Linear viscoelasticity of disordered polystyrene-polyisoprene block copolymer based layered-silicate nanocomposites

The melt-state linear viscoelastic properties for a series of intercalated nanocomposites are examined. The nanocomposites are based on a short disordered polystyrene-polyisoprene diblock copolymer and varying amounts of dimethyldioctadecylammonium modified montmorillonite. The linear dynamic oscillatory moduli and the stress relaxation moduli are in quantitative agreement and suggest that at short times the relaxation of the nanocomposites is essentially unaffected by the presence of the layered-silicate. However, at long times (or equivalently low frequency), the hybrids exhibit dramatically altered viscoelastic behavior. Hybrids with silicate loadings in excess of 6.7 wt % exhibit pseudo-solidlike behavior, similar to that observed in previous studies of exfoliated end-tethered nanocomposites. On the basis of simple phenomenological arguments, the long time behavior is attributed to the presence of anisotropic stacks of silicate sheets randomly oriented and forming a percolated network structure that is incapable of relaxing completely. These arguments are further supported by the ability of large-amplitude oscillatory shear to orient these nanocomposites and to increase their liquidlike character.