According to the Lambert-Beer's law of absorption, the relationship between absorption coefficient α and ε can be written as A =log(I/I 0 ) = εCL = log 10 eαL, where A is the absorption, I and I 0 are the transmitted and incident light intensities, ε is the molar absorption coefficient, C is the concentration, L is the optical path length. However, due to the lack of detail molecular information of polymer materials, it is not easy to find the exact molar concentration C, and we use the ratio absorbance/film thickness A/L as α in this study.