Effects of Molecular Variables and Architecture on the Rheological Behavior of Dendritic Polymers

Abstract

Comparisons of simple shear, oscillatory, and creep flow rheological behaviors are presented for several poly(propylenimine) dendrimers (DAB), hyperbranched (HBP) polyesters, and poly(ether−imide) (PEI) HBPs. Variables examined include molecular weight, degree of branching, nature of peripheral groups, and molecular architecture. In simple shear, DAB dendrimers and HBP polyesters exhibited a Newtonian behavior, and both showed a similar relationship between zero shear rate viscosity, η0, and molecular weight, Mw. In oscillatory experiments, amine end-capped DAB dendrimers and HBP polyesters exhibited Newtonian behavior, while the nitrile end-capped DAB dendrimers showed a Rouse-like frequency dependence of both moduli. A fit of Fox and Flory theory to the fractional free volume data obtained by WLF analysis of DAB dendrimers suggests that the volume free fraction of end groups is independent of generation number. The data on PEI HBPs suggest that at the lowest molecular weight (Mn ∼ 4300) the rheological behavior is Newtonian; however, the higher molecular weight solutions showed shear thinning and normal stress effects, both of which increased with the decrease in the degree of branching (DB). From the concentration dependence of storage and loss moduli, it is observed that intermolecular interactions of PEI HBPs are dominated by hydrodynamics at low concentrations (≤30 wt %), by coil overlap at intermediate concentrations (∼35 wt %), and by entanglements at higher concentrations (≥40 wt %).