Biology and Chemistry Research
Optimizing the Synthesis of Flat-Sheet Phase Inversion Polyvinylidene Fluoride (PVDF) Membranes for Membrane Distillation Margaret Pan ABSTRACT Accompanying the worldâ€™s ever-growing population is an increased demand for efficient and environmentallyfriendly means of producing potable water. Membrane distillation (MD) is a rising technology combining distillation and filtration techniques that seeks to address this issue. This project aimed to optimize the synthesis of porous polyvinylidene fluoride (PVDF) membranes for MD by evaluating the synergistic effect of various synthesis parameters on membrane morphology. PVDF was selected as the polymeric material due to its high hydrophobicity, chemical resistance, thermal stability, and mechanical strength. PVDF was dissolved in N,N-dimethylacetamide to form solutions of varying concentration, and water was utilized as a non-solvent to induce membrane precipitation. Based on literature regarding coagulation kinetics, it was hypothesized that increasing the coagulation bath temperature and lowering the concentration of PVDF in the casting solution would result in membranes with a higher percentage of finger-like pores. These offer straighter channels for water vapor transport to increase flux. SEM imaging, supported by porosity and hydrophobicity data, has both indicated membrane viability and suggested the presence of differing degrees of influence among synthesis parameters. These data, coupled with further studies, may shed light upon such parameter synergy to allow for the holistic optimization of PVDF membrane synthesis. Introduction 1.1 Rationale As the worldâ€™s population continues to grow throughout the twenty-first century, there is an ever-increasing need for an environmentally and economically-friendly means of producing potable water. In fact, according to data provided by the World Health Organization (WHO), approximately 1.1 billion people in the world today do not have access to healthy drinking water . Since seawater constitutes 97% of the Earthâ€™s surface water, desalination is a particularly promising science. However, current desalination methods such as reverse osmosis and traditional distillation require high energy input for operation . Membrane distillation (MD), which has emerged as a promising means of addressing this issue, is a method of water treatment driven by differences in vapor pressure across a porous, hydrophobic membrane. A quasi-hybrid of filtration and distillation, MD is a highly versatile and potentially-advantageous alternative to traditional desalination technology due to its ability to effectively harness waste heat to produce clean water .
Since the membrane physically governs the passage of water vapor, the efficiency of a MD system mostly depends upon the properties of the hydrophobic membrane utilized. Therefore, the purpose of this research project was to optimize the synthesis process of porous PVDF membranes for application in MD. Polyvinylidene fluoride (PVDF) was selected as the material for membrane synthesis due to its high hydrophobicity, chemical resistance, thermal stability, and mechanical strength (fig. 1) . This project was undertaken with the goal of both qualitatively and quantitatively determining the effects of various synthesis parameters on membrane characteristics. It was hypothesized that increasing the coagulation bath temperature and lowering the concentration of PVDF in the casting solution would result in membranes with a higher percentage of finger-like pores. Finger-like pore structures are thought to be more desirable for MD than their spongy counterparts because the fingers offer straighter channels for vapor transport to potentially increase water vapor flux. 1.2 Background The simplest and most commonly utilized MD configuration for pilot testing is known as Direct Contact Membrane Distillation (DCMD), where the membrane is in direct contact with both the feed and purified permeate streams (fig. 2). This is the system under which experimenVolume 3 | 2013-2014 | 47
Biology and Chemistry Research tation was carried out for this project. Other configurations of MD differ in how the resulting permeate is collected and processed by the system, and these may factor in air gaps, vacuums, and sweeping gases on the permeate side .
A DCMD setup consists of five basic components: a hot feed stream, a cold permeate stream, a membrane module containing a hydrophobic membrane, and pumps for cycling water (fig. 3). At the membrane interface, the minimal temperature difference necessary to drive water vapor transport is approximately 20 ºC, though some systems may apply greater heat to their feed streams to achieve temperature differences of up to 50 ºC . Much like how steam rises from a hot cup of coffee, water vapors from the hot feed will naturally diffuse through the pores, while the hydrophobicity of the membrane prevents the liquid phase from passing through, leaving only purified water on the permeate side. Since such systems can utilize waste heat to generate the small temperature gradient necessary for mass transfer, MD has the potential to be far more energy efficient than distillation and reverse osmosis .
