Polystyrene and its copolymers have found important applications as films, foams, and structural components. Copolymers with diene monomers are cross-linked to give thermoplastic elastomers. Acrylonitrile-containing copolymers have improved impact strength relative to polystyrene. Polystyrene film is highly transparent to visible radiation and has a high refractive index. Vinyl aromatic polymers, which are analogs of polystyrene, are also included in this category.