EFFECT OF THE WATER ON THE MECHANICAL PROPERTIES OF REGENERATED FIBROIN FILM

The mechanical properties of regenerated silk fibroin films were studied in relation to the effect of water. The sorption isotherm was separated into an absorption curve C1 and a dissolution curve C2. The amount of sorbed water in film at various humidities was more than that of silk fiber. From the BET plot obtained from the S-shaped sorption isotherm, the internal surface area Vm was calculated to be 218m2/g. A composite curve could be obtained by shifting stress relaxation curves at various humidities along the logarithm time axis. This shows that the time-humidity superposition is well adapted for fibroin. The relation between the shift factor ac and water content was expressed by Fujita-Kishimoto's equation, which is based on the free volume theory. The increase of free volume was only effected by C2 component of sorbed water. The parameter β calculated from Fujita-Kishimoto's equation was 0.06 in the range of C2 from 0 to 0.06 (0-55% R. H.) and 0.16 in the range of C2 higher than 0.06 (55% R. H.). The change of the parameter β may indicate an abrupt increase of the mobility of chain segments at about 55% R. H.