DSC and TMA Studies of Polysaccharide Physical Hydrogels

Various kinds of polysaccharides found in a wide variety of plants, bacteria, crustaceans and insects form hydrogels via physical aggregation in aqueous media. The major mass of hydrogels is water filled, ca. 95 - 99.5%, in a network structure, although the solid shape of the gel is maintained. In this paper, firstly the wide range of gelation mechanisms are briefly described, and then the thermal analysis of representative gel-forming polysaccharides, such as carrageenan, alginate, galactomannan, and pectin, is introduced. By differential scanning calorimetry (DSC), gel-sol and the sol-gel transition temperature of thermoreversible hydrogels are measured and phase diagram is established. It is suggested that binary systems showing sinusoidal gel-sol-gel transition are capable of being assembled. By thermomechanical analysis (TMA), the dynamic modulus (E') at around 1 × 10⁴ Pa of thermo-irreversible hydrogels was obtained using a sample holder designed to measure the viscoelastic properties in water. Reliable coordination is shown between the results obtained by DSC and TMA. In this review, the current research and several topics on concerning the thermal properties of polysaccharide physical hydrogels are introduced.