Theoretical and Experimental Studies on Elementary Reactions in Living Radical Polymerization via Organic Amine Catalysis

The reaction mechanism of living radical polymerization using organic catalysts, a reversible complexation mediated polymerization (RCMP), was studied using both theoretical calculations and experiments. The studied catalysts are tetramethyl­guanidine (TMG), triethyl­amine (TEA), and thiophene. Methyl 2-iodoisobutyrate (MMA-I) was used as the low-molar-mass model of the dormant species (alkyl iodide) of poly­(methyl methacrylate) iodide (PMMA-I). For the reaction of MMA-I with TEA to generate MMA^• and ^•I-TEA radicals (activation process), the Gibbs activation free energy for the inner-sphere electron transfer mechanism was calculated to be 39.7 kcal mol^–1, while the observed one was 25.1 kcal mol^–1. This difference of the energies suggests that the present RCMP proceeds via the outer-sphere electron transfer mechanism, i.e., single-electron transfer (SET) reaction from TEA to MMA-I to generate MMA^• and ^•I-TEA radicals. The mechanism of the deactivation process of MMA^• to generate MMA-I was also theoretically studied. For the studied three catalysts, the theoretical results reasonably elucidated the experimentally observed polymerization behaviors

Elapsed times of partitioning algorithms versus clutter density.

<p>Elapsed times of partitioning algorithms versus clutter density.</p

Ordered and Ultralong Graphitic Carbon Nitride Nanotubes Obtained via In-Air CVD for Enhanced Photocatalytic Hydrogen Evolution

Metal-free graphitic carbon nitride (g-C3N4) has become one of the most up-and-coming photocatalyst candidates for the hydrogen evolution reaction. However, the improvement in photocatalytic property is strongly suppressed by the limited active reaction sites due to the bulk microstructure of g-C3N4. On this basis, we exploit a moderate and economical approach to prepare an ordered and one-dimensionally ultralong carbon nitride nanotube (CN-NT) via the in-air chemical vapor deposition (CVD) with SiO2 nanofiber templates synthesized by electrostatic spinning. Due to the uniform size, fluffiness, and easy removal, SiO2 nanofiber templates are conducive to prepare ordered and tubular CN-NT. The obtained CN-NT sample exhibits an excellent photocatalytic hydrogen evolution rate (HER) of 4605.2 μmol·h–1·g–1 under visible light, which is 33.4 times higher than that of the original bulk g-C3N4. The apparent quantum efficiency reaches 6.49% at 420 nm. The enhancement in the photocatalytic activity is ascribed to the increased specific surface area, faster electron transfer pathway, advanced light absorption ability, and furthermore the lower recombination rate of photogenerated electrons

Direct Measurement of Negative Capacitance in Ferroelectric/Semiconductor Heterostructures

Negative capacitance (NC) is now an attractive research topic owing to its potential applications. For better integration, investigation about the phenomenon and mechanism of NC in ferroelectric materials on semiconductor substrates is important. In this work, ferroelectric BaTiO3 (BTO) films are deposited on the low-resistance Si(100) substrates to constitute Pt/BTO/p-Si/Pt samples with the metal/ferroelectric/semiconductor/metal (MFSM) structure, on which NC are directly measured at low frequencies with a large DC bias. Because of the unique asymmetric interface, the NC value is tunable by the polarity and magnitude of the DC bias. Analysis based on the impedance and ferroelectric characteristics reveals that, in addition to the displacement current related to the electric polarization, there is also relaxation current caused by interface charge injection and oxygen vacancy migration. This work provides another idea for studying miniaturized and low-energy devices utilizing NC, which is of great significance for the development of silicon-based ferroelectric devices

Photocontrolled Organocatalyzed Living Radical Polymerization Feasible over a Wide Range of Wavelengths

