Hardness evaluation of cured urea–formaldehyde resins with different formaldehyde/urea mole ratios using nanoindentation method

To understand the influence of formaldehyde/urea (F/U) mole ratio on the properties of urea–formaldehyde (UF) resins, this study investigated hardness of cured UF resins with different F/U mole ratios using a nanoindentation method. The traditional Brinell hardness (H_B) method was also used for comparison. The H_B of cured UF resin films with different F/U mole ratios was determined after exposing the films to different post-curing temperatures. The nanoindentation method was employed for these films to measure Meyer hardness (H_M) and reduced modulus (E_r) which have been used to calculate the elastic modulus (E_s) of cured UF resins. As the F/U mole ratio decreased, the H_B decreased continuously, indicating a less rigid network structure in low F/U mole ratio UF resins. The higher the post-curing temperature, the greater the value of H_B. The H_M value also showed a similar trend as a function of F/U mole ratio. However, the E_r and E_s did not show a consistent trend as exhibited by H_M and H_B. Both H_M and E_r showed much greater variation in the coefficient of variation (COV) at lower F/U mole ratios 1.0 and 1.2, indicating a more heterogeneous composition of these resins. Linear relationships between H_M and E_r indicate that heterogeneity of the surface composition of samples contributes greatly to variations in the measured values. This variability is discussed in terms of crystal structures present in the cured UF resins of low F/U mole ratios.     

Metamagnetism in DyBaCo2O5+x, x≈0.5

The temperature dependence of the paramagnetic susceptibility χ_m(T) taken in 2500 Oe, the resistivity ρ(T), and the thermoelectric power α(T) of DyBaCo₂O_5+x, which has Ba and Dy ordered into alternate (001) planes of an oxygen-deficient perovskite, have revealed a phase segregation in the compositional range 0.3?x<0.5. Orthorhombic DyBaCo₂O_5.51 has, in addition, oxygen vacancies ordered into alternate rows of the DyO_0.51 (001) planes; a cold-pressed polycrystalline sample exhibits a first-order insulator-metal transition at T_IM=320 K, a Curie temperature T_C=300 K, and a broadened metamagnetic transition temperature T_M≈265 K in 2500 Oe. A ferromagnetic M-H hysteresis curve fails to saturate at 5 T, and a minority ferromagnetic phase below T_M has a volume fraction that decreases with decreasing temperature, vanishing below 50 K. Oxygen vacancies in the DyBaCo₂O_5.5 phase suppress the metallic state; interstitial oxygen does not. A thermoelectric power α(T)>0 of DyBaCo₂O_5.51 changing continuously across T_IM is interpreted to manifest a metallic minority phase crossing a percolation threshold; α(T) also provides evidence for a progressive excitation of higher-spin Co(III) with increasing temperature from below 50 K to above T_IM. A previous model of the RBaCo₂O_5.5 phase is extended to account for the Ising spin configuration below T_C, the magnetic order in the presence of higher-spin octahedral-site Co(III), and the α(T) data.     

Synthesis of homologously pure bacteriochlorophyll-e and f analogues from BChls-c/d via transformation of the 7-methyl to formyl group and self-aggregation of synthetic zinc methyl bacteriopheophorbides-c/d/e/f in non-polar organic solvent

Homologously pure methyl bacteriopheophorbides-e and f (BPhes-e/f_M) were prepared from modification of naturally occurring bacteriochlorophylls-c and d (BChls-c/d), respectively, by transformation of the methyl to formyl group at the 7-position. The absolute configuration of the 1-hydroxyethyl group at the 3-position of (Zn-)BPhes-e/f_M was determined from comparison with structurally known BChl-c/d epimers. Visible spectra of synthetic (Zn-)BPhe-c/d/e/f_M showed that the 7¹-oxidation and the 8²/12¹/20-methylation affected Soret, Q_x and Q_y bands of both the monomeric (in a polar organic solvent) and oligomeric species (in a non-polar solvent).     

