Synthesis, characterization, and interaction strengths of difluorocarbene-modified polystyrene-polyisoprene block copolymers

The effect of difluorocarbene (CF₂) modification on the self-assembly of polystyrene-b-polyisoprene (PS-b-PI) copolymers was investigated. We prepared a set of fully CF₂-modified PS-b-PI copolymers (PS-b-FPI) with different molecular weights (8-13 kg/mol) and located their order-disorder transition temperatures (T_ODT) by rheology and static birefringence. Using both mean-field and fluctuation theory, we determined the temperature dependence of the interaction parameter between the styrene and CF₂-modified isoprene segments (χ_SFI). Two series of polystyrene-b-(partially CF₂-modified polyisoprene) (PS-b-(FPI-s-PI)) materials with different levels of CF₂ modification were also prepared from the reaction between difluorocarbene and two nearly symmetric PS-b-PI precursors with molecular weights of 18 and 61 kg/mol, respectively. A lamellar morphology was established for all of the PS-b-(FPI-s-PI) copolymers below the T_ODT by small-angle X-ray scattering (SAXS) and rheology. The effective interaction parameter (χ_eff) between the PS and FPI-s-PI blocks, calculated from the domain spacing determined by SAXS, passes through a minimum before increasing smoothly by a factor of 4 upon complete CF₂ modification. Using the binary interaction model originally developed for homopolymer/copolymer blends, we were able to understand the dependence of the χ_eff on the CF₂-modification extent in a quantitative manner and extracted the three pairwise interaction parameters. Rheology and static birefringence were also used to locate the T_ODT for the lower molecular weight PS-b-(FPI-s-PI) series. The T_ODT was found to behave in a manner similar to the χ_eff: a decrease was observed at initial CF₂ modification, and a sharp increase was observed upon further CF₂ modification.