bluuebunny/crossref_metadata_2025_split huggingface dataset:
π‐conjugated columnar polyacetylenes prepared with Rh complex catalysts
Authors: Masayoshi Tabata , Takeyuki Sone , Yasuteru Mawatari , Daigo Yonemoto , Atsushi Miyasaka , Takashi Fukushima , Yoshikazu Sadahiro 
Published: April 2003

AbstractHighly stereospecific polymerization of monosubstituted acetylenes was carried out using the Rh complex, [Rh(norbornadiene)Cl]2 catalysts. The resulting polyacetylenes were characterized in detail by 1HNMR, ESR, laser Raman, diffuse reflective UV, and wide angle X‐ray diffraction methods. The data showed that the Rh complex were the preferred catalyst to selectively yield the corresponding cis‐transoid polymers even at room temperature when alcohol, triethylamine or water was used as the polymerization solvent. Additionally, the resulting cis polyacetylenes were found to have a helical form whose polymer is amorphous or composed of pseudohexagonal structures called π‐conjugated columnar as self‐assembly or super structure. Further compression of the amorphous cis polymers resulted in cis to trans isomerization at room temperature under vacuum, breaking rotationally the cis C=C bonds giving π‐radicals called solitons as the origin of a polymer magnet. On the other hand, the π‐conjugated columnar was also found to show an extremely longer wavelength absorption compared with that of the amorphous one, although the absorption maximum was shifted to a shorter wavelength when the columnar was destroyed by the compression. Therefore, the formation of the π‐conjugated columnar can be considered as a new and quite useful control method concerning color of such conjugated polymers, i.e., a new concept concerning the color of conjugated polymers.
Low molar mass poly(styrene‐co‐acrylic acid) amphiphilic block copolymers: synthesis and characterization
Authors: Karakatsanis Ekaterini , Focke Walter , Summers Gabriel 
Published: April 2003

AbstractLow molar mass (∼ 4000) di‐ and triblock copolymers of styrene and tert‐butyl acrylate were synthesized by atom transfer radical polymerization (ATRP) in bulk and solution conditions. A CuBr/N, N,N′,N″,N″‐pentamethyldiethylenetriamine (PMDETA) catalyst system in conjunction with an alkyl‐halide initiator were used to control the synthesis of the polystyrene macroinitiator and the subsequent copolymerization with tert‐butyl acrylate. Hydrolysis of the tert‐butyl acrylate blocks to acrylic acid blocks in the presence of trifluoroacetic acid resulted in the formation of an amphiphilic block copolymer. Size exclusion chromatography (SEC) and matrix assisted laser desorption ionization ‐ time of flight ‐ mass spectrometry (MALDI‐TOF‐MS) were used to determine the molar mass and molar mass distribution of the polystyrene macroinitiators and the block copolymers. 1H NMR was used to characterize the polystyrene macroinitiators and the block copolymers, and to confirm hydrolysis of the poly(tert‐butyl acrylate) blocks to poly(acrylic acid).
Molecular characterization of functional polymers and oligomers by liquid chromatography
Authors: Trathnigg Bernd , Rappel Christina , Hoedl Renate 
Published: April 2003

AbstractDifferent chromatographic regimes can be applied to the separation of functional polymers and oligomers: Size Exclusion Chromatography (SEC), Liquid Adsorption Chromatography (LAC), Liquid Chromatography under Critical Conditions (LCCC) and Liquid Exclusion‐Adsorption Chromatography (LEAC). All oligomers of non‐ionic surfactants can be separated to the baseline by combination of these techniques. In some cases, gradient elution is required to separate all possible homologous series.
Molecular geometry and chain entanglement: parameters for the tube model
Authors: He Tianbai , Porter Roger S. 
Published: April 2003