Photocontrolled organocatalyzed living radical polymerization was conducted over a wide range of irradiation wavelengths (350–750 nm). The polymerization was induced and controlled at the desired wavelengths by exploiting suitable organic catalysts. This system was finely responsive to the irradiation wavelength; the polymerization was instantly switched on and off, and the polymerization rate was sensitively modulated by altering the irradiation wavelength. The polymer molecular weight and its distribution (<i>M</i>_w/<i>M</i>_n = 1.1–1.4) were well controlled for methacrylate monomers up to fairly high conversions in many cases. The monomer scope encompassed various functional methacrylates, and their block copolymers were obtained. The feasibility of such a wide range of wavelengths and the fine response to the wavelength are unprecedented features. As a unique application of the wavelength-responsive nature of this system, we demonstrated “one-pot” selective regulation of living radical polymerization and another type of polymerization (ring opening polymerization), where the regulation was achieved by simply altering the irradiation wavelength. Facile operation and applicability to a wide range of polymer designs are advantages of this polymerization

Performances of partitioning algorithms versus clutter density.

<p>Performances of partitioning algorithms versus clutter density.</p

Performance comparison in combination with RF when <i>r</i>_e = 10(MHz).

<p>Performance comparison in combination with RF when <i>r</i>_e = 10(MHz).</p

Joint Effects of Granule Size and Degree of Substitution on Octenylsuccinated Sweet Potato Starch Granules As Pickering Emulsion Stabilizers

The granules of sweet potato starch were size fractionated into three portions with significantly different median diameters (<i>D</i>₅₀) of 6.67 (small-sized), 11.54 (medium-sized), and 16.96 μm (large-sized), respectively. Each portion was hydrophobized at the mass-based degrees of substitution (DS_m) of approximately 0.0095 (low), 0.0160 (medium), and 0.0230 (high). The Pickering emulsion-stabilizing capacities of modified granules were tested, and the resultant emulsions were characterized. The joint effects of granule size and DS_m on emulsifying capacity (EC) were investigated by response surface methodology. For small-, medium-, and large-sized fractions, their highest emulsifying capacities are comparable but, respectively, encountered at high (0.0225), medium (0.0158), and low (0.0095) DS_m levels. The emulsion droplet size increased with granule size, and the number of freely scattered granules in emulsions decreased with DS_m. In addition, the term of surface density of the octenyl succinic group (SD_‑OSG) was first proposed for modified starch granules, and it was proved better than DS_m in interpreting the emulsifying capacities of starch granules with varying sizes. The present results implied that, as the particulate stabilizers, the optimal DS_m of modified starch granules is size specific

Cyanuric Acid-Assisted Synthesis of Hierarchical Amorphous Carbon Nitride Assembled by Ultrathin Oxygen-Doped Nanosheets for Excellent Photocatalytic Hydrogen Generation

Amorphous carbon nitride with typical short-range order arrangement as an effective photocatalyst is worth exploring but remains a great challenge because its disordered structure induces severe recombination of photogenerated charge carriers. Herein, for the first time, we demonstrate that a hierarchical amorphous carbon nitride (HACN) with structural oxygen incorporation can be synthesized via a cyanuric acid-assisted melem hydrothermal process, accompanied by freeze-drying and pyrolysis. The complex composed of melem and cyanuric acid exhibiting a unique 3D self-supporting skeleton and significant phase transformation is responsible for the formation of an interconnected hierarchical framework and amorphous structure for HACN. These features are beneficial to enhance its visible light harvesting by the multiple-reflection effect within the architecture consisting of more exposed porous nanosheets and introducing a long band tail absorption. The well-designed morphology, band tail state, and oxygen doping effectively inhibit rapid band-to-band recombination of the photogenerated electrons and holes and facilitate subsequent separation. Accordingly, the HACN catalyst exhibits exceptional visible light (λ > 420 nm)-driven photoreduction for hydrogen production with a rate of 82.4 μmol h–1, which is 21.7 and 9.5 times higher than those of melem-derived carbon nitride and crystalline nanotube carbon nitride counterparts, respectively, and significantly surpasses those of most reported amorphous carbon nitrides. Our controlling of rearrangement of the in situ supramolecular self-assembly of melem oligomer using cyanuric acid directly instructs the development of highly efficient amorphous photocatalysts for converting solar energy into hydrogen fuel

Elapsed times of partitioning algorithms versus expected number of measurements.

<p>Elapsed times of partitioning algorithms versus expected number of measurements.</p