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Porous polycarbonate phase-inversion membranes
Journal of Membrane
Science,
60 (1991)
25
25-40
Elsevier Science Publishers B.V., Amsterdam
Porous polycarbonate phase-inversion membranes Michal Bodzek and Jolanta Bohdziewicz Technical University of SLlesla, Institute Glawrce, ul Kuczewsklego 2 (Poland) (Received
of Water, Sewage and Wastes Engmeering,
44-101
May 11,1989; accepted in revised form March 8,199l)
Abstract This paper presents a method of preparation of phase-inversion porous polycarbonate membranes as well as experimental data on their structure and transport and separation properties. From a casting solution containing 8% of polycarbonate in a mixture of dioxane and dimethylformamide (mass ratio 3 2)) it is possible to obtain porous membranes by the phase-inversion method. Gelation should be carried out in an atmosphere of air saturated with water vapour. The porosity of the membranes can be controlled by the time and temperature of gelation. The obtained membranes are reasonably symmetrical, having a spongy structure with identical pore sizes and a skinned external structure. They retam macromolecules and suspended substances to a degree which depends on the nature of the solutes and on the pressure. Keywords membrane preparation tion; polycarbonate membranes
and structure;
microporous
and porous membranes;
ultrafiltra-
Introduction
According to data to be found in the literature [ 11,polycarbonate (PC) is considered to be suitable for the production of microporous membranes. It has been used to prepare microfiltration membranes by means of the “track-etching” method [2] and also by making use of the phase-inversion method to prepare composite membranes for reverse osmosis [ 31. Besides the homopolymer, block copolymers of PC are used for the production of membranes, mainly with polyethylene glycol [ 11 and silicones [ 41. In copolymers the flexibility of the chains is much greater than in the homopolymer, thus a polymer with a higher molar mass can be synthesized and provided with hydrophilic properties [ 11. PC copolymers with silicones are used to produce membranes for gas separation [ 11. PC displays good chemical and mechanical properties [ 51. Since it is a polyester of carbonic acid, it is not alkali-resistant, although it does resist the effect of acids and neutral substances. The solvents of PC are usually halogen derivatives of short-chain aliphatic hydrocarbons (chloroform, dichlorome-
26
thane and others). The solubility of the polymer decreases considerably with increase in the molar mass. For the preparation of membranes, most often polymers with a high molar mass are used [ 61. It is the purpose of this paper to investigate the possibility of using polycarbonate produced in Poland (Bistan AF) for the preparation of phase-inversion membranes and to determine the structure and the transport and separation properties of such membranes. Experimental
Materials
The raw material used for the production of PC membranes was Bistan AF, obtained from a pilot plant installation developed by the Institute of Industrial Chemistry in Warsaw. In order to characterize the polymer, its molar mass was determined by means of viscometry. The intrinsic viscosity [q] was determined with an Ubbelohde viscometer by extrapolation after measuring the time of outflow of the PC solution in chloroform with a concentration of 2-8 kg/m3 at a temperature of 298 K. The intrinsic viscosity determined in this way amounted to 104 cm”/g. In the case of the Polish PC Bistan, the constant values in the Mark-Houwink formula suggested by Sitamaiarak [ 41 were used. The average viscometric molar mass ( MV) was calculated by making use of the following formula: [~]=1.2x10-2
(Mq)082
The molar mass of Bistan AF applied in these investigations amounted to nearly 63,000. It may thus be considered to belong to those PC with high molar mass [ 51 recommended for the production of membranes [ 1 ] and films [ 51. Preparation
of membranes
Membranes were obtained by means of the phase-inversion method [ 1,7,8] using as a solvent a mixture of 1,4-dioxane and N,N-dimethylacetamide (DMAc) with a mass ratio of 3 : 2. The mass ratio of the components of the solvent was selected empirically. Membranes obtained with the use of a solvent containing more dioxane were impermeable at a pressure up to 0.5 MPa, whereas a large amount of DMAc in the solvent generated membranes characterized by very little mechanical resistance and a heterogeneous surface and were unsuitable for testing in an ultrafiltration apparatus. The mechanical properties of the obtained membranes were determined qualitatively by observation of pin-holes and resistance to flow at a pressure of 0.5 MPa. In our investigations the casting solutions used contained 8% of PC (maximum solubility ) in a mixture of solvents. The polymer content was chosen from the viewpoint of solubility and viscosity. Flat membranes were produced in the following way. Freshly prepared cast-
21
ing solution was filtered through a ceramic filter (l-2 pm) and the membrane was formed on a glass plate by means of an applicator with a gap of adjustable height (0.1-0.3 mm). This process was accomplished at ambient temperature. The glass plate with the cast film was subjected to a two-stage process of gelation: in an atmosphere of air saturated with water vapour at temp. of 283,293 and 313 K for 600,900 and 1200 set; in a bath of deionized water at ambient temp. (293 K) for 3600 sec. Membranes obtained in such a way were rinsed in water in order to remove the residual solvent. Besides wet membranes (stored in water), dry membranes were also obtained. These membranes were prepared by drying wet membranes in a vacuum for 24 hr at a temperature of 313 K or in air of low relative humidity at ambient temp. [ 91. The formation of PC membranes thus consists of five stages: preparation of a casting solution with a concentration of 8% by mass in a mixture of dioxane and DMAc (mass ratio 3 : 2 ) ; casting of a film of appropriate thickness; . gelation in air saturated with water vapour at a given temperature and time; additional gelation in a water bath for 3600 set at 293 K; if needed, drying of the wet membrane in a vacuum at 313 K. Double-stage gelation was applied in order to decrease the rate of phase separation. Membranes gelled by immediate immersion in a bath containing a non-solvent (water) were characterized by a very low permeability at a pressure of 0.3 MPa. l
l
l
l
l
l
Investigation of membrane structure The physical structure of the membranes was determined by analysing photographs of their cross-sections taken by means of a scanning electron microscope of type JSM-35 from the Japanese firm JEOL. After the membranes had been precisely dehydrated (drying in a vacuum for 24 hr at a temperature of 313 K), they were fractured in liquid nitrogen and then sputtered with gold in order to confer adequate contrast on the membrane cross-section. The characteristics of the structure of the membrane surface, as well as the pore size distribution, were determined by the bubble-point method [ 1,101. Isobutanol was used as a wetting liquid and water as a permeable medium. The measurements were carried out in the pressure range 0.01-0.06 MPa. A skin layer thickness of 0.5 pm has been assumed to calculate the pore size distribution. The microporous structure (specific surface area and pore volume) was determined by measuring the adsorption and desorption isotherms of nitrogen at 77 K, applying the Gravimat sorption equipment, model 4303, produced by Sartorius. The technique used is of static volumetric type, according to which
28
the gas volumes adsorbed or desorbed are calculated by measuring the pressure variation resulting from the adsorption or desorption of a portion of a known volume of gas. Before the nitrogen adsorption and desorption isotherms were measured the membrane samples were evacuated and flushed with nitrogen. Membranes were also dehydrated using the same procedure as above. The method of interpreting the isotherms has been presented elsewhere [ 111. Investigations of transport and separation properties Measurements of transport and separation properties of the membranes with respect to water and macromolecular and colloidal solutions were carried out by means of Amicon ultrafiltration equipment (type S76-400) (Fig. 1) with a volume of 0.4 dm3 and a membrane diameter of 76 mm. The membrane cell is designed for ultrafiltration or microfiltration at pressures not exceeding 0.5 MPa. The apparatus is equipped with a magnetic stirrer to minimize concentration on the membrane surface, and with a vent/relief valve. Filtering pressure is supplied from an external source.
