Flow-induced crystallisation of polymers containing nucleant particles
thesisposted on 31.01.2017, 04:09 authored by Phillips, Andrew
Thermoplastic polyolefins are typically processed into their final form by injection moulding or extrusion technologies. These processes impart complex flow and temperature histories to the solidifying polymer which in turn can dramatically alter the final physical properties of the component. In particular, if the flow intensity is sufficient, a highly anisotropic crystalline architecture, termed shish-kebab can form. A highly oriented thread-like core is considered to form first (shish), which then provides a template for the formation and growth of oriented chain folded crystalline lamellae (kebabs). Therefore, shish formation is one the key components to understanding morphology development under flow conditions. The crystallisation of semicrystalline polymers is further complicated by the intentional addition of nucleating agents. Nucleants are used to enhance or override polymer crystallisation with the aim of (a) speeding up processing and (b) altering the solid-state morphology and hence final properties of the produced component. However, their influence on flow-induced crystallisation, in particular on shish formation and relaxation, are currently not well understood. Accordingly, the present study aimed to provide new insight into how nucleating agents influence flow-induced crystallisation. This project employed in-situ time-resolved small and wide angle x-ray scattering (rheo-SAXS/WAXS) and Fourier transform infrared spectroscopy (rheo-FTIR) techniques to investigate shish formation and relaxation as well as the morphology development during subsequent crystallisation. Complementary morphological information was obtained using optical microscopy (OM), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) techniques. Isotactic polypropylene (iPP) was selected as the model matrix material due to its following advantageous characteristics: its general relevance to industrial processes, its convenient range of crystallisation kinetics and its ability to form different crystalline polymorphs. Four particulate nucleating agents which had substantially different effects on the iPP matrix, were investigated. During shear flow, surprising sharp meridional streaks were observed in conjunction with the more commonly observed equatorial streaks in the SAXS patterns. No associated lamellar scattering was observed in the SAXS patterns and no crystalline reflections were observed in the WAXS patterns. These results showed that shish contain non-crystalline structural singularities along their length while they form. This provides strong evidence that shish initially form from the aggregation of point-like nuclei into oriented threads. This view contradicts the classical view that shish form from the densification of extended chains or segments of extended chains produced by an abrupt coil-to-stretch transition. All nucleants examined in this thesis (a) decreased the critical shear rate required to form shish and (b) for a given shear rate, increased the density and dimension of shish formed. Therefore, the experimental results revealed that particles assist shish formation under flow conditions. Particle aspect ratio was shown to have the greatest influence on shish formation. This was explained by the manner in which different shaped particles interact with the flow field and modify the local strain environment near the surface of the particle. iPP is known to crystallise in three polymorphic forms: alpha-iPP (monoclinic), beta-iPP (trigonal) and gamma-iPP (orthorhombic). The specific phase formed and its associated morphology is known to have a large influence on the final properties of the component. Therefore, the combined influence of flow condition and nucleant on the polymorphic behaviour of iPP was also investigated in this study. In pure iPP, flow was found to induce beta-iPP. However, flow was surprisingly found to suppress beta-iPP in nucleated material, even when the nucleant specifically induced high quantities of beta-iPP during crystallisation in the absence of flow. In addition, the combined influence of flow and nucleant was found to promote gamma-iPP. These results therefore have important implications for the processing of iPP containing nucleants, the addition of which can influence both the level of orientation achieved as well as the specific crystalline phases formed in the final component.