Effect of water on the thermal properties of silk fibroin

Silk fibroin films cast from water solution, and containing bound water, are quantitatively studied in this work. First, to obtain the solid and liquid heat capacities of the pure dry silk fibroin, cyclic heat treatment was used to monitor the process of removing the bound water. After water removal, the glass transition of pure non-crystalline silk was observed at 451 K (178 °C). The solid and liquid heat capacities of the pure silk fibroin were then measured using differential scanning calorimetry (DSC), temperature-modulated DSC (TMDSC), and quasi-isothermal TMDSC, and found to be: C_p(T)^solid = 0.134 + 3.696 × 10^-3 T J/g K and C_p(T)^liquid = 0.710 + 3.47 × 10^-3 T J/g K over the temperature region from 200 to 450 K. These heat capacities were used to construct the underlying baseline heat capacity for the combined silk-water system. When the combined silk-water system is studied, bound water is lost from the film during heating, and the loss of mass is quantified using thermogravimetric analysis (TGA). Bound water in the silk film acts as a plasticizer, and a lower glass transition of the silk-water system is observed. Comparison of the measured heat capacity of the silk-water system to the calculated total baselines was made in the vicinity of the water-induced glass transition. Results show that the total solid specific heat capacity is in good agreement with the calculated solid baseline in the low-temperature region below about 240 K. As temperature increases above the lower glass transition, all bound water eventually leaves the silk, and the free volume and the silk mobility are reduced. This allows the upper glass transition of the dried silk to be observed.