- Email: [email protected]
liquid crystal composite membrane with unique permselective characteristics
Journal of Membrane Science, 36 (1988) 243-255 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
243
NOVEL POLYMER/LIQUID CRYSTAL COMPOSITE MEMBRANE WITH UNIQUE PERMSELECTIVE CHARACTERISTICS*
TISATO KAJIYAMA, HIROTSUGU KIKUCHI Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Hakozaki, Fukuoka 812 (Japan) and SEIJI SHINKAI Department of Synthetic Chemistry, Faculty of Engineering, Kywhu University, Hakozaki, Fukuoka 812 (Japan) (Received October 17,1986; accepted in revised form May l&1987)
Summary A series of built-up thin films composed of polymer and liquid crystal (LC) was prepared by spreading a single drop of mixture solution on a water surface. The weight fraction of LC in the composite membrane was 60%. The thickness and the aggregation state of the water-cast membrane can be controlled by the kind of solvent and the concentration of the mixture solution. Liquid crystalline material forms a continuous phase in the three-dimensional spongy network of the polymer matrix. Therefore, the liquid crystalline phase can play the role of a low viscous diffusing phase for permeants such as gases or metal ions. The novel polymer/LC composite membranes can be applied to oxygen enrichment, molecular filtration, active transport of metal cations and complete thermocontrol of ion permeation.
Introduction Permeation of gases and liquids through polymer films has been studied in relation to the chemical nature, aggregation state and thermal molecular motion of polymeric chains [ 1,2]. Thermal molecular motion of polymeric chains reflects the diffusion behavior of permeable molecules in a remarkable way. In our previous papers [ 3-61, we reported on functional membranes displaying discontinuous jumps of water or gas permeation in a temperature range corresponding to the primary relaxation process or the phase transition of the membrane polymeric material. Recently, functional characteristics of liquid crystals (LC) have been studied in many fields because of their unique orientation behavior and hydrodynamic properties. Orientation of nematic liquid crystals is easily controlled by *Paper presented at the Fifth International Symposium on Synthetic Membranes in Science and Industry, Ttibingen, F.R.G., September 2-5,1986.
0376-7388/88/$03.50
0 1988 Elsevier Science Publishers B.V.
244
applying electric or magnetic fields. The molecular axes or liquid crystals having positive dielectric anisotropy will orient along the direction of an electric field. The author has reported on a polymer/LC system as novel material for preparation of permselective membranes [ 5-81. Polymer/LC composite membranes, in which liquid crystalline material is embedded in a polymer matrix, exhibit a distinct jump in permeability since an abrupt change in thermal molecular motion occurs at the crystal-liquid crystal phase transition temperature. Therefore, the characteristic orientation and the hydrodynamic properties [ 91 of nematic liquid crystalline materials can be applied for the control of permeation of molecules or ions. The authors have also already demonstrated that K + permeation through a crown-ether-containing composite membrane can be thermocontrolled; the rate of permeation is largely governed by the dispersion state (homogeneous or phase-separated) of the immobilized crown ethers [lo]. The present work is primarily concerned with the preparation of a water-cast membrane composed of a mixture of a polymer and a liquid crystal, and also with an investigation of the permeation characteristics of the polymer/LC composite membranes. Experimental
The chemical structures of the polymers, liquid crystals, fluorocarbon monomers and crown ethers are shown in Fig. 1. A mixture of polymer, LC and a third functional material ( fluorocarbon monomer (FC 1, azobenzene-bridged crown ether ( AZO-CR) or amphiphilic crown ether (AMP-CR) ) was dissolved in a solvent mixture of tetrahydrofuran (THF) , toluene and/or chloroform. Ultrathin composite membranes composed of a secondary system (polymer/LC) or a ternary system (polymer/LC/FC, AZO-CR or AMP-CR) were prepared by a water-casting method. In the preparation of water-cast membranes the composition of the solvent mixture and the concentration of a solute were varied in order to find the optimum conditions for preparation of ultrathin membranes. A drop of a solution of PVC and EBBA (40:60 wt/wt ) in THF/toluene was spread on a water surface at 283 K. The aggregation state of PVC and EBBA was investigated by transmission electron microscopy (TEM) , X-ray spectroscopy and differential scanning calorimetry (DSC) . Thirty-thirty-five layers of water-cast membrane, each of 50 nm thick, were built up into a thin film for a study of the photoresponsive active transport of K+. Polymer/LC composite membranes were also cast on a glass plate to obtain thicker composite films to be used for the investigation of oxygen enrichment, molecular filtration and complete thermocontrol of K+ permeation. A polymer/LC/FC ternary composite membrane was prepared for oxygen enrichment studies. Perfluorotributylamine ( PFTA) and tris (lH,lH,5H-octafluoropentyl) phosphate (TPP) were chosen as the FC monomers because
245 (a)
( C)
POLYMER
l)Polycarbonate(PC)
FLUOROCARBON
l)Perfluorotributylamine(PFTA) (CF$F2CF2CF2)
,N
2)TriS(lH,lH,SH-Octafluoropentyl)phosphate(TPP) Z)Poly(vinyl
chloride)
(PVC)
tCHCH,k il
(d)
CROWN
ETHER
lIAzobenzene-bridged
(b) LIQUID CRYSTAL l)N-(4-ethoxybenzyliclene)-4'-
crown
ether(AZO-CR1
butylanilline(EBBA)
CH~CH~O -@H=N K-N
304K,
N-I
2)4-cyano-4'-pentyl
-@CHAT+, 355K biphenyl(CPBl
W’II WCtN K-N
296K,
Zjmphiphilic
N-I
crown
ethercAMP-CR)
308K
3)4-cyano-4'-heptyloxy
biphenyl(CHOB)
C~H,~O~CEN K-N
325K,
N-I
344K,4
Fig. 1. Chemical structures crown ethers (d) .
