Lamellar diblock copolymers under large amplitude oscillatory shear flow: Order and dynamics

Abstract

AbstractA survey is given on the effects of large amplitude oscillatory shear (LAOS) flow on the orientation behavior of lamellar polystyrene‐block‐polyisoprene (PS‐b‐PI) diblock copolymers with emphasis on the identification of key parameters that govern the observed behavior. For LAOS at temperatures close to the order‐disorder transition temperature, TODT, three orientation regimes related to the dynamic shear viscosity of the polymers are observed: parallel orientation at high frequencies, perpendicular orientation at intermediate frequencies, and parallel orientation again at very low shear frequencies. In annealing experiments it is found that the low frequency orientation behavior depends strongly on the thermal history of the samples. In conceptually new “dual frequency” experiments the effect of the superposition of two deformation components with frequencies in two different orientation regimes is described. From further studies identifying strain as the control parameter in these nonequilibrium experiments an orientation diagram for the long time orientation behavior of lamellar PS‐b‐PI diblock copolymers is constructed as a function of strain and frequency. For intermediate time scales in the experiments it is shown that the third orthogonal orientation component, the transverse orientation, can be obtained as a transient state. For all orientation components possible orientation mechanisms are discussed.