Heat capacities of both PMMA stereomers: Comparison between atomistic simulation and experimental data

The domains of time and space generally covered by full atomistic simulation (AS) to represent the glass transition temperature, T_g, are very small. Physical interpretations of the phenomena occurring at this transition are inevitably limited. To specifically address such limitation, behavior of the heat capacity that accounts for the freezing of the degrees of freedom as temperature is decreased, is investigated. The selected polymer is poly(methyl methacrylate) since it offers the opportunity to exhibit a different T_g according to the tacticity of its chain. AS and experimental data are thus compared to a theoretical model that takes into account three contributions to the leap in the heat capacity occurring at T_g. The comparison discloses that an excellent agreement is obtained between simulated and experimental contributions from vibrations and free volume. However, from an AS viewpoint changes in the conformation weakly contribute to this leap. Despite this discrepancy local contributions to the glass transition as predicted by atomistic simulation, are sufficient to determine T_g.     

Catalyst Screening through Quantum Chemical Calculations and Microkinetic Modeling: Hydrolysis of Carbon Dioxide

Quantum chemical calculations are emerging as an effective way to screen catalysts for particular applications. In this contribution, we demonstrate the power of density functional theory to study CO₂ hydrolysisby six carbonic anhydrase mimics, evaluating thermodynamic and kinetic parameters at the mechanistic level. A microkinetic model was then built based on the kinetics and thermodynamics calculated from first principles. The intrinsic reaction rate constant was calculated from the results of the microkinetic model and compared with experimental data. Overall, the rate constants were in good agreement with experimental values, except for zinc-tri and complex b, which were overestimated. This was ascribed to their ineffective complexation with Zn²⁺. How the reaction rate constants vary with time was also investigated. From 0 to 12 ms, the rate constants of complexes a and d decreased to 50 and 67% of their initial values, respectively; the rate constants of complexes b and f2 were almost invariant with time; the rate constant of complex f1 showed an unusual double sigmoidal shape. The pK_a values of these six carbonic anhydrase mimics as well as three additional mimics were calculated. A correlation between pK_a values and the binding free energy of OH-was obtained by fitting data from five zinc(II) aza-macrocyclic complexes. The reaction rate constants were found to increase linearly with the pK_a value, indicating CO₂ adsorption is the rate-limiting step.     

Structural,magnetic, and optical properties of nano-sized Ni0.85Se

Nanocrystalline Ni_0.85Se was synthesized by a hydrothermal method. X-ray diffraction analysis by Rietveld method indicated that Ni_0.85Se has a NiAs-type hexagonal structure. Narrow crystallite size distribution with an average area-weighted size of 〈ɛ_F〉 = 8.5 nm is obtained by Warren-Averbach method. Structural analysis revealed deformed Se atoms octahedron with the shortest distance between Se atoms in adjacent planes 〈Se–Se〉_adj smaller than between nearest neighbors in layer plane 〈Se–Se〉_nea, and a rather short interatomic distance between the transition metal atoms 〈Ni–Ni〉. The optical property of Ni_0.85Se was studied by UV-spectroscopy. Magnetic measurement shows a ferromagnetic phase transition for Ni_0.85Se below 14 K.     

Measurement of polar orientation in poled side-chain NLO polymers using infrared spectroscopy

Infrared spectroscopy has been used to provide an independent estimate of dipole orientation in poled films of a side-chain polymer based on poly(methyl methacrylate) with an oxynitrostilbene side group attached via an aliphatic spacer. In particular, we used the dichroism of the absorption band assigned to the NO₂ symmetric stretching vibration as a measure of the orientation of the nitrostilbene group. This yields 〈P₂(cos θ)〉, where θ is the angle between the transition dipole moment and the symmetry axis of the sample. We then used a Langevin-type model to calculate 〈cos θ〉 from 〈P₂(cos θ)〉 and concluded that values of 〈cos θ〉 as high as 0.5 are being achieved. We find, however, that 〈P₂(cos θ)〉 obtained by comparison of normal incidence measurements on unpoled and poled films does not agree with 〈P₂(cos θ)〉 obtained from tilted-film measurements on poled films. We propose that this discrepancy arises because the local field in a poled sample affects the polarizability of the molecules. To confirm this, we applied high fields to unpoled films in the infrared spectrometer at room temperature. The results clearly show a large reduction in absorbance with the applied field when the infrared electric field vector has a component perpendicular to the plane of the film. A major error therefore occurs as a consequence of the internal field in poled samples if 〈P₂(cos θ)〉 is derived from the tilted film measurements, whereas 〈P₂(cos θ)〉 derived from normal incidence measurements is essentially correct. © 1994 John Wiley & Sons, Inc.     

