1. A Novel Chemical Surface Treatment To Enhance The Hydrophobicity Of Polyethersulphone Membranes
A. Rastegarpanah, H.R. Mortaheb*
Chemistry and Chemical Engineering Research Center of Iran, Tehran, Iran
Abstract
A novel surface treatment method for fabrication of highly hydrophobic Polyethersulphone (PES) membrane is introduced in the present research.
The method applies tetraethyl orthosilicate (TEOS) and trimethylcholorosilane (TMSCl) in a basic media. TMSCl is selected for grafting of the membrane as a substitute of costly fluoroalkyl silane to modify the surface of PES.
The hydrophobicity of the membrane is crosschecked by contact angle measurements. At the optimum conditions, the contact angle of about 150° is obtained for the resulted membrane.
The ATR-IR analysis also confirms the presence of hydrophobic functional groups on the surface of the membrane.
Finally, the AFM images confirm increasing in roughness of the membrane which enhances the hydrophobicity.
The glass plate with the cast film on it is immediately immersed in the coagulation bath for all the trials. The prepared film is then immersed again in the coagulation bath in 40 °C. The formed layer is then stored in distilled water for 48 h to allow the water-soluble components in the film to be leached out. As the final stage, the film is dried by the filter paper for at least 48 h at room temperature.
Proper amount of 32 wt% NH4OH as the catalyst is added into anhydrous ethanol, and kept stirring 30 min at 60°C, and then TEOS is added. The mix solution is stirred for more 90 min at 60°C. The prepared membrane sheet is dipped in the solution, and then TMSCl is added and mixed for 24 h at 60°C. Finally, the membrane is put in an oven for 6 h, and washed with ethanol.
The ATR-IR spectra of the membrane surface after crosslinking reaction are shown in Fig 1. The figure confirms the presence of Si-Si, Si-O, and Si-CH3 bonds on the surface of the membrane.
Water contact angle on the prepared membrane (Fig. 2) was measured by 5-µL water droplets onto five different spots, where the average value was adopted for each sample. The hydrophilic ether bond in PES chains results in water contact angle of about 75° for this membrane. However, the treated PES membrane provides the water contact angle about 149° as shown in Fig.2b.
The variation of membrane roughness was analyzed by AFM as shown in Fig. 3. The figure shows that treatment causes increase in the roughness from about 60 nm to above 300 nm. Since the hydrophobicity is increased by increasing the roughness, the above result confirms enhancing in hydrophobicity of the membrane.
Results and discussion
The prepared membranes were characterized with three different methods including Attenuated Total Reflection Infrared Spectroscopy (ATR-IR), Atomic Force Microscopy (AFM), and Optical Contact Angle measurement (OCA).
Introduction
In recent years, hydrophobic porous membranes have widely used in new separation processes of membrane such as membrane distillation attention because of their peculiar surface structures and physiochemical properties [1, 2]. A solid is often called super-hydrophobe when the contact angle of water on its surface is larger than 150° [3]. The first investigations in super-hydrophobic surfaces can be traced back to 1950s. Subsequently, in 1997, Wilhelm Barthlott’s research group found a relationship between the lotus leaf hydrophobicity and the self-cleaning based on a series of experiments on the lotus leaf super-hydrophobic phenomenon [4]. Generally, the surface hydrophobicity is attained by both the binary surfaces structures at micro/nano scale and low surface energy. These two factors decrease the contact area between surfaces and water droplets.
Surface modifications such as chemical vapor deposition, ultraviolet, plasma polymerization or modification and membrane preparation methods e.g. stretching, thermally induced phase separation, etching, sol–gel method, and non-solvent induced phase separation have also been explored to further increase membrane hydrophobicity. Grafting fluorinated groups on the solid surface has attracted a great attention in recent years for preparation of super-hydrophobic surfaces [5]. However, these pathways are costly processes and need several pretreatment steps.
PES membranes are applicable in membrane separation process due to their excellent chemical stability, mechanical strength, and ease of fabrication by phase inversion method [6]. However, owning to the presence of ether bonds in the PES chains, these membranes are hydrophilic.
In this work, a hydrophobic PES membrane was prepared by a chemical surface treatment. Trimethylcholorosilane (TMSCl) was selected as the grafting agent to modify the surface of PES. In comparison to other high-hydrophobic PES membranes, this modified membrane is can be prepared economically by a simple procedure.
Figure 1. ATR spectra of treated membrane surface.
Conclusions
In this work, a chemical surface treatment technique was used to create a high-hydrophobic PES membrane. The enhancement in the hydrophobicity was confirmed by characterization analyses of the treated membranes. This method can be used for modification of membranes in the applications, which require hydrophobic membranes.
References
Figure 2. Water droplet on membrane surface
a)Untreated PES membrane (~75˚); b) Treated PES membrane (~150 ˚)
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Experimental
Polyethersulfone (PES) (Ultrason E6020, Mw=58,000) was supplied from BASF, Germany. Polyvinylpyrrolidone (PVP) (K90, Mw=360,000), Anhydrous N, N-dimethylacetamide (DMAc), Anhydrous Ethanol, 32 wt% Ammonia solutions (NH4OH) were obtained from Merck, Germany. Trimethylcholorosilane (TMSCl) was purchased from Fluka, Switzerland. Tetraethyl orthosilicate (TEOS) was provided from Aldrich, USA. All materials were used as received without further treatments.
PES membrane is prepared using phase inversion technique as described in our previous study [7]. Homogeneous polymer solution including 15% w/w PES, 3% w/w PVP, and 20 ml DMAc as the solvent is cast on a smooth glass plate by a casting knife at room temperature.
Figure 3. AFM topographical images of membranes.
a)Untreated PES membrane, b) Treated PES membrane