bluuebunny/crossref_metadata_2025_split huggingface dataset:
Shear of a planar polyelectrolyte brush
Authors: Birshtein Tatyana M. , Zhulina Ekatarina B. 
Published: April 2003

AbstractThe behavior of a planar polyelectrolyte brush subjected to normal and tangential external forces is considered. A new “polyelectrolyte effect” is predicted: shear of a free polyelectrolyte brush leads to a decrease in brush thickness contrary to the case of a free neutral brush. Such behavior is equivalent to that of a neutral brush subjected to an external normal stretching force. In the case of a polyelectrolyte brush this force is created by the osmotic pressure of mobile counterions neutralizing grafted chain charges. Addition of salt diminishes the polyelectrolyte effect and changes the sign of correlation between brush thickness and tangential deformation.
Liquid‐crystalline ordering in two‐dimensional systems with discrete symmetry
Authors: Mercurieva Anna A. , Birshtein Tatyana M. 
Published: April 2003

AbstractThe mean‐field theories of liquid‐crystalline (nematic) ordering developed for three‐dimensional systems are applied to describe two‐dimensional systems of both geometrically anisotropic and anisotropically interacting particles. Systems with discrete symmetry (lattice models) for which long‐range order is possible are considered on the base of the Landau free‐energy expansion. It is shown that the Hamiltonian describing the energy of intermolecular interactions may be written in a common form for lyotropic and thermotropic systems. The mean‐field theory gives a continuous phase transition (second‐order phase transition) for a square lattice, whereas for a triangular lattice it gives a phase transition with latent heat (first‐order phase transition) like for three‐dimensional systems. These results are compared with results of the exact theories (two‐dimensional Ising and Potts models). It is concluded that for realistic two‐dimensional models the orientational in‐plane ordering is not sharper than a second‐order phase transition.
Multiblock copolymers as polymeric surfactants: are “pancakes” better than “dumbbells”?
Authors: Noolandi Jaan 
Published: April 2003

AbstractIt is suggested that multiblock copolymers which form a pancake at the interface between two incompatible homopolymers would be more efficient as compatibilizers than di‐ or triblock copolymers. Less of the multiblock copolymers would be lost into the bulk phases as micelles or mesophases than di‐ or triblock copolymers if some degree of irregularity is included in the multiblock architecture, either in block length or chemical sequence distribution, without compromising the thermodynamic driving forces which localize the copolymers at the interface. This molecular design strategy is aimed at reducing the ability of the surfactant to form a separate phase, thereby increasing the interfacial activity in a nonequilibrium system with a large interfacial area. The multiblock loops which would preferentially orient into the most compatible homopolymer phases would have to exceed a certain critical size in order to avoid the entropic penalty of having an extremely flat pancake, and to provide good adhesion between the bulk phases.
Conformational analysis and electronic structure calculations of S‐1,2‐bis(2‐dibutenoyl)glycerol and S‐1,2‐bis(2,4‐hexadienoyl)glycerol, two analogues of 1,2‐bis(2,4‐octadecadienoyl)‐sn‐glycero‐3‐phosphorylcholine
Authors: Faupel Peter , Buß Volker 
Published: April 2003

AbstractAs a model for 1,2‐bis(2,4‐octadecadienoyl)‐sn‐glycero‐3‐phosphorylcholine, a doubly unsaturated membrane‐forming lipid molecule, force‐field (MMP2) calculations were performed on S‐1,2‐bis(2‐dibutenoyl)glycerol, and CNDO/S‐calculations2CNDO: Complete neglect of differential overlap, a semi‐empirical quantum‐mechanical method.
on the derived minimum‐energy conformations of S‐1,2‐bis(2,4‐hexadienoyl)glycerol. The energy hypersurface especially with respect to the dihedral angles along the C(1)‐C(2) and the two CO ester bonds was explored and the rotational strength as a function of these angles was calculated. The two gauche‐forms were found to be most stable, with a slight preference for the g−‐form. The experimental circular dichroism data obtained for 1,2‐bis(2,4‐hexadienoyl)‐sn‐glycero‐3‐phosphorylcholine, the corresponding phosphorylcholine, indicate a dynamic equilibrium between two forms of opposite chirality possibly involving the g+‐and the g−‐forms.
Intramolecular phase separation of a copolymer chain with mobile primary structure
Authors: Dormidontova Elena E. , Grosberg Alexander Yu. , Khokhlov Alexei R. 
Published: April 2003

