**Abstract**

Polymer–nanoparticle composites (PNCs) play an increasing role in technology. Inorganic or organic nanoparticles are usually incorporated into a polymer matrix to improve material properties. Polyurea is a spontaneously occurring PNC, exhibiting a phase segregated structure with hard nanodomains embedded in a soft (elastically compliant) matrix. This system shows two glass transitions at *T*_{g1} and *T*_{g2}. It has been argued that they are related to the freezing of motion of molecular segments in the soft matrix (usual polymer α-glass transition at *T*_{g1}) and to regions of restricted mobility near the hard nanodomains (α′-process) at *T*_{g2}, respectively. We present detailed dynamic mechanical analysis (DMA) measurements for polyurea networks with different segmental lengths *l*_{c} (2.5, 12.1, 24.5 nm) of the polymer chains, *i.e.* different volume fractions *ϕ*_{x} (0.39, 0.12, 0.07) of the hard domains. The two glass transitions show up in two distinct peaks in tan*δ* at *T*_{α} and *T*_{α′}. Analysing the data using a Havriliak–Negami term for the α- and α′-relaxation, as well as Vogel–Fulcher dependencies for the corresponding relaxation times, it is found that the α-glass transition at *T*_{g1} increases strongly (up to Δ*T* = 70 K) with increasing *ϕ*_{x}, whereas the α′-transition at *T*_{g2} remains unchanged. At *ϕ*^{c}_{x} ≈ 0.19 the two curves intersect, *i.e. T*_{g1} = *T*_{g2}. This value of *ϕ*^{c}_{x} is very close to the percolation threshold of randomly oriented overlapping ellipsoids of revolution with an aspect ratio of about 1:4–1:5. We therefore conclude that around 19% of the hard nanodomains polyurea changes from a system of hard nanoparticles embedded in a soft matrix (*ϕ*_{x} ≤ *ϕ*^{c}_{x}) to a system of soft domains confined in a network of percolated hard domains at *ϕ*_{x}≥ *ϕ*^{c}_{x}.