Generation Of Polymer-Based Interface Materials With Special Wettabilities: Surface Chemistry And Topography

Wettability of a solid surface is an important property governed by both the surface energy and the geometric structure. In point of surface energy, it is known that wetting behavior of a solid surface is determined by its outermost molecular-level structure, which is one way for controlling the wettability of a surface. Another way to tune the wettability is to fabricate micro- and/or nano-structures on the surface. In recent years, much research effort has been devoted to generating various special wettabilities by combining the fabrication of geometric structure and the chemical modification. This dissertation describes research that is done in order to obtain an in-depth understanding of anisotropic wetting and superhydrophobicity and how to create these special wettabilities by introducing surface topography and/or changing surface chemistry. The surface structure and composition were characterized by using atomic force microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy. Contact angle analysis indicated the surface wettability.Anisotropic wetting, the phenomenon that the drop shape will be distorted on geometrically or chemically anisotropic structures, has been demonstrated on submicrometer chemically patterned surfaces (200 nm). Although anisotropic wetting on geometrical structures has been studied using micron-scale features, there has been no deliberate effort to demonstrate whether submicrometric, or less, features would also lead to anisotripic wetting. Herein, periodic grooved structures (minimum period, 318 nm) were prepared on azobenzene-containing multi-arm star liquid crystal polymer (LCP) films by laser interference. Anisotropic wetting was found on such small-scale surface structures. A thermodynamic model was developed to analyze the anisotropic wetting mechanism. It was found that when the three-phase contact line (TPCL) moves perpendicularly to the grooves, the system must overcome a number of energy barriers, however, there is no energy barrier for the TPCL moving along the grooves. With the groove depth increasing, both the degree of wetting anisotropy and the contact angle hysteresis perpendicular to the grooves increased as a result of the increase in the energy barrier. On the other hand, with the wavelength decreasing, the contact angle hysteresis increased due to the increase in the number of energy barriers. The theoretical critical point where the anisotropic wetting disappears was determined to be 16 nm for grooved structure with period of 396 nm. These results may provide theoretical evidence for the design and application of anisotropic wetting surface.For LCP films, periodic grooved structures were generated on the surface by laser irradiation. However, to our best knowledge, the effect of laser irradiation on the surface of thin polymer bilayer film LCP/PMMA has not yet been reported. PMMA has no absorption of the laser light. Here the first example of the formation of wavelike surface patterns on polymer bilayer films induced by laser irradiation was presented. The formation of such patterns was influenced by the thickness of the upper layer and the laser fluence. The wavelike pattern was also able to be guided to have a specific orientation by placing a PDMS mold on the surface of bilayer film prior to laser irradiation. The property of the laser irradiation, i.e., the selectivity through mask-projection systems, allowed us fabricating complicated micropatterns for novel microdevices. After modification with n-dodecanethiol, the wavelike pattern showed a high contact angle of 148o.Little attention has been paid to superhydrophobic modification of polyimide films probably due to the somewhat inert surface of polyimides that may prevent the formation of stable coating via conventional deposition techniques. To solve this problem, an inner-outer process via treatment with KOH, ion-exchange with AgNO3 and thermal treatment, different from the conventional external deposition, was used to generate silver layers on polyimide films. Further modification with n-dodecanethiol led to hydrophobic surfaces. Different morphologies of the silver layers, which result in the variety of hydrophobicity, can be tailored by controlling the thermal treatment temperature. Surfaces prepared at 320°C showed sticky property that water drops did not slide off even when the sample was held upside down. Surfaces prepared at 340°C had a contact angle of 151°and a sliding angle of 32°. A remarkable superhydrophobicity, as evidenced by a very large water contact angle of 162°and a very small sliding angle of 7°, was achieved at 360°C. Such superhydrophobic surfaces also have superoleophilic property and may be utilized to separate oil and water. Although good adhesion between the polyimide matrix and the silver layer can be obtained via the above method, silver is expensive. It is necessary to seek another economical route. Herein, submicron-scale spherical structure was generated on polymide films via replica molding method by using PS sphere (500 nm) array as a template to fabricate PDMS replica mold. The polyimide film with spherical structure was obtained by casting precursor against the PDMS mold. Polyelectrolyte and SiO2 nanoparticles (40 nm) were then deposited on the polyimide spheres via layer-by-layer assembly to prepare a hierarchical raspberry-like structure. The polyimide film with raspberry-like surface structure showed superhydrophilicity with a water contact angle less than 10°. After modification with fluoroalkylsilane, the surface showed superhydrophobicity with a water contact angle of 160°and a sliding angle less than 10°.The cations of ionic liquids can assemble on hydrolyzed polyimide films (negatively charged) via electrostatic interaction. It was reported that the anions of ionic liquids can be used to tune the surface wettabilities. Comparing with using anions as wettability controlling agents, using the cations to tailor the surface properties has the advantage that their physical and chemical properties can be easily changed by structural modifications. Here we have demonstrated that the surface wettability of negatively charged polyimide films could be tuned in the range of 27o to 80o by 13 different ionic liquids based on imidazolium and ammonium salts via electrostatic self-assembly. The assembly of longer-substituent cations was characterized by the formation of spherical nanoparticles that were formed due to sequent aggregation of cations on those electrostatically assembled ones via hydrophobic interaction. For shorter-substituent cations, due to the less hydrophobic interaction than the electrostatic repulsive force between the cations, no aggregates were formed. This method can also be used to quantify the hydrophobicity of ionic liquids.