Hyun Hoon Song

Carboxymethylation of corn starch and characterization using asymmetrical flow field-flow fractionation coupled with multiangle light scattering

Asymmetrical flow field-flow fractionation (AsFlFFF) was coupled online with multiangle light scattering (MALS) to study the changes in the molecular weight and the size distribution of the corn starch during carboxymethylation. A corn starch was derivatized with sodium chloroacetate in alcoholic medium under alkaline condition to produce carboxymethyl starches (CMS) having various degrees of substitution (DS). The change in thermal characteristics and granule structure of the native corn starch and CMS were compared using Thermogravimetric analysis and scanning electron microscope. The ionic strength of the carrier liquid (water with 0.02% NaN(3)) was optimized by adding 50mM NaNO(3) to minimize the interactions among the starch molecules and between the starch molecules and the AsFlFFF membrane. A field-programmed AsFlFFF allowed determination of the molecular weight distribution (MWD) of starches within about 25min. It was found that carboxymethylation of starch results in reduction in the molecular weight due to molecular degradation by the alkaline treatment. The weight-average molecular weight (M(w)) was reduced down to about 4.4x10(5) from about 7.2x10(6) when DS was 0.14. It seems AsFlFFF coupled with MALS (AsFlFFF/MALS) is a useful tool for monitoring the changes taking place in the molecular weight and the size of starch during derivatization.

Novel bent-shaped liquid crystalline compounds: 1. Synthesis and structure analysis of dimesogenic compounds with azo units

Abstract A new series of bent-shaped liquid crystalline compounds (2An), which were composed of two identical mesogens (azobenzene-type), terminal alkyloxy chains (n=6, 8, 10, 12) and 2-hydroxy-1,3-dioxypropylene as central linking unit, have been prepared. Their mesomorphic properties were characterized by polarized light microscopy, differential thermal analysis, and X-ray diffraction (XRD). For 2An series, n=8, 10 and 12 compounds showed enantiotropic SmC phase, but n=6 compound did not show liquid crystal phase, as was confirmed by the XRD patterns.

Novel bent-shaped liquid crystalline compounds II. Synthesis and properties of the 1,3-bis{4-[4-(4-alkyloxybenzoyloxy)benzylidene]aminophenoxy}propan-2-ols

A series of new symmetric dimer compounds was synthesized, constaining 2-hydroxy-1,3-dioxypropylene as the central linkage and terminal alkyl chains with different lengths. The chemical structures of the liquid crystal dimers (2ES-n) were examined by FTIR and 1H NMR spectroscopy. Their mesomorphism, thermodynamic properties and optical textures were investigated by differential scanning calorimetry, polarizing optical microscope and X-ray diffraction. For homologues with terminal propyloxy and butyloxy chains, no liquid crystalline phase was observed. Homologues with pentyloxy and hexyloxy terminal chains showed nematic phases, while those with heptyloxy, octyloxy, nonyloxy and decyloxy terminal chains displayed nematic phases and smectic phases. The results confirmed that the liquid crystalline phase changes from nematic to smectic as the terminal chain length increases.

Preparation of rigid-rod poly(p-phenylene benzobisthiazole) films of single crystalline texture

Abstract Thin films of rigid-rod poly(p-phenylene benzobisthiazole) having single crystalline texture were prepared. An above critical concentration polymer solution of 6 wt% in methane sulfonic acid was sheared to orient the chains (c-axis) along the shear axis in the solution state. The sheared solution was then solidified into thin films via coagulation in the water. Self-assembling phenomenon, previously found in rigid-rod PBZT polymer molecules during the coagulation process, was utilized as the driving mechanism for the alignment of crystallographic a and b -axes in the process. WAXS and FT i.r. spectroscopy with the polarized radiation confirmed the presence of single crystal-like orientation in the film specimen with the crystallographic c -axis preferentially oriented parallel to the shear direction, b -axis to the transverse direction, and a -axis normal to the film surface.

Synthesis of Symmetric Liquid Crystal Dimers Based on Azo and Imine Groups and Investigation of Phase Behaviour by Varying Alkoxy Terminal Chain Length

With the objective to study the effect of alkoxy terminal chain length on mesomorphic properties of liquid crystals, we have synthesized two (Azo and Imine) different series of dimesogens by varying terminal alkoxy chain length (n = 6–12) with a short spacer unit in between two mesogens. Transition temperatures and phase characterization were studied by DSC, POM and XRD analysis. It was found that all the dimers show mesomorphic properties and the change in terminal alkoxy chain length has pronounced effect on the smectic phase window. In the DSC cooling scan, the smectic phase window of azo compounds increased from 6.4°C (2A6) to 16.0°C(2A12), whereas in Schiff base compounds, it increased from 9.2°C (2S6) to 31.0°C(2S12). Further, the 2A11 dimer was found to undergo photo induced configurational changes.

