Study of the dielectric β-relaxation in poly(ethylene terephthalate) and ethylene isophthalate terephthalate copolyesters

The dielectric β-relaxation in poly(ethylene terephthalate) and ethylene isophthalate terephthalate copolyesters with various ethylene isophthalate contents was investigated over a wide range of frequencies. Isothermal spectra and normalized plots show that the dielectric β-relaxation is characterized by the molecular structure between the aromatic rings. The equilibrium polarizability of the motional segment, which was estimated from the dielectric increment obtained by the best fit of the Havriliak-Negami equation to the Cole-Cole plot, indicates that torsional vibrations of the main chain are correlated over a length corresponding to one monomer.     

Torsional interaction in 2-haloethanols: infrared data,ab initio results and treatment of a two-dimensional model

The splittings of the hydroxyl torsional absorption bands observed in the gas phase IR spectra of 2-haloethanols, XCH₂-CH₂-OH are reported for X = Cl, Br, I. The satellite absorptions, assigned to excited states of the skeletal torsion, appear on the low-frequency side of the fundamental, possibly because of weakening of the internal hydrogen bond. The torsional potential surface is investigated by calculations using a two-dimensional model based on the known experimental data for the compound with X = Cl, and by ab initio calculations for the haloethanols with X = F and X = Cl. Models based on the assumption of pure torsion and simple dipole-dipole or central force interaction terms failed to reproduce the observed data for 2-chloroethanol, but good agreement was obtained after local modification of the potential surface interpolated from the ab initio calculations. The results may help devise the phenomenological theory of potential energy needed for quantitative study of the internal hydrogen bond.     

The effect of torsional vibrations superimposed on a creep loaded adhesive joint on its final tensile properties

Aluminum - epoxy single lap joints were subjected at different temperatures to torsional vibrations at constant amplitude superimposed on a creep load. This combination of dynamic and static stresses was chosen in order to simulate to a certain extent the real service conditions of an ordinary bonded joint. The shear strength of these joints was checked in tension at room temperature after their removal from the special device in which the superimposed stresses were applied. It was found that the shear strength of the joint is very dependent on its thermo-mechanical history. DSC analyses and SEM micrographs of the failure surface were used in an attempt to find some correlation between the mechanical properties and the microstructure of the adhesive.     

1H, 2H and 13C NMR spectra and molecular dynamics of some solid alkylene-aromatic polyesters

Molecular dynamics of some mesomorphic main–chain alkylene–aromatic polyesters have been investigated by means of NMR spectra of various nuclei over a wide temperature range. In solid polymers regions of different molecular mobilities coexist and their fractions are determined by the sample temperature and thermal history. The sample annealing leads to the growth of rigid fraction. It was found that below the glass transition temperature the only forms of large–scale mobility are the torsional vibrations and flips of para–phenylene groups, while spacer groups are virtually rigid. Above the glass temperature almost all phenylene rings undergo flipping motions and methylene groups of the spacer take part in complicated motions of both anisotropic and isotropic character.     

Calculation of the temperature dependence of the negative thermal expansion of polyethylene chains by means of the self-consistent harmonic approximation

A parameter-free model based on molecular mechanics with zero Kelvin potentials is developed to describe quantitatively the shortening of the unit cell of polyethylene crystals in chain direction with increasing temperature. Based on the well-known conception that an increase of the amplitudes of the torsional vibrations will cause the thermal shrinkage, we introduce a self-consistent phonon approach to calculate the temperature dependence of the angular displacement correlation function taking into account the anharmonicity of the potential. By means of these functions, we calculate the mean value of the length of a representative chain segment and compare the results with those obtained from x-ray diffraction measurements. © 1996 John Wiley & Sons, Inc.     

Structure of acetyl chloride molecule isotopomers CH<Subscript>3</Subscript>COCl and CD<Subscript>3</Subscript>COCl in the ground and lowest excited singlet and triplet electronic states: a quantum mechanical study

The structures of isotopomers of conformationally flexible acetyl chloride molecule, CH₃COCl and CD₃COCl, in the ground (S₀ and lowest excited singlet (S₁) and triplet (T₁) electronic states were calculated by the RHF, MP2, and CASSCF methods. The equilibrium geometric parameters and harmonic vibrational frequencies of the molecules in these electronic states were estimated. According to calculations, electronic excitation causes considerable conformational changes involving rotation of the CH₃ (CD₃) top and a substantial deviation of the CCOCl fragment from planarity. The results of calculations agree with experimental data. Two dimensional torsional inversion sections of the potential energy surface were calculated and analyzed. Vibrational problems for large amplitude vibrations (torsional vibration in the S₀ state and both torsional and inversion vibrations in the T₁ and S₁ states) were solved in one- and two-dimensional approximations.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 62–70, January, 2005.     

