Water Diffusion in a Soft Contact Lens Polymer and Its Tolerance to UV Radiation Studied by Positron Lifetime Technique M. C. Thimmegowda, H. B. Ravi Kumar, C. Ranganathaiah Department of Studies in Physics, University of Mysore, Manasagangotri, Mysore-570 006, India Received 23 July 2003; accepted 17 November 2003 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 life- time technique. It was found that the ortho-positronium lifetime increases in the beginning of sorption because of microstructural swelling of the polymer matrix. After reach- ing a maximum, the lifetime decreases and becomes con- stant, 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. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1355–1366, 2004 Key words: diffusion; swelling; visco-elastic properties INTRODUCTION From a researcher’s perspective, designing a good contact lens polymer is not a trivial task. The contact lens material must satisfy a number of strict require- ments. It must be transparent, possess chemical and thermal stability and biocompatibility, and be wetta- ble to tears. It must also have suitable mechanical properties. For hydrogels, this includes low modulus of elasticity (softness) for patient’s comfort and good shape memory for handling and proper functioning. Finally, the material must also have good oxygen per- meability. Owing to a lack of blood vessels within the corneal framework, the cornea must get oxygen di- rectly from air. When the contact lens comes in the way of the eye and air, it reduces the oxygen supply, which will cause corneal odema. Excessive prolonged levels of odema may lead to a number of physiological responses including microcysts, inflammation, infec- tions, and corneal ulcers. 1 Furthermore, the contact lens is constantly in touch with the lachrymal fluid, which contains not only water but also lipids, sodium, carbon bicarbonate, and enzymes, which tend to form a thin film on the contact lens if the lens is hydropho- bic. This reduces the transparency and causes infec- tion and irritation to the eye. If the contact lens has good wettability or high hydrophilicity, these prob- lems can be overcome. The dream of realizing such a contact lens came true with the advent of world’s first soft contact lens poly- mer, poly(2-hydroxyethyl methacrylate) (PHEMA), in the 1960s. PHEMA has proven itself to be a promising material, even today, despite the entry of very many new materials to the contact lens industry. It is a transparent hydrogel, which, when hydrated, absorbs up to 40% water but is insoluble in water because of the presence of crosslinkers that form a three-dimen- sional network. 1 Under equilibrium water content conditions, it has glass transition temperature (T g ) be- low the room temperature and, hence, it has apprecia- ble softness. 2 Hydrogels have high polymer–polymer segment interactions. This provides stability of the gel from collapsing on swelling. 3 The polarity of the seg- ments is responsible for the hydrophilicity of the gel. In a hydrogel, polymer–solvent interactions are stron- ger than the polymer–polymer interactions. However, there exists a balance between the two at equilibrium sorption to control the swelling. PHEMA is prepared by free-radical polymerization of HEMA monomer by using either a thermal or a UV initiation system. It is also prepared by a spin-cast process, which involves polymerizing HEMA monomer in a spinning concave mold where the lens power is controlled by the spin rate and mold optics. The degree of swelling is char- acteristic of its composition and crosslink density. The gel tends to absorb or expel fluid efficiently with changes in the environmental conditions such as os- Correspondence to: C. Ranganathaiah (crang1@rediffmail.com). Contract grant sponsor: University of Mysore. Journal of Applied Polymer Science, Vol. 92, 1355–1366 (2004) © 2004 Wiley Periodicals, Inc.