Hyun-jong Paik

Colored single-chain polymeric nanoparticles via intramolecular copper phthalocyanine formation

We have synthesized single-chain polymeric nanoparticles through the intramolecular formation of copper phthalocyanines from polystyrene-co-poly[4-((4-vinylbenzyl)oxy)phthalonitrile] under diluted conditions. Linear copolymer precursors carrying phthalonitrile pendant groups were prepared by reversible addition–fragmentation chain transfer (RAFT) polymerization followed by intramolecular macrocyclization reactions to form Cu-metallated phthalocyanines, which concurrently enabled single chain nanoparticle formation. The preparation of copper phthalocyanines within a single chain nanoparticle was confirmed using FT-IR and UV/Vis spectroscopy. The intramolecular chain collapse was characterized by SEC and AFM.

Styrene-butadiene-glycidyl methacrylate terpolymer/silica composites: dispersion of silica particles and dynamic mechanical properties

Styrene-butadiene-glycidyl methacrylate terpolymer (GMA-SBR) was synthesized by emulsion polymerization for the fuel efficient tire tread composite. The chemical structure of the GMA-SBR was analyzed using infrared spectroscopy, 1H NMR, gel permeation chromatography, and differential scanning calorimetry. The GMA-SBR/silica composite is the first instance introduced covalent bonds between silica filler and rubber molecules by in-chain modification of styrene-butadiene molecules. After compounding, the curing characteristics, the mechanical and dynamic mechanical properties of the composites were analyzed. The GMA-SBR/silica composite exhibited higher wear resistance of 32.9% and lower rolling resistance of 25.7% than the styrene-butadiene rubber 1721/silica composite. These results are due to the improvement of silica dispersion in the composite as the covalent bonding increased the filler–rubber interaction and the countervailing effects of less filler flocculation. The proposed approach assists in finding a solution to improve the performances of tires for fuel efficiency and the reduction of greenhouse gases from the vehicles.

Conjugation of proteins by installing BIO-orthogonally reactive groups at their N-termini.

N-terminal site-specific modification of a protein has many advantages over methods targeting internal positions, but it is not easy to install reactive groups onto a protein in an N-terminal specific manner. We here report a strategy to incorporate amino acid analogues specifically in the N-terminus of a protein in vivo and demonstrate it by preparing green fluorescent protein (GFP) having bio-orthogonally reactive groups at its N-terminus. In the first step, GFP was engineered to be a foldable, internal methionine-free sequence via the semi-rational mutagenesis of five internal methionine residues and the introduction of mutations for GFP folding enhancement. In the second step, the N-terminus of the engineered protein was modified in vivo with bio-orthogonally functional groups by reassigning functional methionine surrogates such as L-homopropargylglycine and L-azidohomoalanine into the first methionine codon of the engineered internal methionine-free GFP. The N-terminal specific incorporation of unnatural amino acids was confirmed by ESI-MS analysis and the incorporation did not affect significantly the specific activity, refolding rate and folding robustness of the protein. The two proteins which have alkyne or azide groups at their N-termini were conjugated each other by bio-orthogonal Cu(I)-catalyzed click chemistry. The strategy used in this study is expected to facilitate bio-conjugation applications of proteins such as N-terminal specific glycosylation, labeling of fluorescent dyes, and immobilization on solid surfaces.

Photodetachment of aryl moieties from covalently functionalized single-walled carbon nanotubes by UV laser irradiation

Photodetachment of aryl moieties from covalently functionalized SWNTs was studied by temperature-dependent and UV Raman spectroscopic tools. The sidewall functionalized SWNTs were prepared via diazonium reactions from 4-bromoaniline and isoamyl nitrite. I–V conductivity measurements were performed for the functionalized SWNTs, after purification from pristine SWNTs using their different solubility. Temperature dependent Raman data appeared to be in line with thermal gravimetric (TGA) data exhibiting removal of aryl moieties above 590 °C. Raman spectra of the functionalized SWNTs were examined using ultraviolet excitation at 244 nm (5.08 eV) and 325 nm (4.82 eV) as well as visible irradiation at 633 nm (1.96 eV). Our experimental results indicated that the disorder-induced D mode should change significantly, whereas the tangential G modes do not become different under UV irradiation. The D mode for the functionalized SWNTs was found weakly at ∼1410 cm−1 at a low power of UV irradiation, whereas it was observed strongly at ∼1310 cm−1 upon 633 nm irradiation. The attached aryl moieties appeared to be removed by UV irradiation as indicated from almost identical spectra with those of pristine SWNTs.

Characterization of AN-SBR/Silica compound with acrylonitrile as a polar group in SBR

AbstractIn this study, acrylonitrile-styrene-butadiene rubber (AN-SBR), in which polar monomer acrylonitrile was introduced to emulsion styrene-butadiene rubber (SBR) as the 3rd monomer, was applied to silica compound to examine the various properties that are required for tire tread compound. The mechanical properties of the AN-SBR compounds, the modulus, and the abrasion resistance were improved. These results are attributed to the increase in the degree of cross-linking of the compounds. In particular, the abrasion resistance result is attributed to the increase of filler-rubber interaction and the increased dispersion of silica in the compound by the application of poly(ethylene glycol) (PEG). When the PEG was applied to the AN-SBR compound, the values of tan δ were increased at 0 °C and decreased at 60 °C in dynamic viscoelastic characteristics. These results are due to the improvement of silica dispersion in the compound as the H-bonding increased the filler-rubber interaction, and the adsorption of the PEG onto silica reduced the strong filler-filler interaction between particles.