Fig. 1. Apparatus
for membrane testing: ( 1) clamp assembly, (2) fill plug “0” ring, (3) pressure plug, (5) pressure fitting, (6) tubing, (7) “0” ring, (8) barrel, (9) stirring
reliefvalve, (4) fill rod, (10) spm bar, top.
(11) base, (12) hose nozzle,
(13) support dwc, (14) rod tip assembly,
(15)
29
The volumetric water flux per surface area of the membrane was determined by measuring the volume of water permeating in a given unit of time at a pressure of 0.1-0.3 MPa and a temperature of 298 K. Before the measurements, the membranes were conditioned by passing water through them at a pressure of 0.2 MPa for 3 hr. The rejection properties were determined by introducing into the apparatus 0.3 dm3 of a solution containing the test substances. The solution was pressurized by nitrogen and, as soon as measurement conditions were reached (pressure and temperature), 5% of the permeate was sampled, in which the content of the test substance and the experimental retention coefficient were determined. Two types of macromolecular compound were used in these investigations, differing in their molar mass and the shape of the macromolecule (protein: yglobulin, and carbohydrate: blue dextran), as well as an emulsion of oil in water (emulsifiable oil ES ). y-Globulin with a molar mass of 160,000 is characterized by its globular particle shape. Solutions with a concentration of 0.1 and 0.5 g/ dm3 in phosphate buffer were used. The protein concentration in the permeate was determined spectrophotometrically within the ultraviolet range (280 nm). Blue dextran with a molar mass of 2,000,OOOhas a branched structure. Its concentration in the permeate was determined calorimetrically at a wavelength of 610 nm. The investigations were carried out with aqueous dextran solutions with a concentration of 0.01 and 0.05%. Solutions of emulsions with concentrations of 0.1,0.2 and 0.5% were prepared with deionized water and an appropriate amount of emulsifiable oil ES. The concentration of oil in the permeate was determined by means of nephelometry. Results and discussion Membrane formation Table 1 presents the results of determining the volumetric water flux; the aim of the measurements was the proper choice of optimal parameters of membrane formation, viz., time and temperature of gelation in air saturated with water vapour, as well as the blade gap setting. The water flux increases with the time of gelation; this means that the membranes become more and more open, and results from the fact that if the membrane is gelled for 900-1200 set the phase separation rate is slow, which favours the generation of open structures. This seems to be proved by photographs of the cross-sections of the membranes as well as by results of investigations on the pore size distribution by means of the bubble-point method (see later). In both cases larger pores were obtained when the membranes were gelled for 1200 set (PC-8-20 membrane) than after gelation for only 600 set (PC-8-10 membrane). It seems, therefore, that the time of gelation of PC membranes with water vapour from the gaseous phase might be a parameter controlling
30 TABLE
I
Transport of water through polycarbonate surements 298 K) A.
Influence of gelation temperature mm, gelation time: 1200 set) Gelatlon temperature (K) Volume water flux (m3/m2-day)
C.
(pressure 0.2 MPa, temperature
of mea-
Influence of gelation time in air saturated with water vapour (casting thickness: 0 2 mm, gelatlon temperature 293 K) Gelation time (set) Volume water flux (m3/m2-day)
B
membranes
Influence of membrane
thickness
Castmg thickness (mm) Volume water flux (ma/m’-day)
600 14.4
900 31.2
1200 35.1
m an saturated with water vapour (casting thickness. 0.2
283 17 5
293 35.1
313 0
(gelatron time 1200 set, gelation temperature 0.1 22 4
0.2 35.1
293 K)
0.3 35.1
the compactness of the membranes. Doubling the gelation time from 600 set to 1200 set doubles the water flux (Table 1A). The water flux decreases with an increase in the gelation temperature in the range from 283 to 313 K (Table 1B). PC membranes gelled at 313 K become impermeable to water. At higher temperatures the phase separation rate is also higher, so that more compact structures are formed. Decreasing the temperature of gelation from 293 K (20’ C ) to 283 K (10’ C ), the water flux more than doubles. The gelation temperature of PC membranes in air saturated with water vapour may also be a parameter which controls their transport properties. The water flux of PC membranes depends on the membrane thickness. However, permeability remains approximately the same for a casting gap thickness of 0.2 or 0.3 mm (Table 1C). The lower water flux for 0.1 mm thickness is connected with membrane compactness. The optimal thickness of the casting gap is 0.2-0.3 mm, taking into account the rate of water transport. The real thickness of the membrane is always 2-3 times smaller than the height of the applicator gap. Table 2 presents the results concerning the water flux of wet and dry PC membranes of differing compactness (PC-810 and PC-8-20 membranes). The drying stage does not influence their transport properties. It has been found that there is no difference in the water flux through dry and wet low porosity membranes (PC-8-10)) whereas the volume water flux passing through more highly porous wet membranes (PC-8-20) is greater by ca. 20% than in the case
31 TABLE
2
Comparison temperature Kind of membrane
PC-8-10 PC-8-20
of water transport rate in wet and dry polycarbonate membranes (pressure 0.2 MPa, of measurements 298 K, casting thickness 0.2 mm, gelation temperature 293 K) Volume water flux
JwlJ,
(m3/m2-day)
Wet membrane,
Dry membrane,
JW
J.
12.6 33.0
12.6 27.4
1.00 1.20
of dry membranes. The possibility of obtaining dry membranes is of great practical importance, as it enables easier transport and storage of membranes and membrane modules. Summing up, it may be said that from a PC solution in a mixture of dioxane and DMAc, wet and dry membranes with a high water flux can be produced. The compactness of the membranes can be adjusted first of all by means of temperature and the time of gelation in air saturated with water vapour. The optimal parameters of the preparation of membranes are as follows: concentration of the polymer: 8% by mass; solvent: 1,4-dioxane and N,N-dimethylacetamide, mixed in proportions of 3 : 2 by mass; temperature of gelation with water vapour from gaseous phase: 283-293 K; time of gelation with water vapour: 600 or 1200 set; gelation conditions in water bath: temp. 293 K, time 3600 set; thickness of casting gap: 0,2-0.3 mm (actual thickness 100 pm). Structure and mechanism of formation Figures 2 and 3 show the cross-sections of PC membranes with different porosities, designated as PC-8-10 and PC-8-20, magnified 240, 800 and 2400 times. These membranes were obtained by gelling a film cast in air saturated with water vapour at 293 K for 10 and 20 min, respectively. The thickness of the casting gap was 0.2 mm. An analysis of the photographs of the cross-sections has led us to the conclusion that the membranes are practically symmetrical, although in the case of the PC-8-10 membrane (Fig. 2) a very thin, irregular skin layer is to be observed. In both these membranes we have a homogeneous spongy structure, i.e. spherical or ellipsoidal pores with similar diameters. The diameters of the macropores inside the structure of the membrane PC8-10 (with low porosity) amount approximately to 6.7-8.3 pm, whereas the pore sizes of the membrane PC-8-20 range from 10-11.6 pm. The sizes of the macropores were calculated from the cross-sectional micrographs. This proves
32
Fig. 2. Cross-sections
of PC-8-10
membranes;
magnification:
(a) 240X;
(b) 8(
3:
Fig. 3. Cross-sections of PC-8-20 membrane; magnification: (a) 240X; (b) 800X; (c) 2400X.
