bluuebunny/crossref_metadata_2025_split huggingface dataset:
An ab initio and force field study on the conformation and chain flexibility of the dichlorophosphazene trimer
Authors: Jaeger Raimund , Vancso G. Julius 
Published: April 2003

AbstractAb initio molecular orbital calculations have been used to study the conformation, valence electron charge density, and chain flexibility of a dichlorophosphazene trimer (CH3[NP(Cl2)]3CH3). The calculations were carried out at the restricted Hartree‐Fock level with the 6–31 G* basis set. The dichlorophosphazene trimer adopts a planar transcis conformation. The valence electron charge distribution indicates strong charge separations along the backbone of the molecule, and is in agreement with Dewar's island delocalization model for bonding in linear and cyclic phosphazenes. In order to determine the height of the torsional barrier (2,5 kcal/mol), the torsional potential of a central PN bond of the trimer was studied with a rigid rotor scan and geometry optimizations of selected rotamers. The flexibility of the PNP bond angle contributes significantly to the chain flexibility. Based on the results of the ab initio calculations, an empirical force field for the dichlorophosphazene trimer was developed. The energy expression includes bond stretch, angle bend, electrostatic, van der Waals, and torsional potential terms. A relaxed scan with the force field achieves good agreement with the ab initio results for the torsional potential in the vicinity of the stable conformation, and an excellent agreement with the ab initio results on changes in the P2N2P3 bond angle and the N1P2  N2P3 dihedral angle during a full rotation around the N2  P3 bond.
Perturbation expansion of macromolecules with realistic pair interactions in three and four dimensions
Authors: Carl Wilfried 
Published: April 2003

AbstractWe calculated the first‐order perturbation expansion of the mean‐square end‐to‐end distance of polymers in three and four space dimensions. The segments of the chain were assumed to interact via a pair potential with a short‐range repulsive and an additional attractive part. For a purely repulsive potential, the well‐known results of the δ‐function‐pseudopotential approach are recovered, whereas the inclusion of an attractive part leads to a non‐vanishing contribution even for a vanishing binary cluster integral.
Aggregation processes in self‐associating polymer systems: A comparative analysis of theoretical and computer simulation data for micelles in the superstrong segregation regime
Authors: Khalatur Pavel G. , Khokhlov Alexei R. , Nyrkova Irina A. , Semenov Alexander N. 
Published: April 2003

AbstractWe present an extension of our previous theory describing aggregation processes in self‐associating polymer systems, i.e., in copolymers with strongly attracting groups. In particular, the formation and properties of micelles are studied in detail for the superstrong segregation regime. We have explicitly taken into account the effect of the radius of attraction, rc, and the structural defects of the micellar core on the micellar shape and stability. It has been shown that for a given aggregation number, Q, the shape of the micellar core is determined by a balance between the interfacial free energy and the energy of structural defects. As the effective energy of attraction increases, the most stable shape of the core can change from spherical to disk‐like (oblate ellipsoid). A transition between spherical and disk‐like core geometries takes place at nearly equal interfacial and defect free energies. In the case of long‐range attraction (i.e., for large rc), we have shown that the surface term must dominate, thus stabilizing a spherical shape of the core. Alternatively, for small, rc, the non‐spherical (disk‐like) geometry of the micellar core becomes most stable. The critical radius of transition, r, theoretically predicted, is in a reasonable agreement with computer simulations presented in our previous paper. If rc is further decreased and the effective energy of attraction is fixed, then the large aggregate can disintegrate and form smaller disk‐like splinters. We have located this equilibrium transition point. The critical value r thus obtained is in very good agreement with molecular dynamics calculations performed by us. At last, the theory predicts the formation of strongly elongated (stripe‐like) micelles in concentrated solutions. Such structures were also observed in our computer experiments.
Quantum chemical studies of propene, ethylene, acetylene and dihydrogen reactivity in the insertion reaction into the titanium‐alkyl bond
Authors: Zakharov Ivan I. , Zakharov Vladimir A. , Zhidomirov Georgii M. 
Published: April 2003

