Non-linear viscoelastic behavior and damping factor of polypropylene/clay nanocomposites

Abstract

The non-linear viscoelastic properties of pure polypropylene and its clay nanocomposites are studied to establish structure–property relationship in conjunction with clay concentrations. First, flow birefringence is performed through a slit-die to obtain centerline principal stress difference during elongational flow for clay nanocomposites. The centerline stress profile of clay nanocomposite reveals additional viscoelastic nature even at low silicate concentrations, while similar short-time chain relaxation is observed. The effects of higher clay concentrations are further examined during the simple shearing flow to consider damping properties of the clay nanocomposites. The step strain, dynamic shear and steady shear are performed. All the samples show time-strain separable melt flow behavior adequately demonstrated through Wagner’s exponential damping function. The damping coefficient is found to be strongly dependent on clay percentage revealing viscoelastic differences therein. We have also used a time-strain separable Kaye-Bernstein Kearsley Zapas (K-BKZ) type constitutive equation to predict steady shear stress. The suggested constitutive model satisfies simple shear at lower fractions of clay while the damping function behaves similar to pure polymer thought to result from the absence of filler–filler interactions and chain length degradation. The unusual rheological behavior for maximum clay concentration studied is explained on experimental as well as theoretical basis. Thus, the results of this investigation would improve the theoretical understanding of possible molecular orientations at different clay concentrations during elongational and shearing flows.