Deformation mechanisms in lamellar S–S/B–S triblock copolymers

The deformation behaviour of two lamellar polystyrene-block-poly(styrene-co-butadiene)-block-polystyrene (S–S/B–S) triblock copolymers was investigated by small-angle X-ray scattering (2D-SAXS) during uniaxial stretching. The samples differed by the styrene content in the statistical S/B middle block, being 50 and 70 wt.% for LN50 and LN70 respectively. The positions of the SAXS reflections were estimated in order to relate the microscopic deformation behaviour with the macroscopic strain. The samples show different deformation mechanisms. A four-point SAXS pattern was found in LN50, which is typical for lamellae undulation and tilting. At higher strains, the contribution of lamellae tilting is decreasing until it is negligible at strains above 200%. During strain hardening, the deformation was affine, indicating triaxial deformation of the domains. In contrast, LN70 shows affine deformation in the whole investigated range. Thus, triaxial deformation is expected to dominate in LN70. The remarkable differences of the deformation mechanisms are expected to result from the different styrene contents in the S/B middle block. This leads to (i) a lower stiffness contrasts between the soft and hard phase and (ii) a reduced long-range order of the lamellae. Both factors may hinder lamellae tilting in LN70, which results in the absence of the four-point SAXS-pattern.