. Okamoto, Pervaporation of organic liquid mixtures through poly(ether imide) segmented copolymer membranes, J. Membr. Sci. 104 (1995) 181±192. H.R. Acharya, S.A. Stern, Z.Z. Lin, I. Cabasso, Separation of liquid benzene/cyclohexane mixtures by perstration and pervaporation, J. Membr. Sci. 37 (1988) 205. J. Hao, K. Tanaka, H. Kita, K. Okamoto, The pervaporation properties of sulfonyl-containing polyimide membranes to aromatic/aliphatic hydrocarbon mixtures, J. Membr. Sci. 132 (1997) 97±108. T. Yamaguchi, S. Yamahara, S. Nakao, S. Kimura, Preparation of pervaporation membranes for removal of dissolved organics from water by plasma-graft filling polymerization, J. Membr. Sci. 95 (1994) 39±49. T. Yamaguchi, A. Tominaga, S. Nakao, S. Kimura, Chlorinated organics removal from water by plasma-graft filling polymerized membranes, AIChE J. 42 (1996) 892±895.
Pervaporation of aromatic/non-aromatic hydrocarbon mixtures through plasma-grafted membranes Hongyuan Wang, Kazuhiro Tanaka, Hidetoshi Kita, Ken-ichi Okamoto* Department of Advanced Material Science and Engineering, Faculty of Engineering, Yamaguchi University, Ube, Yamaguchi 755, Japan Received 19 February 1998; received in revised form 28 August 1998; accepted 11 September 1998
Abstract Using glycidyl methacrylate (GMA) as a grafting monomer and a porous high density polyethylene ®lm as a substrate, plasma-graft polymerized membranes were prepared by the different plasma treatment manners, namely, homogeneous bothside (HBS) and one side (OS) treatments. The poly(GMA)-grafted membranes displayed two different types of the feed composition dependence of permeation ¯ux and separation factor for pervaporation (PV) of benzene/cyclohexane (Bz/Cx) mixtures, depending on the plasma treatment manner and the graft yield. The membranes prepared by the HBS treatment and under mild polymerization conditions displayed the highest performance with a permeation ¯ux of 0.30±0.37 kg/m2 h and a separation factor of 19±22 at feed Bz of 60 wt% and 708C. The membranes exhibited high performance with excellent durability for PV of other aromatic/aliphatic hydrocarbon mixtures. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Composite membranes; Organic separations; Pervaporation; Plasma polymerized membranes; Poly(glycidyl methacrylate)
1. Introduction Membrane-based technology is currently regarded as a new frontier of chemical engineering and has been used for the puri®cation, concentration, and fractionation of ¯uid mixtures. Pervaporation (PV) is commonly considered to be a pro®table complement to distillation for the separation of azeotropic and closeboiling mixtures, which requires at present the use of energy-intensive processes [1]. In the membrane research ®eld, most of the polymeric materials have been found to be selective to water permeation, and *Corresponding author. Tel.: +81-0836-35-9961; fax: +81-083635-9965; e-mail: [email protected]
only a few have been found to be selective to the permeation of organic compounds [1]. However, the separation of organic±organic mixtures such as aromatic/non-aromatic hydrocarbon mixtures offers potentially the largest opportunity for energy and cost savings. So many efforts have been made to prepare PV membranes with good performance and good stability for separation of organic±organic mixtures [2±8]. Yamaguchi et al. [2,3] prepared membranes by a plasma-graft ®lling polymerization technique, where a grafted polymer offered the permselectivity and a porous substrate restrained swelling of the grafted polymer. These membranes showed good performance for separation of organic/organic and organic/water mixtures [2,3,9,10].
