Thin film polymer blends undergoing phase separation and wetting: Identification of early, intermediate, and late stages

Using forward recoil spectrometry and atomic force microscopy, the phase ev olution of a critical blend thin film of deuterated poly(methyl methacrylat e) (dPMMA) and poly(styrene-ran-acrylonitrile) (SAN) is found to develop by three distinct stages. During the early stage, dPMMA-rich wetting layers r apidly grow at the air/polymer and polymer/substrate interfaces. A hydrodyn amic flow mechanism is proposed based on the scaling of the layer thickness with time, t(-1), and the direct observation of an interconnected, biconti nuous morphology across the depletion zone. The lateral wave number of this morphology grows rapidly as t(-1) but slows down to t(-1/3) when the phase size approaches the film thickness. During the intermediate stage, the wet ting layer thins and, concurrently, dPMMA-rich domains spanning the SAN-ric h middle grow as t(-0.41) in good agreement with an interfacially driven gr owth model. During the late stage, these capillary fluctuations eventually cause spontaneous rupturing of the middle layer resulting in an interconnec ted 2D network which eventually coarsens into isolated SAN-rich droplets en capsulated by a thick dPMMA-rich wetting layer. The surface roughness incre ases rapidly initially, reaches a constant value, and then increases at a m uch faster rate than that predicted by a trilayer model based on capillary fluctuations. (C) 2000 American Institute of Physics. [S0021-9606(00)51046- 0].