Films of regenerated Bombyx mori silk are strongly affected by absorbed moisture, a phenomenon studied here by differential scanning calorimetry (DSC). Exposure of previously dried films to environments of controlled relative humidity produces test samples of well-defined equilibrium moisture content. Ultimate moisture uptake is as high as 20–23% (by weight) at 75% relative humidity. The glass transition temperature, Tg, drops by 40°C at moisture uptakes as low as 2%, and Tg depressions as large as 140°C are observed at higher relative humidity. The moisture-induced decrease of Tg is completely reversible, as a film remoistened and then redried possesses an unchanged Tg. Trends in Tg with water uptake correspond reasonably well to predictions of a classical thermodynamic theory, indicating that the plasticization effect of moisture on the combined silk-water system can be satisfactorily explained from macroscopic properties of the constituents without any reference to specific interactions.
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Agarwal et al. (1997) studied this question.