AbstractUsing the ab initio method of SCF MO LCAOSCF MO LCAO: Self‐consistent field molecular orbital linear combination of atomic orbitals.
in a valency‐splitted basis of the Gaussian functions we have studied the addition of various monomers (C3H8, C2H4, C2H2) and dihydrogen to the titanium‐alkyl bond in the complex H2TiCH3. The structure of transition states in the insertion reaction, heats of π‐complex formation and activation energies for the insertion of the coordinated monomers have been calculated. The calculation results show that the reactivity decreases in the order C2H2 > C2H4 > C3H8 > H2. According to the results obtained, the energy of the π*‐antibonding orbital of monomers can serve as an index of relative reactivity in the insertion reaction into the metal‐alkyl bond.
The incorporation of side reactions into pseudostationary polymerization, 1. The closed solution of a kinetic scheme comprising chain‐transfer to monomer or solvent
Authors: Zifferer Gerhard , Olaj Oskar Friedrich 
Published: April 2003

AbstractA procedure is developed that allows the calculation of chain length distributions of polymers prepared by periodic modulation of the initiation process, considering termination by disproportionation and chain transfer. For the case of a (pseudostationary) laserpulse initiated polymerization process a closed solution can be derived for the chain length distribution (cld) of dead polymer and its Oth, 1st and 2nd moments. By analysis of the detectability of the “extra‐peaks” appearing in the cld (which represent the key for the determination of propagation constants) as a function of the parameters chosen the optimum conditions can be predicted for experiments.
The incorporation of side reactions into pseudostationary polymerization, 2. The limiting case of a small amount of chain transfer
Authors: Zifferer Gerhard , Olaj Oskar Friedrich 
Published: April 2003

AbstractApproximate closed expressions for the chain length distribution and the moments of dead polymer prepared by periodic modulation of the initiation process are derived for the limit of a small amount of chain transfer (considering termination by disproportionation). The expressions obtained are given in terms of chain length distribution and moments of a reference system without chain transfer. The range of validity of these approximations is examined for the special case of initiation by laser‐pulses. In addition, the first six moments of the chain‐length distribution of laser‐pulse initiated polymers (in the absence of chain transfer) are given.
Copolymers with controlled distribution of comonomers along the chain, 1. Structure, thermodynamics and dynamic properties of gradient copolymers. Computer simulation
Authors: Pakula Tadeusz , Matyjaszewski Krzysztof 
Published: April 2003

AbstractProperties of copolymer systems with various composition distributions of comonomers A and B along the chain are analyzed by means of computer simulation using the Cooperative Motion Algorithm. Extreme cases of random and block copolymers, as well as the intermediate cases of gradient copolymers with various composition gradients along the chain are considered. Thermodynamic, static and dynamic properties of systems are predicted in a broad temperature range including both the homogeneous and the microphase separation regime. A possibility to control the microphase separation temperature and consequently a possibility to control a shift of the properties along the temperature scale by changing the monomer distribution within copolymer chains is demonstrated.
Random fluorinated copolymers of tetrafluoroethylene: a study of the inclusion/exclusion of defects from the crystal by conformational and packing energy calculations
Authors: Napolitano Roberto , Pucciariello Rachele 
Published: April 2003

AbstractA conformational analysis has been performed on the isolated chains of copolymers of tetrafluoroethylene with hexafluoropropene, chlorotrifluoroethylene or perfluoromethyl vinyl ether, in comparison with polytetrafluoroethylene. The lowest energy conformations in accordance with the chain repeating distance of polytetrafluoroethylene in the low temperature crystal phase have been used in packing energy calculations. The results of both conformational and packing energy calculations suggest that the Cl group is easily tolerated in the crystal phase. The —CF3 group could also be tolerated in the crystal phase but at the cost of conformational and crystal lattice deformations. On the contrary, it can be concluded that the —OCF3 group is excluded from the crystal phase because its presence should require high energy values and wide deformations in the unit cell parameters.
On the evaluation of the anisotropic‐isotropic transition volume fraction for a semirigid polymer
Authors: Figari Giuseppe , Costa Camilla 
Published: April 2003

