research10.1063/1.1493771

Dynamics of star-burst dendrimers in solution in relation to their structural properties

Abstract

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 Deltarho(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) xi, the radius of gyration R-g, the sphere radius R-s, the form polydispersity sigma(s) or analogously, the width of the fuzzy surface region 2sigma(f). 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 R-s determined from the SAXS spectra. Dynamically, the dendrimers behave like "hard", solid spheres. (C) 2002 American Institute of Physics

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