Precursors and nuclei, the early stages of flow-induced crystallization

Flow-induced crystallization (FIC) is the main factor determining the properties of melt-processed semicrystalline polymer products. Therefore it has received much attention in scientific research, both experimental and theoretical. Although the essential phenomena in FIC are slowly being unraveled, a comprehensive theoretical framework, able to explain all these phenomena, is still lacking. Crystallization of polymers can be divided into three regimes: 1. quiescent crystallization, in which spherical structures (spherulites) are formed, 2. flow-enhanced nucleation, leading to a higher number density of spherulites 3. formation of oriented fibrillar nuclei, which are a template of anisotropic crystalline structures. Upon increasing the rate or duration of flow, transitions from regime 1 to regime 2 and regime 3 can be observed. The objective of this thesis is to investigate how flow-enhanced nucleation (regime 2) can be modeled from a rheological point of view, including the coupling between the structure formed and the viscoelastic behavior of the melt. The results of this thesis are twofold. First, the rheology of polymer melts in the late stages of crystallization, characterized, in regime 2, by growth of spherulites, is captured by a viscoelastic suspension model. Secondly, flow-enhanced nucleation in the early stages, which determines the subsequent spherulitic structure development, is modeled. A local and a global formulation of this phenomenon are compared. The local formula- tion offers a consistent theoretical concept for the processes of creation and nucleation of flow-induced precursors (subcritical nuclei). However, it is not yet able to explain experimental observations. The more empirical global formulation, on the other hand, agrees very well with experimental data. Conclusions are drawn from these results and recommendations for future research are given