SHEAR ORIENTATION AND RHEOLOGY OF A LAMELLAR POLYSTYRENE POLYISOPRENEBLOCK-COPOLYMER

A low molecular weight, slightly entangled polystyrene-polyisoprene (P S-PI) diblock copolymer is quenched from the disordered to a locally o rdered but-macroscopically disordered lamellar state and then subjecte d to large-amplitude oscillatory shearing to induce a macroscopically aligned state. Substantial alignment is achieved; as shown by X-ray sc attering; at high frequency and/or low temperature, the lamellae orien t parallel to the shearing surfaces with their normal parallel to the velocity gradient, while at low frequency and/or high temperature, the normal to the lamellae orient perpendicular to both the shearing surf aces and the velocity gradient. These same two alignment directions we re discovered earlier by Koppi et al; in shear-aligned polyolefin-poly olefin diblock copolymers, but in PS-PI, the alignment directions depe nd differently on temperature and frequency than in the polymers of Ko ppi et al. In our PS-PI, the alignment direction at each temperature a nd frequency Is that which as the lower value of the complex modulus G ; and the crossover from parallel to perpendicular alignment in our P S-PI sample occurs at a fixed value of the reduced frequency alpha(T) omega, where alpha(T) is the shift factor used to superimpose the line ar moduli. The linear viscoelastic properties of the two different ali gned states and their shift factors suggest that the parallel orientat ion is favored at high frequency for PS-PI because of the large contra st in the viscoelastic properties between the styrene and isoprene blo cks.