Full copolymer characterization by SEC-NMR combined with SEC-MALDI

Measurements on a series of random copolymers with units of methylmethacrylate, butylacrylate, styrene and maleic-anhydride are performed. A method is used which consists in fractionating the copolymer by size exclusion chromatography (SEC), collecting 30-40 fractions and then recording both the NMR and MALDI spectra of the fractions. In a successive step, bivariate distribution of chain sizes and composition maps are derived from knowledge of the molar mass, weight and composition of the copolymer fractions. The compositional distribution is obtained by summation of the sections of the bivariate distribution which belong to a narrow compositional range.A model for SEC fractionation of copolymers is developed. It allows to predict the composition and D(x) of the SEC fraction (D(x) is the ratio between the number-average and the weight-average molar mass, x is the fraction number).Offline SEC-MALDI measurements are performed, in order to measure experimentally D(x) for the random copolymer. The agreement between theoretical and experimental values for D(x) and for the composition of the SEC fraction (as derived from SEC-NMR measurements) is good. The predictions of the model were also compared with literature data for SEC fractionation of a copolyester with units of butileneadipate and butilenesebacate. The agreement between theoretical and experimental values is more than satisfactory.     

Evaluation of slitlike porosity of carbon adsorbents

Deviations of a pore shape from a slitlike one were analyzed for a variety of carbon adsorbents on the basis of calculations of pore size distributions (PSDs) with respect to the pore volume (f_V(x)) used for estimation of the PSDs related to the specific surface area (f_S(x)) applying several models of pores: individual slitlike pores and mixtures of slitlike and cylindrical pores and gaps between spherical particles. The use of complex pore models allows us to diminish the difference between S_BET and the specific surface area (S_sum) calculated from f_S(x).     

Viscoelastic and dielectric studies on comb- and brush-shaped poly(n-butyl acrylate)

The dynamic compliances J′(ω) and J″(ω) and the dielectric permittivities ?′(ω) and ?″(ω) are reported over a wide range of frequency ω and temperature for comb-branched and brush-shaped poly(n-butyl acrylate) prepared by atom transfer radical polymerization. The analysis here of the viscosity η for the comb- and brush-shaped polymers indicates the need to account for an increase of the persistence length with increasing density of the side chains. Enhanced values of J_S are attributed to a dilution effect arising from the side chains on the values that would otherwise arise from the backbone chain. The dielectric loss ?″(ω) demonstrates a deviation from frequency-temperature superposition at a certain range of frequency, with the deviation increasing with increasing density of the side chains. The deviation occurs for a frequency range for which J′(ω) and J″(ω) are approaching their terminal response, but no corresponding deviation from frequency-temperature superposition is noted for these functions. The dielectric behavior in this region is attributed to a δ-relaxation at frequencies lower than the principal α-relaxation, similar to behavior reported for certain polymers with mesogenic side chains.     

MAX phases Hf2(SexS1−x)C (x = 0–1) and their thermal expansion behaviors

Herein, a series of chalcogen-containing MAX phases, Hf₂(Se_xS_1?x)C, were successfully synthesized, whose lattice parameter change follows the Vegard’s law. The average coefficient of thermal expansion (CTE) can be continuously tuned from 7.59 μK^?1 to 9.93 μK^?1 when the occupancy rate x of Se changes from 0 to 1. The substitution of Se for S effectively soften the crystal structures that is reflected by long average M-A bond in Se-alloying Hf₂(Se_xS_1?x)C. However, the CTEs along a and c axes in all Hf₂(Se_xS_1?x)C MAX phases are almost same which may be find application in thermal barrier coating (TBC) that isotropic volume change is highly demanding.     

