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Formation of poly(vinylidene fluoride) (PVDF) membranes via thermally induced phase separation
Desalination 192 (2006) 160–167
Formation of poly(vinylidene fluoride) (PVDF) membranes via thermally induced phase separation Minghao Gua, Jun Zhanga*, Xiaolin Wangb, Haijun Taoa, Litian Gea a
College of Materials Science and Engineering, Nanjing University of Technology, Nanjing, 210009, China Tel. +86 (25) 8358 7264; Fax: +86 (25) 8324 0205; email: [email protected] b Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Received 14 March 2005; accepted 7 October 2005
Abstract The effects of diluents, thermodynamic interactions, different quenching conditions and coarsening on the morphology of poly(vinylidene fluoride) (PVDF) membranes via thermally induced phase separation were investigated. Scanning electron microscopy indicated that an interconnected spherulitic structure was obtained in the PVDF–DMP system, while the PVDF–DMP–DOS and PVDF–DMP–DOA systems showed a jagged and sharp-edged spherulitic structure. As the interactions decreased, the spherulitic structure was increasingly non-discernable and the value of porosity increased in the order of PVDF–DMP, PVDF–DMP–DOA, PVDF–DBP and PVDF–DMP–DOS. When quenching in 353 K and 368 K, the spherulities became more discernable, and the size of spherulity increased. When quenching in different conditions, the discernible spherulitic structure was obtained in ice water, a greater spherulitic size was obtained in a water bath at 303 K, and PVDF crystallized much slower and improved in an air bath. Keywords: PVDF; Membrane; Thermally induced phase separation; Diluents; Spherulitic structure
1. Introduction PVDF with its semi-crystalline, thermal stability and chemical resistance is a favorable material for membrane products. Many techniques have been used for PVDF membranes. Immerse preci*Corresponding author.
pitation is the main method, as PVDF has good solubility with dimethyl-accetamide and Nmethylpyrrolide at room temperature [1]. PVDF solutions immersed in nonsolvents can form different membranes with celluar, spongy and spherulitic structures [2]. PVDF membranes have been widely used in oil–water separation, gas filtration and wastewater disposal.
Presented at the International Congress on Membranes and Membrane Processes (ICOM), Seoul, Korea, 21–26 August 2005. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.
doi:10.1016/j.desal.2005.10.015
M. Gu et al. / Desalination 192 (2006) 160–167
Thermally induced phase separation (TIPS) is one of the main techniques for the preparation of polymeric porous membranes by controlling phase separation [3].TIPS can be classified into two processes: liquid–liquid (L–L) separation and solid–liquid (S–L) separation [4]. In the TIPS process, a polymer is dissolved in a diluent at a high temperature with the cooling of the solution. When the cooling temperature reaches a binodal line, L–L separation is induced. As for L–L separation, two mechanisms have to be considered: nucleation growth (NG) and spinodal decomposition (SD) [5,6].The NG mechanism occurs in a metastable region in the phase diagram between spinodal and binodal lines, while SD is in an unstable region under spinodal lines. Therefore, membranes formed by SD or NG mechanisms result in a different morphology of membrane porosity and structure of polymer crystallization. When the cooling temperature reaches the crystal lines of the polymer, S–L separation occurs; then the polymer crystallizes and the polymer diluent structure is fixed. Finally, the diluent is removed by extraction, and the porous membrane is obtained. However, PVDF membranes have a limited range of microstructures using the traditional methods in both immerse precipitation and TIPS. PVDF typically forms isotropic, spherulitic microstructures that often include large macrovoids. Lloyd [7] prepared microporous membranes via TIPS by PVDF with diluents of dibutyl-phthalate (DBP), while isotropic spherulitic microstructures that included large macrovoids and irregular porous were formed, dispersed in the spherulities. Hiatt [8]used cyclohexanone, crylic acid and γ-butyrolactone as diluents for PVDF, and an isotropic spherulitic microstructure was also obtained. Therefore, the application problem for PVDF membranes via TIPS focuses on how to control the irregular membrane porous structure. In this work, effects of diluents on membrane morphology, thermodynamic interactions on