On the line intensity ratios of prominent Si II,Si III,and Si IV multiplets

Line intensities of singly, doubly and triply ionized silicon (Si II, Si III, and Si IV, respectively) belonging to the prominent higher multiplets, are of interest in laboratory and astrophysical plasma diagnostics. We measured these line intensities in the emission spectra of pulsed helium discharge. The Si II line intensity ratios in the 3s3p²²D–3s²4p²P^o, 3s²3d²D–3s²4f²F^o, and 3s²4p²P^o–3s²4d²D transitions, the Si III line intensity ratios in the 3s3d³D–3s4p³P^o, 3s4p³P^o–3s4d³D, 3s4p³P^o–3s5s³S, 3s4s³S–3s4p³P^o, and 3s4f³F^o–3s5g³G transitions, and the Si IV line intensity ratios in the 4p²P^o–4d²D and 4p²P^o–5s²S transitions were obtained in a helium plasma at an electron temperature of about 17,000 ± 2000 K. Line shapes were recorded using a spectrograph and an ICCD camera as a highly-sensitive detection system. The silicon atoms were evaporated from a Pyrex discharge tube designed for the purpose. They represent impurities in the optically thin helium plasma at the silicon ionic wavelengths investigated. The line intensity ratios obtained were compared with those available in the literature, and with values calculated on the basis of available transition probabilities. The experimental data corresponded well with line intensity ratios calculated using the transition probabilities obtained from a Multi Configuration Hartree–Fock approximation for Si III and Si IV spectra. We recommend corrections of some Si II transition probabilities.     

Fatigue behavior of polystyrene (PS) analyzed from the Fourier transform (FT) of stress response: First evidence of I2/1(N) and I3/1(N) as new fingerprints

The fatigue behavior of polystyrene (PS) in strain controlled torsion rectangular oscillatory tests was analyzed via Fourier transform (FT) to better understand the time evolution of linear and nonlinear mechanical parameters, and to establish fingerprints related to failure onset. The tests were performed under large amplitude oscillatory shear (LAOS), so the stress response was no longer perfectly sinusoidal and higher harmonics could be detected and quantified in the FT spectra as a function of time or number of cycles N. A linear parameter, the storage modulus (G′(N)), was analyzed, as well as nonlinear parameters of the ratios of the second (I₂(N)) and the third (I₃(N)) harmonics over the fundamental one (I₁(N)). The nonlinear parameter I_2/1(N) is very low for undamaged samples, but its intensity was found to increase when defects were created in the structure to a point where cracks became visible in the sample. On the other hand, the I_3/1(N) parameter increased steadily during a test up to a local maximum where a macroscopic crack occurs in the sample. Both parameters I_2/1(N) and I_3/1(N) are proposed as new criteria to detect the onset of part failure under the conditions tested and can be used as safety limits for partial damage.     

Synthesis of enantiomeric pure lithium and potassium benzamidinate complexes

A new synthesis leading to the chiral amidines (S,S)- and (R,R)-N,N-bis-(1-phenylethyl)benzamidine ((S)- and (R)-HPEBA) in good yields is presented. Further reaction of (S)-HPEBA with n-BuLi gave the chiral lithium salt (S)-LiPEBA. Treatment of KH with (S)-HPEBA in boiling THF afforded the corresponding potassium salt (S)-KPEBA. In contrast by performing the reaction in boiling toluene a fast racemization was observed. In the solid state racemic KPEBA formed a dimer, in which all four nitrogen atoms are in a plane. To each potassium atom a toluene molecule is η⁶-coordinated.     

