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Regenerated cellulose microporous membranes by mixing cellulose cuoxam with a water soluble polymer
/
iournalof •
i "
MEMBRANE SCIENCE
/ I
II ELSEVIER
Journal of Membrane Science 114 (I 996) 149-155
Regenerated cellulose microporous membranes by mixing cellulose cuoxam with a water soluble polymer Guang Yang, Lina Zhang * Department of Chemistry, Wuhan University, Wuhan430072, China
Received 13 February 1995; accepted 8 November 1995
Abstract Regenerated cellulose microporous membranes were satisfactorily prepared by mixing cellulose cuoxam with polyethylene glycol (PEG) and gelatin as pore former. When the PEG molecular weight was smaller than 2000, a mixture of cellulose cuoxam and PEG aqueous solution was miscible, and membranes with a microporous structure were formed. The structure and pore characteristics of the microporous membranes were studied by X-ray diffraction, SEM and a flow rate method. Their mean pore diameter (2~e) and the transmembrane flow of water (J) are more than four times that of unmixed regenerated cellulose membrane. The effects of the water soluble polymer additions on the pore size, structure and mechanical properties of the microporous membrane are discussed. Keywords: Regenerated cellulose; Microporous membrane; Pore former; Mean pore diameter; Transmembrane flow of water
1. Introduction In the last 30 years cellulose membranes have found extensive commercial applications in membrane separation processes, because of their relatively low cost, good compatibility with biological compounds and their remarkable hydrophilic properties [1,2], It is worth noting that utilizing rich plant cellulose as a material can not only reduce loss of limited petroleum resources but also protect the environment. The microporous structure of the membrane is important as it is closely related to transmembrane
* Corresponding author.
flow of water and sieving properties for macromolecules. Recently, Bemberg microporous membranes (BMM) prepared by the microphase separation method have successfully eliminated viruses from human plasma [3,4]. The membrane was prepared using a ternary coagulant mixture of acetone, water and ammonia from a cellulose cuoxam solution. The primary particles grow by amalgamation and under some conditions their radii approach an asymptotic value, which is the radius of the secondary particle [5,6]. In addition, Dunweg [7] showed that a microporous cellulose membrane, regenerated from cuoxam solutions to which polyethylene glycol having an average molecular weight of from 100 to 1500 had been added, had a high ultrafiltration capacity and screening coefficient for proteins. In previous work [8,9], the miscibilities of regenerated
0376-7388/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved SSDI 0376-7388(95)003 14-2
150
G. Yang, L. Zhang/Journal of Membrane Science 114 (1996) 149-155
cellulose membranes blended cellulose from cuoxam/zincoxene and cellulose cuoxam/casein were studied. The experimental results showed that the mean pore diameter (27f) and ethanol permeability (G) of a blend membrane from cellulose cuoxam/zincoxene were lower than those of a nonblend membrane, but the values of the blend from cellulose cuoxam/casein were slightly higher than that of the non-blend one suggesting that the fine microphase separated structure resulted from the hydrophobic R group in the casein provided correspondingly greater pores. This has prompted us to study a simple and powerful method for controlling the pore size of a membrane by the microphase separation of cellulose cuoxam with a water soluble polymer. In this paper, we attempted to prepare microporous membranes by mixing cellulose cuoxam with gelatin or polyethylene glycol (PEG) as pore former. The effects of these additions on the pore size, pore characteristics, morphological structure and mechanical properties of the porous membranes are discussed.
2. E x p e r i m e n t a l
2.1. Materials The linter used was supplied by Hubel Chemical Fiber Manufacture, and its viscosity average molecular weight (M,7) was determined to be 19.6 X 10 4.
