The influence of filler-filler and filler-polymer interactions on the physical properties of silica-filled liquid polyisoprene

The dynamic rheological properties of uncured dispersions of silica in liquid polyisoprene are measured and are used to assess filler-filler networking. An increase in the dynamic storage modulus (G′) is observed with decreasing polyisoprene molecular weight and with increasing silica concentration. For sufficiently high silica concentrations, a low-frequency plateau is observed in the frequency-dependence of G′, indicating the presence of a volume-spanning filler-filler network. The height of this plateau scales with silica volume fraction to the power of 5.2, consistent with several models describing the elasticity of polymer materials filled with fractal clusters of colloidal particles. Treatment of the silica filler surface with a silane coupling agent leads to a decrease in the modulus of the resulting dispersion. The properties of the filler-polymer interface were further investigated by bound polymer measurements and by solid-state ¹H NMR experiments on the bound polymer fractions. The mobility of the polymer segments immediately adjacent to the filler surface is significantly reduced, as evidenced by faster spin-spin relaxation. The bound polymer per m² of silica surface increases with increasing silica concentration, primarily due to entrapment of loosely bound polymer chains in the filler-filler network.