Influence of silicone distribution and mobility on the oxygen permeability of model silicone hydrogels

In order to understand the influence of composition on oxygen permeability, the morphology of model silicone hydrogels in both the dry and hydrated states was characterized using a variety of techniques (AFM, HAADF-STEM, solid-state NMR, and X-ray scattering). The model system studied is heterogeneous on length scales below 20 nm and consists of globular silicone-rich domains that rearrange in response to changes in hydration. In contrast to the well-defined morphologies of block copolymer systems, these radically cured amphiphilic networks are less ordered, showing gradual composition fluctuations. Comprehensive morphology characterization rationalizes the transport behavior of these heterogeneous hydrogels: the non-linear permeability increase with increasing silicone monomer content is not only related to changes in the spatial arrangement of silicone-rich domains but also to their mobility. This understanding is needed for further optimizing soft contact lens materials where oxygen transport and optical clarity are critical features.