Study of free volumes of polymer hydrogel and -silicone-hydrogel contact lenses by means of the positron annihilation lifetime spectroscopy method.

BACKGROUND: Polymer materials based on hydrogel and silicone-hydrogel materials are commonly used in ophthalmology. It is important to research the structure of these materials, mainly the prevalence of free volumes.

OBJECTIVES: The study has been conducted in order to determine the presence of free volume gaps in the structure of polymer hydrogel and silicone-hydrogel contact lenses. In addition, to demonstrate differences in the occurrence of free volumes between types of represented contact lenses.

MATERIAL AND METHODS: Three different hydrogel and three different silicone-hydrogel polymer contact lenses were used as research material. The study was done by means of positron annihilation lifetime spectroscopy (PALS).

RESULTS: As a result of the performed measurements, a graphical curve resulted which describes the relationship between the number of the annihilation acts in the time function. The study revealed the existence of three τ1, τ2 and τ3 components. Significant changes were observed in the ortho-positronium long life component τ3 and their intensities between the examined polymer contact lenses.

CONCLUSIONS: The conducted study using the Tao-Eldrup model indicates the presence of free volume holes in all research materials. The results lead to the following connection: contact lenses of higher oxygen permeability coefficient (silicone-hydrogel contact lenses) have more and larger free volumes than contact lenses of less oxygen permeability coefficient (hydrogel contact lenses).