Size Change of Dendrimers in Concentrated Solution

Solutions of the poly(propylene imine) dendrimers DAB- dendr -(PA) 32 and DAB- dendr -(PA) 64 in methanol are investigated with small angle neutron scattering over the range of dendrimer mass fraction 0.01 ≤ x ≤ 0.80. The single particle scattering function, P ( q ), is used to calculate the structure factor, S ( q ), of the dendrimer solutions and to evaluate the radius of gyration of the dendrimers in dilute solution giving (DAB- dendr -(PA) 32 ) = 12.4 ± 0.2 Å and (DAB- dendr -(PA) 64 ) = (15.6 ± 0.2) Å. In each case the segment density in dilute solution is 0.35 ± 0.03 g cm -3, leaving a large fraction of the dendrimer volume accessible to solvent. We define the “dilute solution regime” for dendrimers to extend to a concentration where the swollen dendrimer volume fraction first equals the value 0.64, which comes at a dendrimer weight fraction of x = 0.25. At this concentration, the spatial arrangement of the dendrimers can be described as a random close packing. For higher concentrations, the experimental scattering functions appear to be self-similar up to a mass fraction of x ≈ 0.60. These observations are consistent with a model of concentrated dendrimer solutions which assumes that individual dendrimers collapse to maintain a volume fraction φ ≈ 0.64, which is the volume fraction of random close packing of hard spheres. Preliminary small X-ray scattering data give further evidence of the dendrimer collapse.