Thermo-viscoelastic response of nanocomposite melts

Observations are reported in shear oscillatory tests with small strains (the frequency-sweep mode) on a hybrid nanocomposite melt [thermoplastic elastomer (ethylene-octene copolymer) reinforced with various concentrations of montmorillonite nanoclay] at temperatures ranging from 150 to 210 °C. A constitutive model is developed for the viscoelastic behavior of a nanocomposite melt at arbitrary three-dimensional deformations with small strains. The melt is treated as an inhomogeneous, permanent polymer network with sliding junctions (entanglements and physical cross-links at the surfaces of nanofiller). It is assumed that macro-deformation induces sliding (plastic flow) of junctions between strands with respect to their reference positions, and the strain energy of the network depends on strain tensors for elastic and plastic deformations. Stress-strain relations are derived by using the laws of thermodynamics. These equations involve four adjustable parameters that are found by fitting the observations. It is demonstrated that (i) the governing equations correctly reproduce the experimental data and (ii) the material parameters change consistently with temperature and concentration of filler.