  Mechanical stretching behavior of
poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS) triblock copolymer (87
wt% polyethylene-co-butylene (PEB) block, 13 wt% polystyrene (PS) block) was
investigated by three different stretching and in situ small angle X ray
scattering (SAXS) measurements. Strain energy density function was investigated
based on the stress stretching ratio ({\lambda}) relationship under uniaxial,
planar extension, and equi-biaxial stretching modes. As the result, cross
effect of strain represented by second invariants of the deformation tensor
(I2) existed and only Ogden model can be used to fit the data. In the cyclic
stretch testing, SEBS exhibited smaller hysteresis during cyclic equi biaxial
stretching mode than for uniaxial stretching one. {\lambda} and stretching
ratio obtained from crystal planes by SAXS ({\lambda}SAXS) were compared to
investigate relationship between microdomain structure change and macroscopic
mechanical property. SAXS measurement revealed that affine deformation occurred
in the smaller {\lambda} region for both uniaxial and equi biaxial stretching
modes and deviation from affine deformation occurred for uniaxial stretching
mode at the larger {\lambda} region. This is because entangled PEB loop chains
could work as cross-linking points when films are stretched by equi-biaxial
stretching mode.
