Deformation and Failure in Semi-Crystalline Polymer Systems

This study deals with the relation between the microstructure of semi-crystalline polymers and their intrinsic deformation and fracture behaviour. For this purpose, the deformation and failure of well-characterised PET, PE and PP samples have been investigated thoroughly. Compression tests on amorphous and cold crystallised PET showed that the strain hardening remained unchanged, despite the development of a crystalline phase by cold crystallisation. This suggests a direct relation between entanglements and strain hardening, similar to amorphous polymers, and the slip of crystallites after yielding. True stress-strain curves of annealed PE confirm that strain hardening is not affected by the degree of crystallinity. However, melt crystallisation and a lower molecular weight grade for PE and PP resulted in a distinctly lower strain hardening modulus. This observation confirms that entanglements affect the strain hardening behaviour, since melt crystallisation and a reduction in molecular weight promotes a loss of entanglements during lamellar folding. Tensile tests were performed to examine the failure behaviour. The melt crystallisation process has proved to be of influence on failure, as its lower strain hardening appeared not to be able to stabilise strain localisation. Moreover, an increase in crystallinity due to an increase in yield stress results in a larger localisation of deformation. Since a higher molecular weight polymer grade hampers the formation of a crystalline phase, localisation after yielding will be less. This and a larger strain hardening modulus enlarge the chance of ductile behaviour in tension for polymers with a larger chain length. Finally, an attempt was made to predict the intrinsic deformation behaviour of PP and PE. The compressible Leonov model appeared to be capable of generating an appropriate description of the intrinsic deformation behaviour over a range of strain rates. For PE, even the peculiar double yield behaviour was modelled adequately by the compressible Leonov model.