Small angle neutron and X-ray scattering by poly(methyl methacrylate) chains

The intensity of radiation scattering by poly(methyl methacrylate) (PMMA) chains is computed as a function of scattering angle over the range $\text{0 < μ = (}\text{4π}\text{λ)}\text{sin}\text{(}\text{υ}\text{2}\text{) < 0.3}\text{A}\text{?}{}^{\mathrm{?1}}$, on the basis of a realistic rotational isomeric state model. The scattering functions F_x(μ), corresponding to Iμ², are developed for chains of x units in terms of the even moments 〈r^2p_ij〉 of the separation distance between pairs of the monomer units i and j. Whereas the theoretical scattering function F_x(μ) for isotactic PMMA increases monotonically with μ, for predominantly syndiotactic PMMA it exhibits a maximum at $\text{μ ≈ 0.05}\text{A}\text{?}{}^{\mathrm{?1}}$. This is in agreement with experimental results on small angle neutron and X-ray scattering by PMMA (in bulk and in solution, respectively). The appearance of the maximum in F_x(μ), heretofore considered anomalous, is shown to be a direct consequence of the preference of racemic diads of PMMA for the trans, trans conformation and of the inequality of the skeletal bond angles at CH₂ and at the doubly substituted C^α.     

An Investigation on the Structure of Polyselenocyanogen [Se2(CN)2] x

The structure of polyselenocyanogen [Se₂(CN)₂] _x is analogous to that of polythiocyanogens, which have the general formula [S _y (CN)₂)] _x (y=1,2,3,...). This result is based on the surprising fact that the FT-IR spectrum of the former compound is similar to the spectra of the latter class of compounds and on other experimental data. Thus, the proposed structure for polyselenocyanogen involves polyazomethine chains connected together by selenium bridges. Selenocyanogen Se₂(CN)₂ polymerizes when treated with certain organic solvents. The polymerization was followed by electronic spectroscopy and involves the formation of peculiar absorption bands in the visible and near infrared portion of the spectrum. Alternatively, selenocyanogen polymerization is promoted by thermal treatment in high boiling solvents like xylenes and decalin. The resulting polymer shows an FT-IR spectrum comparable to that of polythiocyanogens. The thermal stability of selenocyanogen has been studied by thermogravimetric analysis (TGA) in conjunction with differential thermal analysis (DTA). Heating of Se₂(CN)₂ under N₂ to 650°C causes conversion into paracyanogen (CN) _x with the release of selenium.     

The state of tin impurity atoms in vitreous germanium chalcogenides

Tin atoms formed after the radioactive decay of the ^119mmSn and ¹¹⁹Sb impurity atoms in the structure of Ge _x S_{1 ? x} and Ge _x Se_{1 ? x} glasses are stabilized in the form of Sn²⁺ and Sn⁴⁺ centers or in germanium nodes (after ^119mmSn decay), or in the nodes of structural units formed by ¹¹⁹Sb atoms. The Sn²⁺ and Sn⁴⁺ centers correspond to the ionized states of amphoteric two-electron tin center with negative correlation energy. ¹¹⁹Sn atoms formed after the radioactive decay of the ^119mTe atoms in the structure of Ge _x S_{1 ? x} and Ge _x Se_{1 ? x} glasses are stabilized in both chalcogen and germanium nodes.     

Reciprocal distance averages for the necklace model

A method to evaluate reciprocal interbead distance averages is proposed. These averages are computed in the frame of the elastic necklace model with the distribution function of a linear polymer chain distorted by the flow. The resulting set of coupled equations are solved numerically for chains of increasing size, up to Z = 9 beads.     

Solvothermal Synthesis of Ternary Sulfides of Sb2 − x Bi x S3 (x = 0.4, 1) with 3D Flower-Like Architectures

Flower-like nanostructures of Sb_{2 − x}Bi_xS₃(x = 0.4, 1.0) were successfully prepared using both antimony diethyldithiocarbamate [Sb(DDTC)₃] and bismuth diethyldithiocarbamate [Bi(DDTC)₃] as precursors under solvothermal conditions at 180 °C. The prepared Sb_{2 − x}Bi_xS₃ with flower-like 3D architectures were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). The flower-like architectures, with an average diameter of ~4 μm, were composed of single-crystalline nanorods with orthorhombic structures. The optical absorption properties of the Sb_{2 − x}Bi_xS₃ nanostructures were investigated by UV–Visible spectroscopy, and the results indicate that the Sb_{2 − x}Bi_xS₃ compounds are semiconducting with direct band gaps of 1.32 and 1.30 eV for x = 0.4 and 1.0, respectively. On the basis of the experimental results, a possible growth mechanism for the flower-like Sb_{2 − x}Bi_xS₃ nanostructures is suggested.     

