Grafting of polyesters onto carbon black. VI. Copolymerization of alkylene carbonate with cyclic acid anhydride initiated by alkali metal carboxylate groups on carbon black surface

The ring-opening copolymerization of alkylene carbonate with cyclic acid anhydride was found to be initiated by carbon black containing potassium carboxylate (COOK) groups to give an alternating polymer, i.e., polyester. The polyester was propagated from COOK groups and effectively grafted from carbon black surface: e.g., the grafting ratio of polyester from ethylene carbonate (EC) and phthalic anhydride (PAn) went up to over 100%. On the other hand, the initiating activity of alkali metal carboxylate groups increased, depending on the alkali metal countercation, in the following order: COOLi < COONa < COOK < COORb < COOCs. This order was in agreement with that of increasing electropositivity of the counteraction. The activation energy of the copolymerization of EC with PAn was determined to be 26.3 kcal/mol. The rate of the copolymerization was accelerated in an aprotic solvent such as N-methyl-2-pyrrolidone. Furthermore, the effect of solvent and polymerization temperature on the grafting ratio of polyester was investigated.     

Far infrared and millimeter wave studies of impurity scattering within electron-hole liquid in semiconductors

Despite the experimental evidence in far-infrared that impurities are ionized within the electron-hole liquid in silicon and germanium, combination of luminescence data with millimeter wave cyclotron resonance and far-infrared magneto-optics tells us that the impurity-assisted intervalley electron scattering within EHL in silicon can apparently be interpreted on the basis of neutral impurity scattering. Justification of simulating the impurity center within EHL as neutral is grounded for the electron scattering problem, making use of the atomic scattering theory in contrast with a simple ionized impurity model calculation with proper screening.     

Electrostatic Interactions That Determine the Rate of Pseudorotation Processes in Oxyphosphorane Intermediates: Implications with Respect to the Roles of Metal Ions in the Enzymatic Cleavage of RNA

The enzymatic cleavage of RNA takes place via a cyclic pentacoordinate oxyphosphorane intermediate/transition state. We carried out ab initio investigations on the neutral cyclic oxyphosphorane, which exists as a stable intermediate. As a consequence of the conformational preferences of the pentacoordinate trigonal bipyramidal intermediates, the rotation of the P-OH bonds is strongly coupled with the reaction coordinate for the pseudorotation process. In addition, the neutral PF(4)OH species has a higher barrier to pseudorotation than the corresponding anionic species PF(4)O(-). These findings are related to the positive charge of the hydrogen atoms on the equatorial oxygens in the trigonal bipyramidal structures: the hydrogen atoms preferably adopt eclipsed positions relative to the axial ligands. Fixing the cationic species in these regions causes an increase in the barrier heights for pseudorotation processes and, thus, prevents isomerization by pseudorotation. Consequently, metal coordination in the double-metal ion mechanism for enzymatic cleavage of RNA should serve to exclusively stabilize the trigonal bipyramidal intermediate/transition state for the in-line attack and departure process.     

Synthesis,hydrophilicity, and oxygen permeability of poly[(trimethylsilyl)propyne]-gr-poly(N,N-dimethylacrylamide)

N,N-dimethylacrylamide (DMAA) was graft copolymerized on poly[(trimethylsilyl)propyne] (PTMSP) by single electron reduction of PTMSP with potassium naphthalenide (K-Naph), followed by anion polymerization of DMAA from the carbanion formed in the reduction. A hard and practically non-water-swelling PTMSP-gr-poly(DMAA) was obtained under the conditions using controlled amount of K-Naph and DMAA. The graft copolymer was characterized with regard to structure, number-averaged molecular weight, and the amount of grafting poly(DMAA) determined by the relative absorbance of the IR absorption band assigned to the CO and SiC H functionalities (A_CO/A_{SiC H}). The oxygen permeability and water contact angle (θ) of the graft copolymer were evaluated while varying the amount of grafting poly(DMAA). The graft copolymer proved to be highly oxygen permeable (165 Barrers) and hydrophilic (θ = 27°). Its transparency was also elucidated with UV–vis spectra. This graft copolymer was proposed as a promising candidate for use as a hard contact lens material. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 603–610, 1998     

Radiation damage in Si1−x Ge x heteroepitaxial devices

Results are presented of an extended study on the induced lattice defects and their effects on the degradation of Si_1−x Ge _x devices, subjected to a 20 MeV alpha-ray, 1 MeV electron, 1 MeV fast neutron, and 20 and 86 MeV proton irradiations. The degradation of the electrical device performance increase with increasing fluence, while it decreases with increasing germanium content. In the Si_1−x Ge _x epitaxial layers, electron capture levels associated with an interstitial-substitutional boron complex are induced. The radiation source dependence of performance degradation is attributed to the difference of mass and the probability of nuclear collision for the formation of lattice defects.     

Optically controlled confined potential of a vertically aligned double-dot array system

In a vertically coupled dot, it is expected that generalized Kohn theorem is invalid. Then electron–electron interaction can be observed in far-infrared absorption. This interaction depends on the coupling between dots. We fabricated a vertically aligned double-dot array using a double heterostructure to investigate the coupling. The strength of the coupling depends on the confine potential of the dot. In far-infrared magneto-optical absorption measurements under pulsed photoexcitation, we observed peak shift of asymmetry states and increasing of absorption intensity of symmetry states of the dot. This is a result of the confined potential modulation by light. This indicates the possibility of optical control of the coupling.     

An Acceleration Algorithm for Image Reconstruction Based on Continuous-Discrete Mapping Model

In image reconstruction for X-ray computed tomography, images reconstructed by filtered backprojection (FBP) include systematic errors because the FBP method does not take into account some of the properties of the actual imaging system such as the divergence of X-ray beam. One solution to this problem is to use algebraic reconstruction methods, such as generalized analytic reconstruction from discrete samples and natural pixel decomposition. However, in the process of reconstruction using these methods, it is necessary to solve the linear algebraic equations which have a large coefficient matrix. In this paper, we propose a method to accelerate the iteration solving these equations by preconditioning the coefficient matrix using a polynomial function. The results of the computer simulations show the effectiveness of the proposed method.     

Beat structure in spectrally resolved four-wave mixing under crosslinear polarization in GaAs quantum wells

The polarization dependence of beat structure in spectrally resolved four-wave mixing was investigated on 50 Å GaAs multiple quantum wells. Under crosslinear polarization we observed a beating structure at higher energy region of the main spectral peak due to biexciton–exciton transition. The beat has a period of 0.95 ps and is constructive at delay time T=0 ps, which suggests the corresponding double Feynman diagrams to be of the same type. By shortening the central wavelength of the sub-ps laser, we observed the distinctive peak corresponding to the beat period. Even in GaAs system, which is generally believed to have a small biexcitonic effect, there are two effective 2-exciton states with well-defined energies which are required to describe the third-order optical nonlinearly.     

Opto-Electronic Block-Cipher Based on Iteration of the 2-D Toggle Cellular Automata: Algorithm

Parallel architectures and algorithms may offer a solution to the system bottleneck arising from the need to encrypt a very large amount of data without compromising security. In this respect the use of cellular automata with their parallel, simple, regular and modular structure is very promising. We extend the blockcipher algorithm, based on the iterations of so called “toggle” cellular automata rules to two dimensions.The advantages are higher complexity of the crypt-analytical attacks and substantial increase in the speed of the algorithm. Due to its massive parallelism and interconnectivity, the algorithm is very suitable for opto-electronic implementation.