Table 1 The compositionof the membranes MembraneNo. Addition
RC II-l-1 RC II-1-2 RC II-1-3 RC ll-2-1 RC 112-2 RC II-2-3 RC II-2-4 RC II-2-5 RC II-3-1 RC II-3-2
Four grades of polyethylene glycol imported from Japan (PEG400, PEG2000, PEG6000, PEG20000) with molecular weights of 400, 2 × 103, 6 X 103 and 2 X 104, respectively, and gelatin ( M = 8.4 X 104) purchased from Changdu Chemical Reagent Factory were used in this work. They are all water soluble polymers. The main components of the gelatin were analysed to be Gly (22%), Pro (14%), Glu (11%) with an amino acid analyzer (Waters PICO, TAGT). The amino acid residues of the gelatin contained 44.6% hydrophilic groups and 55.4% hydrophobic groups. 2.2. Preparation of membranes A 6 wt% cellulose cuoxam solution (I) was prepared [10]. The desired amount of PEG400 (II-1), aqueous solutions of 20% PEG2000 (II-2), 20% PEG6000 (II-3), 20% PEG20000 (1I-4) and 20% gelatin (III) were respectively added to I. The composition of the membranes prepared by mixing I and II or III are shown in Table 1. The cellulose/polymer mixture solution was filtered, then cast on a glass plate to a depth of 0.25 mm. The cast solution was immersed immediately into a coagulation bath of dilute NaOH aqueous solution. The cellulose membrane regenerated in dilute H2SO 4 was washed in running water, then placed in iso-propanol for 10 h, and finally stored in 20% iso-propanol/2% formaldehyde aqueous solution. The membranes were dried on the glass plate in air for measurement of
Weight ratio (cellulose: addition)
Membraneno.
Addition
Weight ratio (cellulose: addition)
PEG400 PEG400 PEG400 PEG2000 PEG2000 PEG2000
2:1 I: 1 1:2 20:1 10:1 5:1
PEG2000
2:1
PEG2000 PEGt000 PEGt000
1:1 20:1 10:1
RC II-3-3 RC II-3-4 RC II-4-2 RC II-4-3 RC 1I-4-4 RC 1I-4-5 RC III-1 RC Ill-2 RC III-3 RC III-4
PEGt000 PEGt000 PEG20000 PEG20000 PEG20000 PEG20000 Gelatin Gelatin Gelatin Gelatin
5:1 2: I 20:1 10:1 5: i 2:1 20:1 10:1 5:1 2:1
G. Yang, L. Zhang/Journal of Membrane Science 114 (1996) 149-155
their mechanical properties. The membranes are coded in Table 1 and RC-0 represents the membrane prepared by pure cellulose cuoxam. 2.3. Apparatus and measurements
Scanning electron micrographs (SEM) of the membranes were taken by the Material Research and Testing Center of Wuhan Technology University with an SEM ISI-SX-40. The membranes were coated with carbon and gold, then their surfaces and cross sections were observed and photographed. The cross sections of the membranes were prepared by breaking the membrane under liquid nitrogen.
151
The X-ray diffraction pattern was measured with an X-ray diffractometer Rigaku 3015. The X-ray diffraction patterns with C u k a at 35 kv and 25 mA were recorded in the region of 5-45 °. The degree of crystallinity was calculated from X-ray data according to the usual method. The nitrogen contents in gelatin and in the membranes added with gelatin were determined by a model PE-240B elemental analyzer. Tensile strengths (o"b) and breaking elongations (E b) of the dry membrane were measured on a tensile testing machine (Instron-1121) according to a Chinese standard method (GB4456-84). An improved Bruss osmometer based on the flow
Fig. 1. SEM of the surfaceof regenerated cellulose membranes for RC-0 (a), RC II-2-5 (b), RC II-3-2 (c), RC II-4-3 (d), RC III-3 (e), and
RC III-4 (f).
G. Yang, L. Zhang / Journal of Membrane Science 114 (1996) 149-155
152
rate method reported in our previous work [11] was used for measuring the mean pore radius (?f) of the wet membrane. The rf value was given by:
rf =
PrAPiS
k = 3.1(1 - Pr2) 1/2
W Pr= 1
1.5zr dR 2
where 7/ is the absolute viscosity of fluid, ( d v / d t ) i
the rate of flow through the membrane, APi the pressure difference, S the effective area of the membrane, Pr the porosity, k the apparent dimension of the pore distribution, W the weight of the dry membrane, and R the radius of the wet membrane. The transmembrane flow of water ( J ) was measured on a miniature ultrafiltration equipment at 1 atm and 20°C. j=--
V tS
where V is the permeate volume for a given time t and S is the membrane area.
