Cellulose derivative and liquid crystal blend membranes for oxygen enrichment

PERGAMON

European Polymer Journal 35 (1999) 157±166

Cellulose derivative and liquid crystal blend membranes for oxygen enrichment Xin-Gui Li a, *, Mei-Rong Huang a, Ling Hu b, Gang Lin b, Pu-Chen Yang b a

Department of Polymer Materials Science Engineering, College of Materials Science Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, People's Republic of China b Tianjin Institute of Textile Science and Technology, Tianjin 300160, People's Republic of China Received 3 June 1997; accepted 22 January 1998

Abstract Oxygen enriching property and stability of the blend membranes, 10±45 mm thick, fabricated from ethyl cellulose, cellulose nitrate, cellulose diacetate, and cellulose triacetate, with thermotropic cholesteric or nematic liquid crystals (LC), such as cholesteryl oleyl carbonate, p-heptyl-p'-cyanobiphenyl, p-pentylphenol-p'-methoxybenzoate, benzoatecontaining liquid crystal mixture, and triheptyl cellulose, were studied by a variable volume method. The membranes with 1.5±4 wt% LCs showed the maximum oxygen-enriching ability in the temperature range of their LC phase. The oxygen-enriched air ¯ux QOEA and the oxygen concentration YO2 increased simultaneously as the operating pressure increased. The 30 mm-thick triheptyl cellulose/ethyl cellulose (8/92) homogeneous membrane exhibited almost constant oxygen- enriching eciencies of QOEA 1.0  10ÿ4 ml(STP)/s
cm2 and YO2 40% at 308C and 0.41 MPa in a single step for a 35 day operating time. # 1998 Elsevier Science Ltd. All rights reserved.

1. Introduction The liquid crystal (LC)-containing membranes which are simple in structure and are easy to work with [1± 4], can result in much lower capital cost of enriching oxygen directly from air than the carrier facilitated transport membranes and immobilized molten salt membranes which are believed to be highly oxygenenriching membranes [5, 6]. Previously prepared nematic LC membranes containing 40±60 wt% low molecular LC might have a short lifetime due to the loss of LC materials, especially when the membrane thickness is less than 45 mm under a high operating pressure [1]. One way of getting around this problem is by blending a small amount of higher viscosity cholesteric LC with the membrane matrix. Our recent investigations have demonstrated that cholesteric LC membranes show not only higher oxygen enrichment but also higher stability [1, 2, 7±12]. The purpose of

* Corresponding author.

this paper is to further illustrate the enhancement of oxygen enrichment through the blend membranes of the four cellulose derivatives with ®ve types of liquid crystals.

2. Experimental 2.1. Materials Cholesteryl oleyl carbonate (COC) and the benzoate-containing liquid crystal mixture DYC were purchased from the 2nd Chemical Reagent Factory of Tianjin China. p-Heptyl-p'-cyanobiphenyl (7CB) and ppentylphenol-p'-methoxybenzoate (5PMB) were obtained from the Jinghua Special Materials Institute of Shijiazhuang China. The above mentioned four kinds of low molecular weight LCs have the purity of higher than 99%. A polymer liquid crystal, i.e. triheptyl cellulose (THC), was synthesized in our laboratory [13]. The ®ve LCs were selected for this investigation because they are thermotropic and their

0014-3057/98/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 9 8 ) 0 0 0 8 8 - 3

158

Xin-Gui Li et al. / European Polymer Journal 35 (1999) 157±166

solid±liquid crystalline phase transition temperatures approach room temperature. The cholesteric LC phase temperature range of the COC, DYC and THC are 25±398C, 29±328C and 20±1008C, while the nematic LC phase temperature ranges of 7CB and 5PMB are 28.5±408C and 30±438C, respectively. Four cellulose derivatives, ethyl cellulose (EC), cellulose nitrate (CN) made in China, cellulose diacetate (CDA) and cellulose triacetate (CTA) obtained from Daicel Chemical Company in Japan, were used. The 5 wt% EC solution viscosity in ethanol/toluene (1/1) is ca. 0.06 Pa.s. The falling sphere viscosity of the CN is 25 sec and its nitrogen content is 12%. The intrinsic viscosity of the CDA in chloroform is 28 dl/g. The degree of substitution and the number-average molecular weight of the CTA are 2.7 and 86751, respectively.

