Dukjoon Kim

Sulfonated polystyrene grafted polypropylene composite electrolyte membranes for direct methanol fuel cells

Abstract Polypropylene (PP)-g-sulfonated polystyrene (SPS) composite membranes were prepared by grafting polystyrene (PS) on microporous polypropylene membranes via plasma-induced polymerization. Grafting of polystyrene was established not only inside the pores but also on the surface of PP membranes, followed by the sulfonation reaction. The chemical and physical structure of PS-g-PP membranes was investigated using FTIR and SEM. The thickness and weight of the composite membrane increased with increasing grafting time. Ion exchange capacity (IEC), ion conductivity, and methanol permeability coefficient were measured and analyzed according to grafting reaction and sulfonation time. While both the ion conductivity and methanol permeability coefficient increased with grafting amount, the characteristic factor was comparable to that of Nafion®.

Batch and column separation characteristics of copper-imprinted porous polymer micro-beads synthesized by a direct imprinting method

Copper (II) ion-imprinted porous polymethacrylate micro-particles were prepared. Two functional monomers, methacrylic acid and vinyl pyridine, formed a complex with the template copper ion through ionic interactions. The self-assembled copper/monomer complex was polymerized in the presence of an ethylene glycol dimethacrylate cross-linker by a suspension method. After the imprinting sites were provided through removal of the template, the micro-porous particles, of approximate size 200 microm, were obtained for batch and column separation applications. The chemical structure and morphology of the Cu(II)-imprinted micro-porous particles were analyzed using FTIR, SEM, and BET. The adsorption capacity and adsorption kinetics of the imprinted beads for the template Cu(II) ion were significantly affected by particle size, copper ion concentration, pH, and flow rate of the feed solution. The imprinted particles showed high selectivity for the copper ion over other metal ions such as Ni and Zn. The selectivity of the present imprinted polymers for the copper ion was at least 10 times as high as those from commercial sources.

Drug releasing characteristics of thermo- and pH-sensitive interpenetrating polymer networks based on poly(N-isopropylacrylamide)

Interpenetrating polymer networks (IPNs) of poly(N-isopropylacrylamide)/polyurethane (PNIPAAm/PU) and poly(N-isopropylacrylamide)/poly(acrylic acid) (PNIPAAm/PAA) were synthesized to investigate the swelling and drug releasing behavior. The presence of urethane network in PNIPAAm/PU IPNs improved the mechanical strength, but reduced the swelling and drug releasing rates because of its hydrophobic characteristics. The swelling transition temperatures of PNIPAAm gels were little affected by the incorporation of PU networks in IPN structures. The drug releasing process was analyzed with a simple exponential expression of time dependent fractional drug release. The swelling and drug releasing behavior of PNIPAAm/PAa IPNs was significantly affected by the variation of PAA compositions. The drug release process changed from anomalous to dual type via zero-order mode with increasing PAA concentration due to the competitive swelling rates between PNIPAAm and PAA during release process. The releasing rate decreased in the buffer solution of pH 7.4, but increased in that of pH 5.0 with increasing PAA concentration at both 28 and 37°C because the swelling power of PAA in pH 5.0 was much less than that in pH 7.4.

SAXS and NMR analysis for the cast solvent effect on sPEEK membrane properties.

The cast solvent effect on the structure and properties of sulfonated poly(ether ether ketone) (sPEEK) was studied. PEEK was sulfonated to have different sulfonation degrees of 65, 70, and 75%, and its membrane was prepared using the two types of solvents, N,N-dimethylacetamide (DMA) and 1-methyl-2-pyrrolidinone (NMP). Ionic cluster size was analyzed using small-angle X-ray scattering (SAXS), and it was correlated with a few essential membrane properties such as water uptake, methanol permeability, proton conductivity, and cell performance in direct methanol fuel cells (DMFCs). Synchrotron SAXS and solid state NMR data revealed the structural difference between the sPEEK membranes prepared using NMP and DMA, regarding the cluster dimensions of 3.22 and 2.70 nm, respectively. Although the water uptake, methanol permeability, and proton conductivity of the membranes prepared with NMP were higher than those with DMA, the overall cell performance was vice versa. The dimensional instability associated with high water swelling as well as high methanol permeability were the main causes for this inferior cell efficiency of NMP cast membranes. This report demonstrates the importance of selection of cast solvent in preparation of SPEEK electrolyte membranes for DMFC application.

Synthesis of lactide from oligomeric PLA: Effects of temperature, pressure, and catalyst

Lactide was produced from oligomeric PLA by back-biting reaction of the OH end groups. For optimization of the reaction conditions, the effects of temperature, pressure, PLA molecular weight, and catalyst type on the lactide synthesis were examined. The fraction of D,L-lactide decreased with increasing temperature. Among the various Sn-based catalysts, the D,L-lactide fraction was maximized when SnO was used. A higher yield with lower racemization was observed at lower pressure. The conversion of PLA was maximized at an oligomeric PLA molecular weight of ca. 1380. The yield of lactide increased but the fraction of D,L-lactide decreased with increasing molecular weight. The highest conversion with the lowest racemization degree was obtained at a catalyst concentration of 0.1 wt%. The lactide was more sensitive to racemization because of the entropic effect.

