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Effects of nucleating agents and extractants on the structure of polypropylene microporous membranes via thermally induced phase separation
Desalination 192 (2006) 142–150
Effects of nucleating agents and extractants on the structure of polypropylene microporous membranes via thermally induced phase separation Benzhe Luoa, Jun Zhanga*, Xiaolin Wangb, Yuan Zhoua, Jianzhi Wenc 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 c Shandong Zhaoyuan Motian Group Co., Ltd., Zhaoyuan 265400, China Received 15 March 2005; accepted 7 October 2005
Abstract The effects of nucleating agents and extractants on the microstructure of polypropylene (PP) membranes via thermally induced phase separation were investigated. The nucleating agents were dibenzyl sorbitol, adipic acid and benzoic acid. The extractants were methanol, ethanol, formic acid, acetone, butane, n-hexane, dichloroethane and xylene. The spherulitic size of the membrane greatly decreased and the degree of crystallization and enthalpy of crystallization remarkably increased when adding dibenzyl sorbitol; therefore, dibenzyl sorbitol was a perfect nucleating agent for a PP/soybean oil/dibutyl phthalate (DBP) system. Moreover, with the addition of 5 wt‰ of dibenzyl sorbitol, the membrane had the smallest spherulitic size. It was also proved that ethanol, acetone, butane, n-hexane and dichloroethane are proper extractants for a system of PP/DBP/soybean oil. Keywords: Thermally induced phase separation; Polypropylene; Nucleating agent; Membrane
1. Introduction Polypropylene (PP) microporous membranes are widely used in the manufacture of lithium batteries and artificial lungs, and for water purifi*Corresponding author.
cation, chemical separation and in the textile industry [1–3]. Many techniques have been used for PP membrane preparation, such as meltspinning and cold-stretching, blending-stretching, sintering and thermally induced phase separation (TIPS). The TIPS process is a versatile technique
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.2005.10.013
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for making semi-crystallized polymer membranes. In this method, polymer can dissolve in the diluent at a high temperature and form a homogeneous solution, while phase separation is induced by decreasing temperature. After diluents are removed, microporous membranes can be obtained [1,2]. Many studies have shown that the microstructure of membranes results not only from the effects of quenching diluent, temperature and coarsening [6–8], but also from nucleating agents and extractants [8–10]. Adding nucleating agents, PP crystals with a center of nucleating agents, is called heterogeneous nucleation. Owing to a large enhancement of the crystal nucleus, the sizes of spherulites decrease; moreover, microstructure features change [9]. Except for nucleating agents, the microstructure of PP membranes was affected by different extractants [10]. Three nucleating agents (dibenzyl sorbitol, adipic acid, benzoic acid) and eight extractants (methanol, ethanol, formic acid, acetone, butane, n-hexane, dichloroethane, xylene) were used in the preparation of PP microporous membranes. The effects of nucleating agents and extractants on the microporous membranes via TIPS were studied at a constant composition of weight — 30/28/42 — for PP/soybean oil/dibutyl phthalate (DBP).
extractants, purchased from the market, were analytical reagents. All the chemicals were not further purified.
2. Experimental
2.4. Porosity
2.1. Materials
PP membranes were immersed in i-butanol for 24 h, and weighed immediately after removing the i-butanol on the surface. The porosity was calculated according to the formula:
PP (T30S, MFR = 6 g/10 min) was supplied by Qilu Petrochemical (China). Soybean oil was common cooking oil (Sino-Fore Joint Venture Donghai Grain Oil Industrial, China). DBP was purchased from Shanghai Agent. The nucleating agents were benzoic acid (AR, Guangdong Longxi Chemical, China), adipic acid (AR, Shanghai Reagents, China), and dibenzyl sorbitol (AR, NG-2, Tianan Feida Chemical Agent Factory, China). All of the nucleating agents and
2.2. Sample preparation techniques Soybean oil (30 wt%) and PP (70 wt%) were premixed as the mixed diluent. Appropriate amounts of PP and mixed diluent were weighed into a tube, which was placed into an oil bath, holding at 180EC for 4 h with stirring. The mixture was then quenched in the air for 20 min, yielding a solid polymer-diluents sample. The solid sample was chopped into small pieces about 1 mm thick and placed in a tailor-made test tube. After reheating in oven at 180EC for 10 min, it was taken out to quench in a water bath. Finally, the diluents were extracted from the membrane with methanol and n-hexane. 2.3. Scanning electron microscopy (SEM) SEM micrographs were taken using a Kash SX-40 scanning electron microscope. Each of the samples was fractured under liquid nitrogen, and coated with gold using a sputtering coater. One hundred pore sizes were measured of every SEM micrograph using the Sigmascan software. The most probable pore size and the pore size variances (δ2) were calculated by Statistica software.
where W1 is the initial membrane weight, W2 the immersed membrane weight; ρ1 the density of PP, and ρ2 is the density of i-butanol.
