Thermomechanical analysis of calcium alginate hydrogels in water

Spherical hydrogels of Ca alginate (CaAlg) were prepared by conversion of Na alginate (NaAlg) to CaAlg in CaCl₂ aqueous solution. The degree of conversion was varied by changing the conversion times. NaAlg samples with mannuronic and guluronic component ratios (M/G ratio) 0.21 and 0.88 were used. The degrees of conversion and water contents (W_c=mass of water in CaAlg/mass of dry CaAlg) were measured by weighing. The degrees of conversion from NaAlg to CaAlg increased with increasing immersion time and the content of G. W_c of CaAlg hydrogels decreased with increasing immersion time and levelled off at 75 g g^-1 when immersion time exceeded about 10 min. Viscoelastic properties of CaAlg hydrogels in water were measured using a thermomechanical analyzer (TMA) equipped with a quartz compression probe. Dynamic modulus (E') of CaAlg with M/G=0.88 increased with increasing immersion time when immersion time is less than 5 min. When immersion time exceeded 5 min, E' reached a constant value at 4.5*10⁴ Pa. In contrast, E' of CaAlg with M/G=0.21 increased with increasing immersion time. This suggests that densely crosslinked hydrogels are formed when the guluronate sequence of the samples is rich and Na ions are fully converted to Ca ions. This revised version was published online in July 2006 with corrections to the Cover Date.