The Molecular Characteristics of Poly(propyleneimine) Dendrimers As Studied with Small-Angle Neutron Scattering, Viscosimetry, and Molecular Dynamics

Abstract

The molecular characteristics of the first five generations of poly(propyleneimine) dendrimers with two different types of end groups have been investigated using small-angle neutron scattering (SANS), viscosimetry, and molecular dynamics simulations. The dimension of the poly(propyleneimine) dendrimers, as measured by both SANS and viscosimetry, increases linearly with the generation number and roughly as M 1/3, where M is the molar mass of the dendrimer. This relationship proves to be independent of the character of the end group and the solvent used and is indicative of a compact (space-filling) structure with a fractal dimensionality of approximately 3. The distinct maximum in the Kratky representation of the scattering data and the observed relation (i.e., R η ≃ ) between the hydrodynamic radius R η and the radius of gyration R g are consistent with such a structure. Molecular dynamics simulations for two different solvent qualities are in good accordance with the acquired experimental results. The probability distributions of the amine end groups, based on these simulations, exhibit a substantial degree of backfolding. The corresponding radial density distributions show a constant density plateau and a monotonic decrease of the density toward the exterior of the molecule. The above results indicate that the poly(propyleneimine) dendrimers can be considered as flexible molecules with a relatively homogeneous radial density distribution. This view clearly deviates from both the dense shell and dense core models.