Flow and structure

The properties of materials not only depend on the intrinsic material characteristics but also on the microstructure. The importance of microstructure is particularly clear for the case of immiscible polymer blends. For example, to obtain a low cost material with good barrier properties by mixing a small amount of a barrier polymer with a commodity polymer, a layered structure is required whereas increasing the impact resistance of a polymer part by addition of rubber particles requires a very fine dispersion of the latter.

Although many materials are used in the solid state, processing generally occurs in the liquid state. In processing operations such as extrusion, injection moulding, 3D printing, etc. the material is subjected to flow, which leads to changes in the microstructure. In the case of polymer blends, the dispersed phase (droplets) can deform or breakup in intense flows and retract or coalesce in weak flows. Subsequently, the microstructure is frozen in during solidification of the material due to for example crystallization or cross-linking. The interaction between flow and microstructure provides an excellent opportunity to tailor structure by designing appropriate flow conditions. However, this is not a straightforward task since structure development depends on the one hand on the flow properties of the material (viscosity, elasticity) and, in the case of multi-phasic materials, the interactions between the components and on the other hand the flow type and flow intensity.

In addition, interactions between flow and material characteristics should be taken into account since flow properties also affect flow during processing. An example of this is the pressure-driven flow in a tube in which the flow rate and hence velocity in the tube depend on the fluid viscosity. It is the purpose of the course “Flow and Structure” to present the tools and techniques to analyze and design structure development in flow with the aim of optimizing product properties. Thereto, three main aspects are of importance namely the analysis of the flow in processing equipment, either by analytical or numerical methods, the relationships between flow and structure development and the characterization of flow properties via rheology.

Rheology is the study of the deformation and flow of materials. In rheology, constitutive equations, which are relations between stress and deformation rate are developed and the rheological parameters in these equations are experimentally determined in the part of rheology which is called rheometry. By incorporating structural information in the constitutive equations, rheology also allows to link flow behaviour and structure. Hence, rheology can also be used as a tool to probe microstructure. For example, in immiscible polymer blends, the presence of droplets causes an extra elasticity, which can be described by the Maxwell model and allows to determine the droplet size from a small amplitude oscillatory measurement. It should be noted that whereas during processing the flow is mostly very complex, in rheology model flows such as shear flow or uniaxial elongational flow are applied to allow a separate characterization of the different rheological parameters. The relation between flow, structure and material characteristics is schematically depicted in Figure B.1.