Fabrication of PMDA-ODA hollow fibers with regular cross-section morphologies and study on the formation mechanism

Abstract

Since PMDA-ODA polyimide is insoluble, its prepolymer (PMDA-ODA polyamic acid) has to be used to prepare asymmetric membranes which are then converted to polyimide using the imidization method. However, the phase inversion process of polyamic acid (PAA) in water is slow, which causes difficulties in the process of spinning PMDA-ODA hollow fiber (HF). In this paper, a variety of spinning parameters, including the PAA concentration, the content of ethanol additive, the bore fluid composition, the dope and bore flow rates, the HF take-up speed, the air gap distance, the temperature and size of coagulation bath have been adjusted to investigate their impacts on the HF cross-section morphologies. Based on our observations, the guidelines of spinning PMDA-ODA HF with regularly round inner and outer surfaces are given. That is, first, increase the phase inversion rate of the PAA dope; second, increase the residence period of the HF in the water coagulant; and third, maintain a relatively low coagulant temperature. Moreover, the viscosities, the grown rates of the precipitation and convection fronts of different PAA dopes, and the ternary phase diagram have been measured. The formation mechanism of the cross-section morphologies of the PAA HFs have been well explained using the abovementioned results.