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We present neutron spin echo (NSE) experiments on the single chain dynamic structure factor of long poly(ethylene oxide) (PEO) linear chains in the presence of poly(methyl methacrylate)-based single-chain nanoparticles (SCNPs). A complementary structural characterization of the system discards a significant interpenetration of the components and reveals a close to globular conformation of the SCNPs when surrounded by PEO chains. Analogous NSE measurements on blends of PEO with the linear precursor chains of the SCNPs are taken as a reference for the dynamics study. For short times (below approximately 5 ns) PEO in both mixtures exhibits slowed down Rouse dynamics with respect to bulk PEO behavior. The similar deceleration observed in both environments suggests this effect to be due to the large dynamic asymmetry in the mixtures as evidenced by DSC experiments. More interestingly, the NSE results at longer times reveal a spectacular increase of the explored volume of PEO chains in the all-polymer nanocomposite that is absent in the blend with linear precursor chains: assuming the reptation model, the apparent tube diameter confining PEO chain motions in the nanocomposite is about 80% larger than in bulk PEO. This can be considered as the first NSE direct observation of disentanglement in a nanocomposite