Two structurally and chemically different polyhedral oligomeric silsesquioxane (POSS) molecules with different solubility parameters, dodecaphenyl POSS and trisilanolphenyl POSS, were melt-blended with polyphenylsulfone (PPSU), and the effects on the material processing, rheology, and thermomechanical properties were investigated. Process monitoring and capillary rheometry revealed enhancements in the processing melt flow and a reduction in the nanocomposite viscosity with only a small addition of POSS. The magnitude and concentration dependence of rheological modification were shown to depend on POSS structure. Although small increases in the tensile modulus were observed, reductions in the tensile elongation properties were observed at high POSS loading levels. Analysis of the nanocomposite fracture behavior and surface morphology provided evidence for the surface phase segregation of both types of POSS, but the extent of segregation was related to the POSS structure. The degree of interfacial adhesion in the POSS/polymer composites and the resultant rheological behavior were related to the POSS composition and predicted solubility parameters. No measurable effect on the glass-transition temperature or molecular weight of PPSU was observed, whereas small improvements in the thermal degradation behavior were obtained with the incorporation of POSS. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121:2945-2956, 2011