Two-dimensional coarsening and phase separation in thin polymer solution films

A comprehensive experimental study on the late stages of phase separation in two dimensions was performed by phase separating thin (1.5 and 10 μm) films of off- and near-critical polymer solutions. The resulting domain-size growth rate (coarsening rate) of the dispersed phase was directly measured for several systems using optical microscopy. Domain sizes of off-critical solutions coarsen predominantly via Ostwald ripening, growing approximately as ${\mathrm{Kt}}^{1\mathrm{/}3}$ , regardless of system. For near-critical solutions, domain sizes grow approximately as ${\mathrm{Kt}}^{1\mathrm{/}3}$ at early times, with coarsening via both coalescence and Ostwald ripening, and feature a crossover to an ultimate ${\mathrm{t}}^{2\mathrm{/}3}$ growth rate. The crossover domain size decreases with increasing quench depth in a manner consistent with a transition from a thermally driven to a flow-driven coarsening mechanism. In general, K increases with area fraction of the dispersed phase for a given off-critical or near-critical system. Both average and crossover domain sizes increase with increasing film thickness, and coalescence is significantly dependent on film thickness while Ostwald ripening is not. The substrate polarity does not greatly affect off-critical coarsening, although near-critical coarsening (particularly near the crossover) is greatly affected. Other effects of polymer molecular weight, solvent viscosity, and quench depth are discussed.