Synthesis, characterization, and morphology of model graft copolymers with trifunctional branch points

Well-defined graft copolymers with polyisoprene backbones and polystyrene branches, having trifunctional branch points, of the type S₂IS₂ (H-shaped) and (SI)I(SI) (π-shaped) were synthesized by anionic polymerization high-vacuum techniques. The synthetic strategy involves the preparation of the outer parts of the molecules, carrying one reactive Si-Cl bond, followed by coupling with difunctional living poly(isoprenyllithium) chains. In this way, the number and placement of the branches can be precisely controlled. Molecular characterization of the fractionated samples by size exclusion chromatography with UV and RI detection, membrane osmometry, low-angle laser light scattering, and ¹H-NMR spectroscopy confirmed that the materials exhibit narrow molecular weight distributions and low compositional heterogeneity. The strongly microphase-separated morphologies of these two samples were characterized using TEM and SAXS. The π architecture with a PS volume fraction of 0.21 was found to form body-centered cubic spheres, while the H architecture with a PI volume fraction of 0.64 was found to form a lamellar morphology. The observed morphology for these architectures was rationalized by formally dividing the π and H architectures into component simple (single) graft block copolymers which were mapped onto Milner's morphology diagram for simple graft copolymers.