Morphology and mechanical properties of poly(vinyl alcohol) and starch blends prepared by gelation/crystallization from solutions

 In an attempt to produce biodegradation materials, poly(vinyl alcohol) (PVA)–starch (ST) blends were prepared by gelation/crystallization from semidilute solutions in dimethyl sulfoxide (Me₂SO) and water mixtures and elongated up to 8 times. The content of mixed solvent represented as Me₂SO/H₂O (volume percent) was set to be 60/40 assuring the greatest drawability of PVA homopolymer films. The PVA/ST compositions chosen were 1/1, 1/3, and 1/5. The elongation up to 8 times could be done for the 1/1 blend but any elongation was impossible for blends whose ST content was beyond 50%. When the blends were immersed in water at 20 or 83 °C, the solubility became considerable for an undrawn blend with 1/5 composition and a drawn 1/1 blend with λ=8. To avoid this phenomenon, cross-linking of PVA chains was carried out by formalization under formaldehyde vapor. Significant improvement could be established by the cross-linking of PVA chains. For the 1/1 blend, the amount of ST dissolved in water at 23 °C was less than 3% for the undrawn state and 25% for the drawn film. The decrease in the ST content was enough for use as biodegradation materials. Namely, the water content relating to the biodegradation in soil is obviously different from such a serious experimental condition that a piece of blend film was immersed in a water bath. At temperatures above 0 °C, the storage modulus of the formalization blends became slightly higher than those of the nonformalization blends. The Young's modulus of the drawn films with a draw ratio of 8 times was 2 GPa at 20 °C. Received: 23 June 2000 Accepted: 30 October 2000     

Characterization of Natural Chromite Samples from Ophiolite Complexes in the Philippines by 57Fe Mössbauer Spectroscopy

Mössbauer spectroscopy was applied to natural chromite samples from ophiolite complexes in the Philippines. Chemical and structural characterization of the chromite samples was also carried out using X-ray absorption fine structure (XAFS), X-ray diffraction (XRD) and X-ray fluorescence spectrometry (XRF). The Mössbauer spectra of the samples consisted of quadrupole doublets ascribable to Fe³⁺ in octahedral site, Fe³⁺ in tetrahedral site, and Fe²⁺ in tetrahedral site. The relative percentage of Fe³⁺ and Fe²⁺ ions suggested that these Philippine samples were formed under relatively high oxygen fugacity.     

Synthesis of Dendrimer-Like Star-Branched Poly(methyl methacrylate)s of Generations Consisting of Four Branched Polymer Chains at Each Junction by Iterative Methodology Involving Coupling and Transformation Reactions

A series of dendrimer-like star-branched poly(methyl methacrylate)s (PMMA)s of generations consisting of four branched segments at each junction have been successfully synthesized by developing an iterative methodology. It involves two reaction steps in each iterative reaction sequence, (a) a coupling reaction of the α-functionalized living anionic PMMA with four tert-butyldimethylsilyloxymethylphenyl (SMP) groups with benzy bromide (BnBr)-chain-functionalized PMMA and (b) a transformation of SMP group into BnBr functionality. By repeating the reaction sequence thrice, the above-mentioned dendrimer-like star-branched (PMMA)s of up to third-generation made up of 4, 20, and 84 PMMA segments and 16, 64, and 256 BnBr termini were synthesized by the first, second, and third iterative processes, respectively. The resulting polymers all were well-defined in branched architecture and precisely controlled in chain length.     

Characterization of PVA and Chitosan/PVA Blends Prepared from Aqueous Solutions of Various Na2SO4 Concentrations

Uniplaner orientation of a particular crystal plane along the surface of a film was investigated for poly (vinyl alcohol) (PVA) film prepared by a coagulation bath with concentrated aqueous solution containing 100 ∼ 300g of Na₂SO₄ against 1 ℓ of water. The orientation distribution functions of the three crystallographic principal axes of the dried films were obtained by the X-ray diffraction technique. The same treatment was carried out for the films prepared by stretching biaxially of the fresh gel and then by drying the resultant fresh gel. The very high preferential orientation of the crystal chain axes and amorphous chain segments could be realized by the biaxially elongation. Accordingly, the techniques were applied to the biaxially stretching of chitosan and PVA blend films with high Young's modulus. The planer orientation of the chain axes of chitosan and PVA crystallites could be confirmed. The morphology of the film surface was estimated by measurements of contact angle and electron spectroscopy for chemical analysis. The results suggested that the admixture of chitosan decreases wet ability of the specimen and this tendency was slightly enhanced by the biaxially elongation.     