AbstractThe tube diameter in the reptation model is the distance between a given chain segment and its nearest segment in adjacent chains. This dimention is thus related to the cross‐sectional area of polymer chains and the nearest approach among chains, without effects of thermal fluctuation and steric repulsion. Prior calculated tube diameters are much larger, about 5 times, than the actual chain cross‐sectional areas. This is ascribed to the local freedom required for mutual rearrangement among neighboring chain segments. This tube diameter concept seems to us to infer a relationship to the corresponding entanglement spacing. Indeed, we report here that the critical molecular weight, Mc, for the onset of entanglements is found to be Mc = 28 A/(〈R2〉0/M), where A is the chain cross‐sectional area and 〈R2〉0 the mean‐square end‐to‐end distance of a freely jointed chain of molecular weight M. The new, computed relationship between the critical number of backbone atoms for entanglement and the chain cross‐sectional area of polymers, Nc = A0,44, is concordant with the cross‐sectional area of polymer chains being the parameter controlling the critical entanglement number of backbone atoms of flexible polymers.
Thermodynamics of homogeneous equilibrium copolymerization from the point of view of the Flory‐Huggins theory
Authors: Szymanski Ryszard 
Published: April 2003

AbstractA general treatment of the thermodynamics of binary copolymerization from the point of view of the Flory‐Huggins theory is given. The relations allow one to determine, on the basis of experimental data, the thermodynamic parameters of copolymerization and the parameters of intercomponent interaction (χ). On the other hand, when these parameters are known, it is possible to predict, for any initial set of conditions, the equilibrium concentrations of comonomers and the equilibrium microstructure of a copolymer. The presented simulations indicate that positive values of interaction parameters increase the volume fractions of copolymer units in equilibrium with comonomers, in comparison to an analogous copolymerization system without intercomponent interactions.
Comment on theories of interfacial tension of polymeric interfaces
Authors: Noolandi Jaan , Chen Zheng Yu 
Published: April 2003

AbstractA simple scaling argument is shown to give the correction to the interfacial tension of two infinite‐molecular‐weight immiscible homopolymers arising from the translational entropy of finite‐sized chains. It is suggested that the qualitative nature of the Flory‐Huggins model, as shown by Binder and coworkers, precludes meaningful detailed comparison of experimental data with current theoretical expressions for the interfacial tension.
Homopolymer surfactant for immiscible homopolymer blends
Authors: Noolandi Jaan 
Published: April 2003

AbstractIt is commonly believed that block copolymers are the only type of polymeric surfactants available for incompatible homopolymers(1). We explore the conditions for which a homopolymer will also lower the surface tension between two incompatible homopolymers by lowering the enthalpic contribution to the interfacial free energy, noting that the resulting three‐component interface will not necessarily have the same mechanical strength as a block copolymer‐homopolymer interface.
Exact distribution functions in two‐dimensional lattice polymers. Comparison with maximum entropy distributions
Authors: Poland Douglas 
Published: April 2003

AbstractToeplitz matrices are used to calculate the complete distribution functions for such quantities as the fraction of a particular conformational state and the end‐to‐end distance in 2‐dimensional lattice polymer models with a finite range of intrachain correlation. These are compared with approximate distribution functions obtained using the maximum entropy principle. The two‐parameter maximum entropy distribution (giving the exact first and second moments) in most cases gives a good approximation to the exact distribution and always gives a better approximation than a simple Gaussian approximation. We illustratesome extreme forms of the distribution functions when the intrachain interactions are either very attractive or very repulsive.
Monte Carlo simulation of pulsed laser polymerization
Authors: O'Driscoll Kenneth F. , Kuindersma Michael E. 
Published: April 2003

AbstractPulsed laser polymerization (PLP) has been simulated using a Monte Carlo procedure. From the results of numerous simulations it has been shown that the molecular weight distribution (MWD) consists primarily of two superimposed distributions. One distribution, a relatively broad background, represents the termination reactions during the dark period; the other, a rather sharply peaked distribution, represents the termination reactions occurring as a consequence of the large number of small radicals produced during the laser pulse. The postulate that the inflection point on the sharp peak can be used to calculate that the propagation rate constant was tested and found to be accurate to within 3%. The relative position of the broad and sharp distributions on the chain length scale determines the qualitative appearance of the overall MWD and is in turn governed by the rate of photoinitiation and the relative values of termination and propagation rate constants. This explains the qualitatively different shapes of MWD which have been experimentally observed. Finally, it is shown that the occurrence of chain length dependent termination reactions precludes the use of an analytical expression to deduce quantitative or qualitative information about the termination reaction from PLP data.
Brownian dynamics simulation of grafted polymer brushes
Authors: Neelov Igor M. , Binder Kurt 
Published: April 2003

