The viscoelastic and viscoplastic behavior of polymer composites: polycarbonate reinforced with short glass fibers

Abstract

Observations are reported on polycarbonate filled with various amounts of short glass fibers (i) in tensile tests with a constant strain rate and (ii) in oscillatory torsion tests at room temperature. Constitutive equations are derived for the viscoelastic and viscoplastic responses of a polymer composite. The composite is treated as an equivalent network of chains bridged by junctions (entanglements and glass fibers). The network is thought of as an ensemble of meso-regions with arbitrary shapes and sizes. With reference to the theory of cooperative relaxation, the viscoelastic response is attributed to rearrangement of meso-domains that occurs at random times when the regions are thermally activated. The viscoplastic behavior is associated with displacements of meso-regions with respect to each other. The rate of sliding of meso-domains is proportional to the rate of macro-deformation. Constitutive equations for isothermal deformation of a polymer composite are derived by using the laws of thermodynamics. The stress–strain relations are determined by five adjustable parameters that are found by fitting the experimental data. The effect of the filler content on the material parameters is studied in detail.