Physical, electrical, and optical material properties are critically dependent on the spatial arrangement of the material’s constituent atoms, molecules, and aggregates on length-scales ranging from sub-nanometer to micrometer. My research goals are to investigate the structure-morphology-property relationships on these different length scales in the field of soft materials. Fulfillment of these goals will require the multifaceted utilization of a variety of equipment and experimental expertise. Therefore, I briefly summarized my past and current researches with expertise in transmission electron microscopy (TEM), selected area electron diffraction (SAED), and wide-angle/small-angle X-ray diffraction (WAXD, SAXS) techniques.
1. Molecular Self-Assembly on Different Length Scales |
Self-assembly is fundamental to generating structures on all scales from molecules to galaxies. It is defined as a reversible process in which disordered pre-existing parts (building blocks) of a pre-existing system form ordered structures and patterns. Intelligent materials can be tailored from specifically designed molecular components through self-assembly that are programmed under the control of directional noncovalent physical interactions.
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1.1) Helical Superstructures for Biosensors
A biosensor is a device for the detection of a biological analyte with a physico-chemical detector. In this research project, we developed simple biosensors which can detect the handedness of both chemical and biological materials using the so-called “sergeant and soldier effect” which is well-known in chiral liquid crystal self-assembly. Recently, we constructed both right-handed and left-handed propeller-patterned chiral nematic droplets together with achiral droplets from achiral biphenyl carboxylic acid compounds (Figure 1). When the two propeller-patterned chiral nematic droplets with the identical handedness merge, a bigger chiral propeller-patterned nematic droplet with the same handedness is formed. The purpose of this project was first to build helical structures on different length scales through supramolecular self-assembly processes, then second to find the origin of helix formation from achiral/chiral building blocks. This will provide insight into new ways to construct molecules that can form chiral structures which will finally enable these helical concepts to be used in practical applications. The advantage of this biosensor is its high sensitivity, simple device construction and easy morphological detection. |
Figure 1. Chiral propeller-patterned liquid crystal droplets with chirality
Photovoltaic arrays use solar energy to provide electricity for human activities. To date, these cells could be beneficial for some applications where mechanical flexibility and disposability are important. In this research project, we would like to improve the efficiency of solar cells using molecular self-assembly techniques. Since columnar discotic liquid crystals show many extraordinary properties such as one-dimensional electrical conductivity, fast photoconductivity and ferroelectricity, the structures and dynamics of discotic liquid crystals have been studied extensively. By controlling the direction of columnar long axis, we can significantly increase the one-dimensional electric conductivity and photoconductivity which are the main drawback in the polymer solar cells. Recently, we achieved ion-promoted, automorphogenic self-assembly of nanoscale composite fibers using a structurally rigid hexameric macrocycle possessing photonic and electron storage potential (Figure 2). We realized that the molecular wires are constructed by an alternating stack of rigid polycationic macrocycles and spherical polyanionic dendrimers. These are expected to be able to improve electron conduction thus increase the efficiency of solar cells.
Figure 2. Molecular wires constructed by alternating self-assembly of macrocycles and dendrimers
Due to their outstanding electrical and optical properties, conjugated organic materials have been considered as promising materials for a broad range of applications, such as organic photovoltaic devices, organic thin film transistor, and organic light emitting diode. Since the applications of the conjugated organic materials mostly rely on their luminescent, photoconductive and optical properties, the understandings of working physics behind these properties and of structure-property relationships are critical for the development of next-generation organic devices. Among the various organic materials, disc-shaped liquid crystal molecules have been widely studied because of their excellent electrical and optical properties in addition to the high purity and excellent processability. In the practical application, device engineers and physicists would like to avoid molecular aggregations since the π-π interactions can generate excimers, which may decrease the photoefficiency of the system. Broken coplanarity can affect the photophysical properties on the different environments (Figure 3). The photophysical properties of the organic π-conjugated small molecules can be tuned by controlling the molecular packing structures in the different phases.
Figure 3. Photoluminescence change by manipulating the molecular packing structures
The control of liquid crystal alignment is one of the most important technologies for high-performance devices such as light shutters, sensors and modulators. With the photo-functionalized moieties, the formation and deformation of self-assembled hierarchical superstructures can be modulated by the photochemical isomerization upon irradiating the ultraviolet and visible light.The individual azobenzene conformational changes are translated to the macroscopic amplifications resulting in the transformation of molecular packing symmetry. The photo-reversible property of azobenzene has been the basis for smart materials with remote-controllable applications in switchable delivery systems, rewritable hologram films, and optically active fibers. The main objective of this study is to fully utilize the different photo-functional groups on a single molecule (Figure 4).Since the light-driven phase structural behaviors strongly rely on their molecular packing structures, understanding the supramolecular assembly of photo-functionalized anisotropic molecule is significantly important. Therefore, the manipulation of hierarchical superstructures with the fine control of intermolecular interactions between the programmed molecular building blocks are very important to achieve the targeted properties of soft materials.
Figure 4. Photo-modulation of self-assembled superstructure
Nanoscale manufacturing and self-assembly techniques have found entrance into the area of molecular alignment. Since the interaction between nanoparticles with substrates is higher than that between nanoparticles and liquid crystal compounds, the nanoparticles mixed with liquid crystal hosts at the initial state can be phase-separated against the liquid crystal media and diffused onto the substrates in which is a thermodynamically more stable state. The diffused and deposited nanoparticles on the substrates form the planar or vertical alignment layer.We newly proposed and synthesized a photopolymerizable reactive amphiphilic surfactant. We programmed the polyoligomeric silsesquioxane with cyanobiphenyl mesogen for controlling the interaction with liquid crystal molecules, a nanoparticle for automatically forming the vertical alignment layer on the substrates, and a photopolymerizable vinyl function for stabilizing the system as well as for controlling the electro-optical characteristics such as anchoring energy and response time (Figure 5).By optimizing the content of nanosurfactat, we successfully fabricated the automatic and robust molecular alignment layer with a strong surface anchoring energy, which can allow us to reduce the manufacturing cost and to open new doors for electro-optical applications.
Figure 5. Nanosurfactant for automatic molecular alignments
2. High Performance Smart Polymeric Materials |
Smart polymers have one or more properties that can be significantly altered in a controlled fashion by external stimuli, such as stress, temperature, moisture, pH, electric or magnetic fields. Suitably designed structures made from these smart polymers can therefore be made that bend, expand or contract when an external stimulus is applied.
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2.1) Polymers with Excellent Elastic Recovery and Strength Due to their anisotropic properties and the self-assembling capabilities, main-chain liquid crystalline polymers have been used in a range of mechanical, electrical, optical and biological applications (Figure 6). In order to tune these properties, main-chain liquid crystalline polymers, comprised of flexible alkylene spacers between rigid and controlled aromatic units, with various chemical structures and molecular architectures for both mesogenic groups and flexible spacers have been designed. Forming their liquid crystalline phases is dependent of the configuration of the linkages in the mesogenic groups. The spacer length can cause periodic changes in the thermodynamic properties of a material based on whether there are an even or odd number of unit in the liquid crystalline polymer spacers. Similar to the bent-core liquid crystalline small molecules, even numbered polyesters exhibited a limited crystal twisting on the micrometer length scale because of configurational bent. In this research project, we can develop high performance smart polymers with excellent elastic recovery and strength by introducing a conformational bend and/or a configurational bend in the mesogenic groups and/or flexible spacers. |
Figure 6. Hierarchically self-assembled dendronized polymer and programmed mechanical work
Ordered mesoporous polymeric materials are of significant interest in areas such as photonics, separations, and catalysis, which require organized microporous structures on the order of sub-micrometer to micrometer length-scales (Figure 7). When a thin film of block copolymer is cast using an appropriate solvent on a solid substrate, a highly ordered and microporous honeycomb structures with a characteristic length scale is formed. In addition, super-hydrophobic materials have attracted great interest because of their practical application in water repellency, self-cleaning, and antifouling coating. Preparation of the rough surface and subsequent coating of the surface with low surface energy material such as fluorine compounds is an essential process in fabricating super-hydrophobic surfaces. However, the use of the fluorine compound comes with certain disadvantages such as low oil repellency, high toxicity and high cost. In this research project, we attempt to form hierarchical structures from a honeycomb structure in common/selective mixed solvents to a super-hydrophobic surface simply by changing the selective solvent content of the non-fluorinated polymer solution without the need for low-surface energy modification.
Figure 7. Morphological images of honeycomb structure from self-assembled polymeric materials and its surface property
Lyotropic chromonic liquid crystal molecules at a low concentration have been used as pharmaceutical drugs and dyes. Because the self-assembled nanocolumns in liquid crystal phases can be uniaxially oriented in the macroscopic length scales by applying external forces, such as mechanical, electrical or magnetic forces with and without the support of surface alignment layers, their applications have been dramatically magnified and expanded in various fields, such as dichroic light-polarizing materials, biosensing active materials, and electro-optic devices. Our research group recently used the photopolymerizable ionic monomers as solvents rather than water for the fabrication of polymer-stabilized robust coatable thin film polarizers (Figure 8). The photopolymerization of ionic monomers right after the mechanical orientation of lyotropic liquid crystal columns significantly increased mechanical stabilities, such as the adhesion strength with substrates and the surface hardness. The macroscopically oriented and patterned film with robust chemical and mechanical stabilities was successfully fabricated on the flexible polymer substrates. Additionally, the polymer-stabilized film exhibited excellent chemical stabilities.
Figure 8. Photopolymerization of lyotropic chromonic mesogens for flexible and patterned thin film polarizer
2.4) Heat Transfer Organic Materials
Rapid advancements in electronic devices and energy storage systems require high powers and fast processing times within the limited spaces. Since the amount of heat accumulation is proportional to the power density, the effective heat management is of paramount importance to improve the reliability and lifetime of devices. In this regard, most electronic packages contain a metallic heat spreader or heat sink. The introduction of conductive nanoparticles is very effective in maximizing the thermal conducting properties of composite systems at low filler contents, but the uniform dispersion with reduced interfacial resistance remains a big challenge. Another approach is based on the use of an advanced heat transfer organic material as a continuous matrix. The polymeric matrix with high order tends to possess a higher thermal conductivity through the effective lattice vibration and the suppression of phonon scattering. The newly developed discotic liquid crystalline materials with an outstanding thermal conductivity as well as with an excellent mechanical sustainability can be applied as directional heat dissipating materials in electronic and display devices (Figure 9).
Figure 9. Polymer films with the outstanding thermal conductivity fabricated by the polymerization of oriented reactive discogens
Color is a powerful and often rapid means for intuitive communications. Color is visualized in a variety of ways including transmission, diffraction, absorption, and reflection of light. Reflective materials are particularly interesting. The reflection can arise in periodic media with a variation in the refractive index to produce a photonic band gap. Here, we focus on materials that form the cholesteric liquid crystal phase. The chiral nematic phase self-organizes into a one dimensional photonic band gap material. Compared with conventional photonic crystals made from hard materials, soft materials have attracted much attention due to good processability, cost effectivity, and lightweight operation conditions. To securely preserve the homogeneous helical pitch distribution in the robust polymer network, acrylate functions with an identical reactivity are introduced both to chiral guest and to nematic host for suppressing the phase separation via photopolymerization (Figure 10). In this contribution, we report the reflective and flexible polymer films with red, green, and blue reflection colors.
Figure 10. Photopolymerization of helical nanostructures for chirophotonic crystal reflectors