AbstractIt has been found that a method allowing the calculation of an anisotropic‐isotropic transition volume fraction for a given semirigid polymer can be derived from the equations recently proposed by Blonski and coworkers to evaluate the equilibrium compositions of isotropic and anisotropic phases coexisting within ternary systems semirigid polymer/flexible polymer/solvent. By using the term anisotropic‐isotropic transition volume fraction of a semirigid polymer we will refer to its smallest volume fraction giving an anisotropic character to liquid ternary phases also including a flexible polymer and a solvent. Its evaluation simplifies the construction of the corresponding ternary diagram and can also be seen as a fast and quantitative way of estimating the influence exerted by the semirigid polymer, through its intrinsic anisotropy, on ternary phases.
Local chain dynamics of a model polycarbonate near glass transition temperature: A molecular dynamics simulation
Authors: Fan Cun Feng , Çagin Tahir , Shi Wen , Smith Kennith A. 
Published: April 2003

AbstractConstant pressure constant temperature molecular dynamics method is employed to investigate the atomistic scale dynamics of a model Bisphenol A polycarbonate in the vicinity of its glass transition temperature. First, the glass transition temperature and the thermal expansion coefficients of the polymer are predicted by performing simulations at different temperatures. To explore the significance of different modes of motion, various types of time correlation functions are utilized in analyzing the trajectories. In these nanosecond scale simulations, the motion of the chain segments is found to be highly localized with little reorientation of the vectors representing these segments. Detailed analysis of trajectories and the correlation functions of the backbone dihedrals and side methyl groups indicates that they exhibit numerous conformational transitions. The activation energies of the conformational transitions obtained from the simulation are generally larger than the potential barriers for the rotations of these dihedrals, however, both show the same trend. We also have estimated the phenylene ring flip activation energy as 12.6 kcal/mol and the flip frequency as 0.77 MHz at 300 K. These values fall either fall within the range determined by various NMR spectroscopy experiments or slightly out of the range. The study shows that the conformational transitions between the adjacent dihedrals are strongly correlated. Three basic cooperative modes are identified from the simulation. They are: a positive synchronous rotation of two phenylene rings, a negative synchronous rotation of two phenylene rings, and a carbonate group rotation. Above the glass transition temperature, the large scale cooperative motions become much more significant.
Flow effects in the polymer cyclization reaction
Authors: Kolb Alexander , Marques Carlos M. , Fredrickson Glenn H. 
Published: April 2003

AbstractWe investigate the effect of shear and uniaxial extensional flows on the rate of the cyclization reaction in polymers. We find that for such an intramolecular diffusion‐controlled reaction the flow always reduces the reaction rate, in contrast to the previously studied case of intermolecular reactions, where linear flows enhance the reaction rate.
Macromolecular reaction and interdiffusion in a compatible polymer blend
Authors: Yashin Victor , Kudryavtsev Yaroslav , Govorun Elena , Litmanovich Arkady 
Published: April 2003

AbstractA theory describing slow macromolecular reaction and interdiffuion in a compatible polymer blend is suggested based on the linear non‐equilibrium thermodynamic principles. A simple model system is considered. In a blend consisting initially of homopolymers A and B, the transformation A → B proceeds with the B units accelerating the reaction. A system of diffusive‐reaction equations for relevant macroscopic variables is derived. The randomness of the reacting chains' structure gives rise to a new interdiffusion mode in addition to the reacting polymer‐homopolymer B interdiffusion. Numerical calculations reveal that the diffusive intermixing of reacting chains of different composition may significantly affect the reaction rate and the local compositional heterogeneity as well. It is possible to discriminate the fast‐ and the slow‐mode theories of interdiffusion using reaction kinetics data. Under certain conditions, the reaction may proceed in a non‐trivial autowave‐like regime.
Molecular modeling of adsorption and ordering at solid interfaces
Authors: Hentschke Reinhard 
Published: April 2003

