Rheology and Shear-Induced Alignment of Lamellar Diblock and Triblock Copolymers

The linear viscoelastic behavior of diblock and triblock copolymers of styrene and isoprene was studied in the lamellar phase both before and after alignment by large-amplitude oscillatory shear. The orientation of the lamellae, as determined by X-ray scattering, is controlled by the temperature and frequency of alignment. For triblocks, a high degree of alignment with the lamellar normal perpendicular to both the flow and shear gradient directions (perpendicular alignment) can be achieved, so that nearly terminal rheological behavior is observed at low frequencies. The diblock never aligns to this extent. Unlike the diblock, we were unable to align the triblock with the lamellae parallel to the shearing surfaces, presumably because triblocks can form bridges across lamellar domains and so hinder the sliding of lamellae past each other. The diblock lamellae have no bridges and can achieve parallel orientation at low temperatures and high alignment frequencies. This contrasts with the poly(ethylene-propylene)-poly(ethylethylene) (PEP-PEE) system, in which parallel alignment is only seen at low frequencies and low temperatures. 1