Uniform Segment Research Articles

Intrinsic charge and Donnan potentials of grafted polyelectrolyte layers determined by surface conductivity data

In order to characterize grafted polyelectrolyte layers based on electrokinetic measurements a theory of the surface conductivity K σ was developed, starting from the model of thick polyelectrolyte layers with uniform segment distribution and dissociable groups with an unknown p K value. According to this model the inner part of the polyelectrolyte layer adjacent to the substrate is considered to be isopotential while the potential decay occurs in a zone near the solution side of the layer. A simple equation for the Donnan potential Ψ D as a function of pH, p K, electrolyte concentration C 0, and volume charge density ρ was obtained. In the derived equation K σ is directly related to Ψ D , while the other terms have less influence on the magnitude of K σ and can be accounted for in a second approximation using Ψ D as determined from the measured K σ . Evaluation of the suggested model indicates that K σ measurements provide an effective method to characterize polyelectrolyte layers by analyzing the dependence of Ψ D on pH and C 0: The magnitude of K σ yields information about the surface charge at complete dissociation of the ionizable groups. The dependence of K σ on pH and C 0 can be used for the determination of the p K value of the dissociating functions and the segment volume fraction of the polyelectrolyte can be estimated using the measured value of ρ.

Splash-saltation trajectories of soil particles under wind-driven rain

It is usually recognized that relatively large amounts of soil particles cannot be transported by raindrop splashes under windless rain. However, the splash-saltation process can cause net transportation in the prevailing wind direction since variations in splash-saltation trajectory due to the wind are expected in wind-driven rain. Therefore, determining the combined effect of rain and wind on the process should enable improvement of the estimation of erosion for any given prediction technique. This paper presents experimental data on the effects of slope aspect, slope gradient, and horizontal wind velocity on the splash-saltation trajectories of soil particles under wind-driven rain. In a wind tunnel facility equipped with a rainfall simulator, the rains driven by horizontal wind velocities of 6, 10, and 14 m s −1 were allowed to impact three agricultural soils packed into 20×55 cm soil pans placed at both windward and leeward slopes of 7%, 15%, and 20%. Splash-saltation trajectories were measured by trapping the splashed particles at distances downwind on a 7-m uniform slope segment in the upslope and downslope directions, respectively, for windward and leeward slopes. Exponential decay curves were fitted for the mass distribution of splash-saltation sediment as a function of travel distance, and the average splash-saltation trajectory was derived from the average value of the fitted functions. The results demonstrated that the average trajectory of a raindrop-induced and wind-driven soil particle was substantially affected by the wind shear velocity, and it had the greatest correlation ( r=0.96 for all data) with the shear velocity; however, neither slope aspect nor slope gradient significantly predicted the splash-saltation trajectory. More significantly, a statistical analysis conducted with nonlinear regression model of C 1( u * 2/ g) showed that average trajectory of splash saltation was approximately three times greater than that of typical saltating sand grain.

Molecular characterization of a hyperbranched polyester. I. Dilute solution properties

AbstractA hyperbranched polyester was fractionated by precipitation to produce 10 fractions with molecular weights between 20 × 103 and 520 × 103 g mol−1. Each of these fractions was examined by size exclusion chromatography, dilute‐solution viscometry, intensity, and quasi‐elastic light scattering in chloroform solution at 298 K. High‐resolution solution‐state 13C NMR was used to determine the degree of branching; for all fractions this factor was 0.5 ± 0.1. Viscometric contraction factors, g′, decreased with increasing molecular weight, and the relation of this parameter to the configurational contraction factor, g, calculated from a theoretical relation suggested a very strong dependence on the universal viscosity constant, Φ, on the contraction factor. A modified Stockmayer–Fixman plot was used to determine the value of (〈r2〉o/Mw)1/2, which was much larger than the value for the analogous linear polymer. The scaling relations of the various characteristic radii (Rg, Rh, RT, and Rη) with molecular weight all had exponents less than 0.5 that agreed with the theoretical predictions for hyperbranched polymers. The exponent for Rg was interpreted as fractal dimension and had a value of 2.38 ± 0.25, a value that is of the same order as that anticipated by theory for branched polymers in theta conditions and certainly not approaching the value of 3 that would be associated with the spherical morphology and uniform segment density distribution of dendrimers. Second virial coefficients from light scattering are positive, but the variation of the interpenetration function, ψ, with molecular weight and the friction coefficient, ko, obtained from the concentration dependence of the diffusion coefficient suggests that chloroform is not a particularly good solvent for the hyperbranched polyester and that the molecules are soft and penetrable with little spherical nature. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1339–1351, 2003

