Investigation of the Degree of Disorder of the Structure of Polymer Soft Contact Lenses Using Positron Annihilation Lifetime Spectroscopy PALS.

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. OBJECTIVES: 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. MATERIAL AND 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.

Ageing phenomenon in acrylic polymer dental materials detected by means of positron annihilation lifetime spectroscopy.

BACKGROUND: Polymer materials based on acrylic monomers are commonly used in dentistry. It is important to research the structure of dental filling materials towards the material ageing. OBJECTIVES: The study has been conducted in order to determine the presence of free volume gaps in the structure of polymer materials. MATERIAL AND METHODS: Brand new, acrylic polymer based samples of dental Dipol materials were used as a research material. The study was conducted by means of the positron annihilation lifetime spectroscopy (PALS). RESULTS: As a result of the conducted measurements, curves describing numbers of counts of the acts of annihilation in the time function were obtained. The conducted studies revealed the existence of four components τ1, τ2, τ3 and τ4. The τ3 and τ4 components are attributed to the pick-off annihilation of o-Ps orthopositronium trapping by free volume gaps and provide information about geometrical parameters of the volumes. LT 9 computer program was used to calculate components. Free volume holes were determined from empirical relationship between the radius of free volume and the long lifetime components. CONCLUSIONS: Conducted studies indicate the presence of free volume holes in the research materials. It has been noted that a new long lifetime component is assigned to a new kind of free volumes that exist in the structure of material related to the material ageing.

Structural studies of polymer hydrogel and silicone hydrogel contact lenses by means of positron lifetime spectroscopy methods.

UNLABELLED: PURPOSE OF JOB: Currently, there isa need to increase comfort and visual acuity man. Simultaneously improving biocompatibility and minimizing the impact of the material on the physiology of the cornea is the primary driving force behind the evolution of materials used in the manufacture of contact lenses. Despite progressive development of modern materials science, there is still the problem of reducing the level of oxygen available to the cornea resulting in pathological changes in the cornea. Therefore, structural studies increases interesting in relating to the amorphous contact lenses polymeric materials. MATERIALS AND METHODS: The aim of this work is structural investigation of polymer hydrogel and silicone hydrogel contact lenses made in the technology of PC (Phosphoryl Choline). The study method was used positron lifetime spectroscopy PALS. RESULTS: As a result of the measurements obtained curve describing the dependence of the number of counts of acts of annihilation as a function of time. CONCLUSIONS: The study of PALS showed the existence of three components. Component tau1 is responsible for the annihilation of free positrons and the annihilation of electrons vacancy-type point defects. Component tau2 is associated with defects in the volume of grain boundaries formed, dislocations or clusters of vacancies. The results of calculations of mean values positron lifetime samples, showed longtime component tau3 in the spectrum of positron lifetime. Component tau3 is assigned to pick-off annihilation of ortho-positronium o-Ps trapping by free volume and providing information on the geometric parameters of the volume.