Effect of selective perfluoroalkylation on the segregation strength of polystyrene-1,2-polybutadiene block copolymers

A series of partially fluorinated (C₆F₁₃H-modified) polystyrene-b-1,2-polybutadiene (PS-b1,2-PB) copolymers were prepared by the reaction of a model PS-b-1,2-PB copolymer with n-C₆F₁₃I followed by substitution of hydrogen for the iodine by hydrogenolysis. The morphologies and domain spacings of the ordered samples at 120 and 195 °C were determined as a function of C₆F₁₃H modification extent using small-angle X-ray scattering (SAXS). From the domain spacings an effective interaction parameter (χ_eff) between the PS and partially C₆F₁₃H-modified 1,2-PB blocks was estimated. The χ_eff at 120 °C increases significantly with fluorination, from ca. 0.05 (0 mol% C₆F₁₃H modification) to ca. 0.75 (80 mol % C₆F₁₃H modification). The order-disorder transition temperature (T_ODT) for a partially C₆F₁₃H-modified PS-b-1,2-PB (25 mol %) was determined to be 186 ± 3 °C, using rheology and static birefringence. From self-consistent mean-field theory χ_eff for this block copolymer is estimated to be 0.16 at T_ODT, which agrees well with the value of 0.21 estimated from the domain spacing. These estimated interaction parameters demonstrate that a selective perfluoroalkylation approach can strongly enhance the incompatibility of PS-b-1,2-PB copolymers and is a powerful method for examining large regions of the block copolymer phase diagram with a single precursor molecule.