Structural rigidity control in arborescent graft polymers

Successive chloromethylation-anionic grafting sequences on polystyrene have led to well-defined hyperbranched macromolecules. Polymers with branching functionalities f > 5000 and molar masses over 10⁷ g/mol were thus prepared while maintaining a ratio of mass- to number-average molar masses M_w/M_n = 1,1-1,3. Three series of styrene polymers were prepared by varying the density of grafting sites along the chain, resulting in controllable stiffening of the molecular structure. Characterization of the branched molecules by size-exclusion chromatography/low-angle laser light scattering and light scattering confirmed a geometric increase in molar mass for successive generations, and a low ratio of mass- to number-average molar masses. Static and dynamic light scattering experiments showed that the molecules behave like hard spheres in dilute solutions. Measurements in the semidilute range, however, showed a progressive structural stiffening effect as the branching density increases.     

Gold nanorods: Synthesis and optical properties

The main results of studying the synthesis, growth mechanisms, and optical properties of gold nanorods published in the last 5–8 years are briefly reviewed. Hydrosols of gold nanorods with variable axial ratios are synthesized in the micellar solution of ionic surfactants by sead-mediated growth procedure using the stage of particle separation in the glycerol concentration gradient. Results of synthesis in systems containing one surfactant, albeit with different Ag/Au molar ratios and different amounts of gold seeding particles, agree with the published data. It is shown that, in the case of the mixture of two surfactants, the Ag/Au ratio is an efficient controlling parameter of the synthesis of nanorods with large axial ratios. The extinction and differential light scattering, spectra dynamic light scattering, and the depolarization of laser light scattering at 90° are used for the optical control of synthesis. Three fractions are observed in separated samples. One of these fractions is characterized by the only short wavelength plasmon resonance at 570 nm corresponding, in agreement with the published data, to cubic particles. Measurements of the extinction spectra of nanorods in water-glycerol mixtures reveal higher sensitivity of the longitudinal plasmon resonance to the dielectric environment relative to the transverse resonance. It is shown theoretically and experimentally that the relative shift of plasmon resonance is proportional to the relative increment of refractive index of the surrounding medium. To calculate optical properties of nanorods, we employed a model of cylinders with semispherical ends (s-cylinders) corresponding to the shape of real particles and admitting the exact solution by the T-matrix method with a computational burden that is an order of magnitude lower than that used in the discrete dipole method. The set of dependences of the longitudinal resonance wavelength on the axial ratio of different-thickness particles complies with our data and published measurements. Theoretical and experimental values of depolarization ratio I _VH/I _VV for nanorods and nanospheres with different sizes prepared with both citrate (15–46 nm) and original thiocyanate (90 nm) reduction of HAuCl₄ are compared. It is shown that the depolarization parameter of light scattered by a nanorod suspension can exceed the theoretical limit (1/3) for common dielectric particles. The measured 10%-depolarization ratio for 90-nm spheres was far beyond the set of “size-depolarization” measurements for 15–46-nm-dia particles prepared by the citrate method and is indicative of the improved spherical morphology of 90-nm particles. This assumption was confirmed by TEM data, which also revealed both the presence of a noticeable amount of nanorods with a large axial ratio and “nanowires” of about the same thickness. A new analytic calibration for determining the diameter of spherical particles (5–100 nm) by the spectral position of the sol extinction maximum is proposed.     

The size dependent shift of the surface plasmon absorption band of small spherical metal particles

It has been previously established that the surface plasmon of small spherical silver particles, which are embedded in a noble gas matrix, shifts to higher energies (blue shift) as the mean diameterD of the particles decreases (100 Å>D>20 Å). This blue shift has also been found for supported silver particles, and quite recently we observed it by elastic light scattering in the gas phase. This latter experiment proves unambiguously that the blue shift in small silver particles is not induced by interactions with the environment, the presence of which is clearly recognized in less inert matrices such as O₂ or CO. From self-consistent calculations of the surface response of planar jellium surfaces one would expect a red shift, which is also directly confirmed by a few calculations for selected jellium spheres. The contradiction between the observed blue shift for small particles and the predicted red shift for jellium spheres disappears, if one accounts for thed-electrons of silver in a very simple approximation.     

