The films were cut into a rectangle piece and directly placed against one side of the spectrophotometer test cell, using an empty test cell as the reference. The opacity of the films was calculated by the equation O ¼ Abs600/x, where Abs600 is the value of absorbance at 600 nm and x is the film thickness in mm. Measurements were done at least in triplicate using a UV-1601 spectrophotometer(Model CPS-240, Shimadzu, Kyoto, Japan).