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Surface characterization of ethylene-vinyl alcohol copolymer membranes prepared under various conditions
Journal of Membrane Science, 15 (1992) 163-170
163
Elsevier Science Publishers B.V., Amsterdam
Surface characterization of ethylene-vinyl alcohol copolymer membranes prepared under various conditions Katsuhiko
Nakamae,
Takashi
Miyata and Tsunetaka
Matsumoto
Department of Industrial Chemistry, Faculty of Engineering, Kobe University, Rokko, Nada, Kobe 657 (Japan)
(Received January 17,199l; accepted in revised form July 31,1992)
Abstract Surface characteristics of EVA membranes were investigated using ESCA. The membranes were prepared by dipping polymer solution cast on various substrates into a coagulation bath (the phase-inversion method). The ESCA results demonstrated that there were significant differences in the surface composition of EVA membranes prepared under various conditions. The more hydrophobic the substrate, the more the substrate side surface was enriched in the ethylene component. Thus the surface characteristics can be controlled by the kind of the substrate without changing the membrane structure such as pore size. For membranes subjected to hot water treatment, the composition of the substratecontacting surface gradually approached that of the water side surface with increasing temperature. These results suggest that the ethylene and vinyl alcohol components migrated in order to lower the surface free energy at the surface of the membrane. Keywords: membrane
preparation
and structure; ESCA; surface characteristics;
ethylene-vinyl
alcohol
copolymer
1. Introduction Polymeric membranes are utilized in water treatment from the point of economization in power and environmental sanitation. Most of them are asymmetric membranes of the type described by Loeb and Sourirajan [ 11,and are prepared by the phase-inversion method. The asymmetric membrane consists of a dense “skin layer” and a porous “substrate layer”. High permselectivity and rapid permeation are atCorrespondence to: Prof. K. Nakamae, Department of Industrial Chemistry, Faculty of Engineering, Kobe University, Rokko, Nada, Kobe 657, Japan.
0376-7388/92/$05.00
tained by the asymmetric structure. Ethylene-vinyl alcohol copolymer (EVA) has hydrophilic vinyl alcohol segments and hydrophobic ethylene segments in each molecule. We have previously investigated the physical and structural properties of EVA, such as crystallinity [ 21, fine structure [ 3,4], solubility [ 51, surface potential [ 61, and viscoelasticity [ 71. We have also studied the EVA membranes prepared by spreading the polymer solution on a water surface [ 8,9] and by dipping the solution cast on a glass substrate into water [lo]. These membranes were also asymmetric and they had both high permselectivity and rapid permeation.
0 1992 Elsevier Science Publishers B.V. All rights reserved.
164
Recently many investigations on various immobilized enzymes have been performed, and membranes have been used as a support for the immobilization of enzymes. In this respect the surface characteristics of the membrane are very important. In the polymer materials, the characteristics at the surface are different from those in the bulk phase [ 11-131. Therefore the surface characteristics are interesting in a membrane consisting of amphiphilic copolymers such as EVA. The distribution of the functional groups in the membrane is also thought to influence the properties of the membrane. For example, EVA membranes are used for blood dialysis and there it is required that little protein is adsorbed on the surface of the membrane. Thus, the surface characteristics of the membrane must be controlled in accordance with its utilization. Electron spectroscopy for chemical analysis (ESCA) has been used as a technique for studying such surface characteristics. Many polymers have been elaborated with ESCA by D.T. Clark et al. [ 141. The utility of ESCA to elucidate changes in the structure and chemical features of a modified surface is well substantiated. In this study, the surface characteristics, especially the location of the functional groups, of EVA membranes prepared under various conditions were investigated using ESCA. 2. Experimental 2.1.Materials Ethylene-vinyl alcohol copolymer (EVA) had a vinyl alcohol content of 67 mol% and a degree of polymerization of 1100 (Kuraray Co., Ltd.), and the degree of saponification of the vinyl acetate was more than 98%. Dimethylacetamide (DMAc ) as the solvent was obtained from Nakarai Co., Ltd. All reagents used in this work were technical grade. Various plates [glass, poly (methyl methacrylate ), poly (sty-
K. Nakamae et al./J. Membrane Sci. 75 (1992) 163-170
rene ) , poly (ethylene terephtalate), and poly (ethylene) ] were used as substrate for casting of the polymer solutions. The surfaces of these substrates were characterized by contact angle measurement as described below. 2.2. Measurement of the contact angle of the substrates Contact angles of water on the substrate were measured by the droplet method [ 151. The contact angles were determined from advancing and receding contact angles. The advancing contact angle (0,) was measured while the droplet was getting bigger, and the receding contact angle (&) was measured while the droplet was getting smaller. These contact angles were calculated from eqn. (1) where h is the height of droplet, and x is the diameter of droplet. B e = Ztan-‘(Wx) a, * { 90” +con-‘[+h/(h2+
(&90°) (x/2)‘)]
(9190”) (1)
The contact angle was calculated from eqn. (2 ) e=cos-’
{ (8,+8,)/2}
(2)
2.3. Preparation of the membrane Ten grams of EVA were dissolved in 50 ml DMAc at 90 oC. The resulting solution was kept at 30’ C for 24 hr. The solution was cast on the substrate and then dipped into water. Membrane formation was complete within 30 min; the membrane was kept in the water for more than 5 hr. The procedure was described in detail in a previous paper [lo]. 2.4. Post- treatment Dry heat treatment and hot water treatment were performed by heating the membrane at
K. Nakamae et al./J.Membrane Sci. 75 (1992) 163-170
165
TABLE 1 Permeation solutes Solute
Radius (A)
Medium
I (nm)a
PS emulsion Blue dextran Hemoglobin Vitamin B12 L-Tryptophan
550 130 41 11 6
Distilled water Distilled water Tris-CH,COOH buffer (pH 7.4) C8H,COOHCOOK-NaOH buffer (pH 4.8) Distilled water
250 610 405 550 290
“Wavelengths at which the adsorbance was measured.
various temperatures for one hour in an oven and hot water, respectively.
electrons (8=900).
