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  • 1
    Digitale Medien
    Digitale Medien
    Creep of selenium near the glass temperature (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 9337-9342 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Creep experiments were carried out on amorphous selenium (Se) at temperatures in the vicinity of the glass temperature. The recoverable compliance lacks a plateau, indicating Se chains are too short to form an entanglement network. The measured compliance function was thermorheological complex, even after subtraction of the glassy level and normalizing by the steady state compliance. The temperature dependence determined from the viscosity was in accord with previous viscosity data, although weaker than the near-Arrhenius dependence deduced from the stress relaxation of Se. Based on a comparison to other, small-molecule glass-formers, the dynamic fragility calculated from the viscosity was larger than expected from Se's thermodynamic fragility (i.e., steepness of the normalized Kauzmann curve). In contrast, although polypropylene (PP) is substantially more dynamically fragile than Se, PP is less thermodynamic fragile. Thus, when compared to either small-molecule liquids or polymers, Se exhibits a disconnect between dynamic and thermodynamic measures of fragility. © 1999 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.479846
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  • 2
    Digitale Medien
    Digitale Medien
    The application of the energy landscape model to polymers (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 5593-5598 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The energy landscape model of the glass transition has received increasing attention, due to its potential for providing a thermodynamic interpretation of the dynamics of glass-forming liquids. Herein, the idea that the temperature-dependence of the dynamics near the glass temperature ("fragility") can be related to thermodynamic properties is tested for polymers. Previously, for several homologous series, we found deviations from a proposed correlation between fragility and the heat capacity increment at the glass temperature. A survey of 17 polymers likewise indicates no correlation between these two quantities. More recently, the landscape approach to the dynamics of glass-formers was extended to a proposal that fragility can be determined a priori from thermodynamic information. However, for the 11 polymers for which sufficient thermodynamic and relaxation data were available, we find that thermodynamic properties bear no relationship to fragility. Thus, it appears that the status of the glass transition as an important unsolved problem in condensed matter physics remains intact.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.479861
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  • 3
    Digitale Medien
    Digitale Medien
    Relationship between the primary and secondary dielectric relaxation processes in propylene glycol and its oligomers (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 11585-11591 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Dielectric relaxation measurements were performed on propylene glycol (PG) and oligomers having different number of repeat units (N=2, 3, and 69). The primary α-relaxation had the Kohlrausch–Williams–Watts (KWW) form, with a stretch exponent (1−n) which decreased with increasing N. The temperature dependence of the α-relaxation time, as reflected in the fragility index, increased with N. A broad, rather symmetric secondary β-relaxation was observed at higher frequencies in the dielectric loss spectrum for all samples with N〉1. This is the first observation of the β-relaxation peak in dipropylene glycol (N=2) and tripropylene glycol (N=3). The separation between the α- and β-relaxations increased with increasing N. This trend indicates that the separation is minimal in PG, which makes it difficult to resolve the β-relaxation from the more intense α-relaxation. This, together with the fact that the strength of the β-relaxation decreases with the molecular weight of PPG, as found by Johari and coworkers, explains the absence of an observable β- peak or shoulder in isothermal or isochronal dielectric measurement on PG. It is proposed that the deviation of the dielectric loss peak for PG from either the KWW or the Cole–Davidson functions at higher frequencies is due to a β-relaxation masked by the α-process. The same conclusion was reached by Johari and co-workers, based on evidence from their "difference isochrone method." Finally, we show that the data from oligomers of propylene glycol support the recently proposed correlation of τβ(Tg) with (1−n) and the rough agreement between τβ(Tg) and the primitive relaxation time of the coupling model at temperatures above Tg.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.478006
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  • 4
    Digitale Medien
    Digitale Medien
    Short-time viscous and density relaxation in glycerol and ortho-terphenyl (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 1187-1190 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Coupling model analyses of short-time relaxation data of glycerol from quasielastic neutron scattering and ortho-terphenyl (OTP) by molecular dynamic simulation are shown to account for both the time dependence of the correlation function and its temperature dependence. The latter is expressible over a limited temperature range as an Arrhenius relation with an apparent activation energy for the fast α-relaxation process, with EA=6.2 and 3.9 kcal/mol for glycerol and OTP, respectively. The viscosities of these glass formers have also been measured up to temperatures sufficiently high that the mean viscosity relaxation time, 〈τη(T)〉, becomes short, approaching the crossover time, tc(approximate)2 ps, of the coupling model. When this condition is realized, 〈τη(T)〉 becomes comparable to τD(T) in both magnitude and temperature dependence. Hence, the asymptotic high temperature activation energy of 〈τη(T)〉 approximately equals the apparent activation energy of τD. The experimental data, supporting this connection between macroscopic variables such as viscosity and microscopic dynamics from neutron scattering and molecular dynamics simulation, indicate the significance of the fast α-relaxation process in both fragile and strong liquids.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.473215