the feed pressure was 0.06±0.48 MPa. In order to provide more signi®cant information guiding membranebased oxygen enrichment, the air from the air compressor was directly used as the test gas. The oxygen concentration in the oxygen-enriched air (OEA) through the membranes was measured using an Industrial Gas Analyzer, Model 491, manufactured by Tianjin Factory of Glass Instrument of China. The membrane e€ective area was 50 cm2. A detailed method for the calculation of oxygen-enrichment parameters, including the OEA ¯ux QOEA, the OEA permeability POEA, the oxygen permeability PO2, the oxygen concentration YO2, the actual separation factor ASF, is given in our earlier papers [1, 8, 12]. 3. Results and discussion

2.2. Membrane preparation The blend membranes were prepared by a solution casting method of LC and a cellulose derivatives blend solution. A ¯at and uniform blend membrane, with a thickness ranging between 10 and 45 mm was obtained through casting the tetrahydrofuran or dichloromethane solution of EC and CDA or CTA blend with LCs on a glass plate. 2.3. Oxygen-enrichment evaluation The measurements of the oxygen-enriching performance through the membranes were performed using a permeability stainless-steel cell of the variable volume type [12]. The permeate pressure was atmospheric and

3.1. Dependence of oxygen enrichment on LC content The e€ects of the LC content in the homongeneous dense blend membranes on the oxygen enrichments were examined by increasing LC content from zero to 45 wt% at 308C and an operating pressure 0.43 MPa. The relationship between the LC content and the oxygen enrichment is shown in Table 1 and Fig. 1. Both the ¯ux QOEA, the permeability POEA and oxygen concentration YO2 of the oxygen-enriched air through the membranes increase ®rst and then decrease slowly as the LC content increases from zero to 45 wt%. When the LC content ranges between 1.5 and 12 wt%, the QOEA and YO2 values appear to reach the maxima of 1.59  10ÿ4 ml(STP)/s cm2 and 41%, respectively.

Table 1 E€ect of LC content in the dense blend membranes on oxygen enriching properties at 308C under operating pressure 0.43 MPa COC/CN (wt/wt) Membrane thickness (mm) QOEA105 (cm3(STP)/s cm2) PO21010 (cm3(STP) cm/cm2 s cmHg) YO2 (%) THC/EC (wt/wt) Membrane thickness (mm) QOEA105 (cm3(STP)/s cm2) PO21010 (cm3(STP) cm/cm2 s cmHg) YO2 (%) 7CB/EC (wt/wt) Membrane thickness (mm) QOEA105 (cm3(STP)/s cm2) PO21010 (cm3(STP) cm/cm2 s cmHg) YO2 (%) a

0/100 30 0.81 1.98 34.5 0/100 18 5.91 9.90 37.4 0/100 18 6.42 10.65 37.2

4/96 25 1.25 3.32 40.3 1.5/98.5

10/90 30 2.06 5.39 36.0 4/96

17 7.22 13.43 41.0

20 10.1 20.59 39.4

4/96

9/91

17 8.70 16.18 41.0

22 8.01 18.33 39.9

10/90 20 11.4 21.12 37.3 12/88 25 6.47 16.48 40.3

20/80 a

36 6.41 21.40 37.2 16/84 27 6.10 17.16 39.9

These may be the thinnest membranes which are pinhole-free under an operating pressure 0.43 MPa.

30/70 35a 6.05 18.96 36.5 38/62 a

35 5.60 19.19 38.0

45/55 40a 6.99 27.47 38.6

Xin-Gui Li et al. / European Polymer Journal 35 (1999) 157±166

Fig. 1. Plots of POEA (a) and YO2 (b) vs temperatures through the blend membranes with six 7CB/EC ratios at the transmembrane pressure di€erence of 0.4 MPa.

