EFFECT OF MOISTURE ABSORPTION ON THE THERMAL-PROPERTIES OF BOMBYX-MORI SILK FIBROIN FILMS

Films of regenerated Bombyx mori silk are strongly affected by absorbe d moisture, a phenomenon studied here by differential scanning calorim etry (DSC). Exposure of previously dried films to environments of cont rolled relative humidity produces test samples of well-defined equilib rium moisture content. Ultimate moisture uptake is as high as 20-23% ( by weight) at 75% relative humidity. The glass transition temperature, T-g, drops by 40 degrees C at moisture uptakes as low as 2%, and T-g depressions as large as 140 degrees C are observed at higher relative humidity. The moisture-induced decrease of T-g is completely reversibl e, as a film remoistened and then red-led possesses an unchanged T-g. Trends in T-g with water uptake correspond reasonably well to predicti ons of a classical thermodynamic theory, indicating that the plasticiz ation effect of moisture on the combined silk-water system can be sati sfactorily explained from macroscopic properties of the constituents w ithout any reference to specific interactions. (C) 1997 John Wiley & S ons, Inc.