MD can be applied to purify geothermally-heated hydrofracking wastewater, as well as to create self-contained systems of producing fresh water for passengers aboard a ship by harnessing heat generated by ship boilers. MD setups constructed near industrial factories can make effective use of heat produced by machinery. The versatility of such a system in capturing various heat sources contributes greatly to its energy efficiency. Though still in the 48 | 2013-2014 | Volume 3
developmental phases, commercial pilot systems for MD have attained maximum fluxes of approximately 50 liters per square meter of membrane per hour . Flat-sheet PVDF membranes for MD are commonly synthesized by a process known as immersion precipitation (IP) phase inversion, wherein the polymer solution is cast on a substrate and then immersed in a coagulation bath. The coagulation bath consists of a solution in which the polymer cannot dissolve, and this is referred to as the “non-solvent.” In most cases, the non-solvent is deionized (DI) water, which induces the precipitation of the dissolved polymer. Precipitation is exothermic, and the energy released is referred to as “heat of mixing.” A high heat of mixing indicates a faster coagulation rate, which is important in the determination of pore morphology . Pores are created as the solvent coalesces and leaches out of the newly-forming membrane. The kinetics of the solvent/non-solvent exchange determine membrane morphology. By altering conditions of phase inversion, the rate of polymer precipitation will also change, and this sensitive relationship allows for more controlled development of membrane properties . Previous literature illustrates that faster precipitation of the membrane, which can result from high coagulation temperatures, is conducive to the formation of asymmetric membranes with finger-like pores, as in figure 4. In an asymmetric membrane, the surface in contact with the non-solvent coagulates first, and a dense upper skin with smaller pores is formed. This is known as the active side, and it is oriented facing the feed solution in an MD setup to allow for greater selectivity. Finger-like pores, if present, will form adjacent to the upper skin, providing channels for vapor transport.
Biology and Chemistry Research A recent study by Zhao et al.  produced highly spongy membranes using low coagulation temperatures of 25 ºC, while Wang et al.  was able to synthesize membranes with finger-like character at a higher coagulation temperature of 50 ºC. It has also been suggested that lower polymer concentration in the casting solution will allow the surface layer of the precipitating polymer to properly rupture and begin the propagation of finger voids . Though there has been substantial research conducted in the field of PVDF membrane synthesis, previous literature has yet to fully elucidate the synergistic effect of multiple synthesis parameters on membrane morphology. By investigating polymer concentration and coagulation temperature concurrently, this research hopes to shed light on such synergy and achieve optimization of PVDF membrane synthesis. Materials & Methods 2.1 Materials Membrane synthesis was carried out using commercial Kynar 741 polyvinylidene fluoride (MW = 254,000 g/ mol), N,N-dimethylacetamide (DMAc, reagent grade) as the solvent, and DI water as the non-solvent. 2.2 Membrane Preparation Basic experimental methods for the IP phase inversion synthesis of flat-sheet PVDF membranes were adapted from Khayet & Maatsura , with alterations made in specific parameters such as polymer concentration, temperature, and coagulation bath composition. Polymer solutions of 100 g were first prepared at concentrations of 10%, 15%, and 20% by weight. To ensure complete polymer dissolution, PVDF powder was dissolved in DMAc at 50 ºC for 1 hr and subjected to constant magnetic stirring. The three solutions, pale yellow in color after heat exposure, were then allowed to degas and cool to room temperature prior to casting (fig. 5).
