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 (λ_SAXS) 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.