Dynamic Interfacial Fluctuations and Phase Separation Mechanisms Captured in situ with Environmentally Controlled Atomic Force Microscopy

Block copolymers are of broad interest to both fundamental science and technology applications, providing a platform to study phase separation and self-assembly phenomena or for applications in nanolithography and templating. Here, I present the use of environmentally controlled and high-speed atomic force microscopy (AFM) to study the dynamics of poly(styrene-block-methyl methacrylate) (PS-b-PMMA) thin films in situ above the glass transition temperatures (T_g). Imaging under these experimentally challenging thermal annealing conditions enables in situ observation and measurement of the domain dynamics and self-assembly, including interfacial fluctuations, pattern roughness, and two-dimensional phase separation mechanisms. Topographic confinement in nanoscale lithographic channels induces ordering in the PS-b-PMMA nanopattern. By disabling the slow-scan axis, we achieve an effective imaging time resolution to 50 ms per scan, allowing in situ observation of the dynamic interfacial fluctuations and pattern roughness when imaging at temperatures above T_g. Additionally, we utilize environmentally controlled AFM to track the mesoscale structural evolution of incommensurate PS-b-PMMA thin films without topographic confinement, imaging as the film undergoes thermal annealing to analyze the growth of terraces and holes in situ. From ensemble growth statistics acquired from time series AFM images, we see homogeneous nucleation and growth consistent with classical nucleation theory. Scaling behavior in the coarsening period differs from predictions, but this discrepancy is resolved through image analysis, revealing that Ostwald ripening and coalescence contribute to coarsening simultaneously and we estimate the relative contributions of each mechanism. These experiments together demonstrate the utility of high-speed AFM for capturing polymer dynamics across length and time scales.