Fig. 1(b) suggests that the YO2 values through all ®ve 7CB/EC membranes seem to go through a maximum close to a particular temperature range from 27.8± 308C, which partially overlaps the temperature region (28.5±408C) of the liquid crystalline phase of the 7CB liquid crystal. These indicate that the introduction of the LCs can enhance the oxygen-enriching capacity of the cellulose derivatives [11±14]. It must be noticed that QOEA decreases as a result of the increase in the thickness of the membranes containing more than 20 wt% LCs. A larger thickness for the membranes containing 20 wt% LCs are necessary to prevent the formation of the pinholes in the membranes under a higher operating pressure. 3.2. Dependence of oxygen enrichment on LC variety The oxygen-enriching properties of the blend membranes prepared from cellulose triacetate (CTA), cellulose diacetate (CDA) and four di€erent LCs of 9 wt% are presented in Table 2. It is found that all of these LCs can e€ectively improve the oxygen-enriching properties of the CTA and CDA membranes, though their

159

molecular structures, liquid crystalline phases, and LC transition temperatures are di€erent from one another. The oxygen-enriching properties through the ethyl cellulose (EC) and cellulose nitrate (CN) membranes are also improved by the four kinds of LCs [1]. It is well known that the viscosity of LCs will rapidly decrease upon transforming into the anisotropic phase state. This will lead the cellulose derivative and LC molecules in the blend membranes to move and rotate easily, therefore the membranes possess more free volume which bene®ts the oxygen permeation. A small amount of LC-enriched domain dispersed in the membranes might be the main path of oxygen in the LC temperature range. Furthermore, the mechanism of oxygen transport may change from di€usion to solution control. In the eight types of membranes, the CDA/cholesteryl oleyl carbonate (COC) membrane shows the highest QOEA (1.0  10ÿ4 ml(STP)/s cm2) and POEA (3.3  10ÿ10 ml(STP) cm/cm2 s cmHg) which are nearly double that of the pure CDA membrane, and the CTA/p-pentylphenol-p'-methoxybenzoate (5PMB) membrane displays the highest YO2 (40.5%) and actual separation factor (ASF) (2.56). The former might be attributed to both the higher oxygen permeability of CDA than CTA and the lower LC phase temperature of COC which is responsible for higher ¯owing ability and lower viscosity at the same temperature 278C. The latter may be caused by the chemical in®nities of oxygen towards the ester groups in the CTA and 5PMB. The YO2 through the 5PMB/CTA membrane is as large as 40.3%, indicative that the membrane has the potential to approach the limiting YO2 of 47% in a single stage if the operating pressure increased [1]. It might be predicted that the OEA containing 97.6% oxygen could be produced in a simple ®ve-step enriching operation. 3.3. Dependence of oxygen enrichment on cellulose derivative variety The oxygen-enriching properties across the four kinds of cellulose derivative membranes containing 9 wt% p-heptyl-p'-cyanobiphenyl (7CB), benzoate-containing liquid crystal mixture DYC, and COC, are shown in Table 3. The 7CB/EC membrane in the midst of the membranes exhibits the highest QOEA (1.43  10ÿ4 ml(STP)/s cm2) and POEA (9.52  10ÿ10 ml(STP) cm/cm2 s cmHg) regardless of their similar polymeric skeletons. This may be related to the large amount of random structures of EC with ethyl sidegroups, and to the low density which is responsible for the larger free volume. As seen in Table 3, the 7CB/ CTA membrane shows the highest YO2 (40.5%) and ASF (2.56) which is attributed to the high oxygen over nitrogen selectivity of the CTA among the four cellulose derivatives.

