Understanding of the mechanical response of semicrystalline polymers based on the block-like substructure of crystalline lamellae
We investigated the mechanical response of a range of PE and sPP samples as models of semi-crystalline polymer systems under tensile deformation. Splitting the total tensile deformation into elastic and plastic parts shows four characteristic points (A,B,C and D) from the true stress-true strain curves, where the differential compliance and recovery property change. It can be concluded that 1.the critical strains at the points A and B are mainly due to the crystalline phase representing the coupling and slips of blocks as well as lamellae. 2.the critical strain at C reflects a property of the amorphous phase. It is the maximum elasticity of the ideal semi-crystalline polymer system. At point C, a critical stress is reached which destroys the lamellar crystallites, followed by a formation of fibrils. different crystallinity and testing conditions do not change the entanglement state of the amorphous regions and thus the critical stains. 3.the critical strain D is related to the molecular weight, the molecular chain structure and the testing temperature. It is thus a property of the polymer chain itself. Intralamellar coarse slip mechanisms provide sufficient degrees of freedom to accomplish a homogeneous strain distribution between crystalline and amorphous phase. It is the basic consequence of the critical strain law. However, the situation changes at point C. When the deformation is larger than the strain at point C, the strain distribution is no longer homogeneous and the dominant deformation mechanism changes to a destruction(melt) and construction (recrystallization) process. The coupling between the blocks and lamellae provides the physical basis of the modulus and the yield stress in a semi-crystalline polymer system. The investigations of the dynamic mechanical relaxation show the existence of an alpha process in sPP which can only be understood in terms of the block slip motions inside the lamellar crystallites.
semi-crystalline polymer, deformation mechanism, polyethylene, syndiotactic polypropylene, mechanical property
Networked Digital Library of Theses and Dissertations