Orientational Ordering of a Poly(oxyethylene)−Poly(oxybutylene) Diblock Copolymer Gel under Steady Shear Flow

Orientational ordering was investigated in the hexagonally packed rod phase formed by a poly(oxyethylene)−poly(oxybutylene) diblock copolymer in concentrated aqueous solution. The resulting soft gel was subjected to steady shear in a Couette cell, with simultaneous small-angle X-ray scattering (SAXS) experiments. Orientation function coefficients were extracted from SAXS patterns to quantify the degree of orientation. It is found that the averaged Legendre polynomial orientational order parameters increase logarithmically with a linear increase in shear rate. A limiting degree of orientation of P̄ 2 ∼ 0.6 is reached for a shear rate of γ = 100 s -1, above which an increase in shear rate does not significantly improve the orientation. Time-resolved measurements reveal that the steady-state degree of orientation was developed within the gel over a time scale t ∼ 30 s and upon cessation of shear relaxed to an intermediate orientation level over a time scale of t ∼ 1 h.