Nonlinear time-dependent response of polypropylene/nanoclay melts: Experiments and modeling

Observations are reported on polypropylene/compatibilizer/nanoclay hybrid melts with a fixed clay/compatibilizer proportion 1:2 and various concentrations of filler (from 0 to 5 wt.%) in start-up shear tests with a constant strain rate and relaxation tests at various strains (in the interval of shears between 1 and 20). It is shown that growth of nanoclay content leads to formation of stress overshoot in start-up tests and slowing down of the relaxation process (which is strongly affected by strain). A constitutive model is derived for the time-dependent response of nanocomposite melts under three-dimensional deformations with finite strains, and its adjustable parameters are found by fitting the experimental data. It is demonstrated that some critical concentration of nanoclay exists (about 1 wt.%), at which the mean activation energy for rearrangement of chains and its standard deviation are minimal. The presence of a threshold concentration of filler is confirmed by observations in uniaxial tensile tests on solid nanocomposites.