Conformations and dynamics of adsorbed protein-like chains

Conformations and dynamics of adsorbed protein-like chains are investigated by using Monte Carlo simulation based on the modified orientation-dependent monomer-monomer interaction (ODI) model. The chain size and shape of adsorbed protein-like chains, such as mean-square end-to-end distance 〈R²〉, mean-square radius of gyration 〈S²〉_xy (or 〈S²〉_z), shape factors , and 〈δ^*〉 are discussed here. At the same time, fraction of adsorbed segment f_a and average orientation of bond 〈P₂(cos θ)〉 are also investigated. The adsorbed protein-like chains trend to be more flat when adsorption interaction energy becomes strong. Different kinds of interactions (such as contact interaction, sheet interaction, spin-spin interaction, helical interaction, and adsorbed interaction) are considered in detail. Dynamics of adsorbed protein-like chains are investigated by calculating their diffusion coefficients, and we find that there exist the relationships of Dxy∼N−γxyand Dz∼N−γz, and the values of γ_xy and γ_z are 4-5 times larger than that of general self-avoiding walk (SAW) chains. These investigations may provide some insights into adsorption of proteins.     

Synthesis,characterization and magnetic properties of polythiophene/SrFe12-x(ZrZn)0.5xO19 nanocomposite as a microwave absorber

Ferrites are an important group of magnetic materials which are used as absorbers. The incorporation of ferrite and conducting polymer achieves great enhancement in microwave absorption properties. The nanocomposites of hexagonal ferrites embedded by conducting polymers such as polypyrrole, polyaniline and polythiophene (PTH) have been paid much attention. In the present study, strontium hexagonal ferrite doped by Zr and Zn with the final formula of SrFe_12-x(ZrZn)_0.5xO19 considering x = 0.9 and embedded by PTH was produced to achieve a nanocomposite with the highest microwave absorbing ability. In this study, after synthesis of SrFe₁₂O₁₉(ZrZn)_0.5xO19 and PTH, the nanocomposite was prepared by in situ polymerization. Wrapping the ferrite particles and PTH chains could form nanocomposite properly, and therefore acceptable interactions were observable between SrFe_12-x(ZrZn)_0.5xO19ferrite particles and PTH polymer chains in the composites. Assessing the X-ray diffraction (XRD) patterns of SrFe_12-x(ZrZn)_0.5xO19, PTH, and PTH/SrFe_12-x(ZrZn)_0.5xO19 nanocomposite indicated that the PTH characteristic peak shifts slightly and its peak intensity reduces, which may be attribute to the coating of PTH polymer chains onto SrFe_12-x(ZrZn)_0.5xO19 particles. We revealed also lower magnetic properties in the obtained nanocomposite. The morphological assessment also suggested that PTH could effectively coat the SrFe_12-x(ZrZn)_0.5xO19 particles. The synergistic effect of SrFe_12-x(ZrZn)_0.5xO19 particle plus PTH leads to microwave absorption percentage higher than 95% by PTH/SrFe_12-x(ZrZn)_0.5xO19 nanocomposite. Overall, nanocomposite creating by coupling interaction between SrFe_12-x(ZrZn)_0.5xO19 particles (x = 0.9) and PTH can effectively lead to achieve the highest rate of absorption of electromagnetic waves.     

