Overview
Polyvinylidene fluoride is one of the polymers that are used as membrane material. This is because of its outstanding properties. Besides its comparatively high amount of fluorine, the polymer also has a high level of crystallinity. The result of these properties are exhibited in the performance of the Polyvinylidene fluoride polymer. For instance, the chemical stability of Polyvinylidene fluoride in chlorinated solvents and environments that are highly caustic and acidic is very high. Another property of the polymer is its hydrophobicity. This means that it is not compatible with water. This would create challenges if the polymer were to be used for aqueous applications. However, the hydrophilic properties of polyvinylidene fluoride can be enhanced using surface modification (Jang, Song, Kim & Kwom, 2014). This paper will explore the surface medication processes that can be employed to enhance the hydrophilic properties of Polyvinylidene fluoride membranes.
Surface Treatment Methods
Different methods are used for the surface treatment of Polyvinylidene fluoride in order to improve its hydrophobicity properties. One of the methods used, and the method to be explored in this paper is the preparation of ultrafiltration membranes as an approach to improving the hydrophobicity properties of the polyvinylidene fluoride. The most commercially used method for the preparation of ultrafiltration membranes for the polyvinylidene fluoride is the phase inversion method where the induction method used is the immersion precipitation (Mulder, 2000).
In the phase inversion methods of membrane preparation, the transformation of the polymer is performed from a solution for to its solid form through a controlled process. Various techniques are used in the phase inversion method of membrane preparation. The immersion precipitation technique is most commonly used in the phase inversion method of membrane preparation (Mulder, 2000).
The transformation of the polymer is started by casting the polymer solution on an appropriate support. The polymer solution is made by dissolving the polyvinylidene fluoride in an appropriate solvent. The solvent solution that is mounted on a proper support is then dipped in a coagulation bath. The contents in the coagulation bath are made of an appropriate nonsolvent. For instance, by Razzaghi, Safekordi, Tavakolmoghadam, Rekabdar & Hemmati (2014) performed a study where they used the phase inversion method and the immersion precipitation technique to prepare a polymer membrane of polyvinylidene fluoride and cellulose acetate. The solvent that was used to dissolve polyvinylidene fluoride and cellulose acetate into a solution was N, N-dimethylacetamide. The nonsolvent or the coagulant that was used was water (Razzaghi at al., 2014). The exchange of the nonsolvent and solvent results in the precipitation in a process that will be explained below.
The Membrane Formation Process
Figure 1 above shows the setup of the immersion technique that is used with the phase inversion method of membrane preparation. As shown through the arrow marked J2, the solvent, through diffusion, moves into the coagulation bath (Mulder, 2000). The arrow marked J1 shows that the nonsolvent, through diffusion, moves into the cast film (Mulder, 2000). This is the exchange of the nonsolvent and solvent that was highlighted earlier. This process occurs until the thermodynamic stability of the solution in the coagulation bath is offset.
At this point, demixing occurs (Mulder, 2000). It is at this point that the solid polymeric film is created. The structure of the membrane at this point is asymmetric (Mulder, 2000 However, it can be affected by varying factors such as the polymer used, the solvent and nonsolvent, the contents of the coagulation bath, the contents of the casting solution, the time taken for evaporation, the temperatures of the solution in the coagulation bath and the casting solution, the demixing behavior and where the liquid-liquid demixing gap is located (Mulder, 2000). Varying of the factors will yield either less dense and porous membrane or a very dense membrane that is also nonporous and all the permutations in between (Mulder, 2000).
Morphological Characteristics
As intimated above, the structure of the membranes that are formed is influenced by the varying of the factors listed. One particular factor is influential in determining the morphological characteristics of the resultant membranes. The demixing mechanisms that occur influence the morphological characteristics of the membrane (Mulder, 2000). The instantaneous occurrence of the liquid-liquid demixing results in polymer membranes whose top surfaces are relatively porous. The instantaneous liquid-liquid demixing mechanism will result in polymer membranes that are porous. This is the process that is used in the production of ultrafiltration and microfiltration membranes (Mulder, 2000).This is illustrated in Figure 2 below in part A. The schematic shows that some parts of the film that are below the top layer which is indicated as t feature past the binodal curve (Mulder, 2000). This is indicative that the liquid-liquid demixing is instantaneous. This implies that the demixing begins as soon as the cast film is immersed into the coagulation bath (Mulder, 2000). This is what happens when the ultrafiltration and microfiltration membranes are being formed. The resultant membranes are porous (Mulder, 2000).
Figure 2 showing the schematic presentation of what happens to the cast film soon after it is immersed in the coagulation bath
Source: (Mulder, 2000).
The part B of the schematic presentation shows that the contents that are below the top layer are miscible. The implication is that demixing has not occurred yet. It can be concluded that the liquid-liquid demixing takes time to occur after the cast film has been immersed into the coagulation bath (Mulder, 2000). The contents that are below the top layer take time to go past the binodal curve. In this part of the schematic, the liquid-liquid demixing will occur after some units of time depending on the dynamics of the other contents in the coagulation bath and the cast film (Mulder, 2000).
