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Effect of Polymer Concentration on the Morphology and Mechanical Properties of Asymmetric Polysulfone (PSf) Membrane

Effect of Polymer Concentration on the Morphology and Mechanical Properties of Asymmetric... J. Applied Membrane Science & Technology, Vol. 21, December 2017, 33-41 © Universiti Teknologi Malaysia Effect of Polymer Concentration on the Morphology and Mechanical Properties of Asymmetric Polysulfone (PSf) Membrane a* a a a b N. M. Ismail , N. R. Jakariah , N. Bolong , S. M. Anissuzaman , N. A. H. M. Nordin , A. R. Razali Faculty of Engineering, Universiti Malaysia Sabah, 88400 UMS Kota Kinabalu, Sabah, Malaysia Department of Chemical Engineering, Universiti Teknologi PETRONAS (UTP), 32610 Bandar Seri Iskandar, Perak, Malaysia Manufacturing Focus Group, Faculty of Mechanical Engineering, Universiti Malaysia Pahang, 26600 Pekan Pahang, Malaysia ABSTRACT Polymer concentration had been identified as one of the important parameters tailoring the membrane properties. In this work, the effects of polymer concentration on the morphological and mechanical properties of membrane were investigated at three different polymer concentrations, i.e., 20, 25 and 30 wt%. The viscosity of dope solutions were determined to estimate the optimum polymer concentration. The morphological properties of the fabricated membrane were determined using SEM whereas the mechanical properties of the membrane were investigated using tensile tester. Results show that an increase in the polymer concentration could lead to the improvement of the morphological and mechanical properties of the membrane. The tensile strength of the membrane determined for PSf-20, PSf-25 and PSf-30 are 5.73, 6.59 and 7.03 MPa, respectively whereas the elongation at break measured for the membranes are 46.99%, 69.18% and 36.27%, respectively. As shown in this work, the polymer concentration played a significant role to alter on membrane morphology and mechanical strength. Keywords: Membrane, morphology, mechanical properties, asymmetric flat sheet 1.0 INTRODUCTION contains contaminates majorly carbon dioxide [1]. Basically, the associated Associated gas refers to the natural gas gas obtained from oil well is not purely that comes from oil well [1]. This gas natural gas as it contains other can exist either separately from oil in component such as carbon dioxide and the formation which is known as free hydrogen sulphide. The presence of gas, or dissolved in the crude oil, also this contaminates unfortunately lower known as dissolved gas. During the energy value of natural gas which petroleum crude oil extraction, lead to a lower selling price [1]. This associated gas will be separated from challenge can be overcome if the the crude oil by using a separator as natural gas can be successfully purified this gas may cause corrosion inside the from the contaminants. pipeline. The extracted crude oil will Nowadays, studied had shown that then be further processed before being membrane separation is one of the stored meanwhile the separated most efficient alternative for gas associated has to be flared as it is as it separation [2]. This is proven as * Corresponding to: N. M. Ismail (email: [email protected]) 34 N. M. Ismail et al. membrane technology has comparative current study. Thus, it is essential to advantages over other technologies determine the optimum concentration including compactness and light of the polymer for the improvement of weight, easier expansion to increase membranes performances for gas capacity due to modular system, low separation industry which will lead to maintenance, low energy requirements optimum selectivity and permeability. as it does not require phase transition Therefore, this study reports on the for separation to occurs, low cost, and fabrication of polymeric (PSf) environmental friendly [3,4]. asymmetric membrane by using the Asymmetric membrane is phase inversion technique which can preferable morphology for gas be used in the natural gas separation. separation due to their higher gas The objectives of this project are to productivity [5]. Basically, an investigate the effect of polymer asymmetric membrane consists of two concentration on: (a) the viscosity of layers comprising the dense skin layer the dope solution (b) the overall that is responsible for the selective membrane morphology such as micro separation and a sublayer (porous and macro voids formation with structure) that functions as a different polymer concentrations (c) mechanical support for the membrane. mechanical properties of the The membrane performance depends membrane structure. It was predicted greatly on the thickness of the dense that higher polymer concentration skin layer of the asymmetric structure. results in higher skin layer thickness, Theoretically, thicker dense layer decrease of larger macro-voids within would result in membrane which is the membrane and improves the more selective towards CO2, while membrane mechanical strength. having lower