Structure and rheology of hyperbranched and dendritic polymers. I. Modification and characterization of poly(propyleneimine) dendrimers with acetyl groups

Commercially available fourth and fifth generation poly(propyleneimine) (PP I) dendrimers were functionalized with acetyl chloride and deuterated acety l chloride. Their solution properties in water and D2O were measured with d ilution viscometry, densitometry, rheology, and small-angle neutron scatter ing (SANS) and compared to molecular modeling. Both the acetylated and PPI dendrimers exhibited Newtonian rheology in solution at all concentrations, but the functionalized dendrimers were less viscous than the nonacetylated dendrimers at an equal weight fraction (50 wt %). The acetylated dendrimers exhibited a pronounced structure peak in SANS, however, that was not evide nt for PPI in solution and a greatly enhanced solubility. This structure pe ak, evident at concentrations as low as 0.2 wt %, was evidence for long-ran ge electrostatic interdendrimer forces, which were screened by added salt. A quantitative agreement was obtained between the dilute-limiting absolute scattering spectra of both the nonacetylated and acetylated dendrimers in s olution with model calculations via a homogeneous spherical model and input parameters independently obtained from dilution viscometry or direct calcu lation. The combined measurements verified significant solvent penetration for both dendrimer types. The form factors measured in this manner were als o in good quantitative agreement with the results of molecular dynamics sim ulations, which pointed to significant backfolding of the terminal groups. SANS and rheology measurements at higher concentrations suggested dendrimer clustering and interpenetration with increasing concentration, leading to less structure and lower viscosity than would be predicted from the dilute- limiting behavior. (C) 2000 John Wiley & Sons, Inc.