Dong-Gyun Kim

The increase of antifouling properties of ultrafiltration membrane coated by star-shaped polymers

Star-shaped polymers were prepared by atom transfer radical polymerization (ATRP) using poly(ethylene glycol) methyl ether methacrylate (PEGMA) and methyl methacrylate (MMA) as monomers and polyhedral oligomeric silsesquioxane (POSS) as a core material. Linear copolymers from PEGMA and MMA were also prepared for comparison purposes. Polysulfone (PSf) ultrafiltration membranes coated with the star-shaped polymer showed larger fouling resistance and flux recovery than those coated with the linear-shaped polymer. The improved antifouling properties of the star-shaped polymer compared with those of the linear polymer were ascribed to its larger oxygen to carbon ratio and smaller interactive forces with proteins.

Dual Effective Organic/Inorganic Hybrid Star-Shaped Polymer Coatings on Ultrafiltration Membrane for Bio- and Oil-Fouling Resistance

Amphiphilic organic/inorganic hybrid star-shaped polymers (SPP) were prepared by atom transfer radical polymerization (ATRP) using poly(ethylene glycol) methyl ether methacrylate (PEGMA) and 3-(3,5,7,9,11,13,15-heptacyclohexyl-pentacyclo[9.5.1.1³,⁹.1⁵,¹⁵.1⁷,¹³]-octasiloxane-1-yl)propyl methacrylate (MA-POSS) as monomers and octakis(2-bromo-2-methylpropionoxypropyldimethylsiloxy)-octasilsesquioxane (OBPS) as an initiator. Star-shaped polymers (SPM) having PEGMA and methyl methacrylate (MMA) moieties were also prepared for comparative purposes. Polysulfone (PSf) ultrafiltration membranes coated with the SPP showed higher bio- and oil-fouling resistance and flux-recovery ability than the bare PSf membrane. Moreover, the SPP-coated membranes exhibited better antifouling properties than the SPM-coated membrane when they were used for oil/water emulsion filtration. The dual effective antifouling properties of the SPP were ascribed to the simultaneous enrichment of hydrophilic PEG and hydrophobic POSS moieties on the membrane surfaces resulting in the decrease in interactions with proteins and the increase in repellence to oils.

Mussel-Inspired Dopamine- and Plant-Based Cardanol-Containing Polymer Coatings for Multifunctional Filtration Membranes

A series of copolymers [PCD#s, where # is the weight percentage of dopamine methacrylamide (DMA) in polymers] containing mussel-inspired hydrophilic dopamine and plant-based hydrophobic cardanol moieties was prepared via radical polymerization using DMA and 2-hydroxy-3-cardanylpropyl methacrylate (HCPM) as the monomers. PCD#s were used as coating materials to prevent flux decline of the membranes caused by the adhesion of biofoulants and oil-foulants. Polysulfone (PSf) ultrafiltration membranes coated with PCD#s showed higher biofouling resistance than the bare PSf membrane, and the bactericidal properties of the membranes increased upon increasing the content of HCPM units in the PCD#s. Serendipitously, the PSf membranes coated with the more or less amphiphilic PCD54 and PCD74, having the optimum amount of both hydrophilic DMA and hydrophobic HCPM moieties, showed noticeably higher oil-fouling resistance than the more hydrophilic PCD91-coated membrane, the more hydrophobic PCD0-coated membrane, and the bare PSf membrane. Therefore, multifunctional coating materials having biofouling- and oil-fouling-resistant and bactericidal properties could be prepared from the monomers containing mussel-inspired dopamine and plant-based cardanol groups.

Photo-cross-linkable star-shaped polymers with poly(ethylene glycol) and renewable cardanol side groups: synthesis, characterization, and application to antifouling coatings for filtration membranes

Star-shaped polymers (SPCs) containing poly(ethylene glycol) (PEG) and renewable cardanol side groups were synthesized by atom transfer radical polymerization (ATRP). Water-soluble SPCs were turned into water-insoluble materials by self-cross-linking reaction of the unsaturated hydrocarbon chains of cardanol moieties upon UV irradiation. The SPC-coated membranes with UV curing exhibited noticeably higher bio- and oil-fouling resistance than the bare polysulfone (PSf) membrane during filtration experiments, whereas the SPC-coated membranes without UV curing showed a large flux-decline caused by fouling compared to that of the bare membrane, because SPCs were washed out during filtration. The enhanced antifouling properties of the SPC-coated membranes with UV curing were ascribed to a large quantity of PEG moieties on the surfaces stabilized by the cross-linked polymeric structure, leading to decreased interactions with proteins and oils.

Control of Pretilt Angle in Liquid Crystal and Photocurable Monomer System

The pretilt angles of liquid crystals (LCs) could be controlled over the range, 0o to 90°, using LC/photocurable monomer (NOA65) mixture system through UV irradiation. The pretilt angles could be controlled using LC/NOA65 mixture in planar polyimide coated LC cell, by changing the mixture ratio ranging from 99.9/0.1 to 99.0/1.0 and UV irradiation time (10 and 20 min). Planar and tilted alignment was observed in LC/NOA65 mixture of 99.9/0.1 and 99.8/0.2, respectively, after UV irradiation for 20 min. Finally, homeotropic (or vertical) alignment was observed in weight ratio of over 99.7/0.3. The LC alignment behavior was well correlated with the wettability of the alignment films due to the surface morphology on the alignment layer surfaces by photopolymerization-induced phase separation. This study contributes to the latest efforts to develop new method for pretilt angle control.

