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 epsilon (0) and elongation t ime t on a polystyrene-block-poly(ethylene butylene)-block-polystyrene-bloc k-poly(ethylene butylene) tetrablock copolymer with a weight fraction of po lystyrene (PS) of 0.205, which displays spherical morphology. The measureme nts are carried out at high temperatures between the glass transition tempe rature of PS and the order-disorder transition temperature (T-ODT similar t o 190 degreesC) of the block copolymer under uniaxial elongation with epsil on (0) between 0.01 and 1.0 s(-1). The data exhibit strain-induced softenin g under high epsilon (0) (similar to1.0 s(-1)) at low temperatures, but str ain-induced hardening under low epsilon (0) (similar to0.01 s(-1)) at high temperatures. The stress-optical coefficient C(epsilon (0); t) is almost co nstant under high epsilon (0) at low temperatures, close to the value of lo w-density polyethylene melt (similar to2.2 x 10(-9) Pa-1), whereas it incre ases by approximately 10-50 times under low epsilon (0) at high temperature . The plots of the C(epsilon (0); t) vs, t/a(T) (a(T) being the WLF shift f actor) are roughly fitted into a single curve, indicating that the C(epsilo n (0); t) depends on t/a(T), rather than Hencky strain epsilon. Such behavi or, especially under low epsilon (0), reflects the contribution of form bir efringence Deltan(f) of the deformed PS domains. Small angle X-ray scatteri ng and transmission electron microscopy observation reveal that under high epsilon (0), the spherical PS-domains are not appreciably changed, whereas under low epsilon (0), they are deformed into cylinders and oriented along the direction of elongation, thereby resulting in the large contribution of Deltan(f). (C) 2000 Elsevier Science Ltd. All rights reserved.