spontaneously peeled off of the glass slides as precipitation took place (fig. 6). Membranes cast on fabric supports were 10 mils (254.0µm) in thickness to compensate for the presence of the support. These membranes, rather than peeling, became more opaque as precipitation proceeded. Supports were utilized primarily to combat membrane shrinkage, which can hinder membrane characterization processes. Coagulation baths were prepared at temperatures of 25 ºC, 40 ºC, and 55 ºC, and utilized in the creation of membranes for each polymer concentration and thickness. Membranes of each initial condition were synthesized in triplicate, resulting in a total of 54 membranes. Immersed casts were left undisturbed for 5 minutes to allow for complete precipitation. The fully-precipitated membranes were then leached in DI water for 24 hrs to remove residual solvent and dried and stored until further use. 2.3 SEM Surface and cross-sectional membrane morphology were examined using a scanning electron microscope (FEI XL30 SEM-FEG). Membrane samples were fractured using liquid nitrogen to ensure a clean cross-sectional view, and all samples were sputter-coated in a thin layer of gold to improve conductivity. Cross-sectional images were only obtained from unsupported membranes because fabric supports would not yield to fracturing by liquid nitrogen. Surface images were obtained from both. 2.4 Porosity Porosity is the total volume occupied by membrane pores. In order to measure porosity, the membrane was first massed, and then the total volume of the membrane was determined by water displacement. Porosity, ε, was calculated according to the following equation adapted from literature [6,7]:
Membranes were cast using a commercial casting knife (Gardco Universal Blade Applicator) on either glass slides or non-woven fabric supports. To minimize air exposure, casts were immediately immersed in a coagulation bath of DI water at varying temperatures. Unsupported membranes were cast at 7 mils thickness (177.0µm), and these Volume 3 | 2013-2014 | 49
Biology and Chemistry Research Here, m is the mass of the membrane in grams, V_mem is the measured volume of the membrane obtained by water displacement, and d_PVDF is 1.78g/mL, as per literature . 2.5 Hydrophobicity Hydrophobicity, which is vital to the production of non-wetting MD membranes, was measured in terms of the contact angle between a drop of water and the membrane surface (fig. 7) [2,6]. These values were obtained using a contact angle goniometer (KrĂźss EasyDrop). With this instrument, microliter aliquots of water were deposited onto the membrane surface and the subsequent angles formed were measured via integrated image analysis capabilities.
der to measure LEP, the membrane was supported by a porous metal disk and then placed at the bottom of the stirred-cell filled with ~100mL of water. The stirred cell was then attached to a tank of compressed air. The air was slowly released, and the pressure reading at which water began exiting the spout of the stirred-cell was taken as the experimentally-determined value for LEP. Results and Discussion 3.1 SEM SEM was the primary means by which membrane morphology was assessed. Surface SEM imaging was performed to gather information about pore size, a factor that directly impacts membrane selectivity. Figure 8 depicts a surface SEM image from a membrane of 10% PVDF cast at 55 ÂşC. As evident in the bottom right-hand corner of the image, there is some lack of uniformity within surface pore structures of the PVDF membranes fabricated. This can likely be attributed to procedural error. For example, if the immersion of a cast in the coagulation bath was not performed with a swift and uninterrupted sweeping motion, the undulation of the water in the bath could adversely impact surface morphology and result in the smears evident on this specimen.
Greater angles equate to greater hydrophobicity since contact angles assess how well a surface resists the natural adhesion of water molecules. Unsupported membranes experienced membrane shrinkage that prevented accurate contact angle values from being obtained, so hydrophobicity was instead determined in all supported membranes. From each of the triplicate supported membranes, 15 contact angles measurements were made. These 45 values were then averaged and statistically-analyzed to find representative hydrophobicity measures for membranes synthesized under each set of initial conditions. 2.6 LEP Liquid entry pressure (LEP) is the maximum pressure a membrane can withstand before allowing water to permeate its pores. If the hydrostatic pressure of the feed solution in an MD setup exceeds the LEP, then the membrane will be wetted, and contaminants from the feed solution will pass through the pores and taint the permeate. LEP for the membrane specimens were determined using an HP4750X Stirred Cell (Sterlitech Co.). In or50 | 2013-2014 | Volume 3
Though specific pore sizes vary within the sample, all are within the micrometer scale, which will provide the selectivity necessary for effective salt rejection and water purification in an MD system. Similar trends in pore sizes and surface morphology were evident in the remaining surface SEM images as well. In order to evaluate the validity of the initial hypothesis that higher coagulation temperatures and lower polymer concentrations would result in membranes with a greater percentage of finger like pores, cross-sectional SEM im
