Deformation and failure of semi-crystalline polymer systems : influence of micro and molecular structure

Abstract

This thesis is composed of four papers in the field of deformation and failure of semicrystalline polymer systems. The main goal is to identify the relationship between mechanical properties and internal micro and molecular structure, which strongly depends on processing conditions applied. In order to achieve this, three main experimental aspects are addressed: (1) morphological characterization, by means of X-ray scattering techniques, (2) determination of intrinsic deformation behaviour, from true stress-strain measurements in uniaxial compression tests, and (3) the macroscopic response in tensile and impact toughness tests. In Chapter 2 the influence of crystallinity, lamellar thickness and molecular weight on intrinsic properties, like yield stress, strain softening and strain hardening are studied for three semi-crystalline polymers: poly(ethylene terephthalate), polyethylene and polypropylene, differing in glass transition temperature relative to ambient temperature. The yield stress is found to be proportional to lamellar thickness and the strain softening is found to be a property of the amorphous phase. Although a direct measurement of the number of entanglements in the solidified polymer is absent, the strain hardening modulus appears to be related to the chain entanglement density. Indications for this relation are found from the influence of crystallization temperature from the melt on the strain hardening behaviour. A decrease in strain hardening is found when higher melt crystallization temperatures are applied. These higher temperatures allowmore pronounced reeling in of the polymer chains upon crystal growth, decreasing the number of trapped entanglements. The fact that themaximum strain hardening modulus found for the poly(ethylene terephthalate) samples equals that of its amorphous sample, indicates a similar relationship as known for the amorphous class of polymers, where strain hardening is proportional to network density. The observation that crystallinity does not influence the strain hardening modulus appears to be related to the fact that, in the polymers investigated, the crystals yield at large deformation, allowing chain transfer and slip without giving a network contribution. Molecular weight and processing conditions are, therefore, found to be the main aspects determining the post yield behaviour. The importance of this intrinsic behaviour on the macroscopic deformation and failure behaviour is demonstrated in Chapter 3, where uniaxial tensile and impact toughness tests are performed on similar polymer samples as studied in Chapter 2. Stability of deformation is qualitatively predicted using a straight-forward analytical modeling approach. Ductile-to-brittle transitions are found either by an increase in yield stress, due to an increase in lamellar thickness, or by a decrease in strain hardening and/or tensile strength, as a result of a lower chain entanglement density. In impact toughness, no brittle-to-ductile transitions were found and only small variations in impact toughness were observed as a result of variations in intrinsic yield stress and strain hardening. In these chapters quiescent processing conditions were used resulting in isotropic structures. In practice, however, semi-crystalline polymers are often subjected to flow in processing conditions, like e.g. injection moulding and extrusion. This results in flowinduced oriented crystalline structures, which influence the deformation and failure behaviour of polymers. In Chapter 4 injection moulding and extrusion techniques are used to introduce oriented structures in polyethylene and polypropylene samples. These oriented structures are characterized along the thickness and flow path of the samples, subjected to different processing conditions (e.g. temperature and flow rate). Deformation is found to be anisotropic and related to the oriented structure. For all polymers studied an increase of extended chains (shish) in the loading direction is proposed to cause an increase in yield stress, and a lamellar structure oriented perpendicular to loading direction leads to an increase in strain hardening. The variations of yield stress and strain hardening from these oriented structures are capable to induce brittle-to-ductile transitions in impact toughness on the moulded samples. Impact toughness enhancement in polyethylene is found to be most efficiently with increasing strain hardening. The effect of orientation on toughness was less pronounced in polypropylene. Using the relations between deformation behaviour and crystalline micro-structure, as identified in the previous chapters, the impact toughness behaviour of calcium carbonate filled polyethylene and polypropylene systems is studied in Chapter 5. Again, the results indicate an influence of flow history on structure development and on resulting mechanical properties. The highest impact toughness, observed in this study, was obtained from injection moulded bars, with intermediate filler content, tested close to the injection gate in flow direction. This behaviour is explained by the presence of an additional flow-induced oriented structure along the sample thickness. This orientation increases strain hardening in flow direction, and, thus decreases localization of strain in the ligaments between the particles. As a result, more energy can be dissipated and toughness is enhanced. The fact that polypropylene is less sensitive to impact improvement by orientation, as found in Chapter 4, explains its modest impact improvement for the calcium carbonate filled blends.