摘要:

AbstractWe calculate the mean‐square end‐to‐end distances and mean‐square gyration radii using the bond fluctuation model for a binary polymer blend in the presence of a wall by Monte Carlo simulation. In the bulk, the size of the minority, low‐concentration polymer species is compressed compared to the majority one. In the vicinity of the wall, where the minority polymer concentration is enriched due to attraction from the wall, the dimensions of the two types of polymers are approximately equal and are essentially the same as in an athermal polymer melt. Thus, the geometric constraint is more important to the structure of the polymers than the polymer‐polymer and polymer‐wall

ISSN:1022-1344    

DOI:10.1002/mats.1992.040010201

出版商:Hüthig&Wepf Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractBy use of the pivot algorithm, star‐branched chains withF= 3–12 arms of lengthn, nF= 480, and linear chains (F= 2) are generated on a tetrahedral lattice. In order to simulate different qualities of the solvent, specific short‐range interactions are taken into account. Whereas in athermal systems a new configuration — which is obtained by rotating that part of an arm which contains the chain end around a randomly selected bond by ± 120° — is accepted if it is self‐avoiding, for non‐athermal systems the Metropolis‐Rosenbluth criterion with respect to energy must be satisfied in addition. Calculating the energy of the configuration, nearest‐neighbour interactions (each contact contributes an energy Φ·kT, Φ0 exothermal ones) are considered only; no energy is introduced to distinguish between trans and gauche bonds. A rather quick response of chain properties to the variation of thermodynamic conditions is demonstrated. The average acceptance fractionf̄has its maximum value for athermal conditions, slightly decreases for exothermal conditions and strongly decreases with increasingly negative Φ‐values. However, for Φ = −0,5 — representing a solution near theta‐conditions — still 25% of attempted moves are accepted for allF‐values examined. The dependence of global properties (characterized by the mean‐square radius of gyration) on Φ andFis in full

ISSN:1022-1344    

DOI:10.1002/mats.1992.040010202

出版商:Hüthig&Wepf Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractA procedure is developed which allows to treat arbitrary periodic initiation profiles (asymmetric and symmetric triangle profiles, sinusoidal profiles, Gaussian profiles etc.) in pseudostationary radical polymerization. Using an iterative method these profiles are transformed into the (likewise periodic) radical profiles and into the chain‐length distributions of the resulting polymer in case of termination by disproportionation. These distributions are analysed for the position of their inflection points which may be used for experimental determination of the elementary rate constant of chain propagationkp. It turned out that for all profiles that have at least one discontinuity (e.g. asymmetric triangle profiles) the position of the point of inflection is a correct measure ofkpfor a conveniently wide range of experimental parameters. In case of profiles without discontinuity (symmetric triangle profiles, sinusoidal and Gaussian profiles) the position of the inflection point is shifted to lower values which means that thekpvalues determined on this basis will be a little too small. In most cases, however, the error introduced by this fact will not exceed the overall error of the experiment so that in practice the method of determiningkpin pseudostationary polymerization is not restricted to those profiles which exhibit discontinuitie

ISSN:1022-1344    

DOI:10.1002/mats.1992.040010203

出版商:Hüthig&Wepf Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractFor a polycondensation reaction with identical structural units AaBb(i.e. each unit has a functional groups of type A and b of type B) the kth radius of gyration\documentclass{article}\pagestyle{empty}\begin{document}$ \begin{array}{*{20}c} {\left\langle {R^2 } \right\rangle _k = \sum\limits_n {n^k \cdot \left\langle {R_n^2 } \right\rangle P_n } }&{({\rm sum over all }n{\rm - mers; see Eqs}{\rm . (11) and (17))}{\rm }} \\ \end{array} $\end{document}is investigated. By means of a differential method, a recursion formula that holds true for both the pre‐gel and the post‐gel state is obtained for the evaluation of the kth rad

ISSN:1022-1344    

DOI:10.1002/mats.1992.040010204

出版商:Hüthig&Wepf Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractThe semi‐empirical SCF MNDO method has been used to calculate the radical reactivities for each reaction site in some neutral furan molecules as the average of the HOMO and LUMO probabilities both in 2pzatomic orbitals1SCF: Self‐consistent field; MNDO: modified neglect of differential overlap; HOMO: highest occupied molecular orbital; LUMO: lowest unoccupied molecular orbital.. The carbon atoms C5and C2of of the furan ring are the most favoured sites except for the furan derivatives holding a double bond in the substituent group. Also the SOMO2SOMO: Single occupied molecular orbital.probabilities of the radicals formed by addition of a hydrogen atom or a vinyl acetate model radical were calculated. Thus, once a radical is added to the C5position the resulting radical can be partially localized on the carbon C2. Furthermore, the enthalpy of reaction for several radicals was estimated by using the calculated heats of formation of the neutral furan molecules and their radicals. The radical addition to the carbon C5resulted in the most exothermic reaction in comparison with other reaction sites of the molecule. However, no correlation was found between the calculated enthalpies and the degradative‐transfer kinetic constants experimentally determined for the radical polymerization of vinyl acetate in the presence of the furan compounds under

ISSN:1022-1344    

DOI:10.1002/mats.1992.040010205

出版商:Hüthig&Wepf Verlag    

年代:1992

数据来源: WILEY