Influence of end groups on dendrimer rheology and conformation

Poly(propyleneimine) (PPI) dendrimers and their end-functionalized derivatives are studied with steady and dynamic rheology to determine the influence of specific end groups. Fourth- and fifth-generation PPI dendrimers exhibit constant shear viscosities over a wide range of shear rates. Functionalizing the molecular end groups with methyl and benzyl acrylate monomers increases the glass transition temperature and leads to a significant elastic modulus and nonlinear rheological behavior at high shear rates. The zero-shear viscosity, corrected for the shift in glass transition, exhibits a unique maximum with molecular weight. The fractional free volume derived from the thermal rheological measurements is independent of the end-group derivatization and substantially larger than that for linear polymers.