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181.
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183.
Franco M. Rabagliati Mónica A. Pérez Rodrigo A. Cancino Marcelo A. Soto Francisco J. Rodriguez Carlos J. Caro 《Macromolecular Symposia》2003,192(1):13-24
Combined systems including diphenylzinc (Ph2Zn), a metallocene, and methylaluminoxane (MAO), have been employed to initiate the copolymerization of styrene (S) with p-alkylsubstituted styrenes and with α-olefins. The copolymerization processes depend largely on the comonomer, the nature of the metallocene included in the initiator system, the presence of Ph2Zn, the polymerization temperature and the solvent used. Titanocenes produced true copolymers for S/p-substituted styrene, but not in S/α-olefin copolymerization. On the other hand zirconocenes either did not copolymerize S/p-substituted styrene or produced very low conversions, while they succeeded in copolymerizing S/α-olefin, depending on the particular zirconocene employed. A low p-methylstyrene (p-MeS) content in the S/p-MeS copolymer and a low p-tertbutylstyrene (p-ButS) content in the S/p-ButS copolymer decreased Tm, making them easier to process material than s-PS. 相似文献
184.
185.
G. A. Appignanesi J. A. Rodriguez Fris F. Sciortino 《The European physical journal. E, Soft matter》2009,29(3):305-310
The picture of liquid water as consisting of a mixture of molecules of two different structural states (structured, low-density
molecules and unstructured, high-density ones) represents a belief that has been around for long time awaiting for a conclusive
validation. While in the last years some indicators have indeed provided certain evidence for the existence of structurally
different “species”, a more definite bimodality in the distribution function of a sound structural quantity would be desired.
In this context, our present work combines the use of a structural parameter with a minimization technique to yield neat bimodal
distributions in a temperature range within the supercooled liquid regime, thus clearly revealing the presence of two populations
of differently structured water molecules. Furthermore, we elucidate the role of the inter-conversion between the identified
two kinds of states for the dynamics of structural relaxation, thus linking structural information to dynamics, a long-standing
issue in glassy physics. 相似文献
186.
187.
The a posteriori estimate of the errors in the numerical solution of ill-conditioned linear systems with contaminated data
is a complicated problem. Several estimates of the norm of the error have been recently introduced and analyzed, under the
assumption that the matrix is square and nonsingular. In this paper we study the same problem in the case of a rectangular
and, in general, rank-deficient matrix. As a result, a class of error estimates previously introduced by the authors (Brezinski
et al., Numer Algorithms, in press, 2008) are extended to the least squares solution of consistent and inconsistent linear systems. Their application to various direct
and iterative regularization methods are also discussed, and the numerical effectiveness of these error estimates is pointed
out by the results of an extensive experimentation. 相似文献
188.
Rodriguez AT Chen M Chen Z Brinker CJ Fan H 《Journal of the American Chemical Society》2006,128(29):9276-9277
We report a simple and direct synthetic method for the preparation of nanoporous carbon nanotubes with larger pores (>10 nm) on the tube wall. The method combines the use of anodic aluminum oxide (AAO) as a template for the tube diameter and block copolymer/carbohydrates self-assembly within thin films confined inside AAO pore channels to form nanopores. It involves coating the AAO inner pore channel surface with block copolymer (polystyrene-co-poly(vinylpyridine)) and carbohydrates in dimethylformamide (DMF) solution. Drying of DMF induced microphase separation of PS-PVP and formation of ordered PS and PVP/carbohydrate domains. Within the coating, the carbohydrates stay specifically only in the pyridine domains surrounding PS domains due to the hydrogen bonding between carbohydrates and pyridine blocks. After carbonization at high temperature (>460 degrees C) in argon, PS was removed, forming the nanopores and carbohydrates, and PVP was carbonized, forming the framework of nanoporous carbon tubes within AAO channels. Removal of AAO led to the formation of individual monodisperse nanoporous carbon nanotubes with a tube wall of approximately 16 nm. The ease with which these nanoporous carbon nanotubes can be fabricated, and the ability to tune tube nanostructures and surface chemistry through the choice of block copolymers used and carbonization temperature, should facilitate investigations of their scope for practical applications. 相似文献
189.
Shi X Bányai I Rodriguez K Islam MT Lesniak W Balogh P Balogh LP Baker JR 《Electrophoresis》2006,27(9):1758-1767
Generation 5 ethylenediamine (EDA)-cored poly(amidoamine) (PAMAM) dendrimers (E5, E denotes the EDA core and 5 the generation number) with different degrees of acetylation and carboxylation were synthesized and used as a model system to investigate the effect of charge and the influence of dendrimer surface modifications on electrophoretic mobility (EM) and molecular distribution. The surface-modified dendrimers were characterized by size-exclusion chromatography, 1H NMR, MALDI-TOF-MS, PAGE, and CE. The focus of our study was to determine how EM changes as a function of particle charge and molecular mass, and how the molecular distribution changes due to surface modifications. We demonstrate that partially modified dendrimers have much broader migration peaks than those of fully surface functionalized or unmodified E5 dendrimers due to variations in the substitution of individual dendrimer surfaces. EM decreased nonlinearly with increases in surface acetylation for both PAMAM acetamides and PAMAM succinamic acids, indicating a complex migration activity in CE separations that is not solely due to charge/mass ratio changes. These studies provide new insights into dendrimer properties under an electric field, as well as into the characterization of dendrimer-based materials being developed for medical applications. 相似文献
190.
Translation-invariant wavelet processing is applied to grain noise reduction in ultrasonic non-destructive testing of materials. In particular, the undecimated wavelet transform (UWT), which is essentially a discrete wavelet transform (DWT) that avoids decimation, is used. Two different UWT processors have been specifically developed for that purpose, based on two UWT implementation schemes: the "à trous" algorithm and the cycle-spinning scheme. The performance of these two UWT processors is compared with that of a classical DWT processor, by using synthetic grain noise registers and experimental pulse-echo NDT traces. The synthetic ultrasonic traces have been generated by an own-developed frequency-domain model that includes frequency dependence in both material attenuation and scattering. The experimental ultrasonic traces have been obtained by inspecting a piece of carbon-fiber reinforced plastic composite in which we have mechanized artificial flaws. Decomposition level-dependent thresholds, which are suitable for correlated noise, are specifically determined in all cases. Soft thresholding, Daubechies db6 mother wavelet and the three well-known threshold selection rules, Universal, Minimax and SURE, are applied to the different decomposition levels. The performance of the different de-noising procedures for single echo detection has been comparatively evaluated in terms of signal-to-noise ratio enhancement. 相似文献