Blends of a 3-miktoarm star terpolymer (3μ-ISD) of isoprene (I), styrene (S), and dimethylsiloxane (D) with PS and PDMS. Effect on microdomain morphology and grain size

The morphologies of a 3-miktoarm star terpolymer of isoprene (I), styrene (S), and dimethylsiloxane (D), 3μ-ISD, and its blends with PDMS and PS homopolymers, having a lower molecular weight than the corresponding blocks of the star, were studied. Combined EF-TEM and 3D electron tomography clearly revealed that three kinds of cylindrical structures, composed of three different blocks, joined to fill the space and that these cylinders were curved collectively into an arc. The grain size of this star terpolymer was extremely small compared with that of other known linear block copolymers. By blending the neat 3μ-ISD with a small quantity of PDMS homopolymer (5 wt %), the morphology remained the same, but the grain size increased dramatically. It seems that the PDMS homopolymer inserts into the PDMS phase of the star terpolymer and straightens the bent cylinders, thereby increasing long-range order. However, the extra PDMS accumulates in line defects, which if controlled could be beneficial as waveguides in photonic crystals. Blending with PS leads to smaller grain sizes and unstable hexagonal symmetry, which disappears upon the addition of extra PS. Therefore, tetragonal symmetry is inherent in the neat star.