Sung-Seen Choi

Microstructural analysis and cis–trans isomerization of BR and SBR vulcanizates reinforced with silica and carbon black using NMR and IR

SBR is composed of styrene, 1,2-unit, cis-1,4-unit, and trans-1,4-unit. Rubber chains were extracted from cured butadiene rubber (BR) and styrene-butadiene rubbers (SBRs) reinforced with silica and carbon black using o-dichlorobenzene, followed by coagulation to analyze their microstructures by nuclear magnetic resonance spectroscopy (NMR) and infrared spectroscopy (IR). Microstructures of the raw and cured samples were compared and their differences were discussed. The styrene contents of cured samples were greater than those of the raw samples because of a pendent group formed by a cure accelerator. The trans-1,4-unit contents of the cured samples were greater than those of the raw samples; whereas for the samples with high trans-1,4-unit content, the cis-1,4-units of cured samples were greater than those of the raw samples. This is because of cis–trans conversion, which is more favorable than the trans-to-cis conversion. By increasing the curatives content, the cis–trans conversion is activated. The 1,2-unit contents of cured samples were lower than those of the raw samples. The merits and demerits of the microstructural analyses using NMR and IR were also compared.

Analysis of cyclic pyrolysis products formed from amino acid monomer.

Amino acid was mixed with silica and tetramethylammonium hydroxide (TMAH) to favor pyrolysis of amino acid monomer. The pyrolysis products formed from amino acid monomer were using GC/MS and GC. 20 amino acids of alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine were analyzed. The pyrolysis products were divided into cyclic and non-cyclic products. Among the 20 amino acids, arginine, asparagine, glutamic acid, glutamine, histidine, lysine, and phenylalanine generated cyclic pyrolysis products of the monomer. New cyclic pyrolysis products were formed by isolation of amino acid monomers. They commonly had polar side functional groups to 5-, 6-, or 7-membered ring structure. Arginine, asparagine, glutamic acid, glutamine, histidine, and phenylalanine generated only 5- or 6-membered ring products. However, lysine generated both 6- and 7-membered ring compounds. Variations of the relative intensities of the cyclic pyrolysis products with the pyrolysis temperature and amino acid concentration were also investigated.

Formation of metal complex ions from amino acid in the presence of Li+, Na+ and K+ by electrospray ionization: metal replacement of hydrogen in the ligands

Alkali metal cations easily form complexes with proteins in biological systems; understanding amino acid clusters with these cations can provide useful insight into their behaviors at the molecular level including diagnosis and therapy of related diseases. For the purpose of characterization of basic interaction between amino acids and alkali metal, each of the 20 naturally occurring amino acids were ionized in the presence of lithium, sodium and potassium cations by electrospray ionization, and the resulting product ions were analyzed. We focus our attention on the gas phase alkali metal ion-proton exchanged complexes in current study, specifically complexes with serine, threonine, asparagine and glutamine, which share characteristic pattern unlike other amino acids. All amino acids generated [M + H](+) and [M + Na](+) ions, where M stands for the neutral amino acid. Serine, threonine, asparagine and glutamine generated cluster ions of [nM - nH + (n + 1)Na](+) and [nM - (n - 1)H + (n - 1)Na + K](+) , where n = 1-7. While the (M - H + Li) and (M - H + K) species were not observed, the neutral (M - H + Na) species formed by proton-sodium cation exchange had a highly stable cyclic structure with ketone and amine ligand sites, suggesting that (M - H + Na) serves as a building block in cluster ion formation. Cluster ion intensity distributions of [nM - nH + (n + 1)Na](+) and [nM - (n - 1)H + (n - 1)Na + K](+) showed a magic number at n = 3 and 4, respectively. Extensive B3LYP-DFT quantum mechanical calculations were carried out to elucidate the geometry and energy of the cluster ions, and they provided a reasonable explanation for the stability and structure of the cluster ions.

