• Rytka, Christian

Abstract

Surfaces of polymer products are increasingly being functionalized with micro- and nanostructures using mass replication techniques such as injection moulding. The complex transcription process re-quires a deep understanding of material and process interrelations in order to accurately replicate the desired functional structures over many hundreds of thousands of production cycles. The goal of this thesis was the investigation of differences in filling and demoulding behaviour of macro-, micro-, and nanostructures, focussing on the influences of the type of injection moulding process, processing pa-rameters and polymer properties. For this purpose the replication quality of functional structures was investigated using four different moulding processes employing polymers differing in rheology and wetting behaviour. Representative 2D and 3D micro- and nanostructures were transferred into various mould insert materials (nickel, brass, steel and high performance polymers) and replicated by iso- and variothermal injection moulding with and without compression and compared with filling simulations. Importantly, it was found that a parallel compression phase reduces the internal pressure and stresses in the cavity leading to less demoulding damage but without significant influence on replication fideli-ty. Moreover, variothermal heating was favourable especially for filling of high aspect ratio structures. However, clear differences in replication were shown between the glass transition and no-flow tem-perature as upper mould temperature, also in context of the influence of holding pressure. With regards to geometric correlations, a parabolic relationship is demonstrated between the replicated height and the structure width for the replication of microstructures. On this scale, the polymer melt viscosity is clearly more relevant than capillary effects as flow resistance and frozen layer formation are the main reasons for incomplete filling. On the nanoscale, capillary effects become increasingly dominant depending on the surface energy of the polymer and the parabolic correlation is superim-posed by wetting phenomena. The dewetting potential Ωs of a polymer is proposed as a simple rationale for estimation of the repli-cability. The value of Ωs was determined by integrating the spreading coefficient over the range from melt processing temperature to no-flow temperature. Ωs correlates well with the replicated height for four different structures covering both the micro and the nano range for different mould surfaces and polymers with different spreading coefficients. It is clearly shown that a lower Ωs leads to a better replication accuracy. As flow simulations can cut down both costs and time for the development of polymer parts with func-tional surfaces, experimental trials were compared with detailed 3D multiscale Moldflow simulations. Additionally, thermal transfer and 2D filling simulations were carried out with Comsol. By adjusting the heat transfer coefficient and the transition temperature it was possible both for micro- and nanostructures to achieve a good correlation with experimental findings at different processing condi-tions. The macroscopic model with a microstructure can be scaled down in volume and number of elements to save computational time as long as boundary conditions such as the flow front speed are correctly transferred. Finally, the developed knowledge of the filling and demoulding, focussing on heat transfer and no-flow temperature was successfully applied to the reproducible production of nanostructured samples for cell growth tests, and a fast prototyping method based on pattern transfer onto high performance polymer inserts was established. The novel experimental and simulated findings about micro- and nanostructure replication provide a valuable contribution to the development of injection moulded surface-structured polymeric products in the areas of life science and optics / photonics.

Topics

• impedance spectroscopy
• microstructure
• surface
• polymer
• nickel
• simulation
• melt
• glass
• glass
• laser emission spectroscopy
• steel
• transmission electron microscopy
• surface energy
• brass
• melt viscosity