Robust surface modified polyetherimide hollow fiber membrane for long-term desalination by membrane distillation
Graphical abstract
Introduction
One of the drawbacks of membrane distillation (MD), a thermally driven membrane separation process, is the low liquid entry pressure (LEP) of the porous membrane, resulting in high risk of pore wetting and reduction of separation efficiency, by which its operating period is limited [[1], [2], [3], [4]]. Among the various attempts to prepare suitable MD membranes, modification of the membrane surface has been found effective to render the membrane not only hydrophobic but also superhydrophobic [5]. However, based on the Laplace equation, the most important membrane parameter affecting the LEP is the maximum pore size that must be reduced to near the mean pore size by narrowing the pore size distribution [1].
For hollow fiber membranes, surface functionalization has been carried out by employing different techniques such as surface coating, grafting, plasma treatment, physical or chemical vapor deposition. Most of these procedures involve one or more post-treatment steps and the use of hazardous restricted chemicals. In addition, post-processing involves some undesirable effects such as pore blockage, high risks to damage the hollow fiber structure, which may further result in uncontrollable amount and thickness of functional molecules on either the internal or external hollow fiber surfaces. A simple approach for surface functionalization of hollow fibers is through spontaneous surface segregation of fluorinated active additives with low surface free energy, which enables those to migrate to either the internal, external or both surfaces of the hollow fiber during the dry/wet spinning and change the hollow fiber's physico-chemical characteristics [[6], [7], [8], [9], [10], [11], [12], [13]], It is generally accepted that the fluorinated end groups tend to migrate towards the air/polymer interface because of their higher hydrophobicity than the host polymer, which is generally hydrophilic [[6], [7], [8], [9], [10], [11], [12], [13]]. Thus, the need for an extra step along the spinning line can be avoided.
For flat-sheet membranes, it was observed that one of the important parameters affecting surface migration of fluorinated active additives to the top membrane surface is the solvent evaporation period between polymer solution casting and coagulation step [[6], [7], [8], [9]]. This period can be controlled as needed when the flat sheet membrane is fabricated by the phase-inversion method. However, for hollow fibers the evaporation period depends on the air gap distance and this is not long enough to allow the surface migration. In fact, some studies have been carried out investigating the air gap effect on the morphological and chemical characteristics of hollow fibers using different hydrophilic host polymers, polyetherimide (PEI) and polyethersulfone (PES), with fluorinated surface modifying macromolecules [10,11]. These studies revealed that the outer surface of the hollow fibers became more hydrophobic as the air gap distance was increased while no significant change occurred in the internal structure of the hollow fibers [10,11]. It is worth quoting that compared to the studies performed for surface modification of hollow fibers, most researches have been carried out on surface modification of flat sheet membranes to render them more hydrophobic using fluorinated active additives [[6], [7], [8], [9], [10], [11], [12], [13]].
The objective of this study is to prepare a robust hydrophobic hollow fiber membrane with high LEP in a single casting step by surface segregation. Specifically, the effects of changing the hydrophilic polymer concentration in the spinning solution on the characteristics and MD performance of the fabricated hollow fiber membranes were investigated for desalination and a long-term test was performed.
Section snippets
Materials
Polyetherimide (PEI, Ultem® 1010-1000, General Electric Company, GE Plastics Canada Ltd.) was used as a host hydrophilic polymer to prepare the surface modified hollow fiber. PEI was first heated at 90 °C during 24 h in a vacuum desiccator placed in a heating mantle (Selecta) and connected to a vacuum pump (Vacuubrand brand, model MZ2C). The polymer blends were prepared by dissolving the PEI in a mixture of the solvent N-methyl-2-pyrolidone (NMP) and the additive hydroxybutyric acid γ-lactone
Morphological structure, elemental composition and hydrophobic character of the hollow fibers
Fig. 2 shows the cross-section of the prepared hollow fibers. It can be seen that all exhibit almost the same structure consisting of two separate layers of finger-like structure (FLS) with high void volume fraction and denser thin skin layers at the inner and outer surfaces. The inner FLS layer is 1.51, 1.24, 2.03 and 1.45 times thicker than the outer FLS layer for the PEI-14, FPA/PEI-14, FPA/PEI-15 and FPA/PEI-17 hollow fibers, respectively. The FPA/PEI-17 prepared with the highest PEI
Conclusions
Robust surface modified PEI hollow fibers were prepared by surface segregation of a fluorinated polyurethane additive (FPA). Three different hollow fibers were prepared by changing the PEI concentration from 14 to 17 wt% while keeping the FPA concentration at 2 wt% and their properties and DCMD performances were compared with the pristine PEI hollow fiber of 14 wt% PEI concentration.
It was noticed that adding the FPA improved significantly both the mechanical strength and the LEP of the hollow
Acknowledgments
The authors gratefully acknowledge the financial support of the Spanish Ministry of Economy and Competitiveness (MEC) (CTM2015-65348-C2-2-R).
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