Disordered modifications of cobalt molybdate

The structures of recently discovered new high-temperature modifications of cobalt molybdate, a′-and a″-CoMoO₄, were determined. a′-and a″-CoMoO₄ appear after the phase a-CoMoO₄ is heated above the temperature range 700–1000°C. They seem to be the disordered modifications of a-CoMoO₄ with metal atoms distributed at random in an a-CoMoO₄ oxygen network.The F(hkl) values, calculated for variously disordered a-CoMoO₄ structure, were compared with the observed intensities of diffraction lines changing in the course of a → a′ and a → a″ transitions. It was concluded that a″-CoMoO₄₄ has a completely disordered structure with random distribution of both Co and Mo atoms in oxygen interatomic voids. The a′-CoMoO₄ is a partly disordered modification, with random distribution of some cations only.The temperature and the kind of order-disorder transition depend on the method of preparation of a-CoMoO₄ samples.The disordered modifications of cobalt molybdate may be supercooled—even to room temperature—before it transforms rapidly into low-temperature b-CoMoO₄ form.     

A model for transitional plane Couette flow

A simplified model of plane Couette flow is derived by means of a cross-stream (y) Galerkin expansion in terms of trigonometric functions appropriate for idealized stress-free boundary conditions at the plates. A set of partial differential equations is obtained, governing the in-plane (x–z) space-dependence of a velocity field taken in the form: u=U₀(x,z,t)+[1+U₁(x,z,t)]sin(πy/2), v=V₁(x,z,t)cos(πy/2), w=W₀(x,z,t)+W₁(x,z,t)sin(πy/2). Beyond Lorenz-like Waleffe's modeling (Waleffe 1997), this Swift–Hohenberg type of approach is expected to give an access to the microscopic mechanism of spatiotemporal intermittency typical of the transition to turbulence in plane Couette flow (Pomeau 1986, Bergé et al. 1998).     

An equation of state for gases and liquids

We present an equation of state that can represent within experimental error most individual sets of published PVT data for most fluids, whether in the range of vapor at moderate pressures, or compressed liquids, or gases at very high temperatures and densities, any region in fact except the vicinity of the critical point. In terms of pressure the equation is P = DRT [1 + (D/T) (c₁T + c₂D - 1) / (c₃ + c₄T^{$\text{sol1}\text{2}$} + c₅D + c₆D²)] where D = 1/V, the density in mole 1^?1. The coefficients are readily determined by a least squares fit of the data. An additional term is sometimes needed if the D range is very wide, say several times D_c. Different fluids can be simultaneously represented over a limited range, such as the compressed liquid region, by a single reduced form of the equation in which all but three of the constants are the same for all, and these three (a reducing T, 1/c₁, a reducing D, 1 / c₂, and a dimensionless parameter) are characteristic of each individual fluid. The equation can also simultaneously represent many data sets for a single fluid from many labs and covering various T and D ranges. From this, a consistent representation of its thermodynamic properties can be derived.     

Phase formation, crystal structures and magnetic properties of perovskite-type phases in the system La2Co1+z(MgxTi1−x)1−zO6

Perovskite-type cobaltates in the system La₂Co_1+z(Mg_xTi_1−x)_1−zO₆ were studied for z=0≤x≤0.6 and 0≤x<0.9, using X-ray and neutron powder diffraction, electron diffraction (ED), magnetic susceptibility measurements and X-ray absorption near-edge structure (XANES) spectroscopy. The samples were synthesised using the citrate route in air at 1350 °C. The space group symmetry of the structure changes from P2₁/n via Pbnm to R3¯c with both increasing Mg content and increasing Co content. The La₂Co(Mg_xTi_1−x)O₆ (z=0) compounds show anti-ferromagnetic couplings of the magnetic moments for the Co below 15 K for x=0, 0.1 and 0.2. XANES spectra show for the compositions 0≤x≤0.5 a linear decrease in the L₃/(L₃+L₂) Co-L_2,3 edge branching ratio with x, in agreement with a decrease of the average Co ion spin-state, from a high-spin to a lower-spin-state, with decreasing nominal Co²⁺ ion content.     