Matrix IR spectra and normal coordinate analysis for methanesulfenyl chloride and isotopic analogs

The IR spectra (4000-250 cm⁻¹) of CH₃SCl and CD₃SCl in solid argon have been obtained. Fundamental vibrations, except the torsional vibrations, have been assigned. Normal coordinate analysis has been carried out omitting the torsional modes.     

A class of exactly solvable matrix models

Some exactly solvable matrix models are discussed. Possible applications to problems in physical chemistry are pointed out, in particular the Hückel problem, the problem of torsional vibrations of polyatomic molecules, and of vibrations of finite polymer chains. This revised version was published online in July 2006 with corrections to the Cover Date.     

Vibronic coupling in organic semiconductors: the case of fused polycyclic benzene-thiophene structures

The nature of vibronic coupling in fused polycyclic benzene-thiophene structures has been studied using an approach that combines high-resolution gas-phase photoelectron spectroscopy measurements with first-principles quantum-mechanical calculations. The results indicate that in general the electron-vibrational coupling is stronger than the hole-vibrational coupling. In acenedithiophenes, the main contributions to the hole-vibrational coupling arise from medium- and high-frequency vibrations. In thienobisbenzothiophenes, however, the interaction of holes with low-frequency vibrations becomes significant and is larger than the corresponding electron-vibrational interaction. This finding is in striking contrast with the characteristic pattern in oligoacenes and acenedithiophenes in which the low-frequency vibrations contribute substantially only to the electron-vibrational coupling. The impact of isomerism has been studied as well.     

Selection rules for Raman scattering from eigenmodes of spherical nanoparticles

The elastic vibrations of spherical nanoparticles have been widely studied by Raman scattering. However, there exist more than one set of selection rules for Raman scattering from these vibrations. For instance, Kanehisa has stated that only torsional modes with angular momentum l = 2 are Raman-active, while Tanaka et al. proposed that spheroidal modes of even l and torsional modes of odd l are Raman-active. These contradict selection rules of Duval which states that only spheroidal modes of l = 0 or 2 are Raman-active. Our present calculations based on a macroscopic model show that all torsional modes have vanishing intensity in the Raman spectrum.     

Impact of neighboring chains on torsional defects in oligothiophenes

Conjugated organic oligomers are central to the development of efficient organic electronic devices and organic photovoltaics. However, the torsional flexibility of many of these organic materials, in particular oligothiophenes, can adversely affect charge transfer properties. Although previous studies have examined the torsional flexibility of oligothiophenes, there have been only limited studies of the effects of interchain interactions on their torsional potentials. B97-D/TZV(2d,2p) was first benchmarked against a CCSD(T)/aug-cc-pVTZ torsional potential for bithiophene as well as SCS-MP2/TZVPP interaction energies for noncovalent sexithiophene (6T) dimers. The effect of neighboring chains on three distinct torsional modes of sexithiophene was studied using B97-D. Complexation with one or more neighboring chains has a dramatic effect on each of these torsional potentials. For example, for two stacked chains, alternated twisting motions are competitive with torsion about a single terminal dihedral angle, and in both cases we predict nonplanar global energy minima and large amplitude torsional motions at room temperature. In other words, the presence of a single neighboring chain induces significant deviations from planarity in oligothiophenes. However, in the environment of crystalline 6T, the trend in predicted torsional potentials match those of isolated chains, but the force constants associated with torsional motions increase by an order of magnitude. Consequently, although individual oligothiophene chains are torsionally flexible and model stacked dimers exhibit extreme deviations from planarity, in crystalline 6T these oligomers are predicted to adopt planar configurations with a steep energetic cost associated with torsional defects.     