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membrane structure and coarsening on the morphology of membranes were studied. 2. Experimental 2.1. Materials Membranes were prepared with systems of PVDF–DMP, PVDF–DMP–DOA, PVDF–DMP– DOS and PVDF–DBP. The polymer was poly(vinylidene fluoride), FR904, purchased from Shanghai 3F New Materials, China. Dioctylsebacate (DOS), dioctyl-phthalate (DOP) and dimethyl-phthalate (DMP) were used as diluents without further purification. These diluents were purchased from Shanghai Reagents, China. 2.2. Membrane preparation Appropriate amounts of polymer and diluent were measured into a test tube, which was placed into an oil bath. The polymer diluent mixture was dissolved for 4 h at about 453 K. Then the mixture was quenched in air for 20 min, yielding a solid polymer diluents sample. The solid sample was chopped into small pieces and placed in a tailor-made test tube. After reheating in an oven for 10 min at 453 K, it was taken out to quench in the water bath. Finally the diluents were extracted from the membrane with methanol or n-hexane. The extractants were evaporated. Membranes were dried further in a vacuum oven at a slightly elevated temperature. 2.3. Phase diagram Different scanning calorimetry (DSC) was used to determine the crystallization temperature for the dynamic phase diagram. The solid polymer diluent samples were sealed in an aluminum DSC pan, melted at 453 K for 10 min, and then cooled at 10 K/min. The onset of the exothermic peak during the cooling was taken as the dynamic
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crystallization temperature. The cloud points were determined by noting the appearance of turbidity under a microscope. 2.4. Scanning electron microscopy observation The structure and morphology of membranes were observed by scanning electron microscopy (SEM). The cross section of membranes was freeze-fractured under liquid nitrogen. The membrane samples were gold sputtered and analyzed using SEM (Kash SX-40). 2.5. Porosity measurements The appropriately measured PVDF membranes were immersed in i-butanol for 24 h and taken out to be weighed immediately after removing i-butanol on the surface. The method calculating the porosity is available in the literature [9]. The porosity was calculated according to the formula:
where W1 is the initial membrane weight, W2 the immersed membrane weight, ρ1 the density of PVDF, and ρ2 the density of i-butanol. 3. Results and discussion 3.1. Phase diagram The compatibility of polymer and diluents directly reflects the thermodynamic properties such as the binodal line and crystallization temperature. The diluents have different effects on the properties of membranes. Interpretation of the interactions of polymer diluents and the structure of TIPS membranes requires a systems equilibrium phase diagram [10]. The phase diagrams of
PVDF–DMP and PVDF–DMP–DOA are shown in Figs. 1 and 2 where the cloud points decreased and crystallization temperatures slightly increased as a fraction of the PVDF contents increased. The monotectic point (φm) of PVDF–DMP was between 25–30%, and the monotectic point of PVDF–DMP–DOA was between 30–40%. The L–L is located left of φm. The S–L region is located below φm. As a result, the spherulitic structure is the system that has undergone S–L TIPS via polymer crystallization with a low PVDF fraction. It was found that the cloud point line and the binodal line of PVDF–DMP–DOA shifted to a higher temperature than the PVDF– DMP system, showing that DMP–DOA has a lower compatibility with PVDF than DMP. Meanwhile the cloud points of PVDF–DMP– DOA shifted to higher polymer contents than PVDF–DMP, which resulted in a long time for pore growth in the cooling process [4]. 3.2. Effects of diluents In the TIPS process, how the diluents should be selected is an important problem for controlling pore size. Diluents with different compatibilities of PVDF have effects on membrane morphology [11]. When the diluents are subjected to a temperature below the equilibrium melting temperature of the polymer solution, the system undergoes S–L separation via polymer crystallization. During the crystallization process, the diluents are rejected to the inter-spherulitic regions. Upon diluents extraction, the interspherulitic regions, diluent-rich regions, become the membrane pores. Table 1 shows the dielectric constant of different diluents. It was showed that the polar of different diluents is in the order of DMP, DBP, DOA and DOS. Fig. 3 shows the effects of three diluents on the membrane. It was found that PVDF–DMP formed the most discernable and largest spherulitic structure because DMP has close polar with PVDF and the strong
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Fig. 1. Dynamic phase diagram of PVDF–DMP systems.
