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  • 1
    ISSN: 0959-8103
    Keywords: anthracene ; fluorescence ; naphthalene ; non-radiative singlet energy transfer ; polyesters ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Steady state fluorescence has been used to study the efficiency of nonradiative single energy transfer for naphthalene → naphthalene and naphthalene → anthracene for polyesters and their bichromophoric model compounds. Polyesters containing only naphthalene groups were derived from 2,6-naphthalene dicarboxylic acid as the rigid unit, and two series of glycols, HO—(CH2)m—OH and HO—(CH2CH2O)n;—OH, where m=2-6 and n=1-4, as flexible spacers. Bichromophoric model compounds were derived from 2-naphthoic acid and the same glycols. In order to study the transfer for naphthalene → anthracene, a first attempt was made by studying bichromophoric model compounds D—(CH2)m—A, where D and A denote 2-naphthoate (donor) and 9-anthranoate (acceptor) groups, respectively. The fluorescence anisotropy measurements in a solid matrix of glassy poly(methyl methacrylate), for the compounds containing only naphthalene groups, and the simple excitation and emission spectra, for the compounds containing naphthalene and antharacene groups, clearly demonstrate the presence of non-radiative singlet-singlet energy transfer, the efficiency of which depends mainly on n (or n). A theoretical treatment using the rotational isomeric state model of the conformatin of these molecules has also been performed. The combination of the experiments and the theoretical analysis establishes that the Förster radii are 1.2 and 1.4 nm for the naphthalene → naphthalene transfer in the bichromophoric model compounds and polyesters, respectively, and 1.6 ± 0.2 nm for the naphthalene → anthracene transfer in the model compounds studied.
    Additional Material: 16 Ill.
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  • 2
    Electronic Resource
    Electronic Resource
    New York : Wiley-Blackwell
    Die Makromolekulare Chemie 186 (1985), S. 633-640 
    ISSN: 0025-116X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Probabilities of configurations conducive to the intramolecular back-biting abstraction of a hydrogen atom have been evaluated for growing unperturbed poly(vinyl acetate) chains. A realistic rotational isomeric state model was used for the chain statistics. Probabilities are found to be smaller than those seen in an earlier treatment of the polyethylene chain. The smaller probabilities of poly(vinyl acetate) contribute to the virtual absence of short branches. The present study therefore provides support for the validity of the Roedel mechanism for the formation of short branches in the free radical initiated polymerization of ethylene.
    Additional Material: 1 Ill.
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  • 3
    ISSN: 0025-116X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Steady state fluorescence measurements in dilute solution have been performed for eight halogenated polystyrenes and for poly(2,5-dimethylstyrene). The monomers for halogenated polymers are o-X-styrene (X = F, Cl, Br), m-fluorostyrene, p-X-styrene, and p-chloro-α-methylstyrene. The position of the excimer band and the ratio of the intensities of the excimer and monomer fluorescence depend on the substitution. The results for poly(p-chloro-α-methylstyrene) and poly(2,5-dimethylstyrene) can be rationalized by a conformational analysis that examines the preferred geometries for face-to-face complexes by rings bonded to chain atoms i and i + 2. The results for the other polymers cannot be explained by a conformational analysis. They appear to arise from a combination of factors that alter the electronic properties of the chromophore.
    Additional Material: 4 Ill.
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  • 4
    Electronic Resource
    Electronic Resource
    New York : Wiley-Blackwell
    Die Makromolekulare Chemie 194 (1993), S. 2851-2860 
    ISSN: 0025-116X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Fluorescence has been measured for six polyesters derived from terephthalate and six 2R,2R′-propanediols, represented by HOCH2—CRR′—CH2OH. The solvent effects on the emission spectra permit separation of the six polymers into two groups. One group consists of the five polyesters in which R and R′ are alkyl groups, and the other group has only one member, that being the polyester with R = R′ = H. Modelling suggests that the origin of the difference between the two groups lies in the access by the polyester with R = R′ = H to a conformation (inaccessible when R and R′ are alkyl groups) that forms a very tight complex between two successive aromatic rings.
    Additional Material: 8 Ill.
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  • 5
    Electronic Resource
    Electronic Resource
    New York : Wiley-Blackwell
    Die Makromolekulare Chemie 189 (1988), S. 2893-2897 
    ISSN: 0025-116X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The characteristic ratio and its temperature coefficient were examined for alternating copolymers of ethylene and propylene. The stereochemical composition covers the entire range from racemic through meso. Two different assignments, τ = 0 and σ, were used for the statistical weight employed when a methylene group participates in simultaneous first-order interactions with another methylene group and a methyl group. The best description of the conformational properties of the alternating copolymers is achieved with the larger value for τ. This assignment is the one that would be expected if the first-order interactions were pair-wise additive.
    Additional Material: 2 Ill.
