Fast and Slow Dynamics of Water-Soluble Dendrimers Consisting of Amido-Amine Repeating Units by Neutron Spin−Echo

Abstract

Neutron spin−echo (NSE) experiments were performed at room temperature on D2O solutions of fifth generation poly(amido amine) (PAMAM) dendrimers with different types of end groups. Intermediate correlation functions I(Q,t)/I(Q,0) at different scattering vectors Q as a function of time t show different dynamics for dendrimer concentrations of 1.0 and 10 wt %, while the dynamics was independent of the chemical species of the end groups, hydroxyl or glucopeptide. The NSE results for a 10 wt % solution followed a single-exponential decay. With increasing Q, the evaluated effective diffusion coefficients Deff decreased and converged to the translational diffusion coefficient D0, which was obtained from dynamic light scattering. It is assumed that the Q-dependence of Deff is caused by the dendrimer-dendrimer interaction, which is observed as the inter-dendrimer structure factor on small-angle neutron scattering. On the other hand, NSE results for a 1.0 wt % solution were well-fitted to a double-exponential function with two decay rates (fast and slow modes). It was observed that the diffusion coefficient of the slow mode corresponds to D0. The fast mode originates from the motion of amido-amine segments in the dendrimer, which is a common internal unit for the two dendrimers.