Microphase Separation in Model 3-Miktoarm⁺Star Copolymers (Simple Graft) and Terpolymers. 1. Statics and Kinetics

The static and kinetic aspects of the order-disorder transition (ODT) in newly synthesized model 3-miktoarm star copolymer (simple graft) of SI2 type, (polystyrene)(polyisoprene)₂, and a 3-miktoarm star terpolymer of SIB type, (polystyrene)(polyisoprene)(polybutadiene), have been studied using smallangle X-ray scattering (SAXS) and rheology. The morphology and the order-disorder transition temperature (T_ODT) have been identified from the two-dimensional SAXS patterns with shear-oriented samples. Hexagonally ordered cylindrical microdomains aligned along the direction of shear and with T_ODT ═ 379 K have been formed for both samples studied. The SAXS profiles at temperatures well above the T_ODT have been fitted to the mean-field theory (MFT) for graft copolymers. Near the ODT deviations from the theory have been observed and the SAXS data provide unambiguous evidence for the existence of fluctuations. The T_ODT obtained from rheology is in excellent agreement with the one from SAXS. Discontinuities in the SAXS peak intensity and in the storage modulus near file T_ODT are more pronounced in these systems as compared to linear diblocks. The ordering kinetics have been studied with rheology and complementary with SAXS. The width of the kinetically accessible metastable region is enlarged as compared to linear diblocks. For shallow quenches the ordering proceeds by heterogeneous nucleation and growth of three-dimensional grains with cylindrical microstructure. Our kinetic studies probe the metastable states near but below the T_ODT.