I
Fig. 2. SEM of the cross section of membranes for RC-0 (A), RC II-4-3 (B), RC II-2-5 (C and D), and RC II-3-4 (E and F).
153
G. Yang, L. Zhang/Journal of Membrane Science 114 (1996) 149-155
lar weights and contents of PEG in the mixture are shown in Figs. 3 and 4. The values of 2 ~f and J increase with the increase of molecular weight of PEG, and reach the highest at M = 2000 then decrease. The values of 27f and J of the membrane with the addition of PEG2000 appreciably increases with the PEG content (Fig. 4). The values of 2 ?f and J are summarized in Table 2. The values of RC II-2-5 are five times that of RC-0. This implies that PEG2000 is the most suitable as the pore former of regenerated cellulose membrane. The result is in good agreement with that from SEM. However, in Fig. 4 the 2 re and J values of the membrane with PEG20000 almost did not change with the amount added. It is regarded that the cellulose cuoxam with PEG20000 is not miscible because of the high molecular weight of PEG. Therefore, pore formation of the microporous membrane prepared by mixture was remarkably influenced by the molecular weight and amount of polymer added. Only polymer added with a certain molecular weight results in the presence of voids in the membrane structure. It coincides with Kamide's theory [13] on particle growth during membrane formation. When the distance between the centers of gravity of two arbitrarily chosen particles is less than the summation of their radii, they are considered to have collided, yielding a new larger particle. So polymer addition with a lower molecular size yields a larger velocity and larger collision frequency. Fig. 5 shows that the 2~f and J values of the membranes mixed with gelatin addition increase with increase of the gelatin content. It is considered that the mixture of gelatin composed of 18 amino acids with hydrophobic R groups and hydrophilic NH~,
3. R e s u l t s a n d discussion 3.1. Microporous structure
SEM of the surface of the membranes are shown in Fig. 1. The membrane of RC-0 displays a smooth surface being consistent with a homogenous component. The SEM of RC II-2-5 membrane containing 50% PEG2000 exhibits a microporous structure produced by fine microphase separation, which is clearly observed on the SEM of the cross section of the membranes shown in Fig. 2. It is similar to the electron micrographs of the RCa membrane reported by Kamide et al. [12]. This suggests that the mixture of the cellulose cuoxam and PEG2000 was miscible, and the microporous membrane was formed by dissolution of PEG as a pore former from cellulose into an aqueous coagulant. However, the membranes of RC 11-3-2 and RC 1I-4-3 mixed with PEG6000 and PEG20000 (M > 6000) appear as very rough with a sunken surface in their SEM (Fig. 1), and in the cross sections of the RC 11-3-4 and RC II-4-3 membranes (Fig. 2) bumps and holes appeared. It seems that PEG with a high molecular weight as a nonsolvent of cellulose cuoxam solution caused phase separation. The SEM shown in Fig. 1 shows that the membrane with gelatin added has a fine microphase separated structure when the content of gelatin is less than 35%, suggesting that the polymer systems formed by cellulose cuoxam and gelatin are miscible. 3.2. Effect of molecular weight and content
The mean pore diameters (2 ~f) and the transmembrane flow of water ( J ) dependence on the molecu-
~LO
-----' 100
4O
,,z 20
-/L m
40
2
I 3
lgM w
I 4
I
5
~
10
I
I
I
3
4
5
IgM w
Fig. 3. PEG molecularweightdependencesof the mean porediameter(27f) and transmembraneflow of water(J).
154
G. Yang, L. Zhang /Journal of Membrane Science 114 (1996) 149-155
Table 2 Membrane characterization: mean porous diameter, transmembrane flow and mechanical properties Membrane No.
Wet
RC-0 RC II-1-2 RC I1-2-5 RC 1I-3-3 RC 11-4-2 RC III-2 RC III-4
Dry
2~f (nm)
J (ml/h m 2 mmHg)
o-b (kg/cm 2)
e b (%)
Xc (%)
8 22 41 20 23 13 15
17.9 24.9 94.6 39.1 38.7 22.9 33.2
598 935 664 527 648 703 627
6 18 15 18 9 7 5
38 44 41 39 40 40 34
40'~
30
'---- 80
f/.