2.5. Electron spectroscopy for chemical analysis (ESCA)
2.6. Ultrafiltration test
The samples for ESCA measurement were prepared by freeze drying of the membrane. ESCA measurements were carried out with a Shimazu ESCA 750 equipped with data system ESCA PAC 760. Typical operation conditions were as follows: MgKa! radiation with 8 kV, 30 mA. The pressure in the instrument chamber was less than 1 x 10m5Pa. No radiation damage was observed during the data collection time. The charge correction in the binding energy scale was done by setting the -CH,- peak in the carbon spectra to 285.0 eV. Overlapping peaks were resolved into their individual components of the core-level spectra, which were well described by gaussians. The full width at halfmaximum (FWHM) of each individual peak was less than 2.0 eV. Angular-dependence measurements of ESCA were carried out by rotating the sample relative to the fixed position energy analyzer by an angle 8 designated as the angle between the sample normal and the entrance slit in the analyzer. The effective sampling depth could be decreased by decreasing the electron take-off angle 8. It was obvious that electrons collected at decreasing angles (8+O” ) are more sensitive to the surface features than
collected
normal to the surface
Ultrafiltration tests were carried out at 20” C using a type MC-2 permeation cell (Bioengineering Co., Ltd.), effective area 7.7 cm2, at a pressure difference of 1 kg/cm2. Rejection of solutes was calculated from the difference in concentration of the solution before and after permeation. The radius of the solute and the wavelength at which the absorbance was measured are shown in Table 1. 3. Results and discussion 3.1. Influences of the substrate on the surface characteristics of E VA membrane Figures 1 and 2 show Ci, ESCA spectra of EVA membrane cast on glass substrate and PE substrate, respectively. The amount of OH groups at the water side surface was the same for both membranes. However, the amount of OH groups at the substrate side surface was very small for the membrane cast on PE substrate, compared with that of a membrane cast on the glass substrate. This means that the hydrophobic ethylene component was located at the substrate surface and that the hydrophilic vinyl alcohol component migrated to the interior of the
166 a
:
I
K. Nakamae et al./J. Membrane Sci. 75 (1992) 163-l 70 water
.
,
,
290 Binding
side
,
I
,
b
,
,
.
285 energy
,
glass
:
I
*
,
290 (eV)
Binding
side
,
1
,
285 energy
.
.
,
(eV)
Fig. 1. C!,, ESCA spectra of EVA membrane cast on glass substrate by the phase inversion method. a
:
I
water
.,,#
290 Binding
side
b
I**,,_
PE side
,,,,,,,.,,,
285 energy
:
290 (eV)
Binding
285 energy
(eV)
Fig. 2. C,, ESCA spectra of EVA membrane cast on PE substrate by the phase inversion method.
spect to surface composition and the amount of OH groups at the substrate side surface is smaller than at the water side surface. Figure 3 shows the relationship between the water contact angle of the substrate and the surface localization factor of EVA membranes prepared on various substrates. The substrates used were glass, PMMA, PET, PS, and PE; the coagulation bath was water at 10°C. The surface localization factor decreased with increasing contact angle, i.e. the more hydrophobic the substrate, the more enriched was the substrate side surface of the membrane in the ethylene component. The membranes prepared on the hydrophobic substrate were very asymmetric in surface composition. In particular, the PE side surface of the membrane had a vinyl alcohol content of 45 mol%. The surface free energy of the vinyl alcohol component is larger than that of the ethylene component. Therefore, these results suggest that each component migrated in a way that lowers the surface free energy between the membrane and the substrate. Figures 4 and 5 show the depth profiles - obtained by the angular dependence measurements - of the surface composition in EVA membranes cast on glass and PE substrate, re-
membrane because of the strong hydrophobicity of the PE substrate. A surface localization factor was defined by the following equation in order to quantify the degree of asymmetry of EVA membranes with respect to surface composition. Surface localization factor = ( [-C-OH]/ (
[-C&l hbstrate [-C-OH]/ [-C&-l Avater
where ( [-C-OH]
/ [ -CH2-] )s&&r&and ( [-COHI/ [-CHz-1Lter are the surface composition of the substrate side surface and the water side surface, respectively. If this value is less than 1, the membrane is asymmetric with re-
1
1
I
I
50
70
90
Contact
angle
(")
Fig. 3. Effect of the contact angle of the substrate on the surface localization factor of EVA membranes cast on various substrates by the phase inversion method.