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  • 6
    Electronic Resource
    Electronic Resource
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 34 (1996), S. 565-573 
    ISSN: 0887-6266
    Keywords: Fiber ; persistence length ; polyimide ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The transformation matrix method and the concept of virtual bonds has been applied to the study of the persistence lengths of two recently developed polyimide fibers, PMDA-PFMB, synthesized from pyromellitic dianhydride (PMDA) and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB), and BPDA-PFMB, synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and PFMB. The elements of the averaged transformation matrices are obtained from a molecular dynamics trajectory of 1 ns and from a consideration of the symmetry of the torsion potential energy function for selected bonds. The persistence length is found to be ∼ 496 AÅ for PMDA-PFMB and ∼ 66 AÅ for BPDA-PFMB. The large persistence length of PMDA-PFMB, comparable to that of the rodlike polymer poly(benzobisoxazole) (PBO), is caused by the colinearity of the virtual bonds in the chain and the stiffness of the bond angles. The inclusion of the electrostatic interactions does not seem to affect the result significantly, although a very slight decrease (∼ 5%) of the persistence length was detected. The effect of substitution of methyl groups for the trifluoromethyl groups in these polyimide fibers is also studied, with an increase of ∼ 14% in the persistence length of PMDA-PFMB and ∼ 1.4% in the persistence length of BPDA-PFMB. Since a good solvent that would be appropriate for use in a light scattering measurement has not been found for either of these polyimide fibers, the theoretical estimates of the persistence length cannot be compared with experiment. © 1996 John Wiley & Sons, Inc.
    Additional Material: 5 Ill.
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  • 7
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Macromolecular Theory and Simulations 3 (1994), S. 931-937 
    ISSN: 1022-1344
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Chains that are unperturbed by the intramolecular excluded volume effect are examined in the free state and under conditions where one of the ends is attached to an impenetrable surface. The density of the attachment of the chains to the surface is so low that interchain interactions can be ignored. The shapes of the distribution functions for the squared end-to-end distance, r2, and squared radius of gyrations, s2, are evaluated in both situations. As is well-known from previous work, attachment of the chains to the impenetrable surface produces an increase in 〈r2〉 and (to a lesser extent) 〈s2〉. It also narrows the distribution function for r2. There is, however, no detectable effect on the shape of the distribution function for s2. The narrowing of the distribution function for r2, coupled with the absence of any effect on the shape of the distribution function for s2, is rationalized with a very simple model.
    Additional Material: 4 Tab.
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  • 8
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Macromolecular Theory and Simulations 6 (1997), S. 949-963 
    ISSN: 1022-1344
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A quasi-static atomistic simulation methodology is used to study the 180° (π) flip motion of phenoxy rings in the amorphous glass of a semicrystalline polyimide. The π-flip is carried out for the rings in various local microenvironments in the polymer. Results show that the activation energy distribution for the flip covers a broad range of values. The energetics, short and long scale matrix movements, and the interrelation between cooperativity and polymer structure are also studied. The results are in qualitative agreement with previous simulation and experimental work on other glassy polymers such as bisphenol-A polycarbonate and polystyrene. The nature of the movement of the amorphous matrix, in response to the ring flip, is far reaching with greater atomic displacements closer to the phenoxy ring. Intermolecular van der Waals and electrostatic interactions contribute significantly to the energy changes associated with the ring flip. Cooperativity is observed for torsion angles along the repeat unit containing the flipped ring. The mean value of the activation energy compares reasonably well to the experimental value for the flip that has been observed previously through solid-state NMR experiments.
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  • 9
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Macromolecular Theory and Simulations 6 (1997), S. 1119-1137 
    ISSN: 1022-1344
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: In the interests of carrying out computer simulations efficiently for systems of rodlike molecules, the model proposed by Berne and coworkers, in which each rod is represented by an anisotropic Lennard-Jones potential, was extensively compared with a site-site model. Using this potential, Monte Carlo simulations were carried out for rodlike molecules with aspect ratio (Ra) ranging from 3 to 50. Short rods such as Ra 〈 10 exhibit an isotropic phase for essentially the entire range of volume fraction φ, while long ones (Ra 〉 20) show an isotropic phase at only very low φ 〈 0.07. The attractive part of the potential makes rodlike molecules associate as well as align even at low φ. At high φ around or above the transition, the repulsive potential dominates and the attractive potential becomes secondary. Based on simulation results, some implications for designing molecular composites are offered.
    Additional Material: 15 Ill.
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  • 10
    ISSN: 1022-1352
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Steady state fluorescence has been used to study the efficiency of nonradiative singlet energy transfer from naphthalene to anthracene in naph-COO—(CH2)m—OOC-anth for m of 2-6. A theoretical analysis has also been performed, using the rotational isomeric state model, of the conformations of these molecules. The experiments clearly demonstrate the presence of nonradiative singlet energy transfer in the bichromophoric compounds, with an efficiency that depends on the solvent and on m. The combination of the experiments and the theoretical analysis establishes that the Förster radius for this system is 1,6 ± 0,2 nm.
    Additional Material: 12 Ill.
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