Stability of polymer films as a 2D system

According to classical theory of phase transition, fluctuations in systems with low dimensions are so violent that the phase boundary between unstable and metastable states would be smeared. In this experiment, we measure the growth of surface fluctuations on an unstable polymer film with a thickness, h₀ = 5.1 nm, which is much less than the spinodal thickness, h _sp (= 243 nm) thereby the film is in the very deep unstable region. We find the film to show rupturing behavior markedly different from that of an unstable film. Specifically, nucleation of holes – a characteristic rupturing feature of metastable films – is prominent, which is surprising for a film in the very deep unstable region even provision is given to thin films being two-dimensional and hence are susceptible to broadening of the phase boundary by fluctuations. Monte Carlo simulation shows that the nucleated holes can be caused by stochastic thermal fluctuations. Our result thus confirms the broadening of the phase boundary in thin films by fluctuations to be extremely large. As a consequence, the phase behavior of thin films cannot be predicted by the mean-field calculated phase boundary, which however has been the general practice so far.