Two glass transitions of polyurea networks: effect of the segmental molecular weight

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 ΔT = 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.