Preparation of the nylon-6 films. Nylon-6 pellets (Sigma Aldrich) were melted on a hot plate at 250 °C and pressed with an aluminum block for a few minutes. The obtained films were then cooled down to the room temperature. After the samples were vacuum dried at 40 C for 1 week, THz spectra of dried samples were measured for references. Next, the sample films are stored in the humid environments at room temperature controlled by water and saturated solutions of inorganic salts for 2 weeks.