Poly(aryl cyanurate)-Based Thin-Film Composite Nanofiltration Membranes

The successful synthesis of poly(aryl cyanurate) nanofiltration membranes via the interfacial polymerization reaction between cyanuric chloride and 1,1,1-tris(4-hydroxyphenyl)ethane (TPE), atop a polyethersulfone ultrafiltration support, is demonstrated. The use of cyanuric chloride allows for the formation of a polymer that does not contain hydrolysis-susceptible amide bonds that inherently limit the stability of polyamide nanofiltration membranes. In order to achieve a thin defect-free cross-linked film via interfacial polymerization, a sufficient number of each monomer should react. However, the reactivities of the second and third chloride groups of the cyanuric chloride are moderate. Here, this difficulty is overcome by the high functionality and the high reactivity of TPE. The membranes demonstrate a typical nanofiltration behavior, with a molecular weight cutoff of 400 ± 83 g·mol^-1 and a permeance of 1.77 ± 0.18 L·m^-2 h^-1 bar^-1. The following retention behavior Na₂SO₄ (97.1%) > MgSO₄ (92.8%) > NaCl (51.3%) > MgCl₂ (32.1%) indicates that the membranes have a negative surface charge. The absence of amide bonds in the membranes was expected to result in superior pH stability as compared to polyamide membranes. However, it was found that under extremely acidic conditions (pH = 1), the performance showed a pronounced decline over the course of 2 months. Under extremely alkaline conditions (pH = 13), after 1 month, the performance was lost. After 2 months of exposure to moderate alkaline conditions (pH = 12), the MgSO₄ retention decreased by 14% and the permeance increased by 2.5-fold. This degradation was attributed to the hydrolysis of the aryl cyanurate bond that behaves like an ester bond.