Mechanical Properties of Asymmetric Polysulfone Membranes

Nov 9, 2001 - The investigated mechanical properties include the tensile strength at break, the elongation at break, and the initial Young's modulus...
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Ind. Eng. Chem. Res. 2001, 40, 5917-5922

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Mechanical Properties of Asymmetric Polysulfone Membranes Containing Surfactant as Additives Hui-An Tsai, Doan-Ho Huang, Ruoh-Chyu Ruaan,* and Juin-Yih Lai Membrane Research Laboratory, Department of Chemical Engineering, Chung Yuan University, Chung Li, 32023 Taiwan, ROC

The mechanical properties of surfactant containing asymmetric polysulfone (PSf) membranes were under investigation. The investigated mechanical properties include the tensile strength at break, the elongation at break, and the initial Young’s modulus. Lipophilic surfactant (Span80) or hydrophilic surfactant (Tween-20) was added in the PSf casting solution. The effects of surfactant concentration and casting temperature on the mechanical properties of resulting membranes were measured and compared. Experimental results showed that all of the three mechanical properties increased with an increase of the Span-80 content. However, when the surfactant additive was substituted by the hydrophilic Tween-20, the tensile strength at break and the initial Young’s modulus decreased with an increase of the surfactant content, despite the increase of the elongation at break. The plasticizing effect might be the major contribution of surfactant Tween-20 on the membrane’s mechanical strength, but surfactant Span-80 affected the membrane’s mechanical strength mostly through alteration of the membrane structure. The addition of Span-80 resulted in the suppression of macrovoids and formation of a thicker skin layer and a denser membrane structure. Therefore, as Span-80 was added in the casting solution, the consolidated membrane structure resulted in an increase in all of the three measured mechanical properties. The above results indicate that we ought to know the effect of surfactants on the membrane structure in order to evaluate how the addition of surfactants affects the mechanical properties of an asymmetric membrane made by the wet inversion method. Introduction Synthetic membrane technology has grown up very fast since the first integrally skinned asymmetric cellulose acetate membrane was developed by Loeb and Sourirajan using a phase inversion process.1 The asymmetric membrane owns a highly porous sublayer and a ultrathin dense skin layer, which allows high flux and retains satisfactory selectivity. For polymeric materials, mechanical behavior involves the deformation of a material under the influence of an applied force. In the membrane separation process, the membrane is at the heart of a process which can be considered as a permselective barrier between two phases. Transport through the membrane will take place as a result of the driving force acting on the individual components in the feed. Substances transport across the membrane either by pressure or by concentration difference.2 To obtain a higher flux, excess pressure will frequently apply to the upstream of the membrane. Consequently, the mechanical properties of the membrane play important roles in prolonged operations. There were only a few studies focusing on how to improve the membrane mechanical properties. Cabasso et al.3 has found that the addition of poly(vinylpyrrolidone) (PVP) in the dope solution of polysulfone (PSf) would decrease the tensile strength of the PSf hollow fiber. Huang et al.4-6 has prepared modified hydroxylterminated polybutadiene (HTPB)-based polyurethanes (PUs) to improve the mechanical properties of PU * To whom correspondence should be addressed. Tel: 8863-4563171 ext. 4121. Fax: 886-3-4563171 ext. 4199. E-mail: [email protected].

membranes. Qin et al.7,8 and Chung et al.9 have demonstrated the effect of the dope flow rate and shear stress within a spinneret on the mechanical properties of a poly(ether sulfone) hollow fiber. A higher dope extrusion speed or shear rate within the spinneret resulted in a lower elongation but higher tensile strength and Young’s modulus. Membranes owning macrovoid morphology often have inferior mechanical properties. Lai et al.10-12 have found that the addition of nonionic surfactants in a casting solution could enhance or suppress the formation of macrovoids. In our previous studies,13 we have shown that the addition of surfactant (Span-80) in the PSf casting solution eliminated the macrovoid structure in the membrane and effectively enhanced the pervaporation performance. The objective of this work focused on the effect of surfactant addition on the mechanical properties of PSf asymmetric membranes and the relation between membrane structure and mechanical properties. In addition, the effects of casting temperature were also investigated. The morphology and surface roughness of the asymmetric PSf membranes was characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Its relationship to the mechanical strength was also discussed. Experimental Section Materials. PSf used in this study was supplied from Amoco Performance Products Inc. (Ridgefield, CT) under the trade name of Udel P-3500. The solvent, N-methylpyrrolidinone (NMP), was of reagent grade and was used without further purification. Distilled water was used as the coagulant. Surfactants, Tween-20 (polyoxyethylene sorbitan monolaurate) and Span-80 (sorbitan

10.1021/ie010026e CCC: $20.00 © 2001 American Chemical Society Published on Web 11/09/2001

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Table 1. Mechanical Properties as a Function of the Tween-20 Contenta

