Influence of Shear on the Alignment of a Lamellae-Forming Pentablock Copolymer

We characterize shear-induced states of order and disorder in a symmetric lamellae forming poly(cyclohexylethylene) (C)−polyethylene (E) CECEC pentablock copolymer. When subjected to a large amplitude reciprocating shear strain (γ = ±600%) in the ordered lamellae state, the sample rapidly aligns in a perpendicular orientation with long-range order.

The order−disorder transition temperature TODT(γ̇) decreases with increasing shear rate γ̇, such that a shear rate of γ̇ = 7 ± 1 s-1 is sufficient to disorder the material nearly 30 °C below the quiescent TODT. Small-angle neutron scattering (SANS) from the sheared disordered state displays a symmetry that suggests a fundamentally different influence of shear compared to earlier observations of shear-induced anisotropy in the disordered states of lamellae forming diblock and triblock copolymers.

Abrupt cessation of shear leads to spontaneous lamellar ordering into a macroscopically well-aligned transverse orientation—an arrangement that previously has been inaccessible via processing. In contrast to this behavior, the perpendicular alignment forms when cooling the sample from above the TODT while applying low shear rates, similar to the response of di- and triblock copolymers.

These findings shed fresh insight into the complex phenomena that govern flow- and deformation-induced alignment of block copolymer melts.