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 $(^3Q0 (𝝎) ≡ lim_{𝜸0 →0} I_{3∕1}∕𝜸^2_0 )$ by a variation of $𝜸_0$ is hindered by the increasing domain alignment at increasing $𝜸_0$ . Thus, an approach for determining $^3Q_0 (T)$ 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 $^3Q_0$ 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 $^3Q_0 (T)$ correlates to domain alignment for $T < T_{ODT}$.