Electrostatic swelling and conformational variation observed in high-generation polyelectrolyte dendrimers

A combined small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) study was conducted to investigate the structural characteristics of aqueous (D₂O) solution of generation 7 and 8 (G7 & G8) polyamidoamine (PAMAM) dendrimer as a function of molecular protonation. A consequent change in the intramolecular dendrimer conformation was clearly quantified by a detailed data analysis separating intermolecular correlations from the intramolecular contribution. Our results unambiguously reveal both an increase in the molecular size and a continuous variation of the intramolecular density profile upon increasing molecular protonation. This observation is contrary to current understanding of high-generation polyelectrolyte dendrimers where steric crowding is supposed to stiffen the local motion of dendrimer segments hindering exploration of available intradendrimer free volume and thereby inhibiting electrostatic swelling. Our observation is relevant to the elucidation of the general microscopic picture of polyelectrolyte dendrimer structure, as well as the development of dendrimer-based packages based on the stimuli-responsive principle.