Über den Einfluß der elektrostatischen Wechselwirkungen auf die α-Helixbildung von basischen Poly-[α-aminosäuren]

A theoretical treatment of the electrostatic interactions influencing the conformation of poly-[α-aminoacids] containing ionogenic side-chains is reported. Using the Gouy-Chapman-equation for the relation between the density of surface chargeσ _e and surfacepotentialψ the latter one will be determined on the surface of anα-helix of a basic poly-[α-aminoacid] in an aqueous electrolyte solution. As it could be shown,ψ decreases with the electrolyte concentrationc. The equilibrium constants for the propagation step of theα-helix-formation is expressed as a function ofψ and a parameterK ₀ containingΔS₀ andΔH₀ dependent onc, considering the electrostatic energy of neighboured basic groups. The temperature at the mid-point of the transitionT _m is a function of the reciproce ionic strengthI. In a solution of an 1-1-electrolyte,T _m depends linearly on 1/√c. The experimental data for Poly-[L-lysine] are in a very good agreement with the straight line given by this relation. However, in the case of a 1∶1 copolymer ofL-lysine andL-leucine the calculated values are much lower than the measured ones, obviously due to neglecting the hydrophobic interactions of the leucine side chains. Furthermore, a relation for the change of theα-helix-contentf _H with 1/T is given. In this case, in the well-known relation (?f _H/? (1/T))=?ΔH⁰/4Rσ ^1/2, the cooperative parameterσ is substituted by a modified parameterΣ depending on the ionic strength. According to this equation and agreeing with the experimental results,T _m increases with the ionic strength, because the electrostatic potentialψ at theα-helix-surface decreases. However, in contrast to reality, the inclination of thef _H-T-curves increases withI orc, respectively. This may be due to an increase ofσ with the electrolyte concentration.     

Packing in honeycomb networks

Molecules arranging themselves into predictable patterns on silicon chips could lead to microprocessors with much smaller circuit elements. Mathematically, assembling in predictable patterns is equivalent to packing in graphs. An H-packing of a graph G is a set of vertex disjoint subgraphs of G, each of which is isomorphic to a fixed graph H. If H is the complete graph K ₂, the maximum H-packing problem becomes the familiar maximum matching problem. In this paper we give algorithms to find a perfect packing of HC(n) with P ₆ and K _1,3 when n is even and thus determines their packing numbers. Further we also study the packing of HC(n) with 1, 3-dimethyl cyclohexane.     

Copolymerization of acrylic acid to nylon-6 by trapped radicals—I. Vapour phase grafting

Film of nylon-6 has been γ-irradiated in vacuo to various doses D. Grafting has been effected by subsequent exposure in vacuo to vapour at 50° of composition 9.1 wt% acrylic acid and 90.9 wt% water. Determinations have been made of total vapour uptake as well as the individual swellings due to monomer and water. For D ? ca. 2 Mrad, the initial rate of grafting R_g increased with D in accord with R_g ∝ D^β with β = 0.45 ± 0.05, thus suggesting bimolecular chain termination. However, for D ? ca. 2 Mrad, there is no further increase in R_g. This is attributed in part to the fact that the radical yield is proportional to dose only for D ? ca. 2.5 Mrad. Diffusion controlled grafting has been predicted elsewhere to be characterized by β = 0.67. Grafting proceeds from the surface and the initial stages are concluded to be essentially free from diffusion control, since (a) β ≠ 0.67, (b) the rate of uptake of monomer vapour >R_g and (c) a large change in film thickness yields only a very small change in R_g.     

Fine tuning of the structure of phosphine–phosphoramidites: application for rhodium-catalyzed asymmetric hydrogenations

Diastereomers of (4-(diphenylphosphino)pentan-2-yl)-N-isopropyl-{dinaphtho[2,1-d:1′,2′-f][1,3,2]dioxa-phosphepin-2-yl}-4-amine, (S)-(2S,4S)-1, and (S)-(2R,4R)-3; the octahydro derivative 4 of (S)-(2S,4S)-1, and derivative 2 of (S)-(2S,4S)-1 containing a 1,3-propanediyl backbone, have been synthesized and used for rhodium-catalyzed asymmetric hydrogenations of prochiral olefins in order to study the role of the stereogenic elements in the backbone and in the terminal moiety. The central chirality in the bridge has been found to determine the configuration of the product with a cooperative effect from the terminal groups. Excellent ee’s (up to 99.9%) were obtained in the hydrogenation of methyl (Z)-α-acetamidocinnamate using a rhodium complex with the matched arrangement (S)-(2S,4S)-1. The hydrogenation is accomplished in a highly enantioselective manner by using green solvents such as propylene carbonate.     

