Swelling behavior of ordered miktoarm star block copolymer-homopolymer blends

We report the morphological characterization of asymmetric miktoarm star block copolymers of the (PS-b-PI)_n, PS type where n = 2, 3 (denoted 2DB and 3DB miktoarm stars, respectively) and a symmetric super H-shaped block copolymer of the (PS-b- PI)₃PS(PI-b-PS)₃ type (denoted SH) which were synthesized by anionic polymerization. The initial volume fraction of PS (φ_PS) for each copolymer was 0.51-0.56, giving a lamellar morphology. Addition of homopolystyrene (hPS) with a molecular weight lower than the respective PS blocks in the neat materials lead to a transition from the lamellar structure to hexagonally packed cylinders. Addition of low molecular weight homopolyisoprene (hPI) on the other hand, only resulted in swollen lamellae even when the overall composition was highly asymmetric (80/20). Changes in the lamellar spacing as well as in the respective PS and PI layer thickness were measured by SAXS. The transition from lamellae to cylinders with increased PS content occurred without the observation of an intervening cubic morphology for the 2DB and 3DB miktoarm stars. However, blends with 30 and 35% hPS (φ_PS)_total = 0.68-0.70) with the super H-shaped block copolymer lead to the observation of lamellar-catenoid structures.