The temperature dependence of phosphorescence lines in glassy matrices

The temperature dependence from 1.8 to 4.2 K of The linewidth of phosphorescence site-selection spectra has been measured with a Fabry-Pérot interferometer. For coronene and 5-bromoacenaphthene in 1-bromobutane, the linewidth follows a T^1.25±0.2 power law. The lowest linewidth measured was 0.0012 cm^?1 for coronene at 1.8 K. Magnetic fields up to 0.32 T did not influence this width.     

Molecular simulation of the reversible mechanical unfolding of proteins

In this work we have combined a Wang-Landau sampling scheme [F. Wang and D. Landau, Phys. Rev. Lett. 86, 2050 (2001)] with an expanded ensemble formalism to yield a simple and powerful method for computing potentials of mean force. The new method is implemented to investigate the mechanical deformation of proteins. Comparisons are made with analytical results for simple model systems such as harmonic springs and Rouse chains. The method is then illustrated on a model 15-residue alanine molecule in an implicit solvent. Results for mechanical unfolding of this oligopeptide are compared to those of steered molecular dynamics calculations.     

Molecular modeling of ceftriaxone activation in the active sites of penicillin-binding proteins 2

Russian Chemical Bulletin - The enzyme—substrate complexes of penicillin-binding proteins PBP2 from FA19, 35/02, and H041 strains of Nisseria gonorrhoeae with ceftriaxone were simulated by...     

Superposition of dynamic mechanical properties in the glassy state

Superposition of the loss tangent curves could be achieved for the β-transition of a series of homologous epoxy resins. It was found that both a vertical and horizontal shift were necessary to achieve superposition when the curves were plotted as the logarithm of the loss tangent versus reciprocal absolute temperature. Resins from the diglycidyl ether of bisphenol A (DGEBA) were prepared with five different curing agents and their loss tangent curves measured on a free-oscillation torsion pendulum (ca. 1 cps). The β transition is caused by DGEBA, which was found via molecular models to contain a mobile group. The intensity of the loss for three of the resins was found to be proportional to the concentration of DGEBA, molecular models revealing that no additional mobile groups were introduced by these curatives. The remaining two curing agents introduced mobile groups into their systems and for these two, no separate transitions were identified but the intensity of the DGEBA β transition was increased. This may be caused by a coupling of the DGEBA mobile groups through the flexibility of the curative-introduced mobile groups.     

Molecular modeling on garuganin-I

Summary The conformation of garuganin-I was analyzed in terms of the mobility of the cyclic structure. Molecular mechanics calculations were applied to show that the interconversion of the optical isomers is not possible at room temperature due to steric interactions. Different possibilities of the distortion of the molecular conformations were calculated and compared with molecular dynamics simulations.

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Molekulares Modelling von Garuganin-I

Zusammenfassung Die Konformation von Garuganin-I wurde bezüglich der Mobilität der cyclischen Struktur analysiert. Es wurden molekularmechanische Berechnungen herangezogen, die zeigten, daß die Interkonversion der optischen Isomeren bei Raumtemperatur wegen sterischer Wechselwirkungen nicht möglich ist. Es wurden verschiedene Möglichkeiten von Konformationsänderungen berechnet und mit einer Simulation der Molekulardynamik verglichen.

     

Emission studies of 9-cyanoanthracene excimers generated in glassy and crystalline matrices

Fluorescence spectra of 9-cyanoanthracene polycrystalline films and glasses of different viscosities and polarities are reported. The 490 nm emitting species is identified as the trans excimer. The blue-shift of 9-CNA crystal emission upon increasing temperature is explained in terms of a temperature-dependent competition between two crystal traps giving rise to cis or trans excimer position.     

Molecular modeling study on surface,thermal, mechanical and gas diffusion properties of chitosan

The motivation of this work is to provide reliable and accurate modeling studies of the physical (surface, thermal, mechanical and gas diffusion) properties of chitosan (CS) polymer. Our computational efforts have been devoted to make a comparison of the structural bulk properties of CS with similar type of polymers such as chitin and cellulose through cohesive energy density, solubility parameter, hydrogen bonding, and free volume distribution calculations. Atomistic modeling on CS polymer using molecular mechanics (MM) and molecular dynamics (MD) simulations has been carried out in three dimensionally periodic and effective two dimensionally periodic condensed phases. From the equilibrated structures, surface energies were computed. The equilibrium structure of the films shows an interior region of mass density close to the value in the bulk state. Various components of energetic interactions have been examined in detail to acquire a better insight into the interactions between bulk structure and the film surface. MD simulation (NPT ensemble) has also been used to obtain polymer specific volume as a function of temperature. It is demonstrated that these V–T curves can be used to locate the volumetric glass transition temperature (T_g) reliably. The mechanical properties of CS have been obtained using the strain deformation method. Diffusion coefficients of O₂, N₂, and CO₂ gas molecules at 300 K in CS have been estimated. The calculated properties of CS are comparable with the experimental values reported in the literature. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1260–1270, 2007     

Processes of luminescence quenching in study of molecular oxygen diffusion in glassy matrices

The methods have been developed for studying oxygen mobility by donor quenching in solutions with uniform concentration and in geminate pairs in a solid phase. It is shown that the kinetics of luminescence variations with time is described by a diffusion model. Oxygen mobility has been studied at low temperatures (77 K) in hard polymer matrices. squalane. glasses of low-molecular hydrocarbons and alcohols. The methods can be used to measure very low oxygen mobility with a diffusion coefficient of about 10(-23) cm2 s(-1).     

