Effect of polycarbonate structure and reduction time on graphene oxide dispersion

Polycarbonate (PC)/graphene oxide (GO) composites with different GO reduction time and PC types were prepared by using a twin screw extruder at 260 °C after solution mixing with chloroform. The chemical reaction degree of PC/GO composites with GO reduction time was confirmed by C–H stretching peak at 3000 cm ^?1, and the chemical reaction degree decreased with GO reduction time. The slope for storage (G′) versus loss (G″) modulus plot decreases with an increase in heterogeneous property of the polymer melts. So we can check the GO dispersion of the PC/GO composites using by the slop for G′–G″ plot. According to the G′–G″ slopes for PC/GO composite with GO reduction time, GO was well dispersed within PC matrix when the reduction time decreased. It was re‐confirmed by atomic force microscope (AFM) results. Based on the degradation temperature by Thermogravimetric analysis, G′–G″ slopes, and surface roughness by AFM, the branched PC was better than linear PC for the GO dispersion within PC matrix. The fact was also confirmed by tensile test results that the Young's modulus increased with the improvement of GO dispersion. Copyright © 2015 John Wiley & Sons, Ltd.     

Double- and zero-quantum NMR relaxation dispersion experiments sampling millisecond time scale dynamics in proteins

TROSY-based NMR relaxation dispersion experiments that measure the decay of double- and zero-quantum (1)H-(15)N coherences as a function of applied (1)H and (15)N radio frequency (rf) fields are presented for studying millisecond dynamic processes in proteins. These experiments are complementary to existing approaches that measure dispersions of single-quantum (15)N and (1)H magnetization. When combined, data from all four coherences provide a more quantitative picture of dynamics, making it possible to distinguish, for example, between two-site and more complex exchange processes. In addition, a TROSY-based pulse scheme is described for measuring the relaxation of amide (1)H single-quantum magnetization, obtained by a simple modification of the multiple-quantum experiments. The new methodology is applied to a point mutant of the Fyn SH3 domain that exchanges between folded and unfolded states at 25 degrees C.     

Effect of ionic sizes on the stability ratio of a dispersion of particles with a charge-regulated surface

The influence of the ionic sizes on the stability of a dispersion of particles, which have an amphoteric, charged-regulated surface, is discussed. A modified Poisson-Boltzmann equation, which takes into account the sizes of ionic species, is adopted to describe the electrical field. An extended DLVO theory, which takes into account the electrical, the van der Waals, and the hydration energies, is used to estimate the stability of a colloidal dispersion. The effects of the key parameters, including ionic strength, pH, and density of surface sites, on the behavior of problem under consideration are examined. The results obtained are qualitatively consistent with experimental findings in the literature.     

The mass-to-charge ratio scale

Increases in the capacity for accurately measuring the mass-to-charge ratio of specific gas-phase ions justify the reconsideration and standard definition of the gas-phase mass-to-charge ratio scale and the clearly denned connection of that scale to condensed phases. We propose that the chemical mass standard for solids and the gas phase be based upon the mass of carbon-12 buckminsterfulierene (¹²C₆₀). The mass-to-charge ratio scale in the gas phase would be based upon the mass of gas-phase ¹²C₆₀, the mass of the electron, and the electron charge in atomic units. As mass measurement accuracy improves, corrections to this mass-to-charge ratio standard are anticipated for the vaporization energy of the 12C60 molecule and its ionization potential or electron affinity. We propose that the positive ion scale be set by the mass-to-charge ratio of ¹²C ₆₀ ⁺ as (+)719.9994514±0.0000004 u per electron charge. We propose that the negative ion mass scale be set by the mass-to-charge ratio of ¹²C ₆₀ ^? as (?)720.0005484±0.0000004 u per electron charge.     

