(327h) Morphology Transitions in Lamellar Block Copolymer Thin Films between Direct Solvent Immersion Annealing and Thermal Annealing

Thin films of symmetric polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) block-copolymers (sym-BCP) assume lamellar microstructure parallel to the Silicon substrate due to preferential wetting of the Si oxide layer by the PMMA block. These sym-BCPs in thin film geometry can be processed with techniques like thermal annealing and direct solvent immersion annealing (DIA) to achieve this lamellar self-assembly. DIA methodology involves immersion of BCP films into a controlled solvent environment (good/intermediate/bad solvents) to cause rapid self-assembly, and has been reported to produce microdomains (~L_o/2) that are nearly 50% smaller in size compared to that produced by thermal annealing (L_o). We report reversible transition from one domain size to the other by switching between these annealing techniques successively. Further, we show that there is a large asymmetry in the reversible kinetics of the processes due to the distinct molecular mechanisms involved in the two different techniques. To explore this, atomic force microscopy and neutron reflectivity were used to characterize the surface and bulk film structure with treatment time, and intermediate photographs of the transition process were captured. The interesting structural crossover between the two distinct states, and the underlying kinetics of the reversibility process between thermal annealing and DIA will be discussed in terms of chain swelling, diffusion and in-plane vs out-of-plane chain junction density evolution.

NSF DMR# 1905996