Role of SBS-Regulated Interfacial Entanglement in Foaming s-SEBS/PMCs/SBS Composites

Foamed composites comprising star-shaped styrene-ethylene/butylene-styrene copolymers (s-SEBS), polymer microcapsules encapsulating liquefied hydrocarbons (PMCs), and styrene-butadiene-styrene copolymers (SBS) were prepared via a continuous extrusion process using a twin-screw extruder, in which PMCs serve as physical blowing agents. The effects of the molecular weight and chain structure of SBS on interfacial compatibility, rheological properties, cell morphology, mechanical performance, and microscale modulus distribution in s-SEBS/PMCs/SBS composites were systematically evaluated. Heterogeneous and homogeneous entanglements between SBS and s-SEBS are proposed as the basis for the SBS-mediated interfacial regulation. Linear SBS-8W exhibits the lowest interfacial tension (0.89 mN/m) and highest work of adhesion (55.23 mN/m) with s-SEBS, suggesting their optimal compatibility. Under these conditions, the composite exhibited the highest complex viscosity and storage modulus at 180 °C, enhancing the melt's cell-stabilizing capability. Atomic force microscopy (AFM) analysis shows that polystyrene (PS) hard segments formed uniformly sized, well-dispersed nanoscale reinforced domains within the blend matrix. This microstructure allowed the s-SEBS/PMCs/SBS-8W foamed composites to achieve the most regular and homogeneous closed-cell morphology (97.8%), the highest cell density (6.03 × 105 cells/cm3), and a favorable balance between specific tensile strength (4.15 MPa·cm3/g) and elongation at break (1400.88%). This study provides insights into the mechanism by which adjusting the molecular weight of a secondary component optimizes interfacial and rheological behaviors in polymer blends, offering a basis for designing and fabricating high-performance thermoplastic elastomer foams.