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Brigitte Neubauer Gerhard Zifferer Oskar Friedrich Olaj 《Macromolecular theory and simulations》1998,7(1):189-195
Alternating and random copolymers in dilute solution are investigated by means of Monte Carlo simulations on a cubic lattice. Each molecule consists of an equal number of A and B segments, either randomly distributed along the chain or forming an alternating sequence. The energy parameters chosen represent selective solvent conditions (the solvent is a good one for monomers of type A and a θ-solvent for B; between A and B repulsive interactions are operative). Comparison with di- and triblock copolymers of equal overall composition reveals that the behaviour of random or alternating copolymers (subject to the same selective solvent) is quite different. Their properties rather resemble those of homopolymers in a solvent of intermediate quality. The absolute chain dimensions (e.g. the mean square radius of gyration, 〈s2〉, and the mean square end-to-end distance, 〈h2〉) of random and alternating copolymers as well as their scaling exponents are significantly larger than those of block copolymers. The ratio between 〈h2〉 and 〈s2〉 as well as the shape of the polymer (expressed by the asphericity δ) are similar to those of athermal polymers indicating that there is no pronounced selectivity of the solvent. In contrast to block copolymers, these parameters exhibit no significant chain-length dependence. The number of the various types of polymer-polymer contacts (A-A, B-B and A-B) is almost independent of the type of contact at least for the solvent conditions investigated. This is in contrast to block copolymers where A-B contacts are widely suppressed and where the number of B-B contacts is approximately twice as high as that of A-A contacts. 相似文献
164.
Following earlier suggestions the values for the rate coefficient of chain termination kt in the bulk polymerization of styrene at 25°C were formally calculated (a) from the second moment of the chainlength distribution (CLD) and (b) from the rate equation for laser-initiated pseudostationary polymerization (both expressions originally derived for chain-length independent termination) by inserting the appropriate experimental data including the rate constant of chain propagation kp. These values were treated as average values, k and k , respectively. They exhibited good mutual agreement, even the predicted gradation (k < k by about 20%) was recovered. The log-log plot of kt vs. the number-average degree of polymerization of the chains at the moment of their termination yielded exponents b of 0.16–0.18 in the power-law kt = A · Pn −b, A ranging from 2.3 × 108 to 2.7 × 108 L · mol−1 · s−1. These data are only slightly affected if termination is not assumed to occur by recombination only and a small contribution of disproportionation is allowed for. 相似文献
165.
Oskar Friedrich Olaj Thomas Petrik Gerhard Zifferer 《Macromolecular theory and simulations》1998,7(6):641-648
Incompatible amorphous polymer blends on a 5-way cubic lattice were simulated by means of Monte Carlo methods and chain-length distributions of both minority and majority components in the two coexisting bulk phases investigated. Furthermore, the chain-length dependence as well as the temperature dependence of the distribution coefficient was explored. A relative preference for shorter chains was noted in the energetically less favourable media leading to mass segregation and resulting in totally asymmetric shapes of the chain-length distributions especially for the minority component. While the absolute concentration of the minority chains depends on the energetic situation as well as on the average chain-length of the surrounding polymer matrix, the distribution coefficient K turned out to be a function of the individual chainlength N of the dissolved species as well as of the temperature T of the assembly according to the law K = exp[-const · N/T]. These results are in full agreement with previous theoretical and experimental findings. 相似文献
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Oskar Perron 《Mathematische Zeitschrift》1918,1(1):159-159
Ohne Zusammenfassung 相似文献
170.
Oskar Hackl 《Fresenius' Journal of Analytical Chemistry》1938,112(5-6):174-179
Ohne Zusammenfassung 相似文献