Stereo-stability and temperature induced transitions of collagen mimics

Collagenous matrix proteins present in almost every part of the vertebrate organism and in many non-vertebrates are expressed in a rich structural variety. The present communication attributes the theoretical interpretation of temperature induced transitions in collagen mimics in terms of stereochemistry, as explained by the modified Zimm and Bragg model. The results are found to be in good agreement with the experimental data as reported independently by Steven K. Holmgren, Jonthan A. Hodges and Cara L. Jenkins. The order of the values of nucleation parameter and enthalpy changes obtained theoretically, is attributed not only to the stereo-chemistry but also to the relative increase in the degree of stability of collagen mimics.     

The 31 Helix-Coil Transition for Polypeptides.: Circular Dichroism Studies

Abstract

Circular dichroism has become a popular method for following conformational transitions induced in optically active polymers. Recent refinement of experimental spectra obtained from model polypeptides by computer fitting to spectra obtained from solutions of proteins of known conformation has verified the applicability of the models chosen for the α helical and β conformations. However, the spectrum required for disordered regions was in conflict with much of the literature and agrees with our assignment based on studies of collagen and collagen models at elevated temperature. This spectrum consists of two troughs, one at ~225 nm and the other at ~200 nm. The latter had previously been associated with random polypeptides and we have shown the former to be sensitive to disorder in polymers approaching the 3₁ helical conformation. This paper presents the results from a study of three polypeptides which undergo a 3₁ helix → disorder transition.     

Critical behavior near the paramagnetic to ferromagnetic phase transition temperature in La0.6Sr0.4MnO3 ceramic: A comparison between sol-gel and solid state process

Critical behavior near the paramagnetic (PM) to ferromagnetic (FM) phase transition in La_0.6Sr_0.4MnO₃ ceramics prepared by sol-gel and solid state reaction methods has been systematically investigated. The XRD result coupled with the structural Rietveld refinement method exhibited that all samples crystallized in a rhombohedral structure with a space group of R-3C, which indicated the formation of the perovskite structure of La_0.6Sr_0.4MnO₃. The magnetic data obtained from magnetization measurements indicated a second-order phase transition from PM-FM phase around Curie temperature (T_C). Critical exponents, β, γ, and δ values are estimated by various theoretical models such as the modified Arrott plot (MAP), the Kouvel-Fisher plot (KF) and the critical isotherm (CI) techniques. Grain size dependence of the critical behavior of La_0.6Sr_0.4MnO₃ samples with different sizes from nano-scale to mic-scale was detected by fitting the experimental data to the theoretical models.     

Growth of CVD heteroepitaxial diamond on silicon (001) and its electronic properties

Microwave CVD heteroepitaxial diamond film on a 4° off-axis Si(100) substrate is obtained by two stages. The first one is to grow oriented 3c-SiC layers on Si(100) using a non-toxic and non-inflammable (CH₃)₆Si₂NH organic compound carried by hydrogen. The following stage is to grow oriented diamond films on them under the atmosphere of CH₄ and H₂. In each stage there are bias and growth processions. The micro-Raman and micro-Auger analyses prove that there is a perfect orientation relationship between the film and substrate as following: diamond 〈001〉//3c-SiC〈001〉//Si〈001〉. The Hall effect indicates that the film is a P type, whose resistivity is 9.4×10⁻³ Ω cm, the Hall coefficient is 2.9 cm³/Q, the hole mobility is 309 cm²/V s and the carrier concentration reaches 2.2×10¹⁸ cm⁻³.     