==+12(323K)
of polymers
(a), liquid crystals
(b) , fluorocarbon
monomers
(c) and
of their excellent oxygen solubility and high thermal stability. The weight ratios of PVC/EBBA/PFTA and PVC/EBBA/TPP were 40/60/7.2 and 40/60/10.8, respectively. Since the axes of 4-cyano-4’ -pentylbiphenyl ( CPB ) will orient preferentially along the direction of an applied electric field as a result of their positive dielectric anisotropy, CPB molecules were chosen to set up a permselective molecular filtration path for isomeric gases (normal-, iso- and neohydrocarbon gases). A solvent-cast membrane in which CPB molecular axes are oriented perpendicular to the membrane surface was prepared with the homemade casting apparatus shown in Fig. 2. Wide-angle X-ray diffraction measurements were performed in order to study the aggregation and orientation states of CPB molecules in the composite membrane. Permeation of isomeric butanes ( C4H10, iso-C,H,,) was measured as a function of the magnitude of the applied voltage by a volumetric method. A pair of electrodes was placed in the permeation cell and an electric field was applied perpendicular to the membrane surface during measurements of gas permeation. In order to investigate the possibility of photo-induced active transport of K+ through the polymer/LC/photoresponsive crown ether ternary composite thin film, a photoresponsive crown ether (AZO-CR) was prepared by the method previously reported [ 111. Trans-cis photoisomerization of AZO-CR in the composite thin film was confirmed by ultraviolet absorption spectros-
246 Trod,e(Cu& =wling
membrane I
I
solution
electrbde(Hg) silicon I stirrer
Fig. 2. Illustration
of the solvent cast apparatus
Fig. 3. Permeation
cell attached
to the irradiation
under application
O-ring
I
stirrer
of an electric field.
system for visible and ultraviolet
light.
copy after irradiation by UV or visible (VIS) light. The permeation experiment was carried out in a permeation cell with an UV and visible light irradiation system attached, as shown in Fig. 3. Basic and acid aqueous phases containing 1 x 10e4 Mpotassiump-toluenesulfonate were separated by the ternary composite thin film. UV and VIS irradiation took place perpendicular to the film surface from the acid and the basic aqueous phase, respectively. The K+ transport experiment was carried out at 313 K, a temperature above the crystal-liquid crystal phase transition temperature of EBBA. The variation of the concentration of K+ in the basic and acid aqueous phases was evaluated by atomic absorption spectrophotometry. In order to design a chemical switch based on flux magnitude, we have preliminarily investigated the complete thermocontrol of ion permeation through a ternary composite PC/LC/amphiphilic crown ether (AMP-CR) membrane. Preparation of the AMP-CR was described previously [lo]. The composite membranes were prepared by evaporating solvent from a dichloromethane solution. The dispersion state of AMP-CR in the composite membrane was investigated by DSC. Results and discussion
1. Preparation of polymer/liquid crystal composite thin membranes Ultrathin membranes were prepared by carefully spreading a single drop of solution on a water surface. After a considerable amount of solvent had evaporated, the ultrathin membrane was left floating on the water surface for a while. The weight ratio of PVC/EBBA was 40/60. The thickness of the watercast membrane can be controlled to a value between approximately 20 and 50 nm by varying the preparation conditions such as choice of solvent and concentration of solute. Figure 4 shows the transmission electron micrographs (TEM) of one layer
247
Fig. 4. Transmission electron micrographs of a PVC/EBBA water-cast membrane prepared under various conditions, after extraction of EBBA with methanol at 333 K.
of the composite thin membrane after extraction of EBBA with methanol at 333 K. The aggregation state and the thickness of the ultrathin membrane were affected by the casting conditions such as type of solvent, concentration of solute and water temperature. The PVC matrix was remarkably porous when the weight ratio of THF/toluene was l/2 and when at the same time the concentration of solute ranged between approximately 8 and 10 wt% as shown in the enlarged TEM photograph in Fig. 5. As the original thin membrane was nonporous before EBBA was extracted, it appears that porous parts of the membrane surrounded by PVC fibrils were filled with EBBA. Since EBBA is present as a continuous penetrating phase in the composite membrane, it can play a role as a transporting or diffusing phase for permeates or carriers. The water-cast membrane prepared using a solvent mixture with a greater fraction of THF or toluene (e.g. THF/toluene= l/l or THF/toluene= l/3) exhibited apparently phase-separated aggregates as shown in Fig. 4. The mechanism of this phase separation has not been clarified yet. Ultrathin membranes for photoresponsive active transport of K+ were prepared from a solution of PVC/LC/AZO-CR (10/15/l in weight) in a solvent mixture consisting of THF/toluene/chloroform ( 2/4/l in weight ) . The aggregation state of the ultrathin membrane was very similar to that of the
248
Fig. 5. Enlarged transmission electron micrograph of a PVC/EBBA water-cast membrane prepared from a 10 wt% solution in THF/toluene (l/2 wt/wt) , after extraction of EBBA with methanol at 333 K.
PVC/EBBA system prepared from a solution of PVC/EBBA in a THF/toluene solvent mixture. The 30 N 35 layers of the water-cast membrane of 20 - 50 nm thick were built up into a thin film for K+ permeation experiments. 2. Oxygen enrichment through the polymer/liquid crystal/fluorocarbon monomer ternary composite membrane Fluorocarbon (FC!) monomers were used to form a condensing layer of oxygen on the top surface of the composite membrane because FC monomers have a tendency to exude to the membrane surface during evaporation of solvents due to the low surface free energy. Concentration of FC monomers on the membrane surface was confirmed by an elemental depth profile based on X-ray photoelectron spectra [ 121. The degree of solubility of oxygen and ni-
249
ID0 I If’
5
I IO-
I. 0 ~EBBAIPFTA 2. •I PVCIEBBNTPP 3. c3 ~WEBBA I I 5 I rig 5 I 08
5
!& / cm31STP)cm“ d’mHg+
Fig. 6. Plots of Po,/PN, vs. PO, for PFTA-containing FC-free (curve 3) composite membranes, including polymers.
(curve 1) , TPP-containing (curve 2) and reference data for various commonly used
trogen in FC monomers was evaluated by gas chromatography after sorption of these gases in FC monomers by bubbling [ 131. The solubility coefficient of oxygen in FC monomer is about 1.5 times higher than that of nitrogen, although FC monomer has considerable affinity for both gases. In addition, it has been reported that incorporation of gases in FC monomer is reversible, obeying Henry’s law [ 7 1. Figure 6 shows plots of Po,/PN2 against PO, for PFTA-containing (curve 1) , TPP-containing (curve 2) and FC-free (curve 3) composite membranes, together with gas separation data for ordinary polymers for comparison [ 141. An increase in PO, corresponds to an increase in measurement temperature. The value of Paz for the PVC/EBBA/FC ternary composite membrane is higher than that of the FC-free membrane. A remarkable oxygen enrichment effect was observed for the ternary composite membrane containing FC monomer. The order of magnitude of PO, was lO-‘w lo-’ cm3 (STP)-cm-‘-set-‘cmHg-’ and the ratio Po,/PN, was 3.5 N 4.0 (PFTA) and 2.8~ 3.5 (TPP) in the temperature range of the nematic or isotropic state of EBBA. This indicates that FC monomers play a role in enhancing the solubility of oxygen in the composite membrane surface. The ternary composite membrane exhibited unique behavior in that the ratio Po,/P,, increased with an increase of PO, above the glass transition temperature of the matrix polymer (marked by an arrow in Fig. 6). This trend may be caused by a desirable combination of the thermal molecular motions from both matrix polymeric chains and liquid crystalline molecules [ 151. The unique relationship between Po,/PN, and PO, and their magnitudes make us expect that the composite thin film is practically applicable as oxygen enrichment membrane in the medical and engineering fields.