AbstractInterfaces between liquids and solid surfaces are of considerable scientific as well as technological interest, in particular in the context of the adsorption and organization of molecular films. In recent years the direct observation of the molecular structure and often even the dynamics of ordered monolayers at such hidden interfaces has been made possible by the rapid development in scanning probe microscopy. Nevertheless, there is still a lack of understanding with respect to the formation and organization of such films and their interaction with the experimental apparatus. Here computer modeling plays an increasing role as both the complexity of the interfaces and the available computer power increase. This article addresses the application of phenomenological molecular modeling to physisorption at solid surfaces with a special emphasis on the liquid‐solid interface. The paper presents an overview over different modeling approaches and illustrates their application in a series of examples ranging from the simulation of adsorption isotherms of small molecules to the prediction of the structure of physisorbed layers for larger molecules.
Molecular force field study concerning the host properties of carbosilane dendrimers
Authors: Lach Christian , Brizzolara Davide , Frey Holger 
Published: April 2003

AbstractThe effect of structural variations at the core of carbosilane dendrimers has been studied using Molecular Dynamics. Several derivatives of the parent dendrimer have been modeled and characterized with respect to the dimensions of the inner cavities which lie on the order of 5–15 Å. The “denseness” of the outer shell has been studied by calculating solvent accessible surfaces and excluded volumes as a function of the radius of the probe sphere. The higher generation dendrimers were found to possess dense outer shells with holes of the order of 2–3 Å. Dendron separation especially with respect to clefts in the molecule is discussed. It was found that in low generation carbosilane dendrimers the dendrons are clearly separated while at higher generations a substantial amount of interpenetration occurs. In addition, results are presented showing that carbosilane dendrimer surfaces are fractal only over a narrow range of length scales. In this range, the fractal dimensions of the surfaces are of the order of 2.1.
Calculation of vapor pressures not requiring the derivatives of the energy of mixing
Authors: Horst Raland 
Published: April 2003

AbstractA method is presented for the calculation of vapor pressures exclusively on the basis of the energy of mixing, the knowledge of chemical potentials is not required. The only condition used for the calculation is the minimum of the energy of mixing of the overall system in equilibrium. The gas phase is treated as an ideal gas, for the liquid phase no specific thermodynamic description is assumed. The method is demonstrated for a mixture of two solvents and one polymer. The system water/poly(ethylene oxide), the thermodynamics of which are described by an equation that can only be solved numerically thus impeding the calculation of chemical potentials, serves as an example. Interaction parameters are determined by fitting calculated vapor pressures to literature data.
Polymer‐polymer interaction parameters by inverse gas chromatography: A novel molecular interpretation of nonrandom partitioning of solvent probes in polymer blends
Authors: Tan Zhanjie , Vancso G. Julius 
Published: April 2003

AbstractNon‐random partitioning of molecular probes in a polymeric mixture was examined with Kirkwood‐Buff‐Zimm (KBZ) cluster integrals. Equations were derived to give relationships of thermodynamic quantities that can be obtained by using inverse gas chromatography (IGC). The mer‐mer correlation structure factor S*(AB) (0) in a binary mixture of polymers A and B in terms of the spatial distribution of a probe, S, around a molecule A and a molecule B is discussed. The derivation allows a direct assessment of the correlation structure factor obtained either from scattering experiments or from IGC measurements. The common polymer‐polymer interaction parameter from the two types of experiments is discussed with respect to criticism concerning IGC. The detailed molecular description of the probe behavior in a polymeric mixture at infinite dilution of the probe given in this work can be used in IGC studies of the microstructure of polymer mixtures or other mixtures of liquids. In order to illustrate our approach, the structure factor and the interaction parameter of the blends of polystyrene (A) and poly(butyl methacrylate) (B) were evaluated using IGC results of DiPaola‐Baranyi and Degre.
Probing chain ends in adsorbed polymer layers
Authors: Johner Albert , Joanny Jean‐François 
Published: April 2003

