Hydration of poly(pyridiniumsulfonatopropyl betaines)

Abstract

AbstractThe hydration of poly(2‐vinyl‐ and 4‐vinylpyridinium‐3‐sulfonatopropyl betaines) (P2VP‐SB (1) and P4VP‐SB (2)) was studied by sorption isothermal analysis (activity of water a = 0,14–0,98) and by differential scanning calorimetry (DSC, weight fraction of water <0,5). The sorption isotherms at 296 K are typical of a multilayer sorption process (Guggenheim‐Anderson‐De Boer equation): the number of site‐bound water molecules per monomeric unit is 1,33 and 1,59 for 1 and 2, respectively; polymer‐water mixing for indirectly bound solvation layers corresponds to a thermodynamically unfavourable process characterized by Flory's interaction parameter χ of 0,85 and 0,75 for 1 and 2, respectively (Takizawa approach); however, clustering of water molecules (Zimm‐Lundberg theory) is never observed. For 1, DSC allows to quantify three types of water: non‐freezable bound water (type I), 5,0 mol per monomeric unit; freezable bound water (type II), 5,5 mol per monomeric unit, showing a two‐peak melting endotherm at 267 ± 1 K and 271 ± 1 K; bulk free water (type III, melting temperature Tm = 274 K), emerging far before the saturation of all the binding sites for type II water. The poly(pyridinium zwitterions) display slightly decreased hydrophilicity with respect to their aliphatic quaternary ammonium homologues.