Dynamics of B-DNA on the microsecond time scale

We present the first microsecond MD simulation of B-DNA. Trajectory shows good agreement with available data and clarifies the mus dynamics of DNA. The duplex is sampling the B-conformation, but many relevant local transitions are found, including S --> N repuckers (up to 7 N-sugars are found simultaneously), local BII transitions (15% of the dinucleotides are in BII-form; some of these forms are stable for up to 7 ns), and sequence-dependent alpha/gamma transitions (happening in the 7-50 ns time scale, and being stable for up to 80 ns). Partial and total openings are often detected, but no base flipping is found. A.T openings happen after amplification of propeller twist movements, while G.C pairs (which can be opened for up to 1 ns) are opened by a complex mechanism which is often catalyzed by cations. A high affinity Na+ binding site is found in the center of the minor groove. Access to this site by cations is difficult (average entry time 400 ns), but once inside, the ion remains for long periods of time (10-15 ns), producing a sizable narrowing of the minor groove. The essential dynamics of DNA fits well with the pattern of deformation needed to (i) sample uncommon right-handed forms and (ii) sample conformations adopted by DNA when bound to proteins. Clearly, DNA has evolved to be not only a stable structure able to maintain and transmit the genetic information but also a flexible entity whose intrinsic pattern of deformability matches its functional needs.     

Dynamic stability ratio of a colloidal dispersion

The stability ratio of a dispersed system containing colloidal particles having amphoteric surfaces is estimated theoretically. We consider the case where the degree of dissociation of the functional groups on particle surface as a response to the variation in the conditions of the surrounding liquid phase is a function of time. The dynamic nature of the distributions of ions in the electrical double layer near a particle is also taken into account. The result of numerical simulation reveals that the dynamic nature of the system under consideration has the effect of increasing its stability. Due to the fact that the diffusivity of protons in the aqueous solution is much larger than that of other ion species, the level of increase in the stability ratio is less than 40%. However, if the rates of surface reactions are slow, the dynamic stability ratio can exceeds ten times the corresponding equilibrium value. Also, the smaller the particle and/or the thicker the double layer, the more significant the dynamic behavior of the system under consideration.     

Structural studies of melting on the picosecond time scale

Ultrafast structural studies of laser-induced melting have demonstrated that the solid-liquid phase transition can take place on a picosecond time scale in a variety of materials. Experimental studies using ?ngstr?m wavelength X-rays from the sub-picosecond pulse source at Stanford (now retired) on non-thermal melting of semi-conductors, such as indium antimonide, employed the decay of a single Bragg-peak to measure the time component of the phase transition. These materials were found to start melting within one picosecond after the laser pulse. Recent computer simulations have described the thermal melting of ice induced by an infrared laser pulse. Here it was shown that melting can happen within a few picoseconds, somewhat slower than non-thermal melting in semi-conductors. These computer simulations are compatible with spectroscopy experiments on ice-melting, demonstrating that simulations form a very powerful complement to experiments targeting the process of phase-transitions. Here we present an overview of recent experimental and theoretical studies of melting, as well as new simulations of ice-melting where the effect of the size of the crystal on scattering is studied. Based on simulations of a near-macroscopic crystal, we predict the decay of the most intense Bragg peaks of ice following heating by laser pulse, by modeling the scattering from the melting sample in the simulations.     

Synthesis and dispersion of isolated high aspect ratio gold nanowires

We report the assembly properties of high density and high aspect ratio metal nanowire arrays (Au, Cu and Ag with diameters ranging from 40 to 250 nm) after release from the anodic alumina oxide (AAO) templates. Individual Ag and Cu nanowires were observed following release from the template, however, in the case of gold nanowires, the dispersion was dependent on size and aspect ratio. 40-100 nm gold wires aggregated to form bundles or disordered mats. We show that a simple cyanide-mediated release from the AAO template, results in isolated dispersion of wires even for the smallest wire diameters. Possible stabilising mechanisms for observed tendency of nanowires dispersion are discussed.     

Effect of flexible polymers on the dielectric dispersion of rod-like macromolecules

The rotational diffusion of rod-like polar macromolecules (poly-γ-benzyl-l-glutamate or poly-n-hexylisocyanate) within entangled non polar polymer coils (polystyrene) is studied by dielectric absorption in solution. The dielectric increment as well as the rotatory diffusion constant is studied as a function of molecular weight of both components and increasing concentration of the non-polar polymer as well as the temperature. The classical theory of rotational diffusion is not obeyed if the viscosity of the polystyrene solution is taken as the medium viscosity. An increase in molecular aggregation on polystyrene addition is suspected.     