-
r-
0
20
10/ 0
I
I
I
20
40
60
2
0
PEG content / Yo
20
40
60
PEG content / 0~
Fig. 4. PEG contentdependencesof the mean pore diameter(2~f) and transmembraneflow of water(J): ( • ) PEG400, (C)) PEG2000, ( @) PEG6000, and (O) PEG20000.
30
15
=14 %
=
13
=
26
c-
]2
0
I
I
I
10
20
30
Gelatin content /
22
18 10 20 30 Gelatin content / e/~
Fig. 5. Gelatin content dependences of the mean pore diameter (2 ~f ) and transmembrane flow of water (J).
G. Yang, L. Zhang / Journal of Membrane Science 114 (1996) 149-155
COO- groups and cellulose cuoxam was miscible and fine microphase separation occurred. Moreover the nitrogen of the microporous membranes with gelatin added were near to zero suggesting that the gelatin plays a role in pore formation as a pore former. In contrast to the membrane blended by cellulose cuoxam/casein [9], the experimental values of nitrogen in the blend membrane were in good agreement with calculation of casein addition, and its strength was markedly higher than that of the nonblend one.
3.3. Mechanical properties The tensile strengths (o-b), breaking elongations (e b) and degrees of crystallinity (X c) of the membranes are summarized in Table 2. It is clear that the values of o"b, Eb and Xc decrease with increasing content and molecular weight of addition. The results are in good agreement with those analyzed by SEM. The decrease of their mechanical properties results from decreasing miscibility of the mixture. When the amount of polymer added was less than 10 wt%, both PEG and gelatin with the cellulose cuoxam are miscible, and their values of o-b are higher than those of RC-0 membrane. In addition, the crb, Eb and Xc values of the membranes with PEG400 addition are markedly higher than that of RC-0 membrane due to its good miscibility. The gelatin is the solvent-type pore former of a dense film. Thus, the degree of crystallinity and tensile strength of the membranes containing gelatin were superior, but increasing pore size was limited.
4. Conclusions The microporous membranes by mixing cellulose cuoxam with aqueous solutions of PEG (M = 2000) or gelatin (M = 8.4 × 104) were satisfactorily prepared. Their mean pore diameter (2~f) and transmembrane flow of water ( J ) are higher than that of non-mixing membrane. The values of 2rf for the membrane with added PEG2000 (50%) are more than four times that of the general regenerated cellulose membrane. With increasing content of water soluble polymer in the membrane, the mechanical properties decrease and the pore size appreciably
155
increases. The water soluble nature of synthetic polymers can be used as pore formers of regenerated cellulose membrane from cuoxam. The pore sizes and transmembrane flow of water of the microporous membrane can be adjusted by change of content and molecular weight of the polymer added.