167
K. Nakamae et al.jJ. Membrane Sci. 75 (1992) 163-l 70
I 0
, 0.2
I
I
I
0.4
0.6
0.8
1 1.0
sin 0 Fig. 4. Depth profile of atomic ratios in ESCA spectra of an EVA membrane cast on glass substrate. (0 ) Glass side, ( 0 ) water side.
the glass substrate, and in the ethylene component in the case of the PE substrate. These results indicate that the surface of the membrane is appreciably influenced by the substrate. Figure 6 shows the relationship between rejection and solute radius for EVA membranes cast on the PE substrate and on the glass substrate. The agreement between the two curves means that these membranes had very similar filtration properties. Therefore it is apparent that the surface characteristics can be controlled by substrate selection, without changing other membrane properties, such as solute rejection. 3.2. Influence of coagulation bath on surface characteds tics
L 0
1
0.2
8
0.4
1
0.6
I
I
0.8
1.0
sin 8
We reported previously that the structures of EVA membranes prepared by the phase-inversion method were influenced by the coagulation temperature [&lo]. Therefore, it is also expected that one might be able to control the surface characteristics of the membrane by selecting the coagulation temperature. Figure 7 shows the effect of the coagulation temperature on the surface characteristics of
Fig. 5. Depth profile of atomic ratios in ESCA spectra of an EVA membrane cast on PE substrate. (0) PE side, (0 ) water side.
100
80
[-C-OH/ In these figures, was determined by ESCA and -CHz- 1surface bulk was calculated from the [ -C-OH/-CH2-] composition of EVA as received (vinyl alcohol content = 67 mol% ) . The amount of OH groups at the water side surface was approximately constant, Therefore, the surface composition may be close to that of the bulk due to a short coagulation time at the water side surface. As the depth of the analysis was decreased, the substrate side surface was found to be enriched in the vinylalcohol component in the case of
spectively.
z 60 s .r + x 40 ‘a lx 20
Solute
radius
(A)
Fig. 6. Relationship between rejection and solute radius for an EVA membrane cast on (0 ) PE substrate and (0 ) glass substrate by the phase inversion method.
168
K. Nakamae et al./J. Membrane Sci. 75 (1992) 163-170
iO.5. rc
0
10 20 30 40 50 Coagulation temperature ("Cl
10 20 Solubility parameter
Fig. 7.Effect of coagulation temperature on the surface Iocalization factor of an EVA membrane cast on (0 ) PE substrate and (0 ) glass substrate by the phase inversion method.
Fig. 8. Reiationships between solubility parameter of coagulation bath and degree of surface localization of EVA membrane cast on PE substrate. (0 ) PE side, (0 ) coagulation bath side.
EVA membranes cast on glass and PE substrates. Using the glass substrate, the surface characteristics were little influenced by the coagulation temperature. For the PE substrate, the surface localization factor decreased with increasing the coagulation temperature from 0 to 10°C. On the other hand, above 20°C its value was 1, and the composition of the water side surface was the same as that of the PE side surface. For coagulation temperatures above 20 ’ C, it was difficult to separate the membrane from the PE substrate. This may be due to the fact that more ethylene components migrated to the PE side surface by the effect of hot water treatment. Consequently, cohesive failure may occur at the interface between the membrane and the substrate. The substrate side surface of these membranes was investigated by scanning electron microscopy (SEM). The SEM study suggested that the cohesive fracture indeed happened at the surface. If that is the case, the composition of the surface would be the same as that of the bulk resulting in the surface localization factor being equal to 1 above 20’ C. Figure 8 shows the relationship between the solubility parameter (SP) of the coagulation bath and the degree of surface localization of
an EVA membrane prepared on PE substrate at 0°C. Solubility parameters of toluene, acetone, n-butyl alcohol, methanol, ethylene glyco1 and water used as the coagulation bath are 8.91, 9.77, 11.3, 14.28, 16.3, and 23.5 (Cal/ cm3) l/*, respectively [ 161. All these chemicals are nonsolvents for EVA. They are all miscible with DMAc with the exception of toluene. The membranes were prepared using the following procedure. After the polymer solution was cast on the PE substrate, it was dipped into the coagulation bath at 0’ C and kept there for 3 hr. The membrane was then immersed in water to completely remove the solvent. In the case of toluene, which is not miscible with DMAc, coagulation occurred later, in the water bath. For a coagulation bath with a SP equal to 14, the composition of the PE side surface was the same as that in the bulk. For other SPvalues, the PE side surface was always enriched in the ethylene component. At the coagulation bath side surface, the ethylene component concentration was highest for SP=12. Such a complex surface behavior is thought to be due to the following. In the region of SP> 12, the ethylene comconcentration increased at the ponent
K. Nakamae et al./J. Membrane
169
Sci. 75 (1992) 163-170
hydrophobic coagulation bath side surface as the SP of the bath decreased. This is due to the fact that each component migrated in order to make the surface free energy lower during the membrane formation. In the case of SP< 10, because the solvent DMAc only little exchanged with the solution in the bath, the solution cast on the PE substrate was dipped in a water bath subsequently, which may have influenced the bath side surface of the membrane. The migrated amount of each component could be different because the rate of the membrane formation is different for different baths. The surface composition is thought to be the same as that in the bulk, because DMAc diffuses into the bath rapidly in the case of the coagulation bath that had nearly the same SP as DMAc. Those are the reasons for the complex behavior of the degree of surface localization. The surface characteristics of the membrane could be controlled by the preparation conditions (hydrophilicity of the substrate, temperature of the coagulation bath and polarity of the coagulation bath).