Diffusion of spherical micronetworks in polymer diluent systems and melts studied by dynamic light scattering techniques

Spherical polystyrene (PS) micronetworks can be prepared in microemulsion with bulk radii of 5–60 nm and different cross-linking densities. The diffusion of these PS spheres has been studied in polymer diluent systems ranging from dilute solutions to plasticized melts by using forced Rayleigh scattering and photon correlation spectroscopy. On increasing the PS concentration, a colloid glass transition is observed at a volume fraction Φ_C ≈ 0.64 of the swollen spheres. At higher concentration inside the “colloid glass” state the sphere diffusion is slowed down and becomes very complex but can be observed up to the limit of a melt of collapsed spheres.     

On the influence of spatial correlations on the rate of chemical reactions in dense systems. II. Numerical results

The explicit expressions for the rate constant k_f and k_b of a dense system obtained in the preceding paper are investigated numerically. The pair correlation functions representing the spatial correlations are calculated from an associate nonreactive hard sphere system by means of the multicomponent Percus—Yevick equation. The rate constants are found to differ in their dependence on density and time up to an order of magnitude from the corresponding dilute gas value. The time behaviour of k_f and k_b was found to depend sensitively on a relation between the total volumes of the reactant and product molecules.     

Investigation of correlation between microstructure and dielectric properties of cyanate ester/silicate tube hybrids by positron annihilation lifetime spectroscopy

The correlation between microstructure and dielectric properties of cyanate ester (CE)/hollow silicate tube (HST) hybrids was investigated by positron annihilation lifetime spectroscopy, coincidence Doppler‐broadening spectroscopy, Fourier transform infrared spectra, and dynamic mechanical analyses. The addition of HST to CE resin brings a multi‐aspect influence (cross‐linked structure and density, free volume, and interfacial action) on the structure of the cross‐linked network and thus results in significantly varied dielectric properties. There is an optimum content of HST in hybrids to get the lowest dielectric constant and loss. When the content of HST is smaller than the percolation threshold, the hybrid has decreased dielectric constant and loss; this mainly results in the reduced size of free volume and orthopositronium intensity (I₃), although when the content of HST is larger than the threshold, the increased I₃ and the size of free volume as well as the interfacial polarization are responsible for the significantly enlarged dielectric constant and loss. Copyright © 2011 John Wiley & Sons, Ltd.     

Dielectric relaxation study of binary mixture of sulfolane and N,N-dimethylformamide in benzene solution using microwave absorption data

The dielectric constant (???) and dielectric loss (???) for dilute solutions of the binary mixture of different molar concentrations of sulfolane and DMF in benzene solution has been measured at 9.885 GHz and different temperatures (25, 30, 35, 40°C) by using standard microwave techniques. Following the single frequency concentration variational method, the dielectric relaxation time (??) and dipole moment (??) have been calculated. It is found that dielectric relaxation process can be treated as the rate process, just like the viscous flow. The presence of solute-solute molecular associations in benzene solution has been proposed. Energy parameters (??H _?, ??F _?, ??S _?) for dielectric relaxation process of binary mixture at 50% mole fraction in benzene at 25, 30, 35, and 40°C have been calculated and compared with the corresponding energy parameters (??H _??, ??F _??, ??S _??) for the viscous flow.     

Dielectric properties of a liquid of dipole hard spheres

The Kirkwood factor g _K of a model polar liquid of dipolar hard spheres (DHSs) was approximated by analytical equations using the approximation of the interaction of the second neighbors within the hindered rotation model. The derived equations describe the temperature and density dependences of the dielectric functions of the DHS liquid.     