Sediment transport from interrill areas under wind-driven rain

In nature, erosive rainstorms are usually associated with high winds. Therefore, a quantification of wind and rain interactions and the effects of wind on detachment and transport processes provides a great opportunity for a given prediction technology to improve the estimation of erosion. This paper presents experimental data obtained on interrill erosion processes under wind-driven rain. In a wind tunnel facility equipped with a rainfall simulator, windless rains and the rains driven by horizontal wind velocities of 6, 10, and 14 m s −1 were applied to three agricultural soils packed into 20 by 55 cm soil pan at both windward and leeward slopes of 7, 15, and 20%. Wind-driven rainsplash transport was measured by trapping the splashed particles at distances on a 7-m uniform slope segment in the upslope and downslope directions, respectively, for windward and leeward slopes. The process was adequately described by relating the rainsplash transport rate to the rainfall parameter, fluxes of rain energy or momentum, and wind shear velocity by log-linear regression technique. Sediment transport by rain-impacted thin flow was measured by collecting sediment and runoff samples at 5-min intervals after runoff onset. This process was also adequately described by rainfall and flow parameters. Comparing the process contribution to the total sediment transport, it is concluded that rainsplash transport is a significant process that should not be neglected in accurately predicting interrill water erosion under wind-driven rain. Mean values for the ratio of rainsplash erosion to the corresponding total sediment transport were 14.6±6.4, 23.9±7.8, and 38.1±8.7% for the rains driven by 6, 10, and 14 m s −1 wind, respectively.

Exploratory Synthesis and Luminescence Study of the First π-Conjugated Tin-Based Alternating Copolymers for Blue Light-Emitting Diodes at the Very Low Operating Voltage

Tin-based alternating copolymers with a uniform π-conjugated segment were synthesized using the Heck reaction between distyrylstannane monomer and various difunctionalized monomers. The UV−vis absorption maximum peaks of the resulting polymers in chloroform solution and in thin films appeared in the wavelength range of 347−394 nm. Upon photoexcitation with light of wavelength 350 nm, their photoluminescence spectra exhibited an emissive maximum peak around 470−502 nm, corresponding to blue light emission. Multilayered light-emitting diodes with ITO/PEDOT (50 nm)/polymer (80 nm)/Ca (50 nm)/Al (200 nm) composition were fabricated. These LED devices exhibited an emissive maximum peak in the range of 464−472 nm. All of the materials exhibited a very low turn-on voltage of less than 4 V. From cyclic voltammetric studies and optical data, the LUMO level is estimated to be 3.20 eV for SnPhFPV and 2.90 eV for SnPhPVK, and the ionization potentials (HOMO level) were estimated to be 5.90 eV for SnPhFPV and 5.60 eV ...

ON THE STATIC INSTABILITY OF FLEXIBLE PIPES CONVEYING FLUID

The dynamic stability of a collar-stiffened pipe conveying fluid was examined by using the Euler–Bernoulli beam theory. The pipe considered consists of identical substructures, or cells, connected in an identical fashion. Each substructure, or cell, comprises a uniform pipe segment and a collar. A finite element model was developed to predict the dynamic stability of the stiffened pipe under the action of the flowing fluid. Stability maps were obtained for clamped-free collar-stiffened pipes of various design parameters. The design parameters included the arrangement and the geometry of the identical cells. The stability maps demonstrated that the collar-stiffened pipe exhibits unique stability characteristics when compared to a uniform pipe. It was found that the stable region in the stability map enlarges for the collar-stiffened pipe when compared to a uniform pipe. To give clearer insight into the pipe dynamic behavior, the dynamic response and eigenvalue branches were presented for a number of collar-stiffened pipes.