0376-7388/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0376-7388(98)00298-1
222
H. Wang et al. / Journal of Membrane Science 154 (1999) 221±228
In our previous papers [4,5], we have reported on the preparation of composite membranes by plasmainduced graft polymerization of glycidyl methacrylate (GMA) onto porous substrates of high density polyethylene (PE) and on their morphology and PV properties. The grafted membranes with different morphological structures could be prepared by different plasma treatment manners (homogeneous both side (HBS) and one side (OS) treatments) combined with mild or intensive graft polymerization conditions. The membranes prepared by the HBS treatment and mild polymerization conditions were composed of middle, intermediate and surface parts. In the middle part, the graft polymerization hardly occurred and the porous structure of PE remained substantially unchanged. In the intermediate parts, the graft polymer chains grew up on the surface and in the inner amorphous region of micro®brils of PE, resulting in the ®lling of the pores, expansion of the amorphous layers and the loosening of the crystallites stacked in layers on both their sides. In the surface parts, more graft chains were formed and the micro®bril structure was completely split, and as a result, lost the ability to restrain the swelling of the graft chains. The morphology changed gradually from the intermediate parts through the surface sides. Under more intensive polymerization conditions, the intermediate parts became narrower, the morphology changed drastically from the intermediate to the surface parts, and the surface parts looked much more like the homo-polymer. In the case of the OS treatment, the grafted poly(GMA) was hardly formed on the untreated surface, but at the inner part of the untreated side the pores were rather ®lled with the graft polymer. The preliminary PV experiments indicated that the membranes prepared by the HBS treatment and the mild polymerization conditions displayed the much higher PV performance for benzene/cyclohexane mixtures. In this study, the PV properties of these GMAgrafted membranes were investigated for aromatic/ non-aromatic hydrocarbon mixtures in detail, compared with the poly(GMA) membranes fabricated on the PE substrate. 2. Experimental Two kinds of porous PE ®lms supplied by Tonen Chemical Co. Ltd. were used as substrates. They had
the same average pore size of 0.03 mm with the thickness of 16 and 25 mm, and the porosity of 60% and 45%, respectively. The plasma-induced graft polymerization process has been described in detail elsewhere [4,5]. A porous substrate of 88 cm2 in size was inserted in a glass tube (30 cm in length and 4.5 cm in inner diameter) and treated for 120 s (HBS) or 60 s (OS) by Ar plasma (0.02 Pa and 10 w at 13.56 MHz). Then, without exposure to air, the substrate was immersed into a graft monomer solution degassed previously. Graft polymerization was carried out at 30±408C in a water bath, with shaking. After the polymerization, the grafted membranes were immersed in acetone overnight to remove the homopolymer and the unreacted monomer, and dried in an oven at 408C for 20 h. The membranes were weighed before and after the graft polymerization in the dry state, and the grafting yield was determined as weight of grafted polymer per unit area of porous substrate (mg/cm2). Poly(GMA) (PGMA) was prepared by a radical polymerization at 808C under nitrogen, using benzoyl peroxide (0.05 wt% to GMA) and dioxane, and puri®ed by reprecipitation from dioxane with methanol. PGMA membranes with thickness of 30±80 mm were prepared by casting a 5 wt% tetrahydrofuran (THF) solution of PGMA onto glass plates, followed by drying at 408C for 10 h and then in vacuum at 408C for 20 h. PGMA-PE membranes with thickness of 40± 80 mm were prepared by casting a 5 wt% THF solution of PGMA onto the porous PE substrate ®lms (25 mm in thickness), followed by drying at room temperature for 10 h and then in vacuum at 408C for 10 h. The PGMA-PE membranes thus prepared were transparent and could not be peeled off. PV was carried out at 40±788C by an ordinary method [6]. The effective membrane area was 19.6 cm2 and the total volume of the feed liquid was 100 ml. Permeate was collected in a cold trap in liquid nitrogen. Feed and permeate composition analysis was performed on a gas chromatography equipped with 3 m long column packed with DOP 30% Uniport R 60/80. The separation factor, , is de®ned as y 1 ÿ x=x 1 ÿ y;
(1)
where x and y denote the weight fraction (%) of aromatic component in the feed and permeate, respec-
H. Wang et al. / Journal of Membrane Science 154 (1999) 221±228
3. Results and discussion
