# Easy Fabrication of Ultrafiltration Membrane via Polyethersulfone-Fumed Silica
## 3 Results and Discussion  
### Membrane Morphology  
Five membrane samples were prepared using varying fumed silica concentrations. Within this set of membranes, one membrane served as a reference without the addition of FS, denoted as FS-0.0. The other blend membranes were referred to as FS-0.1, FS-0.2, FS-0.3, and FS-0.4, corresponding to the FS weight concentration in the PES dope solution. All samples were cast with a wet thickness of 150 microns.  
Figure 2 presents the SEM images of the cross-sections and top surfaces of the manufactured PES/FS membranes. The membranes have three distinct, asymmetric porous layers which comprise a dense spongy structure on the top surface, a finger-like porous sublayer in the middle, and a sparse spongy structure as the bottom layer. The three distinct layers can be explained as follows. The membrane was fabricated using a wet-phase inversion technique, where the dope solution changes phase from liquid to solid when immersed in pure water. During immersion, due to the exchange between NMP as solvent and pure water as non-solvent, precipitation occurs because the casted solution becomes thermodynamically unstable. In the surface, the casted solution is in direct contact with water and instantly vitrifies while allowing water to enter, generating microvoids. [PERSON] et al. explored the effect of coagulation bath composition on Psf membrane performance and stated that when only pure water was used during the immersion process, substantial residual solvent was entrapped in the middle part because of solvent-non-solvent incompatibility [29]. The hydrophilic nature of silica arises from the abundant silanol groups on its surface. [PERSON] et al. [30] found that incorporating hydrophilic additives into the membrane dope solution enhances the exchange rate of solvent and non-solvent during the coagulation process. This, in turn, expands the size of the macrovoids at greater depth. When water infiltrates from the bottom, the coagulation rate differs from the top surface due to the presence of a glass support. As a result, this process leads to spongy structures similar to those on the top surface, but with larger pores. The results align with previous research findings reported elsewhere [31; 32; 33], including a ternary-phase field model simulation for a water/NMP/Psf system [21], which successfully predicts detailed asymmetric membrane structures comprised of a thick skin layer and a permeable sublayer.  
As the funnel silica is introduced into the PES membrane, it induces more dark spots which are visible in the top surface micrograph. As the concentration of fumed silica increases, the quantity of these dark spots also increases. These dark spots correspond to voids within the membrane, which are indicative of open pores. The cross-section micrographs further confirmed that the addition of fumed silica leads to more open pores and the increased connectivity of the polymer wall. Additionally, membranes containing fumed silica exhibit a more organized structure compared to pure PES membrane, with rounder pore shapes. This may be because as the fumed silica content increased, the casting solution became thermodynamically less stable, resulting in an enhanced precipitation rate and the development of a more permeable membrane. The higher concentration of fumed silica disrupts the conglomeration of polymer molecules in the top layer during the coagulation process, resulting in a membrane with a more permeable top layer and potentially a more permeable sublayer [17].