Looking for the simplicity in polymer networks - Structure changes and comparative analysis of theoretical approaches to deformation of semi-crystalline polymers

To establish relationships between the molecular structure of polyolefines and their physical characteristics which determine possible commercial applications, structural changes and tensile deformation response up to deformations beyond the natural draw ratio were investigated using a variety of experimental approaches. True stress–strain curves were measured at different temperatures so as to estimate the available effective network density, which will eventually define the failure mode of the material under investigation. Analysis of the deformation by means of tensile strain hardening, assuming the Haward–Thackray spring dashpot decoupling assumption by means of Edward-Vilgis’ non-Gaussian rubber-elastic slip-link model, reveals the role of transient and fixed network nodes. It was established by differential scanning calorimetry and X-ray diffraction analysis that the transformation from lamellar to fibrillar morphology passes through the several pronounced stages: deformation of initial lamellae (λ < 1.5); destruction of lamellar structure through the tilt; slippage of molecules in the crystallites; simultaneous formation of fibrils with structural characteristics depending on the molecular structure and on deformation conditions; deformation of the formed fibrillar structure; tilting – formation of chevrons for high molecular weight low density polyethylene or slippage of fibrils and void formation. Distinction between fixed and transient slip link network contributions reveals neatly that although there is a slight drop in the fixed link network density with increasing temperature, this contribution remains of the same order of magnitude and predominantly related to the molecular mass. This observation enhances the idea that the network of entanglements that remain fixed on the time scale of the measurement is actually entropic in nature and does not depend greatly on temperature. Considering slip link contributions, one can find them to be prominently present at ambient temperature and their importance becomes negligible at elevated temperature, except for the case of the most crystalline material