Biomedical subjects

Jin Kon Kim

Publications and source records attributed to Jin Kon Kim.

9 recordsLinked to original sources

PubMed2006

Two-dimensional heterospectral correlation analysis of wide-angle X-ray scattering and infrared spectroscopy for specific chemical interactions in weakly interacting block copolymers.

We investigated, via two-dimensional heterospectral correlation analysis of wide-angle X-ray scattering (WAXS) and infrared (IR) spectroscopy, the specific chemical interactions existing in weakly interacting polystyrene-block-poly(n-pentyl methacrylate) copolymers (PS-PnPMA). PS-PnPMA was shown to exhibit a closed-loop-type phase behavior, where, upon heating, a lower disorder-to-order transition (LDOT) was found at lower temperatures, and an upper order-to-disorder transition (UODT) was observed at higher temperatures. The specific interaction between the PS and PnPMA blocks mainly arises from the dipole in the benzene ring of PS and the induced dipole in the PnPMA due to cluster formation with a size of 1 approximately 2 nm. We found that the synchronous 2D WAXS-IR heterospectral correlation spectrum of the ordered state was completely different from that in the two disordered states. The CH group of the main chains of PS and PnPMA did not contribute to the cluster formation in the two disordered states, indicating that the main chains of PS and PnPMA blocks were randomly distributed in the two disordered states. However, only the C=C group in the PS block contributed to the cluster at a disordered state below the LDOT, whereas both the C-C-O group in PnPMA and the entire phenyl ring and C=C group in PS contributed to cluster formation at another disordered state above the UODT. Thus, the probability that PS (and PnPMA) chains were located at their own neighboring chains at one disordered state above the UODT is larger than that at another disordered state below the LDOT.

Journal Article↗

PubMed2006

Block copolymer film with sponge-like nanoporous strucutre for antireflection coating.

We prepared nanoporous films by spin-coating of polystyrene-block-poly(methyl methacrylate) copolymers (PS-b-PMMA) to a glass and irradiating by ultra-violet source followed by selective removal of PMMA blocks with acetic acid. When spin-coated PS-b-PMMA was no longer annealed at high temperatures, microphase separation between two blocks occurred only in the short-range scale. The porous films prepared from PS-b-PMMA with the volume fraction of PMMA block of 0.69 exhibited a spongelike nanoporous structure over the entire film thickness and showed excellent antireflection with a minimum reflection less than 0.1% at visible and near-infrared wavelengths. The observed reflectances were in good agreement with the predictions based on the characteristic matrix theory.

Light↗

PubMed2006

Transient solidlike behavior near the cylinder/disorder transition in block copolymer solutions.

A nearly symmetric polystyrene-block-polyisoprene diblock copolymer dissolved at a concentration of 40% in styrene-selective solvents exhibited a cylinder-to-disorder transition upon heating. The solvents used were diethyl phthalate (DEP) and 75:25 and 50:50 mixtures of DEP with di-n-butyl phthalate (DBP). In DEP, the most styrene-selective of the three solvents, rheological measurements indicated a distinct plateau in the temperature-dependent elastic modulus across the 8 degrees C interval above the order-disorder transition temperature, T(ODT) = 116 degrees C. Previous small-angle neutron scattering measurements in this regime indicated the equilibrium phase to be a liquidlike solution of approximately spherical micelles. An isothermal frequency sweep in this regime indicated a very long relaxation time. Annealing eventually led to the recovery of liquidlike rheological response, over a time scale of hours. Qualitatively similar phenomena were also observed in 75:25 DEP/DBP and 50:50 DEP/DBP solutions, except the fact that the temperature window of the transient response is narrow and the time scale for the recovery diminishes significantly. Neither small-angle X-ray scattering nor static birefringence gave any clear signature of the transient structure. The structure that leads to the transient rheological response is attributed to micellar congestion due to the slow relaxation of anisotropic micelles into an equilibrium distribution of micelles. Possible origins of the remarkable solvent selectivity dependence are also discussed.

Butadienes↗

PubMed2006

An optical waveguide study on the nanopore formation in block copolymer/homopolymer thin films by selective solvent swelling.

Thin films of mixtures of asymmetric poly(styrene-block-methyl methacrylate) (PS-b-PMMA) diblock copolymers and PMMA homopolymers with cylindrical PMMA microdomains oriented normal to the substrate surface were used to couple optical modes in the Kretschmann configuration, and their optical properties were investigated by optical waveguide spectroscopy (OWS). The nanopore formation in the block copolymer (BCP) waveguide layer via selective solvent swelling and subsequent reannealing was monitored in terms of shifts in the coupling mode angles. The sequential swelling/reannealing of the initial mixture film resulted in a number of discrete or partially interconnected pores instead of cylindrical pores with a high aspect ratio. The simultaneous processes occurring inside and on top of the BCP waveguide layer were discerned independently with high selectivity for p- and s-polarization.

