A series of poly(methyl methacrylate) (PMMA) blends with rigid ladderlike polyphenylsilsesquioxane (PPSQ) were prepared at weight ratios of 100/0, 95/5, 90/10, 85/15, and 80/20 by solution casting and then hot-pressing. Their rheological properties have been studied under both dynamic shear and uniaxial elongation conditions. Their rheological properties depend on the compositions. The storage modulus, G', loss modulus, G", and dynamic shear viscosity, eta*, of the PMMA/PPSQ 95/5 blend were slightly lower than those of pure PMMA. However, the values of G', G", and q* for the other PMMA/PPSQ blends are higher than those of PMMA. The G' values increase with an increase in PPSQ content from 5% through 15% PPSQ at low frequencies and then drop as the PPSQ content increases to 20%. Uniaxial elongational viscosity (eta(E)) data demonstrate that PMMA/PPSQ blends exhibit slightly weaker (5% PPSQ) and much weaker (10% PPSQ) strain-hardening than PMMA. In contrast, the PMMA/PPSQ 85/15 blend shows strain-softening. Neither strain-hardening nor strain-softening was observed in the 80/20 blend. The special rheological properties for the 95/5 blend is probably due to a decrease in PMMA entanglements brought by the specific PMMA-PPSQ interactions. Rheological properties of PMMA/PPSQ blends with higher PPSQ content (>= 10%) are mainly affected by formation of hard PPSQ particles. (c) 2007 Wiley Periodicals, Inc.