Concentration dependence of the storage modulus, E ', was investigated for polyvinylacetate (PVAc) filled with hydroxyapatite (HAP) nanoparticles. The filler volume fraction, v(f), varied from 0 to 0.05 and the E ' and loss tangent, tan delta, were measured below neat matrix T-g at -40 degrees C and above neat matrix T-g at +50 degrees C at 1 Hz. The T-g determined as the position of the maximum on the temperature dependence of tan 6 increased by 14 degrees C compared to the neat PVAc (39 degrees C) by adding 5 vol.% of HAP. At -40 degrees C, the observed small increase of E ' with vf was in agreement with the prediction based on the simple Kerner equation. At +50 degrees C, the increase of E ' with v(f) observed was an order of magnitude greater than that predicted using the simple continuum mechanics model. An attempt was made to explain the observed deviation employing the hypothesis of immobilized entanglements. (c) 2007 Published by Elsevier Ltd.