250 5b
-7.5 _
PVC/CPB(40/60)
dint=O.SOm
Voltage/V Fig. 7. Variation with the magnitude of the applied voltage of the permeability C,H,, and iso-&HI0 through the PVC/CPB (40/60) composite membrane.
coefficients
for
3. Motecular filtration through the polymer/liquid crystal composite membrane Since 4-cyano-4’-pentylbiphenyl (CPB) has a positive dielectric anisotropy, CPB molecules orient preferentially along the direction of an applied electric field. A pair of electrodes was placed in the permeation cell in order to apply an electric field during gas permeation measurements. A wide-angle Xray diffraction study confirmed that the CPB molecular axes aligned perpendicularly to the membrane surface when an electric field was applied. Since LC molecules aggregate as a continuous phase in the composite membrane, it may be possible to control diffusivity of gas molecules through the composite membrane by application of an electric field on the basis of the relationship between the dimensions of the channels formed in the intermolecular regions and the sectional dimensions of the permeating gases. Figure 7 shows the dependence on the applied voltage of the permeability coefficients P for C4H10 and isoC*H,,. The difference in diameter of the sections of C4H1,, and iso-C,H,, is only 0.07 nm ( dC4H,0= 0.49 nm and diao_CIHlo = 0.56 nm) . As the applied voltage was increased, the magnitude of P for both C4H10and iso-C,H,, gradually increased. The effect on P of the applied voltage can be explained by the relative difference of diffusion paths. Gas molecules diffuse along a fairly straight path in an oriented composite membrane and, on the other hand, diffuse along a tortuous path in an unoriented membrane. The separation ratio of PC4nloto Piso-CdHm 2 a, was 2 at zero volt (random orientation of liquid crystalline molecules) , and increased to 5 at 730 V (oriented state) when the space between the two electrodes was 5 mm. The increase of a! with the magnitude of the applied voltage indicates that the distribution of channel dimensions among LC molecules becomes narrower as a result of a stronger molecular orientation perpendicular to the membrane surface. This result indicates that the greater degree of orientation of LC molecules induces a more effective separation of C,H,, and iso-C,H,,, even though the difference of their molecular diameters is less than one angstrom.
251
Timelmin
Fig. 8. Time dependence of relative K+ transport from a basic aqueous phase to an acid aqueous phase following photoirradiation and in the dark.
4. Photoresponsive active transport through the polymer/liquid crystalfphotoresponsive crown ether composite thin film Active transport of potassium ion, was investigated with the PVC/LC/photoresponsive crown ether ternary composite thin film. A new photoresponsive crown ether with a crown ether ring (1%crown-6) and an ammoniumalkyl group bridged by an azobenzene group (AZO-CR) was used as K+ carrier. Trans-AZO-CR is transformed into the &s-isomer by irradiation by ultraviolet light (UV) with a wavelength of 360 nm; the latter is transformed into the former by irradiation by visible light (VIS) with a wavelength longer than 460 nm or by heating. Z’rans-AZO-CR extracts K+ effectively 40.5 times more than does cis-AZO-CR, which undergoes intramolecular complexation as shown in Fig. 1 (d-l ) when the amino tail group is protonated [ 111. Since EBBA exists as a continuous phase in the composite thin film, as mentioned before, EBBA may play a role as an effective K+ transporting or diffusing phase for AZO-CR carriers. A photoresponsive permeation experiment was carried out using a permeation cell with VIS and UV irradiation system attached as shown in Fig. 3. Figure 8 shows the time dependence of the amount of K+ transported from a basic aqueous phase into an acid aqueous phase following photoirradiation and, also, in the dark. Both acid and basic aqueous phases contained the same concentration of K+ before the start of the permeation experiment. Since there are two driving forces, photoirradiation and pH gradient, we have to evaluate the magnitudes of K+ permeation upon photoirradiation and in the dark separately, in order to be able to discuss the photoresponsive active transport of K+. It is clear from Fig. 8 that the permeation rate of K+ upon photoirradiation is greater than in the dark. This indicates that photoirradiation results in active transport of K+ from the basic aqueous phase to the acid phase through the ternary composite thin film. This is the first result as regards photo-in-
252 Plasticized PVC Membrane
t--------_-
Basic aqueous phase
lpm
-
Composite Membrane
k 60nm
Acidic aqueous phase
Fig. 9. Schematic representation of a mechanism for the active transport of K+ through a PVC/EBBA/AXO-CR composite thin film following phoroirradiation.
duced active transport of a metal cation through an organic thin film. The appearance of a slight peak in the curve of the amount of transported K+ vs. permeating time is generally observed in batch active transport experiments. This behavior may occur as a result of the balance between passive and active transport. Figure 9 shows a schematic representation of the mechanism of permeation of K+ through the PVC/EBBA/AZO-CR ternary composite thin film upon photoirradiation. At the surface of the composite thin film contacting the basic aqueous phase, trurzs-AZO-CR is stable when irradiated by VIS light. TransAZO-CR easily forms a complex with K+, the resulting complex diffusing to the opposite side (acid aqueous phase) owing to the concentration gradient of the truns-isomer in the ternary composite thin film. At the surface in contact with the acid aqueous phase, the amino tail group of AZO-CR is protonated. When protonated trans-AZO-CR transforms to the &-isomer when irradiated by UV light, the crown ether ring binds intramolecularly to the protonated amino group, so that cis-AZO-CR effectively releases K+ into the acid aqueous phase. Furthermore, cis-AZO-CR returns to the side of the basic aqueous phase, owing to its concentration gradient in the thin film, where it is again transformed into the truns-isomer by irradiation by VIS light or by heating. Even in the dark active transport of K+ was observed, as shown by the filled circles in Fig. 8. When truns-AZO-CR molecules entrapping K+ form an intermolecular complex in the vicinity of the contacting surface with an acid aqueous phase, active transport of K+ becomes possible even in the dark.
253
1.5 5 1.0? ;x 0.5%
3.1 3.2 3.3 3.4 3 5 36 103K/T
Fig. 10. Arrhenius plot of PK+ for the PC/JZBBA/AMP-CR(a) CR (b ) ( 2 ) composite membranes.