34
that the gelation time of the cast film in air saturated with vapour influences the membrane structure. A shorter time of gelation provides membranes with pores of a smaller diameter and an asymmetric structure. A gelation time within 10 min is too short for the formation of the final membrane pore structure. The final formation of the membrane structure with a skin layer takes place during gelation in a bath containing a liquid non-solvent. The formation of a symmetric spongy structure (PC-8-20 membrane) or an asymmetric structure (PC-8-10 membrane) depends on the phase separation rate in the course of gelation [ 12,131. If symmetrical membranes (PC-8-20) are obtained by means of the phase-separation method by gelation in air saturated with water vapour, the process of gelation is rather slow, yielding membranes of a homogeneous structure (identical pore sizes throughout the crosssection of the membrane with no skin layer). The mechanism of the formation of such membranes is easier to understand if we observe the changes in the concentration of the polymer, solvent and non-solvent during the gelation process. Since the diffusion of the non-solvent on the surface of the film is rather slow, changes in its concentration over the whole cross-section of the membrane (during gelation) are negligible. As a result the polymer gels along the entire cross-section of the membrane virtually simultaneously. Changes in the concentration of the non-solvent during the formation of such a membrane are to be seen in Fig. 4(a). From this we may conclude that the gelation of PC-820 membrane (gelation time 1200 set) has been practically completed in the vapour phase; the aim of gelation in the liquid phase is merely the fixation of the spongy symmetric structure. In the case of the PC-8-10 membrane (gelation time 600 see), however, ge-
G+S
PttASE
;T;K$NESS
OF THE UIST
HOtEaLVENT
e&w
Twh4ESS
OF ntE
C&-r
Fig. 4. Changes in the concentration of the non-solvent in a casting solution at various periods of membrane gelatlon (t= 1,2,3,4). (a) symmetrx membranes; (b) asymmetric membranes
35
lation was only initiated in the first stage of gelation (from the gaseous phase), whereas the final structure of the membrane was formed in the course of gelation in the liquid phase (water bath - second stage of gelation), as proved by the occurrence of a skin layer on the membranes. The skin layer is formed after the cast film has been immersed in the bath containing a non-solvent. In such a case gelation is a rather fast process and the obtained membrane has an asymmetric structure, including a compact skin layer of small thickness. In the case of the PC-%10 membrane the thickness of the skin layer may be ca. O.l0.3 pm. The scheme of the formation of the structure presented by the PC-& 10 membrane is shown in Fig. 4(b). After the immersion of the cast film in a non-solvent at the interface of the casting solution, the concentration of the non-solvent reaches a value at which the phase separation takes place; the concentration inside the membrane is rather low. The result is an increase of the polymer concentration in the surface layer, including the formation of a compact skin layer, which is characteristic for asymmetric membranes. The generated skin layer restricts the further transport of the non-solvent into the interior of the membrane cross-section as well as that of the solvent in the opposite direction. This becomes a barrier limiting the diffusion of the nonsolvent and gives rise to considerable differences in its concentrations over the cross-section of the membrane. The mechanism of the formation of membranes under the skin layer is similar to that when symmetric membranes are formed. Depending on the permeability of the skin, the rate of diffusion of both the non-solvent and the solvent may vary, and thus also a concentration gradient of the non-solvent over the cross-section of the membrane may be achieved. If the difference of concentration of the non-solvent over the crosssection is not too large, and if the time in which the system reaches the point at which a gel is beginning to form is the same over the entire cross-section, asymmetric membranes are formed with a spongy structure characterized by pores of identical size. Summing up, we may say that PC dissolved in a mixture of solvents (dioxane and DMAc) yields, if the phase-separation method is applied, both symmetric and asymmetric membranes, depending on the operation of the phase-separation process. In every case, however, the membrane has an isoporous structure. Membrane
porosity
Figure 5 and Table 3 show the mean pore radius and pore size distribution as well as the pore density (the number of pores with a given radius per surface unit of the membrane) of the skin layers for both PC-%10 and PC-8-20 membranes, determined by means of the bubble-point method. These data are consistent with the results obtained by SEM analysis of cross-sections (Figs. 2 and 3 ) and with those from permeability measurements (Table 1) . The mean pore radius (determined by the bubble-point method) of PC-8-10 is 81.4 nm
36
Fig. 5 Pore size distribution water system: (a) PC-8-10 TABLE
determined membrane;
by means of the bubble-point (b) PC-8-20
method in an isobutanol-
membrane.