AbstractWe present results of computer simulations by the method of Brownian dynamics of polymeric brushes attached to impenetrable planes. For testing both model and method we have used one polymer brush attached to a repulsive plane and compare some results with Monte Carlo results of Lai and Binder on the bond fluctuation model. We have also studied two polymeric brushes attached to two parallel planes at different distances between planes, and investigate the interplay between the interpenetration of the brushes and the configurational properties of the grafted chains.
Force‐length relations in deformed coils above and below the theta state. Possible bimodality in the chain distribution function
Authors: Cifra Peter , Bleha Tomáš 
Published: April 2003

AbstractStretching of flexible macromolecules by an external force, acting on the chain ends, in solvents of variable quality was simulated by the Monte Carlo method on a tetrahedral lattice. The forcelength relations and the stress‐induced changes in the population of the gauche conformers were calculated for coils in theta and athermal solvents. In poor solvents, the stretching of a collapsed coil involves a rather abrupt upturn of the force‐length curve explained as a transition of a globular coil into an extended coil. In conformity with this interpretation, a bimodal shape of the chain vector distribution function P(R) was found in the intermediate range of deformation of collapsed coils. The significance of the results to the stress‐strain behavior of networks with pronounced interaction (such as collapsed gels) are discussed.
Analyzing TREF data by stockmayer's bivariate distribution
Authors: Soares João B. P. , Hamielec Archie E. 
Published: April 2003

AbstractThe Stockmayer bivariate distribution is used to qualitatively and quantitatively interpret TREF (temperature rising elution fractionation) spectra of polyolefins made by multiple site type catalysts. TREF spectra simulated by the proposed method can adequately fit experimental TREF data presented in the literature and can be used to help understand the mechanism of TREF separation and the nature of multiple site types catalysts.
Free volume distributions in amorphous polymers
Authors: Chow T. S. 
Published: April 2003

AbstractA new free volume distribution function for amorphous polymers is derived on the basis of a lattice model. It provides a good description of the non‐Gaussian spreading observed very recently in positron annihilation experiments. This new theory shows that the non‐Gaussian distribution is a result of the large density fluctuations of holes from their equilibrium. The Gaussian distribution is recovered in the limit of small fluctuations. The size and volume distribution of the holes is uniquely defined by two structural parameters: the energy of hole formation and the lattice volume. Our theory predicts that increases in temperature will broaden the distribution and shift its lower peak to higher free volume. Increases in pressure have the opposite effect.
Chain overlap and entanglements in dilute polymer solutions: Brownian dynamics simulation
Authors: Brostow Witold , Drewniak Marta , Medvedev Nikolai N. 
Published: April 2003

AbstractWe have analyzed chain conformations and the existence — or otherwise — of chain overlaps and entanglements in dilute polymer solutions (at concentrations c < C*, c* = critical concentration). The fundamental problem of existence of chain overlaps in dilute solutions is also related to the drag reduction phenomenon (DR). Some experimental results pertinent to DR are explained in terms of entanglements even for solutions at concentrations defined in ppm. We report results of Brownian dynamics simulations of polymer solutions in which the equations of motion of the chains are solved by using the Langevin equation. Chains move according to actions of a systematic frictional force and a randomly fluctuating force w(t), where t is time. In addition, a shear flow field can be introduced into the model. To evaluate the structure of polymer chains in solution we have devised a measure of interchain contacts and two different measures of entanglements. The results for c = 0.3 c* demonstrate that both chain entanglements and overlaps take place even in dilute solution. They also confirm predictions from an earlier combinatorial model.
Monte Carlo simulation of chain length distribution in radical polymerization with transfer reaction
Authors: He Junpo , Zhang Hongdong , Yang Yuliang 
Published: April 2003