(1) and the PC/EBBA/AMP-
Since protonated cis-AZO-CR is hydrophilic, the composite film surface contacting the acid aqueous phase was coated with a plasticized thin PVC membrane of 60 nm thick in order to prevent leakage of protonated AZO-CR into the acid aqueous phase. 5. Complete thermocontrol of ion permeation through the polymer/liquid crystallamphiphilic crown ether composite membrane The composite membranes for study of complete thermocontrol of ion permeation were prepared by evaporating solvent from a dichloromethane solution. The dispersion state of the amphiphilic crown ether (AMP-CR) was investigated by differential scanning calorimetry (DSC ) . The thickness of the membrane was 60 pm. The PC/EBBA (40/60) binary composite membrane had a DSC peak at 305 K, comparable with the crystal-nematic LC phase transition of EBBA ( TKN= 304 K) . The peak was not affected by the addition of AMP-CR(a) ( R=CH3( CH,) 1,,-) (2.9 mol% EBBA) to the composite membrane. In contrast, the PC/AMP-CR (b ) (R= ( CHB( CHZ) 150CH2 ) 2CHOCH,-) binary composite membrane (2.9 mol% EBBA) showed a new peak, in addition to the one of EBBA, at 313 K, which is very close to a DSC peak of AMP-CR (b ) in water (T,=317 K) [ 151. The DSC results support the assumption that AMP-CR (a) is homogeneously dispersed in the composite membrane, whereas AMP-CR (b ) exists as phase-separated aggregates not only in water but also in the composite membrane. Figure 10 shows the dependence of the K+ permeability, Pk+ , on the reciprocal absolute temperature. Permeation of K+ through the PC/EBBA/AMPCR (a) composite membrane was observed both below and above TKN,and the plot of Pk+ vs. reciprocal absolute temperature ( Arrhenius plot) exhibits a break point at around T KN. From this Arrhenius plot, the magnitude of the
254
activation energy for K+ transport was evaluated: E,=1.5 kJ-mol-l below TKN and 2.1 kJ-mol-’ above TKN. Carrier-mediated K+ permeation must be directly affected by the thermal motion of LC molecules. In contrast, K+ permeation through the PC/EBBA/AMP-CR (b) composite membrane was completely suppressed below TKNand increased with increasing temperature above TKN;the magnitude of E, was 3.6 kJ-mol-l. It can be concluded that ion permeation below TKN is largely governed by the dispersion state of the carrier and that the rate of K+ permeation can be controlled sensitively by a thermoswitch. Conclusion
Composite membranes composed of polymer, liquid crystal ( LC ) and a third functional material (FC, AZO-CR, AMP-CR) were prepared by solvent-casting and water-casting methods. The membrane thickness and the aggregation state are strongly dependent on the kind of solvent and on solution concentration. The thickness of a water-cast membrane can be controlled to range from about 20 to about 50 nm. Since LC material forms a continuous phase with very low viscosity in the composite membrane containing 60 wt% LC, the LC phase may play a role as transferring or diffusing phase for permeants such as gases or metal ions. FC monomer may enhance oxygen solubility in the surface of the composite membrane. The ternary composite membrane exhibited the unique behavior that Po,/PN, increased with an increase in PO2 at about the glass transition temperature of the matrix polymer. Using the intermolecular regions (channels) between LC molecules oriented under application of an electric field, the polymer/LC composite membrane can be applied as a molecular filtration membrane for separation of isomeric gases. Since a crown ether cation carrier shows a marked difference in metal-binding ability on the two film surfaces depending on the trarzs-cis conformations induced by photoirradiation, active transport of K+ through the polymer/LC/AZO-CR composite thin film occurs upon photoirradiation. Ion permeation through the ternary composite membrane containing an amphiphilic crown ether (AMP-CR) can be completely thermocontrolled depending on the dispersion state of AMP-CR. A composite membrane with phase-separated aggregates of AMP-CR can be used as a thermoswitch because of the complete suppression of K+ permeation below the crystal-nematic LC phase transition temperature.
References 1
A. Takizawa, T. Hamada, H. Okada, S. Imai and S. Kadota, Permeability of a series of alcohols through poly(methyl-~-glutamate), Polymer, 15 (1974) 157.
255 2
V.A. Tochin, R.A. Shlyakhov and D.N. Sapoxhnikov, Diffusion of oxygen in polyethylene at low temperatures, Polym. Sci. U.S.S.R., 17 (1976) 2548. 3 T. Kajiyama, Y. Nagata, E. Maemura and M. Takayanagi, Molecular motion-permeability relationships in polycarbonate/liquid crystal (EBBA) composite membrane, Chem. Lett., 1979 (1979) 679. 4 T. Kajiyama, Gas and liquid permeabilities through polymer/liquid crystal blend membranes, Membrane, 4 (1979) 299; Membrane structure and permeation property of polymer/liquid crystal composite, Membrane, 6 (1981) 265. 5 T. Kajiyama, Y. Nagata, S. Washizu and M. Takayanagi, Characterization and gas permeation of polycarbonate/liquid crystal composite membrane, J. Membrane Sci., 11 (1982) 39. 6 S. Washizu, I. Terada, T. Kajiyama and M. Takayanagi, Gas permeation through polymer/ liquid crystal composite membrane, Polym. J., 16 (1984) 307. T. Kajiyama, S. Washizu and M. Takayanagi, Membrane structure and permeation properties of poly (vinyl chloride) /liquid crystal composite membrane, J. Appl. Polym. Sci., 29 (1984) 3955. 8 T. Kajiyama, S. Washizu, A. Kumano, I. Terada and M. Takayanagi, Permselective characteristics of polymer/liquid crystal composite membrane, J. Appl. Polym. Sci., Appl. Polym. Symp., 41 (1985) 327. 9 G.W. Gray and P.A. Winsor, Liquid Crystals and Plastic Crystals, John Wiley and Sons, London, 1974. 10 S. Shinkai, S. Nakamura, S. Tachiki, 0. Manabe and T. Kajiyama, Thermocontrol of ion permeation through composite membranes composed of polymer/liquid crystal/amphiphilic crown ethers, J. Amer. Chem. Sot., 107 (1985) 3363. 11 S. Shinkai, M. Ishihara, K. Ueda and 0. Manabe, On-off-switched crown ether-metal ion complexation by photoinduced intramolecular ammonium group ‘tail-biting’, J. Chem. Sot., Chem. Commun., 1984 (1984) 727. 12 A. Takahara, T. Kajiyama, N. Higashi and T. Kunitake, Aggregation state and surface chemical composition of composite thin films composed of poly (vinyl alcohol) and fluorocarbon amphiphiles, Macromolecules, in press. 13 Y. Ohomori and T. Kajiyama, Oxygen enrichment effect of polymer/liquid crystal composite membrane containing fluorocarbon, J. Chem. Sot. Jpn., Chem. Ind. Chem., 1985 (1985) 1897. 14 V.T. Stannett, W.J. Koros, D.R. Paul, H.K. Lonsdale and R.W. Baker, Recent advances in membrane science and technology, Adv. Polym. Sci., 32 (1979) 69. 15 T. Kajiyama, S. Washizu and Y. Ohomori, Oxygen permselective characteristics of poly (vinyl chloride) /liquid crystal/fluorocarbon ternary composite membrane, J. Membrane Sci., 24 (1985) 73.
