Nonlinear Influence of Blending Star and Triblock Copolymers on Morphological and Mechanical Properties of Thermoplastic Elastomers

To tune the mechanical properties of thermoplastic elastomers, a linear Poly(styrene (S)-b-isoprene (I)-b-styrene (S)) triblock copolymer (SIS) is blended with well-defined symmetric star-shaped PS-PI block copolymers with 8 and 15 arms, respectively. The model systems are synthesized via anionic polymerization and grafting-onto techniques and have a lamellar morphology with the long-range order L₀ for the SIS larger than the stars. Morphological analysis using small angle X-ray scattering (SAXS) and atom force microscopy (AFM) reveals a nonlinear dependency of L₀ as a function of the star content, ϕ_star. For small star contents, L₀ of the blends increases above L₀ of the individual compounds. The stars show significantly higher mechanical performance than the SIS, by an increased elongation at break, ε_b, by +15% and +38%, and an increased ultimate tensile strength, σ_UTS, by about a factor of 2. For the blends, the mechanical properties highly depend on the star content, and can be fully uncoupled from the ones of the SIS and the star: at small star contents ε_b is significantly improved beyond ε_b,star, but σ_UTS decreases below σ_UTS,SIS. For higher star contents, σ_UTS strongly increases toward σ_UTS,star _, whereas ε_b shows a minimum around ϕ_star = 50 wt.%, before increasing to ε_b,star.