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 Na
2
SO
4
(97.1%) > MgSO
4
(92.8%)
> NaCl (51.3%) > MgCl
2
(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 MgSO
4
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.