Changes in Physical Properties and Structure of Sol-Gel Derived SiO2 Films in an Environment of High Temperature and High Humidity

Abstract

The effects of addition of poly (ethylene glycol) (PEG), to gel films on the optical properties and structure of the resultant SiO₂ coating films have been studied before and after being subjected to an environment of high temperature and high humidity. The larger amount and the larger molecular weight of PEG produced the larger volume of pores in the resultant SiO₂ films on the decomposition of PEG during the heat-treatment. The refractive index of such porous films was found to increase due to water adsorbed in the films. On the other hand, the porosity and the amount of adsorbed water of the films decreased after the treatment in an environment of high temperature and high humidity. The decreases in porosity and the amount of adsorbed water were more significant in the films prepared with the addition of the larger amount of PEG with a larger molecular weight. These findings can be ascribed to the decrease in silanol groups and densification of the films with the treatment at high temperature and high humidity, as confirmed by the changes in IR absorption spectra and the results of TEM observation.