0.71
0
20 40 60 80 Treatment temperature
100
120
(“C)
Fig. 9. Effect of heat treatment temperature on the degree of surface localization of OH groups in an EVA membrane cast on PE substrate by the phase inversion method. (0 ) PE side, (0 ) water side. 1.21
I
3.3. Effect of post-treatment Figure 9 shows the effect of heat treatment on the surface composition of EVA membranes cast on PE substrate and coagulated at 0” C. The surface composition of both sides did not depend on heat treatment temperature. It seems that treatment for one hour was not sufficient to rearrange the components of EVA at the surface. Figure 10 shows the effect of hot water treatment on the surface composition of EVA membranes cast on PE substrate at 0’ C. In contrast to the dry heat treatment, the amount of OH groups at the PE side surface increased as the temperature of the hot water treatment increased. The composition of the PE side surface of the membrane was equal to that of the
0.71 0
20 40 60 80 100 Treatment temperature (‘C)
Fig. 10. Effect of hot-water treatment temperature on the degree of surface localization of OH groups in an EVA membrane cast on PE substrate by the phase inversion method. (0 ) PE side, (0 ) water side.
water side surface at water temperatures over 70” C. These results indicate that OH groups migrated to lower the interfacial free energy between the PE side surface and hot water under conditions in which the polymer chains could move easily. Furthermore, the surface composition changed, even at a temperature less than 68’ C ( Tg of EVA), because the apparent TBbecame lower because of membrane swelling. On the other hand, the surface composition was
170
K. Nakamae et al./J. Membrane Sci. 75 (1992) 163-170 2
constant at the water side surface. This result is due to the fact that the vinyl alcohol component of EVA could fully migrate in the process of the membrane formation to lower the interfacial free energy between the water side surface and hot water. Consequently, the surface characteristics of EVA membranes could be controlled by the preparation conditions or by the post-treatment.
5
4. Conclusion
6
The surface characteristics of EVA membranes prepared by dipping polymer solution cast on various substrates into a coagulation bath were investigated using ESCA. The surfaces of EVA membranes prepared on a glass substrate were quite different from those of membranes prepared on a polyethylene (PE) substrate. Furthermore, the compositions of the coagulation bath surfaces and substrate side surfaces were different for the membranes prepared on the PE substrate. Surface characteristics of EVA membranes can therefore be controlled by proper substrate selection. The surface characteristics were also altered by hot water treatment but not by dry heat treatment. Consequently, the surface characteristics of the membrane could also be changed by posttreatments. Acknowledgement The authors wish to thank Dr. Takehisa Matsuda, Dr. Hiroo Iwata, and Dr. Akio Kishida of National Cardiovascular Center Research Institute for ESCA measurements.
3
4
7
8
9
10
11
12
13
14
15
References 1
S. Loeb and S. Sourirajan, Sea water demineralization by means of an osmotic membrane, Adv. Chem. Ser., 38 (1963) 117.
16
T. Matsumoto, K. Nakamae, N. Ogoshi, S. Kawasoe and H. Oka, The crystallinity of ethylene-vinyl alcohol copolymer, Kobunshi Kagaku, 28 (1971) 610. T. Matsumoto, K. Nakamae, H. Oka, S. Kawasoe and T. Ochiumi, The microstructure of ethylene-vinyl alcohol copolymer (I), Sen’i Gakkaishi, 30 (1974) T-391. T. Matsumoto, K. Nakamae, H. Oka, S. Kawasoe and T. Ochiumi, The microstructure of ethylene-vinyl alcohol copolymer(II), Sen’i Gakkaishi, 31 (1975) T152. T. Matsumoto, K. Nakamae and T. Ochiumi, Solubility of ethylene-vinyl alcohol copolymers, Sen’i Gakkaishi, 30 (1974) T-398. T. Matsumoto, K. Nakamae, Y. Nakano and K. Nonaka, Potential of ethylene-vinyl alcohol copolymers, Sen’i Gakkaishi, 30 (1974) T-559. T. Matsumoto, K. Nakamae, T. Ochiumi, S. Kawasoe and T. Shioyama, The microstructure and the viscoelastic properties of ethylene-vinyl alcohol copolymers, Sen’i Gakkaishi, 33 (1977) T-49. Y. Fujimura, N. Masutani, K. Nakamae and T. Matsumoto, Mechanism of membrane formation of ethylene-vinyl alcohol copolymer ultrathin membrane spread on water, Kobunshi Ronbunshu, 41 (1984) 77. K. Nakamae, Y. Fujimura, S. Shimatani and T. Matsumoto, Properties of ultrathin membranes prepared from ethylene-vinyl alcohol copolymers, J. Membrane Sci., 29 (1986) 267. K. Nakamae, T. Ochiumi, M. Tsukada, T. Sekido and T. Matsumoto, Effect of the coagulation temperature on membrane properties of ethylene-vinyl alcohol copolymer, Kobunshi Ronbunshu, 42 (1985) 143. R.G. Azrak, Surface property variations in meltformed thermoplastics, J. Colloid Interface Sci., 47 (1974) 779. A. Noshay and J.E. McGrath, Block Copolymers, Overview and Critical Survey, Academic Press, New York, NY, 1977. J.F.M. Pennings and B. Bosman, Analysis of copolymer surfaces by surface reactions and XPS, Colloid Polym. Sci., 258 (1980) 1099. D.T. Clark and H.R. Thomas, Application of ESCA to polymer chemistry. XVII. Systematic investigation of the core levels of simple homopolymers, J. Polym. Sci. Polym. Chem. Ed., 16 (1978) 791. K. Sumiya, T. Taii, K. Nakamae and T. Matsumoto, Adhesion of the vacuum-deposited cobalt thin films to polymer films, J. Adhesion Sot. Japan, 18 (1982) 345. C.M. Hansen and A. Beerbower, Encyclopedia of Chemical Technology, Suppl. Vol., Interscience, New York, NY, 1967, pp. 889.
