" Microfilled" dental restorative resins are reinforced with a colloidal form of silica. They are polishable, highly translucent, and have low initial wear compared to other composites, but are not x-ray opaque. This colloidal silica is difficult to disperse homogeneously in the monomer due to interparticle chaining that dramatically increases viscosity at modest filler loadings, with compromises workability and adaptability. The use of pre-cured resin filler to circumvent the viscosity problem imposes a non-homogeneous morphology, and leads to poor long-term wear due to breakdown between the pre-cured resin filler particles and the clinically cured matrix. Therefore, the objective of this project is to develop a new class of reinforcing particles in the nanometer size range, with high atomic number that can be used in composites with low-shrinkage liquid crystal monomers to overcome deficiencies of current restorative materials. The hypothesis is that metal oxide nanoparticles can be made and surface modified such that they can be homogeneously dispersed and strongly coupled with crosslinked liquid crystal (LC) monomers, and thereby substantially enhance the inherent advantageous properties of LC-polymers such as low polymerization contraction, relatively high modulus, orientation, transformation toughening mechanisms, and fracture resistance. Thus, we propose to develop homogeneous, radiopaque, nanocomposites with near-zero cure-shrinkage that are highly translucent and have increased strength, toughness, and wear resistance over current microfilled composites. The rationale is that radiopaque, non-agglomerated metal oxide nanoparticles can be synthesized and homogeneously dispersed in a LC-dimethacrylate monomer by attaching LC-monomethacrylates to the surfaces. A toughened anocomposite can be produced by means of a ductile, LC-thermoplastic applied to the particle surfaces to dissipate mechanical stresses. Strength, fracture toughness and high modulus will be maintained by a mutually interpenetrating network (IPN) interphase between the nanoparticle LC thermoplastic coating and the highly crosslinked LC matrix. To achieve the objective, three Specific Aims will be carried out: 1. Develop an experimental/theoretical modeling approach to optimized the mechanical properties of nanoparticle composites. 2. Develop model composite systems based on silica nanoparticles. And 3. Develop radiopaque nanocomposites based on Zr-oxide and/or Zr-phosphate reinforced LC-polymer.