gas productivity (permeance), and vice versa. Thus, finding the balance between gas 2.0 METHODS selectivity and permeance by controlling the dense layer thickness is 2.1 Materials very crucial in developing gas separation membrane. To fabricate these asymmetric Notably, polymer concentration in a polymeric membrane, Polysulfone dope solution was identified as the (PSfUdel® P-3500), yellowish most important parameter for tranparent granule was purchased from modifying those properties of Solvay Plastic. Commercially available membrane [6]. This is due to the fact n-methyl-2-pyrrolidone (NMP) that, increasing polymer concentration supplied by Sigma Aldrich was used as in a dope solution would lead to a a solvent whereas the non-solvent used formation of denser skin layer that has for post treatment was methanol a higher selectivity [6,7]. Ideally, an supplied by Fisher Scientific. asymmetric membrane with a hyper- thin skin layer will exhibit high 2.2 Dope Solution Formulation permeation property [9]. However, studies had suggested that Firstly, polysulfone was dried in an polymeric membrane which are highly oven with air circulation at 40 C for 24 permeable to gases will have a low h to remove any present moist. selectivity and vice versa [10]. This Secondly, the required corresponding condition remained as the main amount of PSf and NMP were weighed challenge and motivation for the using weighing balance. In this work, Effect of Polymer Concentration on PSf Membrane Properties 35 the required polymer concentration to The cross-section images of the fabricate membrane are 20%, 25% and membrane were observed at 15 kV of 30 wt.%. The PSf polymer was poured potential with magnifications of 600x, into the bottle containing NMP 1500x and 3000x. Firstly, for the solvent. Then, the mixture were stirred samples preparation, the membrane at constant speed of 215 rpm using was immersed in liquid nitrogen to motor stirrer. The polymer was break them into a small rectangular continuously stirred for several hours shape before being mounted on sample until a homogeneous solution was stubs. Then, the samples were coated formed. Next, the dope solution was by using a mixture of gold and immersed in a shaking water bath for platinum in a sputter-coater. To approximately 40 min at room investigate the effect of polymer temperature. Finally, the dope solution concentration on the morphological was left for 24 h to remove any air properties of the membrane, the cross- bubbles before proceeding with the section images of each fabricated membrane casting step. membrane samples were analysed by using SEM. 2.3 Membrane Fabrication Tensile test on membrane was conducted using a tensile test machine A small amount of dope solution was (Shimadzu Trapezium Lite X Version poured sufficiently on the top sides of 1.1.2) at room temperature. Three the tapped glass plate. Secondly, the samples with a dimension of 25x50 dope solution was spread evenly on the mm were prepared and tested with a surface of the glass plate which was cross head speed of 5 mm/min. The quickly immersed in a water bath. The force at break and difference between membrane formed was transferred and the initial length and length at break immersed to a different basin for each sample were obtained. Finally, containing water bath (tap water) and the tensile strength and elongation at left overnight with the purpose of break values were analysed. removing the solvent left in the membrane. After that, the membrane was immersed in methanol for 4 h to 3.0 RESULTS AND DISCUSSION remove the remaining solvent [11]. Finally, the membrane was dried in a 3.1 Effect of Polymer Concentration room temperature for 3 days. on Dope Viscosity 2.4 Membrane Characterization Result of the viscosity is presented in Figure 1. From this figure, it can be The viscosity of dope solutions with observed that the viscosity of five different polymer concentrations PSf/NMP solution was increased as of 20, 22.5, 25, 27.5 and 30 wt% were PSf concentration increased. In measured by using (a) Brookfield addition, the trend of the increasing Programmable DV-II + Viscometer viscosity was almost doubled from the using spindle number 3 whereas, the previous viscosity reading as each of spindle speed was adjusted according the PSf concentration was increased by to the dope solution viscosity. 