Small-angle neutron scattering and surface force investigations of poly(amido amine) dendrimer with hydroxyl end groups

Atomic force microscopic image on mica substrate, which was dipped for 10 min in an aqueous solution of fifth-generation poly(amido amine) dendrimer with hydroxyl end groups, displayed a flatter texture of dendrimer-adsorbed surface. On the external contrast variation of small-angle neutron scattering for aqueous dendrimer solutions, the apparent radius of gyration of dendrimer decreased with increasing H2O content in H2O/D2O mixed solvent, indicating the water penetration into dendrimer. The calculation based on small-angle neutron scattering theory and a five-layer model of dendrimer with water penetration was fitted with the observed one. In connection with the ability of dendrimer doping small molecules, it is confirmed that the segment density profile in dendrimer is owing to the compensation of the segment density increase by the segment branching and the density decrease due to the segment chain extension, although the water penetration depends on the latter fact. The surface force-separation curves between glass beads adsorbed dendrimers in aqueous solutions depended on the adsorption time. At a medium adsorption time, the electrostatic repulsion force between glass surfaces diminished apparently owing to the adsorption of dendrimers, Moreover, dendrimers sandwiched between glass surfaces raised the adhesive attraction force with glass surfaces. At a long adsorption time, both glass surfaces were covered by adsorbed dendrimers, and the repulsive force acted between dendrimer layers. Same profiles of the surface force-separation curves were observed at a shorter adsorption time for the more concentrated solution and for the higher-generation dendrimer. It is confirmed from the theoretical analysis that the interaction force between dendrimer layers is dominated by the osmotic pressure effect in the steric repulsion force. This suggests the utilization of poly(amido amine) dendrimer with a hydroxyl end groups as a surface-improvement agent promoting the dispersion stability of fine particles in medium.