Table 3. Mechanical Properties as a Function of the Span-80 Contenta

Tween-20 content (wt %)

tensile strength at break (MPa)

elongation at break (%)

initial Young’s modulus (MPa)

Span-80 content (wt %)

tensile strength at break (MPa)

elongation at break (%)

initial Young’s modulus (MPa)

0 5 10 15

2.20 1.40 1.21 1.22

7.65 11.00 14.56 16.67

32.7 18.80 16.56 13.34

0 5 10 15

2.20 2.54 2.94 3.27

7.65 15.54 23.13 55.80

32.7 38.7 37.7 49.2

a Membrane: PSf (10 wt %)/NMP + Tween-20/water, casting at 70 °C.

a Membrane: PSf (10 wt %)/NMP + Span-80/water, casting at 70 °C.

Table 2. Tg (°C) of PSf Membranesa

monooleate), were purchased from Showa Chemicals Inc., Japan. The hydrophilic lipophilic balance (HLB) values were 16.7 and 4.3, respectively. Membranes Preparation. Surfactant (0-15 wt %) was added in NMP to form a solvent mixture. PSf was well dissolved in the above solvent mixture to form a casting solution at a predetermined temperature. The

surfactant content (wt %) Span-80 Tween-20

0

5

10

15

176.9 176.9

165.2 168.8

164.0 169.0

162.8 166.8

a Membrane: PSf (10 wt %)/NMP + surfactant/water, casting at 70 °C.

Figure 1. Cross section of PSf (10 wt %)/NMP + Span-80/water membranes, and casting at 70 °C. Span-80: (A) 0%, (B) 5%, (C) 10%, and (D) 15%.

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Figure 2. Effect of Span-80 on PSf (10 wt %)/NMP + Span-80/water membrane skin layer, casting at 70 °C. Span-80: (A) 0%, (B) 5%, (C) 10%, (D) 15%.

degassed casting solution was cast on a preheated glass plate to a predetermined thickness of 300 µm by a Gardner knife and then immersed into a freshly distilled water coagulation bath at the same temperature overnight; the distilled water tank was refreshed at least four times during the day. The obtained membranes were peeled off and air-dried completely at ambient temperature. Mechanical Properties Measurement. The tensile strength at break and elongation at break were measured according to the ASTM D412 standard method by a tensile test machine (Gotech Testing Machines Inc., GT-7010-D2) at a crosshead speed of 20 mm/min, with a clamp distance of 3 in. The initial Young’s modulus was measured by a dynamic mechanical analyzer (DMA 7e, Perkin-Elmer) of the extension model, damping from 100 to 4000 mN at 100 mN/min. Thermal Properties Measurement. Glass transition temperature (Tg) measurements were carried out by using a Perkin-Elmer differential scanning calorimeter (model DSC 7) at a heating rate of 10 °C/min from 50 to 200 °C. Morphology Analysis. The membrane structures were examined by Hitachi (models S570 and S4700) scanning electron microscopes (SEMs). The membrane samples were fractured in liquid nitrogen and then sputtered with Pt. The AFM used to image the membrane surfaces is a multimode scanning probe microscope with a Nanoscope IIIa controller, supplied by Digital Instruments (USA). The membrane surface is scanned in tapping mode with an oscillating tip. Small pieces of membrane were gulled onto metal disks and attached to a magnetic sample holder, located on top of the scanner tube.

The membrane surface roughness can be expressed by the difference between the highest and the lowest points within the given area (Z), the standard deviation of the Z values within the given area (Rq or RMS), and the mean roughness (Ra) that represents the mean value of the surface relative to the center plane, the plane for which the volumes enclosed by the image above and below this plane are equal. All of the surface roughness was calculated from the AFM images using an AFM software program. Results and Discussion Effect of the Addition of Hydrophilic Surfactant. Various amount of a hydrophilic surfactant, Tween-20, were added into a 10 wt % PSf/NMP casting solution. After being cast at 70 °C, the nascent membrane was immediately immersed in a 70 °C water bath. Table 1 shows the effects of the Tween-20 content on the mechanical properties of the membrane. It was found that the “elongation at break” increased but the “tensile strength at break” and the “initial Young’s modulus” decreased with an increase in the Tween-20 content. It was suspected that the change in mechanical properties by introduction of Tween-20 was due to the plasticizing effect and the alteration of membrane morphology. Plasticization means the introduction of a small molecule which increases the mobility of the polymer chain. Therefore, the plasticizing effect could be confirmed by the depression of Tg after Tween-20 addition. Table 2 showed the Tg measurements of Tween-20 added membranes. The glass transition temperature decreased with the Tween-20 content in the casting solution. Tween20 might act as a plasticizer in the membrane; therefore,