3
Characteristics of the porosity of polycarbonate isobutanol, permeable medium water) Membrane
membrane
(bubble-point
Membrane
PC-&10
method: wetting agent
PC-%20
Radius
Pore density
Radms
(nm)
(m-‘)
(nm)
Pore density (m-“)
306 204
1.95 x lo9 1 15x 10’”
306 204
5.84 x lo9 2.64 x 10”
153 102 76.5
3.00 x 1O’O 3.30x 10” 2.07 x 10”
153 122 102
3 65 x 10” 2 28 x 10”’ 1 05x 10”’
612 Mean radius: 81,4 nm
3.04 x 10’” Mean radius
172,6 nm
and that of the PC-8-20 membrane is 173 nm, which means that the porosities of these membranes are different. The largest pore radius on the other hand is ca. 200 nm for the PC-8-10 membrane and 300 nm for PC-8-20. The pore size distribution of the PC-8-10 membrane is characterized by a narrow maximum: 83.7% with radius close to 76.5 nm. Also, 97% of all the pores are contained within the range of radii from 76.5 to 102 nm; 1.8% of the pores have a radius exceeding 102 nm and only 1.2% a radius of 61.2 nm. Membrane PC-8-20, being more open, is characterized by a broader pore size distribution: 54% and 39.5% of the pores have radii close to 153 and 204 nm, respectively, whereas 0.9% of them have a radius exceeding 204 nm. By means of the nitrogen adsorption/desorption method, the specific surface area and volume of pores in PC-8-10 and PC-8-20 membranes were deter-
37
mined. Both membranes are characterized by an approximately identical specific surface area (4.6-5.2 m”/g), but the pore volume in the case of the less porous membrane is about twice as large as that of the PC-&20 membrane (PC-8-10: 6 x lop3 cm3/g; PC-8-20: 1.1 X lop2 cm”/g ). Differences between pore size determinations made by means of the bubblepoint method and the gas adsorption/desorption method are connected with the differences of the skin layer and membrane matrix structure. With the bubble-point method the skin layer pore structure is determined, while the nitrogen adsorption/desorption method encompasses the whole pore structure (skin layer and membrane matrix). Retention properties The separation properties of a given ultrafiltration and microfiltration membrane depend mainly on the nature of the solutes, the intensity of the concentration polarization and on environmental conditions. The experimental retention coefficients of the solutes mentioned in the experimental part have been determined for two membranes of different porosity (PC-&10 and PC-S-20) as functions of the pressure and concentration of the solutes. The results of these tests are presented in Tables 4 and 5. Pressure has a large effect on the retention coefficients of these membranes. The retention coefficient for all the solutes investigated decreases considerably with increasing pressure, depending on the nature of the solute and, to a lesser degree, upon its concentration. The greatest decline has been observed in the case of dextran, whose molecules are more easily deformed by pressure than those of cross-linked globular protein. The decrease of the retention coefficient of macromolecules and colloids affected by pressure is also due to the concentration polarization (i.e. the thickness and concentration of the polarized layer) TABLE 4 Separation properties of polycarbonate membranes, PC-&10 type (temperature of measurements 298 K) Test substance
Concentration
P=O 05 (MPa)
P=O.l
(MPa)
FlUX
FhlX (m/d)
R (%)
1 10 152 182 2 00 181 2 41 3.01
89 8 58.3 21.6 27 0 75.0 60.0 60.0
R (%)
(m/d) y-Globulin
0 5 (g/dm3) 0.1 (g/dm3) Blue dextran 005 (%) OOl(%) ES od emulsron 0 5 ( % ) 02 (%) 0.1 (W)
0.95 1.31 132 1.54 180 2 21 2.81
90 I 66 I 34 3 210 71.5 74.0 70 0
Note: P= drfference of pressure on the two srdes of the membrane. R = retentron coefficient.
P=O 2 (MPa)
P=O 3 (MPa)
Flux
R(%)
Flux (m/d)
R (%)
74.1 33.3 4.90 4 00 12 0 56.0 50.0
125 1.75 2.05 2 20 2.10 2.65 3 30
35 2 20 8 3.90 2 00 500 50.0 40 0
(m/d) 120 1.68 1.91 2 15 2 05 2 60 3.20
38 TABLE 5 Separation properties of polycarbonate
membrane, PC-B-20 type (temperature of measurements 298
E) Test substances
Concentration
P=O 05 (MPa)
P=O.l
Flux
Flux
R (%)
Blue dextran ES oil emulsion
0 5 (g/dm”) 0 1 (g/dm”) 005 (%) OOl(%) 0.5 (% ) 02 (%a) Ol(%)
R (%)
(m/d)
(m/d) I’-Globulm
(MPa)
1.81 2.45 2.55 3.50 2.50 3.30 5.10
72.3 52.4 35.7 28.5 70.0 87.5 95.0
1.95 3.00 2.98 4.15 2 75 3.81 5.35
P=O 2 (MPa)
I’=0
Flux
Flux
R (%)
(m/d) 68 8 47.6 27 5 23 0 50 0 75 0 90 0
2 01 3.15 3 15 4 35 2 85 3 90 5 45
3 (MPa) R (%)
(m/d) 614 12 4 18.6 10.4 40.0 65.0 85.0
2 11 3 20 3 25 4 50 3.00 4 05 5 50
44 6 6 70 12 2 4 70 40.0 55 0 70.0
poly (vmyl chloride)
(PVC),
Note. P=dlfference of pressure on the two sides of the membrane. R = retention coefflclent TABLE
6
Comparison of retention coefficients and PC membranes [ 14-161 Test
Retention
of polyacrylonitrile
coefficient
(PAN),
(% )
substance
I)-Globulin Blue dextran ES or1 emulsion
PAN
PVC
PC
100 100 100
95-100 100 100
ca. 70 ca 25 ca. 70
which increases with pressure, which in turn leads to an increase in the concentration of the test substance in the permeate, thus reducing the experimental retention coefficient. An analysis of the data presented in Tables 4 and 5 shows that colloidal substances (oil-water emulsion) and proteins are retained at a pressure of 0.1 MPa to a considerable extent. This is connected with the considerable size of the oil particles in the emulsion (0.55 pm) and with the rigid, globular structure of y-globulin. In spite of the high molar mass of blue dextran (Z,OOO,OOO) its retention coefficient is low. The reason is the aforesaid elastic and chainlike structure of the carbohydrate molecules. It has been observed that the retention coefficient increases with increase in the concentration of the solution used for testing the membranes. The same relationship as described above has also been observed in the case of polyacrylonitrile and poly (vinyl chloride) membranes, which we dealt with
39
in our previous papers [ 14-161. Comparing the results obtained in the case of these membranes with those concerning PC membranes, however, it should be stated that the values of the retention coefficient are lower, particularly at higher pressures (Table 6). This proves that the obtained membranes of PC are more open and that their transport and separation properties are intermediate between those of ultrafiltration and microfiltration membranes. Conclusions (1) Bistan-type polycarbonate is potentially an adequate material for the preparation of porous membranes by the phase-inversion method, in which case the gelation step should be carried out in an atmosphere of air saturated with water vapour. (2) Membranes are obtained from a casting solution containing 8% of PC in a mixture of the solvents 1,4-dioxane and N,N-dimethylacetamide (mass ratio 3:2). (3 ) The porosity of the membranes can be controlled by the time and temperature of gelation in air saturated with water vapour. (4) The structure of PC membranes obtained by means of the phase-inversion method can be either symmetrical or asymmetrical, depending on the way in which the phase-inversion process is run. The structure is spongy with identical pore sizes. (5) The structure of the surface layer of the membranes is characterized by a relatively narrow pore size distribution. (6) The obtained membranes retain 50-90% of y-globulin, 27-35% of blue dextran and 70-95% of oil in an emulsion at a pressure of 0.05 MPa. The flux, however, drops considerably in comparison with the pure water flux.
References R.E. Kesting, Synthetic Polymeric Membranes - A Structural Perspective, Wiley, New York, NY, 1985. Nuclepore Corp., Pleasanton, CA. L.T. Rozelle, J.E. Cadotte, R.D. Corneliussen and E.E. Erickson, Development of new reverse osmosis membranes for desalination, Office of Saline Water Research and Development Progress, Report No. 359, US Government Printing Office, Washmgton, DC, 1968. A. Noshay and M. McGrath, Block Copolymers - Overview and Critical Survey, Academic Press, New York, NY, 1977. Poliweglany (Polycarbonates ) , WNT, Warsaw, 1971 (in Polish). D. Kalinska, Tworzywa Sztuczne Med., 2 (1962) 3 (in Polish). S. Loeb and S. Sourirajan, High-flow semipermeable membranes for separation of water from saline solutrons, Adv. Chem. Ser., 38 (1961) 117. S. Sourirajan, Reverse Osmosis and Synthetic Membranes, National Research Council, Ottawa, 1977.