References [1] F. Mouror and M. Oliver, Comparative evaluation of ultrafiltration membranes for purification of synthetic peptides, Sep. Sci. Technol., 24(5/6) (1989) 353. [2] J.H. Hanemaij, T. Robbertsen, Th. Van den Boomgaard, C. Olieman, P. Both and D.G. Schmidt, Characterization of clean and fouled ultrafiltration membranes, Desalination, 68 (1988) 93. [3] Y. Hamamoto, S. Harada, S. Kobayashi, H. lijima and S. Manabe, A novel method for removal of human immunodeficiency virus: filtration with porous polymeric membrane, Vox Sanguinis, 56 (1989) 230. [4] S. Sekiguchi, K. Oto, M. Kobayashi and H. Ikeda, Possibility of hepatitis B virus (HBV) removal from human plasma using regenerated cellulose hollow fiber (BMM), Membrane, 14(4) (1989) 101. [5] K. Kamide, H. lijima and S. Matsuda, Thermodynamics of formation of porous polymeric membrane by phase separation method I. Nucleation and growth of nuclei, Polym. J., 25(11) (1993) 1113. [6] K. Kamide, H. lijima and H. Shirataki, Thermodynamics of formation of porous polymeric membrane by phase separation method II. Particle simulation approach by monte carlo method and experimental observations for the process of growth of primary particles to secondary particles, Polym. J., 26(1) (1994) 21. [7] G. Dunweg, Microporous Cellulose Membrane, UK Pat., GB7086798, 1982. [8] L. Zhang and G. Yang, Regenerated cellulose membrane from cuoxam/zincoxene blend, J. Membrane Sci., 56 (1991 ) 207. [9] L. Zhang, G. Yang and L. Xiao, Blend membranes of cellulose cuoxam/casein, J. Membrane Sci., 103 (1995) 65. [10] L. Zhang, G. Yang, S. Yan and H. Liu, Regenerated cellulose membrane with water resistance, Chinese Pat., CN 1091144A, 1993. [11] L. Zhang and G. Yang, Study on rejection polystyrenes in toluene through regenerated cellulose membranes, Chin. J. Appl. Chem., 8(3) (1991) 17. [12] K. Kamide, S. Nakamura, T. Akedo and S. Manabe, An electromicrographical method for evaluating layer structure of polymeric hollow fiber membrane, Polym. J., 21(3) (1989) 241. [13] K. Kamide and S. Manabe, Role of microphase separation phenomena in the formation of porous polymeric membranes, Materials Science of Synthetic Membrane, 1985, p. 197.
iournalof •
i "
MEMBRANE SCIENCE
/ I
II ELSEVIER
Journal of Membrane Science 114 (I 996) 149-155
Regenerated cellulose microporous membranes by mixing cellulose cuoxam with a water soluble polymer Guang Yang, Lina Zhang * Department of Chemistry, Wuhan University, Wuhan430072, China
Received 13 February 1995; accepted 8 November 1995
Abstract Regenerated cellulose microporous membranes were satisfactorily prepared by mixing cellulose cuoxam with polyethylene glycol (PEG) and gelatin as pore former. When the PEG molecular weight was smaller than 2000, a mixture of cellulose cuoxam and PEG aqueous solution was miscible, and membranes with a microporous structure were formed. The structure and pore characteristics of the microporous membranes were studied by X-ray diffraction, SEM and a flow rate method. Their mean pore diameter (2~e) and the transmembrane flow of water (J) are more than four times that of unmixed regenerated cellulose membrane. The effects of the water soluble polymer additions on the pore size, structure and mechanical properties of the microporous membrane are discussed. Keywords: Regenerated cellulose; Microporous membrane; Pore former; Mean pore diameter; Transmembrane flow of water
1. Introduction In the last 30 years cellulose membranes have found extensive commercial applications in membrane separation processes, because of their relatively low cost, good compatibility with biological compounds and their remarkable hydrophilic properties [1,2], It is worth noting that utilizing rich plant cellulose as a material can not only reduce loss of limited petroleum resources but also protect the environment. The microporous structure of the membrane is important as it is closely related to transmembrane
* Corresponding author.
flow of water and sieving properties for macromolecules. Recently, Bemberg microporous membranes (BMM) prepared by the microphase separation method have successfully eliminated viruses from human plasma [3,4]. The membrane was prepared using a ternary coagulant mixture of acetone, water and ammonia from a cellulose cuoxam solution. The primary particles grow by amalgamation and under some conditions their radii approach an asymptotic value, which is the radius of the secondary particle [5,6]. In addition, Dunweg [7] showed that a microporous cellulose membrane, regenerated from cuoxam solutions to which polyethylene glycol having an average molecular weight of from 100 to 1500 had been added, had a high ultrafiltration capacity and screening coefficient for proteins. In previous work [8,9], the miscibilities of regenerated
0376-7388/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved SSDI 0376-7388(95)003 14-2
150
G. Yang, L. Zhang/Journal of Membrane Science 114 (1996) 149-155
cellulose membranes blended cellulose from cuoxam/zincoxene and cellulose cuoxam/casein were studied. The experimental results showed that the mean pore diameter (27f) and ethanol permeability (G) of a blend membrane from cellulose cuoxam/zincoxene were lower than those of a nonblend membrane, but the values of the blend from cellulose cuoxam/casein were slightly higher than that of the non-blend one suggesting that the fine microphase separated structure resulted from the hydrophobic R group in the casein provided correspondingly greater pores. This has prompted us to study a simple and powerful method for controlling the pore size of a membrane by the microphase separation of cellulose cuoxam with a water soluble polymer. In this paper, we attempted to prepare microporous membranes by mixing cellulose cuoxam with gelatin or polyethylene glycol (PEG) as pore former. The effects of these additions on the pore size, pore characteristics, morphological structure and mechanical properties of the porous membranes are discussed.