Deuterium effect on ionization and fragmentation patterns of monosaccharides ionized by atmospheric pressure chemical ionization

Glucose, galactose, and mannose in H(2)O and D(2)O were ionized by an atmospheric pressure chemical ionization (APCI) method. Isotope effects on fragmentation patterns of the monosaccharides were examined by deuterium replacement of the -OH groups to distinguish the isomers with a single mass spectrometer. The most abundant ions were the [M+H(2)O](+)() and [M(D5)+D+D(2)O](+) for using H(2)O and D(2)O as solvent and eluent, respectively. Major fragment ions were the [M-OH](+) and [M-OH-H(2)O](+) in H(2)O, while those in D(2)O were the [M(D5)+D-D(2)O](+) and [M(D5)+D-2D(2)O](+). The differences in the product ions generated in H(2)O and D(2)O were due to enhancement of the strength of hydrogen bonding by the deuterium replacement. Variations of the ion intensity ratios of the [M-OH](+)/[M-OH-H(2)O](+) and [M(D5)-OD](+)/[M(D5)-OD-D(2)O](+) with the fragmentor voltage showed different trends depending on the kind of monosaccharides. By comparing the ion intensity ratios of the [M+H(2)O](+)()/M(+)(), [M(D5)+D+D(2)O](+)/[M(D5)+D](+), [M-OH](+)/[M-OH-H(2)O](+), and [M(D5)+D-D(2)O](+)/[M(D5)+D-2D(2)O](+), it was possible to distinguish the isomers of monosaccharides.

Fragmentation patterns of protonated amino acids formed by atmospheric pressure chemical ionization

RATIONALE Dissociation reactions of protonated amino acids (AAs) can be used as models for the fragmentation of protonated peptides. Atmospheric pressure chemical ionization mass spectrometry (APCI-MS) provides a great deal of structural information in a short analysis time. METHODS In APCI-MS, the fragmentation patterns can be obtained by varying the cone voltage and some fragment ions are produced that can be used to identify the structure of an analyte. In general, the fragmentation of AAs has used liquid chromatography/tandem mass spectrometry (LC/MS/MS). However, we studied the fragmentation of protonated AAs using a single quadrupole mass spectrometer. RESULTS The principal fragment ions were [M + H - H(2)O - CO](+), [M + H - H(2)O](+), and [M + H - NH(3)](+). AAs that only generated [M + H - H(2)O - CO](+) were alanine, glycine, histidine, isoleucine, leucine, proline, phenylalanine, and valine. AAs that generated [M + H - H(2)O](+) and [M + H - H(2)O - CO](+) were aspartic acid, glutamic acid, serine, and threonine, while AAs that generated [M + H - NH(3)](+) and [M + H - H(2)O - CO](+) were asparagine, cysteine, glutamine, methionine, tryptophan, and tyrosine. Arginine and lysine generated [M + H - H(2)O](+) and [M + H - NH(3)](+). CONCLUSIONS The relative abundances of the fragment ions increased with increase in the cone voltage. The experimental results were explained by the favorability of the intermediate structure and the stability of the fragment ion structure. The specific fragmentation patterns could be used for differentiating underivatized AAs.

Resole-cured NR vulcanizates with thermally reacted p-t-octylphenol formaldehyde resole

Crosslink dissociation of resole-cured NR vulcanizates was studied using steam aging. Four carbon black-filled NR vulcanizates were prepared to investigate the influence of heated p-t-octylphenol formaldehyde resole (at 160°C for 0.0, 1.5, 3.0, and 6.0 h) on the cure characteristics and stability of the crosslinks. By increasing the heating time of the resole, dimethylene ether linkages of the resole decrease while o-methylene quinone intermediates increase. The cure rate of the NR vulcanizates with the heated resole for 1.5 h is faster but those for 3.0 and 6.0 h are slower than that with the unheated resole. In comparison with the vulcanizates containing the nonheated resole, the delta torque of the NR vulcanizate with the heated resole for 1.5 h increases, while those for 3.0 and 6.0 h decrease. The swelling ratios of the four NR vulcanizates decrease after steam aging at 95°C for 7 days. The differences of the swelling ratios before and after steam aging decrease with increase of the heating time of the resole. The decrease of the swelling ratio after the steaming is due to the dissociation of the dimethylene ether linkage in the vulcanizates.

Thermal Aging Behaviors of Weather Resistant Rubber Composites of EPDM, IIR, and BIIR

EPDM, IIR, and BIIR composites were thermally aged and the crosslink density changes were investigated. Crosslink densities of the EPDM composite increased with increasing the aging time and temperature, whereas those of IIR and BIIR composites for long-term aging at high temperatures tended to decrease. Activation energies for the crosslink density changes of the EPDM composite were higher than those of the BIIR one. The experimental results were explained with the number of allylic hydrogens, activation of the zinc complex, the steric hindrance effect, and oxidation of rubber chain.

Influence of Aging Media and Filler System on Recovery Behaviors of Natural Rubber Composites

Difference in recovery behaviors from the circular deformation of natural rubber (NR) composites aged in air and distilled water, respectively were investigated. Recoveries of the samples aged in air were larger than those of the samples aged in distilled water. Recovery rates of the samples reinforced with filler were faster than those of the unreinforced ones. Recovery rates of the carbon black-filled samples were faster than those of the silica-filled ones. Difference in the recovery behaviors according to the aging media can be explained by the crosslinking density changes and the annealing effect.

Role of the UV absorber as a matrix in matrix-assisted laser desorption/ionization mass spectrometric analysis of a mixture of a UV absorber and a stabilizer

A mixture of a UV absorber (Tinuvin 234 or Tinuvin 329) and a UV stabilizer (Tinuvin 770) was analyzed using matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) without any matrix. Fragmentation patterns of the UV absorbers and stabilizer were also investigated. The mass spectra showed the [M+H](+) ions and some fragment ions. Tinuvin 234, Tinuvin 329, and Tinuvin 770 generated three (m/z 119, 370, 432), one (m/z 252), and two (m/z 124 and 140) fragment ions, respectively. These fragment ions can be used to identify the chemical structures of the UV absorbers and stabilizer. Since the UV absorber performed a role as the matrix, the ion abundance of the UV stabilizer was enhanced by mixing with the UV absorber. When organic materials extracted from polypropylene (PP) containing the UV absorber and stabilizer were directly analyzed using MALDI-MS without any matrix, the protonated molecule of the UV stabilizer was detected in abundance but the product ions of the UV absorber were not observed. When 2,5-dihydroxybenzoic acid was used as a matrix, the protonated molecule of the UV absorber was observed.

Atmospheric pressure chemical ionization and fragmentation of aminomonosaccharides in H2O and D2O.

Aminomonosaccharides (glucosamine, galactosamine, and mannosamine) in H2O and D2O were ionized by atmospheric pressure chemical ionization (APCI) and their fragmentation patterns were investigated to identify them. All the aminomonosaccharides showed the same fragment ions but their relative ion intensities were different. Major product ions generated in H2O were [M + H]+, [M + H - H2O]+, and [2M + H - 3H2O]+, while in D2O were [M(D6) + D]+, [M(D6) + D - D2O]+, and [2M(D6) + D - D2O - 2HDO]+. At a high fragmentor voltage above 120 V, the relative ion intensities of the major product ions showed different trends according to the aminomonosaccharides. For the use of H2O as solvent and eluent, the order of the ion intensity ratio of [M + H - H2O]+/[2M + H - 3H2O]+ was galactosamine > mannosamine > glucosamine. When using D2O as solvent and eluent, the order of the ion intensity ratios of [M(D6) + D - D2O]+/[MD6 + D]+ and [2M(D6) + D - D2O - 2HDO]+/[M(D6) + D]+ was mannosamine > galactosamine > glucosamine. It was found that glucosamine, galactosamine, and mannosamine could be distinguished by the specific trends of the major product ion ratios in H2O and D2O.