Microdomain structure change and macroscopic mechanical response of styrenic triblock copolymer under cyclic uniaxial and biaxial stretching modes

The mechanical stretching behavior of the poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS) triblock copolymer (87 wt% polyethylene-co-butylene (PEB), 13 wt% polystyrene (PS)) was investigated under three different stretching modes and through in situ small-angle X-ray scattering (SAXS) analysis. The strain energy density function was investigated based on the stress and stretching ratio (λ) relationship under uniaxial, planar extension, and equi-biaxial stretching modes. As a result, the cross-effect of strain represented by the second invariants of the deformation tensor (I2) was identified, and only the Ogden model found to fit the data. In the cyclic stretching tests, SEBS exhibited smaller hysteresis during cyclic equi-biaxial stretching compared to uniaxial stretching. In other words, the Mullins effect was found to be more obvious for uniaxial stretching than equi-biaxial stretching. The λ and the stretching ratio obtained from the crystal planes by SAXS (λSAXS) were compared to investigate the relationship between the change in the microdomain structure and the macroscopic mechanical properties. Thus, affine deformation was found to occur in the smaller λ region for both uniaxial and equi-biaxial stretching and deviation from affine deformation occurred for uniaxial stretching in the larger λ region. This is because the entangled loops of PEB chains serve as cross-linking points when the films are stretched under equi-biaxial stretching. The mechanical stretching behavior of poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS) was investigated under three different stretching modes and through in situ SAXS analysis. SEBS exhibited smaller hysteresis during cyclic equi-biaxial stretching than that of uniaxial stretching. SAXS measurement revealed that affine deformation occurred in the smaller stretching ratio (λ) region for both uniaxial and equi-biaxial stretching modes and deviation from affine deformation occurred for uniaxial stretching mode at the larger λ region. This is because entangled loops of poly(ethylene-co-butylene) chains serves as cross-linking points when films are stretched under equi-biaxial stretching mode.