tively. The GMA-grafted membranes were conditioned with benzene [5]; only the feed side of the membranes was kept in contact with benzene, and the permeate side was kept in a dry state in vacuum at 708C for 24 h. Both permeability and selectivity increased after the conditioning. PGMA membrane pieces of about 20 mg in weight were used in sorption experiments. Vacuum-dried membrane pieces were immersed in benzene (Bz), cyclohexane (Cx) and Bz/Cx mixtures. The closed containers were placed in a bath at 708C for 30 h to achieve the sorption equilibrium. Then the membrane pieces were taken out and carefully wiped with ®lter paper to remove the surface free solvents, and subsequently placed in a sample bottle containing a small amount of N-methyl pyrrolidone (NMP). The bottle was closed at once and then weighed on a micro balance. The composition of Bz and Cx in solventswollen membranes were determined by means of head space gas chromatography. The sorption amount is de®ned as S 100 Ws ÿ Wd =Wd ;
223
3.1. PV properties of the PGMA-grafted membranes for Bz/Cx system The Tg of grafted PGMA for membrane no. 2 in Table 1 was 758C and a little higher than that of a PGMA ®lm (708C). This may be due to higher polymerization degree of the grafted PGMA and/or its reduced segmental mobility in the pores. Under most of the PV experiments, the grafted PGMA was in the glassy state at least in the permeate side. The results of PV of Bz/Cx mixture are listed in Table 1. Fig. 1 shows the feed composition dependence of the permeation ¯ux and the separation factor. For every membrane, the permeation ¯ux of benzene qBz increased signi®cantly with an increase in benzene fraction in feed XBz. On the other hand, there were observed two different types of feed composition dependence of the permeation ¯ux of cyclohexane qCx and . As shown in Fig. 1(a), qCx increased by a factor of about 5 for membrane no. 3 with increasing XBz from 20 to 80 wt%, and decreased signi®cantly (type A). For membrane no. 5, qCx hardly changed with an increase in XBz, and at ®rst increased a little and then turned to decrease (type B). The value was much larger for membrane no. 3 than for no. 5 at XBz of 20 wt%, but it was only a little larger for no. 3 at XBz of 80 wt%. Among the membranes listed in Table 1, nos. 2±4 and 6 belong to type A, whereas nos. 1, 5, 7 and 8 belong to type B. It is noted that the membranes
(2)
where Wd and Ws denote the weight of dry and solventswollen membranes, respectively. Differential scanning calorimetry (DSC) was measured with a Seiko Instruments DSC-5200 at a heating and cooling rate of 10 K/min. Glass transition temperature Tg was determined from the onset point of a DSC signal on the ®rst heating.
Table 1 Characterization and PV separation results of PGMA-grafted membranesa No.
1 2 3 4 5 6 7 8 a
Graft yield (mg/cm2)
0.7 1.7 1.9 2.4 2.2 3.0 2.0 1.7
Polymerization conditionsb
Thickness (mm)c
Conc. (vol%)
Temp. (8C)
Time (min)
3 3 3 3 15 15 3 5
30 30 30 30 30 40 30 40
35 33 35 60 300 170 155 20
17 26 (29) (32) (26) 26 26 37
PV resultsd Q (kg/m2 h)
(dimensionless)
0.46 0.37 0.30 0.35 0.18 0.06 0.23 0.36
13.5 19 22 22 13.3 23 14.3 13.7
Thickness of PE: nos. 1±7: 16 mm; no. 8: 25 mm. Plasma treatment manner: nos. 1±6: HBS; nos. 7 and 8: OS. Shaking speed: nos. 1 and 4: slow, 80 rpm; nos. 2 and 3, 5±8: middle, 120 rpm. c Measured with TEM or (SEM). d At XBz60 wt% and 708C. For nos. 7 and 8, the untreated surface was in contact with the feed solution. b
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Fig. 1. Feed composition dependence of qBz, qCx and to Bz/Cx mixture at 708C for the PGMA-grafted membranes: (a) no. 3 (*/*) and no. 5 (~/~); (b) no. 1 (~); no. 2 (&); no. 6 (5/!); no. 8 (*/*).
prepared by the HBS treatment and with the larger grafting yield displayed a behavior of type A, whereas the membranes either prepared by the HBS treatment and with a smaller grafting yield or prepared by the OS treatment behaved as type B. The behavior of type A is usually observed for polymeric membranes [6,8], and is explained by the increased swelling of the membranes with an increase in XBz. On the other hand, the behavior of type B might be attributed to the presence of some defects in the membranes, which act as pinholes in the unswollen state rather than in the swollen state. Channels of the pores incompletely ®lled with the graft polymer chains are considered as such defects. Fig. 2 shows the temperature dependence of qBz, qCx, and at XBz of 60 wt%. The activation energies of qBz and qCx, EBz and ECx are summarized in Table 2. Both EBz and ECx were much larger for membrane no. 6 than for the other membranes. For membrane nos. 6 and 3, EBz was smaller than ECx, and increased signi®cantly with a decrease in
Table 2 Activation energies of permeation flux E of PGMA-grafted membranes at xBz of 60 wt% Membrane no.
