The effects of surface tension and entanglements in polymer films: Capillary driven flows, The Marangoni effect and crazing

Abstract

This is a 'sandwich thesis' consisting of four publications I contributed to during my M.Sc. work. These papers are the results of three types of experiments.

Paper 1 studies the formation of non-uniform spin-cast polymer films. Spincoating is widely used to prepare thin polymer films of reproducible thickness. Typically spincoating produces highly uniform films, however in certain circumstances the process results in films with non-uniform surface topographies. The origin of such topographies is not fully understood and the formation of non-uniform films represents a practical problem in both research laboratories and industrial settings. In Paper 1 we find that the formation of non-uniform films is dependent on temperature. Furthermore, our results indicate that surface instabilities form as a result of the Marangoni effect. Finally, we demonstrate that non-uniformities in spin-cast films can be avoided simply by spincoating at lower temperatures.

In Papers 2 and 3 we study the capillary driven levelling of polymer films with non-uniform surface geometries and compare our results to the theoretical predictions of the two-dimensional capillary-driven thin film equation. In Paper 2 we prepare polymer films with small surface perturbations and track their evolution above $T_g$ as the surface flattens. We find that all perturbations approach a universal self-similar attractor at long times, as predicted by theory. Our results also show that the time taken for the perturbations to convergence to the attractor depends on the initial volume of the perturbation. In Paper 3 we prepare samples with a rectangular trench geometry and follow their evolution above $T_g$ as surface forces cause the trench to fill in. At long times we observe a change in the levelling dynamics that is associated with a change in the boundary conditions governing the flow. In Paper 4 we use crazing experiments to probe two types of non-equilibrium entanglement networks. First, we study spincast polymer films and find that chains are stretched compared to equilibrium Gaussian chains. Furthermore, we find that the entanglement network relaxes on timescale on the order of one reptation time. Next, we stack two films in the glassy state to create a bilayer. Chains on either side of the mid-plane of the bilayer suffer a loss of entropy because of their restricted conformations. In the melt, the interface heals. We find it takes less than one reptation time for the bilayer film to become indistinguishable from a single film.