2.5%. This is likely due to polymer The morphological structures of the chain entanglement that causes the fabricated PSf membrane were sudden increase in viscosity. examined by using scanning electron The method of determining the microscope (SEM) (TM3000, Hitachi). critical PSf concentration is simply by 36 N. M. Ismail et al. extrapolating two straight lines which substructure. Hypothetically, the is drawn based on the slope of the increase of the polymer concentration viscosity whereby the viscosity curve in a dope solution will lead to a denser becoming more inclined [12]. Then, formation of skin layer and lesser and the critical concentration is determined smaller formation of macro-voids from the crossed point between the two structure [14]. straight lines. In this work, the critical In this study, all the fabricated concentration of PSf polymer is 26% asymmetric PSf membrane presented as shown in Figure 1. two distinct layers in their cross- Chung et al. [13] has reported in section; the dense skin layer and the their work that chain entanglement in a support structure as presented in Figure polymeric-solvent mixture plays an 2 a(i), b(i) and c(i). Moreover, it can important role in tailoring the be observed that all of the membranes performance of the membrane. From contain macro-voids structure that their work, they suggested that the varies according to the polymer significant increase in the degree of concentration. chain entanglement occurs at a The formation of the thin skin layer concentration of about 35 wt% is due to the instantaneous demixing of (defined as the critical concentration) solvent and non-solvent during the for PES/NMP. phase inversion [15]. Furthermore, the development of the large finger-like macro-voids in the porous sublayer is mainly due to the non-solvent diffusion rate into polymer-poor phase is faster than the rate of solvent diffusion outward [16]. Thus, it indicates that the formation of larger finger-like macro- voids is due to the lower polymer concentration. For PSf-25 membrane, (Figure 2 b(i) and b (ii)), a thin- skinned layer was also observed similar to PSF-20. However, a distinct difference of skin layer thickness between the PSf-20 and PSf-25 membrane cannot be differentiated clearly due to the limitation in the equipment. In addition, finger-like Figure 1 Dope solution viscosity for macrovoids observed in the porous various concentrations sublayer has decreased in length as most of the macro-voids observed are shorter compared to the PSf-20 3.2 Effect of Polymer Concentration membrane. However, few large finger- on Morphology like macro- voids are still present. This suggests that, an increase in polymer The effect of polymer concentration on concentration in the dope solution lead the morphological properties of to a reduction in the macro-voids asymmetric membrane can be structure. observed from the thickness of skin The PSf-30 membrane presented the layer formed and the development of densest skin layer compared to the macro-voids structures in the porous PSf-20 and PSf-25 fabricated Effect of Polymer Concentration on PSf Membrane Properties 37 membrane which as shown in the the macro-voids formed in the porous Figure 2 c(i) and c(ii). Additionally, sublayer also decrease in number as a (i) b (i) c (i) a (ii) b (ii) c (ii) a (iii) b (iii) c (iii) Figure 2 The cross-section of Asymmetric PSf membrane at different polymer concentration; (a) 20 wt% PSf, (b) 25 wt% PSf and (c) 30 wt% PSf for (i) 500x magnification, (ii) 3,000x magnification and (iii) bottom layer with 1,500x magnification the concentration of PSf polymer in the The changes in the morphological dope solution is increased whereas the structure of the PSf-30 are mainly due geometry in which the macro-voids are to the viscosity of the dope solution forming is a teardrop structure which is that coincides with the higher polymer different compared to the two-previous concentration. This is because, higher membrane observed. Further, it can viscosity of the dope solution would also be noticed that the cross-section of prevent the diffusion exchange the membrane sublayer porous between solvent (NMP) and non- structure is forming a sponge-like solvent (water) at the membrane structure as shown in Figure 2 c(iii). sublayer which lead to the fast phase 38 N. M. Ismail et al. separation at the skin layer and slows concentration is increased from 20 to precipitation rate at the membrane 30%. sublayer occurred [6]. As a result, a denser skin layer is formed as shown in 7.03 the Figure 2 c(i) and c(ii). 6.59 Apart from that, the formation of 5.73 spongy-like structure of the PSf-30 membrane is also mainly due to the higher viscosity of the dope solution. Considering the kinetics during the diffusion exchange, higher viscosity of dope solution causes demixing to slows down and as a consequence, the membrane morphology changes to a more spongy structure [8]. Overall, the 1 effect of polymer concentration on the morphological properties of the 20 25 30 membrane can be observed on the PSf Concentration (%) changes occurred in the thickness of the skin layer a well as bottom layer Figure 3 Tensile Strength of Asymmetric and on the formation of the macro- PSf Membrane at Different Polymer voids structure. As the polymer Concentration concentration increased, the thickness of skin and bottom layer also increased whereas the macro-voids structure Basically, the increased in the tensile changes from a large finger-like strength of the membrane is due to the structure to a teardrop structure [8]. improvement in the morphological structure of the membrane. As the 3.3 Effect of Polymer Concentration polymer concentration increased, the on Mechanical