Influence of Spinning Modes on the Structure and Properties of Polyethersulfone Hollow-Fiber Ultrafiltration Membrane

The effect of the main parameters of the spinning process of polyethersulfone (PES) hollow fiber (HF) membranes using the free-fall spinning technique has been studied. The influence of the flow rate of the polymer dope solution, the internal coagulant (bore fluid), the air gap distance, and the bore fluid temperature on the geometric dimensions, structure, and transport properties of the membranes has been analyzed. It has been shown that the variation of these spinning parameters makes it possible to control the transport properties of the obtained membranes in a wide range: the pure water flux (PWF) within 120–950 L/m² h and the rejection coefficient with respect to PVP K-30 in the range of 20–93%. It has been established that the PWF of PES hollow fiber membranes increases linearly with an increase in the fiber extrusion ratio (ER) regardless of the spinning parameter that is varied. As shown by SEM and AFM, an increase in ER leads to an increase in the proportion of interconnected pores on the selective surface of PES HF membranes and the transformation of the pore shape from round to slitlike. A comparative analysis of the structure and properties of hollow fiber and flat sheet membranes obtained from the same composition of the dope solution has shown that the flat sheet membranes are characterized by an anisotropic spongy structure and the hollow fiber membranes have the support layer filled with finger-shaped vacuoles (macrovoids). It is concluded that the formation of macrovoids, as well as an order of magnitude higher water flux of the PES hollow fiber membranes, is due to a violation of the integrity (microfractures) of the selective layer being formed as a result of fiber drawing in the air gap.