Rheology of Dendrimers. 2. Bulk Polyamidoamine Dendrimers under Steady Shear, Creep, and Dynamic Oscillatory Shear

Abstract

Rheological behavior of the first eight generations of bulk polyamidoamine (PAMAM) dendrimers, having nominal molecular weights from about 500 to over 116 000, was investigated under steady shear, shear creep, and dynamic oscillatory shear within the temperature range from T g + 15 °C to Tg + 105 °C. It was found that these dendrimers exhibit (a) constant viscosity at small deformations regardless of the type of stress applied and (b) temperature-/generation-dependent non-Newtonian response at higher shear rates/frequencies. The latter was characterized by finite moduli of elasticity at all generations and by onset of complex-viscosity thinning at some generation-dependent critical temperature and shear frequency. These results represent the first observation of elasticity in one dendrimer family, and they indicate that at rest in bulk these dendrimers collapse, deform, and organize into transient, secondary (i.e., hydrogen)-bonded, quasi-network supramolecular microstructure. They also reveal a distinct change from single-relaxation-mode to a multirelaxation-mode Maxwell-type behavior at generation 4, which is consistent with the closure of dendrimer molecular surface upon itself and the earlier proposed soft interior-dense shell model of intramolecular dendrimer morphology. Further support for this model resulted from an analysis of dendrimer free volume, which exhibited significant contribution not only from the dendrimer end groups but also from their interior building blocks. To account for these observations, a model is proposed that involves dynamics of structural elements that are smaller than the overall dendrimer molecules.

Physical and thermal properties of UV curable waterborne polyurethane dispersions incorporating hyperbranched aliphatic polyester of varying generation number

“…Despite this importance, some recent studies on the melt rheology of certain commercial aliphatic hyperbranched polyester indicates that such materials with lower generations may exhibit shear-thinning behavior due to the interactions between the functional groups on the molecular periphery, whilst Newtonian behavior was observed for materials with larger molecular weight (higher generations) [34,35]. Non-Newtonian phenomena with supermolecular interdendrimer hydrogen bonding have also been reported recently by Uppuliri et al [36].…”

Internal Dynamics of Dendritic Molecules Probed by Pyrene Excimer Formation

“…NMR [46,51,59,60], neutron scattering [61][62][63], dielectric relaxation spectroscopy [64,65], rheology [66], fluorescence anisotropy [67], and molecular and Brownian dynamics computation [68][69][70][71] have provided some information about the mobility of the different segments constituting a dendrimer, generally via the rotational correlation time (τ C ) of the segments. The overall conclusion of these studies is that the chain ends are more mobile than the internal segments, and that their rotational correlation time is not much affected by increasing G values.…”

Orientational mobility and relaxation spectra of dendrimers: Theory and computer simulation

“…At the same time their relaxation properties, which are very important for many practical applications, are poorly understood. [9][10][11][12][13] Computer simulation [14][15][16][17][18][19][20][21][22] is one of the perspective methods to study the dendrimer relaxation properties. Obtained simulation results correlate well with the existing experimental data on large-scale relaxation processes, such as diffusion 14 and intrinsic viscosity.…”