Properties formation of macro-voids structure become lesser and smaller which The mechanical properties of the improve the structure of the membrane membrane different polymer as observed in Figure 2 [17-18]. As a concentration are measured based on result, the tensile strength of the the tensile strength and elongation at membrane also increases. Moreover, break to study the effect of polymer the increase in polymer concentration concentration on the strength of the affected the tensile strength due to the membrane as shown Figure 3 and 4. fewer formation of macro-voids [19]. Figure 3 shows the tensile strength The change in elongation at break result obtained from the tensile test different polymer concentration is analysis conducted on PSf-20, PSf-25 presented in Figure 4. The results for and PSf-30 membrane. The result different PSf concentration shows a shows that an increased in polymer slightly different trend compared to the concentration leads to the increased in tensile strength. Initially, the tensile strength of the membrane. As elongation at break increases when the shown in Figure 3, the tensile strength polymer concentration is increased of the membrane increased about 15% from 20 wt% to 25 wt% similar to the as the polymer concentration is tensile strength. Then, at higher increased from 20 to 30 wt% and polymer concentration of 30 wt%, the increased about 23% when the polymer elongation at break decreases, Tensile Strength (Mpa) Effect of Polymer Concentration on PSf Membrane Properties 39 contradicting to the tensile strength skin layer with large finger-like macro- result. Since elongation at break voids structure to a denser skin layer represent the how long the membrane with rounder tear-drop macrovoids can be stretched before it breaks, this structure. In addition, this study phenomena happened likely due to the experimental result also revealed that rigidified membrane structure with there is an improvement in the increasing polymer concentration. mechanical properties. The tensile Thus, the decreased in elongation is strength of the membrane increases as observed. the polymer concentration is increased. Finally, this study concluded that the increasing polymer concentration significantly improves the 69.18 morphological and mechanical properties of the membrane. 46.99 ACKNOWLEDGEMENT 36.27 The authors would like to extend their gratitude to Universiti Malaysia Sabah (UMS) for the research fund through Research University Grant vot no. SBK0290-TK-2016. Also, we sincerely thanked Faculty of Engineering UMS as well as Advanced 20 25 30 Membrane Technology Research PSf Concentration (%) Centre (AMTEC) UTM for their assistance towards completion of this Figure 4 Elongation at Break of project. Asymmetric PSf Membrane at Different Polymer Concentration REFERENCES 4.0 CONCLUSION [1] B. Shimekit and H. Mukhtar. 2012. Natural Gas Purification In this study, asymmetric PSf/NMP Technologies–Major CO membranes were successfully Separation and Future fabricated with three different polymer Directions. Universiti of concentrations ranging from 20 wt%, Teknologi Petronas. 235-270. to 30 wt%. The viscosity analysis [2] S. Mokhatab and W. Poe. 2012. showed that as the polymer Handbook of Natural Gas concentration is increased, the dope Transmission and Processing. solution viscosity is also increased. Second Edition. Elsevier B.V. Moreover, from the extrapolation of 253-290. the dope viscosity, the optimum [3] D. F. Sanders, Z. P. Smith, R. polymer concentration obtained was 26 Guo, L. M. Robeson, J. E. wt% PSf. Furthermore, the SEM McGrath, D. R. Paul and B. D. micrographs showed that an addition in Freeman. 2013. Energy Efficient polymer concentration from 20 to 30 Polymeric Gas Separation wt% changed the morphological Membranes for a Sustainable structure of the membrane, from a thin Elongation at Break (%) 40 N. M. Ismail et al. Future: A Review. J. Polym. 54: Separation Processes in 4729-4761. Petrochemical Industry: A [4] R. Abedini and A. Review. Desalination. 235: 199- Nezhadmoghadam. 2010. 244. Application of Membrane in Gas [11] N. M. Ismail, A. F. Ismail and A. Separation Processes: Its Mustafa. 2015. Sustainability in Suitability and Mechanisms. J. Petrochemical Industry: Mixed Pet. Coal. 52: 69-80. Matrix Membranes from [5] N. M. Ismail, A. F. Ismail, A. Polyethersulfone/cloisite15A Mustafa, T. Matsuura, T. Soga, for the Removal of Carbon K. Nagata, T. Asaka. 2015. Dioxide. Procedia CIRP. 26: Qualitative and Quantitative 461-466. Analysis of Intercalated and [12] N. Peng, T. S. Chung and K. Y. Exfoliated Silicate Layers in Wang. 2014. Macrovoid Asymmetric Evolution and Critical Factors to Polyethersulfone/Cloisite15a Ò Form Macrovoid-free Hollow Mixed Matrix Membrane for Fiber Membranes. J. Memb. Sci. CO /CH Separation. Chem. 318: 363-372. 2 4 Eng. J. 268: 371-383. [13] T. S. Chung, S. K. Teoh, and X. [6] S. Hamzah, N. Ali, M. M. Hu. 1997. Formation of Ultrathin Ariffin, A. Ali and A. W. High- Mohammad. 