Hierarchical Structure of Polystyrene/Styrene–Butadiene Block Copolymer System Tailored by Extensional Stress and Its Relationship with Ductility–Stiffness Balance

The trade-off dilemma between stiffness, ductility, and transparency in polystyrene (PS) films significantly hinders the expansion of their applications. In this work, mechanical rejuvenation of PS films by different degrees of extensional stress field coupled with styrene–butadiene block (SB) copolymer is applied to improve the ductility without sacrifice of stiffness. The effects of strain rate and stretching temperature are systematically investigated to uncover the synergetic relationship between hierarchical structure and properties of the PS/SB copolymer system. The strain rate is beneficial to obtain nanofibrils with various aspect ratios, which distinctly affect optical transparency, while the stretching temperature acts intensively on the formation of highly robust molecular chain networks and thereby restricts chain mobility. The mechanically rejuvenated films exhibit a yield strength of 56.6 MPa and an elongation at break of 99.7%, representing increases of 1.8 and 20.0 times, respectively, compared with the unstretched films. Robust chain networks and oriented amorphous chains result in the transformation of the crack propagation. The fracture morphology changes from transverse crack to 45° diagonal crazing, which ultimately evolves into longitudinal tearing and nanofibrillation. The nanofibrils break and are pulled out of the matrix, resulting in high ductile properties. This work presents a promising and scalable industrial approach for manufacturing high-performance PS materials.