Effect of conductive fillers on the cyclic stress-strain and nano-scale free volume properties of silicone rubber

The effect of carbon black(CB) and graphite(G) powders on the macroscopic and nano-scale free volume properties of silicone rubber based on poly(di-methylsiloxane)(PDMS) was studied through thermal and cyclic mechanical measurements, as well as with positron annihilation lifetime spectroscopy(PALS). The melting temperature of the composites(Tm) and the endothermic enthalpy of melting(?Hm) were estimated by differential scanning calorimetry(DSC). Tm and the degree of crystallinity(χc) of PDMS composites were found to decrease with increasing the CB content. This can be explained due to the increase in physical cross-linking which results in a decrease in the crystallite thickness. Besides, χc was found to be dependent on the filler type. Cyclic stress-strain behavior of PDMS loaded with different contents of filler has been studied. Mullins ratio(RM) was found to be dependent on the filler type and content. It was found that, RM increases with increasing the filler content due to the increase in physical cross-linking which results in a decrease in the size of free volume, as observed through a decrease of the o-Ps lifetime τ3 measured by PALS. Moreover, the hysteresis in PDMS-CB composites was more pronounced than in PDMS-G composites. Furthermore, a correlation was established between the free volume Vf and the mechanical properties of PDMS composites containing different fillers. A negative correlation was observed between Vf and RM.     

Micro-structure of gamma-ray irradiated polyethylenes studied by positron annihilation

Effects of gamma-ray irradiation on high density (HDPE) and low density (LDPE) polyethylenes were measured by positron annihilation at temperatures between 100 and 420 K. The effect of the irradiation on the intensity I₃ and the lifetime τ₃ of the longest lived component was significant below the glass transition temperature T_g, while they were little affected above T_g. For LDPE a marked minimum was observed in the I₃ vs T curve. The lower edge of the minimum, corresponding to glass II and glass I transitions, was found to shift to lower temperatures by the irradiation. In HDPE both I₃ and τ₃ decreased on irradiation mainly due to radiation induced free radicals. The gel fraction of HDPE was small when irradiated below T_m, while it rose sharply on irradiation near T_m. Positron lifetime parameters of this highly crosslinked HDPE showed a distinct difference compared to HDPE irradiated below T_m. Usefulness of the positron annihilation method is discussed in conjunction with studying micro-structure of polymers.     

Glass transition temperatures of stereoblock,isotactic and atactic polypropylenes of various chain lengths

The effect of sample molecular weight on the glass transition temperature has been examined for isotactic, stereoblock and atactic polypropylene samples. Asymptotic values of T₉ (∞) (isotactic) = 272°K, T₉ (∞) (atactic) = 266°K, and T_g (∞) (stereoblock) = 266°K were found. Deviation from T_g (∞) was observed when M?_n was below 10⁴; the dependence of T_g on M?_n has been discussed in relation to the Gibbs-DiMarzio treatment of the glass transition. The possible effects of both tacticity and crystallinity on T_g were examined; comparison of the data obtained with those of other workers was made. It was concluded that molecular order in polypropylene samples could affect T_g significantly and that this was particularly obvious in short chain stereoblock fractions.     

Contradictory effect of gold nanoparticle-decorated molybdenum sulfide nanocomposites on amyloid-β-40 aggregation

The effect of gold nanoparticle-decorated molybdenum sulfide (AuNP-MoS₂) nanocomposites on amyloid-β-40 (Aβ₄₀) aggregation was investigated. The interesting discovery was that the effect of AuNP-MoS₂ nanocomposites on Aβ₄₀ aggregation was contradictory. Low concentration of AuNP-MoS₂ nanocomposites could enhance the nucleus formation of Aβ₄₀ peptides and accelerate Aβ₄₀ fibrils aggregation. However, although high concentration of AuNP-MoS₂ nanocomposites could enhance the nucleus formation of Aβ₄₀ peptides, it eventually inhibited Aβ₄₀ aggregation process. It might be attributed to the interaction between AuNP-MoS₂ nanocomposites and Aβ₄₀ peptides. For low concentration of AuNP-MoS₂ nanocomposites, it was acted as nuclei, resulting in the acceleration of the nucleation process. However, the structural flexibility of Aβ₄₀ peptides was limited as the concentration of AuNP-MoS₂ nanocomposites was increased, resulting in the inhibition of Aβ₄₀ aggregation. These findings suggested that AuNP-MoS₂ nanocomposites might have a great potential to design new multifunctional material for future treatment of amyloid-related diseases.     

