Non-Einstein Rheology in Segmented Polyurethane Nanocomposites

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.