243
NOVEL POLYMER/LIQUID CRYSTAL COMPOSITE MEMBRANE WITH UNIQUE PERMSELECTIVE CHARACTERISTICS*
TISATO KAJIYAMA, HIROTSUGU KIKUCHI Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Hakozaki, Fukuoka 812 (Japan) and SEIJI SHINKAI Department of Synthetic Chemistry, Faculty of Engineering, Kywhu University, Hakozaki, Fukuoka 812 (Japan) (Received October 17,1986; accepted in revised form May l&1987)
Summary A series of built-up thin films composed of polymer and liquid crystal (LC) was prepared by spreading a single drop of mixture solution on a water surface. The weight fraction of LC in the composite membrane was 60%. The thickness and the aggregation state of the water-cast membrane can be controlled by the kind of solvent and the concentration of the mixture solution. Liquid crystalline material forms a continuous phase in the three-dimensional spongy network of the polymer matrix. Therefore, the liquid crystalline phase can play the role of a low viscous diffusing phase for permeants such as gases or metal ions. The novel polymer/LC composite membranes can be applied to oxygen enrichment, molecular filtration, active transport of metal cations and complete thermocontrol of ion permeation.
Introduction Permeation of gases and liquids through polymer films has been studied in relation to the chemical nature, aggregation state and thermal molecular motion of polymeric chains [ 1,2]. Thermal molecular motion of polymeric chains reflects the diffusion behavior of permeable molecules in a remarkable way. In our previous papers [ 3-61, we reported on functional membranes displaying discontinuous jumps of water or gas permeation in a temperature range corresponding to the primary relaxation process or the phase transition of the membrane polymeric material. Recently, functional characteristics of liquid crystals (LC) have been studied in many fields because of their unique orientation behavior and hydrodynamic properties. Orientation of nematic liquid crystals is easily controlled by *Paper presented at the Fifth International Symposium on Synthetic Membranes in Science and Industry, Ttibingen, F.R.G., September 2-5,1986.
0376-7388/88/$03.50
0 1988 Elsevier Science Publishers B.V.
244
applying electric or magnetic fields. The molecular axes or liquid crystals having positive dielectric anisotropy will orient along the direction of an electric field. The author has reported on a polymer/LC system as novel material for preparation of permselective membranes [ 5-81. Polymer/LC composite membranes, in which liquid crystalline material is embedded in a polymer matrix, exhibit a distinct jump in permeability since an abrupt change in thermal molecular motion occurs at the crystal-liquid crystal phase transition temperature. Therefore, the characteristic orientation and the hydrodynamic properties [ 91 of nematic liquid crystalline materials can be applied for the control of permeation of molecules or ions. The authors have also already demonstrated that K + permeation through a crown-ether-containing composite membrane can be thermocontrolled; the rate of permeation is largely governed by the dispersion state (homogeneous or phase-separated) of the immobilized crown ethers [lo]. The present work is primarily concerned with the preparation of a water-cast membrane composed of a mixture of a polymer and a liquid crystal, and also with an investigation of the permeation characteristics of the polymer/LC composite membranes. Experimental
The chemical structures of the polymers, liquid crystals, fluorocarbon monomers and crown ethers are shown in Fig. 1. A mixture of polymer, LC and a third functional material ( fluorocarbon monomer (FC 1, azobenzene-bridged crown ether ( AZO-CR) or amphiphilic crown ether (AMP-CR) ) was dissolved in a solvent mixture of tetrahydrofuran (THF) , toluene and/or chloroform. Ultrathin composite membranes composed of a secondary system (polymer/LC) or a ternary system (polymer/LC/FC, AZO-CR or AMP-CR) were prepared by a water-casting method. In the preparation of water-cast membranes the composition of the solvent mixture and the concentration of a solute were varied in order to find the optimum conditions for preparation of ultrathin membranes. A drop of a solution of PVC and EBBA (40:60 wt/wt ) in THF/toluene was spread on a water surface at 283 K. The aggregation state of PVC and EBBA was investigated by transmission electron microscopy (TEM) , X-ray spectroscopy and differential scanning calorimetry (DSC) . Thirty-thirty-five layers of water-cast membrane, each of 50 nm thick, were built up into a thin film for a study of the photoresponsive active transport of K+. Polymer/LC composite membranes were also cast on a glass plate to obtain thicker composite films to be used for the investigation of oxygen enrichment, molecular filtration and complete thermocontrol of K+ permeation. A polymer/LC/FC ternary composite membrane was prepared for oxygen enrichment studies. Perfluorotributylamine ( PFTA) and tris (lH,lH,5H-octafluoropentyl) phosphate (TPP) were chosen as the FC monomers because
245 (a)
( C)
POLYMER
l)Polycarbonate(PC)
FLUOROCARBON
l)Perfluorotributylamine(PFTA) (CF$F2CF2CF2)
,N
2)TriS(lH,lH,SH-Octafluoropentyl)phosphate(TPP) Z)Poly(vinyl
chloride)
(PVC)
tCHCH,k il
(d)
CROWN
ETHER
lIAzobenzene-bridged
(b) LIQUID CRYSTAL l)N-(4-ethoxybenzyliclene)-4'-
crown
ether(AZO-CR1
butylanilline(EBBA)
CH~CH~O -@H=N K-N
304K,
N-I
2)4-cyano-4'-pentyl
-@CHAT+, 355K biphenyl(CPBl
W’II WCtN K-N
296K,
Zjmphiphilic
N-I
crown
ethercAMP-CR)
308K
3)4-cyano-4'-heptyloxy
biphenyl(CHOB)
C~H,~O~CEN K-N
325K,
N-I
344K,4
Fig. 1. Chemical structures crown ethers (d) .
==+12(323K)
of polymers
(a), liquid crystals
(b) , fluorocarbon
monomers
(c) and
of their excellent oxygen solubility and high thermal stability. The weight ratios of PVC/EBBA/PFTA and PVC/EBBA/TPP were 40/60/7.2 and 40/60/10.8, respectively. Since the axes of 4-cyano-4’ -pentylbiphenyl ( CPB ) will orient preferentially along the direction of an applied electric field as a result of their positive dielectric anisotropy, CPB molecules were chosen to set up a permselective molecular filtration path for isomeric gases (normal-, iso- and neohydrocarbon gases). A solvent-cast membrane in which CPB molecular axes are oriented perpendicular to the membrane surface was prepared with the homemade casting apparatus shown in Fig. 2. Wide-angle X-ray diffraction measurements were performed in order to study the aggregation and orientation states of CPB molecules in the composite membrane. Permeation of isomeric butanes ( C4H10, iso-C,H,,) was measured as a function of the magnitude of the applied voltage by a volumetric method. A pair of electrodes was placed in the permeation cell and an electric field was applied perpendicular to the membrane surface during measurements of gas permeation. In order to investigate the possibility of photo-induced active transport of K+ through the polymer/LC/photoresponsive crown ether ternary composite thin film, a photoresponsive crown ether (AZO-CR) was prepared by the method previously reported [ 111. Trans-cis photoisomerization of AZO-CR in the composite thin film was confirmed by ultraviolet absorption spectros-
246 Trod,e(Cu& =wling
membrane I
I
solution
electrbde(Hg) silicon I stirrer
Fig. 2. Illustration
of the solvent cast apparatus
Fig. 3. Permeation
cell attached
to the irradiation
under application
O-ring
I
stirrer
of an electric field.
