Elongational flow-induced morphology change of block copolymers Part 1. A polystyrene-block-poly(ethylene butylene)-block-polystyrene-block-poly(ethylene butylene) tetrablock copolymer with polystyrene spherical microdomains

Simultaneous measurements of transient tensile stress and birefringence are conducted as a function of Hencky strain rate ϵ̇0 and elongation time t on a polystyrene-block-poly(ethylene butylene)-block-polystyrene-block-poly(ethylene butylene) tetrablock copolymer with a weight fraction of polystyrene (PS) of 0.205, which displays spherical morphology. The measurements are carried out at high temperatures between the glass transition temperature of PS and the order–disorder transition temperature (TODT∼190°C) of the block copolymer under uniaxial elongation with ϵ̇0 between 0.01 and 1.0 s−1. The data exhibit strain-induced softening under high ϵ̇0 (∼1.0 s−1) at low temperatures, but strain-induced hardening under low ϵ̇0 (∼0.01 s−1) at high temperatures. The stress-optical coefficient C(ϵ̇0;t) is almost constant under high ϵ̇0 at low temperatures, close to the value of low-density polyethylene melt (∼2.2×10−9 Pa−1), whereas it increases by approximately 10–50 times under low ϵ̇0 at high temperature. The plots of the C(ϵ̇0;t) vs. t/aT (aT being the WLF shift factor) are roughly fitted into a single curve, indicating that the C(ϵ̇0;t) depends on t/aT, rather than Hencky strain ϵ. Such behavior, especially under low ϵ̇0, reflects the contribution of form birefringence Δnf of the deformed PS domains. Small angle X-ray scattering and transmission electron microscopy observation reveal that under high ϵ̇0, the spherical PS-domains are not appreciably changed, whereas under low ϵ̇0, they are deformed into cylinders and oriented along the direction of elongation, thereby resulting in the large contribution of Δnf.