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How Does Microstructural Design Affect the Dynamics and Rheology of Segmented Polyurethanes?

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journal contribution
posted on 24.06.2020, 20:31 by Hamid Reza Heydarnezhad, Naser Mohammadi, Arantxa Arbe, Angel Alegria
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 (ζeff) and BDS (ζeffRouse) were compared at the same temperatures. For highly microphase-mixed system with liquid-like terminal behavior, ζeffRouse was consistent with ζeff. For microphase-separated PUs with their nonterminal behavior, however, a fairly large difference between ζeff and ζeffRouse 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.