Wetting and phase separation in polymer blend films: Identification of four thickness regimes with distinct morphological pathways

Using forward recoil spectrometry and atomic force microscopy, the phase evolution of a critical blend thin film of deuterated poly(methyl methacrylate) (dPMMA) and poly(styrene-ran-acrylonitrile) (SAN) is found to depend on film thickness. Four regimes are identified as film thickness l(0) decreases from semi-infinite to below the radius of gyration, R-g. In the semi-infinite limit or regime IV ((l)0 much greater than R-g), surface directed spinodal decomposition is observed and found to agree with cell dynamic simulations. In regime III (10 R-g < (l)0 < 150 R-g), three stages of evolution are observed. During the early stage, wetting dominates and produces a dPMMA-rich/SAN-rich/dPMMA-rich structure. During the intermediate stage, the surface phase flows back into the middle layer inducing lateral phase coarsening. During the late stage, capillary fluctuations rupture the middle layer by spinodal dewetting, resulting in a final morphology with SAN-rich droplets encapsulated by dPMMA-rich wetting layers. Although regime II ( R-g < l(0) < 10 R-g) films also exhibit a tri-layer early stage, correlated holes in the middle layer spontaneously grow suggesting that this layer is too thin to support fully developed capillary fluctuations. Three stages of roughening are observed with a final morphology similar to regime III. In regime I (l0 < Rg), films roughen almost immediately after annealing in contrast to the other regimes. Initially, the surface roughness increases logarithmically with time before reaching a constant value of 2R(g). The final average droplet height, 29.5 nm, is in good agreement with a simple interfacial energy model. Whereas the final morphology for regimes I, II and III are identical, the pathways by which films roughen are distinct suggesting that erroneous conclusions can be made by simply analyzing the final morphology.