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25-40
Elsevier Science Publishers B.V., Amsterdam
Porous polycarbonate phase-inversion membranes Michal Bodzek and Jolanta Bohdziewicz Technical University of SLlesla, Institute Glawrce, ul Kuczewsklego 2 (Poland) (Received
of Water, Sewage and Wastes Engmeering,
44-101
May 11,1989; accepted in revised form March 8,199l)
Abstract This paper presents a method of preparation of phase-inversion porous polycarbonate membranes as well as experimental data on their structure and transport and separation properties. From a casting solution containing 8% of polycarbonate in a mixture of dioxane and dimethylformamide (mass ratio 3 2)) it is possible to obtain porous membranes by the phase-inversion method. Gelation should be carried out in an atmosphere of air saturated with water vapour. The porosity of the membranes can be controlled by the time and temperature of gelation. The obtained membranes are reasonably symmetrical, having a spongy structure with identical pore sizes and a skinned external structure. They retam macromolecules and suspended substances to a degree which depends on the nature of the solutes and on the pressure. Keywords membrane preparation tion; polycarbonate membranes
and structure;
microporous
and porous membranes;
ultrafiltra-
Introduction
According to data to be found in the literature [ 11,polycarbonate (PC) is considered to be suitable for the production of microporous membranes. It has been used to prepare microfiltration membranes by means of the “track-etching” method [2] and also by making use of the phase-inversion method to prepare composite membranes for reverse osmosis [ 31. Besides the homopolymer, block copolymers of PC are used for the production of membranes, mainly with polyethylene glycol [ 11 and silicones [ 41. In copolymers the flexibility of the chains is much greater than in the homopolymer, thus a polymer with a higher molar mass can be synthesized and provided with hydrophilic properties [ 11. PC copolymers with silicones are used to produce membranes for gas separation [ 11. PC displays good chemical and mechanical properties [ 51. Since it is a polyester of carbonic acid, it is not alkali-resistant, although it does resist the effect of acids and neutral substances. The solvents of PC are usually halogen derivatives of short-chain aliphatic hydrocarbons (chloroform, dichlorome-
26
thane and others). The solubility of the polymer decreases considerably with increase in the molar mass. For the preparation of membranes, most often polymers with a high molar mass are used [ 61. It is the purpose of this paper to investigate the possibility of using polycarbonate produced in Poland (Bistan AF) for the preparation of phase-inversion membranes and to determine the structure and the transport and separation properties of such membranes. Experimental
Materials
The raw material used for the production of PC membranes was Bistan AF, obtained from a pilot plant installation developed by the Institute of Industrial Chemistry in Warsaw. In order to characterize the polymer, its molar mass was determined by means of viscometry. The intrinsic viscosity [q] was determined with an Ubbelohde viscometer by extrapolation after measuring the time of outflow of the PC solution in chloroform with a concentration of 2-8 kg/m3 at a temperature of 298 K. The intrinsic viscosity determined in this way amounted to 104 cm”/g. In the case of the Polish PC Bistan, the constant values in the Mark-Houwink formula suggested by Sitamaiarak [ 41 were used. The average viscometric molar mass ( MV) was calculated by making use of the following formula: [~]=1.2x10-2
(Mq)082
The molar mass of Bistan AF applied in these investigations amounted to nearly 63,000. It may thus be considered to belong to those PC with high molar mass [ 51 recommended for the production of membranes [ 1 ] and films [ 51. Preparation
of membranes
Membranes were obtained by means of the phase-inversion method [ 1,7,8] using as a solvent a mixture of 1,4-dioxane and N,N-dimethylacetamide (DMAc) with a mass ratio of 3 : 2. The mass ratio of the components of the solvent was selected empirically. Membranes obtained with the use of a solvent containing more dioxane were impermeable at a pressure up to 0.5 MPa, whereas a large amount of DMAc in the solvent generated membranes characterized by very little mechanical resistance and a heterogeneous surface and were unsuitable for testing in an ultrafiltration apparatus. The mechanical properties of the obtained membranes were determined qualitatively by observation of pin-holes and resistance to flow at a pressure of 0.5 MPa. In our investigations the casting solutions used contained 8% of PC (maximum solubility ) in a mixture of solvents. The polymer content was chosen from the viewpoint of solubility and viscosity. Flat membranes were produced in the following way. Freshly prepared cast-
21
ing solution was filtered through a ceramic filter (l-2 pm) and the membrane was formed on a glass plate by means of an applicator with a gap of adjustable height (0.1-0.3 mm). This process was accomplished at ambient temperature. The glass plate with the cast film was subjected to a two-stage process of gelation: in an atmosphere of air saturated with water vapour at temp. of 283,293 and 313 K for 600,900 and 1200 set; in a bath of deionized water at ambient temp. (293 K) for 3600 sec. Membranes obtained in such a way were rinsed in water in order to remove the residual solvent. Besides wet membranes (stored in water), dry membranes were also obtained. These membranes were prepared by drying wet membranes in a vacuum for 24 hr at a temperature of 313 K or in air of low relative humidity at ambient temp. [ 91. The formation of PC membranes thus consists of five stages: preparation of a casting solution with a concentration of 8% by mass in a mixture of dioxane and DMAc (mass ratio 3 : 2 ) ; casting of a film of appropriate thickness; . gelation in air saturated with water vapour at a given temperature and time; additional gelation in a water bath for 3600 set at 293 K; if needed, drying of the wet membrane in a vacuum at 313 K. Double-stage gelation was applied in order to decrease the rate of phase separation. Membranes gelled by immediate immersion in a bath containing a non-solvent (water) were characterized by a very low permeability at a pressure of 0.3 MPa. l
l
l
l
l
l
Investigation of membrane structure The physical structure of the membranes was determined by analysing photographs of their cross-sections taken by means of a scanning electron microscope of type JSM-35 from the Japanese firm JEOL. After the membranes had been precisely dehydrated (drying in a vacuum for 24 hr at a temperature of 313 K), they were fractured in liquid nitrogen and then sputtered with gold in order to confer adequate contrast on the membrane cross-section. The characteristics of the structure of the membrane surface, as well as the pore size distribution, were determined by the bubble-point method [ 1,101. Isobutanol was used as a wetting liquid and water as a permeable medium. The measurements were carried out in the pressure range 0.01-0.06 MPa. A skin layer thickness of 0.5 pm has been assumed to calculate the pore size distribution. The microporous structure (specific surface area and pore volume) was determined by measuring the adsorption and desorption isotherms of nitrogen at 77 K, applying the Gravimat sorption equipment, model 4303, produced by Sartorius. The technique used is of static volumetric type, according to which
28
the gas volumes adsorbed or desorbed are calculated by measuring the pressure variation resulting from the adsorption or desorption of a portion of a known volume of gas. Before the nitrogen adsorption and desorption isotherms were measured the membrane samples were evacuated and flushed with nitrogen. Membranes were also dehydrated using the same procedure as above. The method of interpreting the isotherms has been presented elsewhere [ 111. Investigations of transport and separation properties Measurements of transport and separation properties of the membranes with respect to water and macromolecular and colloidal solutions were carried out by means of Amicon ultrafiltration equipment (type S76-400) (Fig. 1) with a volume of 0.4 dm3 and a membrane diameter of 76 mm. The membrane cell is designed for ultrafiltration or microfiltration at pressures not exceeding 0.5 MPa. The apparatus is equipped with a magnetic stirrer to minimize concentration on the membrane surface, and with a vent/relief valve. Filtering pressure is supplied from an external source.
