The light absorbance of the film samples (1 cm 6 cm 0.1 mm) was measured at wavelengths ranging from 200 to 700 nm using UV-evis spectrophotometer (Pharmacia, Uppsala, Sweden). The opacity of the films was calculated by the
following equation: opacity (nm/mm) = Abs420/film thickness. Abs420 is the value of absorbance at 420 nm