The morphological characteristics of the membranes resulting from the schematic in part B are different from the membranes resulting from the processes in the schematic in part A (Mulder, 2000). The membranes in this instance have a dense top layer. This makes them nonporous. Unlike the other membranes, these membranes can only be used for per-vaporation and gas separation (Mulder, 2000).
Comparison of Membrane Performance
The versatility of the polyvinylidene fluoride polymer makes it appropriate and useful in many engineering applications. The surface treatment processes discussed above are aimed at enhancing its hydrophilic properties so that it can be used in aqueous applications. There is a need to compare the performance of the treated polymers and the untreated polymers to determine whether there is an improvement in performance. The comparison will use empirical data from two studies where the polyvinylidene fluoride was combined with different solvent to form the polymer solution.
In the study by Razzaghi et al., (2014), the researchers used the polyvinylidene fluoride as the polymer and cellulose acetate as the solvent, the N, N-dimethylacetamide and water as the solvent and coagulant respective. The researchers created polymer membranes using the phase inversion method and the immersion precipitation as the technique. The aim of the study was to investigate the morphology characteristics, the anti-fouling properties, and the blend ration during the permeation, and the pore size of the resultant membranes (Razzaghi et al., 2014). Different methods were used in measuring the variables referred to above.
The researchers found that the resultant membrane had enhancer hydrophilicity properties when compared to the untreated membranes. This implies that the surface treatment of the membranes results in reduced hydrophobicity (Razzaghi et al., 2014). When the contact angle was analyzed further, the researchers found that when the blend ratio of the membrane made of cellulose acetate and polyvinylidene fluoride exceeded 20%, the reduction of the contact angle stopped (Razzaghi et al., 2014).
The researchers also noted a similar occurrence when they analyzed the permeation flux. The observed trend is due to the changes in the morphological characteristics and porosity of the membrane (Razzaghi et al., 2014). The researchers further tested the performance the membranes in aqueous application. In this respect, the membranes were used for sewage treatment. The researchers then compared the size of the pores and the composition of the surface of the membrane after the sewage treatment and the baseline data that was collected before (Razzaghi et al., 2014). The results indicated that a blend ratio of 30 parts of cellulose acetate to 70 parts of polyvinylidene fluoride resulted in an increase in the size of the pores, and by that, the porosity of the membrane. However, the least total fouling ration and the biggest pure water flux was achieved at a blend of 20 parts of parts of cellulose acetate to 80 parts of polyvinylidene fluoride (Razzaghi et al., 2014).
The study by Zhao, Xu, Chen & Yang (2013) offers further empirical basis for the comparison of the performance of neat polymers and treated polymer membranes. In their study, Zhao et al., (2013) used graphene oxide as the solvent in which the polyvinylidene fluoride was dissolved to form a polymer solution. The immersion precipitation technique of the phase inversion process method was used. The results revealed that the blending of the graphene oxide introduced the alcohol group that resulted in the improvement of the surface hydrophilicity of the polyvinylidene fluoride membrane (Zhao et al., 2013).
The results also showed that there was improved water flux after the blending of polyvinylidene fluoride with graphene oxide. More precisely, 2 wt% of graphene oxide resulted in a pure water flux of 26.49 L/m2h and a permeation flux peak value of 14.21 L/m2h (Zhao et al., 2013). These peak values represent an improvement of 79% of the peak value of the pure water flux and 99% of the peak value for the permeation flux in neat polymers (Zhao et al., 2013). These empirical findings show that there is an improved performance of the surface treated polymers when compared to the neat polymers.
Recommendations
The superior properties of polyvinylidene fluoride as a polymer make it a valuable property in engineering and material science. Owing to these properties, the polyvinylidene fluoride has applications in water treatment, membrane reactors, pollutants, and gas separation (Chen et al., 2016). However, its surface hydrophobicity makes the polymer vulnerable to pollutants, low permeability and high maintenance costs whenever it is used (Chen et al., 2016). The paper has shown that these setbacks can be prevented through surface treatment. The resultant membranes have a better performance compared to the neat polymers. However, the permeability of the ultrafiltration membranes is not satisfactory (Chen et al., 2016). The mechanical properties of the polymer are also weakened. Improvements are required in this area to improve the hydrophilicity of the polymer without compromising its mechanical properties.
References
Chen, G., Xu, S., Zhu, W., Wu, Q., Sun, W., and Xu, Z. (2016). Improved surface hydrophilicity of polyvinylidene fluoride ultrafiltration membranes. Retrieved from http://www.4spepro.org/view.php?article=006561-2016-06-17#B11
Jang, H., Song, D., Kim, I. and Kwom, Y. (2014). Fouling control through the hydrophilic surface modification of poly (vinylidene fluoride) membranes. Journal of Applied Polymer Science. 132(21): DOI: 10.1002/app.41712
Mulder, M. (2000). Phase inversion membranes. Enschede. Academic Press.
Razzaghi, M., Safekordi, A., Tavakolmoghadam, M., Rekabdar, F. and Hemmati, M. (2014). Morphological and separation performance study of PVDF/CA blend membranes. Journal of Membrane Science. 470(15): 547-557.
Zhao, C., Xu, X., Chen, J. and Yang, F. (2013). Effect of graphene oxide concentration on the morphologies and antifouling properties of PVDF ultrafiltration membranes. Journal of Environmental Chemical Engineering. 1(3): 349-354.