Fig. 1. Apparatus
for membrane testing: ( 1) clamp assembly, (2) fill plug “0” ring, (3) pressure plug, (5) pressure fitting, (6) tubing, (7) “0” ring, (8) barrel, (9) stirring
reliefvalve, (4) fill rod, (10) spm bar, top.
(11) base, (12) hose nozzle,
(13) support dwc, (14) rod tip assembly,
(15)
29
The volumetric water flux per surface area of the membrane was determined by measuring the volume of water permeating in a given unit of time at a pressure of 0.1-0.3 MPa and a temperature of 298 K. Before the measurements, the membranes were conditioned by passing water through them at a pressure of 0.2 MPa for 3 hr. The rejection properties were determined by introducing into the apparatus 0.3 dm3 of a solution containing the test substances. The solution was pressurized by nitrogen and, as soon as measurement conditions were reached (pressure and temperature), 5% of the permeate was sampled, in which the content of the test substance and the experimental retention coefficient were determined. Two types of macromolecular compound were used in these investigations, differing in their molar mass and the shape of the macromolecule (protein: yglobulin, and carbohydrate: blue dextran), as well as an emulsion of oil in water (emulsifiable oil ES ). y-Globulin with a molar mass of 160,000 is characterized by its globular particle shape. Solutions with a concentration of 0.1 and 0.5 g/ dm3 in phosphate buffer were used. The protein concentration in the permeate was determined spectrophotometrically within the ultraviolet range (280 nm). Blue dextran with a molar mass of 2,000,OOOhas a branched structure. Its concentration in the permeate was determined calorimetrically at a wavelength of 610 nm. The investigations were carried out with aqueous dextran solutions with a concentration of 0.01 and 0.05%. Solutions of emulsions with concentrations of 0.1,0.2 and 0.5% were prepared with deionized water and an appropriate amount of emulsifiable oil ES. The concentration of oil in the permeate was determined by means of nephelometry. Results and discussion Membrane formation Table 1 presents the results of determining the volumetric water flux; the aim of the measurements was the proper choice of optimal parameters of membrane formation, viz., time and temperature of gelation in air saturated with water vapour, as well as the blade gap setting. The water flux increases with the time of gelation; this means that the membranes become more and more open, and results from the fact that if the membrane is gelled for 900-1200 set the phase separation rate is slow, which favours the generation of open structures. This seems to be proved by photographs of the cross-sections of the membranes as well as by results of investigations on the pore size distribution by means of the bubble-point method (see later). In both cases larger pores were obtained when the membranes were gelled for 1200 set (PC-8-20 membrane) than after gelation for only 600 set (PC-8-10 membrane). It seems, therefore, that the time of gelation of PC membranes with water vapour from the gaseous phase might be a parameter controlling
30 TABLE
I
Transport of water through polycarbonate surements 298 K) A.
Influence of gelation temperature mm, gelation time: 1200 set) Gelatlon temperature (K) Volume water flux (m3/m2-day)
C.
(pressure 0.2 MPa, temperature
of mea-
Influence of gelation time in air saturated with water vapour (casting thickness: 0 2 mm, gelatlon temperature 293 K) Gelation time (set) Volume water flux (m3/m2-day)
B
membranes
Influence of membrane
thickness
Castmg thickness (mm) Volume water flux (ma/m’-day)
600 14.4
900 31.2
1200 35.1
m an saturated with water vapour (casting thickness. 0.2
283 17 5
293 35.1
313 0
(gelatron time 1200 set, gelation temperature 0.1 22 4
0.2 35.1
293 K)
0.3 35.1
the compactness of the membranes. Doubling the gelation time from 600 set to 1200 set doubles the water flux (Table 1A). The water flux decreases with an increase in the gelation temperature in the range from 283 to 313 K (Table 1B). PC membranes gelled at 313 K become impermeable to water. At higher temperatures the phase separation rate is also higher, so that more compact structures are formed. Decreasing the temperature of gelation from 293 K (20’ C ) to 283 K (10’ C ), the water flux more than doubles. The gelation temperature of PC membranes in air saturated with water vapour may also be a parameter which controls their transport properties. The water flux of PC membranes depends on the membrane thickness. However, permeability remains approximately the same for a casting gap thickness of 0.2 or 0.3 mm (Table 1C). The lower water flux for 0.1 mm thickness is connected with membrane compactness. The optimal thickness of the casting gap is 0.2-0.3 mm, taking into account the rate of water transport. The real thickness of the membrane is always 2-3 times smaller than the height of the applicator gap. Table 2 presents the results concerning the water flux of wet and dry PC membranes of differing compactness (PC-810 and PC-8-20 membranes). The drying stage does not influence their transport properties. It has been found that there is no difference in the water flux through dry and wet low porosity membranes (PC-8-10)) whereas the volume water flux passing through more highly porous wet membranes (PC-8-20) is greater by ca. 20% than in the case
31 TABLE
2
Comparison temperature Kind of membrane
PC-8-10 PC-8-20
of water transport rate in wet and dry polycarbonate membranes (pressure 0.2 MPa, of measurements 298 K, casting thickness 0.2 mm, gelation temperature 293 K) Volume water flux
JwlJ,
(m3/m2-day)
Wet membrane,
Dry membrane,
JW
J.
12.6 33.0
12.6 27.4
1.00 1.20
of dry membranes. The possibility of obtaining dry membranes is of great practical importance, as it enables easier transport and storage of membranes and membrane modules. Summing up, it may be said that from a PC solution in a mixture of dioxane and DMAc, wet and dry membranes with a high water flux can be produced. The compactness of the membranes can be adjusted first of all by means of temperature and the time of gelation in air saturated with water vapour. The optimal parameters of the preparation of membranes are as follows: concentration of the polymer: 8% by mass; solvent: 1,4-dioxane and N,N-dimethylacetamide, mixed in proportions of 3 : 2 by mass; temperature of gelation with water vapour from gaseous phase: 283-293 K; time of gelation with water vapour: 600 or 1200 set; gelation conditions in water bath: temp. 293 K, time 3600 set; thickness of casting gap: 0,2-0.3 mm (actual thickness 100 pm). Structure and mechanism of formation Figures 2 and 3 show the cross-sections of PC membranes with different porosities, designated as PC-8-10 and PC-8-20, magnified 240, 800 and 2400 times. These membranes were obtained by gelling a film cast in air saturated with water vapour at 293 K for 10 and 20 min, respectively. The thickness of the casting gap was 0.2 mm. An analysis of the photographs of the cross-sections has led us to the conclusion that the membranes are practically symmetrical, although in the case of the PC-8-10 membrane (Fig. 2) a very thin, irregular skin layer is to be observed. In both these membranes we have a homogeneous spongy structure, i.e. spherical or ellipsoidal pores with similar diameters. The diameters of the macropores inside the structure of the membrane PC8-10 (with low porosity) amount approximately to 6.7-8.3 pm, whereas the pore sizes of the membrane PC-8-20 range from 10-11.6 pm. The sizes of the macropores were calculated from the cross-sectional micrographs. This proves
32
Fig. 2. Cross-sections
of PC-8-10
membranes;
magnification:
(a) 240X;
(b) 8(
3:
Fig. 3. Cross-sections of PC-8-20 membrane; magnification: (a) 240X; (b) 800X; (c) 2400X.
