Molecular mechanism of gelation upon the addition of water to a solution of poly(acrylonitrile) in dimethylsulfoxide

Abstract

The solidification of a solution of poly(acrylonitrile) (PAN) in dimethylsulfoxide (DMSO) upon introduction of water into the solution is studied by Raman spectroscopy. In the absence of water, DMSO molecules are found to produce dipole-dipole bonds with PAN molecules. Upon the introduction of water, DMSO molecules produce hydrogen bonds with it and bands at 1005 and 1015 cm−1 appear in the Raman spectrum, which are assigned to the valence vibrations of S=O bonds involved in the hydrogen bonds. Simultaneously, water molecules produce hydrogen bonds with PAN molecules: R-C≡N...H-O-H...N≡C-R, where R is the carbon skeleton of a PAN molecule. Accordingly, a band at 2250 cm−1 arises in the Raman spectrum, which is assigned to the valence vibrations of C≡N bonds producing hydrogen bonds with a water molecule. When the water content is low and the DMSO concentration is high, the length of the hydrogen bonds varies in wide limits and the band at 2250 cm−1 is wide. As the water content rises, DMSO molecules come out of PAN, the variation of the hydrogen bond length in it decreases (the band at 2250 cm−1 narrows), and a high-viscosity system (gel) arises that consists of PAN molecules bonded to water molecules via “equally strong” hydrogen bonds.