Poly(aryl cyanurate)-Based Thin-Film Composite Nanofiltration\nMembranes

Abstract

The successful synthesis\nof poly­(aryl cyanurate) nanofiltration\nmembranes via the interfacial polymerization reaction between cyanuric\nchloride and 1,1,1-tris­(4-hydroxyphenyl)­ethane (TPE), atop a polyethersulfone\nultrafiltration support, is demonstrated. The use of cyanuric chloride\nallows for the formation of a polymer that does not contain hydrolysis-susceptible\namide bonds that inherently limit the stability of polyamide nanofiltration\nmembranes. In order to achieve a thin defect-free cross-linked film\nvia interfacial polymerization, a sufficient number of each monomer\nshould react. However, the reactivities of the second and third chloride\ngroups of the cyanuric chloride are moderate. Here, this difficulty\nis overcome by the high functionality and the high reactivity of TPE.\nThe membranes demonstrate a typical nanofiltration behavior, with\na 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\nbehavior Na₂SO₄ (97.1%) > MgSO₄ (92.8%)\n> NaCl (51.3%) > MgCl₂ (32.1%) indicates that the\nmembranes\nhave a negative surface charge. The absence of amide bonds in the\nmembranes was expected to result in superior pH stability as compared\nto polyamide membranes. However, it was found that under extremely\nacidic conditions (pH = 1), the performance showed a pronounced decline\nover the course of 2 months. Under extremely alkaline conditions (pH\n= 13), after 1 month, the performance was lost. After 2 months of\nexposure to moderate alkaline conditions (pH = 12), the MgSO₄ retention decreased by 14% and the permeance increased by 2.5-fold.\nThis degradation was attributed to the hydrolysis of the aryl cyanurate\nbond that behaves like an ester bond.