system for visible and ultraviolet
light.
copy after irradiation by UV or visible (VIS) light. The permeation experiment was carried out in a permeation cell with an UV and visible light irradiation system attached, as shown in Fig. 3. Basic and acid aqueous phases containing 1 x 10e4 Mpotassiump-toluenesulfonate were separated by the ternary composite thin film. UV and VIS irradiation took place perpendicular to the film surface from the acid and the basic aqueous phase, respectively. The K+ transport experiment was carried out at 313 K, a temperature above the crystal-liquid crystal phase transition temperature of EBBA. The variation of the concentration of K+ in the basic and acid aqueous phases was evaluated by atomic absorption spectrophotometry. In order to design a chemical switch based on flux magnitude, we have preliminarily investigated the complete thermocontrol of ion permeation through a ternary composite PC/LC/amphiphilic crown ether (AMP-CR) membrane. Preparation of the AMP-CR was described previously [lo]. The composite membranes were prepared by evaporating solvent from a dichloromethane solution. The dispersion state of AMP-CR in the composite membrane was investigated by DSC. Results and discussion
1. Preparation of polymer/liquid crystal composite thin membranes Ultrathin membranes were prepared by carefully spreading a single drop of solution on a water surface. After a considerable amount of solvent had evaporated, the ultrathin membrane was left floating on the water surface for a while. The weight ratio of PVC/EBBA was 40/60. The thickness of the watercast membrane can be controlled to a value between approximately 20 and 50 nm by varying the preparation conditions such as choice of solvent and concentration of solute. Figure 4 shows the transmission electron micrographs (TEM) of one layer
247
Fig. 4. Transmission electron micrographs of a PVC/EBBA water-cast membrane prepared under various conditions, after extraction of EBBA with methanol at 333 K.
of the composite thin membrane after extraction of EBBA with methanol at 333 K. The aggregation state and the thickness of the ultrathin membrane were affected by the casting conditions such as type of solvent, concentration of solute and water temperature. The PVC matrix was remarkably porous when the weight ratio of THF/toluene was l/2 and when at the same time the concentration of solute ranged between approximately 8 and 10 wt% as shown in the enlarged TEM photograph in Fig. 5. As the original thin membrane was nonporous before EBBA was extracted, it appears that porous parts of the membrane surrounded by PVC fibrils were filled with EBBA. Since EBBA is present as a continuous penetrating phase in the composite membrane, it can play a role as a transporting or diffusing phase for permeates or carriers. The water-cast membrane prepared using a solvent mixture with a greater fraction of THF or toluene (e.g. THF/toluene= l/l or THF/toluene= l/3) exhibited apparently phase-separated aggregates as shown in Fig. 4. The mechanism of this phase separation has not been clarified yet. Ultrathin membranes for photoresponsive active transport of K+ were prepared from a solution of PVC/LC/AZO-CR (10/15/l in weight) in a solvent mixture consisting of THF/toluene/chloroform ( 2/4/l in weight ) . The aggregation state of the ultrathin membrane was very similar to that of the
248
Fig. 5. Enlarged transmission electron micrograph of a PVC/EBBA water-cast membrane prepared from a 10 wt% solution in THF/toluene (l/2 wt/wt) , after extraction of EBBA with methanol at 333 K.
PVC/EBBA system prepared from a solution of PVC/EBBA in a THF/toluene solvent mixture. The 30 N 35 layers of the water-cast membrane of 20 - 50 nm thick were built up into a thin film for K+ permeation experiments. 2. Oxygen enrichment through the polymer/liquid crystal/fluorocarbon monomer ternary composite membrane Fluorocarbon (FC!) monomers were used to form a condensing layer of oxygen on the top surface of the composite membrane because FC monomers have a tendency to exude to the membrane surface during evaporation of solvents due to the low surface free energy. Concentration of FC monomers on the membrane surface was confirmed by an elemental depth profile based on X-ray photoelectron spectra [ 121. The degree of solubility of oxygen and ni-
249
ID0 I If’
5
I IO-
I. 0 ~EBBAIPFTA 2. •I PVCIEBBNTPP 3. c3 ~WEBBA I I 5 I rig 5 I 08
5
!& / cm31STP)cm“ d’mHg+
Fig. 6. Plots of Po,/PN, vs. PO, for PFTA-containing FC-free (curve 3) composite membranes, including polymers.
(curve 1) , TPP-containing (curve 2) and reference data for various commonly used
trogen in FC monomers was evaluated by gas chromatography after sorption of these gases in FC monomers by bubbling [ 131. The solubility coefficient of oxygen in FC monomer is about 1.5 times higher than that of nitrogen, although FC monomer has considerable affinity for both gases. In addition, it has been reported that incorporation of gases in FC monomer is reversible, obeying Henry’s law [ 7 1. Figure 6 shows plots of Po,/PN2 against PO, for PFTA-containing (curve 1) , TPP-containing (curve 2) and FC-free (curve 3) composite membranes, together with gas separation data for ordinary polymers for comparison [ 141. An increase in PO, corresponds to an increase in measurement temperature. The value of Paz for the PVC/EBBA/FC ternary composite membrane is higher than that of the FC-free membrane. A remarkable oxygen enrichment effect was observed for the ternary composite membrane containing FC monomer. The order of magnitude of PO, was lO-‘w lo-’ cm3 (STP)-cm-‘-set-‘cmHg-’ and the ratio Po,/PN, was 3.5 N 4.0 (PFTA) and 2.8~ 3.5 (TPP) in the temperature range of the nematic or isotropic state of EBBA. This indicates that FC monomers play a role in enhancing the solubility of oxygen in the composite membrane surface. The ternary composite membrane exhibited unique behavior in that the ratio Po,/P,, increased with an increase of PO, above the glass transition temperature of the matrix polymer (marked by an arrow in Fig. 6). This trend may be caused by a desirable combination of the thermal molecular motions from both matrix polymeric chains and liquid crystalline molecules [ 151. The unique relationship between Po,/PN, and PO, and their magnitudes make us expect that the composite thin film is practically applicable as oxygen enrichment membrane in the medical and engineering fields.
