Toshihiro Hiejima

Screw-sense inversion characteristic of α-helical poly(β-p-chlorobenzyl L-aspartate) and comparison with other related polyaspartates†

This is one of a series of studies on the reversal of the helix sense of polyaspartates originated from the pioneering work of Goodman and his associates in 1960s. Poly(β‐p‐chlorobenzyl L‐aspartate) (PClBLA) is one of the well‐studied polyaspartate derivatives in both solution and the solid state. The chemical structure of PClBLA differs from those of poly(β‐benzyl L‐aspartate) (PBLA) and poly(β‐phenethyl L‐aspartate) (PPLA) only at the terminal of the relatively long side chain. PBLA takes a left‐handed form (L) in conventional helicoidal solvents and does not exhibit any screw‐sense inversion. In contrast to PBLA, both PClBLA and PPLA form a right‐handed helix (R) in chlorinated alkane solvents and exhibits a reversal of α‐helix sense at higher temperatures. Yet the transition behaviors in the presence of denaturant acid are quite different between these two polymers. While PPLA exhibits transitions such as R → L → coil by lowering temperature, PClBLA directly goes into the coil state without showing the reentrant L form. The cause of these phenomenological differences among these polymers has been investigated by constructing the phase diagram.

Formation of Positronium in Cup-Stacked Carbon Nanofibers

The positron-lifetime spectrum for cup-stacked carbon nanofibers (CNFs) is composed of three components: 0.125 ns (fixed) (6%), 0.345 ns (75%), 1.21 ns (19%). The longest-lived component assigned to ortho-positronium (o-Ps) verifies a prominent yield of o-Ps, which is in contrast with the fact that no o-Ps is detected for uncapped cylindrical multi-walled carbon nanotubes. CNFs heat-treated to 1073 K in vacuo showed a positron-lifetime spectrum the same as that of untreated CNFs, which demonstrates that the thermal detachment of some functional groups from the surface of the CNFs does not change the Ps yield. The yield is found almost independent of temperature between 10 and 280 K. A single CNF, ca. 50 nm in outer diameter, observed by transmission electron-microscopy, comprises stacked cups of 9~12 truncated conical graphene sheets at ca. 20 o with respect to the fiber axis, so that all edges of graphene sheets are found on the zigzag outeras well as inner-surfaces of the fiber. The Raman spectrum for CNFs exhibits a band of a disorder-induced mode at 1349 cm -1 (D-band) and a band of the E2g2 in-plane mode at 1577 cm -1 (G-band). The intensity-ratios of a D-band to a G-band are 0.17, 0.25 and 0 for a mat of CNFs, the edge plane and the basal plane of a highly oriented pyrolitic graphite block, respectively. For graphite materials, Ps is formed from the positrons trapped in the defects originating from edges of graphene sheets. Introduction Graphite has been widely investigated by means of positron annihilation, and its lifetime spectrum has been reported to consist of three components. The intensity of a long-lived component (1.3~2.0 ns) attributed to orthoositronium (o-Ps) has been shown to be 1.5% for pyrolytic graphite, but 15.6% for graphite powder [1]. Graphite is composed of graphene sheets (hexagonal carbon networks) piling up along the c-axis, and a block of highly oriented pyrolytic graphite (HOPG) consists of crystallites with the well-oriented c-axis and random orientation in the basal plane. The crystallite size is ca. 25 μm along the a-axis and larger than 7 μm along the c-axis. Recently graphite materials of structure different from graphite have been prepared: multi-walled carbon nanotubes (MNTs) and cup-stacked carbon nanofibers (CNFs). MNTs consist of concentric microtubules of cylindrical graphene sheets, a single MNT being a few tens of nanometers in diameter and ca. 1 μm in length [2]. CNFs have a stacking form of truncated conical graphene sheets with a long hollow core around the fiber axis [3]. The positron-lifetime spectrum for uncapped MNTs is fitted with a single exponential term of 0.382 ns with 2 1.1, but no term assignable to o-Ps is separated [4]. Here we studied the annihilation of positrons in CNFs in order to elucidate the formation of Ps in graphite materials. Experimental CNFs (Carbere 24HT, GSI Creos Co.) were heat-treated to 623, 803 or 1073 K in vacuo to Materials Science Forum Online: 2004-01-15 ISSN: 1662-9752, Vols. 445-446, pp 331-333 doi:10.4028/www.scientific.net/MSF.445-446.331

Molecular Dynamics in Chiral Smectic Liquid-Crystalline Phases Studied by Solid State 13 C-NMR Measurements

