A mixture of two epoxy resins, tetraglycidyl 4,4'-diaminodiphenyl methane and bisphenol-A diglycidylether, cured with 4,4'-diaminodiphenyl sulfone, was used as matrix material for high-performance epoxy hybrid nanocomposites containing organophilicly modified synthetic fluorohectorite and compatibilized liquid six-arm star poly(propylene oxide-block-ethylene oxide) (abbreviated as PPO). The hydroxy end groups of the poly(propylene oxide-block-ethylene oxide) were modified, yielding a six-arm star PPO with an average of two pendant stearate chains, two phenol groups, and two hydroxy end groups. The alkyl chains of the stearate end groups played an important role in tailoring the polarity of the polymer. Its phenol end groups ensured covalent bonding between liquid polymer and epoxy resin. Two different organophilic fluorohectorites were used in combination with the functionalized PPO. The morphology of the materials was examined by transmission electron microscopy. The hybrid nanocomposites were composed of intercalated clay particles as well as separated PPO spheres in the epoxy matrix. As determined by dynamic mechanical analysis, the prepared composites possessed glass-transition temperatures around 220degreesC. Although the tensile moduli remain unaltered, the tensile strengths of the hybrid materials were significantly improved. The relatively high fracture toughness of the neat resin, though, was not preserved for the hybrid resins. Scanning electron microscopy of the fracture surfaces revealed extensive matrix shear yielding for the neat resin, whereas the predominant fracture mode of the hybrid nanocomposites was crack bifurcation and branching. (C) 2004 Wiley Periodicals, Inc.