Sequential Thermal and Solvent Immersion Annealing for Self-Assembly of Block Copolymer Films: Hiking the Free Energy Landscape

The kinetics and morphology of block copolymer (BCP) assembly in films are highly dependent on processing pathways that may include thermal annealing (TA), solvent vapor annealing (SVA), and direct immersion annealing (DIA). Thus, it is only to be expected that sequential/parallel annealing of the same BCP film by two or more techniques can produce unique synergies of ordering kinetics, morphology evolution, and asymmetry in reversibility mechanisms. Here we demonstrate a combinatorial DIA and TA method, where an instantaneous DIA rapidly induces weak parallel lamellar ordering with highly reduced domain sizes that rapidly transform into standard TA equilibrium domain structures when subjected to TA. Overall, the DIA plus TA shows much faster processing kinetics than TA alone for a similar degree of ordering. We then explored the reverse ordering process of TA followed by DIA and observed very different kinetics and morphology evolution, with highly irregular intermediate structures but with the same final state of DIA metastable structure. Considering the structural crossovers observed, a chain rearrangement mechanism for transition between the two distinct morphologies is proposed, and the underlying dynamics of this reversibility process are analyzed in terms of chain swelling, diffusion, and in-plane vs. out-of-plane interfacial evolution. The transitions are plotted on a free-energy diagram to elaborate on the asymmetry in processing kinetics.