Poly[isobutyl methacrylate-co-butanediol dimethacrylate-co-3-methacrylyl-propylheptaisobutyl-T8-polyhedral oligomeric silsesquioxane] [P(iBMA-co-BDMA-co-MA-POSS)] nanocomposites with different crosslink densities and different polyhedral oligomeric silsesquioxane (MA-POSS) percentages (5, 10, 15, 20, and 30 wt%) were synthesized by radical-initiated terpolymerization. Linear [P(iBMA-co-MA-POSS)] copolymers were also prepared. The viscoelastic properties and morphologies were studied by dynamic mechanical thermal analysis, confocal microscopy, and transmission electron microscopy (TEM). The viscoelastic properties depended on the crosslink density. The dependence of viscoelastic properties on MA-POSS content at a low BDMA loading (1 wt%) was similar to that of linear P(iBMA-co-MA-POSS) copolymers. P(iBMA-co-1 wt% BDMA-co-10 wt% MA-POSS) exhibited the highest dynamic storage modulus (E') values in the rubbery region of this series. The 30 wt% MA-POSS nanocomposites with 1 wt% BDMA exhibited the lowest E'. However, the E' values in the rubbery region for P(iBMA-co-3 wt% BDMA-co-MA-POSS) nanocomposites with 15 and 30 wt% MA-POSS were higher than those of the parent P(iBMA-co-3 wt% BDMA) resin. MA-POSS raised the E' values of all P(iBMA-co- 5 wt% BDMA-co-MA-POSS) nanocomposites in the rubbery region above those of P(iBMA-co-5 wt% BDMA), but MA-POSS loadings < 15 wt% had little influence on glass-transition temperatures (T-g's) and slightly reduced T-g values with 20 or 30 wt% POSS. Heating history had little influence on viscoelastic properties. No POSS aggregates were observed for the P(iBMA-co-1 wt% BDMA-co-MA-POSS) nanocomposites by TEM. POSS-rich particles with diameters of several micrometers were present in the nanocomposites with 3 or 5 wt% BDMA. (C) 2004 Wiley Periodicals, Inc.