160

Xin-Gui Li et al. / European Polymer Journal 35 (1999) 157±166

Table 2 The oxygen-enriching properties through the LC/CTA or CDA(9/91) blend membranes at 278C under 0.44 MPa operating pressure Membranes

Membrane thickness (mm)

QOEA105 (cm3(STP)/s cm2)

(POEA/PO2)  1010 (cm3(STP) cm/cm2 s cmHg)

YO2 (%)

ASF

Pure CTA COC/CTA DYC/CTA 7CB/CTA 5PMB/CTA Pure CDA COC/CDA DYC/CDA 7CB/CDA 5PMB/CDA

18 13 12 11 14 11 12 11 12 15

2.69 5.67 5.81 5.45 4.91 6.01 10.0 9.46 6.05 5.53

1.45 2.20 2.08 1.80 2.06 1.97 3.58 3.11 2.17 2.48

36.1 40.2 40.2 40.3 40.5 35.1 33.7 32.6 39.1 37.3

2.13 2.53 2.53 2.54 2.56 2.04 1.91 1.82 2.42 2.24

4.12 7.53 7.12 6.16 7.12 5.36 9.13 7.52 7.07 7.45

3.4. Dependence of oxygen enrichment on operating pressure

3.5. Dependence of oxygen enrichment on operating temperature

Figs. 2, 3 and 4 show the plots of QOEA and YO2 versus pressure di€erence across the cellulose derivative/LC membranes. There are concurrent increases in QOEA and YO2 for the membranes when increasing the operating pressure from 0.1 to 0.45 MPa. The dependency of QOEA on the operating pressure is nearly linear whereas the enhancement of the YO2 is exponential with the operating pressure, which is in agreement with that of ordinary membranes. Note that the largest QOEA and YO2 values produced simultaneously for the DYC/EC(9/91) membrane are 1.0  10ÿ4 ml(STP)/s cm2 and 39.9%, respectively. It could be estimated that the QOEA through the DYC/EC(9/91) membrane might reach 2.0  10ÿ4 ml(STP)/s cm2 under the operating pressure of 1.0 MPa.

The plots of QOEA, POEA and YO2 vs, temperature shown in Figs. 5, 6, 7, 8, 9 and 10 indicate that the POEA and YO2 values through the LC/cellulose derivative (9/91) membranes depend strongly on the temperature ranging from 10±708C. It is interesting that more rapid QOEA and POEA increases and a maximum YO2 value were observed in the LC state temperature range of the LCs. This phenomenon might result from the strong sensitivity of the LC viscosity in the ordered LC state to the temperature change. Above the isotropic temperature (T1), the LCs will transform into a disorder isotropic phase, resulting in a further increase in QOEA and POEA but a decrease in YO2. The permeation activation energy of the OEA across the LC membranes are higher in the liquid crystalline state tem-

Table 3 The oxygen-enriching properties through the LC/cellulose derivatives(9/91) blend membranes at 308C under 0.47 MPa operating pressure Membranes

Membrane thickness (mm)

QOEA105 (cm3(STP)/s cm2)

(POEA/PO2)  1010 (cm3(STP) cm/cm2 s cmHg)

YO2 (%)

ASF

7CB/EC 7CB/CDA 7CB/CTA DYC/EC DYC/CDA DYC/CTA COC/EC COC/CDA COC/CTA COC/CN

16 12 11 20 11 12 18 12 13 30

14.3 6.86 6.14 10.3 10.7 6.57 12.8 11.6 6.38 2.41

6.41 2.30 1.89 7.51 3.30 2.21 6.45 3.90 2.32 2.02

38.5 39.2 40.5 39.9 36.1 40.0 38.9 33.0 39.8 39.5

2.36 2.44 2.56 2.50 2.13 2.51 2.40 1.86 2.50 2.47

19.4 7.19 6.24 18.5 9.00 7.05 19.9 9.27 7.43 6.37

Xin-Gui Li et al. / European Polymer Journal 35 (1999) 157±166

Fig. 2. Plots of QOEA and YO2 versus operating pressure for the LC/EC(9/91) blend membranes containing the LC: (w) no LC, (.) 7CB, (r) 5PMB, (R) COC, (t) DYC.