Biology and Chemistry Research ages were taken from all unsupported membranes. Representative images from each of the triplicate sets are organized as follows:
These images suggest an overall increase in finger-like structure with increases in polymer concentration. The membranes cast from 10% PVDF solution display greater spongy characteristics, and the slight finger structures that do form throughout the samples appear irregular and poorly-defined. More uniformity is present in the 15% PVDF membranes, but the structure is still predominantly spongy. Within the 20% PVDF samples, finger-like pore structures are uniform, clearly-defined, and constitute approximately 50-60% of the membrane thickness. Spanning all of the membrane samples are occasional macrovoid formations, particularly evident in membranes M2, M3, M4, and M6 from figure 9 above. These small air gaps are clearer in structure than simple spongy membrane but lack the depth element of finger structures. As there is no clear trend in macrovoid formation, it is likely that these are merely byproducts of the casting process. For example, if a solution was not fully degassed and miniscule bubbles were present in the cast prior to coagulation, macrovoid formation would be likely. Further testing of LEP and flux in MD system should give insight on the precise effect of macrovoids on membrane efficacy. Little change in membrane morphology is exhibited across the coagulation temperature gradient. Though slight differences are present, no clear trend is common to all of the membrane samples. Within the 10% PVDF samples, there appears to be larger void sizes with increases in coagulation temperature, but the pores formed are not definitively structured like fingers. Variations within the 15% and 20% samples are minimal. While both occurrences are inconsistent with what was initially hypothesized, this gives rise to the examination of other factors that may have affected membrane morphology. Upon reevaluating literature sources, it was noted that the organic solvent utilized, N-N, dimethyl-
acetamide (DMAc), is mildly hygroscopic, meaning that it will actively pull moisture from the air . Since water was the non-solvent, it is likely that the humidity of the surrounding environment and the elapsed time between solution preparation and casting would affect the resulting membrane morphology. Introducing moisture from the environment would prematurely initiate polymer precipitation, slowing the overall rate of coagulation and causing deviations from ideal conditions for synthesizing fingerlike pore structures. The time of air exposure between membrane casting and non-solvent immersion may also have similar effects if the air was of substantial humidity. The SEM images indicate that there exists a direct relationship between polymer concentration and percentage of finger-like pores produced, where higher polymer concentrations equate to higher percentages of finger-like pores. This directly opposes that which is claimed by Berghof . The data obtained from this experiment then raise a question regarding the boundaries for polymer solution concentrations for which any given observation is valid. Since literature does not specify a range of polymer concentrations, there could hypothetically exist a â€œthresholdâ€? lower bound for concentration, beyond which finger-like pores cannot form. The low viscosities of low PVDF concentrations may negatively impact finger propagation, so if said threshold falls between 15% and 20%, then the concentrations tested in this experiment would not accurately depict the relationship between polymer concentration and morphology. The validity of both experimental and literature-obtained data must be assessed in further trials with increased sample size and a wider range of concentrations for testing. Likewise, a greater coagulation temperature gradient may be necessary to induce desired changes in membrane morphology. 3.2 Porosity Displayed in Table 1 are measures of porosity for membranes cast from each initial condition of solution concentration and coagulation temperature:
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Biology and Chemistry Research There exists some variation between overall porosities and the trends indicated visually by SEM imaging, but this was to be expected due to the implicit error associated with the water-displacement procedure utilized. An alternate method proposed by current literature involves calculating volume as a function of membrane thickness, but this method was not employed due to the presence of slight variations in thickness across the samples . Low values of standard deviation illustrate that the general trend of decreasing porosity with increasing polymer concentration may be of significance. However, obtaining precise values is not as vital as affirming the presence of substantial porosity in the membrane, which indicates permeability. These data donâ€™t directly factor into determinations of membrane efficacy, but rather serve as supplementary confirmation for the presence of pore structures, which can be assessed visually from SEM imaging. 3.3 Hydrophobicity Hydrophobicity is a measure highly dependent upon surface morphology. Because rougher surfaces offer minimal opportunities for contact between the water droplet and the membrane surface, hydrophobicity increases with surface roughness. Figure 10 displays a contact angle taken at the upper extreme of the hydrophobicity measures for membrane M1 from an area of high surface roughness.