Studies of molecular relaxation of poly(propylene oxide) solutions by dielectric relaxation and brillouin scattering

Dielectric relaxation and Brillouin scattering are jointly used in studying molecular relaxation in poly(propylene oxide) (PPO) and its solutions in methylcyclohexane. The dielectric method was applied to the more concentrated (100%, 80%, 60%, by volume) solutions over a wide temperature and frequency range (30 Hz to 8 GHz) in order that the variation in activation energy characteristic of a glass-forming substance could be delineated. The present work extends previous work on the undiluted polymer to higher frequencies so that range of 12 decades in the dielectric loss maximum f_max as a function of temperature is now available. The “Antoine” equation is found to represent the behavior of log f_max, of the bulk concentrated solutions very well. The more dilute (40%, 20%) solutions were studied only in the high-frequency (GHz) region since phase separation occurred at low temperatures. Both the temperature and dilution effects were interpreted in terms of free-volume theory. Brillouin scattering spectra were obtained at several scattering angles and a wide range of temperatures. A maximum in the curve of hypersonic attenuation versus temperature was observed in each polymer solution. The attenuation maximum shifts toward lower temperature upon dilution, in agreement with the dielectric relaxation result. The Brillouin scattering follows different activation parameters and evidences a more rapid process than does the dielectric relaxation. It is speculated that it monitors a secondary or subglass relaxation, due perhaps, to damped torsional oscillations.     

X-ray scattering for the determination of fat content in dairy products

The scattering of X-rays from biological samples has been shown to produce characteristic profiles, which depend on their molecular structure. The highly ordered fat molecules in an adipose tissue result in a relatively sharp scattering peak at 1.1 nm^?1 with a scattering profile, which is considerably different from the scattering profile of a water-rich tissue. The latter is characterized by a broad scattering peak at about 1.6 nm^?1. A biological sample consisting of a mixture of both adipose and a water-rich tissue is expected to show a scattering profile, which is directly linked to the relative contribution of each component and would reflect the percentage by volume of each component in the mixture. In this work, X-ray scattering profiles of a number of dairy products and water are measured. The values of two selected X-ray scattering characterization parameters (I₁/I₂% and areas A₁/A₂% of the scattering peaks at 1.1 and 1.6 nm^?1, respectively) are plotted against the fat content of each of the measured dairy samples. Results show a strong linear dependence of each of the X-ray scattering parameters and the fat content of the investigated dairy products. These results suggest a possible use of such technique as a new, simple and straight forward method for determination of fat content of dairy products that would join and support the currently available techniques.     

Binary clusters: homogeneous alloys and nucleus-shell structures

Extinction spectra of Ag_x-Au_(1?x)-alloy particles embedded in glass are compared to spectra of gold coated silver clusters and silver coated gold clusters. It is shown, that the optical extinction of alloy particles is described by the Mie theory applying the homogeneous dielectric function \(\hat \varepsilon \) (ω,R, x) measured by Paquet. The absorption of core-shell clusters was calculated deriving an extension of the Mie theory to spheres with arbitrary numbers of shells of arbitrary materials. Comparison to measured spectra points to s-electron motion in the clusters with only slight scattering at the interior interface. The appearance of two distinct Mie peaks however proves the existence of the sharp Ag-Au interface. The extended Mie formalism was also applied to a multilayer system consisting of sodium and a dielectric substance as an example for a spherical hetero structure.     

Crystal structure of thermosensitive gel spheres of poly(N-isopropylacrylamide) in the deionized aqueous suspension as studied by the static light-scattering measurements

Static light-scattering measurements of deionized suspensions of the thermosensitive gels of poly(N-isopropylacrylamide) with various degrees of cross-linking and sizes were made at 20 and 40 °C. Sharp scattering peaks are observed in the scattering curve, and they were attributed to the face-centered cubic (fcc) and/or body-centered cubic lattices (bcc) in the distribution of gel spheres. The fcc and bcc crystal structures formed in the stable and unstable conditions, respectively, i.e., the former formed more favorably at high sphere concentrations and/or low temperatures. The closest intersphere distances were much longer than the hydrodynamic diameters of the gel spheres especially at low sphere concentrations. These experimental results emphasize the important role of the extended electrical double layers in the crystallization of gel spheres, though the contribution of the double layers in gel systems is weak compared with that in the typical colloidal spheres.     