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2.5. Diagrams of differential scanning calorimetry (DSC) analysis DSC analysis was conducted with a PerkinElmer DSC-7C differential scanning calorimeter. In an isothermal crystallization run, the following procedure was employed: a sample was firstly heated to 220EC at 10EC/min and kept at this temperature for about 5 min to remove thermal history. Then the sample was cooled to 50EC at 10EC/min. The degree of crystallization was calculated according to the formula:
where is the heat released during the melting process and is the heat of fusion of a perfect crystal taken as 209 J/g [11] for PP. 3. Results and discussion 3.1. Effects of different nucleating agents on the structure of PP microporous membranes The microstructure of PP membranes prepared by adding 5 wt‰ (content compared with PP) nucleating agents, including dibenzyl sorbitol, adipic acid and benzoic acid respectively, is shown in Fig. 1. Fig. 2 is a partially enlarged detail of Fig. 1. As shown in Fig. 2, the pore sizes are about 1.95, 1.72, 1.80, and 1.97 µm of the membranes from the systems without a nucleating agent, or with dibenzyl sorbitol, adipic acid and benzoic acid. The pore size of the membrane with dibenzyl sorbitol was the smallest one of the four mentioned above, although the difference is not obvious. The δ2 of pore sizes were 0.90, 0.58, 0.49, and 0.64 µm, respectively. This indicates that a nucleating agent promotes the forming of narrow distribution of pore sizes. Correspondingly as Fig. 1 shows, the spherulitic sizes of the four membranes were around 55, 20, 50, and 50 µm. When dibenzyl sorbitol was added, the spherulitic sizes largely decreased and a majority
of the spherulites were uniform in size, though adipic acid and benzoic acid have effects on the spherulite of PP. As the melting points of dibenzyl sorbitol, adipic acid and benzoic acid are 250, 152, and 122.4EC, the latter two nucleating agents melted in diluent (soybean oil/DBP) at 180EC, and then the induced amounts of spherulites hardly increased. Because dibenzyl sorbitol maintained its solid state at that temperature, the presence of dibenzyl sorbitol greatly affected the crystallization of PP as multi-nuclei, leading to more spherulites. Furthermore, heterogeneous nucleation increased the driving force for crystallization, which restricted coarsening of droplets of the polymer-lean phase and resulted in smaller pore sizes. Diagrams of melting curves of four blends of PP/diluent are shown in Fig. 3. All exothermic peaks shifted to the low temperature region, attributed to the presence of a high composition of diluent. However, no marked differences in the position of exothermic peaks were detected. With the addition of 5 wt‰ different nucleating agents, the degrees of crystallization changed to 49.3%, 44.5% and 30.6%, respectively. The presence of dibenzyl sorbitol was the most in favor of nucleation. The cooling curves of different nucleating agents are given in Fig. 4. The crystallization peak of adding nucleating agents shifted to higher temperatures, and the figures of the peak became more slanted due to heterogeneous nucleation. The sample with dibenzyl sorbitol had the highest crystallizing point (114.3EC), the smallest width (5.8EC) of crystallization peak, and the smallest (2.8EC) of the four samples. Because actually indicated half-time of crystallization, the sample with dibenzyl sorbitol had the fastest crystallization rate when cooling at a given rate. When nucleation increased, as proven by SEM (Fig. 1), the space for the growth of spherulite decreased. Thus, the DSC data are consistent with the microstructure photographs. From these results, it is clear that dibenzyl sorbitol is an
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Fig. 1. SEM photographs of the PP/soybean oil/DBP blend with different nucleating agents (1 K).
Fig. 2. SEM photographs of the PP/soybean oil/DBP blended with different nucleating agents (3 K).