Fig. 2. Dynamic phase diagram of PVDF-DMP-DOA systems.
Fig. 3. Micrographs of cross section of a PVDF (25%) membrane prepared with systems of different diluents quenched at 303 K after melting at 453K (×3 k).
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Table 1 Dielectric constant of different diluents [15] Diluent
Dielectric constant
DMP DBP DOA DOS
8.5 6.4 4–5 <4
interactions of C=O in DMP with -F- in PVDF [12], which has advantages for the rich polymer phase to flow, congregate and form a discernable spherulitic structure [13].While DBP has a lower polar than DMP, the PVDF–DBP system showed less discernable spherulity than PVDF–DMP. Meanwhile the PVDF–DMP–DOS and PVDF– DMP–DOA systems showed a jagged and sharpedged spherulitic structure because the added DOS or DOA lowered the interactions in the polymer solution with their weak compatibility with PVDF, which resulted in certain limitations in PVDF crystallization, an unobvious spherulitic structure and lower spherulity size. As DOS has a lower polar than DOA, which means DMP– DOS showed lower compatibility with PVDF than DMP–DOA, the size of the spherulites was more irregular [14]. As shown in Table 2, the value of porosity of the above four systems was in the order of PVDF–DMP, PVDF–DMP–DOA, PVDF–DBP and PVDF–DMP–DOS. The stronger the interactions of diluents with PVDF, the easier the polymer-rich phase can flow and congregate. The area between the spherulites became more compacted as the interactions increased. When the structures of the systems were fixed quenching at 303 K, the area became the membrane pore as the effect of extraction, and the value of porosity decreased as the interactions increased. From Table 1, it is seen that the interactions of diluents with PVDF were in the order of DMP, DMP– DOA, DBP and DMP–DOS, which was coincident with the order of the value of porosity.
Table 2 Porosity of PVDF diluent systems with PVDF fraction of 25 wt% PVDF system
Porosity
DMP DMP–DOA DMP–DOS DBP
0.42 0.59 0.68 0.63
Meanwhile Fig. 3 shows that the area between the spherulites was increased in the order above. 3.3. Effect of quenching conditions and temperatures Although different diluents have effects on membrane morphology, different quenching conditions also affect the polymer crystallization structure [16]. In Fig. 4 it is seen that when membranes are quenched in ice water, there was discernable spherulitic structure. As the cooling rate was extremely high with a strong crystallization potential, the phase separation occurred in the unstable region (SD), and the structure was fixed without any coarsening. When the quenching temperature increased to 303 K, the cooling rate was lower than in ice water, and the spherulite size increased. When quenching at room temperature, the polymer spherulity grew slower and the spherulite size was greater than quenched at ice water. As the cooling rate was slower than when quenched at 303 K, PVDF crystallized much slower and improved, and the spherulity became more regular in size [17]. As shown in Fig. 5, 353 K is below the crystal line, and the quenching times were set in 1 min, 3 min, 5 min and 20 min. As quenching times increased, the spherulities were more discernable and the size of spherulities increased. As polymer crystallized, which inhibited droplets to grow, the diluent was rejected and the small pores were obtained in the spherulites.
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(a)
(b)
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(c)
Fig. 4. Membrane structure of PVDF–DMP formed at different quenched conditions with a PVDF concentration of 25%. a, ice water; b, water bath (303 K); c, air bath at room temperature (303 K). (×1 k).
Fig. 5. Scanning electron micrographs of the cross section of PVDF–DMP membranes as a function of coarsening time at 353 K with a PVDF concentration of 20% (×5 k).
Fig. 6. Scanning electron micrographs of the cross section of PVDF–DMP membranes as a function of coarsening time at 368 K with a PVDF concentration of 20% (×5 k).
