title: Comparison of Occurrence of Free Volumes for Rigid Gas Permeable and Soft Contact Lenses.
abstract: BACKGROUND
  The polymeric materials: hydrogel, silicone--hydrogel and methacrylic acid are used in ophthalmology for the manufacture of contact lenses. It is important to research the structure of these materials, mainly the prevalence of free volumes.
  
  
  OBJECTIVE
  The study has been conducted in order to comparison the presence of free volume gaps in the structure of the polymer soft contact lenses: Etafilcon A (hydrogel), Narafilcon A (silicone-hydrogel) and the polymer rigid gas permeable (RGP) contact lens (Fluor-Silicon-Methacrylat-Copolymer). In addition, to demonstrate differences in the occurrence of free volumes between types of represented contact lenses.
  
  
  METHODS
  Three types of polymer contact lenses were used as materials: Etafilcon A, Narafilcon A and Fluor-Silicon-Methacrylat-Copolymer. 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. 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. There is a clear difference in the free volume sizes and their fractions between measured contact lenses are connected with oxygen permeability in these lenses. The results lead to the following connection: contact lenses of higher oxygen permeability coefficients and a water content of less, have more and larger free volumes than contact lenses of less oxygen permeability coefficient.
authors.forename: Katarzyna
authors.initials: K
authors.lastname: Filipecka
authors.forename: Mariusz
authors.initials: M
authors.lastname: Budaj
authors.forename: Bogdan
authors.initials: B
authors.lastname: Miskowiak
authors.forename: Małgorzata
authors.initials: M
authors.lastname: Makowska-Janusik
authors.forename: Jacek
authors.initials: J
authors.lastname: Filipecki
mesh_terms: D003261:Contact Lenses / Q000145:classification*; D003263:Contact Lenses, Hydrophilic; D004583:Electrons; D020136:Hydrogel, Polyethylene Glycol Dimethacrylate / Q000737:chemistry; D008422:Materials Testing; D008689:Methacrylates; D008956:Models, Chemical; D010100:Oxygen / Q000737:chemistry*; D010539:Permeability; D011108:Polymers / Q000737:chemistry; D012828:Silicones / Q000737:chemistry
chemical_list: D008689:Methacrylates; D011108:Polymers; D012828:Silicones; C114176:etafilcon; D020136:Hydrogel, Polyethylene Glycol Dimethacrylate; D010100:Oxygen