Identification of Delamination in a Composite Beam Using a Damaged Spectral Element

An identification procedure and sensitivity studies on single and multiple delaminations in laminated composite beams are presented. A Spectral Finite Element Model (SEM) based on the Fast Fourier Transform (FFT) is employed. A single damaged spectral element is used to model a uniform composite beam segment with a single delamination, irrespective of its length, by only specifying the delamination location and size. A damage force indicator is used to stimulate the identification procedure. In a realistic identification task, the advantage of the proposed procedure is that only two measurement nodes are sufficient as opposed to several measurement nodes in a standard finite element model with a single delamination. For multiple delaminations, the computation time to perform the identification task shows cubic polynomial complexity and hence is tractable for health monitoring of large skeletal structures. Sensitivity of a damage force indicator with respect to variations in delamination location and size is also presented. The analysis efficiently identifies the severity of delaminated configurations through the damage force indicator. This basic information can be useful in choosing optimal measurement points and the discretized structural model to minimize redundancy in the presence of several other factors such as high noise to signal ratio, damping etc. Conclusions are made on many important issues, which can be addressed with further development based on this preliminary work.

Palladium-Catalyzed Direct Synthesis, Photophysical Properties, and Tunable Electroluminescence of Novel Silicon-Based Alternating Copolymers

A new class of silicon-based alternating copolymers with a uniform π-conjugated segment regulated by organosilicon units was synthesized using the Heck reaction between distyrylsilane monomers and appropriate arylene dibromides. The incorporation of organosilicon units with the aromatic or aliphatic groups on the silicon atoms into π-conjugated systems improved their processability and interrupted the π-conjugation length. They could be spin-cast onto various substrates to give highly transparent homogeneous thin films. Their glass transition temperatures were in the range 94−127 °C. They show strong absorption bands around 356−416 nm, which corresponds to the π−π* transition of the conjugated segments. The maximum photoluminescence appeared around 440−526 nm in the blue or green emission region. Their optical properties depend on the arylene dibromides in the Heck reaction. The single-layered light-emitting diode of an Al/SiPhPPV or SiHMPPV/ITO glass exhibits a threshold voltage of 10−12 V from the I−V c...

Free Vibration of a Spinning Stepped Timoshenko Beam

The finite element method is employed in this paper to investigate free-vibration problems of a spinning stepped Timoshenko beam consisting of a series of uniform segments. Each uniform segment is considered a substructure which may be modeled using beam finite elements of uniform cross section. Assembly of global equation of motion of the entire beam is achieved using Lagrange’s multiplier method. The natural frequencies and mode shapes are subsequently reduced with the help of linear transformations to a standard eigenvalue problem for which a set of natural frequencies and mode shapes may be easily obtained. Numerical results for an overhung stepped beam consisting of three uniform segments are obtained and presented as an illustrative example. [S00021-8936(01)00101-5]

Spatial and temporal variability of turbulent mixing in an estuary

Observations with a microstructure/CTD profiler and an ADCP during five cruises in 1994/95 provide a glance at the turbulent mixing climate of a partially mixed estuary, the Hudson River. The cruises took place under widely varying conditions of river discharge and external forcing by water level fluctuations in New York Bight. Tidal and fortnightly variability patterns of stratification, shear and mixing characteristics were qualitatively repeatable, largely following the description of Peters (1997). (i) Longer-term variations of buoyancy frequency and Richardson number (Ri) during flood were weakly correlated with the river discharge. (ii) All flood tides had substantial mixing. Cruise-to-cruise variations of turbulent dissipation rate, salt flux, and stress were large and correlated with variations in Ri. (iii) During ebb tides, mixing was weak in response to stable stratification during neaps, and it increased dramatically toward spring tides parallel with the occurrence of low Ri. Cruise-to-cruise variations of mixing became small toward spring tide. (iv) The largest vertical salt flux occurred during spring ebbs, which provided about 30% of the total fortnightly salt flux; floods throughout the fortnightly cycle providing most of the remainder. (v) The observed level of small-scale mixing, i.e. stress and vertical salt flux, was consistent with the integral momentum and salt budgets in the uniform segment of the Hudson where the bulk of the measurements were taken. (vi) One topographically forced flow feature examined, lee waves behind a trench across the river, was associated with elevated mixing.

