Effects of adding different ethanol amines during membrane preparation on the performance and morphology of nanoporous PES membranes
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In this study, different ethanol amines (EAs) were added at various concentrations during the production of membranes. The effects of adding these EAs on the performance and morphology of membranes 200 μm and 280 μm thick were investigated. The membranes were obtained via a phase-inversion procedure using polyethersulfone as the base polymer, DMAc as the solvent, and polyvinylpyrrolidone as a pore former. The flux behavior and rejection abilities of these membranes were studied using a crossflow setup. The effects of adding the different ethanol amines during the preparation of membranes on the flux and rejection of these membranes varied significantly. The results showed that membrane performance in the presence of these additives is strongly related to the thickness of the casting film as well as the type of ethanol amine added. Cross-sectional SEM images indicated that these additives have striking effects on the membrane morphology. Diethanol amine is able to increase the fraction of Na2SO4 rejected by the membrane from 70 to near 90 %. The data obtained in this work illustrate that among all of the EAs tested, diethanol amine exerts the greatest effect on membrane performance.
KeywordsMembrane preparation Phase inversion Ethanol amines Morphological study
Polymeric membranes are used for a variety of industrial applications, such as microfiltration, ultrafiltration, nanofiltration, reverse osmosis, and gas separation . The high-Tg polymer polyethersulfone (PES), which exhibits high mechanical, thermal, and chemical resistance, is widely used in the manufacture of asymmetric membranes [2, 3]. PES shows favorable temperature resistance, tolerates a wide range of pH values, and exhibits good resistance to chlorine and other chemicals .
There are several ways to fabricate polymeric membranes, such as track etching, sintering, stretching, and phase separation. The morphology of the resulting membrane strongly depends on the properties of the materials used to fabricate it and the process conditions employed. One well-known procedure for fabricating membranes is immersion precipitation [5, 6, 7]. In this process, an asymmetric structure comprising a dense top layer and a porous sublayer is created. Adding tiny amounts of organic/inorganic additives to the dope solution produces significant effects on membrane morphology and performance [8, 9, 10]. Polyethyleneglycol (PEG) and polyvinylpyrrolidone (PVP) are widely used in casting solutions to change the morphology and performance of the membrane produced [11, 12, 13]. Adding these additives leads to low membrane fouling, high flux, selectivity, and other advantages [14, 15, 16, 17].
Materials and apparatus
Polyethersulfone (PES Ultrason E6020P with MW = 50,000 g/mol) was supplied by BASF (Ludwigshafen, Germany). Polyvinylpyrrolidone (PVP, 40,000 g/mol), ethanol amine, diethanol amine, triethanol amine, and acrylic acid were from Merck (Darmstadt, Germany). Dimethylacetamide (DMAc) was purchased from Scharlau (Sentmenat, Spain). Na2SO4 was used to investigate the ion rejection capabilities of the membranes. Distilled water was used throughout the study.
Membrane composition and preparation procedure
Casting solutions were prepared by mixing 18 wt% PES in dimethylacetamide (DMAc) with one of three ethanol amines (either ethanol amine, diethanol amine, or triethanol amine) at one of three different concentrations (either 3, 5, or 10 wt%), as well as 3 wt% polyvinylpyrrolidone (PVP) used as the pore former. Stirring was performed at 200 rpm for 5 h at 40 °C. After a homogeneous solution had been generated, the dope solution was held at ambient temperature for around 24 h to remove air bubbles. Afterwards, the dope solution was cast onto a glass support at a thickness of either 200 μm or 280 μm using a film applicator at room temperature without evaporation. After coating, the casting film was immersed into a distilled water bath for at least 20 h to remove most of the solvent and water-soluble additives.
The cross-section of each membrane was examined using a scanning electron microscope (SEM). The samples of the membranes were frozen in liquid nitrogen and fractured. After sputtering with gold, they were viewed with a Philips (Eindhoven, The Netherlands) SEM at 25 kV.
Membrane performance evaluation
Results and discussion
Membranes with a thickness of 200 μm
Immersing the cast film into a distilled water bath initiates precipitation. Solvent/nonsolvent exchange takes place and nuclei of the polymer-poor phase form. These nuclei continue to grow until the polymer concentration at the pore/solution interface becomes high enough for solidification to occur. Under instantaneous demixing conditions, the composition of the nuclei remains stable for a long period. Generally, macrovoids are formed where instantaneous demixing takes place . The conversion of big macrovoids to finger-like pores reduces flux. Increasing the amount of ethanol amine probably changes the diffusion rates of the solvent and nonsolvent, leading to delayed demixing. The occurrence of delayed or instantaneous demixing is influenced by thermodynamic stability and kinetic processes. When an additive is added to the casting solution, its thermodynamic stability is changed. On the other hand, the addition of additives to the casting solution can affect the solvent/nonsolvent exchange rate, resulting in kinetic effects [13, 14]. According to the literature, additives with different structures exert different effects on the properties of the membrane. These effects are due to the attraction of the additive to the polymer chains and its affinity for the solvent or nonsolvent. These effects are important for EAs with –OH or –NH functional groups. For instance, the presence of these hydrophilic functional groups in the casting solution increases the affinity and the exchange rate of nonsolvent for diffusion through the membrane bulk.
Based on the discussion above, when 10 wt% diethanol amine is added, the increase in additive concentration causes a small increase in the solvent/nonsolvent exchange rate in the resulting membrane, which results in a slight decrease in flux.
Membranes with a thickness of 280 μm
Among the three ethanol amines added in this work, adding diethanol amine led to the best-performing membranes with suitable rejection parameters for Na2SO4. This is probably due to the properties of diethanol amine, specifically (i) its basicity and (ii) the number of –OH groups present. The basicity and the number of –OH groups decrease for the ethanol amines in the order: triethanol amine > diethanol amine > ethanol amine. Maybe increasing the basicity affects the interactions between polymer chains and reduces interchain attraction. A more open structure may then be formed. On the other hand, increasing the number of –OH groups can affect the miscibility of the solvent and nonsolvent, leading to delayed demixing. However, diethanol amine is a relatively weak base with two –OH functional groups, so it has only a moderate effect during membrane formation, causing a macrovoid-free membrane when the casting solution film is relatively thick.
This study clearly shows that the effects of additives on membrane performance and morphology depend strongly on the casting solution thickness as well as the type of additive used. Different ethanol amines (ethanol amine, diethanol amine, and triethanol amine) used as additives were found to have different effects on the membrane morphology, and their effects also varied for casting solution films with different thicknesses. For thicker membranes, the kinetic process is dominant. On the other hand, thermodynamic instability overcomes the kinetic process when the casting solution film is relatively thin. Membrane rejection was optimal for a 280 μm thick membrane with diethanol amine as an additive.
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