The synthesis,characterization, and weathering behavior of polycarbonates derived from bis(P-hydroxyphenyl) dimethylsilane (BPSI)

Polycarbonates and copolycarbonates based on BPSi and BPA were easily made using the standard interfacial technique. Molecular weight and NMR data indicate that up to 4 mol % of the BPSi decomposes during polymerization via a base-catalyzed rearrangement of BPSi to arylsiloxanes. Oxygen index studies show an increased flame resistance with increasing BPSi monomer content. DSC measurements show a decrease in glass transition with BPSi monomer concentration while TGA shows an increase in char residue with BPSi monomer content. In addition, TGA indicates no change in either thermal or thermooxidative stability with the relative BPSi/BPA concentrations. Outdoor weathering of BPSi PC and BPA PC films indicates that BPSi PC is ca. 5 times more weathering resistant than BPA PC. This latter result can be attributed to a combination of BPSi PC's lower UV absorptivity, higher water repellency, and lower likelihood for side chain photooxidation. © 1993 John Wiley & Sons, Inc.     

Wavelength dependence in photochemical vapor deposition of aluminum film using dimethylaluminum hydride

In photochemical vapor deposition of aluminum film on silicon using dimethylaluminum hydride, (CH₃)₂AlH, a surface reaction dominated below a (CH₃)₂AlH pressure of 0.3 m Torr at 200°C, which was induced only with the 160 nm band emitted from a deuterium lamp. A gas-phase reaction occurred above 0.3 mTorr at 200°C, which could be induced by both 160 nm and 240 nm emission bands from the lamp. To distinguish between surface ad gas-phase reactions, a thickness profile was used. At 240°C the surface reaction could be induced even by the 240 nm band, while the deposits formed under illumination of the two bands were thinner than those obtained with only the 240 nm band, indicating occurrence of vacuum ultraviolet (VUV)-enhanced desorption. The mechanism responsible for the observed wavelength dependence in unclear. The electrical resistivity of the films deposited at 200°C was 4.5 μΩ cm, which did not change with wavelength.     

Echo planar imaging of perfluorocarbons

Emulsions of perfluorotributylamine (FTBA) and perflubron were evaluated for their utility in ¹⁹F echo planar imaging. Fluorine images of the emulsions were obtained in a phantom and two mice that had been predosed. Both agents, but particularly perflubron, show potential for fluorine echo planar studies because of the long spin-spin relaxation times of the CF₃ resonances. High resolution thin slice images obtained in as little as 26.6 ms are presented.     

A facile synthesis of per (poly )fluoroalkyl substituted α-amino acid derivatives

Initiated by CrCl3/Fe redox couple, per(poly)fluoroalkyl iodides added to methyl a-acetylamino acrylate, giving B-per(poly)fluoroalkyl a-amino acid derivatives in good yields.     

Riccati-based preconditioner for computing invariant subspaces of large matrices

Summary. This paper introduces and analyzes the convergence properties of a method that computes an approximation to the invariant subspace associated with a group of eigenvalues of a large not necessarily diagonalizable matrix. The method belongs to the family of projection type methods. At each step, it refines the approximate invariant subspace using a linearized Riccati's equation which turns out to be the block analogue of the correction used in the Jacobi-Davidson method. The analysis conducted in this paper shows that the method converges at a rate quasi-quadratic provided that the approximate invariant subspace is close to the exact one. The implementation of the method based on multigrid techniques is also discussed and numerical experiments are reported. Received June 15, 2000 / Revised version received January 22, 2001 / Published online October 17, 2001     

深紫外无机非线性光学晶体硼铍酸锶及其分子工程学探索方法

阐述了紫外无机非线性光学晶体分子工程学探索方法的基本特点，具体分析深紫外无机非线性光学晶体硼铍酸锶（ＳＢＢＯ）以氟硼铍酸钾（ＫＢＢＦ）为主要参考晶体的分子设计方法，随后根据晶体结构研究、单晶培养、和非线性光学性能测定等实验结果讨论ＳＢＢＯ作为新型深紫外无机晶体的主要优点，即它既具有更短的紫外吸收边（接近１５５ｎｍ）和较大的非线性光学系数（ｄ２２（ＳＢＢＯ）＝０６×ｄ２２（ＢＢＯ）＝１３８ｐｍ／Ｖ），同时晶体无明显层状习性，并肯有良好的化学稳定性和机械性能     

Electron-stimulated desorption of D (H) from condensed D2O (H2O) films

The electron-stimulated desorption (ESD) of D⁻ and H⁻ ions from condensed D₂O and H₂O films is investigated. Three low-energy peaks are observed in the ESD anion yield, which are identified as arising from excitation of ²B₁, ²A₁ and ²B₂ dissociative electron attachment (DEA) resonances. Additional structure is observed between 18 and 32 eV, which may be due to ion pair formation or to DEA resonances involving the 2a₁ orbital. The ion yield resulting from excitation of the ²B₁ resonance increases as the film is heated. We attribute the increase in the ion yield to thermally induced hydrogen bond breaking near the surface, which enhances the lifetimes of the excited states that lead to desorption.     

Gelation of poly(n-butylisocyanate) in benzene

Poly (n-butylisocyanate)-benzene solutions prepared by solubilization at 45°C, followed by aging at room temperature were found to be metastable for months, although, eventually, they separated into a birefringent polymer-rich phase and an isotropic solution. These metastable solutions, as well as isothermally phase-separated biphasic samples, flowed and exhibited dynamic moduli indicative of low polymer connectivity. By contrast samples prepared by a freeze-thaw cycle were uniformly and highly birefringent and showed network (gel) behavior at room temperature. The mechanism of gel formation is most likely the exclusion of the polymer from the benzene crystal during crystallization, forcing the polymer to align and exist at grain boundaries. Films formed from solutions have different moduli than those formed from gels, and are consistent with the proposed mechanism.