A versatile protocol is outlined to create A-block-(B-random-C) block copolymers (BCPs) with variable Flory-Huggins parameters, χ, while maintaining an equal surface energy (γ) of the two blocks. Polystyrene-block-poly(1,2-butadiene) (PS-b-PB) serves as a starting material for subsequent BCP modification. Thiol-ene "click" chemistry is used to modify the PB block to rapidly create random copolymers of PB and thiol-modified PB (P(B-r-Bthiol)) with different molar fractions of thiol (φ) in the PB block. The effect of thiol components and φ on χ and the γ of the blocks of the BCPs is sysmatically investigated by characterizing the phase-separated morphology and period, the wetting behavior, and the directed self-assembly (DSA) behavior of the BCPs. The φ of the P(B-r-Bthiol) block that results in a similar γ to the PS block is estimated with contact angle measurents. Small angle X-ray scattering is used to determine the effect of thiol modification on the BCP morphology. Two different PS-b-P(B-r-Bthiol) BCPs are identified from the same parent PS-b-PB via the thiol-ene “click” reaction with either mercaptoethanol or 1-thioglycerol. Both polymers show a linear dependence of γ with φ and a second order dependence of χ with φ, enabling χ to be fully decoupled from the processing restraints required for topcoat-free DSA with thermal annealing. After thiol modification, a radical scavenger, butylated hydroxytoluene, is added to suppress the cross-linking of unreacted double bonds, enabling defect-free patterning over 1 μm with DSA.

Facile Control of block surface energy via clickable modification of polystyrene-block-polybutadiene enabling topcoat-free directed self-assembly with thermal annealing

ACS Fall 2021

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