Luminescent decay kinetics of malachite green on a picosecond time scale

Luminescence measurements after laser pulse excitation are reported for the dye malachite green in solutions of different viscosity with a time resolution of 10 ps.     

Effect of orientation on the relative dispersion of PMDA-ODA polyimide and polypropylene

The relative dispersion of birefringence, D_b, is determined for isotactic polypropylene and Kapton HA PMDA-ODA polyimide films. D_b is then used to identify the dependence of dispersion on the orientation of polymers and is interpreted in terms of fringe jumping. Optical relationships between the in-plane birefringence and wavelength are formulated to predict the in-plane birefringence of oriented Kapton HA PMDA-ODA polyimide films at any wavelength. These relations can be used for the direct comparison of the in-plane birefringence of Kapton PMDA-ODA polyimide films obtained from different optical techniques (i.e., polarized microscopy, polarized refractometry, wave guide coupling, etc.). © 1995 John Wiley & Sons, Inc.     

Effect of nanoplatelet dispersion on mechanical behavior of polymer nanocomposites

By manipulating processing conditions, three levels of exfoliation of synthetic α-zirconium phosphate (α-ZrP) nanoplatelets in epoxy matrices have been achieved. Transmission electron microscopy and wide angle X-ray diffraction were utilized to confirm the three different levels of exfoliation of nanoplatelets in epoxy/α-ZrP nanocomposites. As expected, it was found that modulus and strength of the nanocomposite are affected by how well the nanoplatelets disperse. It was also found that the operative fracture mechanisms depend strongly on the state of the nanoplatelets dispersion. The crack deflection mechanism, which leads to a tortuous path crack growth, was only observed for poorly dispersed nanocomposites. Delamination of intercalated nanoplatelets and crack deflection were observed in a moderately dispersed system. In the case of fully exfoliated system, the crack only propagated in a straight fashion, which indicates that the fully exfoliated individual nanoplatelet can not affect the propagation of crack at all. The implication of the present findings for structural applications of polymer nanocomposites is discussed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1459–1469, 2007     

Revisiting random walks in fractal media: on the occurrence of time discrete scale invariance

This paper addresses the kinetic behavior of random walks in fractal media. We perform extensive numerical simulations of both single and annihilating random walkers on several Sierpinski carpets, in order to study the time behavior of three observables: the average number of distinct sites visited by a single walker, the mean-square displacement from the origin, and the density of annihilating random walkers. We found that the time behavior of those observables is given by a power law modulated by soft logarithmic-periodic oscillations. We conjecture that logarithmic-periodic oscillations are a manifestation of a time domain discrete scale iNvariance (DSI) that occurs as a consequence of the spatial DSI of the substrate. Our conjecture implies that the logarithmic periods of oscillations in space and time domains are linked by a dynamic exponent z, through z=log(tau)/log(b(1)), where tau and b(1) are the fundamental scaling ratios of the DSI symmetry in the time and space domains, respectively. We use this relationship in order to compute z for different observables and fractals. Furthermore, we check the values obtained with independent measurements provided by the power-law behavior of the mean-square displacement with time [R(2)(t) proportional variant t(2/z)]. The very good agreement obtained between both computations of the z exponent gives strong support to the idea of an intimate interplay between spatial and time symmetry properties that we expect will have a quite general scope. We expect that the application of the outlined concepts in the field of dynamic processes in fractal media will stimulate further research.     

Effect of inititiator on the molecular weight distribution in dispersion polymerization of styrene

Dispersion polymerization of styrene in the particle size range of 10 μ with lauroyl peroxide as initiator results in a double-peak molecular weight (MW) distribution. The high-MW fraction was due to emulsion polymerization. The same phenomenon also exists in AIBN and benzoyl peroxide initiation, although it is less obvious. The kinetics of the reaction for dispersion polymerization was dependent on the concentration of the dispersing agent and the nature of the initiator.