2. E x p e r i m e n t a l
2.1. Materials The linter used was supplied by Hubel Chemical Fiber Manufacture, and its viscosity average molecular weight (M,7) was determined to be 19.6 X 10 4.
Table 1 The compositionof the membranes MembraneNo. Addition
RC II-l-1 RC II-1-2 RC II-1-3 RC ll-2-1 RC 112-2 RC II-2-3 RC II-2-4 RC II-2-5 RC II-3-1 RC II-3-2
Four grades of polyethylene glycol imported from Japan (PEG400, PEG2000, PEG6000, PEG20000) with molecular weights of 400, 2 × 103, 6 X 103 and 2 X 104, respectively, and gelatin ( M = 8.4 X 104) purchased from Changdu Chemical Reagent Factory were used in this work. They are all water soluble polymers. The main components of the gelatin were analysed to be Gly (22%), Pro (14%), Glu (11%) with an amino acid analyzer (Waters PICO, TAGT). The amino acid residues of the gelatin contained 44.6% hydrophilic groups and 55.4% hydrophobic groups. 2.2. Preparation of membranes A 6 wt% cellulose cuoxam solution (I) was prepared [10]. The desired amount of PEG400 (II-1), aqueous solutions of 20% PEG2000 (II-2), 20% PEG6000 (II-3), 20% PEG20000 (1I-4) and 20% gelatin (III) were respectively added to I. The composition of the membranes prepared by mixing I and II or III are shown in Table 1. The cellulose/polymer mixture solution was filtered, then cast on a glass plate to a depth of 0.25 mm. The cast solution was immersed immediately into a coagulation bath of dilute NaOH aqueous solution. The cellulose membrane regenerated in dilute H2SO 4 was washed in running water, then placed in iso-propanol for 10 h, and finally stored in 20% iso-propanol/2% formaldehyde aqueous solution. The membranes were dried on the glass plate in air for measurement of
Weight ratio (cellulose: addition)
Membraneno.
Addition
Weight ratio (cellulose: addition)
PEG400 PEG400 PEG400 PEG2000 PEG2000 PEG2000
2:1 I: 1 1:2 20:1 10:1 5:1
PEG2000
2:1
PEG2000 PEGt000 PEGt000
1:1 20:1 10:1
RC II-3-3 RC II-3-4 RC II-4-2 RC II-4-3 RC 1I-4-4 RC 1I-4-5 RC III-1 RC Ill-2 RC III-3 RC III-4
PEGt000 PEGt000 PEG20000 PEG20000 PEG20000 PEG20000 Gelatin Gelatin Gelatin Gelatin
5:1 2: I 20:1 10:1 5: i 2:1 20:1 10:1 5:1 2:1
G. Yang, L. Zhang/Journal of Membrane Science 114 (1996) 149-155
their mechanical properties. The membranes are coded in Table 1 and RC-0 represents the membrane prepared by pure cellulose cuoxam. 2.3. Apparatus and measurements
Scanning electron micrographs (SEM) of the membranes were taken by the Material Research and Testing Center of Wuhan Technology University with an SEM ISI-SX-40. The membranes were coated with carbon and gold, then their surfaces and cross sections were observed and photographed. The cross sections of the membranes were prepared by breaking the membrane under liquid nitrogen.