Membranes, Artificial↗

PubMed2006

Porous structures of polymer films prepared by spin coating with mixed solvents under humid condition.

We investigate the effects of interfacial energy between water and solvent as well as polymer concentration on the formation of porous structures of polymer films prepared by spin coating of cellulose acetate butyrate (CAB) in mixed solvent of tetrahydrofuran (THF) and chloroform under humid condition. The interfacial energy between water and the solvent was gradually changed by the addition of chloroform to the solvent. At a high polymer concentration (0.15 g/cm3 in THF), porous structures were limited only at the top surfaces of CAB films, regardless of interfacial energies, due to the high viscosity of the solution. At a medium concentration (approximately 0.08 g/cm3 in THF), CAB film had relatively uniform pores at the top surface and very small pores inside the film because of the mixing of the water droplets with THF solution. When chloroform was added to THF, pores at the inner CAB film had a comparable size with those at the top surface because of the reduced degree of the mixing between the water droplets and the mixed solvent. A further decrease in polymer concentration (0.05 g/cm3 in THF) caused the final films to have a two-layer porous structure, and the size of pores at each layer was almost the same.

Cellulose↗

PubMed2005

Broad-band antireflection coating at near-infrared wavelengths by a breath figure.

We prepared a porous thin film by spin-coating of a cellulose acetate butyrate (CAB) solution in tetrahydrofuran under a humid environment. Due to evaporative cooling during spin-coating, condensed water droplets were formed by a breath figure on the CAB solution, and these developed a porous structure after complete drying. By varying the solution concentration and rotating speeds, two distinct morphologies were generated: top and bottom layers with higher and lower porosities, respectively. We found that the two-layer porous film coated on glass exhibited low reflectance of less than 1% in the near-infrared (NIR) regime corresponding to wavelengths between 900 and 2200 nm. Since the porous structure was very uniform over a large area, the film could be easily employed for broad-band antireflection coating at NIR wavelengths.

Journal Article↗

PubMed2004

Breath figure patterns prepared by spin coating in a dry environment.

We introduce a novel method for fabricating breath figure patterns on a homopolymer film by spin coating of polymer solutions with various solvents. The homopolymers employed in this study were cellulose acetate butyrate, monocarboxylated end-functional polystyrene, and poly(methyl methacrylate). Breath figure patterns were generated even when a water-miscible solvent such as tetrahydrofuran (THF) was used as a solvent. We even succeeded in generating breath figure patterns by spin coating even under a dry environment (relative humidity less than 30%), when water was directly added into THF solution. With the combination of the spin coating method, pores with a few hundred nanometers to several micrometers have been generated. We found that the pore size becomes larger with increasing water content in THF solution and decreasing rotating speed. This is equivalent to increasing humidity and decreasing evaporation speed, respectively, in the conventional method, which is direct solvent evaporation under a humid environment. Thus, compared with the conventional method for making breath figure patterns, this method would be very convenient for fabricating large-scale films with various pore sizes.

Journal Article↗

PubMed2003

Effect of hydrostatic pressure on closed-loop phase behavior of block copolymers.

The effect of hydrostatic pressure (P) on closed-loop phase behavior of deuterated polystyrene-block-poly(n-pentyl methacrylate) copolymers [dPS-PnPMA] was investigated by using small-angle neutron scattering and birefringence. For P<20.7 bar, dPS-PnPMA exhibited a lower disorder-to-order transition temperature (T(LDOT)) at 175 degrees C, and then an upper order-to-disorder transition temperature (T(UODT)) at 255 degrees C. With increasing pressure both T(LDOT) and T(UODT) were markedly changed, where dT(LDOT)/dP was 725 degrees C/kbar and dT(UODT)/dP was -725 degrees C/kbar. These are consistent with predictions by the Clausius-Clapeyron equation using measured values of the volume and enthalpy changes of both transitions. The large pressure coefficients imply that the closed-loop phase behavior observed for PS-PnPMA is an entropic-driven phase transition.

Journal Article↗

PubMed2002

Closed-loop phase behaviour in block copolymers.

Closed-loop phase diagrams are known in systems with specific intermolecular interactions. In weakly interacting systems, however, such behaviour has never been observed. Here, diblock copolymers formed from polystyrene covalently linked to poly (n-pentylmethacrylate), P(S-b-nPMA), which have only weak segmental interactions, are shown to exhibit a closed-loop phase behaviour over a narrow range of molecular weight. The endothermic transitions from the disordered to ordered and back to the disordered state, as a function of increasing temperature, are dominantly entropic in origin. The morphology and rheological properties of P(S-b-nPMA) undergo characteristic changes at the transitions. Whereas the disorder-to-order transition temperature increases with decreasing molecular weight, the order-to-disorder transition temperature decreases. At a limiting molecular weight, the closed-loop vanishes and no ordering occurs. These findings provide quantitative insight into an elusive transition in weakly interacting multicomponent systems.

Crystallography, X-Ray↗