Kinetics of Block Copolymer Phase Segregation during Sub-millisecond Transient Thermal Annealing

Abstract

Early stage phase segregation of block copolymers (BCPs) critically impacts the material’s final structural properties, and understanding the kinetics of these processes is essential to intentional design of systems for practical applications. Using sub-millisecond lateral gradient laser spike annealing and microbeam grazing incidence small-angle X-ray scattering, the ordering and disordering kinetics of cylinder forming poly(styrene-b-methyl methacrylate) (PS-b-PMMA) were determined for peak annealing temperatures up to 550 °C for dwells (anneal durations) ranging from 250 μs to 10 ms. These temperatures, far in excess of the normal thermal decomposition limit, are enabled by the short time scales of laser annealing. From initially microphase-segregated films, disordering was observed near the equilibrium order–disorder transition temperature (TODT) for dwell times above 10 ms but was kinetically delayed by diffusion for shorter time scales, resulting in suppression of observed disordering by over 70 °C. The onset of ordering from initially disordered films was also kinetically limited for short dwells. For anneals with peak temperatures well above TODT, the block copolymer fully disorders and quenches to a history-independent final state determined by the quench rate. This kinetic behavior can be represented on an effective Tg and TODT phase map as a function of the heating time scale. These results then potentially enable BCP processing to retain or intentionally modify the initial state while accelerating kinetics for other chemical or structural alignment processes.