Fig. 3. Plots of QOEA and YO2 versus operating pressure for the LC/CDA(9/91) blend membranes containing the LC: (w) no LC, (.) 7CB, (r) 5PMB, (R) COC, (t) DYC.

161

Fig. 4. Plots of QOEA and YO2 versus operating pressure for the LC/CTA(9/91) blend membranes containing the LC: (w) no LC, (.) 7CB, (r) 5PMB, (R) COC.

Fig. 5. Plots of QOEA versus operating temperature for the LC/EC(9/91) blend membranes containing the LC: (w) no LC, (.) 7CB, (r) 5PMB, (R) COC, (t) DYC.

162

Xin-Gui Li et al. / European Polymer Journal 35 (1999) 157±166

Fig. 6. Plots of QOEA versus operating temperature for the LC/CDA(9/91) blend membranes containing the LC (w) no LC (.) 7CB, (r) 5PMB, (R) COC, (t) DYC.

perature range than in the solid LC or isotropic states since the viscosity of liquid-crystalline state reduces more rapidly as the temperature increases, as seen in Table 4. The slight uncertainties in permeation activation energy in Table 4 might be related to distinctly non-linear behaviour of some plots shown in Fig. 8(a), 9(a) and 10(a). It is noted from Figs. 1 and 8±10 and Table 5 that the LC blend membranes exhibit the

Fig. 7. Plots of QOEA versus operating temperature for the LC/CTA(9/91) blend membranes containing the LC (w) no LC (.) 7CB, (r) 5PMB, (R) COC, (t) DYC.

Fig. 8. Plots of POEA (a) and YO2 (b) versus operating temperature for the LC/EC(9/91) blend membranes containing the LC (w) no LC, (r) 7CB, (t) 5PMB, (r) COC, (+) DYC.

maximum oxygen-enriching ability at the transition temperature (TKN or TKCh) from crystalline solid to liquid crystalline phase of the LC components regardless of cellulose derivative matrix and their LC content to a certain extent. The three pure cellulose derivate membranes containing no LC hardly ever show the maximal YO2 value with the variation of temperature. The maximum oxygen-enriching ability of the LC membranes might result from the more free volume, ¯owing ordered phase and higher thermal-expansion coecient of the LCs [7]. The ordering phase structure within the liquid crystal system might be due to a uniform and dense anisotropic structure having fewer defect-like solid crystals than the isotropic non-liquid crystalline phase, resulting in larger YO2 values. Note that an obvious sudden jump of OEA permeability through the blend membranes containing 7CB and COC liquid crystals was sometimes seen in the temperature ranging from 26±308C, as shown in Fig. 1(a), 6, 7, 9(a) and 10(a). This temperature range

Xin-Gui Li et al. / European Polymer Journal 35 (1999) 157±166

Fig. 9. Plots of POEA (a) and YO2 (b) versus operating temperature for the LC/CDA(9/91) blend membranes containing the LC (w) no LC, (r) 7CB, (t) 5PMB, (r) COC, (+) DYC.

163

Fig. 10. Plots of POEA (a) and YO2 (b) versus operating temperature for the LC/CTA(9/91) blend membranes containing the LC (w) no LC, (r) 7CB, (t) 5PMB, (r) COC, (+) DYC.

Table 4 Permeation activation energy of the oxygen-enriched air containing 36±40% oxygen across the LC/cellulose(9/91) derivative membrane under operating pressure 0.44 MPa Permeation activation energy (kJ/mol) at the temperature T Membrane COC/EC 5PMB/EC 7CB/EC DYC/EC 5PMB/CDA DYC/CDA 7CB/CDA COC/CDA COC/CTA 7CB/CTA 5PMB/CTA DYC/CTA

T < TKN or TKCh 14.4 25.5 25.5 29.5 17.7 20.3 25.5 10.0 16.0 23.9 19.2 26.8

TKN or TKCh
T>T1 12.0 8.50 18.2 16.6 16.5 21.1 15.3 17.3 23.0 21.5 23.9 25.5

30 33.7 29 39.5 26 33.0 28 40.0 31 37.5

26 38.9

28 39.6

DYC CDA DYC EC COC CTA COC CDA COC EC 5PMB CTA 5PMB CDA

30 35.3 30 39.6 29±30 40.3 Temperature (8C) Maximum YO2 (%)

