Nonlinear rheological behavior of multiblock copolymers under large amplitude oscillatory shear

The nonlinear responses of olefin multiblock copolymers (OBCs) melts were investigated under large amplitude oscillatory shear (LAOS) using Fourier-Transform rheology in this work. Time-temperature superposition (TTS) was examined using linear viscoelastic functions, as well as nonlinear viscoelastic functions like the intrinsic Q factor (Q 3,0) and the dynamic moduli of the third harmonic ( G 33 , 0 ′ and G 33 , 0 ″ ). It is found that the intrinsic Q factor (Q 3,0) exhibits similar TTS behavior as those of storage moduli and loss moduli. However, the deviation from typical liquidlike behavior of homogeneous melt in Q 3,0 is much more evident than that in the storage moduli, indicating more sensitive of Q 3,0 to the microstructure than linear viscoelastic functions. In addition, we suggested a new plot, the intrinsic phase angle at third harmonic (δ3,0) versus the intrinsic complex moduli at third harmonic ( | G 33 , 0 * | ), which is named as the vGP3 plot. Using the vGP3 plot, the much weaker mesophase separation can be distinguished from the homogeneous state successfully. The high sensitivity of vGP3 to the failure of TTS makes it a new candidate in studying the thermorheological behaviors of complex fluids. Finally, the frequency dependency of intrinsic nonlinearity under LAOS was modeled by combining the molecular stress function (MSF) model and emulsion model. The MSF model can describe the experiment data of linear dynamic moduli and Q 3,0 quite satisfactorily for homogeneous OBCs. For heterogeneous melt, the MSF model and the simple emulsion model account for the homogeneous contribution and the interfacial contribution, respectively. The predictions of the combined model agree quite well with the frequency dependency of the experimental data for both linear storage moduli and intrinsic Q factor (Q 3,0).