Preparation and Crystal Structures of 4-(4′-Pyridylthio)-1-methylpyridinium Salts: Assembly of a Donor-Acceptor-Type Molecule Containing a Sulfide Bridge

A series of ionic 4-(4′-pyridylthio)-1-methylpyridinium salts with different counteranions (1, I⁻; 2, BF⁻₄; 3, PF⁻₆; and 4, OTf⁻, where OTf=trifluoromethanesulfonate) have been prepared. Structural analysis reveals that the cation exhibits a variety of stacking structures dependent on the anion. Compound 1 crystallizes in space group P2_1/n (#14), with a=10.764(3) Å, b=9.601(5) Å, c=13.105(3) Å, β=108.35(2), V=1285.4(8) ų, and Z=4. In this compound, each cation moiety is stacked in a helical arrangement along the c-axis. Compound 2, which is isomorphous to 1, has space group P2_1/n (#14), with a=11.647(2) Å, b=9.203(3) Å, c=13.232(2) Å, β=108.42(2), V=1345.6(5) ų, and Z=4. Compound 3 crystallizes in space group P2_1/n (#14), with a=8.06(1) Å, b=17.43(1) Å, c=10.30(1) Å, β=103.0(1), V=1410(3) ų, and Z=4. In this salt, the cation molecules assume a head-to-tail stacking arrangement, forming a polar pseudo 1-D chain. Compound 4 crystallizes in space group Pb? (#2), with a=7.585(4) Å, b=15.443(7) Å, c=6.775(4) Å, α=99.33(4), β=108.35(2)^o, γ=98.37(4), V=756.6(7) ų, and Z=2. The structure of 4 consists of a columnar stacking of pyridine moieties, with the cation moieties surrounded by the counteranions. Calculations show that the 4-(4′-pyridylthio)-1-methylpyridinium cation may be a good building block for second harmonic generation (SHG) materials, even though salts 1-4 crystallized in centrosymmetric structures and were SHG inactive.     

Fine structure and transition probabilities of the 2p 5 nl bound excited states of Ne atom

We present a semi-empirical calculational procedure for thep ⁵ nl bound excited states of rare gas atoms based on the use of an LS-dependent orbital for an excitednl electron outside a frozen Hartree-Fockp ⁵ core. The spin-orbit interaction is accounted for approximately using a localized Hartree potential. The contribution from the long-range core polarization is estimated by using a parametrized potential. A model potential is also introduced to represent partially the short-range multi-electron interactions for thep ⁵ np levels. The energy levels are calculated by diagonalizing the Hamiltonian matrix following anLS →jl transformation. The calculated fine structure of the Nep ⁵ ns,p ⁵ np,p ⁵ nd, andp ⁵ nf ¦ (jl)KJ〉 levels are in close agreement with the observed level splittings. The transition probabilities are also in agreement with earlier theoretical and experimental results.     

Electronic spectra of o-, m- and p-tolunitrile—substituent effect on internal rotation of the methyl group

The S₁ ← S₀ fluorescence excitation spectra and the S₁→S₀ dispersed fluorescence spectra of o-, m-and p-tolunitrile were measured in supersonic jets. Low-frequency bands due to internal rotation of the methyl group were observed in m- and p-tolunitrile. Observed band positions and relative intensities of the internal rotational bands were reproduced by a calculation using a free rotor basis set. From the analysis, the potential curve of the internal rotation was determined in both S₁ and S₀. It was found that the barrier height increases in going from S₀ to S₁ in m-tolunitrile, while it decreases in p-tolunitrile. In contrast, no low-frequency band was found in o-tolunitrile. It is concluded that the potential curve in o-tolunitrile does not change in going from S₀ to S₁. The change of the barrier height by electronic excitation in tolunitriles differs greatly from that observed in other toluene derivatives. It is suggested that the electronic properties of a substituent are important for the methyl rotation in the excited state.     

