T. Kotaka et al., Elongational flow-induced morphology change of block copolymers. 2. A polystyrene-block-poly(ethylene butylene)-block-polystyrene triblock copolymer with cylindrical microdomains, POLYMER, 42(7), 2001, pp. 3223-3231
Elongational flow behavior of a polystyrene-block-poly(ethylene butylene)-b
lock-polystyrene (SEBS) triblock copolymer melt with cylindrical morphology
is investigated by elongational Row opto-rheometry (EFOR), transmission el
ectron microscopy (TEM) and small angle X-ray scattering (SAXS). The SEES f
ilm is first roll-processed to align the PS cylinders in a preferred direct
ion. The uniaxial elongation is applied either parallel (denote Case I) or
perpendicular (Case II) to the cylinder axis. Transient tensile stress sigm
a((epsilon) over dot(0); t) and birefringence Deltan((epsilon) over dot(0);
t) are measured with a constant Hencky strain rate (epsilon) over dot(0) r
anging from 0.01 to 1.0 s(-1) at various temperatures between PS glass tran
sition, T-g(PS) and the order-disorder transition, T-ODT, of the SEBS. The
data suggest that either the PS or FEB domains is preferentially elongated
in the early stage of elongation, depending on the initial alignment of the
cylinder phase. On further elongation, the elongational viscosity of the C
ase I melt exhibits strain-induced softening behavior in the final stage of
elongation, whereas that of the Case II melts clearly displays strain-indu
ced hardening behavior. The TEM and SAXS data of the samples elongated with
a (epsilon) over dot(0) = 1.0 s(-1) show that the cylinders are mostly inc
lined approximately by 40-50 degrees to the direction of elongation, wherea
s they are mostly aligned, parallel to the elongation direction on slower e
longation. The morphology of highly elongated SEES melts is governed by the
applied strain rate and temperature, regardless of the initial orientation
of the cylinders. (C) 2001 Elsevier Science Ltd. All rights reserved.