Investigation of structural and hydrogen absorption properties in the LaNi5−xPtx-H2 system

The substitution of nickel by platinum in the binary LaNi₅ compound (CaCu₅ structure type, a=5.019(1) Å, c=3.981(1) Å, space group P6/mmm) and its effect on the hydrogenation properties was studied. The phase LaNi_5−xPt_x has a homogeneity domain ranging from x=0 to 5. For x<3, platinum substitutes almost exclusively on site 3g and also replaces nickel on site 2c for x>3. Contrary to what is observed in other systems, the hydrogen absorption plateau pressure was found to increase as a function of the cell volume. Powder neutron diffraction experiments were conducted for two deuterated compounds with x=0.25 and 0.75. Deuterium partial ordering occurs in the case of x=0.25 leading to a symmetry decrease to the space group P6mm (LaNi_4.75Pt_0.25D_5.23, a=4.225(1) Å, c=5.357(1) Å, Z=1, R_Bragg=3.3%). For x=0.75, an orthorhombic superstructure based on the CaCu₅-type lattice was found (LaNi_4.25Pt_0.75D_2.61, a_orth=√3a_hex=9.089(1) Å, b_orth=b_hex=5.272(1) Å, c_orth=2c_hex=8.145(1) Å, Z=4, SG Ibam, R_Bragg=6.1%).     

The Influence of Mechanical Milling and Subsequent Calcination on the Formation of Lithium Ferrites

The influence of ball milling and subsequent calcination of a 2.5:1 molar mixture of α-Fe₂O₃ and Li₂CO₃ on the formation of lithium ferrites has been investigated. Premilling was found to considerably lower the temperature at which the lithium ferrites LiFeO₂ and LiFe₅O₈ are formed. A β-to-α phase transition in LiFe₅O₈ was found to take place on cooling from ca. 1000°C depending on the milling history and cooling regime.     

Platinum and molybdenum oxide deposited carbon electrocatalyst for oxidation of hydrogen containing carbon monoxide

Carbon-supported Pt/MoO_x catalysts for use in PEFC anodes were prepared and their catalytic activity for the oxidation of CO-contaminated H₂ was examined based on the fuel cell performance in PEFC single cell arrangements. Based on the XRD pattern and XPS measurements of the prepared Pt/MoO_x/C catalysts, it was found that the deposited MoO_x exists as an amorphous oxide phase. The MoO_x phase shows a redox peak at around 0.45 V, which was revealed by the cyclic voltammogram of the Pt/MoO_x/C in sulfuric acid solution. The PEFC performance of the cell with Pt/MoO_x/C was improved under 100 ppm CO-contaminated H₂ conditions compared to the Pt/C catalyst, and was almost comparable to the PtRu(1:1)/C catalyst.     

Tolerance of glucose electrocatalytic oxidation on NiO x /MnO x /GC electrode to poisoning by halides

This paper is devoted to the investigation of the tolerance of glucose oxidation on NiO _x /MnO _x /GC binary catalyst to poisoning by halide ions. NiO _x /MnO _x /GC electrode was prepared by the consecutive electrodeposition of MnO _x followed by the deposition of NiO _x nanoparticles atop the previously deposited MnO _x nanoparticles. The electrode was characterized electrochemically and morphologically. Interestingly, experimental results showed that the NiO _x /MnO _x /GC electrode shows a superior electrocatalytic activity compared with the NiO _x /GC electrode and a behavior comparable to the NiO _x /GC electrode regarding tolerance to poisoning by halides, even in the presence of high concentrations of Cl^? and Br^? up to 0.3 M. On the other hand, it was found that iodide ions have a significant effect on glucose electrooxidation under the prevailing operating conditions and even at concentrations lower than those of the other two halide ions.     

Structural study of lanthanum nickelate thin films deposited on different single crystal substrates

Fabrications of La₂NiO_4+δ thin film layers by liquid-injection metalorganic chemical vapor deposition were tried on different single crystals substrates: (001)Si, (001)MgO, (001)LaAlO₃ and (001)SrTiO₃. As results of structural characterizations, polycrystalline dendritic layers of La₂NiO_4+δ tetragonal (or orthorhombic) phase were observed on (001)Si substrates while layers of a perovskite-like cubic structure were observed on the other single crystal substrates. From a high-resolution TEM study of a layer deposited on (001)MgO, such a perovskite-like cubic structure exhibits many planar structural faults likely similar to planes of oxygen vacancies of the La₂NiO_4+δ orthorhombic structure. A thin intermediate epitaxial layer of NiO phase was also identified. Using a X-ray texture diffractometer, the layer structure on (001)MgO, (001)LaAlO₃ and (001)SrTiO₃ was confirmed to be of cubic structure with 〈100〉 axes parallel to those of the substrate. The T dependence of the resistivity of a layer deposited on (001)MgO substrate was found to be of a semi-conducting behavior.     

