Dielectric Relaxation of Monodisperse Linear Polyisoprene: Contribution of Constraint Release

<i>cis</i>-Polyisoprene (PI) has the type A dipole parallel along the chain backbone so that the end-to-end fluctuation of PI chains results in slow dielectric relaxation. Comparison of dielectric and viscoelastic data of PI has revealed several interesting features related to the entanglement dynamics, for example, success and failure of the full dynamic tube dilation (DTD) picture for monodisperse linear and star PI, respectively [see a review: Watanabe, H. <i>Polym. J.</i><b>2009</b>, <i>41</i>, 929, for example]. For monodisperse <i>linear</i> PI, recent modeling [Glomann et al. <i>Macromolecules</i> <b>2011</b>, <i>44</i>, 7430] and single-chain slip-link simulation [Pilyugina et al. <i>Macromolecules</i> <b>2012</b>, <i>45</i>, 5728] suggest that the constraint release (CR) mechanism has negligible influence on the dielectric relaxation time τ_ε in the entangled regime, which appears to disagree with the previous data. Thus, we revisited the classical problem: CR contribution to the dielectric relaxation of PI. Specifically, we made dielectric and viscoelastic measurements for PI/PI blends in a wide range of the molecular weights of long and short components, <i>M</i>₂ = 1.1M and <i>M</i>₁ = 21K–179K, and with a small volume fraction of the short component, υ₁ = 0.1 and/or 0.2, to examine the CR contribution in the experimentally clearest way. It turned out that τ_ε of the short component was longer in the blends than in respective monodisperse bulk even for <i>M</i>₁ = 179K. Furthermore, the viscoelastic and dielectric data of the short components (<i>M</i>₁ ≤ 43K) in the blend exhibited identical mode distribution and relaxation time, which confirmed that the CR mechanism was fully suppressed for these components in the blends. These results demonstrate that the CR mechanism <i>does</i> contribute/accelerate the dielectric relaxation in monodisperse bulk PI systems even in the highly entangled regime (<i>M</i>₁/<i>M</i>_e = 36 for <i>M</i>₁ = 179K). This CR-induced acceleration was found to be consistent with the empirical equations for the terminal relaxation time and CR time of monodisperse PI available in the literature, as noted from a simple DTD analysis of the terminal relaxation process (reptation along a partially dilated tube that wriggles in a fully dilated tube)

Alternating Current Electrolysis for Individual Synthesis of Methanol and Ethane from Methane in a Thermo-electrochemical Cell

The individual synthesis of methanol and ethane from methane was investigated using a thermo-electrochemical cell in gas flow mode over the temperature range of 150–200 °C. Methane was directly oxidized at an anode consisting of sub-10 nm Pt and Fe particles. In the electrolysis of humidified methane, methanol was produced through the formation of active oxygen intermediates from water vapor. In the electrolysis of unhumidified methane, ethane was produced via the dissociation of C–H bonds, followed by dimerization of the resultant •CH3 radicals. However, the formation rates for the target products decreased with time during electrolysis because of overoxidation of the anode by the direct-current (DC) voltage. The alternating current (AC) electrolysis of methane avoided this problem. Under optimized AC polarization conditions at a square waveform voltage of ±2.5 V with a pulse time of 5 s, this cell generated methanol and ethane at rates of 5.1 × 10–5 mol m–2 s–1 (92 mmol gcat–1 h–1) and 3.8 × 10–5 mol m–2 s–1 (69 mmol gcat–1 h–1), respectively, without a substantial loss of activity during continuous electrolysis

Structural and Physical Properties of Heavily Doped Yttrium Vanadate: Y_0.6Cd_0.4VO₃

Structural properties of Y0.6Cd0.4VO3 were investigated by electron diffraction and laboratory and synchrotron X-ray powder diffraction methods. Y0.6Cd0.4VO3 crystallizes in space group Pnma (GdFeO3-type perovskite structure) between 12 and 300 K (a = 5.45887(3) Å, b = 7.57250(4) Å, and c = 5.27643(2) Å at 300 K). The lattice parameters showed anomalous behavior on temperature. The c parameter linearly decreased from 12 to 120 K, and then it lineally increased from 160 to 300 K. The b parameter was constant between 12 and 120 K, demonstrated a drop from 120 to 200 K, and then lineally increased from 200 to 300 K. The c/a ratio had a rather sharp maximum at 150 K. In Y0.6Cd0.4VO3 the V−O distances in the ac plane began to split to shorter and longer ones below 150 K, indicating that orbital fluctuations are involved. The phase transition near 150 K in Y0.6Cd0.4VO3 is accompanied by a broad anomaly on the specific heat and change of the slope of the inverse magnetic susceptibility. Other members of the Y1-xCdxVO3 solid solution with x = 0.3, 1/3, and 0.5 did not show this kind of phase transition. This kind of a phase transition has never been detected in other doped vanadates, R1-xMxVO3 (R = Y and rare earths and M = Ca and Sr)

Efficient Hydrogen Production by Direct Electrolysis of Waste Biomass at Intermediate Temperatures

Biomass has been considered as an alternative feedstock for energy and material supply. However, the lack of high-efficiency and low-cost processes for biomass utilization and conversion hinders its large-scale application. This report describes electrochemical hydrogen production from waste biomass that does not require large amounts of energy or high production costs. Hydrogen was produced by the electrolysis of bread residue, cypress sawdust, and rice chaff at an onset cell voltage of ca. 0.3 V, with high current efficiencies of approximately 100% for hydrogen production at the cathode and approximately 90% for carbon dioxide production at the anode. The hydrogen yields per 1 mg of the raw material were 0.1–0.2 mg for all tested fuels. Electrolysis proceeded continuously at plateau voltages that were proportional to the current. These characteristics were attributable to the high catalytic activity of the carbonyl-group functionalized mesoporous carbon for the anode reaction, and that the major components of biomass such as cellulose, starch, lignin, protein, and lipid were effectively utilized as fuels for hydrogen production