Characterization of ultrafiltration membrane prepared from poly ethersulfone by using electron spin resonance technique
Introduction
Since the announcement of the asymmetric cellulose acetate (CA) membrane for seawater desalination in 1960, a number of polymeric materials have been investigated for their feasibility to prepare reverse osmosis (RO) membranes. Even though many polymers exhibited excellent RO performance, there are only two types of polymers that are currently being used for commercial RO membranes; one is CA and the other is aromatic polyamide. There may be a number of reasons for these polymers to be chosen, e.g. thermal, chemical and solvent stability, price, etc. but the most important one is that membranes prepared from these polymers can separate solute from solvent. High water permeability would be another criterion for the choice, but permeability can increase as the effective thickness of the membrane decreases, and it is not as important as the selectivity. For some polymers, it is possible to prepare membranes that exhibit high separations for sodium chloride and organic molecules of low molecular weights while for others it is not possible. Methods to screen polymers without doing RO experiments, which are preceded by a tedious process of membrane preparation, have long been sought for. In one of such attempts, it was found that the polymers, from which RO membranes of high salt selectivities could be prepared, were isolated in a limited range when two dimensional plots were made for hydrogen bonding and dispersion force components of solubility parameters of polymers [1].
It has been known for a long time that excellent ultrafiltration membranes can be prepared from polyethersulfone (PES) by the phase inversion technique [1], [2]. Several attempts have been made to reduce the molecular weight cut-off of PES membranes as much as possible but no membranes with a molecular weight cut-off below 6000 have been achieved. To prepare PES membranes with pore sizes as small as those of RO membranes, one has to sulfonate or carboxylate to provide the polymer with electric charges and to make the polymer more hydrophilic [1]. On the solubility parameter plot, PES is out of the range where polymers for RO membranes are located.
It was reported that the electron spin resonance (ESR) technique could be used to study the structure and transport of asymmetric CA membranes for RO [3]. It was found that ESR spectra of the spin probe, when it was brought into the CA membrane in an RO experiment from the feed solution, consisted of two signals overlapping on each other. One stems from the radicals located in membrane pores and their movement in the pore is weakly restricted, and the other from the radicals that are interacting with the polymer molecule and their movement is highly restricted. The second type of radicals diffused into the intersegmental space in water-swollen polymer matrix. The radical probes could be leached out completely by immersing the membrane in distilled water for 24 h. Both pores and intersegmental spaces in the water-swollen polymer matrix seem to contribute to water flow under RO conditions. On the other hand, only the flow channels created from spaces in the water-swollen polymer matrix seem to contribute to the salt rejection. It was concluded together with RO separation experiments of sodium chloride that more pores were closed at higher shrinkage temperatures, so that sodium chloride rejection increased as the shrinkage temperature increased. In this paper, a similar ESR technique is applied to elucidate the structure and transport of PES membranes.
An attempt is made in this paper to answer the question why no RO membrane can be prepared from PES polymer, while it is possible from CA.
Section snippets
Membrane preparation
PES (Victrex 4100P) supplied by Imperial Chemical Industries was dried in an oven at 160 °C for 24 h prior to use. A solution of PES (15 wt.%) and polyvinyl pyrrolidone (PVP, 15 wt.%) in N-methyl pyrrolidone was prepared as a stock solution and filtered through a 5 μm filter. In some cases, 2,2,6,6-tetramethylpiperridinyloxy (TEMPO) free radical (0.01 wt.%) was blended in the polymer solution. The polymer solution at temperature 10 °C was cast on a glass plate to a thickness of 0.66 mm and the
Results and discussion
Fig. 1 shows the ESR spectra of the membrane casting solution-containing TEMPO. Similar spectra also observed for TEMPO solution in water. This isotropic spectrum is due to the rapid and randomly tumbling nitroxides [4]. The two outermost peaks of the nitroxide radicals are caused by the parallel transitions of the nuclear spin state, M1=±1 [5]. Three hfs lines are almost equal in height; ratio of height of ‘A’ and ‘C’ with respect to middle line ‘B’ is almost one. As the viscosity of the media
Conclusions
From the above study, the following conclusions can be drawn:
- 1
ESR technique can be used to study the structure and cast some light to the transport of RO and UF membranes.
- 2
No significant affect was observed on the performance of the membrane when the membrane was prepared incorporation with TEMPO.
- 3
Water may flow through the pores of PES membranes. The sizes of pores are those of UF membranes. Unlike CA, the polymer matrix is little swollen or not at all swollen by water, and continuous channels
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