To molecularly investigate the role of the microstructure in controlling the dynamics of segmented polyurethanes (PUs), a series of them were systematically designed, synthesized, and experimentally scrutinized. Broadband dielectric spectroscopy and rheometry along with small-angle X-ray scattering and differential scanning calorimetry were applied to provide insights into molecular origins, particularly the role of transitional friction coefficients (ζ), affecting dynamics and rheology of the various microstructures of PUs. In this way, first, the effective ζs of model PUs were extracted from their rheological data using the Bueche−Ferry procedure. Second, the experienced ζ by the hard and soft segments (SS and HS) in pure components and resultant PUs were calculated using BDS data and were applied in a Rouse model based methodology to estimate the effective ζ. Finally, the obtained effective ζ from rheometry (ζ eff ) and BDS (ζ eff Rouse ) were compared at the same temperatures. For highly microphase-mixed system with liquid-like terminal behavior, ζ eff Rouse was consistent with ζ eff . For microphase-separated PUs with their nonterminal behavior, however, a fairly large difference between ζ eff and ζ eff Rouse was found. A universal curve was also prepared to illustrate the relationship between the dynamics and microstructural features of model PUs. According to these results and the SAXS data analyses, the frictional forces acting at the interfaces between SS-rich and HS-rich phases were suggested as a main origin for the deviation from terminal Rouse-like dynamics in the microphase-separated PUs, while bare friction coefficients in their pure components play an insignificant role in the dynamics of these systems.

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To provide a fundamental understanding regarding the potential of nanoparticle-induced viscosity reduction in segmented block copolymer-based nanocomposites, the shear rheological behavior of segmented polyurethane/C60 nanocomposites (NPUs) was studied up to 2 wt % C60, and the results were complemented by several dynamic and structural probes. The same microstructural features and dynamical relaxations were revealed for each matrix (PU) and its corresponding NPUs in the absence of deformation, representing the lack of difference in their dynamical behaviors at all studied C60 content. Under shear, however, interesting changes in the terminal rheological properties of PUs were observed in the presence of a C60 nanofiller. An anomalous terminal shear viscosity (η0) reduction was found for microphase-mixed PUs at a low C60 content of up to 0.5 wt %. However, the microphase-mixed NPUs containing a higher C60 content as well as the microphase-separated NPUs showed a higher η0 than their matrices. Slippage at the polymer/C60 interfaces was suggested as a possible mechanism behind the viscosity reduction, which appeared controllable by the degree of microphase separation, the ratio of slip length to the nanoparticle size, and the stiffness of segments. Accordingly, an effective slip length, b eff, was considered as the key factor in controlling the observed noncontinuum effects regarding the nanoparticle-induced viscosity reduction in the microphase-mixed PUs. When most C60 nanoparticles of radius r are positioned on the hard segments (HSs) of microphase-mixed PUs, b eff/r > 1 can lead to viscosity reduction, while the localization of C60 in the vicinity of soft segments (SSs) increased the matrix’s η0. A semiuniversal curve was also found to roughly estimate the η0 values of various NPUs.