Water diffusion in a soft contact lens polymer and its tolerance to UV radiation studied by positron lifetime technique

Abstract

ABSTRACT:The kinetics of water sorption, the topology or the free-volume changes due to the presence of sorbed water in a soft contact lens polymer, poly(2-hydroxyethyl methacrylate), were investigated by using the positron lifetime technique. It was found that the ortho-positronium lifetime increases in the beginning of sorption because of microstructural swelling of the polymer matrix. After reaching a maximum, the lifetime decreases and becomes constant, maybe because of the filling of the free-volume holes with water molecules. The diffusion process is found to be non-Fickian. By using the dual-mode sorption model, the Fickian-controlled part and the relaxation-controlled part of diffusion were separated. Further, the positron results seem to indicate the existence of water clusters in the sorbed lens material. The tolerance or stability of the soft lens material to UV radiation seems to be satisfactory as revealed by positron results.

New method of determining miscibility in binary polymer blends through hydrodynamic interaction: The free volume approach

“…In the absence of a measured value of C, the relative fractional free volume (F vR ), defined as F vR ¼ V f  I 3 , is generally used to understand the changes in the free volume content of the system under study. 16,17,20,22,23 I 3 is usually meant to represent the number of free volume sites (hole density) in the material. However, there are arguments that I 3 could be influenced by other mechanisms such as quenching and inhibition as well, so one cannot directly use this to represent the number density of free volume holes.…”

Nanoscale Observation of Dehydration Process in PHEMA Hydrogel Structure

“…Each dehydration curve was recorded as the changes in the mass ratio M t /M ∞ of investigated sample in the function of square root of dehydration time, where M t is the mass of the water amount in the time t and M ∞ is the mass of the water content in fully swollen sample. Mass ratios were acquired by the following equation:where m t means the mass of the sample in the time t of dehydration process, m ∞ is the mass of the sample in the equilibrium swelling state and m d is the mass of the dried material [19]. If the (1) is known for values near the 0.5, the calculation of diffusion coefficient is possible as follows [20]: …”

Influence of curing agent and compatibilizer on the physicomechanical properties of polypropylene/nitrile butadiene rubber blends investigated by positron annihilation lifetime technique

“…However, in molecular media like polymers, the positron of o ‐Ps picks up an electron from the surrounding medium and annihilates, thereby its lifetimes gets shortened to a few nanoseconds. Since o ‐Ps is preferentially localized in the free volume holes of polymer systems,23–33, 36–38 the Ps parameters τ 3 and I 3 have been widely used over the last few years in the study of microstructural behavior of this class of materials. It is usual to correlate the free volume characteristics to the viscoelastic properties of the system under investigation.…”

“…Positron lifetime technique is one of the sophisticated tools available for determining directly the nanometer sized free volume holes and their relative number density (concentration) in polymers 23–26. This technique has proved to be useful on studies regarding diffusion kinetics in electron‐irradiated polycarbonate,27 water sorption in contact lens polymers,28 sorption in poly (vinyl alcohol),29 polymers containing silver nanoparticles,30 structural relaxation/physical aging in polymers,31, 32 etc. Even though positron annihilation lifetime spectroscopy (PALS) has been successfully used in the study of polymers for more than two decades, only recently it has been used to study microstructural behavior of polymer blends 33, 34…”