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 ( Q 0 ), one of the MAOS material functions, exhibited a difference between unentangled star and linear PS. When the maximum Q 0 value ( Q 0,max ) was plotted against the entanglement number of span molecules ( Z s ), unentangled star PS exhibited larger Q 0,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.