Dynamics of Shear Alignment in a Lamellar Diblock Copolymer: Interplay of Frequency, Strain Amplitude, and Temperature

Abstract

Rheo-optical methods are used to examine the combined effect of shear frequency, strain amplitude, and temperature on the direction and kinetics of flow-induced alignment in lamellar block copolymers. The development of shear-induced alignment in a nearly symmetric polystyrene-polyisoprene diblock (ODT ≃ 164 °C) is recorded in real time using flow birefringence as a probe of the transient lamellar orientation distribution. As alignment progresses during large amplitude oscillatory shearing, the birefringence shows an initial fast and a later slow change. While increasing strain amplitude (γ o ) generally speeds both the fast and the slow processes, below a critical γ o the slow process is not observed and a well-aligned state is not achieved. The transient birefringence observed at a particular frequency and temperature, but different strain amplitudes, can be partially superposed by scaling time with γ o n(ω) . However, the fast and slow processes require different values of n(ω). Estimates of n(ω) show that effects of strain are highly nonlinear and stronger than the simple rescaling of time in terms of either cumulative strain (∼tγ o ) or cumulative flow energy (∼tγ o 2 ). The effect of temperature enters most strongly through the shift of time scale of molecular relaxations (a T ).