Structural interpretation of the strain-rate, temperature and morphology dependence of the yield stress of injection molded semicrystalline polymers

The yield stress of polypropylene specimens with different initial microstructural states is investigated. These latter were obtained by systematic variations of the processing conditions in injection molding, resulting in different laminated skin-core structures. The morphology of the molded specimens was characterized by polarized light microscopy (full specimen), differential scanning microscopy (core layer) and wide-angle X-ray scattering (skin region). The yield stress was evaluated at different nominal strain-rates (1.67×10−3 to 150 s−1) and temperatures (23, 40 and 60 °C). The experimental results are analyzed in the frame of Eyring's viscous flow and lamellar cluster models, being established the relationships between the activation volume and enthalpy upon the initial morphological state of the specimens. It is proposed that similar deformation mechanisms operate in both skin and core layers, although with distinct temperature and strain-rate sensitivities. The morphology dependence of the yield stress at different temperatures and strain-rates is established in terms of a laminate composite approach. Yielding is interpreted based on the deformation of crystalline lamellae by the pulling out action of both the molecular chains that are anchored in the inter-lamellar amorphous phase and the tie-molecules. The deformation mechanism operating at the skin and core layers are unified by an elastic-beam mechanical analogue that is able of explaining the morphology, temperature and strain-rate dependences of the yield stress.