EBz (kJ/mol)
ECx (kJ/mol)
1 3 5 6 8
55 50 53 65 59
57 65 62 90 61
temperature. On the other hand, for membrane nos. 1 and 8, hardly depended on temperature. It is noted that these membranes had also weak dependence of on feed composition. This suggests that no increase in with decreasing temperature might be attributed to the similar reason, namely, the presence of some defects. The in¯uence of defects may be larger at lower temperature. The results mentioned above lead to the following conclusions concerning the relationship between the
H. Wang et al. / Journal of Membrane Science 154 (1999) 221±228
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Fig. 2. Temperature dependence of qBz, qCx and to Bz/Cx mixture at xBz of 60 wt% for the PGMA-grafted membranes: no. 1 (*/*); no. 3(&/&); no. 5(~/~); no. 6 (5/!).
PV performance and the preparation conditions of the PGMA-grafted membranes. The membranes prepared by the HBS treatment and the mild graft polymerization conditions and having a grafting yield of about 2 mg/cm2, have the best PV performance; that is, total permeation ¯ux Q of 0.30±0.37 kg/m2 h and of 19± 22 for the membrane nos. 2±4 at XBz of 60 wt% and 708C. The membranes prepared by the HBS treatment and the intensive polymerization conditions have a lower permeation ¯ux due to the much less permeable intermediate parts. The membranes prepared by either the HBS treatment and the intensive polymerization conditions or the OS treatment have the lower separation factor even for a grafting yield of 2 mg/cm2. This is probably due to the presence of defects, which act as more serious pinholes in the less swollen state at the lower XBz and/or at the lower temperature. The membrane no. 3 was subjected to a durability test. During the durability test of about seven months, the membrane was always contacted with feed mixtures in a PV apparatus and PV experiments were
Fig. 3. Durability of the PGMA-grafted membrane no. 3 for PV of Bz/Cx mixture.
intermittently carried out for 500 h in total. The results of durability experiment are shown in Fig. 3. Both of qBz and qCx increased with the experimental time up to 20 days (PV running time up to 100 h). Then qBz, qCx and were almost kept constant. The PGMA-grafted membrane showed good stability for PV of Bz/Cx mixtures. 3.2. PV properties of PGMA and PGMA-PE membranes for Bz/Cx system PGMA swelled extremely in benzene at higher temperatures and a PGMA membrane with thickness of 70 mm was used for the PV measurements, of which the results are listed in Table 3. The membrane seemed to be capable of PV measurements for Bz/ Cx mixtures but could not tolerate pure benzene
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Table 3 PV separation results of PGMA membrane (70 mm in thickness) at 708C
Table 4 Sorption amounts and sorption, diffusion and PV separation factors in PGMA-PE membranes at 708C
PV run. Feed composition (wt%)
Permeation flux Separation factor Q (kg/m2 h) (dimensionless)
Feed comp. xBz
1 2 3 4 5
0.04 0.04 0.18 0.79 0.10
60 60 80 100 60
15.0 12.7 8.5 ± 2.0
permeation, and lost its selectivity after the measurement for one week (PV run 5 in Table 3). PV of Bz/Cx mixtures (XBz20±80 wt%) through a PGMA-PE membrane was carried out with the back side (PE side) in contact with the feed solutions, and the results are shown in Fig. 4. The feed composition dependence of qCx and belong to type A, but both of qBz and qCx increased more with an increase in XBz, compared with the plasma-grafted membrane no. 3, indicating much larger increases in the swelling. The values of the PGMA-PE membrane were similar to those of the PGMA membrane, but smaller than those of no. 3, indicating that the swelling-restraint effect of the PE substrate is much higher for the grafting than for the impregnation.
