Dimensional crossover in the phase separation kinetics of thin polymer blend films

The kinetics of phase separation in thin polymer blend films of polystyrene and polybutadiene on a silicon substrate is examined by optical microscopy of the free film boundary. Our observations on 1000 and 200 Angstrom films are consistent with a crossover from three- to two-dimensional spinodal decomposition kinetics in the (off-critical) viscous hydrodynamic regime. In this stage of phase separation the exponent n, characterizing the scale R(t) similar to t(n) of the coarsening pattern, is predicted to change from 1 to a value near 0.46 upon lowering the dimensionality.