Intrinsic aging and effective viscosity in the slow dynamics of a soft glass with tunable elasticity

We investigate by rheology and light scattering the influence of the elastic modulus, G0, on the slow dynamics and the aging of a soft glass. We show that the slow dynamics and the aging can be entirely described by the evolution of an effective viscosity, eta(eff), defined as the characteristic time measured in a stress relaxation experiment times G0. At all times, eta(eff) is found to be independent of G0, of elastic perturbations, and of the rate at which the sample is quenched in the glassy phase. We propose a simple model that links eta(eff) to the internal stress built up at the fluid-to-solid transition.     

Theory and ab initio calculation of radiative lifetime of excitons in semiconducting carbon nanotubes

We present a theoretical analysis and first-principles calculation of the radiative lifetime of excitons in semiconducting carbon nanotubes. An intrinsic lifetime of the order of 10 ps is computed for the lowest optically active bright excitons. The intrinsic lifetime is, however, a rapid increasing function of the exciton momentum. Moreover, the electronic structure of the nanotubes dictates the existence of dark excitons near in energy to each bright exciton. Both effects strongly influence measured lifetime. Assuming a thermal occupation of bright and dark exciton bands, we find an effective lifetime of the order of 10 ns at room temperature, in good accord with recent experiments.     

Photosensitized DNA cleavage promoted by amino acids

A novel class of DNA cleavage agents are reported that derive activity from amino acids tethered to a photoactive intercalator.     

(Z)-3-p-tolylsulfinylacrylonitriles as chiral dipolarophiles: reactions with diazoalkanes

[reaction: see text] The dipolarophilic reactivity of enantiopure (Z)-3-p-tolylsulfinylacrylonitriles (1) has been evaluated with diazoalkanes. 3-Cyanopyrazoles are obtained when R = H, but with R = alkyl (Bn, n-Bu, and t-Bu) only one cycloadduct (4 or 5) is formed in high yield under mild conditions, therefore evidencing a complete control of the regioselectivity and the endo/exo and pi-facial selectivities. These reactions are a new straightforward entry to the synthesis of pyrazolines and related structures and reveal the excellent dipolarophilic features of (Z)-sulfinylacrylonitriles.     

Byte structure variable length coding (BS-VLC): a new specific algorithm applied in the compression of trajectories generated by molecular dynamics

Molecular dynamics is a well-known technique very much used in the study of biomolecular systems. The trajectory files produced by molecular dynamics simulations are extensive, and the classical lossless algorithms give poor efficiencies in their compression. In this work, a new specific algorithm, named byte structure variable length coding (BS-VLC), is introduced. Trajectory files, obtained by molecular dynamics applied to trypsin and a trypsin:pancreatic trypsin inhibitor complex, were compressed using four classical lossless algorithms (Huffman, adaptive Huffman, LZW, and LZ77) as well as the BS-VLC algorithm. The results obtained show that BS-VLC nearly triplicates the compression efficiency of the best classical lossless algorithm, preserving a near lossless behavior. Compression efficiencies close to 50% can be obtained with a high degree of precision, and the maximum efficiency possible (75%), within this algorithm, can be performed with good precision.     

Dependence of pulser driving responses on electrical and motional characteristics of NDE ultrasonic probes

Acoustic performance in ultrasonic transmitters can be improved by means of a suitable electrical driving response and matching/tuning networks. It is important to predict this electrical response, but doing so is not easy because it departs notably from the nominal pattern with the loading probes. In practice, the analysis of HV pulser spikes in NDE applications requires fairly complex models in the transient regime and, in addition, non-linear problems could arise, especially in the case of tuned transmitters. In this paper, the most relevant influences of loading characteristics of NDT ultrasonic probes on the pulser electrical driving responses are evaluated in time and frequency domains. Conventional pulse generators and typical NDE pulsers are considered. Driving responses are analysed across commercial ultrasonic probes and, alternatively, across similar purely electrical loads. Distinct influences on pulser responses from electrical and motional sections of the probes are identified. All these aspects are studied on the basis of experimental and computer results.     

Comment on: “From classical to modern ether-drift experiments: the narrow window for a preferred frame” [Phys. Lett. A 333 (2004) 355]

In a recent article [M. Consoli, E. Costanzo, Phys. Lett. A 333 (2004) 355], M. Consoli and E. Costanzo have investigated classical and modern aether drift experiments and explored the narrow window for detection of a preferred reference frame. This Letter proposes an easy to perform variant of Fizeau's experiment, which may confirm or deny Consoli–Costanzo's claims.     

Thermo‐ and pH‐responsive gradient and block copolymers based on 2‐(2‐methoxyethoxy)ethyl methacrylate synthesized via atom transfer radical polymerization and the formation of thermoresponsive surfaces

A series of gradient and block copolymers, based on 2‐(2‐methoxyethoxy)ethyl methacrylate (MEO₂MA) and tert‐butyl acrylate (^tBA), were synthesized by atom transfer radical polymerization (ATRP) in a first step. The MEO₂MA monomer leads to the production of thermosensitive polymers, exhibiting lower critical solution temperature (LCST) at around room temperature, which could be adjusted by changing the proportion of ^tBA in the copolymer. In a second step, the tert‐butyl groups of ^tBA were hydrolyzed with trifluoroacetic acid to form the corresponding block and gradient copolymers of MEO₂MA and acrylic acid (AA), which exhibited both temperature and pH‐responsive behavior. These copolymers showed LCST values strongly dependent on the pH. At acid pH, a slightly decrease of LCST with an increase of AA in the copolymer was observed. However, at neutral or basic conditions, ionization of acid groups increases the hydrophilic balance considerably raising the LCST values, which even become not observable over the temperature range under study. In the last step, these carboxylic functionalized copolymers were covalently bound to biocompatible and biodegradable films of poly(3‐hydroxybutyrate‐co‐3‐hydroxyhexanoate) [P(HB‐co‐HHx)] obtained by casting and, previously treated with ethylenediamine (ED) to render their surfaces with amino groups. Thereby, thermosensitive surfaces of modified P(HB‐co‐HHx) could be obtained. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011