Hye-Jin Jeon

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.

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.

Microstructural Changes of Starch Aqueous Solutions during Gelatinization and Retrogradation Studied through X-ray Scattering

Microstructural changes of potato and corn starch aqueous solutions associated with their gelatinization and retrogradation processes were examined through the use of small- and wide-angle X-ray scattering. It was found that the structural changes during these two processes were dominantly affected by the amount of phospholipids and phos- phates contained in the starch. The phosphates contained in the amylopectin of the starch granule allow easy penetra- tion of water into the structure, thus causing easy swelling or high solubility. This simultaneously allows greater freedom of chains, thus causing the recrystallization of the chains during the retrogradation process. On the other hand, in corn starch, which contains a lower amount of phosphates and a complex structure formed between phospholipid and amylose, the chain movements are much reduced, resulting in a higher gelatinization temperature and a reduction of recrystallization during retrogradation and a broad distribution of crystal size. In both starches, a micro-phase sepa- rated inhomogeneous structure was suggested in the amorphous state above the gelatinization temperature, which appears to be associated with the presence of highly branched amylopectin units.