WATER SORPTION IN POLY(AMMONIUM SULFOPROPYLBETAINES) .1. DIFFERENTIAL SCANNING CALORIMETRY

The hydration of four amorphous acrylic and methacrylic poly(zwitterions) bearing the ammonium sulfopropylbetaine function as a side-groups N+-(CH2)3-SO3-, mu almost-equal-to 25 D) was studied by differential scanning calorimetry over broad ranges of temperature (150-400 K) and water content (weight fraction W1 < 0.5). Analyses were made of the first-order transitions and heat capacity of sorbed water, glass transition temperature (T(g)) measurements. Nonfreezable bound water, about 7.7 +/- 0.9 mol/monomeric unit, behaves as a single phase: Its mobility, fairly similar to that of bulk liquid water in viscoelastic systems at T > 250 K, decreases with temperature in the glassy systems, but never disappears, even at 185 K. The depression of the glass transition temperature of the hydrated polymers obeys Couchman's equation: T(g) = SIGMA(i)W(i)DELTA-C(pi) T(gi)/SIGMA(i)W(i)DELTA-C(pi). Freezable bound water, about 6.7 +/- 0.9 mole/monomeric unit, shows multipeak melting endotherms in the range 242-272 K. Because of their charged sites, the hydration process of the poly(zwitterions) appears more similar to that of poly(electrolytes) than to that of uncharged hydrophilic polymers.