Formation of highly uniform InGaAs ridge quantum wires by selective molecular beam epitaxy on novel InP patterned substrates

For InP-based InGaAs ridge quantum wires by selective molecule beam epitaxial (MBE) growth on 〈 1 10〉 oriented InP mesa-stripes, the origin of wire inhomogeneity and possible methods to improve the wire uniformity were investigated in detail. Appearance of extra-side-facets on (111)A sidewalls was the major reason for ridge waving, leading to the inhomogeneity of the wire. By introducing mis-orientation into the mesa-direction, the width of the extra-facets were drastically reduced, leading to the suppression of the ridge waving. On the other hand, growth on a mesa-stripe having wide and narrow width regions led to complete removal of the extra-side-facets and the ridge waving from the wide mesa region, resulting in formation of a highly uniform InGaAs wire segment having a length of several microns.

Blue Electroluminescence from Novel Silicon-Containing Poly(Cyanoterephthalylidene) Copolymers

A new class of silicon-containing poly(cyanoterephthalylidene) copolymers with a uniform π-conjugated segment was synthesized using the Knoevenagel reaction between the dialdehyde monomer and the appropriate diacetonitrile. The incorporation of organosilicon units with the flexible alkyl side group into the polymer rigid backbone would afford processable electroluminescent materials and interrupt the regular π-conjugated chains. The resulting polymers were highly soluble in common organic solvents. The Mn and Mw of the resulting polymers are in the range 3800−4200 and 5400−6000 with a polydispersity index range of 1.38−1.43, respectively. Their glass transition temperature was in the range of 82−87 °C. According to the molecular mechanical calculations for single chain models of SiBuPPV and SiBuCNPPV, trans conformations are more stable than cis conformations. The UV−visible absorbance of present polymers show strong absorption bands around 340−360 nm, since the π-conjugated system is regulated by organosilicon units. Their photoluminescence spectra appeared around 460−470 nm in the blue region. The threshold voltage of the SiHMCNPPV was about 7 V. Surprisingly, these polymers exhibit blue light-emitting diodes in the EL emissive band at 480 nm in the blue region, instead of red light-emitting diodes, when an operating voltage of higher than 7 V is applied. These copolymers show a relatively low operating voltage compared with a block copolymer having well-defined structures because the incorporation of electron-withdrawing cyano groups into the π-conjugated system increased the electron affinity of those polymers and decreased the LUMO energy level.

Molecular Dynamics Simulations of Polar Polymer Brushes

Brushes of end-grafted, polar polymer chains in a good, nonpolar solvent are studied via molecular dynamics simulations of a bead-spring model. The monomers are connected by finitely extensible nonlinear springs. The springs connecting consecutive monomers in a chain are labeled with dipoles parallel to the chain contour. We consider the dipoles as limits of charge pairs, replacing each dipole by a pair of charges positioned at the centers of mass of two consecutive beads. Equilibrium simulations are performed, and the static and dynamic properties of the chains are related to the dipole magnitude. It was found that the electrostatic interactions lead to a shorter brush and the collapse becomes more pronounced with increasing segmental dipole moment. The dielectric relaxation of the tethered chains is studied, and the dielectric permittivity is obtained from equilibrium dipole moment correlation functions. Results on the dielectric relaxation of tethered chains from experiments conducted by Yao et al. exhibited a broader dielectric dispersion spectrum than that of free macromolecules in homogeneous solutions. These authors attributed the results to the thermodynamic confinement (to keep uniform segment density) and the spatial confinement that prohibits a chain to cross the tethering domain boundary. The simulations indicate that neither the spatial confinement nor the thermodynamic confinement lead to a broader dielectric dispersion spectrum for the tethered chains, and we attribute the discrepancies to the morphology differences between the experimental system and the simulated one. Weak electric fields parallel and perpendicular to the grafting surface are applied, and the system response is related to the equilibrium fluctuations. We also examine the structural properties of the brushes at steady state under the influence of external electric fields.