Viscoelastic properties and residual stresses in polyhedral oligomeric silsesquioxane-reinforced epoxy matrices

Fiber-filled thermosetting polymer composites are extensively used in aerospace industries. One disadvantage of these materials is cure induced or thermally induced residual stresses in the matrix, which may result in deteriorated performance and premature failure. This article explores the use of epoxy/multifunctional polyhedral oligomeric silsesquioxane (POSS) nanocomposites as resins with reduced thermal stress coefficients that result in mitigated residual stresses. The effect of POSS loading on the thermal stress coefficient of the epoxy/POSS nanocomposite resins was investigated from below the β-relaxation to the α-relaxation, or glass transition temperature, (i.e., from −100 to 180 °C) by measuring the shear modulus and linear thermal expansion coefficient. The thermal stress coefficient of the epoxy/POSS nanocomposites is found to be a strong function of temperature, decreasing rapidly with decreasing temperature through the α-relaxation region, increasing in the vicinity of the β-relaxation, and then decreasing below the temperature associated with the peak in the β-relaxation. With increasing POSS content, the thermal stress coefficient is reduced compared with the neat resin in the vicinity of the α-relaxation; however, the thermal stress coefficient increases with increasing POSS content below the temperature of the β-relaxation peak. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2719–2732, 2008     

Molecular dimensions and chain conformation in glassy syndiotactic polystyrene

For the first time the small-angle neutron scattering (SANS) from mixtures of protonated and totally deuterated syndiotactic polystyrene (sPS) has been studied. Two amorphous samples with similar molecular weights have been measured at various concentrations of the protonated part. All measurements were performed at room temperature using the scattering equipment of two different laboratories. The molecular weight M_w evaluated from SANS data agreed with those obtained by gel permeation analysis (GPC). In the Kratky representation the scattering contribution due to the contrast scattering shows a plateau behavior up to q = 0.45 Å^?1, where q is magnitude of the scattering vector. This observation is in evident contrast to what is expected from the rotational isomeric state (RIS) model. In addition the characteristic ratios C_∞, derived either from the plateau height or from radii of gyration of the Zimm regime and being in reasonable agreement with each other, show strong deviations from the predictions of the RIS model. © 1994 John Wiley & Sons, Inc.     

The effect of densification on the mechanical properties of amorphous glassy polymers

Four polymers, viz. polystyrene, polycarbonate, poly(methyl methacrylate), and poly-(vinyl chloride), were cooled from the melt to room temperature under hydrostatic pressures of 30 and 1000 atm. Cooling under high pressure increased the density by 0.4–0.6%, and the effect of this has been examined for the torsional creep properties, the dynamic properties at 1 Hz, the Charpy impact strength, the thermal expansivity, and the torsional yield stress. It turned out that, in general, densification affects the thermomechanical properties only slightly.     

Finite difference modeling of the gas transport process in glassy polymers

Finite difference modeling has been used to predict the results of gas transport experiments for a concentration-dependent diffusion coefficient. Experiments on the transport of CO₂ in poly(ethylene terephthalate) and poly(ethylene naphthalate) had previously shown a difference between the effective diffusion coefficients for absorption and desorption runs of a double-sided experiment, but this effect had not been seen for single-sided experiments. The finite difference calculations show that such results are to be expected, and the parameters included in the models that attempt to describe the diffusion process in glassy polymers, such as the dual-mode model, and which lead to concentration-dependent diffusion coefficients, can be found by fitting the experimental data for the double-sided experiment using finite difference modeling. The dependence of the effective diffusion coefficient on pressure for the single-sided experiment can be correctly predicted using results from the double-sided experiment for an identical sample. © 1996 John Wiley & Sons, Inc.     

Static luminescence quenching by oxygen and nitric oxide for phenanthrene in glassy matrices

Steady-state and pulsed methods have been applied to static phosphorescence and fluorescence quenching for phenanthrene caused by oxygen and nitric oxide. The characteristic fluorescence quenching distances have been determined for oxygen (8.7±0.5 Å) and nitric oxide (8.9±0,5 Å); the phosphorescence quenching involves an exchange mechanism, and measurements have been made on the characteristic quenching distances R₀ and exchange-interaction attenuation parameters L. Estimates have been made on the rate constants for dynamic quenching of the triplet excited states of phenanthrene in low-viscosity liquids, which agree well with the measured values for hexane.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 6, pp. 667–672, November–December, 1988.     