The temperature dependence of the spin-lattice relaxation time T 1 is investigated in an antiferroelectric liquid crystal, (S)-4-(1-methylheptyloxy-carbonyl)phenyl 4′-octyloxybiphenyl-4-carboxylate (S-MHPOBC) by means of solid-state 13 C-NMR spectroscopy. The spin-lattice relaxation time T 1 of almost all of aromatic carbons has a minimum at the temperature region of the SmC A * phase in the temperature dependent curves, because the correlation time τ c of the molecular motion corresponds to the Larmor frequency ω 0 of 100 MHz in the present measurements. As for the aliphatic carbons in a flexible chain, the value of T 1 increases with increasing temperature, because the molecular motion characterized by τc is faster than the Larmor frequency ω 0 .

Orientational and Conformational Characteristics of Oxyethylene-Type Spacers Involved in Dimer Liquid Crystals

2H-NMR and wide-angle X-ray measurements were performed on dimer liquid crystals (LC) having an oxyethylene (OE)-type spacer such as MS-O(CH2CH2O)x-MS, MS and x respectively denoting mesogenic cores and the number of OE units. When mesogenic cores were properly chosen, these dimer compounds exhibited enantiotropic nematic LC phase over a sizably wide temperature range (∼100°C). The differential scanning calorimetry (DSC) and X-ray analyses of two crystalline samples, one prepared from a chloroform solution and the other obtained via melt crystallization, gave practically the same results. The orientational characteristics of the mesophase were studied by the 2H-NMR method, leading to a conclusion that the OE-type spacer takes spatial arrangements characteristic of the nematic phase over a wide range of temperatures.

Orientational order and thermodynamic properties of mainchain trimer liquid crystals: a combined use of 2H NMR, PVT and high-pressure differential thermal analysis

Abstract The trimer compounds (4,4′-bis[ω-(4-cyanobiphenyl-4′-yloxy)alkoxy]biphenyls) (CBA-T n , n =9, 10) with n indicating the number of carbon atoms in the spacer exhibit a nematic order on cooling from the melt. The orientational characteristics of the central and terminal mesogenic cores were separately investigated in the liquid-crystalline (LC) state for CBA-T10 by the 2 H NMR method. The ratio of the quadrupolar splittings was found to remain nearly constant over the entire range of the nematic state, suggesting that the conformation of the intervening spacer is also stable. The pressure-dependence of the phase boundary curves has been studied for transitions such as crystal↔nematic LC↔isotropic melt by the high-pressure differential thermal analysis. The transition entropies have been obtained according to the Clapeyron relation. A combined use of the PVT data led to an estimate of the constant-volume transition entropies, which is in reasonable agreement with the conformational transition entropies estimated previously by the rotational isomeric state analysis of the spectroscopic data.

Positronium Formation in Alkali Metal-intercalated Cup-stacked Carbon Nanofibers

Positrons injected into alkali metal-intercalated cup-stacked carbon nanofibers convert to positronium (Ps) atoms with a quite high yield of ca. 25%, which is insensitive to insertion of alkali metal intercalant layers and charge transfer from alkali metal to graphene sheets. The high yield is ascribed to the formation of Ps in voids distributed in a large quantity on edge-planes (not on sheet-planes) of graphite materials.

Occlusion of Hydrogen in Alkali Metal-Intercalated Cup-Stacked Carbon Nanofibers

Hydrogen storage in alkali metal-graphite intercalation compounds (AGICs) prepared from cup-stacked carbon nanofibers (A-CNFs, A = K, Rb and Cs) was studied by means of thermal desorption of hydrogen. The hydrogen sorbed to stage-1 K-CNFs amounts to 0.067 dm3 (STP) per gram of carbon, most of which is fixed to KGIC as a chemical component.

Thermodynamics of polymeric fluids - effect of volume on the configurational entropy of chain molecules

Various conformation-dependent properties of chain molecules have been successfully treated within the rotational isomeric state approximation. The conformation entropy is one of such properties which can be readily defined by the partition function, the sum of all possible configurations of the chain. Flexible polymers often exhibit crystallization and in some cases liquid-crystallization as well. In these first-order transitions, changes in the spatial arrangement of polymer chains are considered to be a major factor involved. In order to explicitly determine the conformational contribution to the melting entropy, the latent entropy observed under the isobaric condition must be corrected for the volume change. The entropy separation involves a hypothetical assumption that the volume of the isotropic fluid may be compressed to that of the solid state without affecting the configurational part of the entropy of molecules. Finally thermodynamic significance of the conformation entropy in these transitions is emphasized on the basis of the critical studies of the entropy-volume relation of chain molecules in the liquid state.