Optical properties of low-density foams considered as targets for inertial confinement fusion

An optical method for measuring scattered light, namely by analyzing the transmitted light, is used to characterize the optical properties of low-density foams that are candidates as targets for inertial confinement fusion. This method allows us to simultaneously measure both scattering and intrinsic absorption in the foam. Three types of foams have been examined: two have similar chemistries and are based on either ethylene glycol dimetacrylate (EGDM) or trimethylolpropane trimethacrylate (TMPT), The other one is based on resorcinol formaldehyde (RF). We observe that while the RF foams are orange in color and nearly transparent, the EGDM/TMPT based foams are white and opaque. The opacity of these foams is shown to be due to light scattering by particles having dimensions on the order of 200 nm. In contrast, the light scattering from the RF foams indicates that the structures scattering light are on the order of 20 nm. The orange color of the RF foams is shown to be due to intrinsic absorption of light by the polymer comprising the foam. The onset of absorption is seen to shift towards the blue as the foams age, Index matching to the TMPT foam using ethanol is shown to significantly reduce the scattering. (C) 1998 American Vacuum Society.