(Bi0.5Na0.5)ZrO3 modified KNN-based ceramics: Enhanced electrical properties and temperature insensitivity

To further improve the properties of KNN-based lead-free ceramics, a new ceramic system, (0.98-x)K_0.525Na_0.475Nb_0.965Sb_0.035O₃-0.02 BaZr_0.5Hf_0.5O₃-x(Bi_0.5Na_0.5)ZrO₃(KNNS-BZH-xBNZ) was designed, the relevant properties such as piezoelectricity, strain, and temperature stability were analysed in detail. It was found that the R-T phase boundary can be successfully constructed when x=0.030, and this two-phase coexistence shows relatively good comprehensive properties (d₃₃~410 pC/N, T_C~255 °C, S_uni~0.132%, and d₃₃*~441 pm/V). Meanwhile, its strain property also shows good temperature stability from room temperature to 180 °C (S_uni100°C/S_uniRT~97.5% and S_uni180°C/S_uniRT~83.9%), which is comparatively superior to many KNN-based ceramics and some lead-based ceramics. Therefore, KNNS-BZH-xBNZ ceramics may broaden the practical application of lead-free ceramics.     

Structure of sulfur complexes in zeolite Na<Emphasis Type="Italic">X</Emphasis>(S)

The distribution of sulfur atoms embedded in single crystals of zeolite NaX is investigated using the single-crystal X-ray diffraction analysis. It is shown that, in the unit cell of the NaX(S) zeolite, 67 sulfur atoms are located in the form of S₈ or S₆ rings and S₂ molecules in large cavities and in the form of Na₄S₄ complexes in the sodalite cages.     

Complex formation between S−6 and organic polysulphides in dimethylacetamide

In a dipolar aprotic medium (dimethylacetamide), organic sulphides or polysulphides RS_xR (R = alkyl or phenyl group, x = 1, 2, 3) are not reduced into thiolate ions by S₃⁻ or S²⁻₈, electrogenerated from sulphur, but give complexes RS_xR, S²⁻₆ by shifting the equilibrium between S₃⁻ and S²⁻₆ ions. Complexes stability constants, visible spectra and rates of formation are given. A mechanism for the complex formation is proposed form the kinetic studies.     

Structure and physicochemical properties of the (AsSI)1? x (As2S3) x and (AsSeI)1? x (As2Se3) x glasses

The structure and physicochemical properties of glasses in the (AsSI)_1−x (As₂S₃) _x and (AsSeI)_1−x (As₂Se₃) _x systems are investigated. It is demonstrated using ¹²⁷I M?ssbauer spectroscopy that the structure of the glasses under study is built up of the structural units AsS_3/2 (AsSe_3/2), which correspond to the compounds As₂S₃ (As₂Se₃), and the ternary structural units, which correspond to the compounds AsSI (AsSeI). In turn, these structural units are connected into a three-dimensional network. The proposed model accounts for the observed linear dependences of the physicochemical properties (density, microhardness, glass transition temperature) of the glasses on the composition. Original Russian Text ? V.A. Bordovskii, R.A. Castro, A.V. Marchenko, P.P. Seregin, 2007, published in Fizika i Khimiya Stekla.     

Microstructure and dielectric properties of compositionally graded Ba1-xSrxTiO3 multilayer tunable capacitors

Li₂O/B₂O₃-added Ba_1-xSr_xTiO₃ (B_1-xS_xT) ceramics, where 0.2 ≤ x ≤ 0.35, were well densified at 920 °C with pure perovskite structure. The dielectric constant, tunability, and figure of merit (FOM) of B_1-xS_xT ceramics increased with x because of the decreasing Curie temperature (T_C). The specimen with x = 0.35, whose T_C was close to room temperature, exhibited a large tunability of 27.4 % and FOM of 110 at 10 kV/cm. A compositionally graded multilayer (CGML), which was sintered at 920 °C, was fabricated using B_1-xS_xT thick films to produce a temperature-stable tunable capacitor, and it evinced a dense microstructure and a continuous interface between the B_1-xS_xT thick film and the Ag electrode. This CGML capacitor showed a large tunability (51 %) and FOM (150) at 20 kV/cm. It also exhibited stable tunability (17–28 % at 10 kV/cm) at temperatures between 30–90 °C. Therefore, the B_1-xS_xT CGML capacitor is a suitable candidate for temperature-stable tunable capacitors.     