Influence of defects on the infrared spectra of polymers

For an infinitely repeated regular polymer chain structure the only vibrations which are optically active are those in which the phase of the motion is the same in each unit (the factor-group modes). Frequencies for which the phase difference is nonzero are optically inactive but can become activated by the presence of defects in the chain. Such defects would normally be chemical impurities or conformational irregularities in the chain. A simple theory is developed which shows that for a dilute system of defects the major characteristics determining possible activation of the nonfactor-group modes are: (1) the strength of the coupling between the defect vibration and the vibrations of the neighboring chain, and (2) whether or not the natural frequency of the defect vibration lies inside a lattice band of the regular chain. An analysis of the low- and high-frequency regions of the spectrum of low-density polyethylene, based on the above considerations, indicates that several features of the spectrum can be associated with defect-induced absorption. A similar explanation can account for certain intensity changes in the C? Cl stretching region of syndiotactic poly(vinyl chloroide) when this polymer is submitted to mechanical treatment.     

Mössbauer spectroscopic study of the temperature dependence of the intramolecular mobility of a cross-linked polymeric cation-exchanger based on acrylic acid

Conclusions The temperature transition of a solvated cross-linked polymer (the SGK-7 carboxyl cation-exchanger, which contains iron(III) ions) from the solid state to a state characterized by intense intramolecular motions was detected and studied by Mössbauer spectroscopy. Triggering of two types of motions was demonstrated: high-frequency vibrations of atoms with a higher amplitude (manifested by a sharp decrease in the probability of the Mössbauer effect) and slower (conformational) motions (manifested by a change in the shape of the spectrum). The parameters of the motions were determined within the framework of the lattice model and the effect of the solvating liquid on the nature of the transition was demonstrated.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1740–1745, August, 1985.     

One- and Two-Dimensional Torsion-Inversion Models for the Fluoral Molecule (CF3CHO) in Excited Electronic States

The structure of the conformationally nonrigid fluoral molecule (CF₃CHO) in the ground (S₀) and lowest excited triplet (T₁) and singlet (S₁) electronic states was studied by ab initio quantum-chemical methods. The equilibrium geometric parameters and harmonic vibrational frequencies of the molecule in these electronic states were determined. The calculations demonstrated that the electronic excitation causes substantial changes in the molecular structure involving the rotation of the CF₃ top and the deviation of the CCHO carbonyl fragment from planarity. The quantum-mechanical problems for large-amplitude vibrations, namely, for the torsional vibration in the S₀ state and the torsional and inversion vibrations (nonplanar carbonyl fragment) in the T₁ and S₁ states, were solved in the one- and two-dimensional approximations. A comparison of the results of calculations revealed the correlation between the torsional and inversion motions.     

Quantum-chemical study of the structure of the acetyl fluoride molecule in the ground and lowest excited singlet and triplet electronic states

The structure of the conformationally flexible acetyl fluoride molecule (CH3CFO and CD3CFO) in the ground (S0) and lowest excited triplet (T1) and singlet (S1) electronic states was calculated by different quantum-chemical methods (RHF, UHF, MP2, CASSCF). The equilibrium geometric parameters and harmonic vibrational frequencies of the molecules in these electronic states were estimated. The calculations demonstrated that the electronic excitation causes considerable conformational changes involving the rotation of the CH3(CD3) top and a substantial deviation of the CCFO carbonyl fragment from planarity. For large-amplitude vibrations, namely, for the torsional vibration in the S0 state and the torsional and inversion (nonplanar carbonyl fragment) vibrations in the T1 and S1 states, the quantum-mechanical problems were solved in one-dimensional (1D) and two-dimensional (2D) approximations. The results of calculations are in good agreement with experimental data.     

Molecular dynamics calculation to clarify the relationship between structure and mechanical properties of polymer crystals: the case of orthorhombic polyethylene

The molecular dynamics (MD) technique was used to calculate the temperature dependence of the structure, molecular motion, and mechanical property of the orthorhombic polyethylene (PE) crystal. The potential functional parameters reported by Karasawa et al. (J Phys Chem, 95 (1991) 2260) were refined further so that the vibrational frequencies of infrared and Raman bands, measured by us at ultra-low temperatures for the normal and fully deuterated PE, could be reproduced well. The flip-flop motion around the chain axis and the torsional motion of the skeletal chains were found to start above ca. 350 K and increase the amplitude of these motions progressively. Coupling these two types of chain motion resulted in a steep increase of the thermal vibration parameters or the mean-square-displacements of carbon and hydrogen atoms, corresponding well with the X-ray data. The lattice constants and the related linear thermal expansion coefficients were also found to be in good agreement with the observed data. The calculated Young's modulus along the chain axis decreased gradually with the increasing temperature: 330 GPa at 0 K to 280 GPa at room temperature. The latter was in good agreement with the value of 280–305 GPa evaluated from the Raman measurement of the longitudinal acoustic mode. Young's modulus was found to relate intimately with the chain contraction caused by the skeletal torsional motion. Only 0.3% contraction of the chain resulted in the reduction of the modulus by ca. 35%. A similar behavior was also seen in the trigonal polyoxymethylene and nylon 6 α forms.     

