Confinement induced stabilization in polymer blend thin films

Using atomic force microscopy, film stability is investigated for poly(meth yl methacrylate) (PMMA)/poly(styrene-ran-acrylonitrile) (SAN) thin film ble nds ranging from 10 nm to a few microns thick and deposited on an oxide cov ered silicon substrate. In addition to characterizing the surface morpholog y, the PMMA rich phase which wets the surface and oxide, is selectively etc hed to reveal the underlying or interfacial phase morphology. For 50/50 ble nds at 158 degreesC, films are found to be stable for thickness values betw een 20 and 100 run. Dewetting and phase separation mechanisms are invoked t o understand why films are unstable below and above this range, respectivel y. By annealing 25/75, 50/50 and 75/25 thin film blends between 158 and 200 degreesC, a stability diagram is constructed and shows that blends with lo w PMMA volume fractions (i.e. wetting component) are more stable than blend s rich in PMMA. In some cases, phase separation is observed at temperatures as low as 158 degreesC, which is about 12 degreesC below the bulk lower cr itical solution temperature (LCST). Film stability is analyzed by predictin g the PMMA concentration necessary to produce a surface wetting layer at it s equilibrium thickness. (C) 2001 Elsevier Science Ltd. All rights reserved .