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  • 5
    Digitale Medien
    Digitale Medien
    The anomalous Debye–Waller factor and the fragility of glasses (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 2967-2970 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The correlation between the magnitude of the Debye–Waller anomaly and the temperature dependence of the relaxation time and viscosity of glass-forming liquids (i.e., their fragility) is investigated using the coupling model of relaxation. The correlation is shown to be a natural consequence of the relationship between the noncooperative and intermolecularly cooperative relaxation times of the model. Specifically, the deviation of the mean squared displacement from a linear temperature dependency increases as the fragility (in the Angell sense) of the glass-forming liquid increases because more fragile glasses exhibit substantially more short-time, noncooperative relaxation. This latter fact arises from their shorter noncooperative relaxation times, as deduced from the coupling model. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.471117
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  • 6
    Digitale Medien
    Digitale Medien
    Response to "Comment on ‘Short time dynamics of glass-forming liquids' '' [J. Chem. Phys. 104, 8169 (1996)] (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 8171-8172 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Our J. Chem. Phys. 103, 1152 (1995) article described synthetic data generated from the coupling model, which was analyzed in the framework of mode coupling theory (MCT). The purpose was to demonstrate that someone unaware of how the data was generated could carry out an MCT analysis and find some, though not necessarily all, of the features to be in conformance to MCT. The work thus served as a caution against fixation on only some features of short time experimental data, since this can lead to premature conclusions that MCT has been verified. Due to the limitations of experiments on real materials, most data either does not allow a test of all MCT predictions or, as has been found in polymers, is in disagreement with MCT. These points, which have evidently been missed, are reiterated in the present communication. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.471528
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  • 7
    Digitale Medien
    Digitale Medien
    High frequency relaxation of o-terphenyl (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 4632-4636 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Results of molecular dynamics simulations (MDS) of o-terphenyl, a glass-forming liquid, are analyzed in terms of the coupling model of relaxation. At low temperatures thermally activated relaxation processes are suppressed, whereby the density–density correlation function, C(t), obtained by MDS is determined entirely by vibrational modes. This enables the low temperature data to be used to deduce the vibrational density of states, g(ω). With g(ω) determined, the vibrational contribution, Cpho(t), is calculated at higher temperatures assuming that g(ω) is independent of temperature. At higher temperatures, relaxation makes its appearance and is modeled here by the fast dynamics of the coupling model. Assuming that vibration and relaxation contribute independently, the density–density self-correlation function is given by the product Cpho(t)Crel(t), with the relaxation part obtained from the coupling model. There is good overall agreement between the calculated C(t) and the MDS data. Microscopic parameters, including the energy barrier for reorientation of the o-terphenyl molecule, are extracted from the MDS results. © 1995 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.470651
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  • 8
    Digitale Medien
    Digitale Medien
    Macromolecular diffusion and the autoadhesion of polybutadiene (1985)
    s.l. : American Chemical Society
    Macromolecules 18 (1985), S. 1310-1314 
    Details
    ISSN: 1520-5835
    Quelle: ACS Legacy Archives
    Thema: Chemie und Pharmazie , Physik
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1021/ma00148a046
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  • 9
    Digitale Medien
    Digitale Medien
    Segmental relaxation and the correlation of time and temperature dependencies in poly (vinyl methyl ether)/polystyrene mixtures (1992)
    s.l. : American Chemical Society
    Macromolecules 25 (1992), S. 363-367 
    Details
    ISSN: 1520-5835
    Quelle: ACS Legacy Archives
    Thema: Chemie und Pharmazie , Physik
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1021/ma00027a056
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  • 10
    Digitale Medien
    Digitale Medien
    Segmental relaxation and molecular structure in polybutadienes and polyisoprene (1991)
    s.l. : American Chemical Society
    Macromolecules 24 (1991), S. 5315-5319 
    Details
    ISSN: 1520-5835
    Quelle: ACS Legacy Archives
    Thema: Chemie und Pharmazie , Physik
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1021/ma00019a016
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