Tunable Electroluminescence from Silicon-Containing Poly(p-phenylenevinylene)-Related Copolymers with Well-Defined Structures

A new class of silicon-containing poly(p-phenylenevinylene)-related copolymers with a uniform π-conjugated segment regulated by organosilicon units was synthesized by the well-known Wittig reaction between the appropriate diphosphonium salts and the dialdehyde monomer such as terephthaldicarboxaldehyde, 2,5-thiophenedicarboxaldehyde, and N-(2-ethylhexyl)-3,6-diformylcarbazole. The resulting polymers were highly soluble in common organic solvents. They could spin cast onto various substrates to give highly transparent homogeneous thin films without heat treatment. Their glass transition temperatures were in the range of 104-119 °C. The UV-visible absorbance of the present polymers shows strong absorption bands at around 347-387 nm, which corresponds to the π-π* transition of the conjugated segments. Their maximum photoluminescence wavelengths for SiPPV analogues appeared around 420-480 nm in the blue emission region, except a silicon-containing poly(p-phenylenevinylene)-related copolymer having a thiophene unit showed a strong PL peak at 520 nm in the green emission region. In the case of the polymers containing a carbazole unit, their PL spectra show both the highest peak in the PL emissive band at 450 nm and an additional strong emissive band in the green region. The single-layer light-emitting diode of a Al/SiPhPPV or SiPhPVK/ITO glass is fabricated. The threshold voltage is in the range 6-12 V from the I-V curve. The electroluminescence (EL) spectrum of the SiPhPPV gives the highest peak in the EL emissive band at 450 nm, when the operating voltage of 9 V was applied. For the polymer containing a carbazole unit, the EL spectrum of the polymer shows the highest peak in the EL emissive band at 450 nm as well as an additional strong emissive band in the yellow region, when the operating voltage of 10 V was applied. Interestingly, the SiPhPVK gives a strong white emitting light, when the operating voltage of higher than 12 V was applied.

A SAXS study of the internal structure of dendritic polymer systems

Small-angle x-ray scattering was used to characterize the single-particle scattering factors produced by poly(amidoamine) dendrimers, poly(propleneimine) dendrimers, and polyol hyperbranched polymers in dilute solutions with methanol as solvent. Fits from electron density modeling reveal similar overall densities of the dendrimers as a function of dendrimer generation. The seventh through tenth generation poly(amidoamine) dendrimers exhibit higher order scattering features that require nearly monodisperse, spherical particles with essentially uniform internal segment densities. Dilute hyperbranched polymer solutions exhibit scattering more indicative of the inherent irregularity of internal segment densities and overall sizes to be expected within these systems. Radii of gyration estimated from electron density modeling agree reasonably well with those estimated by standard Guinier methods used in previous studies. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2913–2924, 1997

Novel silicon-containing poly ( p-phenylenevinylene) related polymers for blue light-emitting diodes

A new class of silicon-containing poly ( p-phenylenevinylene) -related polymers with a uniform π-conjugated segment, regulated by organosilicon units, shows two strong absorption bands around 312 and 352 nm, corresponding to the π-π * transition of the carbazole unit and the conjugated segments, respectively. The present polymers exhibit two strong photoluminescent peaks in the blue range of 420–480 nm and the green region of 540 nm. The latter is attributed to the carbazole segment. Their electroluminescence (EL) spectra similarly show two strong emissive bands in the blue region and the yellow region. The current-voltage ( I–V) curve of a typical light-emitting device fabricated with a Al/SiPVKPPV/ITO glass structure shows a typical rectifying characteristic with a threshold voltage of 12 V.