2014. High performancepolyethersulfone Performance of Polysulfone Hollow-fiber Membranes. J. Ultrafiltration Membrane: Effect Memb. Sci. 133: 161-175. of Polymer Concentration. J. [14] M. I. Mustaffar, A. F. Ismail and Eng. Appl. Sci. 9: 2543-2550. R. M. Illias. 2004. Study on the [7] H. Julian and I. G. Wenten. Effect of Polymer Concentration 2012. Polysulfone Membranes on Hollow Fiber Ultrafiltration for CO /CH Separation: State of Membrane Performance and 2 4 the Art. J. Eng. 2: 484-495. Morphology. Reg. Conf. Eng. [8] A. K. Hołda, B. Aernouts, W. Educ. RCEE 2005. 1-12. Saeys and I. F. J. Vankelecom. [15] B. Zhou and Ph.D. 2013. Study of Polymer Massachusetts Institute of Concentration and Evaporation Technology. 2006. Simulations Time as Phase Inversion of Polymeric Membrane Parameters for Polysulfone- Formation in 2D and 3D. based SRNF Membranes. J. [16] H. Sofiah, A. Nora’aini and M. Memb. Sci. 442: 196-205. A. Marinah. 2010. The Influence [9] A. F. Ismail and L. P. Yean. of Polymer Concentration on 2003. Review on the Performance and Morphology of Development of Defect Free and Asymmetric Ultrafiltration Ultrathin Skinned Asymmetric Membrane for Lysozyme Membranes for Gas Separation Separation. J. Appl. Sci. 10: Through Manipulation of Phase 3325-3330. Inversion and Rheological [17] M. Z. Yunos, Z. Harun, H. Basri, Factors. J. Appl. Polym. Sci. 88: A. F. Ismail. 2012. Effects of 442-451. Water as Non-solvent Additive [10] M. T. Ravanchia, T. Kaghazchia on Performance of Polysulfone and A. Kargarib. 2009. Ultrafiltration Membrane. Adv. Application of Membrane Mater. Res. 488-489: 46-50. Effect of Polymer Concentration on PSf Membrane Properties 41 [18] X. Wang, Z. Li, M. Zhang, T. Soc. Chem. 7: 10503-10516. Fan and B. Cheng. 2017. [19] P. Amirabedi, R. Yegani and M. Preparation of a Polyphenylene K. R. Aghjeh. 2013. Sulfide Membrane from a Experimental Design Applied Ternary Polymer/Solvent/Non- toFabrication of PSf Membranes Solvent System by Thermally via NIPS Method. J. Text. Induced Phase Separation. R. Polym. 1: 24-30. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Applied Membrane Science & Technology Unpaywall

Effect of Polymer Concentration on the Morphology and Mechanical Properties of Asymmetric Polysulfone (PSf) Membrane

Ismail, N. M.; Jakariah, N. R.; Bolong, N.; Anissuzaman, S. M.; Nordin, N. A. H. M.; Razali, A. R.
Journal of Applied Membrane Science & Technology – Dec 7, 2017
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2600-9226
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J. Applied Membrane Science & Technology, Vol. 21, December 2017, 33-41 © Universiti Teknologi Malaysia Effect of Polymer Concentration on the Morphology and Mechanical Properties of Asymmetric Polysulfone (PSf) Membrane a* a a a b N. M. Ismail , N. R. Jakariah , N. Bolong , S. M. Anissuzaman , N. A. H. M. Nordin , A. R. Razali Faculty of Engineering, Universiti Malaysia Sabah, 88400 UMS Kota Kinabalu, Sabah, Malaysia Department of Chemical Engineering, Universiti Teknologi PETRONAS (UTP), 32610 Bandar Seri Iskandar, Perak, Malaysia Manufacturing Focus Group, Faculty of Mechanical Engineering, Universiti Malaysia Pahang, 26600 Pekan Pahang, Malaysia ABSTRACT Polymer concentration had been identified as one of the important parameters tailoring the membrane properties. In this work, the effects of polymer concentration on the morphological and mechanical properties of membrane were investigated at three different polymer concentrations, i.e., 20, 25 and 30 wt%. The viscosity of dope solutions were determined to estimate the optimum polymer concentration. The morphological properties of the fabricated membrane were determined using SEM whereas the mechanical properties of the membrane were investigated using tensile tester. Results show that an increase in the polymer concentration could lead to the improvement of the morphological and mechanical properties of the membrane. The tensile strength of the membrane determined for PSf-20, PSf-25 and PSf-30 are 5.73, 6.59 and 7.03 MPa, respectively whereas the elongation at break measured for the membranes are 46.99%, 69.18% and 36.27%, respectively. As shown in this work, the polymer concentration played a significant role to alter on membrane morphology and mechanical strength. Keywords: Membrane, morphology, mechanical properties, asymmetric flat sheet 1.0 INTRODUCTION contains contaminates majorly carbon dioxide [1]. Basically, the associated Associated gas refers to the natural gas gas obtained from oil well is not purely that comes from oil well [1]. This gas natural gas as it contains other can exist either separately from oil in component such as carbon dioxide and the formation which is known as free hydrogen sulphide. The presence of gas, or dissolved in the crude oil, also this contaminates unfortunately lower known as dissolved gas. During the energy value of natural gas which petroleum crude oil extraction, lead to a lower selling price [1]. This associated gas will be separated from challenge can be overcome if the the crude oil by using a separator