Analysis of the structure of dendrimers in solution by small-angle neutron scattering including contrast variation

The analysis of a dendrimer of fifth generation by small-angle neutron scattering (SANS) in solution is presented. The contrast of the solute toward the solvent dimethylacetamide (DMA) is changed systematically by measurements of the dendrimer in mixtures of deuterated with protonated DMA. Additional SANS measurements at highest contrast and varying dendrimer concentrations allowed to determine the structure factor of the dendrimers in solution. SANS intensities measured at different contrast are shown to yield the contrast ρ - ρ m where ρ is the average scattering length density of the dissolved dendrimer and ρ m is the scattering length density of the solvent. This allows to determine the molecular weight of the dendrimer in an unambiguous fashion. The comparison of the measured and the calculated molecular weight demonstrates that the dendritic structure under consideration here is not fully perfect. The analysis of the radial structure of the dendrimer rests on the decomposition of the measured intensities into terms depending on powers of the contrast p - ρ m . The leading term which scales with the square of the contrast leads to the determination of the scattering intensity referring to infinite contrast. This allows to elucidate the radial scattering length density in an unambiguous manner. The analysis demonstrates that the present dendrimer, composed of flexible units, has a compact structure where the density has its maximum at the center of the molecule. This is in accord with recent theoretical deductions.