The effect of sparger geometry on gas holdup and regime transition points in a bubble column equipped with perforated plate spargers

This paper investigates the effect of sparger geometry on flow regime of a bubble column. The experiments presented in this study were performed under atmospheric pressure with water/air in a cylindrical Plexiglas^® column of 33.0 cm i.d. and 3.0 m height. Three different perforated plate spargers were employed. Hole diameter was varied in the range of 1–3 mm, while the free area was 1.0%.The theory of linear stability is used for the prediction of regime transitions in the bubble column and a comparison has been presented between the predictions and the experimental observations. A good agreement between the predictions and the experimental values of transition gas holdup has been obtained.In addition, the data from the literature has been analyzed. Experimental values of transition gas holdups and predictions by the theory of linear stability have been compared with those of literature.A correlation based on dimensionless numbers (Archimedes, Froude, Eötvös and Weber) and the group (d_o/D_C) for the prediction of gas holdup in homogeneous regime is proposed. The average error between the correlation predictions and experimental values remains under ±10%.The proposed correlation is compared with the published data and found to be in fairly good agreement.     

Pressure‐dependent Raman modes near the cubic‐tetragonal transition in strontium titanate

The pressure dependence of the Raman frequency shifts of various Raman modes is calculated at room temperature using the volume data from the literature for the cubic‐tetragonal transition in SrTiO₃. The isothermal mode Grüneisen parameters of those Raman modes are obtained, which decrease with increasing pressure for this molecular crystal. Calculated Raman frequencies are then used to predict the damping constant and the inverse relaxation time of those Raman modes as a function of pressure by means of the pseudospin‐phonon (PS) coupled model and the energy fluctuation (EF) model to describe the cubic‐tetragonal transition in SrTiO₃. Also, the values of the activation energy are extracted for the Raman modes studied using both models (PS and EF). Our predicted damping constant and the inverse relaxation time for the Raman modes, can be compared with the experimental measurements close to the cubic‐tetragonal transition in SrTiO₃.     

Photoluminescence response of colloidal quantum dots on VO2 film across metal to insulator transition

We have proposed a method to probe metal to insulator transition in VO₂ measuring photoluminescence response of colloidal quantum dots deposited on the VO₂ film. In addition to linear luminescence intensity decrease with temperature that is well known for quantum dots, temperature ranges with enhanced photoluminescence changes have been found during phase transition in the oxide. Corresponding temperature derived from luminescence dependence on temperature closely correlates with that from resistance measurement during heating. The supporting reflectance data point out that photoluminescence response mimics a reflectance change in VO₂ across metal to insulator transition. Time-resolved photoluminescence study did not reveal any significant change of luminescence lifetime of deposited quantum dots under metal to insulator transition. It is a strong argument in favor of the proposed explanation based on the reflectance data.

PACS

71.30. + h; 73.21.La; 78.47.jd     

Graphitization effects of CH4 addition on NCD growth by first and second Raman spectra and by X-ray diffraction measurements

Nanocrystalline diamond (NCD) films were grown on silicon substrates by hot filament chemical vapour deposition in Ar/H₂/CH₄ gas mixtures. In the current study, the methane volume concentration varied from 0.5 to 3.5 vol.% in order to estimate its effect on nanodiamond morphology and structure. Film micrograph obtained from scanning electron microscopy showed diamond grain agglomerate, also called ballas diamond, which presented the grain size variation as a function of methane concentration increase. The transition from diamond agglomerate to graphite structure was also observed when CH₄ concentration is higher than 2.0 vol.%, confirmed from second order Raman measurements. The film local stress was estimated from the G-peak shift analyses and showed critical values necessary for the graphite/diamond phase formation. Structural investigations carried out by X-ray diffraction measurements for films deposited from 0.5 up to 2.0 vol.% CH₄ presented characteristic diamond diffraction peaks corresponding to 〈111〉, 〈220〉 and 〈311〉. A preferential orientation, changed from 〈110〉 to 〈111〉, was observed during NCD film deposition as a function of the methane concentration.     

Thermal behavior of nylon 6 copolyamides containing aromatic rings

The thermal behavior of terpolymers of caprolactam (CL) and of hexamethylenediammonium isophthalate (IH) and terephthalate (TH) has been studied in a wide range of compositions by differential thermal analysis. It has been found that either amorphous or crystalline polymers having different crystallization rates can be obtained by changing the composition. The dependence of melting points and glass transition temperatures on the composition has been investigated. The dependence of T_g on the composition for the copolymers CL–IH, IH–TH, and CL–TH has been described by means of the Gordon-Taylor equation. It has been found that this equation fits the experimental data for the IH–TH system only if a parameter which takes into account interactions between the monomeric units is introduced. A ternary iso-T_g map has been obtained through statistical analysis. The influence of the chain stiffness and bulkiness of the monomers on the glass transition temperature is discussed.     

