WATER SORPTION IN BUTYL ACRYLATE ZWITTERIONOMERS OF THE AMMONIOPROPANESULFONATE AND AMMONIOETHOXYDICYANOETHENOLATE TYPE

Water sorption by amorphous statistical butyl acrylate (A unit) zwitte rionomers bearing ammoniopropanesulfonate (B unit, N+--(CH2)(3)--SO3-, mu = 24.0 D) or ammonioethoxydicyanoethenolate (B' unit, N+--(CH2)(2) --O-CO--C-(CN)(2), mu = 25.9 D) dipolar functional groups was studied at 23 degrees 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 th e BET equation for a multilayer process characterized by a number of s ite-bound water molecules per dipolar unit, independent of copolymer c omposition and close to that measured on the corresponding zwitterioni c homopolymers: n(m) approximate to 2.0 and 0.7 for AB and AB', respec tively. The initial kinetics of the diffusion process may be described according to a Fickian model up to the saturation of the first solvat ion layer capacity, with a typical increase of the diffusion coefficie nt for the AB' copolymers of strongly reduced water-zwitterion interac tions. In spite of the complex heterogeneous morphology of these zwitt erionomers, 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 mo nitored by the overall content and the chemical structure of the dipol ar units in the copolymer chains.