Conformational chain statistics in a model lipid bilayer: Comparison between mean field and Monte Carlo calculations

A comparison between a mean field theory of chain packing in membranes and micelles and Monte Carlo simulations is presented for model lipid bilayers. In both approaches the “lipids” are modeled as freely jointed (but self-avoiding) chains of spherical segments. The first segment of the chain represents the head group, anchored to the bilayer interface by a harmonic binding potential. The simulations are performed for symmetric bilayers composed of 200 chains, with periodic boundary conditions. Both pure and mixed bilayers (composed of long and short chains) are analyzed. In the simulation nonbonded segments interact via Lennard-Jones potentials, ensuring nearly uniform segment density in the bilayer core, as assumed in the mean field theory. The lateral pressure profiles governing the probability distribution of chain conformations in the mean field theory are related and compared to the tangential pressure profiles calculated from the simulations using Kirkwood–Buff’s molecular theory. The two pressure profiles show very good agreement. We also calculate two conformational chain properties: end-segment distributions and orientational bond order parameters. The end-segment distributions calculated by the two approaches show excellent agreement. The order parameters compare somewhat less satisfactorily, yet we found that the order parameters derived from the simulations depend rather sensitively on the details of the interaction potential. In general, the results of the simulations support the use of the mean field theory as a (simple) tool for studying conformational chain statistics in confined environments and related thermodynamic properties, such as membrane curvature elasticity.

Electrophoretic Mobility of a Polyelectrolyte-Adsorbed Particle: Effect of Segment Density Distribution

Varoqui's theory [Nouv. J. Chim.6,187 (1982)] for the electrophoretic mobility of a charged colloidal particle adsorbed by neutral polymers with an exponential distribution of the polymer segments is extended to the case where adsorbed polymers are charged. An analytic mobility expression is derived on the basis of the linearized Poisson–Boltzmann equation. Comparison is made with the case of particles coated with a polyelectrolyte layer of fixed thickness with uniform segment distribution.

Storage and retrieval methods to support fully interactive playout in a disk-array-based video server

One of the most important challenges in a videoon-demand (VOD) system is to support interactive browsing functions such as “fast forward” and “fast backward.” Typically, these functions impose additional resource requirements on the VOD system in terms of storage space, retrieval throughput, network bandwidth, etc. Moreover, prevalent video compression techniques such as MPEG impose additional constraints on the process since they introduce interframe dependencies. In this paper, we devise methods to support variable rate browsing for MPEG-like video streams and minimize the additional resources required. Specifically, we consider the storage and retrieval for video data in a diskarray-based video server and address the issue of distributing the retrieval requests across the disks evenly. The overall approach proposed in this paper for interactive browsing is composed of (1) a storage method, (2) sampling and placement methods, and (3) a playout method, in which the sampling and placement methods are two alternatives for video-segment selection. The segment-sampling scheme supports browsing at any desired speed while balancing the load on the disk array, as well as minimizing the variation on the number of video segments skipped between samplings. In contrast, the segment-placement scheme supports completely uniform segment sampling across the disk array for some specific speed-up rates. Several theoretical properties for the problem studied are derived. Finally, we describe experimental results on the visual effect of the proposed frame-skipping approach.

Unsteady flow of blood through narrow blood vessels— a mathematical analysis

Of concern in the paper is a study of unsteady flow of blood in a uniform arteriole segment under various boundary conditions at the wall. The blood vessel is modelled as an initially stressed orthotropic elastic tube filled with a viscous incompressible fluid representing blood which is treated here as a polar fluid. As for the equations governing the motion of the arteriole wall, we have taken a pair of appropriate equations derived in one of our earlier communications, by using suitable constitutive relations and the principle of superimposition of a small additional deformation on a state of known finite deformation. Numerical computations of the derived analytical expressions for velocity distribution, pressure, total angular velocity, wall shear stress, flow rate and resistance to fluid motion have been carried out. Variation of these quantities as functions of frequency and the system parameters have been studied with a view to illustrate the applicability of the mathematical model.