Calculation of the Structural Linear Expansion Coefficient from the Chemical Composition of Glasses in the R2O(RO)–Al2O3–B2O3 (R = Li, Na, Ca, and Ba) System

The regularities of the influence of the glass composition on the structural linear expansion coefficient in the glass transition range are analyzed using the published data obtained in systematic experimental investigations of glasses in the R ₂O(RO)–Al₂O₃–B₂O₃ (R = Li, Na, Ca, and Ba) system. A method is proposed for calculating the structural linear expansion coefficients of these glasses from the chemical composition with a mean relative error of 10%. The possibilities of extending the composition range covered by the calculation are considered.     

A comparative study between epoxy/Titania micro‐ and nanoparticulate composites thermal and mechanical behavior by means of particle–matrix interphase considerations

The aim of the present study is to examine and compare the thermal and mechanical properties of epoxy resin/TiO₂ particle microcomposites (0.2 μm) and nanocomposites (21 nm). Composite materials consisting of epoxy resin reinforced with different amounts of TiO₂ microparticles (1, 5, 10, 15, and 20% wt) and TiO₂ nanoparticles (0.5%, 1%, 3%wt) were prepared. The thermal and mechanical properties of the manufactured composites were investigated and compared through differential scanning calorimetry (DSC) and three‐point bending tests (3PB). Lipatov's Theory was then applied on the DSC results, thus leading to the calculation of the particle‐matrix interphase thickness which was correlated to experimental findings. The glass transition temperature (T_g) of the materials was obtained and the effect of the grain size on the measured T_g values was investigated. The data obtained from DSC tests for both micro‐ and nanoinclusions when normalized with respect to the specific surface area of the particles, resulted in a single continuous curve describing the normalized phase transition enthalpy variation with filler weight fraction. POLYM. ENG. SCI., 58:1146–1154, 2018. © 2017 Society of Plastics Engineers     

Dielectric dispersion near the morphotropic phase boundary of 0.64PMN-0.36PT ceramics

A near morphotropic phase boundary (MPB) composition of PMN-PT ceramic, 0.64Pb(Mg_1/3Nb_2/3)O₃-0.36PbTiO₃, has been synthesized for various piezoelectric, ferroelectric and dielectric applications. The relaxation mechanisms and dielectric characteristics of this solid solution have been investigated with dielectric spectroscopy measurements in the frequency range of 20 Hz–2 MHz. The dielectric properties have shown distinct and prominent Debye type relaxation at the temperatures corresponding to the ferroelectric phase. The peak in dielectric loss parameter has been found to be dominated by the dc conductivity in paraelectric phase. The experimentally obtained values of various parameters have been found in agreement with the values obtained by fitting of the experimental data in Debye model. An excellent agreement of the results with Debye distribution of relaxation times was obtained with the distribution parameter α showing minima around the ferro-to para-electric phase transition. Different activation energies of the relaxation time have been observed in the ferroelectric and paraelectric phases. An average relaxation time has been found to decrease from ～10^?3 s to ～10^?6 s with the increasing temperatures.     

Shock Compression of Boron Carbide: A Quantum Mechanical Analysis

The shock Hugoniot of boron carbide, from 0 to 80 GPa, has been obtained using first principles quantum mechanics (density functional theory) and molecular dynamics simulation. The Hugoniot for six different structures which vary by structure or stoichiometry were computed and compared to experimental data. The effect of stoichiometry, and structural variation within a given stoichiometry, are shown to have marked effects on the shock properties with some compositions displaying bilinear behavior in the computed shock velocity‐particle velocity profiles while others show a continuous Hugoniot curve with no evidence of a phase transition over the pressure range considered in this work. Two structures, B₁₂(CBC) and B₁₁C_p(CCB), have predicted phase transition pressures lying within the 40–50 GPa range suggested experimentally. It is shown that the phase transition is driven by deformation of the 3‐atom chain within the boron carbide crystal structure which induces a discontinuous volume change at the critical shock pressure. The effect of defects, in the form of chain vacancies, on the shock response is presented and the ability of shear to significantly lower the phase transition pressure, in accord with experimental observation, is demonstrated.     

