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Characterization of methyl-substituted polyamides used for reverse osmosis membranes by positron annihilation lifetime spectroscopy and MD simulation



Three kinds of polyamides were synthesized from three diamines and 1,3,5-benzenetricarbonyl trichloride (TMC). The diamines used were m-phenylene diamine, N-methyl-m-phenylenediamine, and N,N′-dimethyl-m-phenylenediamine. The average free volume sizes of the polyamides were measured by positron annihilation lifetime spectroscopy (PALS), and the free volume fractions were evaluated by molecular dynamics (MD) simulations. The methyl substitution on amino groups of diamines brought about an increase in interstitial space of molecular chains of the polyamides. In addition, reverse osmosis (RO) membranes were prepared by interfacial polymerization from the three diamines and TMC. The increase in the degree of methyl-substitution of diamines led to increased chlorine resistance and decreased salt rejections of the polyamide RO membranes. Thus, the methyl-substitution of diamines significantly influenced membrane performance. The vacancy sizes and fractional volumes in polyamides evaluated by PALS measurement and MD simulation were well correlated with salt rejection of polyamide RO membranes. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009