163
Elsevier Science Publishers B.V., Amsterdam
Surface characterization of ethylene-vinyl alcohol copolymer membranes prepared under various conditions Katsuhiko
Nakamae,
Takashi
Miyata and Tsunetaka
Matsumoto
Department of Industrial Chemistry, Faculty of Engineering, Kobe University, Rokko, Nada, Kobe 657 (Japan)
(Received January 17,199l; accepted in revised form July 31,1992)
Abstract Surface characteristics of EVA membranes were investigated using ESCA. The membranes were prepared by dipping polymer solution cast on various substrates into a coagulation bath (the phase-inversion method). The ESCA results demonstrated that there were significant differences in the surface composition of EVA membranes prepared under various conditions. The more hydrophobic the substrate, the more the substrate side surface was enriched in the ethylene component. Thus the surface characteristics can be controlled by the kind of the substrate without changing the membrane structure such as pore size. For membranes subjected to hot water treatment, the composition of the substratecontacting surface gradually approached that of the water side surface with increasing temperature. These results suggest that the ethylene and vinyl alcohol components migrated in order to lower the surface free energy at the surface of the membrane. Keywords: membrane
preparation
and structure; ESCA; surface characteristics;
ethylene-vinyl
alcohol
copolymer
1. Introduction Polymeric membranes are utilized in water treatment from the point of economization in power and environmental sanitation. Most of them are asymmetric membranes of the type described by Loeb and Sourirajan [ 11,and are prepared by the phase-inversion method. The asymmetric membrane consists of a dense “skin layer” and a porous “substrate layer”. High permselectivity and rapid permeation are atCorrespondence to: Prof. K. Nakamae, Department of Industrial Chemistry, Faculty of Engineering, Kobe University, Rokko, Nada, Kobe 657, Japan.
0376-7388/92/$05.00
tained by the asymmetric structure. Ethylene-vinyl alcohol copolymer (EVA) has hydrophilic vinyl alcohol segments and hydrophobic ethylene segments in each molecule. We have previously investigated the physical and structural properties of EVA, such as crystallinity [ 21, fine structure [ 3,4], solubility [ 51, surface potential [ 61, and viscoelasticity [ 71. We have also studied the EVA membranes prepared by spreading the polymer solution on a water surface [ 8,9] and by dipping the solution cast on a glass substrate into water [lo]. These membranes were also asymmetric and they had both high permselectivity and rapid permeation.
0 1992 Elsevier Science Publishers B.V. All rights reserved.
164
Recently many investigations on various immobilized enzymes have been performed, and membranes have been used as a support for the immobilization of enzymes. In this respect the surface characteristics of the membrane are very important. In the polymer materials, the characteristics at the surface are different from those in the bulk phase [ 11-131. Therefore the surface characteristics are interesting in a membrane consisting of amphiphilic copolymers such as EVA. The distribution of the functional groups in the membrane is also thought to influence the properties of the membrane. For example, EVA membranes are used for blood dialysis and there it is required that little protein is adsorbed on the surface of the membrane. Thus, the surface characteristics of the membrane must be controlled in accordance with its utilization. Electron spectroscopy for chemical analysis (ESCA) has been used as a technique for studying such surface characteristics. Many polymers have been elaborated with ESCA by D.T. Clark et al. [ 141. The utility of ESCA to elucidate changes in the structure and chemical features of a modified surface is well substantiated. In this study, the surface characteristics, especially the location of the functional groups, of EVA membranes prepared under various conditions were investigated using ESCA. 2. Experimental 2.1.Materials Ethylene-vinyl alcohol copolymer (EVA) had a vinyl alcohol content of 67 mol% and a degree of polymerization of 1100 (Kuraray Co., Ltd.), and the degree of saponification of the vinyl acetate was more than 98%. Dimethylacetamide (DMAc ) as the solvent was obtained from Nakarai Co., Ltd. All reagents used in this work were technical grade. Various plates [glass, poly (methyl methacrylate ), poly (sty-
K. Nakamae et al./J. Membrane Sci. 75 (1992) 163-170
rene ) , poly (ethylene terephtalate), and poly (ethylene) ] were used as substrate for casting of the polymer solutions. The surfaces of these substrates were characterized by contact angle measurement as described below. 2.2. Measurement of the contact angle of the substrates Contact angles of water on the substrate were measured by the droplet method [ 151. The contact angles were determined from advancing and receding contact angles. The advancing contact angle (0,) was measured while the droplet was getting bigger, and the receding contact angle (&) was measured while the droplet was getting smaller. These contact angles were calculated from eqn. (1) where h is the height of droplet, and x is the diameter of droplet. B e = Ztan-‘(Wx) a, * { 90” +con-‘[+h/(h2+
(&90°) (x/2)‘)]
(9190”) (1)
The contact angle was calculated from eqn. (2 ) e=cos-’
{ (8,+8,)/2}
(2)
2.3. Preparation of the membrane Ten grams of EVA were dissolved in 50 ml DMAc at 90 oC. The resulting solution was kept at 30’ C for 24 hr. The solution was cast on the substrate and then dipped into water. Membrane formation was complete within 30 min; the membrane was kept in the water for more than 5 hr. The procedure was described in detail in a previous paper [lo]. 2.4. Post- treatment Dry heat treatment and hot water treatment were performed by heating the membrane at
K. Nakamae et al./J.Membrane Sci. 75 (1992) 163-170
165
TABLE 1 Permeation solutes Solute
Radius (A)
Medium
I (nm)a
PS emulsion Blue dextran Hemoglobin Vitamin B12 L-Tryptophan
550 130 41 11 6
Distilled water Distilled water Tris-CH,COOH buffer (pH 7.4) C8H,COOHCOOK-NaOH buffer (pH 4.8) Distilled water
250 610 405 550 290
“Wavelengths at which the adsorbance was measured.
various temperatures for one hour in an oven and hot water, respectively.
electrons (8=900).
