Building Bridges by Blending: Morphology and Mechanical Properties of Binary Tapered Diblock/Multiblock Copolymer Blends

Abstract

This work explores the scope and limitations of enhancing the poor mechanical properties of diblock copolymers by blending with tapered multiblock copolymers of styrene (S) and isoprene (I), P(I-co-S)n. Blending of different tapered diblock copolymers (n = 1; Mn = 80 and 240 kg mol−1, 50 wt% polyisoprene (PI) units, lamellar morphologies) affords brittle materials with low elongation at break. An increasing degree of phase separation from (i) miscible P(I-co-S)/P(I-co-S)n copolymer blends, to (ii) partially miscible and (iii) finally immiscible blends is studied. The effect of miscibility on the mechanical properties is studied for two diblock copolymers (Mn = 80 and 240 kg mol−1, domain spacing of 38 and 77 nm, respectively), blended with a series of multiblock copolymers P(I-co-S)n (n = 2–5; domain spacing of 42 to 20 nm) of similar molecular weight. Increasing disparity in the domain spacing results in partially miscible and finally immiscible blends. Immiscibility causes lower elongation at break, albeit superior tensile properties compared to the pure tapered diblock copolymers are maintained. The study shows that the addition of a minor fraction of multiblock copolymers to diblock copolymers is a versatile method toward improved mechanical properties, while retaining an ordered nanophase-separated morphology.