Two distinct mechanisms of transcriptional regulation by the redox sensor YodB

Significance Bacteria sense and protect themselves against oxidative stress using redox-sensing transcription regulators with cysteine residues. Here, we investigate at the molecular level how the YodB protein, a transcription repressor in Bacillus subtilis, monitors and responds to different oxidative stresses. Diamide stress leads to the formation of disulfide bonds between cysteine residues, whereas the more toxic quinone compound methyl-p-benzoquinone forms an adduct on a specific cysteine residue. These chemical modifications lead to considerably different changes in the YodB structure, causing the release of YodB from the DNA of antioxidant genes. The redox-sensing transcription regulator YodB allows B. subtilis to respond to multiple oxidative signals of differing toxicity by adopting different structures. For bacteria, cysteine thiol groups in proteins are commonly used as thiol-based switches for redox sensing to activate specific detoxification pathways and restore the redox balance. Among the known thiol-based regulatory systems, the MarR/DUF24 family regulators have been reported to sense and respond to reactive electrophilic species, including diamide, quinones, and aldehydes, with high specificity. Here, we report that the prototypical regulator YodB of the MarR/DUF24 family from Bacillus subtilis uses two distinct pathways to regulate transcription in response to two reactive electrophilic species (diamide or methyl-p-benzoquinone), as revealed by X-ray crystallography, NMR spectroscopy, and biochemical experiments. Diamide induces structural changes in the YodB dimer by promoting the formation of disulfide bonds, whereas methyl-p-benzoquinone allows the YodB dimer to be dissociated from DNA, with little effect on the YodB dimer. The results indicate that B. subtilis may discriminate toxic quinones, such as methyl-p-benzoquinone, from diamide to efficiently manage multiple oxidative signals. These results also provide evidence that different thiol-reactive compounds induce dissimilar conformational changes in the regulator to trigger the separate regulation of target DNA. This specific control of YodB is dependent upon the type of thiol-reactive compound present, is linked to its direct transcriptional activity, and is important for the survival of B. subtilis. This study of B. subtilis YodB also provides a structural basis for the relationship that exists between the ligand-induced conformational changes adopted by the protein and its functional switch.

Collapse of Homeotropic Liquid‐Crystal Alignment by Increased Molecular Packing on Comb‐Like Polymer Surfaces

We report an unusual alignment behavior of liquid crystals (LCs) on well-ordered comb-like poly(oxyethylene) surfaces. The homeotropic LC alignments that are observed on as-coated surfaces of the polymers are transformed to the random planar type after annealing treatment, even though the molecular structure of the polymer surface becomes more ordered and the surface energy decreases. Studies of the surface properties, such as molecular structure, morphology, and wettability, reveal that such an unexpected alteration of the LC alignment originates from the density of the alkyl side chains being enhanced by localized packing.

Terahertz electromagnetic interference shielding using single-walled carbon nanotube flexible films

We performed time-domain terahertz spectroscopy on flexible films coated by single-walled carbon nanotube. The films demonstrate good shielding of electromagnetic waves in the terahertz range, maintaining good transparency for visible light. The shielding efficiency can be engineered by the thickness control of the film and by the chemical treatments.

Regulatory mechanisms of thiol-based redox sensors: lessons learned from structural studies on prokaryotic redox sensors

Oxidative stresses, such as reactive oxygen species, reactive electrophilic species, reactive nitrogen species, and reactive chlorine species, can damage cellular components, leading to cellular malfunction and death. In response to oxidative stress, bacteria have evolved redox-responsive sensors that enable them to simultaneously monitor and eradicate potential oxidative stress. Specifically, redox-sensing transcription regulators react to oxidative stress by means of modifying the thiol groups of cysteine residues, functioning as part of an efficient survival mechanism for many bacteria. In general, oxidative molecules can induce changes in the three-dimensional structures of redox sensors, which, in turn, affects the transcription of specific genes in detoxification pathways and defense mechanisms. Moreover, pathogenic bacteria utilize these redox sensors for adaptation and to evade subsequent oxidative attacks from host immune defense. For this reason, the redox sensors of pathogenic bacteria are potential antibiotic targets. Understanding the regulatory mechanisms of thiol-based redox sensors in bacteria will provide insight and knowledge into the discovery of new antibiotics.

Anisotropy-Driven High Thermal Conductivity in Stretchable Poly(vinyl alcohol)/Hexagonal Boron Nitride Nanohybrid Films

Controlling the anisotropy of two-dimensional materials with orientation-dependent heat transfer characteristics is a possible solution to resolve severe thermal issues in future electronic devices. We demonstrate a dramatic enhancement in the in-plane thermal conductivity of stretchable poly(vinyl alcohol) (PVA) nanohybrid films containing small amounts (below 10 wt %) of hexagonal boron nitride ( h-BN) nanoplatelets. The h-BN nanoplatelets were homogeneously dispersed in the PVA polymer solution by ultrasonication without additional surface modification. The mixture was used to prepare thermally conductive nanocomposite films. The in-plane thermal conductivity of the resulting PVA/ h-BN nanocomposite films increased to 6.4 W/mK when the strain was increased from 0 to 100% in the horizontal direction. More specifically, the thermal conductivity of a PVA/ h-BN composite film with 10 wt % filler loading can be improved by up to 32 times as compared to pristine PVA. This outstanding thermal conductivity value is significantly larger than that of materials currently used in in-plane thermal management systems. This result is attributed to the anisotropic alignment of h-BN particles in the PVA chain matrix during stretching, enhancing phonon conductive paths and hence improving the thermal conductivity and thermal properties of PVA/ h-BN nanocomposite films. These polymer nanocomposites have low cost as the amount of expensive conductive fillers is reduced and can be potentially used as high-performance materials for thermal management systems such as heat sink and thermal interface materials, for future electronic and electrical devices.