Switching of interfacial instabilities from the liquid/air interface to the liquid/liquid interface in a polymer bilayer

Abstract

A combined experimental and theoretical study uncovers the interfacial morphologies of a thin bilayer composed of a polystyrene (PS) upper layer and a poly-methylmethacrylate (PMMA) lower layer, coated on a Si wafer with 120 nm oxide layer. The bilayers with ultrathin PMMA layer (M_w = 15 kg mol⁻¹ and 365 kg mol⁻¹) under a thin PS (M_w = 2.5 kg mol⁻¹) layer are always found to evolve with a strongly deforming polymer/air interface over a non-deforming polymer/polymer interface. In contrast, the polymer/polymer interface is found to deform more when the PMMA layer is lower viscosity (M_w = 15 kg mol⁻¹) and thicker. With the increase in the M_w of PS films, the bilayers also show larger deformability of the polymer/polymer interface. The linear stability analysis and nonlinear simulations confirm that the increase (decrease) in the viscous resistance due to the larger (smaller) molecular weight and smaller (larger) thickness of the PMMA layer enforces lesser (bigger) deformability in the polymer/polymer interface. The computations mimicking the experiments are presented to demonstrate the switchover of the dominant mode of instability from the polymer/air to polymer/polymer interface varying the thickness of the lower layer and the viscosity of both PS and PMMA layers.