For fabricating a polyamide (PA) selective film by interfacial polymerization (IP), we selected a linear structure of succinyl chloride, an acyl chloride monomer, and of 1,3-diaminopropane, an amine monomer. It was not by conventional IP; instead, the technique of spin-coating was successfully integrated into the process of IP. We called such integrated systems as dynamic IP. Spin-coating provided a strong centrifugal force which induced arrangement and orientation of molecular chains in the horizontal direction. Hence, a dense and thin selective film of PA formed. In characterizing the different layers of the PA composite membrane in terms of free volume elements, we exploited the advantages of probing it with positron annihilation spectroscopy (PAS). PAS data revealed a shorter o-Ps lifetime (τ3 = 1.71 ns) and a lower free volume intensity (I 3 = 7.9%) for the case of the PA film fabricated through the dynamic IP process, in contrast to a longer τ3 of 1.86 ns and a higher I 3 of 9.1% for the film produced from applying the static IP process. Transmission electron miscroscopy (TEM) results complemented the free volume data obtained. TEM described cavities with sizes outside the sensitivity of the PAS instrument. TEM micrographs showed that the cavities present in the PA film formed by applying the IP process without the integration of spin-coating ranged in size between 40 and 60 nm, whereas those in the film obtained from the IP process integrated with spin-coating ranged 15–30 nm. Separation performance data indicated a simultaneous enhancement of flux and selectivity for the composite membrane fabricated through the combined techniques of IP and spin-coating. This outcome is different from the common behavior known as the trade-off phenomenon.

Abstract:The present work investigated the permeation of indium ions through a polymer inclusion membrane (PIM), prepared with cellulose triacetate (CTA) as the base polymer, tris(2-butoxyethyl) phosphate (TBEP) as the plasticizer and di-(2-ethylhexyl)phosphoric acid (D2EHPA) as the extractant. With 5 M HCl aqueous solution as the strip solution, we observed an initial indium permeability of 2.4 × 10 −4 m/min. However, the permeability decreases with time, dropping to about 3.4 × 10 −5 m/min after 200 min of operation. Evidence was obtained showing that hydrolysis of CTA occurred, causing a dramatic decrease in the feed pH (protons transported from strip to feed solutions) and a loss of extractant and plasticizer from the membrane, and then leading to the loss of indium permeability. To alleviate the problem of hydrolysis, we proposed an operation scheme called polymer inclusion membranes with strip dispersion: dispersing the strip solution in extractant-containing oil and then bringing the dispersion to contact with the polymer membrane. Since the strong acid was dispersed in oil, the membrane did not directly contact the strong acid at all times, and membrane hydrolysis was thus alleviated and the loss of indium permeability was effectively prevented. With the proposed scheme, a stable indium permeability of 2.5 × 10 −4 m/min was obtained during the whole time period of the permeation experiment.