Rheological Investigation of the Microscopic Phase Structure in Poly(Styrene‐b‐Isoprene‐b‐Styrene) Thermoplastic Elastomer

This study systematically investigates the correlation between microphase structures and rheological behavior in three poly(styrene‐b‐isoprene‐b‐styrene) (SIS) triblock copolymers with varying polystyrene contents, employing a multi‐technique approach combining time–temperature superposition (TTS), small‐angle X‐ray scattering (SAXS), and transmission electron microscopy (TEM). The results demonstrate that the applicability of TTS in the terminal region is strongly influenced by polystyrene content. For the low‐polystyrene‐content sample (SIS1105, PS wt . = 15%), TTS fails at low angular frequencies ω , with the terminal slope increasing markedly with temperature, indicating temperature‐dependent disruption of its microstructure under dynamic shear. At lower temperatures (130°C–150°C), however, its average terminal slope stabilizes around 0.247 (≈0), consistent with a body‐centered cubic (BCC) arrangement of spherical microdomains. In contrast, high‐polystyrene‐content samples (SIS1209, PS wt . = 29% and SIS5562, PS wt . = 45%) exhibit perfect TTS behavior, with nearly constant terminal slopes averaging 0.330 (≈1/3) and 0.470 (≈1/2), corresponding to hexagonally packed cylindrical (HEX) microdomains and alternating lamellar (LAM) microphase structures, respectively. These results are in excellent agreement with SAXS and TEM observations.