AbstractIn this paper, the basic principle and a Monte Carlo method are described for numerically simulating the chain‐length distribution in radical polymerization with transfer reaction to monomer. The agreement between the simulated and analytical results shows that our algorithm is suitable for systems with transfer reaction. With the simulation algorithm, we confirm that transfer reaction has a similar effect as disproportionation on the molecular weight distribution in radical polymerization with continuous initiation. In the pulsed laser (PL) initiated radical polymerization with transfer reaction, the ‘waves’ on the chain‐length distribution profile become weaker as the ratio of transfer reaction rate constant, ktr, to the propagation rate constant, kp, is increased in the case with either combination‐type or disproportionation‐type termination. Moreover, it seems that the combination termination has a broadening effect on the waves. Therefore, kp can also be determined by precisely locating the inflection point Lo on the chain‐length distribution profile for radical polymerization with transfer reaction, unless ktr is large enough to smear out the waves on the chain‐length distribution.
Calculation of critical points not requiring the derivatives of the Gibbs energy demonstrated for a mixture of two homopolymers with the corresponding copolymer
Authors: Horst Roland , Wolf B. A. 
Published: April 2003

AbstractThe method for the calculation of phase diagrams (spinodal, binodal and tie lines) exclusively on the basis of the Gibbs energy of mixing, ΔG, with no need of calculating its derivatives with respect to the composition variables was extended to determine the critical conditions and the condition of the stability of the critical points. The method is applied to a ternary mixture of two homopolymers with a copolymer consisting of the same monomers. The sequence distribution of the copolymer is kept constant between random and purely alternating, and phase diagrams are calculated for different chemical compositions of the copolymer. Three critical lines were found within a very small interval of the copolymer composition.
Electrostatic rigidity of polyelectrolytes from reparametrization invariance
Authors: Kholodenko Arkady L. , Douglas Jack F. , Vilgis Thomas A. 
Published: April 2003

AbstractThe persistence length lp of a polyelectrolyte chain can be represented as lp = lO + le where lO is the bare persistence and Ie is the electrostatic contribution coming from the effects of electrostatic chain self‐interactions. Using a reparametrization‐invariant path integral model of semiflexible polymers we find that Ie depends on the ionic strength I as Ie ∼ I−1/2. This result accords with experimental observations and recent Monte Carlo simulations. Reparametrization‐invariance is apparently an essential constrainet in selecting acceptable models of semiflexible polymers.
Anisotropic fluctuations in ordered copolymer phases
Authors: Yeung Chuck , Shi An‐Chang , Noolandi Jaan , Desai Rashmi C. 
Published: April 2003

AbstractThe random phase approximation is reformulated to investigate the anisotropic fluctuations about an ordered polymer phase. This very general method is applied to the lamellar phase of block copolymers. The calculated anisotropic scattering intensity captures the main features observed experimentally including the secondary peaks due to fluctuations with hexagonal symmetry. We also determined the limits of metastability of the lamellar phase as well as the bending and elastic moduli of the lamellae.
From the plateau problem to periodic minimal surfaces in lipids, surfactants and diblock copolymers
Authors: Gòz̀dz̀ Wojciech , Holyst Robert 
Published: April 2003

AbstractA novel method is presented for generating periodic surfaces. Such periodic surfaces appear in all systems which are characterized by internal interfaces and which additionally exhibit ordering. One example are systems of AB diblock copolymers, where the internal interfaces are formed by the chemical bonds between the A and B blocks. In these systems at least two bicontinuous phases are formed: the ordered bicontinuous double diamond phase and the gyroid phase. In these phases the ordered domains of A monomers and B monomers are separated by a periodic interface of the same symmetry as the phases themselves. Here we present a novel method for the generation of such periodic surfaces based on the simple Landau‐Ginzburg model of microemulsions. We test the method on four known minimal periodic surfaces, find two new surfaces of cubic symmetry, and show how to obtain periodic surfaces of high genus and n‐tuply continuous phases (n > 2). So far only bicontinuous (n = 2) phases have been known. We point out that the Landau model used here should be generic for all systems characterized by internal interfaces, including the diblock copolymer systems.
Postpolymerization effect on molecular weight distribution generated by pulsed laser radical polymerization
Authors: Nikitin Anatolij N. 
Published: April 2003

