Structural characterization and polyelectrolyte behavior of poly(amido amine) dendrimers investigated by light and neutron scattering methods

Molecular characterization and polyelectrolyte behavior of poly(amido amine) (PAMAM) dendrimers in aqueous solution have been investigated using scattering techniques. The size and particle structure of generations 3-7, and 9, ethylene diamine core dendrimers were determined by small angle neutron scattering. This study provides the first continuous data set to test predictions of theory and simulation. For G = 3-7, a linear dependence of the radius of gyration with generation, not predicted by theory, was found. A smaller R{dollar}sb{lcub}rm g{rcub}{dollar} for G = 9 is interpreted as incomplete generation growth due to segment dense-packing. Dendrimer structure becomes increasingly compact and spherical with G = 9 being well described by a solid sphere. The absence of {dollar}R propto Msp{lcub}x{rcub}{dollar} scaling as well as of a universal scaling of the interior dimensions strongly indicates that PAMAM dendrimers are not fractal.; Dendrimer end units were located using the principle of charge-mirroring by counterion condensation and neutron contrast matching techniques. With titration of the terminal primary amine groups, the counterions are distributed around the 8G PAMAM perimeter. Titration of the interior tertiary amines leads to a counterion distribution which closely matches the dendrimer structure. This gives strong evidence that the dendrimer terminal units are located near the periphery.; The polyelectrolyte behavior of spherical 8G PAMAM dendrimers in dilute salt-free solution was investigated by small angle neutron scattering and static and dynamic light scattering. Static measurements-reveal two length scales in solution: local ordering and long-range "domains." Increasing dendrimer charge enhances liquid-like ordering and domain size. Three diffusive modes are observed; none correspond to dendrimer self diffusion. The fast mode is ascribed to coupled counterion-macroion motion. The intermediate mode appears to reflect dendrimer motion within ordered domains. The slow mode is attributed to diffusion of large-scale domains, with strong evidence provided by the angular dependence of the slow mode and scattered light intensity, and their coincident trend with dendrimer charge. At moderate ionic strength the solution behavior returns to Brownian, neutral polymer behavior.