Mechanical and Thermal Properties of High‐Concentration Hydrogels of Agarose Prepared Using Subcritical Water

High‐concentration hydrogels with concentrations of agarose ( c ) of up to 50 wt% have been successfully obtained by using subcritical water conditions. The obtained high‐concentration hydrogels exhibit definitely high mechanical properties as to Young's modulus ( E 0 ) and the stress at break in uniaxial stretching. The melting point ( T m ), that is, the gel‐to‐sol transition temperature, also increases with c , and T m for the 50 wt% hydrogel exceeds 100°C. Furthermore, investigations of the agarose hydrogels over a wide range of c have revealed a specific boundary concentration lying between 15 and 20 wt%. The profile of the stress–strain curves, the c ‐dependence of E 0 , and the c ‐dependence of the strain at break all change at this boundary concentration. Remarkably, the endothermic enthalpy change per unit weight of hydrogel also turns to decrease above this concentration. These results indicate a strong possibility of changes in the crosslinking structure due to increased concentration. In addition, differences in the mechanical as well as the thermal properties between agarose and κ ‐carrageenan hydrogels have been clarified, which can be explained by their respective crosslinking mechanisms: hydrogen bonding for agarose and electrostatic interactions for κ ‐carrageenan.