Fig. 3. DSC melting curves of the PP/soybean oil/DPB/ dibenzyl sorbitol blend with different nucleating agents. A, dibenzyl sorbitol; B, adipic acid; C, benzoic acid.
excellent nucleating agent for making microporous membranes. 3.2. Effects of amount of nucleating agent on structure of PP microporous membranes The micrographs of the final membrane with different dosage of dibenzyl sorbitol are shown in Fig. 5. Fig. 6 is a partially enlarged detail of Fig. 5. Fig 5 shows that the spherulite size
Fig. 4. DSC cooling curves of the PP/soybean oil/DPB/ dibenzyl sorbitol blend with different nucleating agents.
increased with dibenzyl sorbitol up to 5 wt‰; especially with the 4 wt‰ and 5 wt‰ of dibenzyl sorbitol, the spherulite size obviously decreased to less than 25 µm. Moreover, every spherulite boundary was illegible with an addition above 4 wt‰, and even immeasurable with the addition of 7 wt‰. This is because the more heterogeneous nuclei generated, the more spherulites are impinged in a small space. As 7 wt‰ di-
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Table 1 DSC melting results of the PP/soybean oil/DPB blend with different nucleating agents , EC
Nucleating agent None Dibenzyl sorbitol Adipic acid Benzoic acid
146.4 147.5 144.8 144.6
, EC 152.9 153.9 156.8 153.5
, EC 158.8 160.6 158.5 159.3
∆Tm, EC
∆Hm, J@g!1
C, %
12.4 13.1 13.7 14.7
27.2 30.9 27.9 19.2
43.4 49.3 44.5 30.62
, melting onset temperature; , melting peak temperature; , melting ending temperature; ∆Tm = width of melting peak; ∆Hm, melting enthalpy; C, degree of crystallization.
!
,
Table 2 Crystallization results of the PP/soybean oil/DPB blend with different nucleating agents , EC
Nucleating agent None Dibenzyl sorbitol Adipic acid Benzoic acid
112.9 114.3 113.2 113.8
, EC 109.2 111.5 110.3 110.4
, EC 104.9 108.5 107.2 106.8
, EC 8.0 5.8 6.0 7.0
, EC 3.7 2.8 2.9 3.4
∆Hc, J@g!1 !30.0 !31.2 !28.5 !21.4
, crystallization onset temperature; , crystallization peak temperature; , crystallization ending temperature; = ! , width of crystallization peak; = ! ; ∆Hc, crystallization enthalpy. Table 3 DSC melting results of the PP/soybean oil/DPB/dibenzyl sorbitol blend , EC
Dibenzyl sorbital, ‰ 0 1 3 5 7
146.4 147.8 148.0 147.5 147.0
, EC 152.9 153.2 154.2 153.9 153.2
, EC 158.8 158.8 158.9 160.6 159.1
∆Tm, EC
∆Hm, J@g!1
C, %
12.4 11.0 10.9 13.1 11.1
27.2 30.5 32.8 30.9 23.9
43.38 48.64 52.31 49.28 38.12
, melting onset temperature; , melting peak temperature; , melting ending temperature; ∆Tm = width of melting peak; ∆Hm, melting enthalpy; C, degree of crystallization.
benzyl sorbitol added; on the contrary, the spherulite size was greater than that of 5 wt‰ dibenzyl sorbitol. In this case, the superfluous dibenzyl sorbitol induced to congregate and was enlarged, which decreased the function of the nucleating agent. The pore sizes of the finished membranes with different amounts of dibenzyl
!
,
sorbitol, shown in Fig. 6, were 1.94, 1.98, 1.88, 1.87, 1.72, and 1.69 µm, respectively, and the δ2 of pore sizes were 0.91, 0.97, 0.76, 0.67, 0.58, and 0.63 µm. It is clear that the finished membrane had the smallest and most uniform pores with the addition of 5 wt‰ dibenzyl sorbitol.
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Fig. 5. SEM photographs of the PP/soybean oil/DBP/dibenzyl sorbitol blend (1 K).
Fig. 6. SEM photographss of the PP/soybean oil/DBP/dibenzyl sorbitol blend (3 K).
Diagrams of the melting curve blend with different dosages of dibenzyl sorbitol (0, 1 wt‰, 3 wt‰, 5 wt‰, 7 wt‰) are shown in Fig. 7,
which shows the Tm and the degrees of crystallization were slightly increased from 0 to 3 wt‰ and then decreased from 3 wt‰ to 5 wt‰ so that
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Fig. 8. DSC cooling curves of the PP/soybean oil/DPB/ dibenzyl sorbitol blend with different dosages of nucleating agents.
Fig. 7. DSC melting curves of the PP/soybean oil/DPB/ dibenzyl sorbitol blend with different dosages of nucleating agents.