AbstractThe postpolymerization effect on molecular weight distribution (MWDs) and on the Pulsed Laser Polymerization (PLP) technique for evaluation of kinetic constants is investigated. General expressions for moments are derived for a polymerization scheme that contains the reactions of chain initiation, propagation, and termination by recombination or disproportionation, under polymerization initiation by an arbitrary sequence of radiation pulses. The results of calculation of MWDs and of the weight‐average degree of polymerization (Pw) for methyl methacrylate are presented. It is shown that the Pw value strongly depends on postpolymerization. A new method for determining the rate constants of chain propagation and chain termination from a single experiment by polymerization with packets of laser pulses is presented.
Polymer blends, copolymers and networks. Scattering properties and phase behavior
Authors: Benmouna Mustapha , Briber Robert , Hammouda Boualem 
Published: April 2003

AbstractThe phase behavior and scattering properties of polymer blends, copolymers and networks in solution and in bulk are examined. The theoretical framework used here is based upon the extension of the random phase approximation to polymers proposed by de Gennes and its application to chains with various architectures. The case of blends containing stiff chains is considered and the effect of nematic interaction on the phase behavior and scattering properties is discussed. The compressibility problem is reviewed in connection with the free volume theory models. The coupling between density and composition fluctuations is examined together with the effects of pressure on the structural and thermodynamic properties of blends. The dynamics of copolymers are also examined in the light of the new developments both from the theoretical and experimental fronts. The extent to which the chain architecture affects phase behavior, static scattering and dynamic behavior is discussed. A particular emphasis is put on the case of cyclichomopolymers and copolymers. Free chains in a network and crosslinked blends are also a subject of particular interest in this paper. The interplay between macrophase and microphase transitions due to the crosslinks and the electrostatic forces for charged polymers is also considered. De Gennes' analogy betwen the elastic restoring forces in the network and the Coulomb forces in the dielectric medium is generalized by including the screening phenomenon. This generalization is required in order to account for the experimental observations in the low momentum transfer range. Following Briber et al., it is argued that the new screening length can be related to the initial fluctuations at the temperature of crosslinking.
Langevin dynamics simulations of macromolecules on parallel computers
Authors: Jung Bernd , Lenhof Hans‐Peter , Müller Peter , Rüb Christine 
Published: April 2003

AbstractA parallel algorithm is developed that allows efficient Langevin‐dynamics simulations of macromolecular coils, which is the usual structure of synthetic polymers in solution and in bulk. Contrary to usual so‐called spatial decomposition algorithms, we map the one‐dimensional topology of the chain molecule on the parallel computer. The speedup of the algorithm is measured on different multi‐processor systems. The reliability of the parallel calculations is shown by comparison with sequential simulations.
Statistical derivation of kinetic molecular weight development equations in nonlinear free‐radical polymerization
Authors: Tobita Hidetaka 
Published: April 2003

AbstractWe propose a statistical method to derive the differential equations that describe the weight‐average molecular weight development during nonlinear free‐radical polymerizations, by using the random sampling technique. We consider two types of nonlinear free‐radical polymerization schemes, free‐radical polymerization with chain transfer to polymer and free‐radical crosslinking (co)polymerization. The obtained equations fully agree with those obtained through the kinetic approach using the method of moments. The physical meaning of each term in the differential equations as well as the implication of the functional form is discussed from the point of view of the statistical derivation.
A theoretical approach to the thermochemistry of the polymerization of some derivatives of the monomers CH2X (X = CH2, NH, O)
Authors: Leroy Georges , Dewispelaere Jean‐Pierre , Wilante Claude , Benkadour Hanan 
Published: April 2003

AbstractIn this work, we have calculated the thermodynamic parameters of the polymerization of some derivatives of the species CH2X (X = CH2, NH, O), using ab initio methods of quantum chemistry and the usual formalism of statistical thermodynamics. It is shown that the Gibbs functions ΔG(l, c) corresponding to CH2NOCH3, CNCHNCN, CF2O and all the percyano derivatives are largely positive which indicates that the spontaneous (radical or ionic) chain polymerization of these monomers is thermodynamically prohibited.