Synthesis of polymeric nanocomposites in supercritical carbon dioxide

Supereritical carbon dioxide has been of great interest in various areas of chemical science and engineering during the last decade. In this thesis, the properties of poly (methyl methacrylate) (PMMA)/silica nanocomposites with different surfactant and filler concentrations synthesized through in-situ polymerization in supercritical carbon dioxide were investigated. In addition, ferromagnetic nano-particles were synthesized and functionalized for the preparation of nanocomposites. The thermal stability and surface morphology of PMMA/iron oxide nanocomposites synthesized in supercritical carbon dioxide were first studied in the work. The scanning electron microscopy micrographs show that higher surfactant concentration results in higher number density of polymeric nanocomposite particles and better dispersion of nano-particles in the polymeric composites. Higher concentration of silica nano-particles in the composites results in excessive particle agglomeration. The cause of agglomeration is due to the MPS molecules on different particles react with each other. A better thermal stability of the composites, as revealed by thermal gravimetric analyses and differential scanning calorimetry analyses, was observed due to the enhancement of the surfactant and filler interactions. Surprisingly, composites with PMMA grafted particles did not show an anticipated drastic improvement of the mechanical properties. This may result from the plasticizing effect of the stabilizer used in the dispersion polymerization

Fundamentals, preparation, and characterization of superhydrophobic wood fiber products

In this study, we developed a facile method for preparing a superhydrophobic paper surface using a layer-by-layer deposition of polydiallyldimethylammonium chloride (polyDADMAC) and silica particles, followed by a fluorination surface treatment with 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS, CF3(CF2)5CH2CH2Si(OC2H5)3). The wood fiber products prepared in this study had contact angles of water greater than 150 degree and sliding angles less than 5 degree. Besides their high water repelling property, the superhydrophobic paper products kept a high tensile strength at high relative humidity condition. The superhydrophobic paper products also showed high resistance to bacterial contamination.M.S.Committee Chair: Yulin Deng; Committee Member: Jeffery S. Hsieh; Committee Member: Sujit Banerjee; Committee Member: Zhong Lin Wan

Generalized Fabrication of Monolayer Nonclose-Packed ColloidalCrystals with Tunable Lattice Spacing

Here, we report a simple colloidal transfertechnology that enables scalable fabrication of monolayernonclose-packed silica colloidal crystals on a large variety ofsubstrates. Two-dimensional colloidal crystals with an unusualnonclose-packed structure are first assembled on silicon wafers bya spin-coating technique. A poly(vinyl alcohol) (PVA) film castupon the spin-coated colloidal crystal is used to transfer thenonclose-packed particle arrays onto various substrates. Thelattice spacing of the transferred monolayer colloidal crystal caneasily be adjusted by thermally treating the PVA-silica spherescomposite film for varied durations. We also have demonstratedthe templating fabrication of periodic arrays of gold nanodotsusing a transferred monolayer nonclose-packed colloidal crystal as a structural template. The resultant plasmonic array exhibitshigh surface-enhanced Raman scattering enhancement factor (∼3.8 × 107) for adsorbed benzenethiol molecules

Leafhopper Wing-Inspired Broadband Omnidirectional Antireflective Embroidered Ball-Like Structure Arrays Using a Nonlithography-Based Methodology

Leafhoppers (Thaia rubiginosa) actively coat their wings with embroidered ball-like secretory brochosomes, which act as antireflective structures to enhance camouflage against predators. Inspired by the leafhoppers, we report a scalable nonlithographic approach for self-assembling nonclose-packed embroidered ball-like hierarchical structure arrays. The resulting structures create a gradual refractive index transition at the air/substrate interface, thereby suppressing the optical reflection for wide viewing angles. Compared with a bare substrate, the average reflectance of the structured substrate in the whole visible spectral region is reduced from 9 to 3% at normal incidence, and the average reflectance of that is even reduced by ca. 22% as the incident angle reaches 75°. Moreover, the dependence of the height and the shape of the hierarchical structure on the omnidirectional antireflection performance is systemically evaluated in this research

Self-assembled biomimetic superhydrophobic hierarchical arrays

Here, we report a simple and inexpensive bottom-up technology for fabricating superhydrophobic coatingswith hierarchical micro-/nano-structures, which are inspired by the binary periodic structure foundon the superhydrophobic compound eyes of some insects (e.g., mosquitoes and moths). Binary colloidalarrays consisting of exemplary large (4 and 30 lm) and small (300 nm) silica spheres are first assembledby a scalable Langmuir–Blodgett (LB) technology in a layer-by-layer manner. After surface modificationwith fluorosilanes, the self-assembled hierarchical particle arrays become superhydrophobic with anapparent water contact angle (CA) larger than 150 . The throughput of the resulting superhydrophobiccoatings with hierarchical structures can be significantly improved by templating the binary periodicstructures of the LB-assembled colloidal arrays into UV-curable fluoropolymers by a soft lithographyapproach. Superhydrophobic perfluoroether acrylate hierarchical arrays with large CAs and small CA hysteresiscan be faithfully replicated onto various substrates. Both experiments and theoretical calculationsbased on the Cassie’s dewetting model demonstrate the importance of the hierarchical structure inachieving the final superhydrophobic surface states

Cicada-Wing-Inspired Self-Cleaning Antireflection Coatings on Polymer Substrates

The cicada has transparent wings with remarkable self-cleaning properties and high transmittance over the whole visible spectral range, which is derived from periodic conical structures covering the wing surface. Here we report a scalable self-assembly technique for fabricating multifunctional optical coatings that mimic cicada-wing structures. Spin-coated two-dimensional non-close-packed colloidal crystals are utilized as etching masks to pattern subwavelength-structured cone arrays directly on polymer substrates. The resulting gratings exhibit broadband antireflection performance and superhydrophobic properties after surface modification. The dependence of the cone shape and size on the antireflective and self-cleaning properties has also been investigated in this study

Self-Assembled Mechanochromic Shape Memory Photonic Crystals by Doctor Blade Coating

Mechanochromic shape memory photonic crystals can memorize their original structures and recover the inherent structural colors in response to external stimuli; thereby they have rendered various important optical applications. Unfortunately, most existing shape memory polymers are thermoresponsive, and the corresponding mechanochromic characteristics are limited by the heat-demanding programming process. Besides that, a great majority of current fabrication methodologies suffer from low throughput, hindering the practical applications. Herein, a scalable technology is developed to engineer macroporous shape memory photonic crystals by self-assembling silica colloidal crystals in a polyurethane acrylate/polyethoxylated trimethylolpropane triacrylate/poly(ethylene glycol) diacrylate matrix, followed by a wet etching treatment to selectively remove silica colloids. The as-created photonic crystals display a brilliant structural color, which is reversibly tunable with mechanical deformation at ambient conditions. Upon stretching, the reduced interlayer lattice spacing of the photonic crystals leads to a blueshift of the reflection peak position and a significant color change. Importantly, the stretched macroporous film can fix its temporary structures without applying any contact force and simultaneously recover its original configuration and appearance by applying ethanol evaporation-induced capillary pressures. The reversibility and the dependence of templated silica colloid size on mechanochromic characteristics have also been investigated in the research

Visual and reversible carbon dioxide sensing enabled by doctor blade coated macroporous photonic crystals

With significant impacts of carbon dioxide on global climate change, carbon dioxide sensing is of great importance. However, most of the existing sensing technologies are prone to interferences from carbon monoxide, or suffer from the use of sophisticated instruments. This research reports the development of reproducible carbon dioxide sensor using roll-to-roll compatible doctor blade coated three-dimensional macroporous photonic crystals. The pores are functionalized with amine groups to allow the reaction with carbon dioxide in the presence of humidity. The adsorption of carbon dioxide leads to red-shift and amplitude reduction of the optical stop bands, resulting in carbon dioxide detection with visible readout. The dependences of the diffraction wavelength on carbon dioxide partial pressure for various amine-functionalized photonic crystals and different humidities in the environment are systematically investigated. In addition, the reproducibility of carbon dioxide sensing has also been demonstrated in this research

Octopus-Inspired Assembly of Nanosucker Arrays for Dry/Wet Adhesion

The octopus is capable of adhering to slippery, rough, and irregular surfaces in the marine intertidal zone because of its periodic infundibulum-shaped suckers on the arms. Here, we present a scalable self-assembly technology for fabricating adhesion materials that mimic octopus sucker functionality. By utilizing spin-coated two-dimensional colloidal crystals as templates, non-close-packed nanosucker arrays are patterned on silicone substrates. The resulting nanosuckers can be deformed to exhibit great adhesive capacities on both microrough and flat surfaces in dry and wet environments. This indicates a probable biomimetic solution to the challenge of wound care