Investigation of Dope Characteristics and Take-Up Speed on the Spinning of Asymmetric Hollow Fiber Membranes

Asymmetric hollow fiber (HF) membranes have become ubiquitous in their applications in ultra- and microfiltration, gas separation, and reverse osmosis. HFs illustrate many favorable characteristics, namely the larger surface and separation area due to the geometry of the HFs, relatively high mechanical stability, and are uncomplicated to handle and use [1]. The HF membranes observed in this work were produced through the nonsolvent-induced phase separation (NIPS) process. The NIPS process comprises the interactions between multiple (usually three) components, in which a homogenous system of a chosen polymer (P), solvent (S), and any desired additives (dope solution) undergo phase separation with a specific geometry (dictated by the type of spinneret utilized) in a nonsolvent (NS; most typically water) [2]. Due to the immiscibility of the polymer in the water, diffusion is initiated, leading to phase separation. This, in turn, leads to membrane formation [2]. While many parameters play an essential role in the fabrication of HF membranes, only the dope characteristics and their consequences on forces the fiber experiences during spinning are investigated in this work. These properties have an enormous impact on fiber morphology and porosity, and these, in turn, affect fiber performance. While performance with the appropriate testing equipment (gas separation unit/ultrafiltration unit) was investigated, it is not included in the scope of this work. The dope composition and temperature affect the dope’s viscosity, influencing the kinetics of phase inversion and the S-NS exchange. Additionally, the shear and elongational stresses the fiber experiences due to the take-up speed and the gravitational force in the airgap play a significant role in the fiber morphology and porosity. Unfortunately, it is difficult to determine the impact of each phenomenon unambiguously. Thus, the motivation of this work is two-fold: (1) to highlight the significance of the dope characteristics in the precipitation process and final fiber morphology and (2) to investigate the effects of the different forces mentioned above on the end-product independently.