A hard, solvent free, substantially non-hydrophilic polyurethane elastomer for use in making dental appliances. The elastomer has a hardness of not less than about Shore D65. A polyether or polyester polyol with a functionality greater than two and a molecular weight greater than one hundred is reacted with an isocyanate prepolymer prepared from one or more branched polyether or polyester polyols with a functionality greater than two; and then blending prepolymer and first polyether or polyester polyol with nanoparticles. In one embodiment, the polypropylene glycol triether and the isocyanate prepolymer comprises dicyclohexylmethane 4,4′-diisocyanate.