as natural gas can be successfully purified this gas may cause corrosion inside the from the contaminants. pipeline. The extracted crude oil will Nowadays, studied had shown that then be further processed before being membrane separation is one of the stored meanwhile the separated most efficient alternative for gas associated has to be flared as it is as it separation [2]. This is proven as * Corresponding to: N. M. Ismail (email: [email protected]) 34 N. M. Ismail et al. membrane technology has comparative current study. Thus, it is essential to advantages over other technologies determine the optimum concentration including compactness and light of the polymer for the improvement of weight, easier expansion to increase membranes performances for gas capacity due to modular system, low separation industry which will lead to maintenance, low energy requirements optimum selectivity and permeability. as it does not require phase transition Therefore, this study reports on the for separation to occurs, low cost, and fabrication of polymeric (PSf) environmental friendly [3,4]. asymmetric membrane by using the Asymmetric membrane is phase inversion technique which can preferable morphology for gas be used in the natural gas separation. separation due to their higher gas The objectives of this project are to productivity [5]. Basically, an investigate the effect of polymer asymmetric membrane consists of two concentration on: (a) the viscosity of layers comprising the dense skin layer the dope solution (b) the overall that is responsible for the selective membrane morphology such as micro separation and a sublayer (porous and macro voids formation with structure) that functions as a different polymer concentrations (c) mechanical support for the membrane. mechanical properties of the The membrane performance depends membrane structure. It was predicted greatly on the thickness of the dense that higher polymer concentration skin layer of the asymmetric structure. results in higher skin layer thickness, Theoretically, thicker dense layer decrease of larger macro-voids within would result in membrane which is the membrane and improves the more selective towards CO2, while membrane mechanical strength. having lower gas productivity (permeance), and vice versa. Thus, finding the balance between gas 2.0 METHODS selectivity and permeance by controlling the dense layer thickness is 2.1 Materials very crucial in developing gas separation membrane. To fabricate these asymmetric Notably, polymer concentration in a polymeric membrane, Polysulfone dope solution was identified as the (PSfUdel® P-3500), yellowish most important parameter for tranparent granule was purchased from modifying those properties of Solvay Plastic. Commercially available membrane [6]. This is due to the fact n-methyl-2-pyrrolidone (NMP) that, increasing polymer concentration supplied by Sigma Aldrich was used as in a dope solution would lead to a a solvent whereas the non-solvent used formation of denser skin layer that has for post treatment was methanol a higher selectivity [6,7]. Ideally, an supplied by Fisher Scientific. asymmetric membrane with a hyper- thin skin layer will exhibit high 2.2 Dope Solution Formulation permeation property [9]. However, studies had suggested that Firstly, polysulfone was dried in an polymeric membrane which are highly oven with air circulation at 40 C for 24 permeable to gases will have a low h to remove any present moist. selectivity and vice versa [10]. This Secondly, the required corresponding condition remained as the main amount of PSf and NMP were weighed challenge and motivation for the using weighing balance. In this work, Effect of Polymer Concentration on PSf Membrane Properties 35 the required polymer concentration to The cross-section images of the fabricate membrane are 20%, 25% and membrane were observed at 15 kV of 30 wt.%. The PSf polymer was poured potential with magnifications of 600x, into the bottle containing NMP 1500x and 3000x. Firstly, for the solvent. Then, the mixture were stirred samples preparation, the membrane at constant speed of 215 rpm using was immersed in liquid nitrogen to motor stirrer. The polymer was break them into a small rectangular continuously stirred for several hours shape before being mounted on sample until a homogeneous solution was stubs. Then, the samples were coated formed. Next, the dope solution was by using a mixture of gold and immersed in a shaking water bath for platinum in a sputter-coater. To approximately 40 min at room investigate the effect of polymer temperature. Finally, the dope solution concentration on the morphological was left for 24 h to remove any air properties of the membrane, the cross- bubbles before proceeding with the section images of each fabricated membrane casting step. membrane samples were analysed by using SEM. 2.3 Membrane Fabrication Tensile test on membrane was conducted using a tensile test machine A small amount of dope solution was (Shimadzu Trapezium Lite X Version poured sufficiently