151
The X-ray diffraction pattern was measured with an X-ray diffractometer Rigaku 3015. The X-ray diffraction patterns with C u k a at 35 kv and 25 mA were recorded in the region of 5-45 °. The degree of crystallinity was calculated from X-ray data according to the usual method. The nitrogen contents in gelatin and in the membranes added with gelatin were determined by a model PE-240B elemental analyzer. Tensile strengths (o"b) and breaking elongations (E b) of the dry membrane were measured on a tensile testing machine (Instron-1121) according to a Chinese standard method (GB4456-84). An improved Bruss osmometer based on the flow
Fig. 1. SEM of the surfaceof regenerated cellulose membranes for RC-0 (a), RC II-2-5 (b), RC II-3-2 (c), RC II-4-3 (d), RC III-3 (e), and
RC III-4 (f).
G. Yang, L. Zhang / Journal of Membrane Science 114 (1996) 149-155
152
rate method reported in our previous work [11] was used for measuring the mean pore radius (?f) of the wet membrane. The rf value was given by:
rf =
PrAPiS
k = 3.1(1 - Pr2) 1/2
W Pr= 1
1.5zr dR 2
where 7/ is the absolute viscosity of fluid, ( d v / d t ) i
the rate of flow through the membrane, APi the pressure difference, S the effective area of the membrane, Pr the porosity, k the apparent dimension of the pore distribution, W the weight of the dry membrane, and R the radius of the wet membrane. The transmembrane flow of water ( J ) was measured on a miniature ultrafiltration equipment at 1 atm and 20°C. j=--
V tS
where V is the permeate volume for a given time t and S is the membrane area.
I
Fig. 2. SEM of the cross section of membranes for RC-0 (A), RC II-4-3 (B), RC II-2-5 (C and D), and RC II-3-4 (E and F).
153
G. Yang, L. Zhang/Journal of Membrane Science 114 (1996) 149-155
lar weights and contents of PEG in the mixture are shown in Figs. 3 and 4. The values of 2 ~f and J increase with the increase of molecular weight of PEG, and reach the highest at M = 2000 then decrease. The values of 27f and J of the membrane with the addition of PEG2000 appreciably increases with the PEG content (Fig. 4). The values of 2 ?f and J are summarized in Table 2. The values of RC II-2-5 are five times that of RC-0. This implies that PEG2000 is the most suitable as the pore former of regenerated cellulose membrane. The result is in good agreement with that from SEM. However, in Fig. 4 the 2 re and J values of the membrane with PEG20000 almost did not change with the amount added. It is regarded that the cellulose cuoxam with PEG20000 is not miscible because of the high molecular weight of PEG. Therefore, pore formation of the microporous membrane prepared by mixture was remarkably influenced by the molecular weight and amount of polymer added. Only polymer added with a certain molecular weight results in the presence of voids in the membrane structure. It coincides with Kamide's theory [13] on particle growth during membrane formation. When the distance between the centers of gravity of two arbitrarily chosen particles is less than the summation of their radii, they are considered to have collided, yielding a new larger particle. So polymer addition with a lower molecular size yields a larger velocity and larger collision frequency. Fig. 5 shows that the 2~f and J values of the membranes mixed with gelatin addition increase with increase of the gelatin content. It is considered that the mixture of gelatin composed of 18 amino acids with hydrophobic R groups and hydrophilic NH~,
3. R e s u l t s a n d discussion 3.1. Microporous structure
SEM of the surface of the membranes are shown in Fig. 1. The membrane of RC-0 displays a smooth surface being consistent with a homogenous component. The SEM of RC II-2-5 membrane containing 50% PEG2000 exhibits a microporous structure produced by fine microphase separation, which is clearly observed on the SEM of the cross section of the membranes shown in Fig. 2. It is similar to the electron micrographs of the RCa membrane reported by Kamide et al. [12]. This suggests that the mixture of the cellulose cuoxam and PEG2000 was miscible, and the microporous membrane was formed by dissolution of PEG as a pore former from cellulose into an aqueous coagulant. However, the membranes of RC 11-3-2 and RC 1I-4-3 mixed with PEG6000 and PEG20000 (M > 6000) appear as very rough with a sunken surface in their SEM (Fig. 1), and in the cross sections of the RC 11-3-4 and RC II-4-3 membranes (Fig. 2) bumps and holes appeared. It seems that PEG with a high molecular weight as a nonsolvent of cellulose cuoxam solution caused phase separation. The SEM shown in Fig. 1 shows that the membrane with gelatin added has a fine microphase separated structure when the content of gelatin is less than 35%, suggesting that the polymer systems formed by cellulose cuoxam and gelatin are miscible. 3.2. Effect of molecular weight and content
The mean pore diameters (2 ~f) and the transmembrane flow of water ( J ) dependence on the molecu-
~LO
-----' 100
4O
,,z 20
-/L m
40
2
I 3
lgM w
I 4
I
5
~
10
I
I
I
3
4
5
IgM w
Fig. 3. PEG molecularweightdependencesof the mean porediameter(27f) and transmembraneflow of water(J).