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Figure 3. AFM analysis of PSf (10 wt %)/NMP + surfactant/water membrane, casting at 70 °C: (A) without surfactant, (B) 15 wt % Span-80, (C) 15 wt % Tween-20.

the elongation increased along with the Tween-20 content while the initial Young’s modulus was reduced by Tween-20. Effect of the Addition of Lipophilic Surfactant. When the added surfactant was substituted by a lipophilic surfactant, Span-80, again, the effects of surfactant on the mechanical properties were investigated. Table 3 shows the effects of the Span-80 content on the mechanical properties of the membrane. Unlike what was observed in the case of Tween-20, all of the three mechanical properties (the tensile strength at break, the elongation at break, and the initial Young’s modulus) increased with an increase in the surfactant (Span-80)

content. In general, the tensile strength at break for a polymeric substance increases with the reduction of the elongation at break. However, it was surprising to find an increase in the tensile strength at break with an increase in the elongation at break. Apparently, the mechanical properties of Span-80 containing membranes were totally different from those of the Tween20 containing one. According to the Tg information, as shown in Table 2, the plasticizing effect still existed in the Span-80 containing membranes. The increase in elongation may be explained by the plasticizing effect, but it needs other explanations for the increase in the tensile strength and initial Young’s modulus.

Ind. Eng. Chem. Res., Vol. 40, No. 25, 2001 5921 Table 4. Roughness (nm) of the PSf Membranea membrane

RMS Ra a

PSf/NMP

PSf/NMP + 15 wt % Span 80

PSf/NMP + 15 wt % Tween-20

7.126 5.161

2.345 1.859

7.932 6.316

10 wt % PSf, casting at 70 °C.

Effect of the Membrane Structure on the Mechanical Properties. Strathmann et al.14 has reported that a highly porous membrane was formed when the coagulant entered the nascent membrane faster than the rate of solvent escaping. A dense membrane was formed vice versa. Because Span-80 was a relatively lipophilic surfactant (HLB ) 4.3), it might retard the coagulant (water) inflow to the nascent membrane and subsequently suppress the macrovoids. Furthermore, the size of the macrovoids decreased with an increase in the surfactant content. Figure 1 showed the SEM images of the cross section of Span-80 containing membranes. The size of the macrovoid was significantly supressed by the addition of Span-80. Subsequently, the overall porosity of the membranes decreased with an increase in the surfactant content. The compact membrane structure may be an explanation for the increase in the tensile strength at break. Figure 2 reveals the skin layer of various Span-80 containing PSf asymmetric membranes. It was observed that the addition of Span-80 increased the thickness of the dense skin layer. The skin layer thickness may also contribute to the endurance to the applied force and resulted in an increase in the tensile strength at break. Polymeric materials that possessed deep cavities will induce stress centralization to form weak points under an applied force. The surface roughness of asymmetric PSf membranes was then characterized by AFM analysis. Figure 3 showed the results of AFM analysis, and Table 4 revealed the RMS and Ra values. It was found that the surface roughness of the Tween-20-added membrane was the highest among these membranes. The deep cavities on the surface may form weak points of the membrane. After the addition of Span-80, huge cavities were eliminated and the tensile strength was strengthened. Opposite to the Span-80 containing membrane, the initial Young’s modulus and the tensile strength of Tween-20 containing membrane decreased with the Tween-20 content. These phenomena may also relate to the membrane structure. Figure 4 shows the SEM images of Tween-20 containing PSf membranes. A lot of huge macrovoids were observed from the cross section of the membrane. In comparison with the Span-80 containing membranes as shown in Figure 2, the skin layer of Tween-20 added membranes was thinner and much more porous. A similar observation was also obtained when it was compared with the surfactant-free membrane. Weak points in the membrane might be introduced by the macrovoids and the porous skin layer. Subsequently, the initial Young’s modulus and the tensile strength at break of the Tween-20 containing membranes were reduced. Effect of the Casting Temperature. To further elucidate the effect of the membrane structure, we cast Span-80 containing PSf solution at different temperatures to obtain membranes of different structures but of similar compositions. PSf and Span-80 were dissolved in NMP to form a PSf (10 wt %) and Span-80 (15 wt

Figure 4. Cross section of PSf (10 wt %)/NMP + Tween-20/water membranes, and casting at 70 °C. Tween-20: (A) 5%, (B) 10%, (C) 15%.

