Abstract
The morphology of a family of four 3-miktoarm star terpolymers of polystyrene (PS), polyisoprene (PI), and polymethyl methacrylate (PMMA) was studied. For three of the samples, the molecular weight of the PMMA block was systematically varied while that of the PS and PI blocks were held fixed. In the fourth sample, the PS and PMMA blocks were approximately of the same molecular weight and the PI was the majority component. In all terpolymers, the system displayed a three-phase, two-dimensionally periodic microstructure of an inner PI column with a surrounding PS annulus in a matrix of PMMA. For the two samples with the longer PMMA blocks (218 and 186 kg/mol), the PI-PS and PS-PMMA interfaces were cylindrical, whereas a unique nonconstant mean curvature (non-CMC) concentric diamond prism shape of the PI and PS microdomains occurred for the sample with the lowest molecular weight PMMA block (144 Kg/mol) as well as the polymer with the longer PI block. In these star terpolymers there are three chemically different chains emanating from the same junction point. The interaction parameter between PS and PMMA is relatively low, whereas that between PI and PMMA is the highest. The star molecular architecture gives the molecule the ability to "choose" which arms directly interact in the microphase segregate state. In the present systems, the junctions lie on the PI-PS interface causing partial mixing of the PS and PMMA blocks, while minimizing the highly unfavorable contact between the PI and PMMA blocks.