Determination of the Landau–Placzek Ratio from Small-Angle X-ray Scattering Data

The possibility of determining the Landau–Placzek ratio R _LP by the small-angle X-ray scattering (SAXS) technique is discussed. The intensities of small-angle X-ray scattering by thermal density fluctuations, concentration fluctuations, and a microinhomogeneous structure, as well as the phonon and structural components of scattering by thermal density fluctuations, can be separated from the angular and temperature dependences of the SAXS intensity. This makes it possible to determine the Landau–Placzek ratio R _LP as the ratio between the structural and phonon components of the intensity of scattering by thermal density fluctuations. Within the limits of experimental error, the calculated temperature dependence of the Landau–Placzek ratio R _LP for vitreous boron oxide coincides with the curve obtained from the light scattering data. It is found that the temperature coefficient of the SAXS intensity (phonon component) for boron oxide changes at the temperature of the transition from a supercooled liquid state to a solid noncrystalline state. This suggests a similarity of this transition to second-order phase transitions.     

Molecular weight and chain conformation of amylopectin from rice starch

By using laser light scattering (LS) and size exclusion chromatography combined LS, we have investigated the molecular weight and chain conformation of amylopectin from rice of India (II‐b), japonica (IJ‐b), and glutinous (IG‐b) in dimethyl sulfoxide (DMSO) solution. The weight‐average molecular weight (M_w) and radius of gyration (〈S²〉^½) of amylopectin were determined to be 4.06 × 10⁷ and 128.5 nm for India rice, 7.41 × 10⁷ and 169.6 nm for japonica rice, 2.72 × 10⁸ and 252.3 nm for glutinous rice, respectively. The 〈S²〉^½ values were much lower than that of normal polymers, indicating a small molecular volume of amylopectin, as a result of highly branched structure. Ignoring the difference of degree of branching, approximated dependences of 〈S²〉^½ and intrinsic viscosity ([η]) on M_w for amylopectin in DMSO at 25°C were estimated to be 〈S²〉^½ = 0.30M_w^0.35 (nm) and [η] = 0.331M_w^0.41 (mL g^?1) in the M_w range studied. Moreover, from the 〈S²〉^½ values of numberless fractions obtained from many experimental points in the SEC chromatogram detected with LS, the dependence of 〈S²〉^½ on M_w for the II‐b sample was estimated also to be 〈S²〉^½ = 0.34 M_w^0.347, coinciding with the above results. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Termination rate coefficients for acrylamide in the aqueous phase at low conversion

The kinetics of acrylamide (AAm) free radical polymerization at low conversion of monomer to polymer in the aqueous phase was investigated at 50 °C using γ-radiolysis relaxation, which is sensitive to radical-loss processes. The values of the termination rate coefficients for AAm ranged from 8×10⁶ to 3×10⁷ M⁻¹ s⁻¹ as the weight fraction of polymer ranged from 0.002 to 0.0035, which is significantly lower than the low-conversion values for monomers such as styrene (2×10⁸ M⁻¹ s⁻¹) and methyl methacrylate (4×10⁷ M⁻¹ s⁻¹) in organic media. These can be quantitatively explained by applying a chain-length-dependent model of free-radical polymerization kinetics [Russell GT, Gilbert RG, Napper DH. Macromolecules 1992;25:2459. [19]] in which termination kinetics are expressed in terms of a diffusion-controlled encounter of radicals which ultimately yields an expression for the chain-length-averaged termination rate coefficient, 〈k_t〉. The lower 〈k_t〉 for AAm arises due to a combination of the high k_p value, promoting rapid formation of slower terminating long chains, and the slow diffusion of short propagating chains, relative to other common monomers. The chain transfer to monomer constant for AAm in water at 50 °C, C_M, was estimated using the chain-length-distribution method with correction for band-broadening [Castro JV, van Berkel KY, Russell GT, Gilbert RG. Aust J Chem 2005;58:178. [21]] and found to be 1.2×10⁻⁴ (±10%). The diffusion characteristics for AAm were adapted from those obtained for a similar aqueous system (hydroxyethyl methacrylate) together with a 0.5 exponent for the power law dependence on penetrant degree of polymerization at zero weight fraction polymer. This provides an adequate fit to the 〈k_t〉 data. This is the first application of the chain-length-dependent model to describe experimental termination rate coefficients for an aqueous system at low conversion to polymer. The result that the experimental termination rate coefficients can be reproduced with an a priori model with physically reasonable parameters supports the physical assumptions underlying that model.