on the top sides of 1.1.2) at room temperature. Three the tapped glass plate. Secondly, the samples with a dimension of 25x50 dope solution was spread evenly on the mm were prepared and tested with a surface of the glass plate which was cross head speed of 5 mm/min. The quickly immersed in a water bath. The force at break and difference between membrane formed was transferred and the initial length and length at break immersed to a different basin for each sample were obtained. Finally, containing water bath (tap water) and the tensile strength and elongation at left overnight with the purpose of break values were analysed. removing the solvent left in the membrane. After that, the membrane was immersed in methanol for 4 h to 3.0 RESULTS AND DISCUSSION remove the remaining solvent [11]. Finally, the membrane was dried in a 3.1 Effect of Polymer Concentration room temperature for 3 days. on Dope Viscosity 2.4 Membrane Characterization Result of the viscosity is presented in Figure 1. From this figure, it can be The viscosity of dope solutions with observed that the viscosity of five different polymer concentrations PSf/NMP solution was increased as of 20, 22.5, 25, 27.5 and 30 wt% were PSf concentration increased. In measured by using (a) Brookfield addition, the trend of the increasing Programmable DV-II + Viscometer viscosity was almost doubled from the using spindle number 3 whereas, the previous viscosity reading as each of spindle speed was adjusted according the PSf concentration was increased by to the dope solution viscosity. 2.5%. This is likely due to polymer The morphological structures of the chain entanglement that causes the fabricated PSf membrane were sudden increase in viscosity. examined by using scanning electron The method of determining the microscope (SEM) (TM3000, Hitachi). critical PSf concentration is simply by 36 N. M. Ismail et al. extrapolating two straight lines which substructure. Hypothetically, the is drawn based on the slope of the increase of the polymer concentration viscosity whereby the viscosity curve in a dope solution will lead to a denser becoming more inclined [12]. Then, formation of skin layer and lesser and the critical concentration is determined smaller formation of macro-voids from the crossed point between the two structure [14]. straight lines. In this work, the critical In this study, all the fabricated concentration of PSf polymer is 26% asymmetric PSf membrane presented as shown in Figure 1. two distinct layers in their cross- Chung et al. [13] has reported in section; the dense skin layer and the their work that chain entanglement in a support structure as presented in Figure polymeric-solvent mixture plays an 2 a(i), b(i) and c(i). Moreover, it can important role in tailoring the be observed that all of the membranes performance of the membrane. From contain macro-voids structure that their work, they suggested that the varies according to the polymer significant increase in the degree of concentration. chain entanglement occurs at a The formation of the thin skin layer concentration of about 35 wt% is due to the instantaneous demixing of (defined as the critical concentration) solvent and non-solvent during the for PES/NMP. phase inversion [15]. Furthermore, the development of the large finger-like macro-voids in the porous sublayer is mainly due to the non-solvent diffusion rate into polymer-poor phase is faster than the rate of solvent diffusion outward [16]. Thus, it indicates that the formation of larger finger-like macro- voids is due to the lower polymer concentration. For PSf-25 membrane, (Figure 2 b(i) and b (ii)), a thin- skinned layer was also observed similar to PSF-20. However, a distinct difference of skin layer thickness between the PSf-20 and PSf-25 membrane cannot be differentiated clearly due to the limitation in the equipment. In addition, finger-like Figure 1 Dope solution viscosity for macrovoids observed in the porous various concentrations sublayer has decreased in length as most of the macro-voids observed are shorter compared to the PSf-20 3.2 Effect of Polymer Concentration membrane. However, few large finger- on Morphology like macro- voids are still present. This suggests that, an increase in polymer The effect of polymer concentration on concentration in the dope solution lead the morphological properties of to a reduction in the macro-voids asymmetric membrane can be structure. observed from the thickness of skin The PSf-30 membrane presented the layer formed and the development of densest skin layer compared to the macro-voids structures in the porous PSf-20 and PSf-25 fabricated Effect of Polymer Concentration on PSf Membrane Properties 37 membrane which as shown in the the macro-voids formed in the porous Figure 2 c(i) and c(ii). Additionally, sublayer also decrease in number as a (i) b (i) c (i) a (ii) b (ii) c (ii) a (iii) b (iii) c (iii) Figure 2 The cross-section of Asymmetric PSf membrane at different polymer concentration; (a) 20 wt% PSf, (b) 25 wt% PSf and (c) 30 wt% PSf