34
that the gelation time of the cast film in air saturated with vapour influences the membrane structure. A shorter time of gelation provides membranes with pores of a smaller diameter and an asymmetric structure. A gelation time within 10 min is too short for the formation of the final membrane pore structure. The final formation of the membrane structure with a skin layer takes place during gelation in a bath containing a liquid non-solvent. The formation of a symmetric spongy structure (PC-8-20 membrane) or an asymmetric structure (PC-8-10 membrane) depends on the phase separation rate in the course of gelation [ 12,131. If symmetrical membranes (PC-8-20) are obtained by means of the phase-separation method by gelation in air saturated with water vapour, the process of gelation is rather slow, yielding membranes of a homogeneous structure (identical pore sizes throughout the crosssection of the membrane with no skin layer). The mechanism of the formation of such membranes is easier to understand if we observe the changes in the concentration of the polymer, solvent and non-solvent during the gelation process. Since the diffusion of the non-solvent on the surface of the film is rather slow, changes in its concentration over the whole cross-section of the membrane (during gelation) are negligible. As a result the polymer gels along the entire cross-section of the membrane virtually simultaneously. Changes in the concentration of the non-solvent during the formation of such a membrane are to be seen in Fig. 4(a). From this we may conclude that the gelation of PC-820 membrane (gelation time 1200 set) has been practically completed in the vapour phase; the aim of gelation in the liquid phase is merely the fixation of the spongy symmetric structure. In the case of the PC-8-10 membrane (gelation time 600 see), however, ge-
G+S
PttASE
;T;K$NESS
OF THE UIST
HOtEaLVENT
e&w
Twh4ESS
OF ntE
C&-r
Fig. 4. Changes in the concentration of the non-solvent in a casting solution at various periods of membrane gelatlon (t= 1,2,3,4). (a) symmetrx membranes; (b) asymmetric membranes
35
lation was only initiated in the first stage of gelation (from the gaseous phase), whereas the final structure of the membrane was formed in the course of gelation in the liquid phase (water bath - second stage of gelation), as proved by the occurrence of a skin layer on the membranes. The skin layer is formed after the cast film has been immersed in the bath containing a non-solvent. In such a case gelation is a rather fast process and the obtained membrane has an asymmetric structure, including a compact skin layer of small thickness. In the case of the PC-%10 membrane the thickness of the skin layer may be ca. O.l0.3 pm. The scheme of the formation of the structure presented by the PC-& 10 membrane is shown in Fig. 4(b). After the immersion of the cast film in a non-solvent at the interface of the casting solution, the concentration of the non-solvent reaches a value at which the phase separation takes place; the concentration inside the membrane is rather low. The result is an increase of the polymer concentration in the surface layer, including the formation of a compact skin layer, which is characteristic for asymmetric membranes. The generated skin layer restricts the further transport of the non-solvent into the interior of the membrane cross-section as well as that of the solvent in the opposite direction. This becomes a barrier limiting the diffusion of the nonsolvent and gives rise to considerable differences in its concentrations over the cross-section of the membrane. The mechanism of the formation of membranes under the skin layer is similar to that when symmetric membranes are formed. Depending on the permeability of the skin, the rate of diffusion of both the non-solvent and the solvent may vary, and thus also a concentration gradient of the non-solvent over the cross-section of the membrane may be achieved. If the difference of concentration of the non-solvent over the crosssection is not too large, and if the time in which the system reaches the point at which a gel is beginning to form is the same over the entire cross-section, asymmetric membranes are formed with a spongy structure characterized by pores of identical size. Summing up, we may say that PC dissolved in a mixture of solvents (dioxane and DMAc) yields, if the phase-separation method is applied, both symmetric and asymmetric membranes, depending on the operation of the phase-separation process. In every case, however, the membrane has an isoporous structure. Membrane
porosity
Figure 5 and Table 3 show the mean pore radius and pore size distribution as well as the pore density (the number of pores with a given radius per surface unit of the membrane) of the skin layers for both PC-%10 and PC-8-20 membranes, determined by means of the bubble-point method. These data are consistent with the results obtained by SEM analysis of cross-sections (Figs. 2 and 3 ) and with those from permeability measurements (Table 1) . The mean pore radius (determined by the bubble-point method) of PC-8-10 is 81.4 nm
36
Fig. 5 Pore size distribution water system: (a) PC-8-10 TABLE
determined membrane;
by means of the bubble-point (b) PC-8-20
method in an isobutanol-
membrane.