Table 4 DSC crystallization results of the PP/soybean oil/DPB/dibenzyl sorbitol blend , EC
Dibenzyl sorbitol, ‰ 0 1 3 5 7
112.9 113.8 115.6 114.3 113.8
, EC 109.2 111.1 113.5 111.5 111.1
, EC 104.9 107.4 110.4 108.5 108.5
, EC 8.0 6.4 5.2 5.8 5.3
, EC 3.7 2.7 2.1 2.8 2.7
∆Hc, J@g!1 !30.0 !31.7 !33.8 !31.2 !28.0
, crystallization onset temperature; , crystallization peak temperature; , crystallization ending temperature; = ! , width of crystallization peak; = ! ; ∆Hc, crystallization enthalpy.
the presence of dibenzyl sorbitol promoted PP crystallization at a high content diluent surrounding showed the effect as heterogeneous nuclei. The cooling curves of different dosages of dibenzyl sorbitol were obtained from Fig. 9 which shown that as the addition of dibenzyl sorbitol increased, the temperature of Tc increased from 0 to 3 wt‰, and decreased from 3 wt‰ to 5 wt‰, the change of width of crystallization and , show the same as Tc, and they all reached peaks at 3 wt‰. The data indicate that with the addition of 3 wt‰ dibenzyl sorbitol the crystallization rate of PP increased. Therefore, adding 3 wt‰ dibenzyl sorbitol showed the best nucleating effect in this system. To sum up the above results, to obtain a better membrane, the best dosage of
dibenzyl sorbitol is not the more and the better, but rather for this system is 5 wt‰. 3.3. Effects of different extractants on the structure of PP microporous membranes The extraction ratios (the ratio of the loss of weight) of using different extractants in this system without nucleation are shown in Table 5. The extraction ratios of methanol, formic acid, and xylene were below 70%, which means they were incompletely extracted in the system. Thus, the other five extractants were improper for the mixed diluent. The porosities of membranes extracted by ethanol, acetone, butane, n-hexane and dichloroethane were 66.9%, 71.1%, 71.9%,
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Fig. 9. SEM photographs of the PP/soybean oil/DBP extracted with different extractants.
Fig. 10 SEM photographs of the PP/soybean oil/DBP extracted with different extractants.
Table 5 Extraction ratio of different diluents in the PP/soybean oil/DBP blend Extraction
Extractant ratio
Methanol Ethanol Formic acid Acetone Butane n-Hexane Dichloroethane Xylene
0.48 0.68 !0.02 0.72 0.72 0.71 0.69 0.36
67.5%, and 66.9%, respectively, approaching theoretical values. The microstructures of the five finished membranes are shown in Fig. 9. Fig. 10 is a partially enlarged detail of Fig. 9. As Fig. 9 shows, the extractants hardly have an effect on spherulite size and morphology, but some large pores appeared at interconnecting spherulites using the last three extractants, which is clear in Fig. 10. From these results, it is seen that ethanol and acetone are the best two extractants.
4. Conclusions Adding dibenzyl sorbitol, the spherulite size of PP and the pore size of the membrane decreased, while the degree of crystallization increased. Dibenzyl sorbitol was proved a suitable nucleating agent for producing membranes in the system of PP/soybean oil/DBP. Adipic acid and benzoic acid had nearly no effect on nucleating and were not good nucleating agents for producing membranes with the system of PP/soybean oil/DBP. The pore size of membranes faintly decreased with increasing dibenzyl sorbitol dosage. The spherulite size deceased dibenzyl sorbitol dosage no more than 5 wt‰. On the contrary, if the dosage was over 5 wt‰, the size of the spherulite increased. The porosities of membranes extracted by ethanol, acetone, butane, n-hexane and dichloroethane were near 70%. Thus, uniformity of pore sizes using ethanol and acetone as extractants can be obtained. Acknowledgments This work is supported by Hi-Tech Research
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and Development Program of China (2002AA328020) and National Basic Research Program of China (2003CB615701). References [1] R.A. Griffin, Separator for electrochemical cells. US Patent 4,794,057, 1988. [2] E. Kessler, T. Batzilla, F. Wechs and F. Wiese, Integrally asymmetrical polyolefin membrane. US Patent 6,497,752, 2002. [3] Y. Gao and L. Ye, Basic of Membrane Separation Technology. Chemical Industry Press, Beijing, 1989, pp. 106–109. [4] K. Kamada, S. Mianmi and K. Yoshida, Porous polypropylene hollow filaments and method making the same. US Patent 4,055,696, 1977. [5] A.J. Castro, Methods for making microporous products. US Patent 4,247,498, 1981. [6] D.R. Lloyld and K.E. Kinzer, Microporous membrane formation via thermally induced phase separation. I. Solid-liquid phase separation. J. Membr. Sci., 52 (1990) 239–261.