Sorption amounts
Separation factors
SBz SCx S (g/100 g dry polymer) 0 40 60 80 100
21 27 32 130
0.4 5.5 4.8 3.0
5.4 3.7 2.7
D
5.3 3.8 3.2
28 15 8.6
The sorption amounts of Bz/Cx mixtures in PGMA membranes are shown in Table 4. The sorption separation factor S was calculated using Eq. (1) and is listed in Table 4. S decreased with an increase in XBz. Assuming that the swelling state of the PGMA-PE membrane at the feed side in the PVexperiments is not seriously different from that in the sorption experiments, the diffusion separation factor D was calculated using the values of PV for the PGMA-PE membrane. As shown in Table 4, S and D were almost the same, indicating half-and-half contribution of the sorption and diffusion processes to the PV separation of Bz/Cx mixtures. Poly(ethylene oxide imide) segmented copolymer (PEO-PI) membranes have been reported to have high permeation ¯uxes with rather low separation factors for PV of Bz/Cx mixtures. For example, the PEO-PI membrane with poly(ethylene oxide) (PEO) segment of 48 wt%, of which degree of polymerization was 23 and Tg was ÿ508C, displayed speci®c permeation ¯ux of 13 kg mm/(m2 h) and of 7.0 at XBz of 60 wt% and 508C [6]. In this case, the value was divided into S and D values of 3.2 and 2.2, respectively. Compared with the PEO-PI membrane, the higher of the PGMA-PE membrane is mainly due to the higher D. This is because the permeation matrix is rubbery for the former but glassy for the latter. The lower values reported for the poly(methyl acrylate)-grafted membranes [2,3,5] may be attributed to the similar reason. 3.3. PV of the PGMA-grafted membranes for other organic systems
Fig. 4. Feed composition dependence of qBz, qCx and for Bz/Cx mixture at 708C for the PGMA-PE membrane.
Table 5 lists PV properties of the plasma-grafted membrane no. 3 for the binary systems of Bz/n-
H. Wang et al. / Journal of Membrane Science 154 (1999) 221±228 Table 5 PV results of aromatic/non-aromatic hydrocarbon mixtures for the PGMA-grafted membrane no. 3 Feed solutions
Bz/Cx Bz/n-Hx Tol/n-Oct Tol/i-Oct
PV resultsa Q (kg/m2 h)
0.30 0.22 0.12 0.09
22 12 20 210
a
PV conditions: at the aromatics feed composition of 60 wt% and 708C.
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sorption amount was 57 g/100 g-dry-polymer. For nHx, n-Oct, and i-Oct, the sorption amounts were too low to be determined (less than 0.4 g/100 g-dry-polymer). The values were in the order of Tol/i-Oct>>Tol/n-Oct>Bz/Cx>Bz/n-Hx. The PGMA-grafted membrane showed a very high value of 210 for Tol/i-Oct mixture with a rather low Q value at XTol of 60 wt%. It also showed higher and lower Q for the Tol/n-Oct system than for the Bz/Cx system. This phenomenon is different from the case of sulfonylcontaining polyimide membranes [8]. This is due to the lower swelling state of the membrane in the former system. The Q values were larger for Bz/Cx and Bz/nHx systems. 4. Conclusions 1. The PGMA-grafted membranes with a graft yield of about 2 mg/cm2 prepared by the HBS treatment and the mild graft polymerization conditions displayed the much higher PV performance than the membranes prepared by either the HBS treatment and the intensive polymerization conditions or the OS treatment, e.g., Q of 0.30±0.37 kg/ m2 h and of 19±22 at XBz of 60 wt% and 708C for Bz/Cx mixture. 2. Compared with the PGMA-PE membrane, the PGMA-grafted membranes showed higher PV performance with much better durability. 3. The contribution of the sorption and diffusion processes to the PV separation of Bz/Cx mixture was half-and-half for the PGMA-PE membrane. 4. For the PGMA-grafted membrane, was in the order of Tol/i-Oct Tol/n-Oct>Bz/Cx>Bz/n-Hx, and Q was larger for the benzene system.
Fig. 5. Feed composition dependence of q and for Bz/Cx, Bz/nHx, Tol/n-Oct and Tol/i-Oct mixtures at the feed composition of 60 wt% aromatics and 708C for the PGMA-grafted membrane no. 3.
hexane(n-Hx), toluene(Tol)/n-octane(n-Oct), and Tol/ iso-octane(i-Oct) at 708C and feed composition of the aromatic component of 60 wt%. Fig. 5 shows the feed composition dependence of q and . For toluene, the
Acknowledgements The authors thank Tonen Chemical for supplying the porous high-density polyethylene ®lms. This work was supported partly by a grant-in-aid for Developmental Scienti®c Research (no. 08455367) from the ministry of Education, Science, Sports, and Culture of Japan, and also by Petroleum Energy Center and Japan Chemical Innovation Institute at the R&D project for membrane for petroleum component separation.
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