Flow induced crystallization of polymers : precursors and nuclei

Abstract

Semi-crystalline polymers cover over two thirds of commercial applications of polymeric materials. With most industrial processing technologies, semi-crystalline polymers are brought in the molten state and then deformed for different reasons: i.e. to create specific properties like a high modulus with fiber spinning or to shape geometrically complex parts as with injection molding. The deformation not only accelerates the crystallization kinetics but also can cause crystalline structures to change from isotropic spherulites to the highly oriented cylindrical structures (e.g. shish-kebab) which determine the ultimate properties. Therefore, understanding the interplay between flow fields and the resulting crystalline structures is of great importance. It enables the design of processing procedures and tailoring final product properties. The objective of this thesis is to reveal how a flow field affects the crystal structures by studying the early stages of the crystallization process which are dominated by the creation of shear-induced precursors and nuclei. Polymer crystallization involves two steps: nucleation and growth. Nucleation provides quantitative (more or less) and qualitative (isotropic or oriented) nuclei and these are the templates for further crystal growth that, ultimately, will fill the full space. The nucleation step is very sensitive and is often controlled by additives (nucleating agents) or/and imposing a flow field. The formation and features of nuclei are, therefore, the key factors that determine the crystalline structures. Probing and quantifying nuclei is the main work of this thesis. According to the resulting morphology, nuclei can be divided into two groups: pointlike nuclei and fibrillar nuclei. The former give rise to spherulites, the latter mainly induce oriented structures like the well-known shish-kebab; i.e. fibrils with transverse lamellae. Oriented nuclei can be further classified into row nuclei and shish nuclei by whether they can be directly observed with X-ray characterizations. In the first part of this thesis, the creation of point-like nuclei is studied for an isotactic polypropylene with a nucleating agent, with and without applied shear deformation. For such a system, nucleation is dramatically enhanced by both the nucleating agent (U-Phthalocyanine) and the flow. The nucleation density is that large and, therefore the crystallite size so small that optical microscope is not suitable to count nuclei numbers. Therefore, a suspension-based rheological model is used to quantify the nuclei density. In the second part, precursors of row nuclei in a bimodal Polyethylene (PE) are studied. Since row nuclei cannot be observed directly, the crystallization step is triggered by increasing the pressure which raises the equilibrium melting temperature and thus the under-cooling: a so-called pressure quench. It is shown that shear-induced precursors can be generated at temperatures close to the nominal melting temperature and only relax very slowly. Next, the formation of shish nuclei during flow is studied by using fast (30 frame/s), time resolved X-ray scattering in combination with rheological measurements. It is found that a critical shear rate exists for the formation of shish nuclei within very short flow times (0.25 s). Shish precursors are formed during flow and these were found to develop into shish afterwards, during or after flow depending on the flow strength. Besides the external effects, the nuclei formation is also influenced by the molecular structure. Therefore, in the last part, the effect of the molecular architecture on shearinduced crystallization is studied for isotactic polypropylene (iPP) and two propylene/ethylene random copolymers with varying ethylene monomer contents. These three grades had very similar rheological behavior; there is only a small but important difference in the longest relaxation time. Flow enhanced nucleation density was found to be the lowest for the homopolymer which is due to the shorter longest relaxation time. However the reduced growth rate due to the added ethylene monomer leads to slower overall crystallization kinetics of the random copolymers compared to the homopolymer at similar thermal conditions.