Structure and Mechanical Properties of Chitosan/Poly(Vinyl Alcohol) Blend Films

Summary: The origins of the thermal and mechanical properties of chitosan and poly(vinyl alcohol) (PVA) with inter- and intra-hydrogen bonds were investigated systematically by using X-ray, DSC, positron annihilation and viscoelastic measurements. Based on their individual properties, the characteristics of the blend films were estimated in relation to their morphology and mechanical properties as a function of chitosan content. The characteristics of the blend films were also analyzed in terms of the deviation from a simple additive rule of chitosan and PVA content. These results suggested that the miscibility of chitosan and PVA could be ensured by entanglement of the amorphous chain segments of chitosan and PVA. Further detailed analysis revealed that the chitosan content on the film surface is higher than that of the admixture content of chitosan after elongation, although the chitosan and PVA chains were crystallized independently. The elongation could be achieved for the blend films whose PVA content was higher than 50% and the drawn blend films were transparent. Thus, it may be expected that sufficiently entangled meshes formed between chitosan and PVA amorphous chains within the film, the PVA content being higher than 50%, were maintained under the elongation process.     

Experimental study of internal flow structures in cylindrical rotating detonation engines

The internal flow structures of detonation wave were experimentally analyzed in an optically accessible hollow rotating detonation combustor with multiple chamber lengths. The cylindrical RDC has a glass chamber wall, 20 mm in diameter, which allowed us to capture the combustion self-luminescence. A chamber 70 mm in length was first tested using C₂H₄O₂ and H₂–O₂ as propellants. Images with a strong self-luminescence region near the bottom were obtained, confirming the small extent of the region where most of the heat release occurs as found in our previous research. Based on the visualization experiments, we tested RDCs with shorter chamber walls of 40 and 20 mm. The detonation wave was also observed in the shorter chambers, and its velocity was not affected by the difference in chamber length. Thrust performance was also maintained compared to the longer chamber, and the short cylindrical RDC had the same specific impulse tendency as the cylindrical (hollow) or annular 70-mm chamber RDC. Finally, we calculated the pressure distributions of various chamber lengths, and found they were also consistent with the measured pressure at the bottom and exit. We concluded that the short-chamber cylindrical RDC with equal length and diameter maintained thrust performance similar to the longer annular RDC, further expanding the potential of compact RDCs.     

Mössbauer Spectroscopic Study of a Mural Painting from Morgadal Grande,Mexico

Hyperfine Interactions - In this study, 57Fe Mössbauer spectroscopy has been applied to fragments of a mural painting excavated at Morgadal Grande, Mexico, to characterize the pigments used. A...     

Thermal decomposition of propyne and allene in shock waves

Propyne (p-C₃H₄) or allene (a-C₃H₄) mixtures, highly diluted with Ar, were heated to the temperature range 1200–1570 K at pressures of 1.7–2.6 atm behind reflected shock waves. The thermal decompositions of propyne and allene were studied by both measuring the profiles of the IR emission at 3.48 μm or 5.18 μm and analyzing the concentrations of reacted gas mixtures. The mechanism and the rate constant expressions were discussed from both the profiles and the concentrations of reactant and products obtained. The rate constant expressions for reactions, (1) p-C₃H₄ → a-C₃H₄, (?1) a-C₃H₄ → p-C₃H₄, and (5) p-C₃H₄ + H → CH₃ + C₂H₂ were evaluated.     

(1,4,7,10-Tetraazacyclododecane) (diamine or alanine)-Cobalt(III) complexes

(1,4,7,10-Tetraazacyclododecane) [diamine or (S)-alanine]-cobalt(III) complexes [diamine = ethylenediamine, 2-(aminomethyl)pyridine, (R)-1,2-propanediamine, (R,R)-1,2-diaminocyclohexane, trimethylenediamine and 2-methyl-1,3-diaminopropane] are prepared and characterized spectroscopically. The ligand field transitions occur at lower energies than those of the corresponding tetraamine analogues. Severe distortions caused by the too small size of the cyclic ligand are one of the origins. The distortions also exert influence upon circular dichroism spectra.