3.4. Coarsening effect Three different mechanisms were described for the coarsening of the microstructure in the late stages of phase separation: Ostwald ripening, coalescence and the hydrodynamic flow mechanism [18]. As for the real growth of droplets, the coarsening process is described by the direct ratio d:t α. The exponent α depends on the microscopic
mechanisms of particle growth. Sigga [19] conidered both coalescence and hydrodynamic flow effects in phase separation, and deduced the droplets growth process in three stages: intermedia, flow and gravity-dominated. The coarsening effects studied at 368 K are seen in Fig. 6. The coarsening time was set at 1 min, 3 min, 5 min and 20 min. As shown in
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Fig. 6, 368 K was above the crystallization temperature. Then the droplets grew through the L–L phase region. As a result, the rich polymer phase had the potential to flow and congregate. There is discernable spherulitic structure at 3 min. As time increased, the size of the spherulities increased, while the size of the pore was unvaried as the constraint of polymer crystallization. Comparing Figs. 5 and 6, it was found that the spherulite growth rate of PVDF was higher at 368 K than at 353 K, as droplets had more time to congregate, while the structure and size of pores was unvaried as coarsening or quenching depth increased.
4. Conclusions 1. Phase diagrams were obtained for two PVDF diluent systems. The cloud-point curve was shifted to the lower temperature in the order PVDF–DMP, PVDF–DMP–DOA. This order can be explained by the compatibility between PVDF and diluents. The crystallization temperatures were slightly influenced by the polymer fraction. 2. At different quenched conditions, smaller spherulite sizes were obtained by quenching in ice water, and the improved and regular spherulities were formed when quenching at ambient temperature, while the lesser improved spherulities were obtained at 303 K. 3. Membrane morphology was greatly determined by the interactions between diluents and polymer. As the interactions decreased, the spherulitic structure was less discernable and porosity increased in the order of PVDF–DMP, PVDF–DMP–DOA, PVDF–DBP and PVDF– DMP–DOS. 4. When quenching at 353 K and 368 K, the spherulities became more discernable, and their size increased. The spherulite growth rate of PVDF was higher at 368 K than at 353 K, as the polymer-rich phase had more time to congregate in the L–L phase separation region.
Acknowledgments This work is supported by the Hi-Tech Research and Development Program of China (2002AA328020) and National Basic Research Program of China (2003CB615701). References [1] A. Bottino, G.C. Roda, G. Capannelli and S. Munari, The formation of microporous polyvinylidene difluoride membrane by phase separation. J. Membr. Sci., 57 (1991) 1–20. [2] M.L. Yeow, Y.T. Liu and K. Li, Morphological study of poly(vinylidene fluoride) asymmetric membranes: Effects of the solvent, additive and dope temperature. J. Appl. Poly. Sci., 92 (2004) 1782– 1789. [3] D.R. Lloyd, Membrane materials science—an overview, in: D.R. Lloyd, ed., Materials Science of Synthetic Membranes, Symp. Ser. No. 269, ACS, Washington, DC, 1985. [4] D.R. Lloyd, S. Kim and K.E. Kinzer, Microporous membrane formation via thermally induced phase separation. II. Liquid–liquid phase separation. J. Membr. Sci., 64 (1991) 1– 11. [5] D.R. Lloyd, J.W. Barlow and K.E. Kinzer, Microporous membrane formation via thermally induced phase separation. AIChE Symp. Ser., 84 (1988) 28–41. [6] H. Matsuyama, S. Kudari, H. Kiyofuji and Y. Kitamura, Kinetic studies of thermally induced phase separation in polymer-diluent system. J. Appl. Poly. Sci., 76 (2000) 1028–1036. [7] D.R. Lloyd and K.E. Kinzer, Microporous membrane formation via thermally induced phase separation. І. Solid–liquid phase separation. J. Membr. Sci., 52 (1990) 239–261. [8] W.C. Hiatt and G.H. Vitzthum, Microporous membrane via thermally induced phase separation, in: D.R. Lloyd, ed., Materials Science of Synthetic Membranes, Symp. Ser. No. 269, ACS, Washington, DC, 1985. [9] F. Luo, J. Zhang, X.L. Wang, J.F. Cheng and Z.J. Xu, Formation of hydrophilic EAA copolymer microporous membranes via thermally induced phase separation. Acta Polymerica Sinica, (2002) 566–571.
M. Gu et al. / Desalination 192 (2006) 160–167 [10] M. Kurata, Thermodynamic of Polymer Solutions, Harwood Academic, London, 1982. [11] H. Matsuyama, M. Teramoto, S. Kundari and Y. Kitamura, Effect of diluents on membrane formation via thermally induced phase separation. J. Appl. Polym. Sci., 82 (2001) 169–177. [12] M. Tazaki, R. Wada, M. Okabe and T. Homma, Crystallization and gelation of poly-(vinylidene fluoride) in organic solvents. J. Appl. Polym. Sci., 65 (1997) 1517–1524. [13] M. He, X. Cheng and X. Dong, Polymer Physics, 2nd ed., Fudan University Press , Shanghai, 1990, pp. 58–96. [14] C. Hitesh, H. Vadalia, K. Lee Allen and S. Myerson Thermally induced phase separation in ternary crystallizable polymer solution. J. Membr. Sci., 89 (1994) 37–50. [15] N. Cheng, Handbook of Solvents, Chemical Industry Press, Beijing, 1994, pp. 288–372.
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[16] B.F. Barton and A.J. McHugh, Modeling the dynamics of membrane structure formation in quenched polymer solutions. J. Membr. Sci., 166 (2000) 119– 125. [17] M.C. Yang and J.S. Perng, Effect of quenching temperature on the morphology and separation properties of polyproplene microporous tubular membranes via thermally induced phase separation. J. Polym. Res., 5 (1998) 213–219. [18] S. Wang and J.M. Torkeson, Coarsening effects on the formation of micorporous membrane produced via thermally induced phase separation of polystyrene-cyclohexanol solutions. J. Membr. Sci., 98 (1995) 209–222. [19] E.D. Siggia, Late stage of spinodal decomposition in binary mixtures. Physics Rev. Assoc., 20 (1979) 595–605.
Formation of poly(vinylidene fluoride) (PVDF) membranes via thermally induced phase separation Minghao Gua, Jun Zhanga*, Xiaolin Wangb, Haijun Taoa, Litian Gea a
College of Materials Science and Engineering, Nanjing University of Technology, Nanjing, 210009, China Tel. +86 (25) 8358 7264; Fax: +86 (25) 8324 0205; email: [email protected] b Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Received 14 March 2005; accepted 7 October 2005
Abstract The effects of diluents, thermodynamic interactions, different quenching conditions and coarsening on the morphology of poly(vinylidene fluoride) (PVDF) membranes via thermally induced phase separation were investigated. Scanning electron microscopy indicated that an interconnected spherulitic structure was obtained in the PVDF–DMP system, while the PVDF–DMP–DOS and PVDF–DMP–DOA systems showed a jagged and sharp-edged spherulitic structure. As the interactions decreased, the spherulitic structure was increasingly non-discernable and the value of porosity increased in the order of PVDF–DMP, PVDF–DMP–DOA, PVDF–DBP and PVDF–DMP–DOS. When quenching in 353 K and 368 K, the spherulities became more discernable, and the size of spherulity increased. When quenching in different conditions, the discernible spherulitic structure was obtained in ice water, a greater spherulitic size was obtained in a water bath at 303 K, and PVDF crystallized much slower and improved in an air bath. Keywords: PVDF; Membrane; Thermally induced phase separation; Diluents; Spherulitic structure
1. Introduction PVDF with its semi-crystalline, thermal stability and chemical resistance is a favorable material for membrane products. Many techniques have been used for PVDF membranes. Immerse preci*Corresponding author.
pitation is the main method, as PVDF has good solubility with dimethyl-accetamide and Nmethylpyrrolide at room temperature [1]. PVDF solutions immersed in nonsolvents can form different membranes with celluar, spongy and spherulitic structures [2]. PVDF membranes have been widely used in oil–water separation, gas filtration and wastewater disposal.
Presented at the International Congress on Membranes and Membrane Processes (ICOM), Seoul, Korea, 21–26 August 2005. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.
doi:10.1016/j.desal.2005.10.015
M. Gu et al. / Desalination 192 (2006) 160–167
Thermally induced phase separation (TIPS) is one of the main techniques for the preparation of polymeric porous membranes by controlling phase separation [3].TIPS can be classified into two processes: liquid–liquid (L–L) separation and solid–liquid (S–L) separation [4]. In the TIPS process, a polymer is dissolved in a diluent at a high temperature with the cooling of the solution. When the cooling temperature reaches a binodal line, L–L separation is induced. As for L–L separation, two mechanisms have to be considered: nucleation growth (NG) and spinodal decomposition (SD) [5,6].The NG mechanism occurs in a metastable region in the phase diagram between spinodal and binodal lines, while SD is in an unstable region under spinodal lines. Therefore, membranes formed by SD or NG mechanisms result in a different morphology of membrane porosity and structure of polymer crystallization. When the cooling temperature reaches the crystal lines of the polymer, S–L separation occurs; then the polymer crystallizes and the polymer diluent structure is fixed. Finally, the diluent is removed by extraction, and the porous membrane is obtained. However, PVDF membranes have a limited range of microstructures using the traditional methods in both immerse precipitation and TIPS. PVDF typically forms isotropic, spherulitic microstructures that often include large macrovoids. Lloyd [7] prepared microporous membranes via TIPS by PVDF with diluents of dibutyl-phthalate (DBP), while isotropic spherulitic microstructures that included large macrovoids and irregular porous were formed, dispersed in the spherulities. Hiatt [8]used cyclohexanone, crylic acid and γ-butyrolactone as diluents for PVDF, and an isotropic spherulitic microstructure was also obtained. Therefore, the application problem for PVDF membranes via TIPS focuses on how to control the irregular membrane porous structure. In this work, effects of diluents on membrane morphology, thermodynamic interactions on
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membrane structure and coarsening on the morphology of membranes were studied. 2. Experimental 2.1. Materials Membranes were prepared with systems of PVDF–DMP, PVDF–DMP–DOA, PVDF–DMP– DOS and PVDF–DBP. The polymer was poly(vinylidene fluoride), FR904, purchased from Shanghai 3F New Materials, China. Dioctylsebacate (DOS), dioctyl-phthalate (DOP) and dimethyl-phthalate (DMP) were used as diluents without further purification. These diluents were purchased from Shanghai Reagents, China. 2.2. Membrane preparation Appropriate amounts of polymer and diluent were measured into a test tube, which was placed into an oil bath. The polymer diluent mixture was dissolved for 4 h at about 453 K. Then the mixture was quenched in air for 20 min, yielding a solid polymer diluents sample. The solid sample was chopped into small pieces and placed in a tailor-made test tube. After reheating in an oven for 10 min at 453 K, it was taken out to quench in the water bath. Finally the diluents were extracted from the membrane with methanol or n-hexane. The extractants were evaporated. Membranes were dried further in a vacuum oven at a slightly elevated temperature. 2.3. Phase diagram Different scanning calorimetry (DSC) was used to determine the crystallization temperature for the dynamic phase diagram. The solid polymer diluent samples were sealed in an aluminum DSC pan, melted at 453 K for 10 min, and then cooled at 10 K/min. The onset of the exothermic peak during the cooling was taken as the dynamic
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crystallization temperature. The cloud points were determined by noting the appearance of turbidity under a microscope. 2.4. Scanning electron microscopy observation The structure and morphology of membranes were observed by scanning electron microscopy (SEM). The cross section of membranes was freeze-fractured under liquid nitrogen. The membrane samples were gold sputtered and analyzed using SEM (Kash SX-40). 2.5. Porosity measurements The appropriately measured PVDF membranes were immersed in i-butanol for 24 h and taken out to be weighed immediately after removing i-butanol on the surface. The method calculating the porosity is available in the literature [9]. The porosity was calculated according to the formula:
where W1 is the initial membrane weight, W2 the immersed membrane weight, ρ1 the density of PVDF, and ρ2 the density of i-butanol. 3. Results and discussion 3.1. Phase diagram The compatibility of polymer and diluents directly reflects the thermodynamic properties such as the binodal line and crystallization temperature. The diluents have different effects on the properties of membranes. Interpretation of the interactions of polymer diluents and the structure of TIPS membranes requires a systems equilibrium phase diagram [10]. The phase diagrams of
PVDF–DMP and PVDF–DMP–DOA are shown in Figs. 1 and 2 where the cloud points decreased and crystallization temperatures slightly increased as a fraction of the PVDF contents increased. The monotectic point (φm) of PVDF–DMP was between 25–30%, and the monotectic point of PVDF–DMP–DOA was between 30–40%. The L–L is located left of φm. The S–L region is located below φm. As a result, the spherulitic structure is the system that has undergone S–L TIPS via polymer crystallization with a low PVDF fraction. It was found that the cloud point line and the binodal line of PVDF–DMP–DOA shifted to a higher temperature than the PVDF– DMP system, showing that DMP–DOA has a lower compatibility with PVDF than DMP. Meanwhile the cloud points of PVDF–DMP– DOA shifted to higher polymer contents than PVDF–DMP, which resulted in a long time for pore growth in the cooling process [4]. 3.2. Effects of diluents In the TIPS process, how the diluents should be selected is an important problem for controlling pore size. Diluents with different compatibilities of PVDF have effects on membrane morphology [11]. When the diluents are subjected to a temperature below the equilibrium melting temperature of the polymer solution, the system undergoes S–L separation via polymer crystallization. During the crystallization process, the diluents are rejected to the inter-spherulitic regions. Upon diluents extraction, the interspherulitic regions, diluent-rich regions, become the membrane pores. Table 1 shows the dielectric constant of different diluents. It was showed that the polar of different diluents is in the order of DMP, DBP, DOA and DOS. Fig. 3 shows the effects of three diluents on the membrane. It was found that PVDF–DMP formed the most discernable and largest spherulitic structure because DMP has close polar with PVDF and the strong
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Fig. 1. Dynamic phase diagram of PVDF–DMP systems.
Fig. 2. Dynamic phase diagram of PVDF-DMP-DOA systems.
Fig. 3. Micrographs of cross section of a PVDF (25%) membrane prepared with systems of different diluents quenched at 303 K after melting at 453K (×3 k).
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Table 1 Dielectric constant of different diluents [15] Diluent
Dielectric constant
DMP DBP DOA DOS
8.5 6.4 4–5 <4
interactions of C=O in DMP with -F- in PVDF [12], which has advantages for the rich polymer phase to flow, congregate and form a discernable spherulitic structure [13].While DBP has a lower polar than DMP, the PVDF–DBP system showed less discernable spherulity than PVDF–DMP. Meanwhile the PVDF–DMP–DOS and PVDF– DMP–DOA systems showed a jagged and sharpedged spherulitic structure because the added DOS or DOA lowered the interactions in the polymer solution with their weak compatibility with PVDF, which resulted in certain limitations in PVDF crystallization, an unobvious spherulitic structure and lower spherulity size. As DOS has a lower polar than DOA, which means DMP– DOS showed lower compatibility with PVDF than DMP–DOA, the size of the spherulites was more irregular [14]. As shown in Table 2, the value of porosity of the above four systems was in the order of PVDF–DMP, PVDF–DMP–DOA, PVDF–DBP and PVDF–DMP–DOS. The stronger the interactions of diluents with PVDF, the easier the polymer-rich phase can flow and congregate. The area between the spherulites became more compacted as the interactions increased. When the structures of the systems were fixed quenching at 303 K, the area became the membrane pore as the effect of extraction, and the value of porosity decreased as the interactions increased. From Table 1, it is seen that the interactions of diluents with PVDF were in the order of DMP, DMP– DOA, DBP and DMP–DOS, which was coincident with the order of the value of porosity.
