Effect of Shear on the Face-Centered Cubic Phase in a Diblock Copolymer Gel

Abstract

The effect of steady or oscillatory shear on the orientation of a face-centered cubic micellar phase formed by a poly(oxyethylene)−poly(oxybutylene) diblock copolymer in an aqueous salt solution has been investigated using small-angle X-ray scattering (SAXS). Steady shear was found to orient the mesophase into a polydomain structure with the hexagonal close-packed (hcp) planes both parallel and perpendicular to the shear plane. The hcp layers were found to be randomly stacked along the shear gradient direction. Different flow mechanisms were also observed as the shear rate was increased. At γ̇ = 5 s-1 a sliding mechanism of the 2D hcp layers stacked perpendicular to the shear gradient direction was identified. This produced an increase in domain spacing compared to that for an unoriented gel that is consistent with a transition from an fcc powder to sliding hcp layers. At γ̇ = 50 s-1 a partial reorientation of the {111} planes oriented perpendicular to the shear plane was observed. Upon cessation of shear, melted grains recrystallized in a distinct orientation of an fcc crystal with a [110] direction parallel to the shear. Partial melting of grains with {111} planes oriented parallel to the shear plane only occurred at γ̇ = 250 s-1. The effect of strain amplitude of oscillatory shear was investigated at constant shear rate on samples preoriented by steady shear. The behavior noted for steady shear, specifically, partial reorientation of {111} planes was only observed in the limit of very large strain amplitudes.