Polyethylene glycol: Optical constants from 191 to 1688 nm (0.735–6.491 eV) by spectroscopic ellipsometry

Abstract

Polyethylene glycol (PEG) is an important polymer with numerous industrial and therapeutic applications. Accordingly, the optical constants of PEG should be a useful resource to other scientists and engineers. Herein, we report the optical functions of PEG as determined from reflection spectroscopic ellipsometry (SE) and transmission ultraviolet-visible spectroscopy. These functions were obtained from a commercial liquid sample of PEG that had a molecular weight of 285–315 g/mol. Since this sample is a liquid, the reflection and transmission measurements required special experimental considerations. The reflection SE measurements necessitated roughening (frosting) the inside of the vessel (Petri dish) that contained PEG and leveling the instrument instead of the liquid sample. The transmission measurements were obtained via a dual-cuvette approach that removed the effects of the cuvettes and their interfaces. From these measurements, the extinction coefficient of the PEG was determined between 205 and 1700 nm. Since PEG has very low absorption over the measured wavelength range, and because of the relatively wide wavelength range considered here, the refractive index was fit with a Sellmeier model for wavelengths from 191 to 1688 nm. The interface at the liquid surface was modeled with a Bruggeman effective medium approximation. The optical functions obtained in this work agree well with previously reported values. For example, we obtained the following nx values for the material, where x is the wavelength in nanometers: n300 = 1.493, n500 = 1.459, and n1000 = 1.443.