for (i) 500x magnification, (ii) 3,000x magnification and (iii) bottom layer with 1,500x magnification the concentration of PSf polymer in the The changes in the morphological dope solution is increased whereas the structure of the PSf-30 are mainly due geometry in which the macro-voids are to the viscosity of the dope solution forming is a teardrop structure which is that coincides with the higher polymer different compared to the two-previous concentration. This is because, higher membrane observed. Further, it can viscosity of the dope solution would also be noticed that the cross-section of prevent the diffusion exchange the membrane sublayer porous between solvent (NMP) and non- structure is forming a sponge-like solvent (water) at the membrane structure as shown in Figure 2 c(iii). sublayer which lead to the fast phase 38 N. M. Ismail et al. separation at the skin layer and slows concentration is increased from 20 to precipitation rate at the membrane 30%. sublayer occurred [6]. As a result, a denser skin layer is formed as shown in 7.03 the Figure 2 c(i) and c(ii). 6.59 Apart from that, the formation of 5.73 spongy-like structure of the PSf-30 membrane is also mainly due to the higher viscosity of the dope solution. Considering the kinetics during the diffusion exchange, higher viscosity of dope solution causes demixing to slows down and as a consequence, the membrane morphology changes to a more spongy structure [8]. Overall, the 1 effect of polymer concentration on the morphological properties of the 20 25 30 membrane can be observed on the PSf Concentration (%) changes occurred in the thickness of the skin layer a well as bottom layer Figure 3 Tensile Strength of Asymmetric and on the formation of the macro- PSf Membrane at Different Polymer voids structure. As the polymer Concentration concentration increased, the thickness of skin and bottom layer also increased whereas the macro-voids structure Basically, the increased in the tensile changes from a large finger-like strength of the membrane is due to the structure to a teardrop structure [8]. improvement in the morphological structure of the membrane. As the 3.3 Effect of Polymer Concentration polymer concentration increased, the on Mechanical Properties formation of macro-voids structure become lesser and smaller which The mechanical properties of the improve the structure of the membrane membrane different polymer as observed in Figure 2 [17-18]. As a concentration are measured based on result, the tensile strength of the the tensile strength and elongation at membrane also increases. Moreover, break to study the effect of polymer the increase in polymer concentration concentration on the strength of the affected the tensile strength due to the membrane as shown Figure 3 and 4. fewer formation of macro-voids [19]. Figure 3 shows the tensile strength The change in elongation at break result obtained from the tensile test different polymer concentration is analysis conducted on PSf-20, PSf-25 presented in Figure 4. The results for and PSf-30 membrane. The result different PSf concentration shows a shows that an increased in polymer slightly different trend compared to the concentration leads to the increased in tensile strength. Initially, the tensile strength of the membrane. As elongation at break increases when the shown in Figure 3, the tensile strength polymer concentration is increased of the membrane increased about 15% from 20 wt% to 25 wt% similar to the as the polymer concentration is tensile strength. Then, at higher increased from 20 to 30 wt% and polymer concentration of 30 wt%, the increased about 23% when the polymer elongation at break decreases, Tensile Strength (Mpa) Effect of Polymer Concentration on PSf Membrane Properties 39 contradicting to the tensile strength skin layer with large finger-like macro- result. Since elongation at break voids structure to a denser skin layer represent the how long the membrane with rounder tear-drop macrovoids can be stretched before it breaks, this structure. In addition, this study phenomena happened likely due to the experimental result also revealed that rigidified membrane structure with there is an improvement in the increasing polymer concentration. mechanical properties. The tensile Thus, the decreased in elongation is strength of the membrane increases as observed. the polymer concentration is increased. Finally, this study concluded that the increasing polymer concentration significantly improves the 69.18 morphological and mechanical properties of the membrane. 46.99 ACKNOWLEDGEMENT 36.27 The authors would like to extend their gratitude to Universiti Malaysia Sabah (UMS) for the research fund through Research University Grant vot no. SBK0290-TK-2016. Also, we sincerely thanked Faculty of Engineering UMS as well as Advanced 20 25 30 Membrane Technology Research PSf Concentration (%) Centre (AMTEC) UTM for their assistance towards completion of this Figure 4 Elongation at Break of project. 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