WATER SORPTION IN POLY(AMMONIUM SULFOPROPYLBETAINES) .2. SORPTION ISOTHERMS

Water sorption by four amorphous acrylic and methacrylic poly (zwitterions) bearing ammonium sulfopropylbetaine side groups (- N+ - (CH2)3-SO3-, mu almost-equal-to 25 D) was studied at a constant temperature of 23-degrees-C and over a broad range of water activity (0.14-0.90). Whatever the physical state of the hydrated polymer, glassy or viscoelastic, water diffusion is Fickian (average diffusion coefficient D(s)BAR in the range 2-16 X 10(-8) cm2 s-1), and the sorption isotherms may be quantitatively analyzed according to the Guggenheim-Anderson-De Boer amended BET equation for multilayer sorption processes. The number of site-bound water molecules per monomeric unit is in the range 1.5-2.0, and apparently there is no great energy difference between direct site binding and indirect binding in the successive solvation layers. The polymer-water interaction parameter (-0.6 < chi-Flory < 0.6) is an increasing function of the water content of the hydrated poly (zwitterions) over the whole composition range (water volume fraction < 0.5), without any clear transition from the glassy to the viscoelastic state. Clustering of water molecules (Zimm-Lundberg theory) is never observed, even at high water content. Because of the charged structure of their dipolar units, the poly (zwitterions) show a water sorption process similar to that of the corresponding poly (electrolytes) of the tetra-alkylammonium sulfonate type.