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

Abstract

ABSTRACT:Commercially available fourth and fifth generation poly(propyleneimine) (PPI) dendrimers were functionalized with acetyl chloride and deuterated acetyl chloride. Their solution properties in water and D 2 O were measured with dilution viscometry, densitometry, rheology, and small-angle neutron scattering (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 evident for PPI in solution and a greatly enhanced solubility. This structure peak, evident at concentrations as low as 0.2 wt %, was evidence for long-range 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 solution with model calculations via a homogeneous spherical model and input parameters independently obtained from dilution viscometry or direct calculation. The combined measurements verified significant solvent penetration for both dendrimer types. The form factors measured in this manner were also in good quantitative agreement with the results of molecular dynamics simulations, 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.

Influence of End Groups on Dendrimer Rheology and Conformation

“…Previous work with acetylated PPI dendrimers focuses on solution structure, rheology, and interactions with linear polymers in water. 10,11 In this work, we have examined both the solution architecture and melt rheology of fourth-and fifth-generation poly(propyleneimine) (PPI) dendrimers derivatized with two different acrylate monomers. The results of density measurements along with steady and dynamic rheology are analyzed to draw connections between copolymer composition, molecular structure, and bulk mechanical behavior.…”

Molecular Control of the Viscosity of Model Dendritically Branched Polystyrene Solutions: From Polymeric to Colloidal Behavior

“…It is remarkable that data from all these different systems can be put together in a generic plot when their hydrodynamic radius is considered for the estimation of the c* and thus the effective volume fraction. 34,38 Furthermore, an important finding is the fact that the dendritic samples with the same generation G (Table 1) and similar values of the ratio g (Table 2) exhibit different viscosity-volume fraction behavior (different curves). In particular, it can be observed that at high volume fractions the dendritic polymer with the smallest total molecular weight (PS1:5) exhibits a steeper volume fraction dependence compared to its counterparts, approaching the behavior of multiarm stars and hard spheres.…”

“…In the present case the values of φ m cover the range 1.28 < φ m < 3.87(see Table 3 and Figure 6 below); similar values of φ m have been reported for other dendritic polymers recently. 38 Still, the fit of eq 3 to the data is far from perfect; one possible origin for the deviations could be the not truly spherical (rather wedgelike) shape of the dendritic polymers, which could also lead to a distinct geometrical packing compared to purely spherical objects. 40 Obviously, this equation cannot describe polymeric behavior.…”

Shear-induced effects in hyperbranched-linear polyelectrolyte complexes

“…Because of their distinct structural features, the HPs exhibit a different rheological behavior compared to linear polymers ͑LPs͒, either in bulk [19][20][21][22][23] or in solutions and in mixtures with nonhyperbranched molecules. [24][25][26][27][28][29] The transition from the Newtonian to the shear-thinning regime occurs at higher shear rates, while in the Newtonian regime they exhibit a lower viscosity compared to LP systems of comparable molecular weight.…”

“…[24][25][26][27][28][29] The transition from the Newtonian to the shear-thinning regime occurs at higher shear rates, while in the Newtonian regime they exhibit a lower viscosity compared to LP systems of comparable molecular weight. [30][31][32] These observations have been attributed to their highly branched structure which drastically affects their deformability and their final shape under shear flow.…”