Rheology of dendrimers. 2. Bulk polyamidoamine dendrimers under steady shear, creep, and dynamic oscillatory shear

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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 Tg+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.

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