(551g) Thermodynamic Mixing Rules for Block-Random Copolymers

Block copolymer chemistry is typically selected based on the targeted morphology and desired Flory-Huggins interaction parameters. Chemistry limits the range of temperatures of operability. In addition, the only free design parameter is the degree of polymerization which limits the range of domain spacings. Instead, we show that one can systematically and continuously change the effective Flory-Huggins parameter χ_eff by replacing one of the blocks with a statistically random block comprised of two monomers. This strategy of introducing block-randomness is useful only when the thermodynamic mixing rules to determine the effective parameter χ_eff are known prior to synthesis. In this work, we utilize both computational and experimental approaches to interrogate the thermodynamic mixing rules for obtaining the effective parameter χ_eff for the block-random copolymer using the parameter χ between pure blocks and composition of the random block predicted by regular solution theory. The order-disorder temperatures are identified in simulation and experiment which establishes χ_eff, and its relation to the composition of the random block is established. In experiment, symmetric polyisoprene-poly(isoprene-r-styrene) block-random copolymers are used as the model system and composition is varied. Computational simulations are carried out using a bead-spring model for representing the synthesized copolymers, and the effect of composition and degree of polymerization are established.