Structure and Property Modification of Bimodal Molecular Weight Distribution Polyethylene by Electron Beam Irradiation

Polyethylene of bimodal molecular weight distribution was irradiated with an electron beam. The thermal and mechanical properties were examined by DSC, small and wide angle X-ray scattering and static tensile test according to the crystal morphology of the irradiated samples. The crystal morphology change upon irradiation, as revealed by wide angle X-ray scattering, correlated well with the changes in melting enthalpy, whereas the lamellar thickness and the amorphous gap thickness remained virtually unchanged at irradiation doses up to 500 kGy. Crosslinks in the crystal domains became evident at an energy level of 250 kGy, resulting in reduced crystallinity and crystal size of the (110) and (200) planes. The samples became stiff and brittle with increased irradiation dose, which seem to be more relevant to the amount of crosslinks than the crystal morphology changes.

Solvent effects on self-assembly and superstructures of amide dendrons

AbstractSupramolecular ordering via self-assembly of amide dendrons was investigated in different solvents. The selection of the solvents strongly affected the supramolecular ordering of third generation dendrons depending on the solubility strength while only a minimal effect was seen with the first and second generation dendrons. The number of amide branches and alkyl tails which are responsible for the hydrogen bond and steric hindrance determined the solvent effect. Relatively low numbers of branches in the first and second generation dendrons compared with the third generation resulted in the minimal solvent effect. Solutions of low solubility favoring collapsed and extended conformation of third generation dendrons induced lamellar superstructure, while solutions of high solubility favoring expanded globular conformation formed columnar superstructures. The results were discussed based on the Kamlet-Taft parameters of the solvents.

Synthesis of Symmetric Liquid Crystal Dimers Based on 1,2-bis{-[-(4-alkoxybenzoyloxy)benzylidene]amino)phenoxy}ethane and Investigation of Phase Behaviour by Varying Alkoxy Terminal Chain Length

With the objective to study the effect of alkoxy terminal chain length on mesomorphic properties of liquid crystals, We have synthesized ester and Schiff base dimers from diamine derivative containing 1,2-dioxyethylene as short spacer with aldehyde derivatives having different lengths of terminal alkoxy chains (n = 6–12). The chemical structure of the final products was investigated by fourier transform infrared (FT-IR) spectroscopy and proton nuclear magnetic resonance (1H NMR) spectroscopy. The mesomorphic properties and optical textures of the resultant dimers were characterized by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). It was found that all the dimers show mesomorphic properties and the increase in terminal alkoxy chain length has pronounced effect on the mesomorphic properties. The temperature range of smectic phase window widens and nematic-isotropic phase window narrows sharply with increasing alkoxy chain length.

Poly(2,5‐dihydroxy‐1,4‐phenylene benzobisthiazole)/poly(1,4‐phenylene benzobisthiazole) copolymers: Chain packing and properties

Crystal-packing, optical, and electrical properties of poly(2,5-dihydroxy-1,4-phenylene benzobisthiazole) (DiOH-PBZT) and copolymers of DiOH-PBZT/poly(1,4-phenylene-benzobisthiazole) (PBZT) were examined. Intramolecular hydrogen bonds between the hydroxyl units and the neighboring nitrogen atoms, as evidenced by the IR spectra, led to the formation of a pseudoladder chain structure and changed the chain packing. The (200) and (010) planes were both affected by the copolymer composition, with the (200) plane spacing increasing from 5.895 to 6.482 A and the (010) plane spacing decreasing from 3.539 to 3.404 A with the transition from the unsubstituted PBZT homopolymer to the DiOH-PBZT homopolymer. The cell dimensions of the copolymers were simple averages of those of the individual homopolymers, suggesting the isomorphic crystal structure formation of the two units. The c-axis spacing, however, remained unchanged. The increase in the conjugation length of the copolymers as the dihydroxy content increased was confirmed by the bathochromic shift of the absorption band in the ultraviolet-visible spectra. The intrinsic conductivities of the copolymers were 3 orders of magnitude higher than that of the unsubstituted PBZT.

Strain-induced crystallization of blends of natural rubber and ultra high cis polybutadiene as studied by synchrotron X-ray diffraction

Strain-induced crystallization of a series of natural rubber (NR) and ultra high cis polybutadiene (UBR) blend was studied by the in situ synchrotron wide angle X-ray diffraction. X-ray diffraction peaks from the oriented rubber were decomposed into the three phases (isotropic amorphous (IA), oriented amorphous (OA), and crystalline (CR)) to probe the structure changes upon stretching. Overall chain orientation of the blend upon stretching was speeded up, which was found to be associated with the UBR chains. The strain-induced crystallization was retarded by the UBR chains. It was found out that 100% UBR did not show crystallization and the crystallization in the blend was only associated with NR chains. Furthermore, the early oriented UBR chains do not play a role as nucleation sites for the NR chains to crystallize. The crystallite size became larger with the UBR content, which was attributed to the small number of nuclei formation by the presence of UBR chains.