3
Characteristics of the porosity of polycarbonate isobutanol, permeable medium water) Membrane
membrane
(bubble-point
Membrane
PC-&10
method: wetting agent
PC-%20
Radius
Pore density
Radms
(nm)
(m-‘)
(nm)
Pore density (m-“)
306 204
1.95 x lo9 1 15x 10’”
306 204
5.84 x lo9 2.64 x 10”
153 102 76.5
3.00 x 1O’O 3.30x 10” 2.07 x 10”
153 122 102
3 65 x 10” 2 28 x 10”’ 1 05x 10”’
612 Mean radius: 81,4 nm
3.04 x 10’” Mean radius
172,6 nm
and that of the PC-8-20 membrane is 173 nm, which means that the porosities of these membranes are different. The largest pore radius on the other hand is ca. 200 nm for the PC-8-10 membrane and 300 nm for PC-8-20. The pore size distribution of the PC-8-10 membrane is characterized by a narrow maximum: 83.7% with radius close to 76.5 nm. Also, 97% of all the pores are contained within the range of radii from 76.5 to 102 nm; 1.8% of the pores have a radius exceeding 102 nm and only 1.2% a radius of 61.2 nm. Membrane PC-8-20, being more open, is characterized by a broader pore size distribution: 54% and 39.5% of the pores have radii close to 153 and 204 nm, respectively, whereas 0.9% of them have a radius exceeding 204 nm. By means of the nitrogen adsorption/desorption method, the specific surface area and volume of pores in PC-8-10 and PC-8-20 membranes were deter-
37
mined. Both membranes are characterized by an approximately identical specific surface area (4.6-5.2 m”/g), but the pore volume in the case of the less porous membrane is about twice as large as that of the PC-&20 membrane (PC-8-10: 6 x lop3 cm3/g; PC-8-20: 1.1 X lop2 cm”/g ). Differences between pore size determinations made by means of the bubblepoint method and the gas adsorption/desorption method are connected with the differences of the skin layer and membrane matrix structure. With the bubble-point method the skin layer pore structure is determined, while the nitrogen adsorption/desorption method encompasses the whole pore structure (skin layer and membrane matrix). Retention properties The separation properties of a given ultrafiltration and microfiltration membrane depend mainly on the nature of the solutes, the intensity of the concentration polarization and on environmental conditions. The experimental retention coefficients of the solutes mentioned in the experimental part have been determined for two membranes of different porosity (PC-&10 and PC-S-20) as functions of the pressure and concentration of the solutes. The results of these tests are presented in Tables 4 and 5. Pressure has a large effect on the retention coefficients of these membranes. The retention coefficient for all the solutes investigated decreases considerably with increasing pressure, depending on the nature of the solute and, to a lesser degree, upon its concentration. The greatest decline has been observed in the case of dextran, whose molecules are more easily deformed by pressure than those of cross-linked globular protein. The decrease of the retention coefficient of macromolecules and colloids affected by pressure is also due to the concentration polarization (i.e. the thickness and concentration of the polarized layer) TABLE 4 Separation properties of polycarbonate membranes, PC-&10 type (temperature of measurements 298 K) Test substance
Concentration
P=O 05 (MPa)
P=O.l
(MPa)
FlUX
FhlX (m/d)
R (%)
1 10 152 182 2 00 181 2 41 3.01
89 8 58.3 21.6 27 0 75.0 60.0 60.0
R (%)
(m/d) y-Globulin
0 5 (g/dm3) 0.1 (g/dm3) Blue dextran 005 (%) OOl(%) ES od emulsron 0 5 ( % ) 02 (%) 0.1 (W)
0.95 1.31 132 1.54 180 2 21 2.81
90 I 66 I 34 3 210 71.5 74.0 70 0
Note: P= drfference of pressure on the two srdes of the membrane. R = retentron coefficient.
P=O 2 (MPa)
P=O 3 (MPa)
Flux
R(%)
Flux (m/d)
R (%)
74.1 33.3 4.90 4 00 12 0 56.0 50.0
125 1.75 2.05 2 20 2.10 2.65 3 30
35 2 20 8 3.90 2 00 500 50.0 40 0
(m/d) 120 1.68 1.91 2 15 2 05 2 60 3.20
38 TABLE 5 Separation properties of polycarbonate
membrane, PC-B-20 type (temperature of measurements 298
E) Test substances
Concentration
P=O 05 (MPa)
P=O.l
Flux
Flux
R (%)
Blue dextran ES oil emulsion
0 5 (g/dm”) 0 1 (g/dm”) 005 (%) OOl(%) 0.5 (% ) 02 (%a) Ol(%)
R (%)
(m/d)
(m/d) I’-Globulm
(MPa)
1.81 2.45 2.55 3.50 2.50 3.30 5.10
72.3 52.4 35.7 28.5 70.0 87.5 95.0
1.95 3.00 2.98 4.15 2 75 3.81 5.35
P=O 2 (MPa)
I’=0
Flux
Flux
R (%)
(m/d) 68 8 47.6 27 5 23 0 50 0 75 0 90 0
2 01 3.15 3 15 4 35 2 85 3 90 5 45
3 (MPa) R (%)
(m/d) 614 12 4 18.6 10.4 40.0 65.0 85.0
2 11 3 20 3 25 4 50 3.00 4 05 5 50
44 6 6 70 12 2 4 70 40.0 55 0 70.0
poly (vmyl chloride)
(PVC),
Note. P=dlfference of pressure on the two sides of the membrane. R = retention coefflclent TABLE
6
Comparison of retention coefficients and PC membranes [ 14-161 Test
Retention
of polyacrylonitrile
coefficient
(PAN),
(% )
substance
I)-Globulin Blue dextran ES or1 emulsion
PAN
PVC
PC
100 100 100
95-100 100 100
ca. 70 ca 25 ca. 70
which increases with pressure, which in turn leads to an increase in the concentration of the test substance in the permeate, thus reducing the experimental retention coefficient. An analysis of the data presented in Tables 4 and 5 shows that colloidal substances (oil-water emulsion) and proteins are retained at a pressure of 0.1 MPa to a considerable extent. This is connected with the considerable size of the oil particles in the emulsion (0.55 pm) and with the rigid, globular structure of y-globulin. In spite of the high molar mass of blue dextran (Z,OOO,OOO) its retention coefficient is low. The reason is the aforesaid elastic and chainlike structure of the carbohydrate molecules. It has been observed that the retention coefficient increases with increase in the concentration of the solution used for testing the membranes. The same relationship as described above has also been observed in the case of polyacrylonitrile and poly (vinyl chloride) membranes, which we dealt with
39
in our previous papers [ 14-161. Comparing the results obtained in the case of these membranes with those concerning PC membranes, however, it should be stated that the values of the retention coefficient are lower, particularly at higher pressures (Table 6). This proves that the obtained membranes of PC are more open and that their transport and separation properties are intermediate between those of ultrafiltration and microfiltration membranes. Conclusions (1) Bistan-type polycarbonate is potentially an adequate material for the preparation of porous membranes by the phase-inversion method, in which case the gelation step should be carried out in an atmosphere of air saturated with water vapour. (2) Membranes are obtained from a casting solution containing 8% of PC in a mixture of the solvents 1,4-dioxane and N,N-dimethylacetamide (mass ratio 3:2). (3 ) The porosity of the membranes can be controlled by the time and temperature of gelation in air saturated with water vapour. (4) The structure of PC membranes obtained by means of the phase-inversion method can be either symmetrical or asymmetrical, depending on the way in which the phase-inversion process is run. The structure is spongy with identical pore sizes. (5) The structure of the surface layer of the membranes is characterized by a relatively narrow pore size distribution. (6) The obtained membranes retain 50-90% of y-globulin, 27-35% of blue dextran and 70-95% of oil in an emulsion at a pressure of 0.05 MPa. The flux, however, drops considerably in comparison with the pure water flux.
References R.E. Kesting, Synthetic Polymeric Membranes - A Structural Perspective, Wiley, New York, NY, 1985. Nuclepore Corp., Pleasanton, CA. L.T. Rozelle, J.E. Cadotte, R.D. Corneliussen and E.E. Erickson, Development of new reverse osmosis membranes for desalination, Office of Saline Water Research and Development Progress, Report No. 359, US Government Printing Office, Washmgton, DC, 1968. A. Noshay and M. McGrath, Block Copolymers - Overview and Critical Survey, Academic Press, New York, NY, 1977. Poliweglany (Polycarbonates ) , WNT, Warsaw, 1971 (in Polish). D. Kalinska, Tworzywa Sztuczne Med., 2 (1962) 3 (in Polish). S. Loeb and S. Sourirajan, High-flow semipermeable membranes for separation of water from saline solutrons, Adv. Chem. Ser., 38 (1961) 117. S. Sourirajan, Reverse Osmosis and Synthetic Membranes, National Research Council, Ottawa, 1977.
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