Large-scale Reversible Domain Restructuring Mechanism between Direct (Solvent) Immersion Annealing and Thermal Annealing in Lamellar Block Copolymer Films

Thin films of symmetric block-copolymers (L-BCP) like polystyrene-b-polymethyl methacrylate (PS-b-PMMA) assume lamellar microstructure parallel to the Si substrate due to wetting of the Si oxide layer by the PMMA domain. We investigate the origins of drastically different domain spacing achieved in dried L-BCP films after immersion in a mixture of controlled good/intermediate/poor solvent, termed direct immersion annealing (DIA), compared to thermal melt annealing. DIA has been shown to produce microdomains (~L_o/2) that are nearly 50% smaller in size compared to that produced by thermal annealing (L_o, that is also predicted by equilibrium melt theory). We show it is possible to reversibly go from one domain size to the other by switching successive annealing techniques. There is however, asymmetry in the kinetics of the reversibility processes due to the different molecular mechanisms involved in the two techniques. To explore this, in-situ Neutron Reflectivity was performed for DIA from a thermally annealed state. The interesting structural crossover between the two distinct states and the underlying kinetics of the reversibility process will be discussed in terms of chain swelling, chain diffusion and in-plane vs out-of-plane chain junction density evolution.