title: Investigation of the Degree of Disorder of the Structure of Polymer Soft Contact Lenses Using Positron Annihilation Lifetime Spectroscopy PALS.
abstract: BACKGROUND
  Hydrogel and silicone-hydrogel polymeric materials are widely used in ophthalmology for the manufacture of contact lenses. An important aspect is the investigation of the structure of these materials.
  
  
  OBJECTIVE
  This study has been conducted in order to compare the degree of disorder and presence of free volumes in the internal structure of the polymeric soft contact lenses Omafilcon A (hydrogel) and Comfilcon A (silicone-hydrogel). Differences in the occurrence of trapping centers for positrons and free volumes between the types of investigated contact lenses have been demonstrated.
  
  
  METHODS
  Two types of polymeric contact lenses were used as materials: Omafilcon A (hydrogel) and Comfilcon A (silicone-hydrogel). The study was performed using positron annihilation lifetime spectroscopy (PALS).
  
  
  RESULTS
  When the results of the measurements has been obtained, a graphical curve has created to describe the relationship of the number of annihilation acts in time. Significant changes were observed between the contact lenses investigated in positron trapping in macropores (based on a two-state model) and the presence of free volumes (based on the Tao-Eldrup model).
  
  
  CONCLUSIONS
  The use of the positron annihilation two-state model made it possible to demonstrate that a higher positron trapping rate in macropores occurs in the silicone-hydrogel contact lens. Additionally, calculations using the Tao-Eldrup model show the existence of free volumes in both types of materials. The size and fraction of free volumes is much larger in the silicone-hydrogel contact lens.
authors.affiliation: Institute of Physics, Faculty of Mathematics and Natural Sciences, Jan Dlugosz University, Częstochowa, Poland.
authors.forename: Jacek
authors.initials: J
authors.lastname: Filipecki
authors.affiliation: Institute of Physics, Faculty of Production Engineering and Materials Technology, Częstochowa University of Technology, Częstochowa, Poland.
authors.forename: Katarzyna
authors.initials: K
authors.lastname: Kotynia
authors.affiliation: Institute of Physics, Faculty of Mathematics and Natural Sciences, Jan Dlugosz University, Częstochowa, Poland.
authors.forename: Katarzyna
authors.initials: K
authors.lastname: Filipecka
mesh_terms: D003263:Contact Lenses, Hydrophilic; D004583:Electrons; D020100:Hydrogels / Q000737:chemistry*; D015394:Molecular Structure; D012828:Silicones / Q000737:chemistry*; D013057:Spectrum Analysis
chemical_list: D020100:Hydrogels; D012828:Silicones; C540498:comfilcon A
keywords: contact lenses; defects; free volumes; polymers; positron annihilations