2.5. Electron spectroscopy for chemical analysis (ESCA)
2.6. Ultrafiltration test
The samples for ESCA measurement were prepared by freeze drying of the membrane. ESCA measurements were carried out with a Shimazu ESCA 750 equipped with data system ESCA PAC 760. Typical operation conditions were as follows: MgKa! radiation with 8 kV, 30 mA. The pressure in the instrument chamber was less than 1 x 10m5Pa. No radiation damage was observed during the data collection time. The charge correction in the binding energy scale was done by setting the -CH,- peak in the carbon spectra to 285.0 eV. Overlapping peaks were resolved into their individual components of the core-level spectra, which were well described by gaussians. The full width at halfmaximum (FWHM) of each individual peak was less than 2.0 eV. Angular-dependence measurements of ESCA were carried out by rotating the sample relative to the fixed position energy analyzer by an angle 8 designated as the angle between the sample normal and the entrance slit in the analyzer. The effective sampling depth could be decreased by decreasing the electron take-off angle 8. It was obvious that electrons collected at decreasing angles (8+O” ) are more sensitive to the surface features than
collected
normal to the surface
Ultrafiltration tests were carried out at 20” C using a type MC-2 permeation cell (Bioengineering Co., Ltd.), effective area 7.7 cm2, at a pressure difference of 1 kg/cm2. Rejection of solutes was calculated from the difference in concentration of the solution before and after permeation. The radius of the solute and the wavelength at which the absorbance was measured are shown in Table 1. 3. Results and discussion 3.1. Influences of the substrate on the surface characteristics of E VA membrane Figures 1 and 2 show Ci, ESCA spectra of EVA membrane cast on glass substrate and PE substrate, respectively. The amount of OH groups at the water side surface was the same for both membranes. However, the amount of OH groups at the substrate side surface was very small for the membrane cast on PE substrate, compared with that of a membrane cast on the glass substrate. This means that the hydrophobic ethylene component was located at the substrate surface and that the hydrophilic vinyl alcohol component migrated to the interior of the
166 a
:
I
K. Nakamae et al./J. Membrane Sci. 75 (1992) 163-l 70 water
.
,
,
290 Binding
side
,
I
,
b
,
,
.
285 energy
,
glass
:
I
*
,
290 (eV)
Binding
side
,
1
,
285 energy
.
.
,
(eV)
Fig. 1. C!,, ESCA spectra of EVA membrane cast on glass substrate by the phase inversion method. a
:
I
water
.,,#
290 Binding
side
b
I**,,_
PE side
,,,,,,,.,,,
285 energy
:
290 (eV)
Binding
285 energy
(eV)
Fig. 2. C,, ESCA spectra of EVA membrane cast on PE substrate by the phase inversion method.
spect to surface composition and the amount of OH groups at the substrate side surface is smaller than at the water side surface. Figure 3 shows the relationship between the water contact angle of the substrate and the surface localization factor of EVA membranes prepared on various substrates. The substrates used were glass, PMMA, PET, PS, and PE; the coagulation bath was water at 10°C. The surface localization factor decreased with increasing contact angle, i.e. the more hydrophobic the substrate, the more enriched was the substrate side surface of the membrane in the ethylene component. The membranes prepared on the hydrophobic substrate were very asymmetric in surface composition. In particular, the PE side surface of the membrane had a vinyl alcohol content of 45 mol%. The surface free energy of the vinyl alcohol component is larger than that of the ethylene component. Therefore, these results suggest that each component migrated in a way that lowers the surface free energy between the membrane and the substrate. Figures 4 and 5 show the depth profiles - obtained by the angular dependence measurements - of the surface composition in EVA membranes cast on glass and PE substrate, re-
membrane because of the strong hydrophobicity of the PE substrate. A surface localization factor was defined by the following equation in order to quantify the degree of asymmetry of EVA membranes with respect to surface composition. Surface localization factor = ( [-C-OH]/ (
[-C&l hbstrate [-C-OH]/ [-C&-l Avater
where ( [-C-OH]
/ [ -CH2-] )s&&r&and ( [-COHI/ [-CHz-1Lter are the surface composition of the substrate side surface and the water side surface, respectively. If this value is less than 1, the membrane is asymmetric with re-
1
1
I
I
50
70
90
Contact
angle
(")
Fig. 3. Effect of the contact angle of the substrate on the surface localization factor of EVA membranes cast on various substrates by the phase inversion method.