Spectroscopic study of α- and β-phase isotactic polypropylene and of atactic polypropylene in solution

Raman spectroscopy is used to investigate the conformation and packing of isotactic crystalline α-phase polypropylene compared with lower-order β-phase isotactic polypropylene and to study the solution behavior of atactic polypropylene. The high-frequency region of the spectrum is analyzed in light of a normal-mode calculation that takes into account the methyl-group vibrations. This region is sensitive to both chain conformation and packing, and because of the high intensity of the methyl and methylene high-frequency stretching modes, it can be used to probe small changes in intermolecular or intramolecular order. Differences in the thermal behavior between the two solid isotactic polypropylene samples are explained interms of packing defects which exist in the β-phase form. In the solution study, we demonstrate that, for molecules in which bands sensitive to intermolecular interactions exist, as is the case of the methyl and methylene vibrations of polypropylene, spectroscopic techniques can be used to estimate the minimum overlap concentration.     

Spin labeling methods in molecular biology : G.I. Likhtenshtein,John Wiley & Sons,New York,London, Sydney and Toronto, 1976, pp. xiii + 258, price £26.00

As a consequence of intramolecular vibrations distorted apparent structures may result from an electron diffraction analysis of molecules possessing symmetrical equilibrium configuration. The amount of torsional distortion gives information concerning the barrier height to internal rotation. An approach is suggested to estimate barrier heights on the basis of average torsional angles as determined from electron diffraction, and expressions of the rotation-dependent distances as obtained from a Taylor expansion by neglecting higher order terms.     

Hydrodynamic and Colloidal Interactions in Concentrated Charge-Stabilized Polymer Dispersions

Hydrodynamic and colloidal interactions are explored in concentrated, charge-stabilized colloidal dispersions by measuring the dependence of rheology (e.g., low and high-shear viscosity, high-frequency viscosity, and modulus) and self-diffusivity on salt content, particle size, and concentration. Model, sulfonated polystyrene lactices of varying diameter are prepared and investigated by shear rheology, high-frequency torsional resonance, electrophoresis, titration, and dynamic light scattering. The high-frequency and high-shear viscosity both are dominated by hydrodynamic interactions, but are shown not to be identical, due to the microstructure distortion resulting from high shear rates. The short-time self-diffusion is also shown to be insensitive to direct particle interactions, but has a different concentration dependence than the high-frequency viscosity, further illustrating a predicted violation of a generalized Stokes-Einstein relationship for these properties. The apparent colloidal surface charge is extracted from the high-frequency elastic modulus measurements on concentrated dispersions. The surface charge is in good agreement with results from critical coagulation concentration measurements and perturbation theories, but disagrees with electrophoretic mobility experiments. This indicates that the effective surface charge determined by torsional high-frequency measurements is a more reliable predicter of the salt stability of charge-stabilized dispersions, in comparison to zeta-potentials determined from electrophoretic mobilities. Further, we demonstrate by direct comparison that measurements of the apparent plateau modulus by rotational rheometry underestimate the true, high-frequency modulus and provide unreliable estimates for the surface charge. Copyright 2000 Academic Press.     

Molecular structure of acetyldeyiethylphosphine,MeC(O)PMe2: II. CNDO/2 calculations

The sp, spd and spd' approximations of the CNDO/2 method have been applied to energy calculations for some models of the acetyldimethylphosphine molecule generated during treatment of the electron diffraction data. The results obtained for trial configurations with different angles of rotation of the acetyl group about the PC _ac bond predict two symmetric energy minima separated by a rather low barrier. This is in agreement with the electron diffraction data which are compatible with the suggestion of large amplitude torsional vibrations of the acetyl group. Energy calculations have also been performed for the final electron diffraction models. The calculations, however, fail to remove ambiguity from the question of a preferred model of the acetyldimethylphosphine molecule.