Cationic gel crystals of lightly cross-linked poly(2-vinylpyridine) spheres (170∼180 nm in diameter) in the deionized aqueous suspension

Colloidal crystallization of deionized suspensions of the cationic gel spheres of lightly cross-linked poly(2-vinylpyridine), AIBA-P2VP (170～180 nm in diameter) has been studied from the microscopic observation, morphology, phase diagram, and elastic property. Critical concentrations of melting that coexisted with ion-exchange resins were low compared with those without resins and decreased as the degree of cross-linking decreased. The density of a gel sphere in suspension state (ρ), i.e., weight percent of the gel spheres divided by the corresponding volume percent, was between 0.5 and 0.8, and decreased as the degree of cross-linking of the spheres decreased. The ρ values also decreased with decreasing size of gel spheres, which supports the small P2VP gel spheres being softer than the large ones. The closest intersphere distances of the crystals were much longer than the hydrodynamic diameters of the gel spheres especially at low sphere concentrations. Fluctuation parameters evaluated from the rigidities of the crystals of AIBA-P2VP (0.05～0.09) were similar to those of gel crystals of cationic gel spheres of lightly cross-linked poly(2-vinylpyridine) spheres coated with poly(ethylene glycol), 400 nm in diameter, and thermo-sensitive gel spheres of pNIPAm, poly(N-isopropylacrylamide), but larger than those of typical colloidal hard spheres. The stable crystal phase formed beyond the adsorbed monolayer of cationic gel spheres on the surface of the substrate. These experimental findings support important role of the extended electrical double layers around the cationic gel spheres in addition to the excluded volume effect of the sphere themselves on the crystallization.     

Optical properties of Ho3+: Alkali mixed fluoride glasses

Optical absorption, fluorescence and laser characteristics of Ho³⁺ incorporated (1 % molar) glass systems of 48ZrF₄-23BaF₂-8A1F₃-20RF (RF = LiF-NaF or NaF-KF pair) were examined. Physical properties such as density, refractive index, mean atomic volume, molar refractivity and dielectric constant were measured. The measured absorption intensities were matched with the best fit Judd-Ofelt intensity parameters. The effects of the holmium glass chemical compositions were examined for properties such as spontaneous emission probabilities, branching ratios and radiative lifetimes of lasing transitions by the use of the Judd-Ofelt model. Studies were also carried out to understand the mixed alkali fluoride effects on the measured values of induced emission cross sections for the observed luminescent transitions ⁵I_7.6, ⁵F₅ and ⁵S₂ → ⁵I₈ of holmium glasses.     

Short-time mobility of spherical particles in concentrated aqueous dispersions determined by diffusing wave spectroscopy

Summary The diffusion of optically dense, charge stabilized, polystyrene latex spheres of 500 nm radius in water at 25°C was determined by diffusing wave spectroscopy in the backscattering mode. The diffusion coefficient of the latex spheres had a value of 4.8×10^–13 m² s^–1 at infinite dilution and monotonically decreased as the number density of latex spheres increased. The diffusion coefficient dropped by a factor of 3 at 38 volume % latex and a factor of 20 at 59 volume % latex. Comparison was made to diffusion of sterically stabilized poly(methyl methacrylate) latex in refractive index matched mixed organic fluids as determined by dynamic light scattering. The concentration dependence determined by diffusing wave spectroscopy matched closely to the short-time diffusion determined by dynamic scattering and not to the long-time diffusion. Although the difference in repulsive potential between the charge and the sterically stabilized lattices must be large, it has a surprisingly small effect on the translational diffusion of the latex over distances small compared to the intersphere distance.     

