Pattern formation in thin polymer films
Creators: name: Amarandei, George
Publisher: name: University of Limerick
Year: 2023
Subjects: Classical physics

description: The dewetting of thin polymer films placed on solid substrates is of large technological and scientific importance as it can affect their integrity. The dewetting can proceed via two mechanisms: Impurities, or amplification of specific modes from the capillary spectrum lead to film rupture followed by polymer bead up in less, or more regular patterns, respectively. The overall goal of this thesis is to understand, to control and to trigger the different dewetting mechanisms by coupling strong external electric fields and/or by changing the interaction potential between the two interfaces of thin films. This is performed from both experimental and theoretical perspectives, and new models developed or those from existing theories are able to match and explain partially or completely the experimental findings. The effect of strong electric fields in a new polymer-air-polymer configuration is studied experimentally using combinations of thin polymer films with different thicknesses, viscosities and dielectric constants. The proposed theoretical model (based on coupled long-wave time evolution equations for the two free surface profiles) predicts in agreement with experimental observation that in such systems the instability evolves only through a mirror mode that couples the two surfaces in an anti-phase manner. Two types of topographies of the film surfaces can be tuned by varying the mobility ratio of the two films. In type I both films’ surfaces show labyrinthine patterns from which drop-shaped patterns evolved simultaneously. In type II the less viscous film exhibits a labyrinthine pattern with the more viscous film displaying isolated drop-like features surrounded by smaller secondary structures. In the absence of external fields coupled to the capillary spectrum, the stability of thin polymer films on solid substrates is determined by the interaction between the polymer-air and polymer-substrate interfaces. That interaction is controlled here by placing metal (gold) nanoparticles (NPs) at the polymer-air interface creating substrate/polymer/NP/air systems, their stability being dependent on the areal coverage. Continuous coverage leads to wrinkle formation; large coverage and NP density leads to the development of a spinodal instability; while low coverage and NP density retards nucleation dewetting. The proposed theoretical model captures the formation of an energy barrier and the transition of the system from unstable to metastable regimes. Experimentally, the stability is found to be independent of the adhesion between the particles and the polymer substrates, but the path taken by the system from unstable states to a flat film depends on particle mobility and polymer film thickness. For relatively high adhesion between the particles and thicker polymer films, the time frame of coverage reduction is relatively large and the particles maintain their initial uniform distribution. For ultra-thin films the coverage decreases through NP embedding. Thus, as the particles embed the roughness of the instability decreases while its wavelength remains constant. This can be explained by a relaxation process that occurs once the constraint that determines the instability is switched-off by the embedding. For lower adhesion between the particles and the polymer substrate, NP diffusion, coalescence and aggregation leads to the formation of a fractal network of NPs and to the development of spinodal instabilities of the free surface of the polymer films. For thinner films a single wavelength is observed while for thicker films two wavelengths compete. The aggregation process results in a decrease in coverage, the networks evolve into disordered particle assemblies while the polymer films flatten again. To conclude the study the combination of the different influences leads to a panoply of patterns at different length scales and spatial positions with potential use for surface enhanced Raman spectroscopy. The rapid growth of a single instability mode from the capillary spectrum due to the applied electric field can have a significant influence on the particle aggregation process only if weak adhesion between the NPs and polymer is present.