AbstractThe phenomenon of intramolecular phase separation of a single copolymer chain with mobile primary structure (i.e., a polymer chain with reversibly adsorbable ligands) in dilute solution is investigated on the basis of a Flory‐type interpolation theory of the coil‐globule phase transition. It is shown that under certain critical conditions the stability of the homogeneous primary structure is violated and intramolecular phase separation takes place. Phase diagrams for the cases of rigid and flexible chains are calculated. The main characteristics of a polymer chain in the two‐phase state (such as fraction of monomeric units, swelling coefficient and fraction of units with ligands for coexisting intramolecular phases) are investigated.
Molecular arrangements and conformations of liquid unbranched alkanes in narrow slits
Authors: Vacatello Michele , Auriemma Finizia 
Published: April 2003

AbstractRealistic, atomistic models of liquid tridecane in broad slits (>3 nm) and in narrow slits of thickness 1,2 nm and 1,0 nm have been obtained using the Monte Carlo technique. The setup of the models is such that the molecules in the slits are in equilibrium with the bulk liquid. The surfaces of the plates are modelled as two‐dimensional arrays of hexagonally packed units having the same size and interaction parameters of a methylene group. The regions adjacent to the plates in slits with thickness > 3 nm are characterized by a well defined tendency to form partially ordered layer structures, while molecules at a distance from the plates larger than 1,5 nm are unperturbed. The simultaneous presence of two plates increases the tendency to form layer structures when their distance is 1,2 nm, while this tendency is almost totally destroyed when the slit is squeezed down to a thickness of 1,0 nm. This is also associated with a 10% decrease of the density in the latter slit.
Diffraction modelling of poly(vinyl chloride): An approach to the crystallinity enigma
Authors: Hobson Rachel J. , Windle Alan H. 
Published: April 2003

AbstractThis paper demonstrates that changes which are seen to occur in experimental X‐ray diffraction patterns of poly(vinyl chloride) (PVC) with varying sample tacticity are indicative of a crystal structure in which both isotactic and syndiotactic sequences are incorporated within the same lattice. The isotactic sequences are in a conformation first suggested by Juijn et al. which enables them to emulate the shape of the syndiotactic chain. We present a short summary of molecular and diffraction modelling results which show how the intensities of the 200 and 110 peaks change with crystallite tacticity. The results provide a rational explanation for the long standing enigma that the observed crystallinity of atactic PVC is too high to be accounted for in terms of crystallization of syndiotactic sequences only.
Thermodynamics of swelling of latex particles with two monomers: A sensitivity analysis
Authors: Maxwell Ian A. , Noel Lilian F. J. , Schoonbrood Harold A. S. , German Anton L. 
Published: April 2003

AbstractA sensitivity analysis is performed to determine at what conditions the simplified model for swelling of latex particles by two monomers or two solvents is valid. This model proposes that, inter alia, the fractions of two monomers in the latex particles and in the monomer droplets are equal. The model is a simplified version of that proposed by Ugelstad et al. and results from the use of three assumptions, namely (1) the molar volumes of the monomers are equal, (2) the interaction parameters between the two monomers are equal and (3) the interaction parameters between each of the monomers and the polymer are equal. It is shown by simulation that these assumptions are of algebraic necessity only. The simple model is always valid when utilizing realistic values for all the appropriate parameters for partially water soluble monomers that swell the polymer.
Free‐radical multicomponent polymerization reactors and chemical composition distribution
Authors: Engelmann Uwe , Schmidt‐Naake Gudrun 
Published: April 2003