30 38.3

5PMB EC 7CB CTA 7CB EC

7CB CDA

Fig. 11. Plots of QOEA (a) and YO2 (b) versus operating time at 0.4 MPa for (w) 30 mm-thick THC/EC(8/92) membrane at 408C; (r) 45 mm-thick THC/EC(20/80) blend membrane at 408C; (t) 25 mm-thick 7CB/EC(12/88) membrane at 308C; and (r) 36 mm-thick 7CB/EC(38/62) at 308C.

overlaps the liquid crystalline phase temperature of the 7CB and COC.

Liquid crystal Cellulose derivative

Table 5 The temperature of maximally concentrating oxygen from air by the LC/cellulose derivative membranes

28 39.5

Xin-Gui Li et al. / European Polymer Journal 35 (1999) 157±166

DYC CTA

164

3.6. Dependence of oxygen enrichment on operating time Generally, the more the LC content, the thinner the LC/cellulose derivative blend membrane, and the lower the stability of the membranes [1, 14, 15], especially at an elevated operating temperature and pressure. The variation of the QOEA and YO2 of the OEA enriched by four homogeneous dense blend membranes with high LC content are evaluated as the operating time, as shown in Fig. 11. It is apparent that the 7CB/ EC(38/62) blend membrane whose QOEA declines signi®cantly after 120 h operation exhibits the lowest stability because of the high 7CB content. The 7CB/ EC(12/88) and THC/EC(20/80) membranes have higher stability of performance. The THC/EC(8/92) membrane shows the highest stability because of the low THC content in the membrane and also because THC has much higher molecular weight than other LCs. There is no signi®cant variation in the OEA ¯ux and oxygen concentration with the operating time ranging from 0 to 35 days. After the operating time of 35 days, the YO2 decreases dramatically from 39 to 36% which is higher than the YO2 in the OEA enriched by

Xin-Gui Li et al. / European Polymer Journal 35 (1999) 157±166

165

Fig. 12. Dynamic mechanical loss Tan d at 11 Hz and 28C/min (a) and wide-angle X-ray di€ractograms (CuKa) (b) of the virgin membranes for THC/EC(40/60), COC/EC(9/91) and EC.

poly(1-(trimethylsilyl)-1-propyne) membrane but lower than the YO2 in the OEA enriched through the polycarbonate, polysulfone, and poly(4-methylpentene-1) membranes [16±18]. But the QOEA increases from 1.15  10ÿ4 to 1.35  10ÿ4 ml(STP)/s cm2 which might result from some pinhole formed during the continuous operation at the higher pressure. It is believed that the less LC-containing homogeneous blend membranes

can maintain their defect-free feature over a working life of at least 45 days in the presence of long-term pressurization [17]. 3.7. Distribution of LC components in the matrix The LC/CDA(9/91) blend membrane appears to exhibit phase separation, to a certain extent, because

166

Xin-Gui Li et al. / European Polymer Journal 35 (1999) 157±166

there are many small adhesive and visible LC regions to the naked eye, resulting in lower air-separation performance. As shown in Tables 2, 3 and 5, the average YO2 values through LC/CDA(9/91) membranes are indeed lower than those of the LC/EC and LC/CTA membranes. Fig. 12(a) plots the dynamic mechanical loss with rising temperature for pure EC, COC/EC(9/ 91), and THC/EC(40/60) membranes. Apparently, the two blend membranes are similar to pure EC in dynamic mechanical behaviour. Only the COC/EC blend membranes exhibited broader loss peak in a temperature range of 110±1508C than the EC membrane. But when the LC content is lower than 20 wt%, LC/ EC, LC/CTA and LC/CN membranes are transparent by visual observation and have no adhesive touch. A dark ®eld in the view of the membranes was observed under polarized microscopy, indicating no assembling LC phase. The virgin LC/EC membranes exhibit similar wide-angle X-ray di€ractograms to the EC membrane, as shown in Fig. 12(b) and Ref. [7]. The addition of 40 wt% THC to EC caused the peak at the spacing of 0.46 nm to become lower and wider, indicating interaction between the THC and EC, i.e., the THC could be compatible with EC. These all suggest that the LC components might be essentially uniformly distributed in the cellulose derivative matrix except for CDA. The LCs may play a plasticizing role in the LC blend membranes and enable the membranes to exhibit higher gas permeability.

