Fig. the thick and dense upper layer of

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Last updated: September 14, 2019

Fig. 2 demonstrates the cross-section SEMimages taken from the prepared membranes in two magnification scales.    Figure 2 SEM images of the prepared membranes containing 21 wt. % PES in two magnification scales As the figure suggests, the top layer structureshows completely different membrane structures depending on the amount of thesurfactant present in the solution. Changes in the contents of themarpol surfactant caused morphological changes in the membrane structures sothat by increasing the amount of surfactant from 0 to 8 wt.%, the thick and denseupper layer of the membrane was converted into a low-dense thin layer.

 However, the further increase ofsurfactant from 8 wt.% to 10 wt.% had inverse effects and thickened the surfacelayer.

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The mechanism of membrane formation has already been describedin previous studies, such as Saljoughi and Mousavi 26 and Saljoughi et al. 27-29.  In general, the addition of hydrophilicadditives, such as merpol, to the casting solution has two main effects onmembrane morphology, which are discussed below in brief.  The hydrophilicnonsolvent properties of merpol additive cause an increase in the thermodynamicinstability of the casting solution. Once the cast films are immersed into the coagulationbath, the film surface will be coated by a layer of amphiphilic merpol molecules.

The presence of these molecules will reduce surface tension and, consequently,water molecules will be diffused, easily 24, 26. Low connectivitybetween PES and water molecules causes formation of nuclei of apolymer-poor phase on the surface, which ultimately causes the repulsion of PESchains at presence of surfactant molecules. These nucleiare superficial porous builders. As long as the concentration of the polymer in theboundaries rises significantly and until the moment of solidification, the process of solvent and nonsolvent diffusional exchange willcontinue. This suggests thatthe addition of marpol leads to the formation of more nuclei, resulting in moreporosity in the surface.

The instantaneous demixing in the coagulation bathand the formation of more porous membrane structure is a clear consequence ofthe above effects. Another important effect of marpol is the increasein viscosity of the cast film. This increase in viscosity, during the solidification process, reduces solvent (NMP) and nonsolvent (water) diffusionalexchange rate and prevents instantaneous demixing.

The delayed demixing process suppresses macrovides and creates denser structures.What ultimately determines the characteristics of the membrane’s finalstructure depends on the priority of instantaneous or delayed demixing proccesesm which in turn as discussed earlierdepends on the concentration of merpol surfactant in the cast solution. Two key factors involved in the formation of thickand low porosity membrane structures, which include increased viscosity and priorityof delayed demixing over instantaneous. This condition, in the present study, was observed when the surfactant contentwas increased from 8 wt.

% to 10 wt.%.  

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