One-step immobilization and purification of his-tagged enzyme using poly(2-acetamidoacrylic acid) hydrogel

AbstractNi2+-Complexed poly(2-acetamidoacrylic acid) (PAAA) hydrogel support was developed for the one-step immobilization and purification of recombinant histidine-tagged glutamyl aminopeptidase (His-tagged GAP). Ni2+-PAAA hydrogel was prepared from the polymerization of 2-acetamidoacrylic acid (AAA) and 2,2-[(1,4-dioxo-1,4-butanediyl) diamino] bis(2-propenoic acid) (DBDBPA) with potassium persulfate in dimethyl sulfoxide (DMSO), followed by Ni2+complexation. His-tagged GAP was immobilized directly from the cell lysate onto the Ni2+-PAAA hydrogel support and then purified. Catalytic activity of immobilized His-tagged GAP for the hydrolysis of alanylpara-nitroanilide revealed 90% conversion after 30 min of incubation, indicating sustained catalytic activity. The hydrogel-immobilized enzyme also exhibited enhanced thermal stability of sustained 70% activity after 1 h incubation at 60 °C, while the free enzyme activity was reduced to 50% at the same condition. After four cycles of hydrogel regeneration, the immobilized enzyme lost only 20% of its initial activity. Ni2+-PAAA hydrogel provided a new and convenient immobilization/purification system for His-tag enzymes through easy and simple procedures.

Modification of the surfaces of silicon wafers with temperature-responsive cross-linkable poly[oligo(ethylene oxide) methacrylate]-based star polymers.

Temperature-responsive photo-cross-linkable poly[oligo(ethylene oxide) monomethyl ether methacrylate] (POEOMA)-based star polymers were synthesized by atom transfer radical polymerization (ATRP), for the modification of silicon (Si) wafer surfaces. The polymers showed a lower critical solution temperature (LCST) behavior in aqueous media. The polymers were modified with benzophenone (Bzp) functional groups that were utilized in UV-triggered (λ = 365 nm) cross-linking reactions for the preparation of polymer networks. The star polymers were deposited onto the surfaces of Si wafers by spin coating, and stable polymer films were formed by simple UV irradiation. The stability of thermoresponsive cross-linked polymer films deposited on the Si wafer was confirmed by changing their hydrophilicity by changing the temperature of the environment. In addition, the POEOMA-based star polymers could be utilized for the preparation of photolithography-patterned surfaces. The successful formation of uniform stable polymeric films indicates that Bzp-functionalized POEOMA star polymers can be used for a simple Si surface modification.

Reduced filler flocculation in the silica-filled styrene–butadiene–glycidyl methacrylate terpolymer

This study presents a method to improve the dispersion of silica in rubber compounds using a styrene-butadiene-glycidyl methacrylate terpolymer (GMA-SBR) synthesized by cold emulsion polymerization. It has been demonstrated that silica particles in conventional rubbers tend to agglomerate during storage, as well as at the onset of vulcanization, because of their polarity. GMA-SBR can improve the compatibility with silica by the formation of covalent bonds between the epoxy groups of GMA-SBR and silanol groups on the silica surface. SBR 1721 and GMA-SBR silica-filled compounds were prepared without curatives by a kneader and a two-roll mill. After compounding, the reaction of the epoxy group, filler flocculation, and morphology of the compounds were analyzed using infrared spectroscopy, a rubber process analyzer, and transmission electron microscopy, respectively. In addition, the content of bound rubber in the compounds was determined by extracting the unbound rubber material with toluene. The GMA-SBR silica-filled compounds had a higher bound rubber content and exhibited significantly different filler flocculation and silica dispersion behaviors compared with the SBR 1721 silica-filled compounds.

In situ formation of polymer–protein hybrid spherical aggregates from (nitrilotriacetic acid)-end-functionalized polystyrenes and His-tagged proteins

Polymer–protein hybrid nanostructures are important for their potential applications in pharmaceuticals and biotechnology, and new methods to generate hybrid nanostructures would be advantageous in the above fields. In this study, we present a simple method for the construction and control of polymer–protein hybrid spherical aggregates in situ from nickel complexed (nitrilotriacetic acid)-end-functionalized polystyrene (Ni-NTA-PS) and histidine-tagged green fluorescent protein (His-tagged GFP) through NTA-Ni–His interaction in water–DMF (DMF 4 vol.%) at physiological pH. The generality of the approach was demonstrated using His-tagged GFP and His-tagged lipases. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) measurements revealed that hybrid aggregates of His-tagged GFP were stable up to 15 days. The size of hybrid aggregates depended on the molecular weight of polymers, concentration of polymers, concentration of protein, and also the rate of addition of polymer to the solvent containing protein. After formation of hybrid aggregates, removal of DMF by dialysis from the system resulted in the aggregation and phase separation of aggregates over time. The size of polymer–protein hybrid aggregates decreased with excess of imidazole addition. A possible mechanism for the formation of such polymer–protein hybrid aggregates is also described.

Covalent attachment of polystyrene on multi-walled carbon nanotubes via nitroxide mediated polymerization

A new method to attach polymers on carbon nanotubes has been studied. We used nitroxide mediated polymerization (NMP) to graft polystyrene from multi-walled carbon nanotubes (MWNTs). Carboxyl groups on MWNTs were activated with thionyl chloride (SOCl2) to give acyl chloride derivative (MWNT-COCl). NMP initiator was introduced to MWNT by esterification of 1-hydroxy-2-phenyl-2-(2′,2′, 6′, 6′-tetramethyl-1′-piperidinyloxy)ethane (HO-PE-TEMPO) with acyl chloride groups of MWNTs. The obtained MWNT-PE-TEMPO was used for initiation of bulk polymerization of styrene, yielding polystyrene-grafted MWNTs (MWNT-g-pSt). The resulting composites of MWNT-g-pSt were analyzed by TEM, SEM, FT-IR and TGA.