Time dependent solid-state 13C NMR study on alkaline hydrolysis of polyacrylonitrile hollow fiber ultrafiltration membranes

The effects of alkaline hydrolysis on the chemical structure of polyacrylonitrile (PAN) hollow fiber ultrafiltration (UF) membrane were studied using solid-state 13C NMR spectroscopy. The PAN membrane was hydrolyzed in an NaOH solution of pH 13, which is generally used in water industry for anti-fouling treatments of UF membranes but is much weaker than the alkaline concentrations (0.5–2.5 N) used so far in conventional 13C NMR studies. All of the carbon sites of the PAN were characterized by the solid-state 1H–13C CPMAS and HETCOR NMR spectra. Upon the alkaline treatments, the chemical conversion from the cyano functional groups of the PAN to the carboxylates was observed. The analysis of the 13C CPMAS NMR spectra obtained as a function of the treatment time for 4–96 h showed that the chemical modification occurred very rapidly in the early stage (≤6 h) of the reaction but then proceeded slowly up to 96 h. The relative amount of the hydrolyzed acrylonitrile units after 96 h of the reaction at room temperature was estimated to be only 3.8%. We suggested that the alkaline hydrolysis of the PAN membrane proceeds at two distinct rates, a rapid conversion of the cyano groups on or near the membrane surface and a slow conversion of the bulk.