Fluorescence observations on complex formation between linear and hyperbranched polyelectrolytes in dilute aqueous solutions

The complexation between poly(ethylene imine) (PEI) and poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS) or AMPS copolymers was investigated with the relative excimer emission intensity I_E/I_M of a cationic probe 1-pyrenemethylamine hydrochloride (PyMeA · HCl), fluorescence nonradiative energy transfer (NRET) I_Py/I_Np of naphthalene to pyrene labels, the fluorescence anisotropy r and I_E/I_M of pyrene labels. PEI was a hyperbranched weak polycation in acid solution, which formed complex with anionic polyelectrolytes due to the electrostatic attraction. The I_E/I_M of PyMeA · HCl probe decreased to zero, the intra-, intermolecular NRET I_Py/I_Np and I_E/I_M of pyrene labels on the AMPS copolymers reached their maxima when χ was increased from 0 to 2.4, which was defined as the mole ratio of the amino group in PEI to the AMPS group in the polyanion. These facts indicated the formation of nonstoichiometric complex of the oppositely charged polyelectrolytes when χ = 2.4 at the concentration much lower than their overlap concentrations. The intermolecular aggregate appeared as indicated by an increase in the intermolecular I_Py/I_Np and r with χ up to 2.4 due to neutralizing and hydrophobizing the polyelectrolytes and the bridging effect of the PEI chain bound on different polyanion chains. At high pH, PEI became a neutral polymer and did not bind with the AMPS anion to form the complex as illustrated by the constant value of r for the pyrene labels attached to the AMPS polyanion as that without addition of PEI. The amino group in PEI quenched pyrene and naphthalene emission, resulting a decrease in both I_Np and I_Py.     

Electrochemically inactive organic substance influence on peak height in anodic stripping voltammetry

A simplified method was proposed for calculation of changes of peak height (i) vs. deposition time (t_e) in the presence of a surfactant acting only at the deposition stage. The simplification was based on the assumption that the deposition current has a constant value until time t₁ when it decreases abruptly to a lower level. Hence the dependency of the peak height on the deposition time can be written:i=β₀t₁+β₁(t_e?t₁) Limiting cases were discussed: t₁→0 and β₁→1 leading to three types of i vs. t_e relation. The experimental evaluation was carried out for Pb(II), Cu(II) and Cd(II) in the presence of various polyethyleneglycols. All three types of curves were obtained in the experiments, as well as some which did not fit the simplified theory.     

Role of crystallite size on the photoluminescence properties of SrIn2O4:Eu phosphor synthesized by different methods

Photoluminescence (PL) of Eu³⁺ was studied in SrIn₂O₄ host lattice. A complete solid solubility of Eu³⁺ has been found in the series SrIn_2−xEu_xO₄ [x=0-2.0]. The phase formation at a relatively low temperature and in a very short duration was achieved by combustion synthesis (CS). Concentration quenching of luminescence has been observed in SrIn_2−xEu_xO₄ [x=0.1-2.0] and the critical concentration for maximum emission was found to be with x=0.3. In order to find the role of crystallite size on the PL properties of SrIn₂O₄:Eu³⁺, the results obtained with phosphors synthesized by solid state reaction (SSR) and CS methods were compared.     

Reactivity and Structural Properties of a Mechanochemically Treated Ag2O-V2O5 System in Relation to AgVO3 Polymorphs

Formation and chemical properties of amorphous AgVO₃, which was prepared by mechanochemical treatment of an Ag₂O-V₂O₅ mixture, and crystalline AgVO₃ were studied in relation to AgVO₃ polymorphs. A ball-milled sample of the mixture was assigned as a highly deformed β-AgVO₃ rather than the low density phase α-AgVO₃. Crystalline α-AgVO₃ and β-AgVO₃ were converted into deformed β-AgVO₃ by ball milling, which produced a clear change. δ-AgVO₃ is resistant to mechanical treatment and its structure was not markedly affected. The dissolved chemical species from the ball-milled sample precipitates to form α-AgVO₃ without a seeding crystal, but other polymorphs deposit if they are present; i.e., β-AgVO₃ and δ-AgVO₃ grow on the seeding crystal.     

Solvent influence on the viscosity-temperature relationship for dilute polybutadiene solutions

The temperature dependence of the viscosity of dilute polybutadiene solutions has been described by an Arrhenius expression according to Moore's treatment. The activation energy of flow was found to depend on both concentration and molecular weight through an empirical parameter, K_e. This parameter was positive in toluene, zero in decalin and negative in dioxane. This behaviour is discussed in connection with the temperature dependence of the expansion coefficient (x_η) and the excluded volume parameter (z). A recent Yamakawa-Tanaka expression relating x_η and z was used in the theoretical calculation of K_e.     