Effect of Morphology and Pore Size of Sulfonated Mesoporous Benzene-silicas in the Preparation of Poly(vinyl alcohol)-Based Hybrid Nanocomposite Membranes for Direct Methanol Fuel Cell Application

Sulfonated mesoporous benzene-silicas were introduced into a poly(vinyl alcohol) (PVA) polymer matrix to act as a barrier for methanol crossover, to prepare composite electrolyte membranes for direct methanol fuel cell applications. Highly ordered 2D hexagonal mesoporous benzene-silicas were prepared using 1,4-bis(triethoxysilyl)benzene (BTEB) organosilica precursor and two kinds of organic templates, such as an octadecyltrimethylammonium bromide (ODTMA) and a Pluronic P123 poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer, to investigate the effect of the morphology and the pore size on the methanol permeability and the proton conductivity of the membranes. The sulfonated mesoporous benzene-silica and PVA were mixed with a sulfosuccinic acid (SSA) cross-linker to improve the membrane stability from mechanical and conductive viewpoints. The physical and chemical characterization of the hybrid electrolyte membranes was performed by varying the contents of sulfonated mesoporous benzene-silicas and SSA. All the hybrid membranes studied showed good performance in lowering the methanol crossover (i.e., approximately 68% reduction in comparison with the Nafion117 membrane), and mesoporous benzene-silica with smaller particle morphology and pores (2-3 nm) was observed to be a more effective additive.

Preparation and Characterization of Nafion/Poly(1-vinylimidazole) Composite Membrane for Direct Methanol Fuel Cell Application

The base monomer, 1-vinyl imidazole was impregnated in a Nation 112 membrane and polymerized to poly(l-vinylimidazole) (PVI) by UV irradiation in order to reduce the methanol permeability of the Nation membrane when used in a direct methanol fuel cell. As the PVI content in the composite membrane was increased, the equilibrium water uptake decreased and the size of the hydrated ion clusters was reduced, as confirmed by small angle X-ray scattering analysis. The electrochemical properties of the membrane, such as its proton conductivity, methanol permeability, and electro-osmotic drag, were also affected by the equilibrium water uptake and hydrated pore size. Even a small amount of the base polymer incorporated into the membrane resulted in a significant effect on its proton conductivity and methanol permeability. The methanol transport induced by the electro-osmotic drag was evaluated and the methanol permeability and limiting current density data obtained in this study. Although the absolute number of the electro-osmotic drag was not determined, the trend of change is discussed in relation to the bulk-like water in the composite membranes. The novel composite membrane exhibited improved cell performance compared with a plain Nafion membrane due to its reduced methanol crossover rate.

Preparation and characterization of water-swellable natural rubbers

Water-swellable rubbers were prepared by dispersing the superabsorbent polymer particles, sodium polyacrylate particles, in natural rubber, and their water absorption properties were investigated. Sodium polyacrylate particles were synthesized using the inverse suspension polymerization technique, and their thermal and water absorption properties were characterized. The equilibrium water uptake in sodium polyacrylate particles was strongly dependent on both the salt concentration of aqueous media and crosslinking density of polymer. The dynamic and equilibrium water-swelling behavior of the prepared rubbers were significantly affected by addition of carbon black, hydrophilic polymer, and coupling agent. Those effects were well explained by microphotographic morphologies obtained using a scanning electron microscope.

Three-dimensional cubic (Im3m) periodic mesoporous organosilicas with benzene- and thiophene-bridging groups

Three-dimensional cubic (Im3m) periodic mesoporous organosilicas with aromatic benzene- and thiophene-bridging groups were synthesized using 1,4-bis(triethoxysilyl)benzene and 2,5-bis(triethoxysilyl)thiophene as organosilica precursors in the presence of a F127 PEO–PPO–PEO triblock copolymer under acidic conditions. Highly ordered 3D cubic (Im3m) mesostructure was confirmed by synchrotron small angle X-ray scattering and transmission electron microscopy. Nitrogen adsorption analysis of the aforementioned samples (extracted and additionally calcined in nitrogen at 375–400 °C) revealed their high surface area (460–630 m2 g−1) and accessible ordered large pores (in the range of 6–9 nm). High thermal stability of these benzene- and thiophene-PMO samples (up to 500 and 400 °C, respectively) was confirmed by thermogravimetric analysis, while their framework chemistry was studied by solid-state 13C and 29Si MAS NMR.

Formation of Poly(ethylene glycol)-Poly(ε-caprolactone) Nanoparticles via Nanoprecipitation

Size control of therapeutic carriers in drug delivery systems has become important due to its relevance to biodistribution in the human body and therapeutic efficacy. To understand the dependence of particle size on the formation condition during nanoprecipitation method, we prepared nanoparticles from biodegradable, amphiphilic block copolymers and investigated the particle size and structure of the resultant nanoparticles according to various process parameters. We synthesized monomethoxy poly(ethylene glycol)-poly(ε-caprolactone) block copolymer, MPEG-PCL, with different MPEG/PCL ratios via ring opening polymerization initiated from the hydroxyl end group of MPEG. Using various formulations with systematic change of the block ratio of MPEG and PCL, solvent choice, and concentration of organic phase, MPEG-PCL nanoparticles were prepared through nanoprecipitation technique. The results indicated that (i) the nanoparticles have a dual structure with an MPEG shell and a PCL core, originating from self-assembly of MPEG-PCL copolymer in aqueous condition, and (ii) the size of nanoparticles is dependent upon two sequential processes: diffusion between the organic and aqueous phases and solidification of the polymer.