Unconventional Spinodal Surface Fluctuations on Polymer Films

We study the temporal growth pattern of surface fluctuations on a series of spinodally unstable polymer films where the instability is adjustable by the film thickness, $h_0 $. For the most unstable film studied (whose $\left| {\frac{h_0 -h_{sp} }{h_{sp} }} \right|=0.988$; $h_{sp} $ is the thickness where the second derivative of the interfacial potential of the film equals to zero), the growth rate function of the surface modes as a function of the wavevector fits well to the mean-field theory. As the film thickness is increased such that $\left| {\frac{h_0 -h_{sp} }{h_{sp} }} \right|\le 0.977$, the mean-field theory demonstrates marked disagreement with experiment, notwithstanding provision of the known corrections from high-order terms and thermal noises. We show that the deviations arise from large-amplitude fluctuations induced by homogeneous nucleation, which is not accounted for in the conventional treatments.