Distinguishing Linear from Star-Branched Polystyrene Solutions with Fourier-Transform Rheology

Distinguishing between Linear and Star Polystyrenes with Unentangled Arms by Dynamic Oscillatory Shear Tests

Linear and nonlinear viscoelastic properties of star polystyrene (PS) melts with unentangled arms were measured by using small-amplitude and medium-amplitude oscillatory shear (SAOS and MAOS) tests. For comparison purposes, such tests were also conducted on entangled linear and star PS melts. Interestingly, the linear viscoelastic properties of unentangled star PS were quantitatively described using the Lihktman-McLeish model for entangled linear chains, indicating that unentangled stars were indistinguishable from linear chains by using relaxation spectra. By contrast, the relative intrinsic nonlinearity (Q0), one of the MAOS material functions, exhibited a difference between unentangled star and linear PS. When the maximum Q0 value (Q0,max) was plotted against the entanglement number of span molecules (Zs), unentangled star PS exhibited larger Q0,max values than linear PS, which was quantitatively predicted via the multimode K-BKZ model. Therefore, in the unentangled regime, star PS was concluded to be characterized by intrinsically higher relative nonlinearity than linear PS.

Rheo-chemistry in Reactive Processing of Polyolefin

A brief review of reactive processing of polyolefins, which are the most widely used polymers today, is given in this paper from the role of flow field in processing and the characterization of the modified chain topologies. The materials are mainly focused on polyethylene (PE), polypropylene (PP) and polyolefin elastomer (POE) here. Aspects of reactions, coupling/crosslinking and scission are discussed both in terms of the mechanism and kinetics under different flow conditions.

Analytical Rheology of Polymer Melts: State of the Art

The extreme sensitivity of rheology to the microstructure of polymer melts has prompted the development of “analytical rheology,” which seeks inferring the structure and composition of an unknown sample based on rheological measurements. Typically, this involves the inversion of a model, which may be mathematical, computational, or completely empirical. Despite the imperfect state of existing models, analytical rheology remains a practically useful enterprise. I review its successes and failures in inferring the molecular weight distribution of linear polymers and the branching content in branched polymers.

Liquid-to-solid transition of concentrated suspensions under complex transient shear histories

Suspensions containing noncolloidal aluminum particles in a Newtonian carrier liquid and the effects of shear history on the rheological properties of suspensions are investigated showing a critical concentration phi(c), which is close to the "freezing" packing fraction of spherical particle suspensions. An apparent liquid-to-solid transition was clearly found for suspensions below phi(c) via transient step shear after long shear history as well as via the large amplitude oscillatory shear flow with controlled shear stress. The microstructures or the local flow characteristics of suspensions with phi<phi(c) and with phi>phi(c) are still different according to the Fourier Transform Rheological analysis. Although suspensions with phi<phi(c) show solidlike behaviors after long shear history, the flow behaviors keep unchanged as shear stress increases, and typically show solidlike, liquid-solid coexistence, liquidlike, and jamming behaviors, successively. However, liquidlike regime of concentrated suspensions with phi>phi(c) disappears after preshear.