Though there is variance within these hydrophobicity measurements, indicated by the standard deviation error bars, these data suggest a possible correlation between low polymer concentrations and greater hydrophobicity measures. Such a correlation is supported by the crosssectional SEM images obtained for the membranes. Since hydrophobicity is a function of surface roughness, it is highly dependent upon coagulation kinetics and resulting morphology as well. More sponge-like characteristics toward the active side of the membrane surface, as facilitated by slow coagulation rates, would translate to greater hydrophobicity. As a general trend, a greater percentage of sponge-like character is evident in membranes with lower polymer concentration, so hydrophobicity is in accordance with what is indicated by SEM imaging. 3.4 LEP
Due to the softness of the PVDF material, the membrane surface was subject to unwarranted modifications during the synthesis procedure. Small disruptions of the bath during coagulation could lead to a texturization beyond micro-pores. Therefore, as per existing literature, substantial variability is typically exhibited within contact angle measurements . The values obtained for hydrophobicity of each type of membrane are displayed in the graph as follows: 52 | 2013-2014 | Volume 3
Measures of LEP, which are dependent upon both hydrophobicity and tensile strength, will give insight on the durability of a membrane . Low LEP values indicate that a membrane will not be able to withstand the water pressure applied in actual MD application. If the liquid phase is allowed to pass through the hydrophobic membrane pores, then the membrane is rendered ineffective. Though increases in overall porosity and quantity of voids will theoretically allow for greater rates of water flux, such alterations may also adversely impact the strength of the membrane. Therefore, LEP serves as a constraint for alterations in membrane morphology because more finger-like pores are desirable only if membrane strength is not compromised. Though data have not yet been obtained for LEP of the membrane samples in this experiment, ongoing work will allow for the determination of an optimal balance between membrane morphology and membrane durability.
Biology and Chemistry Research Conclusions and Future Work 4.1 Conclusions The principle achievement of this research was the synthesis and characterization of porous hydrophobic PVDF membranes for potable water production in membrane distillation processes. Under varying initial synthesis conditions, PVDF membranes were successfully fabricated by immersion precipitation phase inversion. It was hypothesized based on literature that lower polymer concentrations and higher coagulation bath temperatures would result in a greater percentage of finger-like pores in the PVDF membranes. While SEM imaging seemed to indicate otherwise, the data obtained give insight regarding the complexity of the relationship between synthesis parameters and pore morphology. SEM illustrated general increases in finger-like structure with greater polymer concentrations, while coagulation temperature had little apparent effect on morphology. In addition to the effect of other factors such as environment humidity and concentration thresholds, differing degrees of influence is a plausible explanation for the trends evident in SEM because the effect of polymer concentration seems to supersede that of coagulation temperature. Nevertheless, highly uniform and well-defined pore structures were evident in membranes synthesized from 20% PVDF, which are promising for future studies. Adequate porosity levels assessed in the membranes are suggestive of high water vapor flux in an MD system, and hydrophobicity, in accordance with the morphology trends noted from SEM, is such that the membranes are expected to successfully bar the passage of tainted feed solution. Though significant progress has been made in evaluating the effects of polymer concentration and coagulation temperature on membrane morphology, more concrete conclusions are still necessary to obtain. Noting such, further studies with increased sample sizes and wider ranges of testing will be employed in order to achieve a clearer understanding of the synergy governing synthesis parameters and resulting membrane morphology. 4.2 Future Work Future work for this project will include performing LEP tests and flux tests on current samples to assess membrane viability in a benchmark DCMD system. Greater sample sizes and more controlled synthesis conditions will be implemented to increase procedural uniformity. In future studies, a mechanized procedure may be applied for membrane casting. Such a system would involve a simple conveyor belt with a casting knife set on top, slightly immersed in the coagulation bath at the bottom. As membranes are cast, they will be transported by conveyor belt into the coagulation bath at a constant rate. This will further regulate the synthesis process, improving repeatability and efficiency to allow for more effective testing of new procedures.
