Unusual melting transition of nitrogen physisorbed on carbon nanotube bundles.

The melting transition of nitrogen physisorbed on close-ended single-wall nanotube bundles was investigated using synchrotron X-ray diffraction measurements. The beta-nitrogen solid diffraction peak was observed above the coverage that corresponded to the monolayer and the average size of the nitrogen solid was approximately 30 A. The diffraction peak was surprisingly maintained above the triple point of the bulk nitrogen solid. The crystal structure of N2 changed from cubic N2 (beta-phase) to hexagonal N2 (beta-phase) at 35.61 K. The melting temperature of the nano-scale solid nitrogen in the experiment was between 80 K and 90 K, however, which is about 20 K higher than the melting temperature of normal bulk nitrogen. The observed extraordinary melting behavior of nitrogen might originate from a combination of two factors, i.e., the substrate field effect of the carbon nanotube surface (the interaction between the single walled carbon nanotubes and the adsorbates) and the capillary condensation. If the substrate field effect is especially prominent, the nitrogen molecules that were adsorbed mainly in the groove region would be under 1,100-Torr pressure from the nanotube bundles, compared to the corresponding melting temperature of the bulk beta-nitrogen solid under a high pressure.

Study of dielectric relaxations of anhydrous trehalose and maltose glasses.

We investigated the frequency dependent dielectric relaxation behaviors of anhydrous trehalose and maltose glasses in the temperature range which covers a supercooled and glassy states. In addition to the α-, Johari-Goldstein (JG) ß-, and γ-relaxations in a typical glass forming system, we observed an extra relaxation process between JG ß- and γ-relaxations in the dielectric loss spectra. We found that the unknown extra relaxation is a unique property of disaccharide which might originate from the intramolecular motion of flexible glycosidic bond. We also found that the temperature dependence of the JG ß-relaxation time changes at 0.95T(g) and it might be universal.

Temperature dependent vibrational modes of glycosidic bond in disaccharide sugars.

We studied the temperature dependent vibrational modes of the glycosidic bond in trehalose, sucrose, and maltose at wavenumbers ranging from 1000 to 1200 cm(-1). We found that the slope of temperature dependent Raman shifts of the glycosidic bond in trehalose and sucrose changed at temperatures around 120 degrees C, indicating a bond length or a bond angle (dihedral and torsional angles) change. However, we did not observe any slope change in maltose because the melting temperature of maltose is very close to 120 degrees C. We also found, at temperatures below 120 degrees C, that Raman shifts of the vibrational modes of the glycosidic bond in trehalose showed the strongest temperature dependence among the three disaccharides.

The glass transition temperatures of sugar mixtures.

We measured the glass transition temperatures of mono-, di-, and trisaccharide mixtures using differential scanning calorimeter (DSC) and analyzed these temperatures using the Gordon-Taylor equation. We found that the glass transition temperatures of monosaccharide-monosaccharide and disaccharide-disaccharide mixtures could be described by the conventional Gordon-Taylor equation. However, the glass transition temperatures of monosaccharide-disaccharide and monosaccharide-trisaccharide mixtures deviated from the conventional Gordon-Taylor equation and the amount of deviation in the monosaccharide-trisaccharide mixtures was larger than those in the monosaccharide-disaccharide mixtures. From these results, we conclude that the size and shape of the sugars play an important role in the glass transition temperature of the mixtures.