By considering the importance of viscoelasticity and the rheological behavior of polymeric solutions in the membrane fabrication process, the control of membrane morphology is considered as an important challenge. Therefore, in this research, it can be investigated the effect of different parameters such as polymer type, solvent type (strong or weak), co-solvent and non-solvent additives on the possibility of formation hydrogen interactions between different components of the solution (physical gelation) to provide a membrane with a desired microstructure, performance and physical properties. NMP as a good solvent for both PSf and PVDF resulted in an increase in the critical polymer concentration and a decrease in the dynamic rheological behavior of polymer solutions such as storage and loss modules, complex viscosity, and zero-shear viscosity. The hydrogen bond is formed between the solvent and polymer into the PSf/2P, PVDF/NMP, and PVDF/DMSO solutions, resulting in the appearance of a network structure of interconnected pores. By adding of 2P at different ratios in the PSf/NMP solution, the mean pore radius increased from 0.6 nm in the M5 membrane (pure NMP) to 41.4 nm in the M2 membrane (pure 2P), but such a similar trend was not observed in pore size of PVDF membrane due to the addition of DMSO into the PVDF/NMP solution. The addition of water to the PSf/NMP system caused to the elimination of finger-like macrovoids due to the occurrence of a delayed liquid-liquid demixing and reduction of membrane pore size from 0.60 nm in M5 membrane to 39.3 nm in M6 membrane. In the PVDF system, the amount of macrovoids decreased and their size increased by the addition of water to the solution and also, the pore size is changed from 19.21 nm in the M11 membrane to 16.29 nm in the M12 membrane. The results of atomic force microscopy (AFM) showed that the type of polymer, as well as the type solvent used, could change the pore size, permeability and separation performances of the membrane, but this change in the PSf membranes was more significant than PVDF membranes. The presence of air gap region and elongation force due to the gravity in the production of PSf and PVDF hollow fiber membranes resulted in the orientation of their outer skin, and consequently, reduction of the pore size and permeability. By applying the air gap region in the production of the H7 membrane (PVDF/DMSO), small crystalline particles are formed in the middle layer of the membrane structure which indicates the appearance of crystalline induced gelation. The results of the ATR-FTIR spectrum of PVDF flat membranes showed that by adding water as a non-solvent additive or DMSO as a co-solvent additive, the peak intensity of a ? crystalline phase disappears. On the other hand, the addition of DMSO into the PVDF/NMP solution, resulted in ? and ? crystalline phases appear to be stronger in the PVDF membranes structure and the addition of water has little effect on the structure of the two crystalline phases. PSf and PVDF flat membranes made from solutions with a lower critical concentration than the other solutions exhibit lower tensile strength and lower Young's modulus. The application of the air gap region in the production of PSf hollow fiber membranes resulted in the arrangement of the outer skin of the membrane, which resulted in increased tensile strength and Young's modulus. While, in the PVDF hollow fiber membranes, the result was completely different and the air distance reduced the physical properties of the membranes, which could be attributed to the formation of crystals in the membrane structure due to the gravity induced elongation force in the air gap.