[en] Using x-ray photon correlation spectroscopy, we examined slow nanoscale motion of silica nanoparticles individually dispersed in entangled poly (ethylene oxide) melt at particle volume fractions up to 42 %. The nanoparticles, therefore, serve as both fillers for the resulting attractive polymer nanocomposites and probes for the network dynamics therein. The results show that the particle relaxation closely follows the mechanical reinforcement in the nanocomposites only at the intermediate concentrations below the critical value for the chain confinement. Quite unexpectedly, the relaxation time of the particles does not further slowdown at higher volume fractions- when all chains are practically on the nanoparticle interface- and decouples from the elastic modulus of the nanocomposites that further increases orders of magnitude.