The structure of dendritic molecules in solution as investigated by small-angle neutron scattering

The analysis of the equilibrium structure of a dendrimer of fourth generati on by small-angle neutron scattering (SANS) is described. The analysis pres ented here is based on measurements of the SANS-intensities at different co ntrasts <(rho)over bar>-rho(m) with <(rho)over bar> being the average scatt ering length density of the dissolved dendrimer and rho m the scattering le ngth density of the solvent dimethylacetamide (DMA). In this study, rho(m) was changed by using mixtures of protonated and deuterated DMA. On varying the contrasts of solute towards solvent, the average scattering length dens ity of the dissolved dendrimer <(rho)over bar> can be obtained unambiguousl y. Contrast variation leads to determination of the molecular weight of the dendrimer which is shown to be in excellent agreement with the calculated value. The measured scattering intensity I(q) (q = (4 pi/lambda) sin(theta/ 2). lambda: wavelength of radiation; theta: scattering angle) can be decomp osed into terms depending on the contrast <(rho)over bar>-rho(m) and a term independent of it. Contribution of the latter is mainly given by incoheren t scattering of the numerous protons present in the structure of the dendri mer. Experimental data presented here demonstrate that this contribution mu st be removed in order to obtain meaningful structural information. The ter m scaling linearly with <(rho)over bar>-rho(m) is negligible within given m argins of error. The structural information is embodied in an contribution which scales with contrast <(rho)over bar>-rho(m) in the square and may be regarded as scattering intensity extrapolated to infinite contrast. Fourier -inversion of this term leads to a radial segment density distribution whic h has its maximum at the center of the molecule. This is in agreement with recent theoretical predictions and shows that the dendrimer of fourth gener ation studied here is a strongly fluctuating structure akin to branched or star polymers.