154
G. Yang, L. Zhang /Journal of Membrane Science 114 (1996) 149-155
Table 2 Membrane characterization: mean porous diameter, transmembrane flow and mechanical properties Membrane No.
Wet
RC-0 RC II-1-2 RC I1-2-5 RC 1I-3-3 RC 11-4-2 RC III-2 RC III-4
Dry
2~f (nm)
J (ml/h m 2 mmHg)
o-b (kg/cm 2)
e b (%)
Xc (%)
8 22 41 20 23 13 15
17.9 24.9 94.6 39.1 38.7 22.9 33.2
598 935 664 527 648 703 627
6 18 15 18 9 7 5
38 44 41 39 40 40 34
40'~
30
'---- 80
f/.
-
r-
0
20
10/ 0
I
I
I
20
40
60
2
0
PEG content / Yo
20
40
60
PEG content / 0~
Fig. 4. PEG contentdependencesof the mean pore diameter(2~f) and transmembraneflow of water(J): ( • ) PEG400, (C)) PEG2000, ( @) PEG6000, and (O) PEG20000.
30
15
=14 %
=
13
=
26
c-
]2
0
I
I
I
10
20
30
Gelatin content /
22
18 10 20 30 Gelatin content / e/~
Fig. 5. Gelatin content dependences of the mean pore diameter (2 ~f ) and transmembrane flow of water (J).
G. Yang, L. Zhang / Journal of Membrane Science 114 (1996) 149-155
COO- groups and cellulose cuoxam was miscible and fine microphase separation occurred. Moreover the nitrogen of the microporous membranes with gelatin added were near to zero suggesting that the gelatin plays a role in pore formation as a pore former. In contrast to the membrane blended by cellulose cuoxam/casein [9], the experimental values of nitrogen in the blend membrane were in good agreement with calculation of casein addition, and its strength was markedly higher than that of the nonblend one.
3.3. Mechanical properties The tensile strengths (o-b), breaking elongations (e b) and degrees of crystallinity (X c) of the membranes are summarized in Table 2. It is clear that the values of o"b, Eb and Xc decrease with increasing content and molecular weight of addition. The results are in good agreement with those analyzed by SEM. The decrease of their mechanical properties results from decreasing miscibility of the mixture. When the amount of polymer added was less than 10 wt%, both PEG and gelatin with the cellulose cuoxam are miscible, and their values of o-b are higher than those of RC-0 membrane. In addition, the crb, Eb and Xc values of the membranes with PEG400 addition are markedly higher than that of RC-0 membrane due to its good miscibility. The gelatin is the solvent-type pore former of a dense film. Thus, the degree of crystallinity and tensile strength of the membranes containing gelatin were superior, but increasing pore size was limited.
4. Conclusions The microporous membranes by mixing cellulose cuoxam with aqueous solutions of PEG (M = 2000) or gelatin (M = 8.4 × 104) were satisfactorily prepared. Their mean pore diameter (2~f) and transmembrane flow of water ( J ) are higher than that of non-mixing membrane. The values of 2rf for the membrane with added PEG2000 (50%) are more than four times that of the general regenerated cellulose membrane. With increasing content of water soluble polymer in the membrane, the mechanical properties decrease and the pore size appreciably
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increases. The water soluble nature of synthetic polymers can be used as pore formers of regenerated cellulose membrane from cuoxam. The pore sizes and transmembrane flow of water of the microporous membrane can be adjusted by change of content and molecular weight of the polymer added.
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