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 Å 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 $R\left(t\right)\sim t^n$ of the coarsening pattern, is predicted to change from 1 to a value near 0.46 upon lowering the dimensionality.

• Received 13 December 1995

DOI:https://doi.org/10.1103/PhysRevLett.76.4368