A laser induced fluorescence study of vibrational energy transfer processes in cyclopropane

Results of infrared laser induced fluorescence studies on cyclopropane are presented. Molecules were excited from the ground state to the v₁₀ level of cyclopropane using a Q-switched CO₂ laser operating on either the P(14) or P(20) transition of the 9.6 μ branch. Fluorescence was observed from the v₆, v₈, v₁₀ + v₁₁ and v₅ + v₁₀ levels of cyclopropane. The self-deactivation of vibrationally excited cyclopropane through V → T/R processes was found to have a rate of 8.0 ± 1.5 ms^?1 torr^?1. Deactivation by rare gas collisions was also studied with comparison to simple V → T and V → R theories. V → V equilibration processes are discussed involving the v₆, v₈, v₁₀, v₁₁, and v₁₀ + v₁₁ levels.     

Classification for molecular aggregation structure of monolayers on the water surface

The melting temperature, T_m, and the crystalline relaxation temperature, Tα_c, of palmitic acid and dipalmitoyl phosphatidylcholine monolayers on the water surface were evaluated by combination of two kinds of measurements: first, the subphase temperature, T_sp, dependence of the monolayer modulus based on the surface pressure-area (π-A) isotherm and second, the T_sp dependence of the electron diffraction, ED patterns of their monolayers. On the basis of their characteristic temperatures of the monolayers, the aggregation structure of the monolayers which were transferred onto a hydrophilic SiO substrate at various surface pressures and T_sps was investigated by means of transmission electron microscopy. The π-A isotherm for the fatty acid monolayer on the pure water surface represented the aggregating process of isolated domains grown right after spreading a solution on the pure water surface. The fatty acid monolayer on the pure water surface was classified into a crystalline monolayer (T_sp < T_m) and an amorphous one (T_sp > T_m). The crystalline monolayer was further classified into two types; crystalline domains were aligned along their crystallographic axes owing to an induced sintering at the interfacial region among monolayer domains by surface compression (T_sp < Tα_c), while not for T_sp > Tα_c. In the case of the phospholipid monolayer, the monolayer was classified into a compressing crystallized monolayer (T_sp < T_m) and an amorphous one (T_sp > T_m). The compressing crystallized monolayer is a monolayer in which crystallization was gradually induced at plateau region on the π-A isotherm by compression. Electron diffraction studies of arachidic acid monolayers in different dissociated states of hydrophilic groups revealed that formation of the compressing crystallized monolayer was attributed to an electrostatic repulsion among ionic hydrophilic groups. It was concluded that the aggregation structure of monolayers on the water surface was systematically classified into ‘the crystalline monolayer’, ‘the amorphous monolayer’ and ‘the compressing crystallized monolayer’, with respect to thermal and chemical (intermolecular repulsive) factors.     

Electronic and steric effects of ligands as control elements for rhodium-catalyzed asymmetric hydrogenation

Electronic and steric effects in the rhodium diphosphinite catalyzed asymmetric hydrogenation were investigated. A series of electronically and sterically modified (S)-BINOL and (S)-H₈-BINOL ligands was synthesized and effects on the catalytic performance were studied. Phosphinite basicity was varied by using p-CH₃O, p-CH₃, p-H, p-CF₃, 3,5-(CH₃)₂, 3,5-(CF₃)₂ substituents on the diphenylphosphine moieties. In the hydrogenation of dimethyl itaconate and methyl (Z)-α-acetamido cinnamate an increase in enantioselectivity and activity was observed with increasing phosphine basicity.     

Influence of chemical structure and chain length on conducting properties of dielectric polymers in metal/polymer/metal structures

The influence of weight-average molecular mass M _w on the maximal width of a conducting layer h _max of polysterene, polymethylmethacrylate, and polyamidine films was investigated. It was shown that h _max increases in proportion to M _w ^S for all investigated polymers. It points to the fact that the h _max value is defined by the macromolecular coil size.     

Studies of the system TaS2−xSex

Single crystal and powder samples of the system TaS_2?xSe_x have been prepared and studied. The range of solubility was found to extend from x = 0 to x = 2.0. X-Ray analysis has shown that mixed anion samples exhibit a series of hexagonal layered polymorphs similar to those found in TaS₂ and TaSe₂, with the a and c lattice parameters increasing monotonically from TaS₂ to TaSe₂. Electrical transport properties were measured on single crystals and found to be similar to the end compositions. Organic molecules such as pyridine and collidine were found to intercalate TaS_2?xSe_x for x ≦ 1.4, and superconducting transition temperatures were measured for both intercalated and unintercalated samples. The highest T_c obtained was 4.1 K in the 4H(c) phase of the sample TaS_1.6Se_0.4.