Spontaneous molecular orientation of polyimides induced by thermal imidization (6). Mechanism of negative in-plane CTE generation in non-stretched polyimide films

The mechanism of negative coefficient of thermal expansion (CTE) generation for non-stretched polyimide (PI) films is proposed in this work. Negative CTE behavior was observed in some miscible binary blend films composed of a major fraction of a rod-like semi-crystalline PI derived from pyromellitic dianhydride (PMDA) with p-phenylenediamine (PDA) and flexible PIs based on 2,3,3′,4′-biphenyltetracarboxylic dianhydride (a-BPDA) whereas homo PMDA/PDA PI film shows a considerably low but a positive CTE value. The results suggest that the negative CTE generation is related to not only a considerably high extent of in-plane orientation of the PMDA/PDA chains but also to the crystallinity of the blends. The present work revealed that some other PIs, a poly(ester imide), and a polybenzoxazole system also display negative CTE and these systems also possess extremely high extents of in-plane chain orientation without exception. In addition to CTE, the morphologies were monitored as a function of imidization temperature for two PI systems, PMDA/2,2′-bis(trifluoromethyl)benzidine and PMDA/m-tolidine by wide-angle X-ray diffraction, FT-IR spectroscopy, birefringence, and film density measurements. The results suggested that the negative CTE phenomenon occurs when PI films possess very high extents of in-plane orientation and a less crystalline morphology simultaneously, thereby significant thermal expansion can be allowed to the thickness direction.     

Predictive elucidation of conformational characteristics and configurational properties of poly(1-methylphosphirane) and poly(1-phenylphosphirane) as a molecular design

Conformational characteristics and configurational properties of poly(1-methylphosphirane) (PMePP) and poly(1-phenylphosphirane) (PPhPP) have been predictively elucidated by the refined rotational isomeric state scheme coupled with ab initio molecular orbital and density functional calculations. The lone pair of the phosphorus atom adopts an sp hybrid orbital. Owing to the high s character (50%), the polyphosphiranes exhibit low proton (hydrogen) affinities, and hence the lone pair does not form any intramolecular attractive interactions with hydrogen. As the meso-diad probability varies from 0 (syndiotactic) to 1 (isotactic), the characteristic ratio of PMePP slightly increases from 6.7 to 7.4, whereas that of PPhPP considerably decreases from 38 to 7.3. The large dimension of syndiotactic PPhPP is chiefly due to π-π attractions formed between adjacent phenyl groups. The trivalent phosphorus atom may be bonded to heavy, noble, and transition metals but readily or gradually oxidized. The usefulness and necessity of the polyphosphiranes have been assessed.     

Structures and electronic transport on silicon surfaces

By utilizing a variety of surface superstructures formed on silicon surfaces and atomic layers grown on them, close correlations between the atomic-scale structures and electrical conduction phenomena at the surfaces have been revealed. State-of-art techniques for analyzing and controlling atomic/electronic structures of surfaces are leading to an understanding of the novel electronic transport properties at surfaces. For example, the electrical conduction through surface-state bands, which are inherent in the surface superstructure, has been confirmed in in-situ measurements. An important phenomenon has also been found, where adatoms donate carriers into the surface-state band, resulting in a remarkable enhancement in electrical conductance. The nucleation of the adatoms diminishes such a doping effect. Furthermore, electrical conduction through atomic layers grown on the surfaces, whose growth structures are sensitive to the substrate surface structures, will be also discussed. In this review, we emphasize that the surface electronic transport properties are closely related to the atomic structures and atomistic dynamics on surfaces. The ultimate two-dimensional electron systems, consisting of the surface-state bands and grown atomic layers, are expected to provide a new stage in surface physics, as well as a precursory stage leading to atomic-scale electronics devices.     

Including Curriculum Focus in Mathematics Professional Development for Middle‐School Mathematics Teachers

This paper examines professional development workshops focused on Connected Math, a particular curriculum utilized or being considered by the middle‐school mathematics teachers involved in the study. The hope was that as teachers better understood the curriculum used in their classrooms, i.e., Connected Math, they would simultaneously deepen their own understanding of the corresponding mathematics content. By focusing on the curriculum materials and the student thought process, teachers would be better able to recognize and examine common student misunderstandings of mathematical content and develop pedagogically sound practices, thus improving their own pedagogical content knowledge. Pre‐ and post‐mathematics content knowledge assessments indicated that engaging middle‐school teachers in the curriculum materials using pedagogy that can be used with their middle‐school students not only solidified teachers' familiarity with such strategies, but also contributed to their understanding of the mathematics content.     