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K. Nakamae et al.jJ. Membrane Sci. 75 (1992) 163-l 70
I 0
, 0.2
I
I
I
0.4
0.6
0.8
1 1.0
sin 0 Fig. 4. Depth profile of atomic ratios in ESCA spectra of an EVA membrane cast on glass substrate. (0 ) Glass side, ( 0 ) water side.
the glass substrate, and in the ethylene component in the case of the PE substrate. These results indicate that the surface of the membrane is appreciably influenced by the substrate. Figure 6 shows the relationship between rejection and solute radius for EVA membranes cast on the PE substrate and on the glass substrate. The agreement between the two curves means that these membranes had very similar filtration properties. Therefore it is apparent that the surface characteristics can be controlled by substrate selection, without changing other membrane properties, such as solute rejection. 3.2. Influence of coagulation bath on surface characteds tics
L 0
1
0.2
8
0.4
1
0.6
I
I
0.8
1.0
sin 8
We reported previously that the structures of EVA membranes prepared by the phase-inversion method were influenced by the coagulation temperature [&lo]. Therefore, it is also expected that one might be able to control the surface characteristics of the membrane by selecting the coagulation temperature. Figure 7 shows the effect of the coagulation temperature on the surface characteristics of
Fig. 5. Depth profile of atomic ratios in ESCA spectra of an EVA membrane cast on PE substrate. (0) PE side, (0 ) water side.
100
80
[-C-OH/ In these figures, was determined by ESCA and -CHz- 1surface bulk was calculated from the [ -C-OH/-CH2-] composition of EVA as received (vinyl alcohol content = 67 mol% ) . The amount of OH groups at the water side surface was approximately constant, Therefore, the surface composition may be close to that of the bulk due to a short coagulation time at the water side surface. As the depth of the analysis was decreased, the substrate side surface was found to be enriched in the vinylalcohol component in the case of
spectively.
z 60 s .r + x 40 ‘a lx 20
Solute
radius
(A)
Fig. 6. Relationship between rejection and solute radius for an EVA membrane cast on (0 ) PE substrate and (0 ) glass substrate by the phase inversion method.
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K. Nakamae et al./J. Membrane Sci. 75 (1992) 163-170
iO.5. rc
0
10 20 30 40 50 Coagulation temperature ("Cl
10 20 Solubility parameter
Fig. 7.Effect of coagulation temperature on the surface Iocalization factor of an EVA membrane cast on (0 ) PE substrate and (0 ) glass substrate by the phase inversion method.
Fig. 8. Reiationships between solubility parameter of coagulation bath and degree of surface localization of EVA membrane cast on PE substrate. (0 ) PE side, (0 ) coagulation bath side.
EVA membranes cast on glass and PE substrates. Using the glass substrate, the surface characteristics were little influenced by the coagulation temperature. For the PE substrate, the surface localization factor decreased with increasing the coagulation temperature from 0 to 10°C. On the other hand, above 20°C its value was 1, and the composition of the water side surface was the same as that of the PE side surface. For coagulation temperatures above 20 ’ C, it was difficult to separate the membrane from the PE substrate. This may be due to the fact that more ethylene components migrated to the PE side surface by the effect of hot water treatment. Consequently, cohesive failure may occur at the interface between the membrane and the substrate. The substrate side surface of these membranes was investigated by scanning electron microscopy (SEM). The SEM study suggested that the cohesive fracture indeed happened at the surface. If that is the case, the composition of the surface would be the same as that of the bulk resulting in the surface localization factor being equal to 1 above 20’ C. Figure 8 shows the relationship between the solubility parameter (SP) of the coagulation bath and the degree of surface localization of
an EVA membrane prepared on PE substrate at 0°C. Solubility parameters of toluene, acetone, n-butyl alcohol, methanol, ethylene glyco1 and water used as the coagulation bath are 8.91, 9.77, 11.3, 14.28, 16.3, and 23.5 (Cal/ cm3) l/*, respectively [ 161. All these chemicals are nonsolvents for EVA. They are all miscible with DMAc with the exception of toluene. The membranes were prepared using the following procedure. After the polymer solution was cast on the PE substrate, it was dipped into the coagulation bath at 0’ C and kept there for 3 hr. The membrane was then immersed in water to completely remove the solvent. In the case of toluene, which is not miscible with DMAc, coagulation occurred later, in the water bath. For a coagulation bath with a SP equal to 14, the composition of the PE side surface was the same as that in the bulk. For other SPvalues, the PE side surface was always enriched in the ethylene component. At the coagulation bath side surface, the ethylene component concentration was highest for SP=12. Such a complex surface behavior is thought to be due to the following. In the region of SP> 12, the ethylene comconcentration increased at the ponent
K. Nakamae et al./J. Membrane
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Sci. 75 (1992) 163-170
hydrophobic coagulation bath side surface as the SP of the bath decreased. This is due to the fact that each component migrated in order to make the surface free energy lower during the membrane formation. In the case of SP< 10, because the solvent DMAc only little exchanged with the solution in the bath, the solution cast on the PE substrate was dipped in a water bath subsequently, which may have influenced the bath side surface of the membrane. The migrated amount of each component could be different because the rate of the membrane formation is different for different baths. The surface composition is thought to be the same as that in the bulk, because DMAc diffuses into the bath rapidly in the case of the coagulation bath that had nearly the same SP as DMAc. Those are the reasons for the complex behavior of the degree of surface localization. The surface characteristics of the membrane could be controlled by the preparation conditions (hydrophilicity of the substrate, temperature of the coagulation bath and polarity of the coagulation bath).