[7] G.B.A Lim, S.S Kim, Q Ye, Y.F. Wang and D.R. Lloyd, Microporous membrane formation via thermally induced phase separation. IV. Effect of isotactic polypropylene crystallization kinetics on membrane structure. J. Membr. Sci., 64 (1991) 31–40. [8] K.S. McGuire, D.R. Lloyd and G.B.A. Lim, Microporous membrane formation via thermally induced phase separation. VII. Effect of dilution, cooling rate, and nucleating agent addition on morphology. J. Membr. Sci., 79 (1993) 27–34. [9] S.S. Kim, D.B.A Lim, A.A. Alwattari, Y.F. Wang and D.R. Lloyd, Microporous membrane formation via thermally induced phase separation. V. Effect of diluent mobility and crystallization on the structure of isotactic polypropylene membranes. J. Membr. Sci., 64 (1991) 41–53. [10] H. Matsuyama, M. Kim and D.R. Lloyd, Effect of extraction and drying on the structure of microporous polyethylene membranes prepared via thermally induced phase separation. J. Membr. Sci., 204 (2002) 413–419. [11] J. Brandrup, E.H. Immergut and E.A. Grulke, Polymer Handbook, 3rd ed., Wiley, New York, 1999, p. V/29.
Effects of nucleating agents and extractants on the structure of polypropylene microporous membranes via thermally induced phase separation Benzhe Luoa, Jun Zhanga*, Xiaolin Wangb, Yuan Zhoua, Jianzhi Wenc 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 c Shandong Zhaoyuan Motian Group Co., Ltd., Zhaoyuan 265400, China Received 15 March 2005; accepted 7 October 2005
Abstract The effects of nucleating agents and extractants on the microstructure of polypropylene (PP) membranes via thermally induced phase separation were investigated. The nucleating agents were dibenzyl sorbitol, adipic acid and benzoic acid. The extractants were methanol, ethanol, formic acid, acetone, butane, n-hexane, dichloroethane and xylene. The spherulitic size of the membrane greatly decreased and the degree of crystallization and enthalpy of crystallization remarkably increased when adding dibenzyl sorbitol; therefore, dibenzyl sorbitol was a perfect nucleating agent for a PP/soybean oil/dibutyl phthalate (DBP) system. Moreover, with the addition of 5 wt‰ of dibenzyl sorbitol, the membrane had the smallest spherulitic size. It was also proved that ethanol, acetone, butane, n-hexane and dichloroethane are proper extractants for a system of PP/DBP/soybean oil. Keywords: Thermally induced phase separation; Polypropylene; Nucleating agent; Membrane
1. Introduction Polypropylene (PP) microporous membranes are widely used in the manufacture of lithium batteries and artificial lungs, and for water purifi*Corresponding author.
cation, chemical separation and in the textile industry [1–3]. Many techniques have been used for PP membrane preparation, such as meltspinning and cold-stretching, blending-stretching, sintering and thermally induced phase separation (TIPS). The TIPS process is a versatile technique
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.2005.10.013
B. Luo et al. / Desalination 192 (2006) 142–150
143
for making semi-crystallized polymer membranes. In this method, polymer can dissolve in the diluent at a high temperature and form a homogeneous solution, while phase separation is induced by decreasing temperature. After diluents are removed, microporous membranes can be obtained [1,2]. Many studies have shown that the microstructure of membranes results not only from the effects of quenching diluent, temperature and coarsening [6–8], but also from nucleating agents and extractants [8–10]. Adding nucleating agents, PP crystals with a center of nucleating agents, is called heterogeneous nucleation. Owing to a large enhancement of the crystal nucleus, the sizes of spherulites decrease; moreover, microstructure features change [9]. Except for nucleating agents, the microstructure of PP membranes was affected by different extractants [10]. Three nucleating agents (dibenzyl sorbitol, adipic acid, benzoic acid) and eight extractants (methanol, ethanol, formic acid, acetone, butane, n-hexane, dichloroethane, xylene) were used in the preparation of PP microporous membranes. The effects of nucleating agents and extractants on the microporous membranes via TIPS were studied at a constant composition of weight — 30/28/42 — for PP/soybean oil/dibutyl phthalate (DBP).
extractants, purchased from the market, were analytical reagents. All the chemicals were not further purified.