250 5b
-7.5 _
PVC/CPB(40/60)
dint=O.SOm
Voltage/V Fig. 7. Variation with the magnitude of the applied voltage of the permeability C,H,, and iso-&HI0 through the PVC/CPB (40/60) composite membrane.
coefficients
for
3. Motecular filtration through the polymer/liquid crystal composite membrane Since 4-cyano-4’-pentylbiphenyl (CPB) has a positive dielectric anisotropy, CPB molecules orient preferentially along the direction of an applied electric field. A pair of electrodes was placed in the permeation cell in order to apply an electric field during gas permeation measurements. A wide-angle Xray diffraction study confirmed that the CPB molecular axes aligned perpendicularly to the membrane surface when an electric field was applied. Since LC molecules aggregate as a continuous phase in the composite membrane, it may be possible to control diffusivity of gas molecules through the composite membrane by application of an electric field on the basis of the relationship between the dimensions of the channels formed in the intermolecular regions and the sectional dimensions of the permeating gases. Figure 7 shows the dependence on the applied voltage of the permeability coefficients P for C4H10 and isoC*H,,. The difference in diameter of the sections of C4H1,, and iso-C,H,, is only 0.07 nm ( dC4H,0= 0.49 nm and diao_CIHlo = 0.56 nm) . As the applied voltage was increased, the magnitude of P for both C4H10and iso-C,H,, gradually increased. The effect on P of the applied voltage can be explained by the relative difference of diffusion paths. Gas molecules diffuse along a fairly straight path in an oriented composite membrane and, on the other hand, diffuse along a tortuous path in an unoriented membrane. The separation ratio of PC4nloto Piso-CdHm 2 a, was 2 at zero volt (random orientation of liquid crystalline molecules) , and increased to 5 at 730 V (oriented state) when the space between the two electrodes was 5 mm. The increase of a! with the magnitude of the applied voltage indicates that the distribution of channel dimensions among LC molecules becomes narrower as a result of a stronger molecular orientation perpendicular to the membrane surface. This result indicates that the greater degree of orientation of LC molecules induces a more effective separation of C,H,, and iso-C,H,,, even though the difference of their molecular diameters is less than one angstrom.
251
Timelmin
Fig. 8. Time dependence of relative K+ transport from a basic aqueous phase to an acid aqueous phase following photoirradiation and in the dark.
4. Photoresponsive active transport through the polymer/liquid crystalfphotoresponsive crown ether composite thin film Active transport of potassium ion, was investigated with the PVC/LC/photoresponsive crown ether ternary composite thin film. A new photoresponsive crown ether with a crown ether ring (1%crown-6) and an ammoniumalkyl group bridged by an azobenzene group (AZO-CR) was used as K+ carrier. Trans-AZO-CR is transformed into the &s-isomer by irradiation by ultraviolet light (UV) with a wavelength of 360 nm; the latter is transformed into the former by irradiation by visible light (VIS) with a wavelength longer than 460 nm or by heating. Z’rans-AZO-CR extracts K+ effectively 40.5 times more than does cis-AZO-CR, which undergoes intramolecular complexation as shown in Fig. 1 (d-l ) when the amino tail group is protonated [ 111. Since EBBA exists as a continuous phase in the composite thin film, as mentioned before, EBBA may play a role as an effective K+ transporting or diffusing phase for AZO-CR carriers. A photoresponsive permeation experiment was carried out using a permeation cell with VIS and UV irradiation system attached as shown in Fig. 3. Figure 8 shows the time dependence of the amount of K+ transported from a basic aqueous phase into an acid aqueous phase following photoirradiation and, also, in the dark. Both acid and basic aqueous phases contained the same concentration of K+ before the start of the permeation experiment. Since there are two driving forces, photoirradiation and pH gradient, we have to evaluate the magnitudes of K+ permeation upon photoirradiation and in the dark separately, in order to be able to discuss the photoresponsive active transport of K+. It is clear from Fig. 8 that the permeation rate of K+ upon photoirradiation is greater than in the dark. This indicates that photoirradiation results in active transport of K+ from the basic aqueous phase to the acid phase through the ternary composite thin film. This is the first result as regards photo-in-
252 Plasticized PVC Membrane
t--------_-
Basic aqueous phase
lpm
-
Composite Membrane
k 60nm
Acidic aqueous phase
Fig. 9. Schematic representation of a mechanism for the active transport of K+ through a PVC/EBBA/AXO-CR composite thin film following phoroirradiation.
duced active transport of a metal cation through an organic thin film. The appearance of a slight peak in the curve of the amount of transported K+ vs. permeating time is generally observed in batch active transport experiments. This behavior may occur as a result of the balance between passive and active transport. Figure 9 shows a schematic representation of the mechanism of permeation of K+ through the PVC/EBBA/AZO-CR ternary composite thin film upon photoirradiation. At the surface of the composite thin film contacting the basic aqueous phase, trurzs-AZO-CR is stable when irradiated by VIS light. TransAZO-CR easily forms a complex with K+, the resulting complex diffusing to the opposite side (acid aqueous phase) owing to the concentration gradient of the truns-isomer in the ternary composite thin film. At the surface in contact with the acid aqueous phase, the amino tail group of AZO-CR is protonated. When protonated trans-AZO-CR transforms to the &-isomer when irradiated by UV light, the crown ether ring binds intramolecularly to the protonated amino group, so that cis-AZO-CR effectively releases K+ into the acid aqueous phase. Furthermore, cis-AZO-CR returns to the side of the basic aqueous phase, owing to its concentration gradient in the thin film, where it is again transformed into the truns-isomer by irradiation by VIS light or by heating. Even in the dark active transport of K+ was observed, as shown by the filled circles in Fig. 8. When truns-AZO-CR molecules entrapping K+ form an intermolecular complex in the vicinity of the contacting surface with an acid aqueous phase, active transport of K+ becomes possible even in the dark.
253
1.5 5 1.0? ;x 0.5%
3.1 3.2 3.3 3.4 3 5 36 103K/T
Fig. 10. Arrhenius plot of PK+ for the PC/JZBBA/AMP-CR(a) CR (b ) ( 2 ) composite membranes.
