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.

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Positron lifetime technique was used to study the physical ageing process in poly(ethylene terephthalate) (PET). Positron lifetime results show that the structural relaxation processes in PET encompass two different time regimes, one short and the other long. The relaxation function constructed from the measured o‐Ps intensity I3 exhibits non‐exponential character, which can be best fitted with two additive exponentials. The Narayanaswamy model (Kohlraush‐William Watt (KWW) function) is invoked to extract the stretching parameter β indicating the extent of deviation from exponential relaxation. Based on the relaxation times, the activation energies calculated seem to label the different kinetic units of PET structure participating in the relaxation process. © 2002 Society of Chemical Industry

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