(747j) Thermodynamic Mixing Rules for Block-Random Copolymers

Block copolymer chemistry is typically selected based on the targeted morphology and desired Flory-Huggins interaction parameters, often limiting the range of temperatures of operability and design of materials. Instead, one can systematically and continuously change the Flory-Huggins interaction parameter χ between two blocks by replacing one of the blocks with a statistically random block comprised of two monomers. However, 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 establish precise thermodynamics mixing rules to obtain the effective parameter χ_eff for the block-random copolymer from the Flory-Huggins interaction parameter χ between pure blocks. In experiment, symmetric polyisoprene-poly(isoprene-r-styrene) block-random copolymers are used as the model system. The order-disorder temperature for the synthesized symmetric block copolymers is used to identify the effective parameter, and its relation to the composition of the random block is established. Computational simulations are carried out using a bead-spring model for representing the synthesized copolymers. Finally, in simulation, we test whether the order-disorder transition temperature is truly described by an “effective” interaction parameter by comparing a pure block copolymer interacting with the effective parameter χ_eff to the block-random copolymer with monomers interaction with parameter χ.