AbstractWe study theoretically the pull‐out of polymer chains from an adsorbed polymer layer by sticking of the chain ends on an opposing surface using scaling arguments and a mean field theory. When only one chain is pulled out from the layer, we extend previous results obtained for a single adsorbed chain and calculate the force necessary to extract the chain from the layer. We then discuss end adsorption from an adsorbed layer of polymers bearing specific end groups onto a second surface. Two bridging regimes are predicted: a diffuse layer regime at weak separations (or/and weak interaction) and a large separation strong interaction regime where the bridges stretch into a brush like structure. Bridging fractions and force profiles are displayed that could be compared to atomic force microscope or surface force apparatus experiments.
On the determination and ramifications of chaos in many‐body systems: A model study of polyethylene
Authors: Newman David E. , Watts Christopher , Sumpter Bobby G. , Noid Donald W. 
Published: April 2003

AbstractComputational methods for elucidating information related to the dynamical behavior of multidimensional systems are presented and applied in the area of macromolecular dynamics. Results indicate that the dimensionality of chaotic dynamics for a model of polyethylene can only be characterized for a very narrow range of extremely low temperature (0–2 K). These results provide evidence that suggests the dynamics of this system are completely chaotic at temperatures as low as 10 K. The preponderance of increasing density of low frequency floppy vibrational modes in large molecular systems provides a facile mechanism for the onset of global chaos. Ramifications of such global chaos to the accurate modeling and simulation of macromolecular dynamics is discussed.
Conformational transitions of end‐adsorbed triblock copolymers in a nonselective solvent
Authors: Haliloglu Turkan , Mattice Wayne L. 
Published: April 2003

AbstractConformational dynamics of triblock copolymers end‐adsorbed onto a surface from a nonselective solvent have been studied by a Monte Carlo lattice simulation technique. The triblock copolymers are A B A with NA = 10 and NB = 5, 10, 20 and 40, at surface interaction parameters ε = −0.5 and −1.0. A is the adsorbing block and B is the nonadsorbing block. The number of chains in the periodic box is varied over the range 100 ≤ n ≤ 500. The triblock copolymer can exist in three states: loop (L), tail (T) and free (F) chain. Fractions of those conformations at different system parameters (n, NB and ε) and the associated lifetimes are calculated from the simulations. A kinetic scheme is constructed and the corresponding transition rate matrix is used in the master formalism equation, the solution of which yields the dynamic modes of the system. A correlation function is defined to produce a single overall view of the rate of transitions between different conformational states. The degree of surface interaction has strong influence on the rate of transitions between the states, in particular, longer average lifetimes and a wider distribution of lifetimes of the loop conformation contribute to the slower decay of the correlation curves. At high surface adsorption, larger NB decrease the rate of all transitions at all values of n. A weak dependence on n is observed for all sizes of NB at both surface energies.
Study of the conformational disorder in polyesters giving liquid‐crystalline phases
Authors: Pirozzi Beniamino , Napolitano Roberto , Tuzi Angela , Centore Roberto 
Published: April 2003

AbstractConformational and packing energy analyses have been performed by molecular mechanics calculations on the polyester derived from the condensation of 2‐(4‐hydroxyphenyl)‐6‐hydroxybenzoxazole with terephthalic acid, which presents a high degree of crystallinity in spite of the head‐to‐tail constitutional disorder. The line repetition groups in accordance with the experimental value of the c axis have been taken into account. Possible conformations of chains having constitutional and conformational disorder in the crystalline phase have been proposed. Packing energy calculations performed on pairs of chains give the close packing distances and indicate the best symmetry elements. The results are in agreement with the unit cell obtained by X‐ray data, the thermodynamical properties of the polymer and the existence of liquid‐crystal phases.