Optical properties of polylactides

Polylactides (PLAs) have been known for several decades but have only recently gained commercial significance as a leading environmentally benign plastic available from renewable resources. Accordingly, it is highly desirable to understand the optical properties of these materials. Optical properties are important in dyeing operations for textile applications and in various packaging applications where coloring or clarity is desirable, however, a description of optical properties is not available in the literature. In this study, ellipsometric measurements were made on a well-characterized set of homopolymers and copolymers spanning a wide range of stereoisomer proportions (L-content). Data were collected and analyzed within the framework of the well-known Cauchy model for the index of refraction. No statistical difference is found for the Cauchy parameters for the various optical compositions. The data for all samples can be accurately described using values of Cauchy coefficients of A=1.445 +/- 7.529x10(-4) and B = 4.8916 x 10(3) +/- 1.426 x 10(2) nm. It was found that over the light wavelengths from 300 to 1300 nm, the index of refraction for PLA decreased from 1.499 to 1.448. The uncertainty in the index of refraction calculated from the Cauchy coefficients increases with increasing wavelength, and ranges from 1.1% at 300 nm to 0.1% at 1300 nm. The final useful result of the study is that the index of refraction of PLA within the range of wavelengths from 300 nm to 1300 nm may be described by n(lambda)=(1.445 +/- 0.00075)+(4892 +/- 143) nm(2)/lambda(2).