Nonlinear Melt Rheology of Lamellae Forming Polystyrene-b-Poly-2-Vinylpyridine Diblock Copolymers

Lamellae forming polystyrene-b-poly-2-vinylpyridine diblock copolymer melts are investigated with linear shear rheology and Fourier transformation rheology (FT rheology) to quantify their nonlinear behavior under oscillatory shear via mechanical higher harmonic contributions such as I_3/1(ω₁, γ₀). The determination of the zero-shear nonlinearity (${}^3{Q}_0\left(\omega \right)\equiv {\lim }_{{\gamma }_0\to 0}{I}_{3/1}/{\gamma }_0^2$) by a variation of γ₀ is hindered by the increasing domain alignment at increasing γ₀. Thus, an approach for determining ³Q₀(T) by a variation of the temperature is developed and used. This approach allows obtaining insights on the nonlinear behavior directly at temperature-dependent phase transitions, such as at the order-disorder transition temperature T_ODT of block copolymers. The maximum of ³Q₀ is found to be close to T_ODT. The nonlinearity originating from the connection of the unequal polymer blocks is shown to dominate the overall nonlinearity, and the maximum of ³Q₀(T) correlates to  domain alignment for T < T_ODT.