AbstractFormulae to calculate the statistically caused instantaneous copolymer composition distribution as well as the chemical distribution of accumulated macromolecules, which is due to polymerization statistics and shifts in mean polymer composition during the reaction process, are derived on the basis of a universal model for free‐radical solution polymerization with any number of monomers proceeding in a batch, semi‐batch or continuous, ideally mixed vessel. The influence of the reactor type on chemical composition distributions is investigated for a copolymerization of different reactive components (methyl methacrylate/styrene/maleic anhydride), a system with nearly equal reactive monomers (methyl methacrylate/styrene), and the ternary polymerization of methyl methacrylate/styrene/maleic anhydride. Though products of constant mean composition are obtainable in a semi‐batch or steady‐state continuous reactors, considerable statistical dispersion cannot be removed in any case.
Short‐chain behaviour of star‐branched nonreversal random walks embedded in a tetrahedral lattice
Authors: Zifferer Gerhard 
Published: April 2003

AbstractAnalytical results are presented for the mean‐square dimensions of star‐branched nonreversal random walk polymers on a tetrahedral lattice considering short‐chain effects and the influence of the core (centre) of the star. They are checked against numerical results obtained from highly precise Monte Carlo calculations. Short‐chain effects are studied in detail. In addition, the deviation from proportionality between mean‐square dimensions and the total chain‐length is evaluated and compared to that for self‐avoiding chains and stars near theta‐conditions, the total chain‐lengths ranging from 125 to 3845 segments.
Static and dynamic molecular mechanics modelling and X‐ray scattering calculations for a cyclic siloxane macromolecule
Authors: Pachter Ruth , Bunning Timothy J. , Crane Robert L. , Adams W. Wade , Socci Edward P. , Farmer Barry L. 
Published: April 2003

AbstractA liquid crystalline system consisting of cyclic penta(methylsiloxane) cores with biphenylyl 4‐propoxybenzoate mesogens is studied with respect to its molecular conformation and the intermolecular ordering of pendant groups by employing molecular mechaincs calculations, while a molecular dynamics simulation indicates the degree of conformational flexibility. The calculated X‐ray scattering patterns for these structures provide insight into the origin of the experimentally observed results. A good agreement is found between the calculated and experimental reflections data, supporting the ‘cylinder’ global topology as the basis for forming a ‘columnar’ structure. Further, a comparison of the calculated and experimental meriodional sections shows a strong similarity of both the meridional intensities and d‐spacings.
Application of conservation law to NMR measurements of poly(propylene) stereoregularity
Authors: van der Burg Marijke W. , Chadwick John C. , Sudmeijer Olof , Tulleken Herbert J. A. F. 
Published: April 2003

AbstractA new method is presented for data reconciliation in the 13C NMR determination of the pentad distribution in regioregular poly(propylene). Deviations of experimentally measured data from mathematically derived conservation laws are corrected by a maximum‐likelihood method. In this method some assumptions are made about the structure of the erros to obtain better estimates of the correct data. The validity of this procedure was demonstrated with Monte Carlo simulation, after which the method has been applied to real 13C NMR data to show what corrections may be obtained. Conservation laws for hexads and heptads are derived as well, for possible future use.
Modelling free‐radical copolymerization kinetics—evaluation of the pseudo‐kinetic rate constant method, 2. Molecular weight calculations for copolymers with long chain branching
Authors: Xie Tuyu Y. , Hamielec Archie E. 
Published: April 2003

AbstractThe full moment equations and equations using pseudo‐kinetic rate constants for binary copolymerization with chain transfer to polymer in the context of the terminal model have been developed and solved numerically for a batch reactor operating over a wide range of conditions. Calculated number‐ and weight‐average molecular weights (M̄n and M̄w) were compared with those found using the pseudo‐kinetic rate constant method (PKRCM). The results show that the weight‐average molecular weights calculated using PKRCM are in agreement with those found using the method of full moments for binary copolymerization when polymeric radical fractions φ1˙ and φ2˙ of type 1 and 2 (radical centers are on monomer types 1 and 2 for a binary copolymerization) are calculated accounting for chain transfer to small molecules and polymer reactions in addition to propagation reactions. Errors in calculating M̄w using PKRCM are not always negligible when polymer radical fractions are calculated neglecting chain transfer to small molecules and polymer. In this case, the relative error in M̄w by PKRCM increases with increase in monomer conversion, extent of copolymer compositional drift and chain transfer to polymer rates. The errors in calculating M̄w, however, vanish over the entire monomer conversion range for all polymerization conditions when chain transfer reactions are properly taken into account. It is theoretically proven that the pseudo‐kinetic rate constant for chain transfer to polymer is valid for copolymerizations. One can therefore conclude that the pseudo‐kinetic rate constant method is a valid method for molecular weight modelling for binary and multicomponent polymerizations.
Isomerism and symmetry of bridged polymeric phthalocyanines
Authors: Knothe Gerhard 
Published: April 2003

