Byoung Gak Kim

Effect of n-alkyl and sulfonyl groups on the wetting properties of comblike poly(oxyethylene)s and stick-slip behavior.

The influence of side chain length and sulfonyl moiety on the molecular structures and wettability behavior of poly(oxyethylene)s with alkyl sulfonyl side chains (CH(3)-nSE, n = 1, 2, 3, 4, 5, 6, 8, 10), where n is the number of the carbon atom in the n-alkyl side group, was investigated. CH(3)-nSEs having shorter side chains (n < 5) do not have ordered structures, and their surfaces were found to be more polar than those of CH(3)-nSEs having longer side chains (n ≥ 5). The CH(3)-nSEs having longer side chains show double-layered lamellar structures (n ≥ 5) with well-aligned side chains and low surface energies in the range 21.2-25.8 mN/m. Interestingly, stick-slip behavior was observed only on the surfaces of CH(3)-3SE and CH(3)-4SE when water was used as the test liquid. The surface deformation at the three-phase line was generated from interactions between water and sulfonyl groups, and the optimum side chain lengths were believed to cause the stick-slip behavior.

Wettability of the morphologically and compositionally varied surfaces prepared from blends of well ordered comb-like polymer and polystyrene

Phase-separated surfaces of blends of polystyrene (PS) and well ordered comb-like polymer, poly[(oxy(decylsulfonylmethyl)ethylene)] (CH(3)-10SE), were prepared by spin casting polymer mixtures. Various surface morphologies, such as holes, islands, connected islands and pillars, were prepared by changing the blend compositions. Due to the influence of the CH(3)-10SE domain with a well ordered molecular conformation, a very low energy surface (≈22mN/m) was created, which is close to the value of the pure polymer (≈20mN/m), even when the blends contained only 20wt.% of the pure polymer. Furthermore, by selective etching the PS domain in the blend surfaces, the advancing contact angles of water and n-hexadecane were highly increased from 113.5° and 43.2° for the pure CH(3)-10SE surface to 133.3° and 67.2° for the CH(3)-10SE structural surfaces with holes prepared using the solvent etching method, respectively. The result of the water advancing contact angles measured on the samples immersed in water over 20days showed that the film stability of CH(3)-10SE could be improved considerably by even adding small amounts of PS.

Sulfonation of PIM-1 — towards highly oxygen permeable binders for fuel cell application

AbstractThe development of alternative, non-fluorinated membranes for polymer electrolyte membrane fuel cells necessitates the co-development of a non-fluorinated electrode catalyst binder to ensure compatibility between membrane and electrode. However, most hydrocarbon based polymers have lower gas permeability than perfluorinated Nafion. In this work we tried to obtain a sulfonated, non-fluorinated binder based on PIM-1 (polymer of intrinsic microporosity 1) which has up to 2000 times higher permeability than Nafion. However, sulfonation was not straightforward and often led to degradative side reactions. Sulfonated polymers were too brittle to give stable membranes and the highest experimental IEC was 1.03 meq/g, significantly lower than the theoretical IEC of 3.2 meq/g (2 sulfonic acid groups per repeat unit).

Ultra-hydrophobic sticky polymer surfaces formed by water-induced surface deformation

Surface properties of poly(oxyethylene)s with alkyl thioether side chains (CH3-nTE; n=carbon atoms in the side chain) were investigated. CH3-nTEs having shorter side chains (n=6, 8, 10) did not have ordered structures, while those with longer chains (n=12, 14) showed well-ordered, lamellar structures with side chain crystalline domains. Accordingly, CH3-12TE and CH3-14TE film surfaces were much more enriched with hydrophobic alkyl side chains than CH3-8TE and CH3-10TE surfaces. However, CH3-12TE and CH3-14TE films had smaller water contact angles (≈114°) than CH3-8TE and CH3-10TE films (>130°). This unexpected behavior is ascribed to the formation of very rough CH3-8TE and CH3-10TE film surfaces (Rrms>100nm) on contact with water. The CH3-10TE/poly(methyl methacrylate) (PMMA) surfaces also showed high water contact angles (>125°), even with low CH3-10TE content (30wt%), because the CH3-10TE moiety roughened the surfaces. Furthermore, ultra-hydrophobic sticky behavior was observed for CH3-nTEs (n=8, 10) and CH3-10TE/PMMA surfaces.

Enhanced dielectric properties of polyimide/BaTiO3 nanocomposite by embedding the polypyrrole@polyimide core-shell nanoparticles

A three-phase polymer composite with a ferroelectric phase (BaTiO3) and conductive core-shell polypyrrole@ polyimide (PPy@PI) nanoparticles embedded in a polyimide (PI) matrix was prepared by using a simple direct mixing and solution casting process. PPy@PI nanoparticles as the conductive filler were synthesized to obtain homogeneous dispersion in the PI matrix. The dependence of the dielectric behavior on the BaTiO3 and PPy@PI contents was studied over a wide frequency range from 10 kHz to 1 MHz. The incorporation of low loading levels of PPy@PI nanoparticles to the PI/BaTiO3 system led to a significant enhancement in the dielectric constant with suppressed loss. The dielectric constant (Dk) of the PI/BaTiO3/PPy@PI (25/75/5) hybrid was 58.53 at 10 kHz, i.e., three times higher than that of the PI/BaTiO3 (25/75) nanocomposite without PPy@PI. The fairly improved dielectric properties were due to the synergistic enhancement of the ferroelectric ceramic filler, BaTiO3, and conductive filler, PPy@PI nanoparticles, as well as due to homogeneous dispersion. Such polymer composites have the potential for use in fabricating embedded capacitors, and specimens with various shapes can be easily obtained owing to the flexibility of the nanocomposite.

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.