ordinary LC-free membranes. The blend membranes can give the highest oxygen-enriching capability of an OEA ¯ux of 1.0  10ÿ4 ml(STP)/s cm2 with an oxygen concentration of 40±41% under the transmembrane pressure di€erence of 0.41±0.43 MPa and 308C in a single step. The thick, self-supporting membranes show higher oxygen concentration than the composite membranes fabricated by laminating the blend thin-®lm with the same composition. These membranes may be applicable for breathing systems in medical ®elds.

Acknowledgements This investigation was supported by the National Natural Science Foundation of P.R. China and Science Technology Development Foundation of Tongji University in Shanghai China.

References [1] [2] [3] [4] [5] [6]

4. Conclusions

[7]

Liquid crystal/cellulose derivative blends have been successfully cast into homogeneous dense blend membranes with the thickness ranging between 10 and 45 mm using a solution casting technique. The oxygenenriching performance of the blend membranes is signi®cantly enhanced compared to that of the pure cellulose derivative membranes. This is veri®ed by the increase in PO2 and YO2 of the LC blend membranes in nearly the entire temperature range examined in this study. The oxygen enrichment of the blend membranes depends strongly on the membrane composition; LC/ cellulose derivative ratio and operating temperature and pressure, but the oxygen enrichment through the less LC-containing blend membranes are slightly dependent of operating time after 35 days. The oxygen enrichment of the LC blend membranes exhibits a unique dependency on temperature with a similar dependency on transmembrane pressure di€erence to

[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

Huang M-R, Li X-G. Gas Sep. Purif. 1995;9:87. Li X-G, Huang M-R. Sep. Sci. Technol. 1994;29:1905. Li X-G, Yang P-C. Polym. Bull. (Beijing) 1990;3:142. Li X-G, Huang M-R, Yang P-C. Chem. Ind. Eng. (China) 1991;8(1):16. Li X-G, Huang M-R, Technol. Water Treatment, 1994;20:99; Chem. Abst., 1995;122:1076992. Huang M-R, Li X-G. Chem. Ind. Eng. (China) 1994;11(4):15. Huang M-R, Li X-G, Lin G. Sep. Sci. Technol. 1995;30:449. Li X-G, Huang M-R, Lin G. J. Membrane Sci. 1996;116:143. Li X-G, Huang M-R, Lin G, Yang P-C. Colloid Polym. Sci. 1995;273:772. Li X-G, Huang M-R, Lin G, Yang P-C. J. Appl. Polym. Sci. 1994;51:743. Chen L, Li X-G, Wang N-C, Polym. Mater. Sci. Eng. (China), 1993;9(4):103; Chem. Abst., 1994;121:1809236; Engineering Index, 1994;32:M007281, 1994:A091493. Huang M-R, Li X-G. Plastics Ind. (China) 1993;2:49. Huang M-R, Li X-G. J. Appl. Polym. Sci. 1994;54:463. Li X-G, Huang M-R. Angew. Makromol. Chem. 1994;220:151. Li X-G, Huang M-R, Lin G, Yang P-C. Sep. Sci. Technol. 1994;29:671. Li X-G, Huang M-R, Lin G, Chen L, Yang P-C. Proceedings of 34th IUPAC Congress. Beijing, China, 1993. p. 645. Li X-G, Huang M-R. Sep. Sci. Technol. 1996;31:579. Li X-G, Huang M-R. J. Appl. Polym. Sci. 1997;66:2139.