AbstractBridged polymeric phthalocyanines exist in different isomeric forms of varying symmetry. The number of isomers and the symmetry depends on the structure, i. e. the size and shape, of the polymeric phthalocyanine. Equations are derived for calculating the number of isomers for bridged polymeric phthalocyanines, and symmetry point groups for various shapes (linear, square, rectangular) are discussed.
Modelling of crosslinking and cyclization in free‐radical copolymerization of vinyl/divinyl monomers
Authors: Zhu S. , Hamielec A. E. , Pelton R. H. 
Published: April 2003

AbstractThe Tobita‐Hamielec kinetic model of the crosslink density distribution is further elaborated and extended to incorporate divinyl loop formation. Divinyl loop formation, primary and secondary cyclization, and their effects on the elastically effective crosslink density distribution are extensively examined as a function of cyclization parameters and double bond reactivities. Under conditions of equal double bond rectivity and negligible cyclization, primary chains born at different conversions have the same crosslink density. The variance of the crosslink density distribution is mainly due to unequal reactivities of double bonds and secondary cyclization. Divinyl loop formation and primary cyclization contribute neither to gelation nor to network elasticity and have an insignificant effect on the variance of the crosslink density distribution.
Monte Carlo simulation of tetrahedral chains, 6. Linear and star‐branched polymers near to theta conditions
Authors: Zifferer Gerhard 
Published: April 2003

AbstractBy use of the pivot algorithm, star‐branched chains with F = 4, 8 and 12 arms of length n and linear chains (F = 2) are generated on a tetrahedral lattice (120 ⩽ nF ⩽ 3 840). By taking into account non‐bonded nearest‐neighbour interactions (each contact contributes an energy ϕ · kT to the total energy of the configuration) a variation of the thermodynamic quality of the solvent is simulated by a variation of the energy parameter ϕ in the range −0,425 to −0,525. The energy parameter ϕ⊙ = −0,475, characteristic of theta conditions, was evaluated by use of an intramolecular criterion (proportionality between mean‐square dimensions and total chain‐length) and confirmed by a crossover scaling analysis. Theta dimensions are found to be larger than those of nonreversal random walks, the deviation growing with increasing number of arms.
Molecular dynamics simulation of semirigid polymers
Authors: Fusco Roberto , Longo Luca , Caccianotti Luciano , Aratari Carola , Allegra Giuseppe 
Published: April 2003

AbstractThe chain rigidity of poly(p‐hydroxybenzoate) was estimated through the theoretical evaluation of its persistence length (Lp). A non‐Brownian molecular dynamics (MD) simulation of an isolated chain with 20 monomeric units was performed. The sampled conformational population was analyzed and the orientational correlation function between monomeric units along the chain was calculated. An algorithm based on the worm‐like chain model was applied to evaluate the persistence length. The results were compared with those obtained from equilibrium models like the freely‐rotating‐chain and the rotational‐matrix method with fluctuations. Equilibrium models give different results depending on the degree of accuracy used in describing the monomeric unit. The inclusion of thermal fluctuations is crucial to obtain realistic results. These coincide with those given by MD simulation when only nearest‐neighbour orientational correlations are taken into account: inclusion of higher‐order correlation terms leads to lower values of the persistence length. The origin of this discrepancy was investigated. The MD simulation results are characterized by an overrepresentation of conformations with a short end‐to‐end distance resulting from an anomalous energy concentration in the first bending mode of the chain. In analogy with previous simulation results from systems characterized by a week coupling amoung their degrees of freedom, failure in the energy equipartition is proposed as a likely explanation of the anomalous dynamical behaviour.
Monte‐Carlo simulation of copolymer mass spectra
Authors: Montaudo Maurizio S. 
Published: April 2003

