Photochemically Induced Marangoni Patterning of Polymer Thin Films: Modeling and Experiments

Abstract

The numerous and highly diverse applications of topographically patterned polymer films have fueled interest in developing new approaches that are highly accessible and versatile, for use in industrial and research pursuits. In that vein, a method called photochemically induced Marangoni patterning (PMP) was recently developed in which topography formation is encoded with photochemically patterned surface-tension-gradients. The environmentally benign nature and spatio-temporal tunability of PMP for creating micrometer and sub-micrometer sized features are highly attractive in a potential surface-patterning technology. PMP involves a complex interplay between various physical processes that determine topography growth and decay attributes. Despite recent efforts aimed towards understanding PMP, many important questions remain unanswered. In this research, we develop a mathematical model that takes into account the principles of fluid mechanics and interfacial phenomena involved in PMP. The model helps identify characteristic regions in the vast process parameter space that would otherwise be experimentally difficult and time-consuming to explore. Simulations are used to understand the role played by the underlying substrate in governing feature evolution, which will help expand the substrate inventory for PMP to include those for flexible electronics applications, or other materials compatible with continuous roll-to-roll processing. With the deeper understanding of polymer-substrate interactions, we explore how various process parameters influence patterning resolution (feature width) and aspect ratio (feature height to width ratio), both key quantities of interest in patterned films, and derive scaling laws to guide experimental design. The predictive capability of the model is tested by performing complementary experiments aimed at achieving both higher feature resolution and aspect ratio. In addition, we demonstrate an approach to pattern functional polymers which are light-insensitive by forming a bilayer comprising a light-sensitive polymer sitting on top of a light-insensitive polymer. Surface-tension gradients are introduced in the top layer and polymer-polymer interface coupling leads to the transfer of patterns to the bottom layer. This study helps expand the library of polymers that can be patterned using PMP. Overall, the thesis work advances the fundamental understanding of polymer thin film behavior to help develop a versatile polymer patterning technology.