Each film specimen was cut into a rectangular piece and placed
directly in a spectrophotometer cell, and measurements were
performed with air as the reference for transparency. A spectrum
of each film was obtained in a UV–vis spectrophotometer (Beckman
DU650, Germany). The area under the absorption curve from
400 to 800 nm was recorded and the opacity of the film (arbitrary
units/mm) calculated by dividing the absorbance at 500 nm by the