AbstractA computer program, MACO5, was developed which employs the Monte‐Carlo approach to simulate the time‐variation of the mass spectral peak intensities of a copolymer sample during the synthesis of an A/B copolyester via reactive blending of a mixture of homopolyesters A and B or when a copolymer undergoes partial degradation. The program was first used to simulate literature data consisting of a series of mass spectra of a butadiene/styrene copolymer subjected to partial ozonolysis. It was also used to investigate the role of terminal groups in reactive blending processes. Two models were developed: the first one describes inner‐group ester‐ester exchange reactions, the second describes the process in which and active terminal groups attacks an ester group. The predictions of the two models are compared with recent experimental mass spectral data concerning a poly(ethylene adipate‐co‐ethylene terephalate) copolyester.
Modelling free‐radical copolymerization kinetics, 3. Molecular weight calculations for copolymers with crosslinking
Authors: Xie Tuyu , Hamielec Archie E. 
Published: April 2003

AbstractA comprehensive model for molecular weight calculations of free‐radical crosslinking copolymerizations was developed using the pseudo‐kinetic rate constants and the method of moments. The moments of copolymer chain distributions are defined in such a way so that the molecular weight averages of crosslinking copolymers can be calculated using the moments. The present model is based on a general crosslinking copolymerization scheme, accounting for chain transfer to small molecules and polymer, bimolecular termination, and crosslinking reactions. The influence of crosslinking reactions on molecular weight development is discussed. The effects of the reactivity of pendant double bonds on the moments development were further demonstrated using model simulations. The simulations results suggest that the higher‐order molecular weight averages are very sensitive to the reactivity of pendant double bonds. It was found that chain transfer to polymer affects the gelation point significantly. The radical fractions must be calculated accounting for chain transfer reactions in addition to propagations in order to properly evaluate pseudo‐kinetic rate constants. The present model was used to predict kinetic behavior and molecular weight development of styrene/m‐divinylbenzene and styrene/ethylene dimethacrylate free‐radical crosslinking copolymerizations in benzene solution at 60°C. It was found that the present model is in excellent agreement with the experimental data published in the literature. Model predictions and experimental data show that the reactivity of pendant double bonds is much lower than that of vinyl and divinyl monomers. The simulation results suggest that the assumption of the same reactivity of functional groups is likely not valid for many free‐radical crosslinking copolymerizations. The present model based on a kinetics approach can be used to predict molecular weight development for vinyl/divinyl free‐radical crosslinking copolymerizations and to estimate kinetic parameters in the pre‐gelation period.
Dependence of flow behaviour of nematics in shear on the form of the mean‐field potential
Authors: Minale Mario , Apuzzo Giuseppe , Greco Francesco , Marrucci Giuseppe 
Published: April 2003

AbstractThe dynamic transition of nematics in (slow) shear from flow‐aligning to tumbling behaviour is here predicted by using the rigid rod‐like molecular model together with an extension of the Maier‐Saupe mean‐field potential. Such an extension, already available in the literature with reference to equilibrium data, includes interactions of rank higher than the basic quadrupolar one considered by Maier and Saupe. The theoretical predictions are then compared with the few existing sets of dynamic data. Although an acceptable qualitative agreement is found, the theory generally overpredicts the value of the relevant ratio α3/α2 of Leslie coefficients.
Monte Carlo studies of the conformational statistics of polymers. Polyethylene
Authors: Raucci Rosa , Vacatello Michele 
Published: April 2003

AbstractThe conformational distribution of unperturbed polyethylene (PE) has been studied utilizing the Monte Carlo approach. The results are in excellent agreement with experiments on the average dimensions of PE chains, as well as with the molecular scattering function obtained in mixtures of deuterated and hydrogenous PE at 400 K. Maps of the free energy as a function of two consecutive torsional angles confirm that an approximate description of the conformational distribution of PE can be given in terms of three rotational isomeric (RIS) states. The location of the states and the corresponding energy and entropy parameters, which can be separately evaluated from the Monte Carlo results, are compared with literature results obtained by internal energy calculations for butane and pentane.