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

Highlights

• PMDA-ODA HFs with regular cross-section morphology are prepared.

• Effects of dope chemistry and spinning parameters on HF morphologies are investigated.

• Increasing the phase inversion rate and residence time of HF in coagulant is crucial for spinning PMDA-ODA PAA.

• Mechanism of the formation of irregular cross-sectional morphologies of HFs are proposed.

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.

Introduction

Aromatic polyimides (PI) are mechanically strong and chemically stable materials because of the rigid polymer backbone structure and the intense intermolecular attracting forces [1], [2]. However, the high chain rigidity and strong inter-chain interactions often lead to the poor solubility and low processability of polyimides [3]. To address this issue, polyimide structures are often tuned by increasing the chain flexibility and/or interrupting the chain packing to make them organosoluble [4]. Moreover, most polyimide-based membranes are fabricated from these soluble polyimides. These membranes may suffer from the problems of plasticization, swelling, or dissolution when they are applied in harsh working conditions such as organic solvent nanofiltration, nature gas sweetening, etc. To increase the stabilities, doping nanoparticles (MOFs [5], TiO₂ [6] and POSS [7]) or post treatments (crosslinking, annealing, and protective coating [1]) are required.

Ideally, membranes with stable separation performance can be obtained by preparing from polymers with high resistance to different organic solvents and chemicals. Polyimides synthesized from rigid dianhydrides such as PMDA, BTDA, etc. are typically insoluble in most organic solvents and have the potential for preparing solvent resistant or anti-plasticization membranes [2]. Since insoluble polyimides are difficult to process, membranes fabricated from them are barely reported compared with soluble PI based membranes. In 1975, Schumann and Strathmann were the first to patent a process to prepare ultrafiltration and reverse osmosis flat-sheet membranes based on insoluble PIs [8]. Their soluble prepolymers, polyamic acids (PAA), were used to make asymmetric membranes, which were then converted to polyimide membranes by either thermally or chemically induced imidization methods. Since then, fabrications and separation properties of insoluble PI based asymmetric membranes have been reported in some patents and papers [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. In six of the literatures [8], [9], [15], [16], [17], [19], PAA membranes are in flat-sheet configurations, while the rest of them [10], [11], [12], [13], [14], [18], [20] are in hollow fiber or capillary forms. Note that, due to the high hydrophilicity of PAA polymers, the phase inversion processes of PAA dopes are very slow in the water coagulant [17]. Therefore, it is relatively easy to cast the PAA dopes or coat the PAA dilute solutions on some substrates to form PAA asymmetric membranes. To our best knowledge, the methods of fabricating self-supporting PAA hollow fiber membranes are only reported in two patents [12], [13] and one paper [20], where only the optimized spinning parameters are presented.

In this study, we intended to systematically study the effects of the spinning conditions including the dope composition, bore fluid composition, flow rates of dope and bore fluids, air gap distances, coagulant temperatures, take up speed, etc. on the PAA hollow fiber morphologies. The target was to give some guidelines as to how to spin integrated PAA hollow fibers with regularly round inner and outer surfaces. The poly(4,4′-oxydiphenylenepyromellitimide) (PMDA-ODA) was select as the membrane material since PMDA-ODA PI was one of the most studied insoluble polyimides and the PMDA-ODA PAA solution was commercially available with stable properties such as the uniform PAA molecular weight of different batches which was important for preparing polymer dopes with reproducible viscosities.