THE MOLECULAR CHARACTERISTICS OF POLY(PROPYLENEIMINE) DENDRIMERS AS STUDIED WITH SMALL-ANGLE NEUTRON-SCATTERING, VISCOSIMETRY, AND MOLECULAR-DYNAMICS

The molecular characteristics of the first five generations of poly(pr opyleneimine) dendrimers with two different types of end groups have b een investigated using small-angle neutron scattering (SANS), viscosim etry, and molecular dynamics simulations. The dimension of the poly(pr opyleneimine) dendrimers, as measured by both SANS and viscosimetry, i ncreases linearly with the generation number and roughly as M-1/3, whe re M is the molar mass of the dendrimer. This relationship proves to b e independent of the character of the end group and the solvent used a nd 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-eta, similar or equal to root(5/3)R-g) between the hydrodynamic r adius R-eta,, 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 resul ts. The probability distributions of the amine end groups, based on th ese simulations, exhibit a substantial degree of backfolding. The corr esponding radial density distributions show a constant density plateau and a monotonic decrease of the density toward the exterior of the mo lecule. The above results indicate that the poly( propyleneimine) dend rimers can be considered as flexible molecules with a relatively homog eneous radial density distribution. This view clearly deviates from bo th the dense shell and dense core models.