0.71
0
20 40 60 80 Treatment temperature
100
120
(“C)
Fig. 9. Effect of heat treatment temperature on the degree of surface localization of OH groups in an EVA membrane cast on PE substrate by the phase inversion method. (0 ) PE side, (0 ) water side. 1.21
I
3.3. Effect of post-treatment Figure 9 shows the effect of heat treatment on the surface composition of EVA membranes cast on PE substrate and coagulated at 0” C. The surface composition of both sides did not depend on heat treatment temperature. It seems that treatment for one hour was not sufficient to rearrange the components of EVA at the surface. Figure 10 shows the effect of hot water treatment on the surface composition of EVA membranes cast on PE substrate at 0’ C. In contrast to the dry heat treatment, the amount of OH groups at the PE side surface increased as the temperature of the hot water treatment increased. The composition of the PE side surface of the membrane was equal to that of the
0.71 0
20 40 60 80 100 Treatment temperature (‘C)
Fig. 10. Effect of hot-water treatment temperature on the degree of surface localization of OH groups in an EVA membrane cast on PE substrate by the phase inversion method. (0 ) PE side, (0 ) water side.
water side surface at water temperatures over 70” C. These results indicate that OH groups migrated to lower the interfacial free energy between the PE side surface and hot water under conditions in which the polymer chains could move easily. Furthermore, the surface composition changed, even at a temperature less than 68’ C ( Tg of EVA), because the apparent TBbecame lower because of membrane swelling. On the other hand, the surface composition was
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constant at the water side surface. This result is due to the fact that the vinyl alcohol component of EVA could fully migrate in the process of the membrane formation to lower the interfacial free energy between the water side surface and hot water. Consequently, the surface characteristics of EVA membranes could be controlled by the preparation conditions or by the post-treatment.
5
4. Conclusion
6
The surface characteristics of EVA membranes prepared by dipping polymer solution cast on various substrates into a coagulation bath were investigated using ESCA. The surfaces of EVA membranes prepared on a glass substrate were quite different from those of membranes prepared on a polyethylene (PE) substrate. Furthermore, the compositions of the coagulation bath surfaces and substrate side surfaces were different for the membranes prepared on the PE substrate. Surface characteristics of EVA membranes can therefore be controlled by proper substrate selection. The surface characteristics were also altered by hot water treatment but not by dry heat treatment. Consequently, the surface characteristics of the membrane could also be changed by posttreatments. Acknowledgement The authors wish to thank Dr. Takehisa Matsuda, Dr. Hiroo Iwata, and Dr. Akio Kishida of National Cardiovascular Center Research Institute for ESCA measurements.
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References 1
S. Loeb and S. Sourirajan, Sea water demineralization by means of an osmotic membrane, Adv. Chem. Ser., 38 (1963) 117.
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T. Matsumoto, K. Nakamae, N. Ogoshi, S. Kawasoe and H. Oka, The crystallinity of ethylene-vinyl alcohol copolymer, Kobunshi Kagaku, 28 (1971) 610. T. Matsumoto, K. Nakamae, H. Oka, S. Kawasoe and T. Ochiumi, The microstructure of ethylene-vinyl alcohol copolymer (I), Sen’i Gakkaishi, 30 (1974) T-391. T. Matsumoto, K. Nakamae, H. Oka, S. Kawasoe and T. Ochiumi, The microstructure of ethylene-vinyl alcohol copolymer(II), Sen’i Gakkaishi, 31 (1975) T152. T. Matsumoto, K. Nakamae and T. Ochiumi, Solubility of ethylene-vinyl alcohol copolymers, Sen’i Gakkaishi, 30 (1974) T-398. T. Matsumoto, K. Nakamae, Y. Nakano and K. Nonaka, Potential of ethylene-vinyl alcohol copolymers, Sen’i Gakkaishi, 30 (1974) T-559. T. Matsumoto, K. Nakamae, T. Ochiumi, S. Kawasoe and T. Shioyama, The microstructure and the viscoelastic properties of ethylene-vinyl alcohol copolymers, Sen’i Gakkaishi, 33 (1977) T-49. Y. Fujimura, N. Masutani, K. Nakamae and T. Matsumoto, Mechanism of membrane formation of ethylene-vinyl alcohol copolymer ultrathin membrane spread on water, Kobunshi Ronbunshu, 41 (1984) 77. K. Nakamae, Y. Fujimura, S. Shimatani and T. Matsumoto, Properties of ultrathin membranes prepared from ethylene-vinyl alcohol copolymers, J. Membrane Sci., 29 (1986) 267. K. Nakamae, T. Ochiumi, M. Tsukada, T. Sekido and T. Matsumoto, Effect of the coagulation temperature on membrane properties of ethylene-vinyl alcohol copolymer, Kobunshi Ronbunshu, 42 (1985) 143. R.G. Azrak, Surface property variations in meltformed thermoplastics, J. Colloid Interface Sci., 47 (1974) 779. A. Noshay and J.E. McGrath, Block Copolymers, Overview and Critical Survey, Academic Press, New York, NY, 1977. J.F.M. Pennings and B. Bosman, Analysis of copolymer surfaces by surface reactions and XPS, Colloid Polym. Sci., 258 (1980) 1099. D.T. Clark and H.R. Thomas, Application of ESCA to polymer chemistry. XVII. Systematic investigation of the core levels of simple homopolymers, J. Polym. Sci. Polym. Chem. Ed., 16 (1978) 791. K. Sumiya, T. Taii, K. Nakamae and T. Matsumoto, Adhesion of the vacuum-deposited cobalt thin films to polymer films, J. Adhesion Sot. Japan, 18 (1982) 345. C.M. Hansen and A. Beerbower, Encyclopedia of Chemical Technology, Suppl. Vol., Interscience, New York, NY, 1967, pp. 889.