An accurate simplified data treatment for the initial adsorption kinetics in conditions of laminar convection in a slit: application to protein adsorption

We present the derivation of a simple approximation for the original expression of the adsorption rate [Langmuir 10 (1994) 3898] in conditions of laminar flow in a slit, to relate the measured initial kinetic constant k with the interfacial kinetic constant k_a and the transport-limited Lévêque constant k_Lev. The same method of derivation is applied here to get a simple approximation of the average kinetic constant 〈k〉 [Biomaterials 20 (1999) 1621]. For the local value, at distance x from the entrance of the slit, we propose k(x)/k_a=(u−1)(au−1)/(bu+1), where u=k(x)/k_Lev, a=0.452, b=−0.625, with a maximal error of 1% in comparison with the exact solution. For the average value over the length of the slit, we propose 〈k〉/k_a=(U−1)(AU−1)/(BU+1), where U=〈k〉/〈k_Lev〉, A=0.203, B=−0.273, with a maximal error of 0.03%. These approximations lead to an easy determination of the adsorption constant and diffusion coefficient D of the solute, as appropriate plots of experimental data provide k_a and D^2/3 as the intercepts of the curve with the ordinate and abscissa axes, respectively. It is pointed out that the linear approximation k⁻¹=k_a⁻¹+k_Lev⁻¹ would lead to the overestimation of both the diffusion coefficient and adsorption kinetic constant. As an example, the application to the analysis of experimental data for adsorption of α-chymotrypsin onto mica plates is provided.     

Theoretical and experimental analysis of the fine structure and hyperfine structure in the configurations 5p6s and 5p6p of the antimony II spectrum

From 17 transitions in the singly ionized Sb II spectrum the hyperfine structure (A andB splitting constants) of the complete excited configurations 5p6s and 5p6p were determined by means of optical interference spectroscopy. In addition, a theoretical analysis both of the fine structure and also of the hyperfine structure was carried out (in the case of 5p6p of the general typenpn′p for the first time in literature). For the 3 levels 5p6p ³ P ₁, 5p7p ³ D ₂ and 5p6p ¹ P ₁ a different classification was found and consistent values for the fine structure parameters, mixing coefficients and single electron hyperfine structure splitting parametersa _nl ^ik andb _nl ^ik were obtained. The three new determinations in Sb II of the quadrupole moment (in barn) of¹²¹Sb (Q(5p6s)=?0.55(5);Q(5p6p)=?0.57(5) from the 5p-electron andQ(5p6p)=?0.7(2) from the 6p-electron) are well agreeing with each other but differ to former values from SbI. The core polarization and isotope shift of the lines, however, are compatible with our former results in SbI.     

Distortion of the coordination polyhedron in mononuclear copper(II) complexes. Optical absorption and electron spin resonance studies on some five-coordinate copper(II) complexes

The preparation and spectral properties are reported for a series of ten mixed-ligand copper(II) complexes of the form [Cu(A)(B)_n](X), where A is acetylacetonate anion, B represents a mono- or bidentate ligand such as morpholine, piperidine, isonicotinic acid hydrazide, 2,2′-bipyridine, 1,10-phenanthroline and X is variously NO^?₃ or ClO^?₄ anion and n = 1 or 2. The coordination of the anions and ligands has been demonstrated by infrared and electronic spectral methods. Electron spin resonance spectral data show the square-pyramidal five-coordinated arrangement around copper(II) in these complexes. Parameters such as g₆, g_⊥, A₆, A_⊥, 〈g〉 and 〈A〉 calculated from electron spin resonance data in solid and solution state at room temperature as well as frozen solution indicate the presence of the unpaired electron in the d_x2_?y2 or d_z2 orbital. These results are in good agreement with electronic and photoacoustic spectral studies.