Non-Einstein Rheology in Segmented Polyurethane Nanocomposites

Abstract

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.

Recent advances on the structure–properties relationship of multiblock copolymers

“…On top of this, the emergence of the normal mode (), arising from long PPG SSs diffusion is another proof of the growing phases incompatibility. By combining these results with DSC, SAXS and rheology experiments in two papers, 40,41 Heydarnezhad et al provide a deep understanding of the structure–properties of their amorphous TPUs and corresponding nanocomposites that one could be very much tempted to reproduce from other MBCs. The task seems, however, more intricate for semicrystalline MBCs because of the lack of mobility of the hard phase.…”

“…(C) Broadband dielectric spectroscopy reveals the degree of phase separation in TPUs based on PPG SSs, adapted from. 41 (D) Hansen solubility parameters sphere diagrams indicating the compatibility of a polyol SS and a MDI/BDO HS with bitumen, adapted from Reference [114] crystallites, provoking their swelling. Papers by Gallu et al, 54,114,129 dealing with complex materials having industrial applications, well illustrate the need for both physical and chemical probes to access a global understanding of MBC based materials.…”

“…(B) Low‐field NMR provides a direct estimation of soft and hard phase relative fractions in PS‐PB diblock (SB) and star (KR) copolymers, adapted from Reference [122]. (C) Broadband dielectric spectroscopy reveals the degree of phase separation in TPUs based on PPG SSs, adapted from 41 . (D) Hansen solubility parameters sphere diagrams indicating the compatibility of a polyol SS and a MDI/BDO HS with bitumen, adapted from Reference [114]…”

“…Besides, thermoplastic polyurethanes (TPUs), based on diisocyanate HSs, are appreciated for their great chemical tunability, involving either crystalline or amorphous hard domains 33–36 . In this case, the word “ordered” is sometimes used to substitute “crystalline”, 37 revealing the uncertainty regarding the molecular arrangement of these complex molecules 38–41 . Lastly, polystyrene (PS) forming amorphous spherical domains in soft PB or PI is used for several decades, 2–8 and until recently, in association with soft poly(phenylene sulfide sulfone) 42 .…”

“…[33][34][35][36] In this case, the word "ordered" is sometimes used to substitute "crystalline", 37 revealing the uncertainty regarding the molecular arrangement of these complex molecules. [38][39][40][41] Lastly, polystyrene (PS) forming amorphous spherical domains in soft PB or PI is used for several decades, [2][3][4][5][6][7][8] and until recently, in association with soft poly(phenylene sulfide sulfone). 42 Note that beyond their chemical nature, their length and their number density, the HSs polymolecularity, their position in the chain and the architecture of the molecule (e.g., comb, stars …) strongly impact the phase diagram of MBCs.…”