Synthesis, characterization, and application to nanofiltration of ionomeric poly (acrylamidocaproic acid) membrane

Abstract

The synthesis, characterization, and application to nanofiltration of poly(acrylamidocaproic acid) neutralized by calcium ion were carried out. The hydrophobic methylene group in side substituent prevents excessive swelling with water and acts as a binding of the chain. The neutralized ionic groups is designed in order to enhance the affinity of the membrane to water. Acrylamidocaproic acid and its corresponding polymer were successfully synthesized, which was evidenced by $^1H$ and $^13C$ NMR spectra. In dry state, the ionic group, calcium carboxylate, aggregates to form multiplets above 20mol% degree of neutralization. The hydrogen bonding of amide group was destroyed during multiplet formation due to the reduction of the motional freedom of the side chain. The extension of the short side chain (side substituent) started to be loosened at 44 mol% neutralization. In this state, the multiplet can be formed without severe side chain stretching due to the proximity of the ionic groups. It is also expected that the more stable multiplet is formed with increasing the degree of neutralization. As the degree of neutralization increases, the affinity to water increases. Since the neutralized polymer is soluble in water when the degree of neutralization exceeds about 40 mol%, application to water permselective membrane is limited for these cases. The study on the nanofiltration is confined into the polymers of which degree of neutralization range from 0mol% to 12mol%. The ionic groups are rather homogeneously dispersed in these ionic contents. Therefore, the effect of ionic aggregation in the water swollen nanofiltration membrane is not considered. The amount of water molecules participated in the hydration shell surrounding an ionic group was increased with increasing the ionic concentration in the membrane. The exponential increase of flux with increasing degree of neutralization would be due to the increase of the number and size of the hydration shell. Wide spect...