%)/NMP casting solution. The membrane was cast at 30, 50, or 70 °C and then immersed in a coagulant (water) bath at the same temperature. Figure 5 shows the structure of membranes cast at various temperatures. The membrane owned fingerlike macrovoids when it was cast at 30 °C, and the macrovoids were almost diminished at 70 °C. Table 5 shows the effect of casting temperature on the mechanical properties of PSf membranes. It was found that all of the three mechanical properties, the tensile strength at break, the elongation at break, and the initial Young’s modulus, increased with an increase of the casting temperature. The SEM images in Figure 5 show that the macrovoids of the membrane cast at 30 °C were larger than those cast at 70 °C. More weak points would be in-

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Figure 5. Effect of the casting temperature on PSf (10 wt %)/NMP + 15 wt % Span-80/water membrane morphology and casting at (A) 30 °C, (B) 50 °C, and (C) 70 °C. Table 5. Mechanical Properties as a Function of the Casting Temperaturea casting temp (°C)

tensile strength at break (MPa)

elongation at break (%)

initial Young’s modulus (MPa)

30 50 70

1.36 3.03 3.27

17.75 35.49 55.80

27.7 38.3 49.2

a Membrane: PSf (10 wt %)/NMP + 15 wt % Span-80/water casting at various temperatures.

troduced by the macrovoids and caused the decrease in mechanical properties. The effect of the casting temperature again showed that the membrane structure has a great influence on the mechanical properties. Conclusions The addition of a surfactant affected the mechanical properties of a polymeric membrane by plasticizing the membrane or altering the membrane structure. The plasticization effect increased the elongation at break and decreased the initial Young’s modulus. A thinner skin layer and porous membrane structure reduced all of the three mechanical properties. The addition of Tween-20 increased the elongation at break and reduced the initial Young’s modulus because of plasticization, but the decrease in the tensile strength at break was probably due to the porous membrane structure. The addition of Span-80 resulted in a denser membrane. All of the three measured mechanical properties increased. The increases in the tensile strength at break and in the initial Young’s modulus could be attributed to the dense membrane structure. The increase in elongation was probably contributed by both the plasticization and the membrane structure. Therefore, one ought to know the effect of surfactants on the membrane structure in order to evaluate how the addition of surfactants affects the mechanical properties of an asymmetric membrane made by the wet inversion method. Acknowledgment The authors sincerely thank the National Science Council of Taiwan, ROC (NSC 89-2216-E-033-013), for the financial support.

Literature Cited (1) Loeb, S.; Sourirajan, S. Seawater demineralization by means of an osmotic membrane. Adv. Chem. Ser. 1963, 38, 117. (2) Mulder, M. Basic Principles of Membrane Technology; Kluwer Academic Publishers: London, 1991. (3) Cabasso, I.; Klein, E.; Smith, J. K. Polysulfone hollow fibers. I. Spinning and properties. J. Appl. Polym. Sci. 1996, 20, 2377. (4) Huang, S. L.; Lai, J. Y. Tensile property of modified hydroxyl-terminated polybutadiene-based polyurethanes. J. Appl. Polym. Sci. 1997, 64, 1235. (5) Huang, S. L.; Lai, J. Y. Structure-tensile properties of polyurethanes. Eur. Polym. J. 1997, 33, 1563. (6) Huang, S. L.; Yu, S. J.; Lai, J. Y. Structure effect on the peel strength of polyurethane. J. Adhes. Sci. Technol. 1998, 12, 445. (7) Qin, J. J.; Chung, T. S. Effect of dope flow rate on the morphology, separation performance, thermal and mechanical properties of ultrafiltration hollow fiber membrane. J. Membr. Sci. 1999, 157, 35. (8) Qin, J. J.; Wang, R.; Chung, T. S. Investigation of shear stress effect within a spinneret on flux, separation and thermomechanical properties of hollow fiber ultrafiltration membranes. J. Membr. Sci. 2000, 175, 197. (9) Chung, T. S.; Qin, J. J.; Gu, J. Effect of shesr rate within the spinneret on morphology, separation performance and mechanical properties of ultrafiltration polyethersulfone hollow fiber membranes. Chem. Eng. Sci. 2000, 55, 1077. (10) Lai, J. Y.; Lin, F. C.; Wu, T. T.; Wang, D. M. On the formation of macrovoids in PMMA membranes. J. Membr. Sci. 1999, 155, 31. (11) Lin, F. C.; Wang, D. M.; Lai, C. L.; Lai, J. Y. Effect of surfactants on the structure of PMMA membrane. J. Membr. Sci. 1997, 123, 281. (12) Wang, D. M.; Lin, F. C.; Wu, T. T.; Lai, J. Y. Formation mechanism of the macrovoids induced by surfactant additives. J. Membr. Sci. 1998, 142, 191. (13) Tsai, H. A.; Li, L. D.; Lee, K. R.; Wang, Y. C.; Li, C. L.; Huang, J.; Lai, J. Y. Effect of surfactant addition on the morphology and pervaporation performance of asymmetric polysulfone membranes. J. Membr. Sci. 2000, 176, 97. (14) Strathman, H.; Kock, K.; Amar, P.; Baker, R. W. The formation mechanism of asymmetric membranes. Desalination 1975, 16, 179.

Received for review January 4, 2001 Revised manuscript received May 24, 2001 Accepted May 24, 2001 IE010026E