Modelling the non-linear mechanics of polymeric microcapsules and hydrogels

Polymeric parts are used in a wide range of products from textile and cosmetics to all types of electronic devices. The quality and performance of these parts depend highly on their mechanical behaviour. Therefore, it is important to understand their mechanics and to be capable of predicting their performance under the conditions specific to their use. In this work the mechanical behaviour of two types of polymeric parts in different applications are investigated. The first type is a sample of polymeric microcapsules that serve as perfume oil containers in washing detergents. During the washing, they are expected to remain stable. However, they should break and release their contents during the storage or the wearing time of the washed textiles, hence extending their softness and fresh smell. %In chapter one, their mechanical behaviour under compression, as a likely breakup scenario, is characterised. In chapter two the dynamics and the deformation of such capsules under shear flow are studied. The second system is a hydrogel actuator which has a bilayer structure. One layer is composed of aligned and the other of randomly oriented poly(N-isopropyl acrylamide) fibers. When the bilayer mat is put in water it folds into a stable 3D structure, which can be used as a scaffold for cell culture and in tissue engineering. In this work that is presented in four chapters, numerical methods are used to understand the mechanics underlying the deformation of the studied polymeric parts. The corresponding simulations complement the mechanical tests, by making the calculation of the mechanical properties possible, and contribute to the explanation of the experimental observations. Chapter one is the extended abstract of the following three chapters that contain the full text of our publications. In addition to present a detailed outline of the contents of each publication, it describes the relationship between them in a broader context. Furthermore, it explains the contribution of the thesis to the topics that are addressed in this research as well as the author's contributions to each publication. In chapter two, finite element analysis (FEA) together with nano-indentation experiments is applied to study the mechanics of perfume containing microcapsules under uniaxial compression. An elastic-perfectly plastic model is proposed to reproduce the results of the mechanical tests. Furthermore, with the help of FEA calculations the corresponding mechanical properties are approximated. Then, using the detailed stress distribution maps, a breakup mechanism is proposed. At the end, this mechanism is validated by predicting the shape of the broken capsules. The focus of chapter three is on the deformation dynamics and the breakup of such capsules in a generic shear flow. In this study, the shells of the capsules have a finite thickness, and their mechanics are described by an elastic or elastic-perfectly plastic constitutive laws. It is shown that under the hydrodynamic stresses, the originally circular elastic…