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    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 117 (2002), S. 4047-4062 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have measured both the static and dynamic structure factors of a single dendrimer with small-angle x-ray scattering (SAXS) and neutron spin-echo spectroscopy under good solvent conditions with the aim of finding a consistent correlation between the structural properties of dendrimers and their dynamic behavior. The samples under investigation were star-burst polyamidoamine dendrimers with generations g=0 to 8 in dilute methanol solutions. A model independent approach employing inverse Fourier transformation and square root deconvolution methods has been used to analyze the SAXS data to obtain the pair distance distribution function p(r) and the radial excess electron density profile Δρ(r). In addition, we formulated a model that takes both the colloidal (globular, compact shape with form polydispersity or fuzzy surface) as well as the loose, polymeric (self-avoiding random walk) character of dendrimers into account. With this model we were able to describe the spectra of all dendrimer generations consistently. Parameters discussed as a function of the dendrimer generation are, among others, the correlation length of the density fluctuations (blob radius) ξ, the radius of gyration Rg, the sphere radius Rs, the form polydispersity σs or analogously, the width of the fuzzy surface region 2σf. Both the model-independent approach and the model fits reveal that at least down to the third generation the dendrimers exhibit a rather compact, globular shape. These findings are in agreement with the dynamic results obtained by NSE spectroscopy which probes length scales both larger and much smaller than the dimension of a single dendrimer. The method reveals that the dynamics throughout is dominated by the center-of-mass diffusion—the internal dynamics is suppressed. The diffusion coefficients obtained are close to the values calculated from the Stokes–Einstein relation using the sphere radius Rs determined from the SAXS spectra. Dynamically, the dendrimers behave like "hard", solid spheres. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have investigated the dynamics of polymers in bimodal polyethylene (PE) melts in the transition region from Rouse- to reptationlike behavior by varying the mass fraction Φt of long tracer chains (N(approximate)3Ne or 4Ne) in a short-chain matrix (N(approximate)Ne=entanglement segment number) over the full concentration range. At short times (ns) the dynamic structure factor for single-chain relaxation was investigated by neutron-spin-echo (NSE) spectroscopy. To obtain information about the long-time (ms) dynamics the tracer diffusion coefficient (DNMR) was measured by pulsed-field-gradient (PFG)-NMR. We discuss our NSE data within a mode analysis which includes the relaxation rates Wp of the independent normal modes of the internal chain dynamics and the center-of-mass diffusion coefficient DNSE as model parameters. Only modes exceeding the Φt-dependent length of a single entanglement strand Ne(Φt) are found to be strongly hindered by topological constraints. The DNSE are Φt-independent and systematically faster than the strong concentration-dependent DNMR, suggesting an effective time-dependent diffusion coefficient. The Hess model, which we have generalized for polydisperse melts, provides a time-dependent diffusion coefficient. Taking chain-end effects into account we get an excellent description of the NSE data. The mobility of the chain ends is much higher than the mobility of the inner segments resulting in an entanglement segment number which increases with decreasing tracer concentration. The concentration dependence of Ne(Φt), as obtained from the mode analysis and the Hess model, is in agreement with our calculation within a self-consistent modification of the model by Kavassalis and Noolandi for entanglement formation. © 1999 American Institute of Physics.
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