The effect of ball size on mill performance

The specific rates of breakage of particles in a tumbling ball mill are described by the equation S_i = ax^α_i(Q(z), where Q(z) is the probability function which ranges from 1 to 0 as particle size increases. This equation produces a maximum in S, and the particle size of the maximum is related to ball diameter by x_m = k₁d². The variation of a with ball diameter was found to be of the form a = k₂/d^1.5. Both k₁ and k₂ vary with mill diameter, and simple power laws have been assumed, k₁ ∝ D^0.1, k₂ ∝ D^0.6. If it is also assumed that the mean overall values of S_i for a mixture of balls is the weighted mean of S_i values for each ball size, equations are derived for calculating this mean value. As an example, the results are used in a mill simulation to show the quantitative effect of different ball mixes in a two-compartment cement mill versus a uniform mix over the whole mill.     

Control of interfacial barriers in n-type FeS2 (pyrite) by electrodepositing metals (Co, Cu) forming isostructural disulfides

Cobalt and copper were electrodeposited on n-type, 100-oriented, pyrite slices. The occurrence of a favorable chemical interaction with the pyrite interface is marked by the appearance of pronounced prebulk deposition (PBD) features in the case of Co. This metal leads to the formation of highly ohmic metallic contact, while deposition of Cu provides a significant Schottky barrier. The effects of electrodeposited Co and Cu can be explained by the formation of the pyrite-type compounds CoS₂ and CuS₂, at the interface, which are n- and p-type metallic compounds, respectively. When time for diffusion is allowed, mixed Co_xFe_1−xS₂ and Cu_xFe_1−xS₂ phases are also to be expected as learned from XPS experiments on (electron beam evaporated) Co/(MOCVD) pyrite layers. A model is proposed which links the electronic properties of these disulfides with their solid state chemical behavior in the pyrite structure and with their ability to form PBD layers (Co).     

Highly ordered bilayer structures of gold(I)–alkanethiolates having two types of alkyl groups

The luminescent gold(I)–alkanethiolates containing two types of alkyl groups, Au(I)–(SR)_x(SR′)_ys, were synthesized by mixing HAuCl₄ with two n-alkanethiols in tetrahydrofuran. Au(I)–(SR)_x(SR′)_ys have highly ordered bilayer structures and the dominant conformations of the alkyl groups were found to be all-trans from X-ray diffraction and FT-IR spectroscopy studies. The gold nanoparticles stabilized by two types of alkanethiolates could be prepared by reducing the Au(I)–(SR)_x(SR′)_ys using tetrabutylammonium borohydride (TBABH) as a reducing agent. Elemental analysis results of Au(I)–(SR)_x(SR′)_ys and gold nanoparticles prepared from Au(I)–(SR)_x(SR′)_ys indicated that the longer alkanethiols are more reactive for the formation of Au(I)–(SR)_x(SR′)_ys and also the longer alkylthiolates are preferentially absorbed on the gold particles compared with the shorter ones.     

Elastic moduli of highly stretched tie molecules in solid polyethylene

The elastic properties of interlamellar bridges in semicrystalline polyethylene (PE) were estimated from the molecular-mechanics calculations on the assumption that the energy loading of a chain backbone represents the principal deformation mechanism. The calculations result in the force-length functions featuring abrupt discontinuities due to sequential annihilation of the defects by the conformational transitions. The correlation of the chain elastic moduli E with the concentration of defects in the chain and with the chain extension ratio x were established. The distribution functions ζ(E) of Young's moduli of interlamellar bridges in semicrystalline PE were calculated by using the literature data on the chain length distributions of tie molecules. The impact of the distribution function of moduli ζ(E) on the overall elastic response of solid PE materials was examined, particularly in cases of the stacked lamellae morphology involving so-called hard elastic PE.