2. Experimental
2.4. Porosity
2.1. Materials
PP membranes were immersed in i-butanol for 24 h, and weighed immediately after removing the i-butanol on the surface. The porosity was calculated according to the formula:
PP (T30S, MFR = 6 g/10 min) was supplied by Qilu Petrochemical (China). Soybean oil was common cooking oil (Sino-Fore Joint Venture Donghai Grain Oil Industrial, China). DBP was purchased from Shanghai Agent. The nucleating agents were benzoic acid (AR, Guangdong Longxi Chemical, China), adipic acid (AR, Shanghai Reagents, China), and dibenzyl sorbitol (AR, NG-2, Tianan Feida Chemical Agent Factory, China). All of the nucleating agents and
2.2. Sample preparation techniques Soybean oil (30 wt%) and PP (70 wt%) were premixed as the mixed diluent. Appropriate amounts of PP and mixed diluent were weighed into a tube, which was placed into an oil bath, holding at 180EC for 4 h with stirring. The mixture was then quenched in the air for 20 min, yielding a solid polymer-diluents sample. The solid sample was chopped into small pieces about 1 mm thick and placed in a tailor-made test tube. After reheating in oven at 180EC for 10 min, it was taken out to quench in a water bath. Finally, the diluents were extracted from the membrane with methanol and n-hexane. 2.3. Scanning electron microscopy (SEM) SEM micrographs were taken using a Kash SX-40 scanning electron microscope. Each of the samples was fractured under liquid nitrogen, and coated with gold using a sputtering coater. One hundred pore sizes were measured of every SEM micrograph using the Sigmascan software. The most probable pore size and the pore size variances (δ2) were calculated by Statistica software.
where W1 is the initial membrane weight, W2 the immersed membrane weight; ρ1 the density of PP, and ρ2 is the density of i-butanol.
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2.5. Diagrams of differential scanning calorimetry (DSC) analysis DSC analysis was conducted with a PerkinElmer DSC-7C differential scanning calorimeter. In an isothermal crystallization run, the following procedure was employed: a sample was firstly heated to 220EC at 10EC/min and kept at this temperature for about 5 min to remove thermal history. Then the sample was cooled to 50EC at 10EC/min. The degree of crystallization was calculated according to the formula:
where is the heat released during the melting process and is the heat of fusion of a perfect crystal taken as 209 J/g [11] for PP. 3. Results and discussion 3.1. Effects of different nucleating agents on the structure of PP microporous membranes The microstructure of PP membranes prepared by adding 5 wt‰ (content compared with PP) nucleating agents, including dibenzyl sorbitol, adipic acid and benzoic acid respectively, is shown in Fig. 1. Fig. 2 is a partially enlarged detail of Fig. 1. As shown in Fig. 2, the pore sizes are about 1.95, 1.72, 1.80, and 1.97 µm of the membranes from the systems without a nucleating agent, or with dibenzyl sorbitol, adipic acid and benzoic acid. The pore size of the membrane with dibenzyl sorbitol was the smallest one of the four mentioned above, although the difference is not obvious. The δ2 of pore sizes were 0.90, 0.58, 0.49, and 0.64 µm, respectively. This indicates that a nucleating agent promotes the forming of narrow distribution of pore sizes. Correspondingly as Fig. 1 shows, the spherulitic sizes of the four membranes were around 55, 20, 50, and 50 µm. When dibenzyl sorbitol was added, the spherulitic sizes largely decreased and a majority
of the spherulites were uniform in size, though adipic acid and benzoic acid have effects on the spherulite of PP. As the melting points of dibenzyl sorbitol, adipic acid and benzoic acid are 250, 152, and 122.4EC, the latter two nucleating agents melted in diluent (soybean oil/DBP) at 180EC, and then the induced amounts of spherulites hardly increased. Because dibenzyl sorbitol maintained its solid state at that temperature, the presence of dibenzyl sorbitol greatly affected the crystallization of PP as multi-nuclei, leading to more spherulites. Furthermore, heterogeneous nucleation increased the driving force for crystallization, which restricted coarsening of droplets of the polymer-lean phase and resulted in smaller pore sizes. Diagrams of melting curves of four blends of PP/diluent are shown in Fig. 3. All exothermic peaks shifted to the low temperature region, attributed to the presence of a high composition of diluent. However, no marked differences in the position of exothermic peaks were detected. With the addition of 5 wt‰ different nucleating agents, the degrees of crystallization changed to 49.3%, 44.5% and 30.6%, respectively. The presence of dibenzyl sorbitol was the most in favor of nucleation. The cooling curves of different nucleating agents are given in Fig. 4. The crystallization peak of adding nucleating agents shifted to higher temperatures, and the figures of the peak became more slanted due to heterogeneous nucleation. The sample with dibenzyl sorbitol had the highest crystallizing point (114.3EC), the smallest width (5.8EC) of crystallization peak, and the smallest (2.8EC) of the four samples. Because actually indicated half-time of crystallization, the sample with dibenzyl sorbitol had the fastest crystallization rate when cooling at a given rate. When nucleation increased, as proven by SEM (Fig. 1), the space for the growth of spherulite decreased. Thus, the DSC data are consistent with the microstructure photographs. From these results, it is clear that dibenzyl sorbitol is an
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Fig. 1. SEM photographs of the PP/soybean oil/DBP blend with different nucleating agents (1 K).
Fig. 2. SEM photographs of the PP/soybean oil/DBP blended with different nucleating agents (3 K).
Fig. 3. DSC melting curves of the PP/soybean oil/DPB/ dibenzyl sorbitol blend with different nucleating agents. A, dibenzyl sorbitol; B, adipic acid; C, benzoic acid.
excellent nucleating agent for making microporous membranes. 3.2. Effects of amount of nucleating agent on structure of PP microporous membranes The micrographs of the final membrane with different dosage of dibenzyl sorbitol are shown in Fig. 5. Fig. 6 is a partially enlarged detail of Fig. 5. Fig 5 shows that the spherulite size
Fig. 4. DSC cooling curves of the PP/soybean oil/DPB/ dibenzyl sorbitol blend with different nucleating agents.
increased with dibenzyl sorbitol up to 5 wt‰; especially with the 4 wt‰ and 5 wt‰ of dibenzyl sorbitol, the spherulite size obviously decreased to less than 25 µm. Moreover, every spherulite boundary was illegible with an addition above 4 wt‰, and even immeasurable with the addition of 7 wt‰. This is because the more heterogeneous nuclei generated, the more spherulites are impinged in a small space. As 7 wt‰ di-
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Table 1 DSC melting results of the PP/soybean oil/DPB blend with different nucleating agents , EC
Nucleating agent None Dibenzyl sorbitol Adipic acid Benzoic acid
146.4 147.5 144.8 144.6
, EC 152.9 153.9 156.8 153.5
, EC 158.8 160.6 158.5 159.3
∆Tm, EC
∆Hm, J@g!1
C, %
12.4 13.1 13.7 14.7
27.2 30.9 27.9 19.2
43.4 49.3 44.5 30.62
, melting onset temperature; , melting peak temperature; , melting ending temperature; ∆Tm = width of melting peak; ∆Hm, melting enthalpy; C, degree of crystallization.
!
,
Table 2 Crystallization results of the PP/soybean oil/DPB blend with different nucleating agents , EC
Nucleating agent None Dibenzyl sorbitol Adipic acid Benzoic acid
112.9 114.3 113.2 113.8
, EC 109.2 111.5 110.3 110.4
, EC 104.9 108.5 107.2 106.8
, EC 8.0 5.8 6.0 7.0
, EC 3.7 2.8 2.9 3.4
∆Hc, J@g!1 !30.0 !31.2 !28.5 !21.4
, crystallization onset temperature; , crystallization peak temperature; , crystallization ending temperature; = ! , width of crystallization peak; = ! ; ∆Hc, crystallization enthalpy. Table 3 DSC melting results of the PP/soybean oil/DPB/dibenzyl sorbitol blend , EC
Dibenzyl sorbital, ‰ 0 1 3 5 7
146.4 147.8 148.0 147.5 147.0
, EC 152.9 153.2 154.2 153.9 153.2
, EC 158.8 158.8 158.9 160.6 159.1
∆Tm, EC
∆Hm, J@g!1
C, %
12.4 11.0 10.9 13.1 11.1
27.2 30.5 32.8 30.9 23.9
43.38 48.64 52.31 49.28 38.12
, melting onset temperature; , melting peak temperature; , melting ending temperature; ∆Tm = width of melting peak; ∆Hm, melting enthalpy; C, degree of crystallization.
benzyl sorbitol added; on the contrary, the spherulite size was greater than that of 5 wt‰ dibenzyl sorbitol. In this case, the superfluous dibenzyl sorbitol induced to congregate and was enlarged, which decreased the function of the nucleating agent. The pore sizes of the finished membranes with different amounts of dibenzyl
!
,
sorbitol, shown in Fig. 6, were 1.94, 1.98, 1.88, 1.87, 1.72, and 1.69 µm, respectively, and the δ2 of pore sizes were 0.91, 0.97, 0.76, 0.67, 0.58, and 0.63 µm. It is clear that the finished membrane had the smallest and most uniform pores with the addition of 5 wt‰ dibenzyl sorbitol.
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Fig. 5. SEM photographs of the PP/soybean oil/DBP/dibenzyl sorbitol blend (1 K).
Fig. 6. SEM photographss of the PP/soybean oil/DBP/dibenzyl sorbitol blend (3 K).
Diagrams of the melting curve blend with different dosages of dibenzyl sorbitol (0, 1 wt‰, 3 wt‰, 5 wt‰, 7 wt‰) are shown in Fig. 7,
which shows the Tm and the degrees of crystallization were slightly increased from 0 to 3 wt‰ and then decreased from 3 wt‰ to 5 wt‰ so that
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Fig. 8. DSC cooling curves of the PP/soybean oil/DPB/ dibenzyl sorbitol blend with different dosages of nucleating agents.
Fig. 7. DSC melting curves of the PP/soybean oil/DPB/ dibenzyl sorbitol blend with different dosages of nucleating agents.
Table 4 DSC crystallization results of the PP/soybean oil/DPB/dibenzyl sorbitol blend , EC
Dibenzyl sorbitol, ‰ 0 1 3 5 7
112.9 113.8 115.6 114.3 113.8
, EC 109.2 111.1 113.5 111.5 111.1
, EC 104.9 107.4 110.4 108.5 108.5
, EC 8.0 6.4 5.2 5.8 5.3
, EC 3.7 2.7 2.1 2.8 2.7
∆Hc, J@g!1 !30.0 !31.7 !33.8 !31.2 !28.0
, crystallization onset temperature; , crystallization peak temperature; , crystallization ending temperature; = ! , width of crystallization peak; = ! ; ∆Hc, crystallization enthalpy.
the presence of dibenzyl sorbitol promoted PP crystallization at a high content diluent surrounding showed the effect as heterogeneous nuclei. The cooling curves of different dosages of dibenzyl sorbitol were obtained from Fig. 9 which shown that as the addition of dibenzyl sorbitol increased, the temperature of Tc increased from 0 to 3 wt‰, and decreased from 3 wt‰ to 5 wt‰, the change of width of crystallization and , show the same as Tc, and they all reached peaks at 3 wt‰. The data indicate that with the addition of 3 wt‰ dibenzyl sorbitol the crystallization rate of PP increased. Therefore, adding 3 wt‰ dibenzyl sorbitol showed the best nucleating effect in this system. To sum up the above results, to obtain a better membrane, the best dosage of
dibenzyl sorbitol is not the more and the better, but rather for this system is 5 wt‰. 3.3. Effects of different extractants on the structure of PP microporous membranes The extraction ratios (the ratio of the loss of weight) of using different extractants in this system without nucleation are shown in Table 5. The extraction ratios of methanol, formic acid, and xylene were below 70%, which means they were incompletely extracted in the system. Thus, the other five extractants were improper for the mixed diluent. The porosities of membranes extracted by ethanol, acetone, butane, n-hexane and dichloroethane were 66.9%, 71.1%, 71.9%,
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Fig. 9. SEM photographs of the PP/soybean oil/DBP extracted with different extractants.
Fig. 10 SEM photographs of the PP/soybean oil/DBP extracted with different extractants.
Table 5 Extraction ratio of different diluents in the PP/soybean oil/DBP blend Extraction
Extractant ratio
Methanol Ethanol Formic acid Acetone Butane n-Hexane Dichloroethane Xylene
0.48 0.68 !0.02 0.72 0.72 0.71 0.69 0.36
67.5%, and 66.9%, respectively, approaching theoretical values. The microstructures of the five finished membranes are shown in Fig. 9. Fig. 10 is a partially enlarged detail of Fig. 9. As Fig. 9 shows, the extractants hardly have an effect on spherulite size and morphology, but some large pores appeared at interconnecting spherulites using the last three extractants, which is clear in Fig. 10. From these results, it is seen that ethanol and acetone are the best two extractants.
4. Conclusions Adding dibenzyl sorbitol, the spherulite size of PP and the pore size of the membrane decreased, while the degree of crystallization increased. Dibenzyl sorbitol was proved a suitable nucleating agent for producing membranes in the system of PP/soybean oil/DBP. Adipic acid and benzoic acid had nearly no effect on nucleating and were not good nucleating agents for producing membranes with the system of PP/soybean oil/DBP. The pore size of membranes faintly decreased with increasing dibenzyl sorbitol dosage. The spherulite size deceased dibenzyl sorbitol dosage no more than 5 wt‰. On the contrary, if the dosage was over 5 wt‰, the size of the spherulite increased. The porosities of membranes extracted by ethanol, acetone, butane, n-hexane and dichloroethane were near 70%. Thus, uniformity of pore sizes using ethanol and acetone as extractants can be obtained. Acknowledgments This work is supported by Hi-Tech Research
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