Structure and rheology of hyperbranched and dendritic polymers. II. Effects of blending acetylated and hydroxy-terminated poly(propyleneimine) dendrimers with aqueous poly(ethylene oxide) solutions

The effect of adding acetylated poly(propyleneimine) dendrimers to the stru cture and rheology of aqueous solutions of high molecular weight poly(ethyl ene oxide) (PEO) was investigated by rheology and small-angle neutron scatt ering in a temperature range of 10-40 degrees C. In the semidilute regime, the steady shear rheology of PEO solutions was unmodified by the addition o f dendrimers at a comparable weight concentration. At the highest concentra tions studied, the addition of acetylated dendrimers suppressed the onset o f a low-frequency elastic modulus at the lowest temperature investigated. F or comparison, the addition of PEO of a comparable molecular weight at the same weight fr action resulted in a milder suppression but, unlike the dend rimers, greatly increased the solution viscosity. The addition of acetylate d dendrimers to a semidilute PEO solution at 10 degrees C substantially red uced the solution turbidity. These effects on the rheology and optical prop erties were confirmed by small-angle neutron scattering measurements of the molecular structure of the mixture. Additional SANS measurements in the di lute regime (0.1 wt % PEG) showed quantitatively that the dendrimers decora ted the PEO chains in a necklace structure, such as that observed previousl y for micelles. The results suggested a mechanism of rheology modification whereby the dendrimers disrupted the association network structure in the P EO solution at lower temperatures by preferentially associating with the PE O chains in solution. (C) 2000 John Wiley & Sons. Inc.