Non-Einstein Rheology in Segmented\nPolyurethane Nanocomposites

Abstract

To provide a fundamental understanding regarding the\npotential of nanoparticle-induced viscosity reduction in segmented\nblock copolymer-based nanocomposites, the shear rheological behavior\nof segmented polyurethane/C₆₀ nanocomposites (NPUs) was\nstudied up to 2 wt % C₆₀, and the results were complemented\nby several dynamic and structural probes. The same microstructural\nfeatures and dynamical relaxations were revealed for each matrix (PU)\nand its corresponding NPUs in the absence of deformation, representing\nthe lack of difference in their dynamical behaviors at all studied\nC₆₀ content. Under shear, however, interesting changes\nin the terminal rheological properties of PUs were observed in the\npresence of a C₆₀ nanofiller. An anomalous terminal shear\nviscosity (η₀) reduction was found for microphase-mixed\nPUs at a low C₆₀ content of up to 0.5 wt %. However, the\nmicrophase-mixed NPUs containing a higher C₆₀ content as\nwell 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\nreduction, which appeared controllable by the degree of microphase\nseparation, the ratio of slip length to the nanoparticle size, and\nthe stiffness of segments. Accordingly, an effective slip length, <i>b</i>_eff, was considered as the key factor in controlling\nthe observed noncontinuum effects regarding the nanoparticle-induced\nviscosity reduction in the microphase-mixed PUs. When most C₆₀ nanoparticles of radius <i>r</i> are positioned on the\nhard segments (HSs) of microphase-mixed PUs, <i>b</i>_eff/<i>r</i> > 1 can lead to viscosity reduction,\nwhile the localization of C₆₀ in the vicinity of soft segments\n(SSs) increased the matrix’s η₀. A semiuniversal\ncurve was also found to roughly estimate the η₀ values\nof various NPUs.