Unusual Rheological Properties of Lightly Crosslinked Polydimethylsiloxane

The rheological behavior of elastomers of polydimethylsiloxane vinyl-terminated has been investigated in both rotational and oscillating modes. Partially crosslinked liquid rubbers have been prepared, varying the curing agent-to-elastomer weight ratio p. Viscoelastic fluids with unusual rheological properties have been obtained. For small p, zero-shear viscosity and storage dynamic modulus G′(ω) at low frequencies are obtained, lower than those of reference liquid elastomers. Outcomes are discussed according to the polymer tube model theory and an extension at nanoscale of the Einstein equation of viscosity proposed. The unentangled crosslinked nanodomain model has been proposed. At light networking, isolated crosslinked nanodomains would form and the longer chains will be preferentially bonded. This will determine a decrease in the entanglement density in the matrix. On the nanodomain surface the dangling chains will not be long enough to give stable entanglements, determining the formation of a free volume at the interface polymer matrix-nanoparticles.