Nonlinearity from FT-rheology for liquid crystal 8CB under large amplitude oscillatory shear (LAOS) flow

Abstract

This study systematically investigated the nonlinear stress behavior of liquid crystal (8CB, 4-4′-n-octyl-cyanobiphenyl) in lamellar smectic A phase under large amplitude oscillatory shear (LAOS) flow. To investigate the nonlinear stress response under LAOS flow, the nonlinearity (I3/1) from Fourier transform-rheology as a function of applied shear time (3600 s) was calculated according to changes in both strain amplitude γ0 and frequency ω. The storage modulus G′(t) and loss modulus G″(t) from the conventional rheometer program under various LAOS flow conditions decreased and reached equilibrium as a function of time. This could be attributed to shear alignment of the lamellar smectic A structure. On the contrary, with G′(t) and G″(t), the nonlinearity I3/1(t) showed three different behaviors depending on the magnitude of strain amplitude: (1) Region I: Increased (increased and reached equilibrium), (2) region II: Increased and decreased (showed maximum value; decreased and reached equilibrium), and (3) region III: Decreased (decreased and reached equilibrium) as a function of time. These three different time-dependent behaviors of nonlinearity (I3/1) were shown to be related with the alignment behavior of the lamellar structure. With stress decomposition method, the viscous and elastic stresses of 3600 s were calculated. Viscous and elastic stresses showed different behavior at region I and region III. With an equilibrium value of 3600 s, the G′, G″, and nonlinearity (I3/1) were plotted as a function of strain amplitude, γ0. Interestingly, I3/1(γ0) increased and then decreased (maximum) even though G′(γ0) and G″(γ0) only decreased with increasing strain amplitude. From these results, it can be concluded that LAOS analysis of nonlinear stress, especially I3/1 from FT-rheology, is more sensitive to microstructure than storage modulus G′ and loss modulus G″.