Hydrophilic Polymers

Abstract

The design of polymers for use in both daily and continuous (day and night) wear contact lenses and the correlation of polymer structure with those properties relevant to contact lens wear are described. Two approaches have been examined:- a) The surface modification of existing polymers having otherwise attractive properties. b) The use of copolymerisation to combine some of the desirable properties of the respective homopolymers. The first approach is restricted to thermoplastic and elastomeric polymers that have potential advantages over the established material, poly (methyl methacrylate), in that they are more flexible and more oxygen permeable. Of the polymers examined, all of which are too hydrophobic for direct use, poly (4-methyl pent-l-ene) proved to be the most readily modified in such a way that its surface became sufficiently wettable to sustain a coherent tear film without reducing its optical properties to an unacceptable level. The successful use of equilibrium advancing water contact angle measurements to provide an 'in vitro' assessment of this behaviour is described. In an attempt to produce more hydrophilic versions of such otherwise acceptable polymers as poly (4-methyl pent-l-ene) copolymerisation of 4-methyl pent-l-ene with hydrophilic monomers (such as 2-hydroxyethyl methacrylate and N-vinyl pyrrolidone) has been attempted with a combined Lewis acid/free radical initiator. The compositions of the resulting polymers are discussed in terms of complex formation between the hydrophilic monomer and the Lewis acid. A class of polymers possessing several advantages over thermoplastics and elastomers, particularly for continuous wear use, are the synthetic hydrogels. These polymers are conveniently described as "water swollen cross-linked polymers" and may be synthesised from various combinations of hydrophilic and hydrophobic monomers to give polymers possessing a range of properties. In the present work conventional free radical polymerisation techniques have been used in conjunction with monomers having broadly similar radical reactivity ratios. The equilibrium water contents of the hydrogels produced in this way were examined and found to be governed by a balance of polar and steric effects. Anomalous water contents which were encountered in hydrogels containing methacrylic acid are described in terms of the strong hydrogen bonding ability of methacrylic acid. A similar type of anomalous behaviour was encountered in the measured values of "dissolved" oxygen permeability coefficients. At water contents of less than 30% these were markedly dependent on the molecular structure and possible water binding ability of the hydrogel polymer, although at higher water contents the oxygen permeability showed a simple dependence on the equilibrium water content. On the basis of this work and available information on corneal oxygen consumption rates predictions of oxygen permeability/thickness requirements for continuous wear contact lenses have been made. In a similar manner initial work has been carried out on the adaption of an 'in vitro' technique for the correlation of the surface hydrophilicity of hydrogels with mucous build up on hydrogel lenses during continuous wear.