Fabrication Method of Functional Surface with Layer Patterns by Material Extrusion Method

Abstract

This thesis delves into fabrication methods for functional surfaces using material extrusion-type three-dimensional (3D) printing technologies. A key characteristic of this technology is the layer-by-layer deposition of materials, resulting in a layered pattern on the surface. These layer patterns are often seen as a drawback due to their impact on the surface quality and the completion level of macroscopic models. Typically, the layer height in standard material extrusion 3D printers is measured in microunits, leading to microstructure forms. These microstructures are actively being researched with the goal of developing various functional surfaces and devices. In particular, micropatterns created with material extrusion 3D printers are replicated using curable polymers to fabricate functional surfaces. Although several printing conditions govern material extrusion 3D printing technology, layer height, and printing angle are the primary factors influencing the shape of the surface structure. This technology tends to form convex patterns on the printed material's surface, with the patterns' spacing and height determined by layer height. This influences the surface roughness and the characteristics of the pattern-replicated polymer. Additionally, the printing angle creates a staircase-like pattern in material extrusion 3D printing samples, which are manufactured layer by layer. This has enabled the creation of a variety of microstructures previously unachievable through layer height variation alone. In this thesis, a new method for fabricating functional surfaces is introduced by applying techniques previously studied with regard to layer height and printing angle. Functional surfaces with unique characteristics due to new printing conditions are also presented. The first part of this dissertation introduces a novel method (printing angle method) utilizing the printing angle technique. As the printing angle of the mold made with a material extrusion-type 3D printer increases, so does the water contact angle (WCA) of the polymer replicating the pattern. A surface with a wettability gradient was fabricated by replicating a semicircular mold with a continuously varying surface slope. Water contact angle measurements and droplet test results demonstrated the characterization of the wettability gradient. Droplets released on a gradient surface inclined at 80° were controlled in their movement; the splash points of the droplets post-collision were tracked. Compared with a general superhydrophobic surface, the distance of the main drop and splash drop was found to be reduced, proving that a surface with a wettability gradient can be fabricated using the printing angle method. In the next part of the dissertation, a novel pattern that utilizes the bottom width, as opposed to the previously used layer pattern, is introduced. To ensure stable attachment to the printing bed, the bottom of the 3D-printed mold is printed flat. However, by printing onto a patterned epoxy substrate, the bottom pattern of the mold was retained, and the pattern in the epoxy was further engraved. The shape of the microscale pattern was controlled by rotating the bottom epoxy substrate, onto which polylactic acid filaments were printed directly. By altering the direction of the bottom epoxy substrate and the printing resolution, a total of 12 different antiadhesive surfaces were formed and tested to characterize their adhesive forces. The surface cast from the mold with a microgrid pattern with an internal angle of 60° and a printing resolution of approximately 800 μm, showed a higher antiadhesive property compared with a flat polymer surface. This confirmed that several functional surfaces can be produced by utilizing various printing conditions of material extrusion 3D printing technology, as well as stacking patterns. The last part of the dissertation proposes a polydimethylsiloxane (PDMS)-PDMS double-casting method for convex-shape surfaces. This includes Raman spectral analysis to verify the principle of PDMS-PDMS double casting. Interfacial separation was performed using the decrease in bonding between the interfaces depending on the curing conditions of the polymer and compared with the plasma method to verify performance. The plasma method showed a lower contact angle than bare PDMS due to surface modification in all molds before and after replication, but the method presented in this part was similar to bare PDMS both before and after replication. Through the double-casting method, it was possible to replicate the same pattern as the mold using the material extrusion method, which can be used in various fields. It can be used to create a functional surface with a convex pattern or as an easy-to-remove mold instead of a mold made using the material extrusion method. The fabrication methods delved into in this thesis present new possibilities for the fabrication of functional surfaces using material extrusion. The layer pattern created by the material extrusion method is produced in a convex one-dimensional shape. Still, by using the methods presented in this thesis, it is expected that molds of not only two-dimensional shapes, but more diverse patterns can be fabricated.