An atomic force microscopy investigation on self-assembled peptide nucleic acid structures on gold(111) surface

We studied interaction of hydrophilic polymer chain and hydrophilic silica nanoparticles in a dilute aqueous system using an idealized model system comprised of a well characterized polyvinyl alcohol of 100 Å R_g and hard spherical LUDOX® silica of 80 Å radii. Interaction among the polymer chains forming polymer clusters with collective polymer structure factor induced by the polymer-mediated potentials of mean force between the nanoparticles, was observed. However, Gaussian nature of individual polymer chain remains unaltered. The dilute system of polymer with low silica volume fraction has the scattering form which was appropriately modeled as the sum of the individual profiles of spherical silica particles and polymer cluster of interchain packing. With increasing silica volume fraction in the dilute solution, the spatial range parameter between the particles is reduced; hence there is a net increase in the mean potential force and consequently to stronger interaction between the silica and polymer. In the dilute systems of high silica with low polymer volume fraction, the polymer chain apparently attracted closer to the silica and concurrently absorbed to the silica hard surface and their scattering data were excellently fit with a model form factor as comprising of one unit forming the core of the spherical silica particles and the interacting polymer as the corona. This result of severe change in polymer interchain conformation in a dilute system corroborated with reduced polymer viscosity observed.     

Prediction of permselectivity of nitrate and acetate ions in the electrodialysis of aqueous solutions

Electrodialysis of aqueous solutions of acetic and nitric acids encountered in the industrial practice of deacidification of raw glyoxal solutions has been carried out to investigate the permselectivity of the system. The dependence of the permselectivity coefficient τ on experimental parameters such as flow velocity (V), current density (I), concentration of anions (C_i), etc. has been evaluated. A correlation between τ and the critical parameters has been obtained. Well-known theoretical equations have been used to calculate τ at limiting current density I_lim and as I approaches zero. Calculated results are compared with observed values. Limitations of theoretical procedures when applied to a system containing a weak acid are explained.     

Template free fabrication of hollow hematite spheres via a one-pot polyoxometalate-assisted hydrolysis process

Uniform hollow hematite (α-Fe₂O₃) spheres with diameter of about 600-700 nm and shell thickness lower than 100 nm are obtained by direct hydrothermal treatment of dilute FeCl₃ and tungstophosphoric acid H₃PW₁₂O₄₀ solution at 180 °C. The hollow spheres are composed of robust shells with small nanoparticles standing out of the surface and present a high-surface area and a weak ferromagnetic behavior at room temperature. The effect of concentration of H₃PW₁₂O₄₀, reaction time and temperature for the formation of the hollow spheres are investigated in series of experiments. The formation of the hollow spheres may be ascribed to a polyoxometalte-assisted forced hydrolysis and dissolution process.     

In situ SAXS studies of a prototypical RAFT aqueous dispersion polymerization formulation: monitoring the evolution in copolymer morphology during polymerization-induced self-assembly

Small-angle X-ray scattering (SAXS) is used to characterize the in situ formation of diblock copolymer spheres, worms and vesicles during reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate at 70 °C using a poly(glycerol monomethacrylate) steric stabilizer. ¹H NMR spectroscopy indicates more than 99% HPMA conversion within 80 min, while transmission electron microscopy and dynamic light scattering studies are consistent with the final morphology being pure vesicles. Analysis of time-resolved SAXS patterns for this prototypical polymerization-induced self-assembly (PISA) formulation enables the evolution in copolymer morphology, particle diameter, mean aggregation number, solvent volume fraction, surface density of copolymer chains and their mean inter-chain separation distance at the nanoparticle surface to be monitored. Furthermore, the change in vesicle diameter and membrane thickness during the final stages of polymerization supports an ‘inward growth’ mechanism.

In situ small-angle X-ray scattering is used to monitor the formation of diblock copolymer spheres, worms and vesicles during reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate.