Comparison between Free Volume Characteristics of Composite Membranes Fabricated through Static and Dynamic Interfacial Polymerization Processes

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. I 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 (tau(3) = 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 tau(3) 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.