Synthesis of manzacidin A and C: efficient construction of quaternary carbon stereocenters bearing nitrogen substituents

An efficient synthetic method for stereoselective construction of asymmetric quaternary carbon stereocenters, bearing nitrogen in the form of Boc-protected allyl amines, has been developed. This methodology is employed in the synthesis of marine alkaloids, manzacidin A and C.     

Chronological Change from Face‐On to Edge‐On Ordering of Zinc–Tetraphenylporphyrin at the Phenyloctane–Highly Oriented Pyrolytic Graphite Interface

The self‐assembled structure of alkoxy‐ and N‐alkylcarbamoyl‐substituted zinc–tetraphenylporphyrin at the liquid–highly oriented pyrolytic graphite (HOPG) interface was observed by using scanning tunneling microscopy. The alkoxy porphyrin showed a phase transition from face‐on to edge‐on ordering. The phase transition requires the close‐packed structure of alkoxy porphyrin. The chronological change of the ordering was traced to show the existence of several types of Ostwald ripening including two‐step phase transition from small edge‐on to face‐on and then further to edge‐on orderings. On the other hand, the N‐alkylcarbamoyl porphyrin showed persistent edge‐on ordering, and the ordering was analyzed by the Moiré pattern. Although the edge‐on ordering is observed only in the nonpolar solvent, the orderings have potential applications in the charge and energy transfer.     

Nonclassical Tunability of Solid‐State CD and CPL Properties of a Chiral 2‐Naphthalenecarboxylic Acid/Amine Supramolecular Organic Fluorophore

The solid‐state chiral optical properties (circular dichroism and circularly polarized luminescence) of a 2‐naphthalenecarboxylic acid/amine supramolecular organic fluorophore can be controlled by changing the aryl unit of the chiral 1‐arylethylamine component of the molecule rather than altering the chirality of the 1‐arylethylamine itself.     

AC Stark-shift in CPT-based Cs miniature atomic clocks

We report on studies on the light-shift in caesium miniature atomic clocks based on coherent population trapping (CPT) using a micro-fabricated buffer-gas cell (MEMS cell). The CPT signal is observed on the Cs D1-line by coupling the two hyperfine ground-state Zeeman sublevels involved in the clock transition to a common excited state, using two coherent electromagnetic fields. These light fields are created with a distributed feedback laser and an electro-optical modulator. We study the light-shift phenomena at different cell temperatures and laser wavelengths around 894.6?nm. By adjusting the cell temperature, conditions are identified where a miniature CPT atomic clock can be operated with simultaneously low temperature coefficient and suppressed light-shift. The impact of the light-shift on the clock frequency stability is evaluated. These results are relevant for improving the long-term frequency stability of CPT-based Cs vapour-cell clocks.     

Isotope-selective ionization utilizing molecular alignment and non-resonant multiphoton ionization

We demonstrate laser nitrogen isotope separation, which is based on field-free alignment and angular-dependent ionization of ¹⁴N₂ and ¹⁵N₂ isotopologues. A linearly polarized short laser pulse (???~?795?nm, ?????~?60?fs) creates rotational wave packets in the isotopologues, which periodically revive with different revival times as a result of different moments of inertia. Another linearly polarized short laser pulse (???~?795?nm, ?????~?60?fs) ionizes one of the isotopologues selectively as a result of their different angular distributions. In the present experiments, the ion yield ratio R [=I(¹⁵N₂ ⁺)/I(¹⁴N₂ ⁺)] can be changed in the range from 0.85 to 1.22, depending on the time delay between the two laser pulses.     

Efficient photo-ionization for barium ion trapping using a dipole-allowed resonant two-photon transition

Two efficient and isotope-selective resonant two-photon ionization techniques for loading barium ions into radio-frequency (RF)-traps are demonstrated. The scheme of using the strong dipole-allowed transition 6s ²? ¹S₀→6s6p? ¹P₁ at λ=553 nm as a first step towards ionization is compared to the established technique of using a weak inter-combination line (6s ²? ¹S₀→5d6p? ³D₁,λ=413 nm). An increase of two orders of magnitude in the ionization efficiency is found favoring the transition at 553 nm. This technique can be implemented using commercial all-solid-state laser systems and is expected to be advantageous compared to other narrowband photo-ionization schemes of barium in cases where highest efficiency and isotope-selectivity are required.