Fabrication of PMDA-ODA hollow fibers with regular cross-section morphologies and study on the formation mechanism
Graphical abstract
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.
Section snippets
Summary of the formation mechanism of HF irregularity
For HF spinning, an imperfect morphology (e.g. an irregular inner/outer shape, ovalization) will decrease the mechanical strength of HF. Nevertheless, some researchers aimed to control the circumferential instability to obtain the aligned grooved inner surface of HFs, which were preferred for guiding the nerve regeneration [21], [22], [23], [24]. Yin et al. built a model that could well predict the number of grooves [21], [22]. The model combined the Marangoni effect and the buckling mechanism
Materials
The 15 wt% PMDA-ODA/N-methyl pyrrolidone (NMP) solution was purchased from Changzhou Runge chemical industrial Co., Ltd. (China). Acetic anhydride and triethylamine were obtained from Tianjin Fu Chen Chemical Reagents Factory (China). Ethanol (EtOH), acetone, isopropanol and n-hexane were got from Beijing Chemical Works (China). All chemicals were used as received.
Dope preparation
The PAA powder was obtained by first precipitating the PAA solution in acetone for preventing the hydrolysis of PAA in water and then
Kinetic studies of the precipitation processes of the PAA dopes
One difficulty to spin PAA hollow fiber is that the phase inversion is slow so that the nascent PAA HF is soft and easy to deform during the spinning process. Although the mechanical strength of the PAA HF can be improved by spinning a high concentrated PAA dope solution, the increased dope viscosity may in return decrease the solvent/non-solvent exchange rate and cause a slower phase inversion process. Therefore, it is crucial to reduce the PAA dope viscosity while maintain a relatively high
Conclusions
HFs based on insoluble PIs have great potentials for the separations in harsh environments. However, the HFs have to be prepared using their soluble prepolymers, polyamic acid, which typically have slow phase inversion rates and are difficult for HF spinning. In this study, we demonstrate that the PMDA-ODA PAA HFs with regularly round inner and outer surfaces can be prepared by carefully selecting the dope composition and spinning parameters. After the chemical imidization treatments, the
Acknowledgement
The authors would like to thank the National Natural Science Foundation of China (51403012) to fund this research.
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