(1) and the PC/EBBA/AMP-
Since protonated cis-AZO-CR is hydrophilic, the composite film surface contacting the acid aqueous phase was coated with a plasticized thin PVC membrane of 60 nm thick in order to prevent leakage of protonated AZO-CR into the acid aqueous phase. 5. Complete thermocontrol of ion permeation through the polymer/liquid crystallamphiphilic crown ether composite membrane The composite membranes for study of complete thermocontrol of ion permeation were prepared by evaporating solvent from a dichloromethane solution. The dispersion state of the amphiphilic crown ether (AMP-CR) was investigated by differential scanning calorimetry (DSC ) . The thickness of the membrane was 60 pm. The PC/EBBA (40/60) binary composite membrane had a DSC peak at 305 K, comparable with the crystal-nematic LC phase transition of EBBA ( TKN= 304 K) . The peak was not affected by the addition of AMP-CR(a) ( R=CH3( CH,) 1,,-) (2.9 mol% EBBA) to the composite membrane. In contrast, the PC/AMP-CR (b ) (R= ( CHB( CHZ) 150CH2 ) 2CHOCH,-) binary composite membrane (2.9 mol% EBBA) showed a new peak, in addition to the one of EBBA, at 313 K, which is very close to a DSC peak of AMP-CR (b ) in water (T,=317 K) [ 151. The DSC results support the assumption that AMP-CR (a) is homogeneously dispersed in the composite membrane, whereas AMP-CR (b ) exists as phase-separated aggregates not only in water but also in the composite membrane. Figure 10 shows the dependence of the K+ permeability, Pk+ , on the reciprocal absolute temperature. Permeation of K+ through the PC/EBBA/AMPCR (a) composite membrane was observed both below and above TKN,and the plot of Pk+ vs. reciprocal absolute temperature ( Arrhenius plot) exhibits a break point at around T KN. From this Arrhenius plot, the magnitude of the
254
activation energy for K+ transport was evaluated: E,=1.5 kJ-mol-l below TKN and 2.1 kJ-mol-’ above TKN. Carrier-mediated K+ permeation must be directly affected by the thermal motion of LC molecules. In contrast, K+ permeation through the PC/EBBA/AMP-CR (b) composite membrane was completely suppressed below TKNand increased with increasing temperature above TKN;the magnitude of E, was 3.6 kJ-mol-l. It can be concluded that ion permeation below TKN is largely governed by the dispersion state of the carrier and that the rate of K+ permeation can be controlled sensitively by a thermoswitch. Conclusion
Composite membranes composed of polymer, liquid crystal ( LC ) and a third functional material (FC, AZO-CR, AMP-CR) were prepared by solvent-casting and water-casting methods. The membrane thickness and the aggregation state are strongly dependent on the kind of solvent and on solution concentration. The thickness of a water-cast membrane can be controlled to range from about 20 to about 50 nm. Since LC material forms a continuous phase with very low viscosity in the composite membrane containing 60 wt% LC, the LC phase may play a role as transferring or diffusing phase for permeants such as gases or metal ions. FC monomer may enhance oxygen solubility in the surface of the composite membrane. The ternary composite membrane exhibited the unique behavior that Po,/PN, increased with an increase in PO2 at about the glass transition temperature of the matrix polymer. Using the intermolecular regions (channels) between LC molecules oriented under application of an electric field, the polymer/LC composite membrane can be applied as a molecular filtration membrane for separation of isomeric gases. Since a crown ether cation carrier shows a marked difference in metal-binding ability on the two film surfaces depending on the trarzs-cis conformations induced by photoirradiation, active transport of K+ through the polymer/LC/AZO-CR composite thin film occurs upon photoirradiation. Ion permeation through the ternary composite membrane containing an amphiphilic crown ether (AMP-CR) can be completely thermocontrolled depending on the dispersion state of AMP-CR. A composite membrane with phase-separated aggregates of AMP-CR can be used as a thermoswitch because of the complete suppression of K+ permeation below the crystal-nematic LC phase transition temperature.
References 1
A. Takizawa, T. Hamada, H. Okada, S. Imai and S. Kadota, Permeability of a series of alcohols through poly(methyl-~-glutamate), Polymer, 15 (1974) 157.
255 2
V.A. Tochin, R.A. Shlyakhov and D.N. Sapoxhnikov, Diffusion of oxygen in polyethylene at low temperatures, Polym. Sci. U.S.S.R., 17 (1976) 2548. 3 T. Kajiyama, Y. Nagata, E. Maemura and M. Takayanagi, Molecular motion-permeability relationships in polycarbonate/liquid crystal (EBBA) composite membrane, Chem. Lett., 1979 (1979) 679. 4 T. Kajiyama, Gas and liquid permeabilities through polymer/liquid crystal blend membranes, Membrane, 4 (1979) 299; Membrane structure and permeation property of polymer/liquid crystal composite, Membrane, 6 (1981) 265. 5 T. Kajiyama, Y. Nagata, S. Washizu and M. Takayanagi, Characterization and gas permeation of polycarbonate/liquid crystal composite membrane, J. Membrane Sci., 11 (1982) 39. 6 S. Washizu, I. Terada, T. Kajiyama and M. Takayanagi, Gas permeation through polymer/ liquid crystal composite membrane, Polym. J., 16 (1984) 307. T. Kajiyama, S. Washizu and M. Takayanagi, Membrane structure and permeation properties of poly (vinyl chloride) /liquid crystal composite membrane, J. Appl. Polym. Sci., 29 (1984) 3955. 8 T. Kajiyama, S. Washizu, A. Kumano, I. Terada and M. Takayanagi, Permselective characteristics of polymer/liquid crystal composite membrane, J. Appl. Polym. Sci., Appl. Polym. Symp., 41 (1985) 327. 9 G.W. Gray and P.A. Winsor, Liquid Crystals and Plastic Crystals, John Wiley and Sons, London, 1974. 10 S. Shinkai, S. Nakamura, S. Tachiki, 0. Manabe and T. Kajiyama, Thermocontrol of ion permeation through composite membranes composed of polymer/liquid crystal/amphiphilic crown ethers, J. Amer. Chem. Sot., 107 (1985) 3363. 11 S. Shinkai, M. Ishihara, K. Ueda and 0. Manabe, On-off-switched crown ether-metal ion complexation by photoinduced intramolecular ammonium group ‘tail-biting’, J. Chem. Sot., Chem. Commun., 1984 (1984) 727. 12 A. Takahara, T. Kajiyama, N. Higashi and T. Kunitake, Aggregation state and surface chemical composition of composite thin films composed of poly (vinyl alcohol) and fluorocarbon amphiphiles, Macromolecules, in press. 13 Y. Ohomori and T. Kajiyama, Oxygen enrichment effect of polymer/liquid crystal composite membrane containing fluorocarbon, J. Chem. Sot. Jpn., Chem. Ind. Chem., 1985 (1985) 1897. 14 V.T. Stannett, W.J. Koros, D.R. Paul, H.K. Lonsdale and R.W. Baker, Recent advances in membrane science and technology, Adv. Polym. Sci., 32 (1979) 69. 15 T. Kajiyama, S. Washizu and Y. Ohomori, Oxygen permselective characteristics of poly (vinyl chloride) /liquid crystal/fluorocarbon ternary composite membrane, J. Membrane Sci., 24 (1985) 73.