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

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.