To address the issue of DMAc’s hygroscopic properties, additional batches of PVDF solutions will be stored in a low humidity environment. A greater range of coagulation temperatures will be utilized, and temperatures will be maintained more consistently with an external heat source. Other parameters of membrane synthesis such as PVDF molecular weight, coagulation bath composition, and dissolution temperature will also be considered for testing. Ideally, the data obtained from these trials will be able to elucidate the holistic effect of various synthesis parameters on the efficacy of PVDF membranes for MD. Acknowledgments This project was conducted over the course of two trimesters and the adjoining summer, and all experimentation was performed in the Wiesner lab at Duke University Department of Civil and Environmental Engineering. I would like to thank the lab and its Principal Investigator, Dr. Mark R. Wiesner, for providing equipment and resources. I would also like to thank Judy Winglee, whom I worked with closely in the Wiesner lab, for her help in selecting a research topic, providing initial lab training, and performing SEM imaging of the membrane specimens. In addition, I would like to thank the Research in Chemistry program at the North Carolina School of Science and Mathematics and my research advisor Dr. Myra Halpin for introducing me to the wonders of scientific inquiry and offering support and guidance throughout the research process. References  D. M. a Alrousan, P. S. M. Dunlop, T. a McMurray, and J. A. Byrne, “Photocatalytic inactivation of E. coli in surface water using immobilised nanoparticle TiO2 films.,” Water research, vol. 43, no. 1, pp. 47–54, Jan. 2009.  L. Camacho, L. Dumée, J. Zhang, J. Li, M. Duke, J. Gomez, and S. Gray, “Advances in Membrane Distillation for Water Desalination and Purification Applications,” Water, vol. 5, no. 1, pp. 94–196, Jan. 2013.  F. Liu, N. A. Hashim, Y. Liu, M. R. M. Abed, and K. Li, “Progress in the production and modification of PVDF membranes,” Journal of Membrane Science, vol. 375, no. 1–2, pp. 1–27, Jun. 2011.  F. Berghof, “The formation mechanism of phase inversion membranes,” vol. 21, pp. 241–255, 1977.  Y.-H. Zhao, B.-K. Zhu, X.-T. Ma, and Y.-Y. Xu, “Porous membranes modified by hyperbranched polymersI. Preparation and characterization of PVDF membrane using hyperbranched polyglycerol as additive,” Journal of Membrane Science, vol. 290, no. 1–2, pp. 222–229, Mar. 2007.  X. Wang, X. Wang, L. Zhang, and Q. An, “Journal of Macromolecular Science , Part B : Physics Morphology and Formation Mechanism of Poly ( Vinylidene Fluoride ) Membranes Prepared with Immerse Precipitation : Ef Volume 3 | 2013-2014 | 53
Biology and Chemistry Research fect of Dissolving Temperature,” no. July 2013, pp. 37– 41.  M. Khayet and T. Matsuura, “Preparation and Characterization of Polyvinylidene Fluoride,” pp. 5710– 5718, 2001.  H. Fan, Y. Peng, Z. Li, P. Chen, Q. Jiang, and S. Wang, “Preparation and characterization of hydrophobic PVDF membranes by vapor-induced phase separation and application in vacuum membrane distillation,” Journal of Polymer Research, vol. 20, no. 6, p. 134, May 2013.  X. Wang, L. Zhang, D. Sun, Q. An, and H. Chen, “Formation mechanism and crystallization of poly(vinylidene fluoride) membrane via immersion precipitation method,” Desalination, vol. 236, no. 1–3, pp. 170–178, Jan. 2009.  C.-L. Li, D.-M. Wang, A. Deratani, D. Quémener, D. Bouyer, and J.-Y. Lai, “Insight into the preparation of poly(vinylidene fluoride) membranes by vapor-induced phase separation,” Journal of Membrane Science, vol. 361, no. 1–2, pp. 154–166, Sep. 2010.  M. G. Buonomenna, P. Macchi, M. Davoli, and E. Drioli, “Poly(vinylidene fluoride) membranes by phase inversion: the role the casting and coagulation conditions play in their morphology, crystalline structure and properties,” European Polymer Journal, vol. 43, no. 4, pp. 1557– 1572, Apr. 2007.  J. Zhang, N. Dow, M. Duke, E. Ostarcevic, J.-D. Li, and S. Gray, “Identification of material and physical features of membrane distillation membranes for high performance desalination,” Journal of Membrane Science, vol. 349, no. 1–2, pp. 295–303, Mar. 2010.  C.-Y. Kuo, H.-N. Lin, H.-A. Tsai, D.-M. Wang, and J.-Y. Lai, “Fabrication of a high hydrophobic PVDF membrane via nonsolvent induced phase separation,” Desalination, vol. 233, no. 1–3, pp. 40–47, Dec. 2008.  Y. Xiao and M. R. Wiesner, “Characterization of surface hydrophobicity of engineered nanoparticles.,” Journal of Hazardous Materials, vol. 215–216, pp. 146–51, 2012.
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