Table 2 Porosity of PVDF diluent systems with PVDF fraction of 25 wt% PVDF system
Porosity
DMP DMP–DOA DMP–DOS DBP
0.42 0.59 0.68 0.63
Meanwhile Fig. 3 shows that the area between the spherulites was increased in the order above. 3.3. Effect of quenching conditions and temperatures Although different diluents have effects on membrane morphology, different quenching conditions also affect the polymer crystallization structure [16]. In Fig. 4 it is seen that when membranes are quenched in ice water, there was discernable spherulitic structure. As the cooling rate was extremely high with a strong crystallization potential, the phase separation occurred in the unstable region (SD), and the structure was fixed without any coarsening. When the quenching temperature increased to 303 K, the cooling rate was lower than in ice water, and the spherulite size increased. When quenching at room temperature, the polymer spherulity grew slower and the spherulite size was greater than quenched at ice water. As the cooling rate was slower than when quenched at 303 K, PVDF crystallized much slower and improved, and the spherulity became more regular in size [17]. As shown in Fig. 5, 353 K is below the crystal line, and the quenching times were set in 1 min, 3 min, 5 min and 20 min. As quenching times increased, the spherulities were more discernable and the size of spherulities increased. As polymer crystallized, which inhibited droplets to grow, the diluent was rejected and the small pores were obtained in the spherulites.
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(a)
(b)
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(c)
Fig. 4. Membrane structure of PVDF–DMP formed at different quenched conditions with a PVDF concentration of 25%. a, ice water; b, water bath (303 K); c, air bath at room temperature (303 K). (×1 k).
Fig. 5. Scanning electron micrographs of the cross section of PVDF–DMP membranes as a function of coarsening time at 353 K with a PVDF concentration of 20% (×5 k).
Fig. 6. Scanning electron micrographs of the cross section of PVDF–DMP membranes as a function of coarsening time at 368 K with a PVDF concentration of 20% (×5 k).
3.4. Coarsening effect Three different mechanisms were described for the coarsening of the microstructure in the late stages of phase separation: Ostwald ripening, coalescence and the hydrodynamic flow mechanism [18]. As for the real growth of droplets, the coarsening process is described by the direct ratio d:t α. The exponent α depends on the microscopic
mechanisms of particle growth. Sigga [19] conidered both coalescence and hydrodynamic flow effects in phase separation, and deduced the droplets growth process in three stages: intermedia, flow and gravity-dominated. The coarsening effects studied at 368 K are seen in Fig. 6. The coarsening time was set at 1 min, 3 min, 5 min and 20 min. As shown in
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Fig. 6, 368 K was above the crystallization temperature. Then the droplets grew through the L–L phase region. As a result, the rich polymer phase had the potential to flow and congregate. There is discernable spherulitic structure at 3 min. As time increased, the size of the spherulities increased, while the size of the pore was unvaried as the constraint of polymer crystallization. Comparing Figs. 5 and 6, it was found that the spherulite growth rate of PVDF was higher at 368 K than at 353 K, as droplets had more time to congregate, while the structure and size of pores was unvaried as coarsening or quenching depth increased.
4. Conclusions 1. Phase diagrams were obtained for two PVDF diluent systems. The cloud-point curve was shifted to the lower temperature in the order PVDF–DMP, PVDF–DMP–DOA. This order can be explained by the compatibility between PVDF and diluents. The crystallization temperatures were slightly influenced by the polymer fraction. 2. At different quenched conditions, smaller spherulite sizes were obtained by quenching in ice water, and the improved and regular spherulities were formed when quenching at ambient temperature, while the lesser improved spherulities were obtained at 303 K. 3. Membrane morphology was greatly determined by the interactions between diluents and polymer. As the interactions decreased, the spherulitic structure was less discernable and porosity increased in the order of PVDF–DMP, PVDF–DMP–DOA, PVDF–DBP and PVDF– DMP–DOS. 4. When quenching at 353 K and 368 K, the spherulities became more discernable, and their size increased. The spherulite growth rate of PVDF was higher at 368 K than at 353 K, as the polymer-rich phase had more time to congregate in the L–L phase separation region.
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