Analysis of thermal reactions of photochromic species in glassy matrices based on the concept of dispersive processes

Thermal decoloration reactions of photochromic spiroindolinonaphthoxazine in glassy polymer matrices have been studied. The non-exponential kinetic behaviour in these systems was analyzed using the phenomenological equation with an exp(-t^α) dependency describing dispersive processes in glassy solids. The dispersive parameter α represents the molecular environmental characteristics of matrix polymers.     

The spin-density-functional formalism for quantum mechanical calculations: Test on diatomic molecules with an efficient numerical method

We have applied the spin-density-functional (SDF ) formalism with the local-spin-density (LSD ) approximation to a number of small molecules with the primary aim of testing the approximation for molecular applications. A new numerical method to solve the one-electron wave equation is developed, utilizing the special features of the SDF formalism. We have calculated energy curves, dissociation energies, and equilibrium distances for some diatomic molecules [H (²Σ, ²Σ), H₂(¹Σ, ³Σ), He (¹Σ), and He₂(¹Σ)] and the vibrational frequencies of H₂. The deviations from the experimental results are typically 1/2 eV for the energies and ≤ 0.1 Å for the distances. We discuss the LSD approximation using the concept of an exchange-correlation hole and make predictions about the applicability to other molecules. The LSD approximation is compared with the Hartree-Fock and multiple-scattering-Xα methods and some difficulties in the latter methods are pointed out. It is argued that the SDF formalism within the LSD approximation has physical advantages compared to the Hartree-Fock and Xα methods and that it should provide a simple and useful method for a broad range of applications.     

Computer simulation of decay of trapped electrons in glassy matrices by tunnelling to scavenger molecules

The kinetics of decay of trapped electrons via tunnelling to acceptor molecules in a glassy matrix was computed by the method of stochastic simulation. The kinetic curves of electron decay were obtained for different concentrations of electron acceptor in the matrix. The results of the simulation are compared with the corresponding results calculated on the basis of certain kinetic models of the electron tunnelling in low-temperature glasses.     

On some mechanical effects in glassy polymers attributed to chain entanglements

The important role of the entanglements in the deformation of high-molecular-weight glassy polymers is demonstrated by two phenomena: the build-up of material resistance in polymethylmethacrylate after chain interpenetration and the intrinsic crazing of polycarbonate which is observed when the entanglement network reaches its limits of extension.Dedicated to Prof. Dr. F. H. Müller.     

A new approach toward modeling of mixed-gas sorption in glassy polymers based on metaheuristic algorithms

Modeling mixed-gas sorption has always been associated with computational challenges due to the existence of two or more conflicting objective functions. This study aims to use an artificial intelligence approach toward modeling mixed-gas sorption in PIM-1 and TZ-PIM polymeric membranes. Non-dominated sorting genetic algorithm (NSGA-II) has been applied to identify the extended Henry-Langmuir (EHL) isotherm based on CO₂-CH₄ mixed-gas sorption data. Also, the group method of data handling (GMDH) neural network is implemented to obtain a formula for the calculation of equilibrium partial pressure corresponding to three effective parameters, which are easily measurable. The formula provides an accurate estimation from the equilibrium relationship between the partial pressure of each gas in the binary gas mixtures over the PIM-1 and TZ-PIM membranes. Eventually, the calculated coefficients of EHL isotherm and obtained formula for computing the partial pressure of each component are simultaneously applied into the isotherm model to predict the mixed-gas sorption behavior. The results showed that the computed lines well reproduce the experimental data points, proving that the applied artificial intelligence approach offers a suitable approximation for mixed-gas sorption.     

Molecular modeling of phenothiazine derivatives: self-assembling properties

This study aims to present a detailed theoretical investigation of noncovalent intermolecular interactions between different π-π stacking phenothiazine derivatives and between different alkane chains varying from propane to decane. Second-order M?ller-Plesset perturbation (MP2), coupled cluster (CC), and density functional (DFT) theories were the quantum chemistry methods used in our calculation. For MP2 and CC methods, the density-fitting and local approximations were taken into account, while in the case of DFT, the M06 and M06-2x hybrid meta-GGA exchange-correlation functionals as well as the semiempirical correction to the DFT functional for dispersion (BLYP-D) was considered. The results obtained with the aforementioned methods were compared with the potential energy curve given by the DF-SCSN-LMP2 theory considered as benchmark. For all these calculations, the correlation-consistent basis sets of cc-pVNZ (where N = D, T, Q) were used. In addition, potential energy curves built using the semiempirical PM6-D2 and the MM3 molecular force field methods were also compared with the benchmark curve and their efficiency was discussed. As the next step, several geometry conformations were investigated for both phenothiazine derivatives and alkane chain dimers. It was found that the conformational stability of these molecular systems is exclusively given by the dispersion-type electron correlation effects. The density functional tight-binding (DFTB) method applied for dimer structures was compared with the results obtained by the higher level local perturbation theory method, and based on these conclusions larger phenothiazine derivative oligomers structures were investigated. Finally, the optimal configuration of the complex molecular systems built by phenothiazine derivative, alkane chain fragments, and thiol groups was determined, and their self-assembling properties were discussed.