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

Abstract

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.

Investigating the void structure of the polyamide active layers of thin-film composite membranes

“…Based on TEM images of the active layer of a brackish water reverse osmosis (RO) membrane in which globular features 20-60 nm in size were visible, Pacheco et al further proposed that active layers are composed of a dense nodular base from which the ridge-and-valley structure extends outwards. Similar features to those interpreted as nodules by Pacheco et al and voids by others [13][14][15][16] are visible in TEM images reported elsewhere [18][19][20], although they were not identified as nodules or voids.…”

“…For each membrane, the region corresponding to the active layer is outlined in red dotted lines. For all membranes, the TEM images show well-defined globular features, lighter in color than the surrounding polyamide, similar to the features identified as voids [13][14][15][16] and nodules [17] in the literature. The grayscale contrast between any two regions in TEM images is produced by the difference in the amount of electrons of the analysis beam that pass through the two regions of the sample [41].…”

“…While there is no agreement in the peer-reviewed literature on whether the globular features in TEM images discussed here are voids or dense nodules [21], the features are significantly lighter in shade than the rest of the active layer when bright-field imaging is used [14][15][16][17][18][19][20], indicating that they have a lower electron density than the surrounding polymer. Therefore, these features are likely voids, instead of dense nodules, or correspond to regions of significantly lower polymer density.…”

Thin film composite membranes comprising of polyamide and polydopamine for dehydration of ethylene glycol by pervaporation

“…The parameters commonly studied are the concentrations of amine and/or acyl chloride reactants, contact time between the two reactants [9][10][11], and annealing temperature and time [12,13]. In another approach, the surface coating method has also been used to fabricate membranes, and spin-coating is shown to be more favorable than a simple dip-coating for fabricating a thin layer with a dense structure [14]. To overcome the problems related to solvent-induced swelling of the membranes, inorganic components, cross-linkers or nanoparticles, may be introduced into the polyamide skin layer during interfacial polymerization.…”

Copolymerization modification of poly(vinyltriethoxysilane) membranes for ethanol recovery by pervaporation

“…Currently, membrane fabrication methods like interfacial polymerization, plasma polymerization, atom transfer radical polymerization (ATRP), dip-coating and also other methods have been applied to the preparation of ultrathin pervaporation membranes. [22][23][24][25] It can be concluded that the dramatic decrease of selective layer thickness has been achieved mostly because small molecules are used instead of polymers, like PDMS, as lm forming monomers in most of these methods. Therefore, in our previous study, Zhang et al 26 have reported the preparation of thin high-ux poly(vinyltriethoxysilane) (PVTES) membranes with vinyltriethoxysilane (VTES) monomers for ethanol recovery, which possessed a ux over 10 000 g m À2 h…”