Abstract
Water sorption by amorphous statistical butyl acrylate (A unit) zwitterionomers bearing ammoniopropanesulfonate (B unit, N^+^(CH~2~)~3~SO, μ = 24.0 D) or ammonioethoxydicyanoethenolate (B′ unit, N^+^(CH~2~)~2~OCOC^−^(CN)~2~, μ = 25.9 D) dipolar functional groups was studied at 23°C over a broad range of water activity (0.22 < a < 0.93). For a < 0.65, the sorption isotherms may be analyzed according to the BET equation for a multilayer process characterized by a number of site‐bound water molecules per dipolar unit, independent of copolymer composition and close to that measured on the corresponding zwitterionic homopolymers: n~m~ ≈ 2.0 and 0.7 for AB and AB′, respectively. The initial kinetics of the diffusion process may be described according to a Fickian model up to the saturation of the first solvation layer capacity, with a typical increase of the diffusion coefficient for the AB′ copolymers of strongly reduced waterzwitterion interactions. In spite of the complex heterogeneous morphology of these zwitterionomers, of its continuous evolution and of the change of physical state of the hard domains from glassy to viscoelastic upon hydration, the water sorption process appears rather simple: it is essentially monitored by the overall content and the chemical structure of the dipolar units in the copolymer chains.