pubs.acs.org/Langmuir © 2010 American Chemical Society
Interplay between Dewetting and Layer Inversion in Poly(4-vinylpyridine)/Polystyrene Bilayers Stuart C. Thickett,†,‡ Andrew Harris,‡ and Chiara Neto*,† †
School of Chemistry F11 and ‡School of Chemical and Biomolecular Engineering J01, The University of Sydney, NSW 2006 Australia Received August 3, 2010. Revised Manuscript Received September 7, 2010
We investigated the morphology and dynamics of the dewetting of metastable poly(4-vinylpyridine) (P4VP) thin films situated on top of polystyrene (PS) thin films as a function of the molecular weight and thickness of both films. We focused on the competition between the dewetting process, occurring as a result of unfavorable intermolecular interactions at the P4VP/PS interface, and layer inversion due to the lower surface energy of PS. By means of optical and atomic force microscopy (AFM), we observed how both the dynamics of the instability and the morphology of the emerging patterns depend on the ratio of the molecular weights of the polymer films. When the bottom PS layer was less viscous than the top P4VP layer (liquid-liquid dewetting), nucleated holes in the P4VP film typically stopped growing at long annealing times because of a combination of viscous dissipation in the bottom layer and partial layer inversion. Full layer inversion was achieved when the viscosity of the top P4VP layer was significantly greater (>104) than the viscosity of the PS layer underneath, which is attributed to strongly different mobilities of the two layers. The density of holes produced by nucleation dewetting was observed for the first time to depend on the thickness of the top film as well as the polymer molecular weight. The final (completely dewetted) morphology of isolated droplets could be achieved only if the time frame of layer inversion was significantly slower than that of dewetting, which was characteristic of high-viscosity PS underlayers that allowed dewetting to fall into a liquid-solid regime. Assuming a simple reptation model for layer inversion occurring at the dewetting front, the observed surface morphologies could be predicted on the basis of the relative rates of dewetting and layer inversion.
Introduction The spreading of a liquid polymer on a solid surface and the stability of the resulting film are issues of major importance in fundamental studies of surface physics, but they also have the potential to benefit a number of high-tech applications such as the design of new nano- and micropatterned surfaces for biosensing, biomaterials, polymer solar cells, and microfluidics. If a liquid wets a surface completely, it spontaneously forms a continuous, uniform film at the interface. Liquids that do not wet a surface because of unfavorable intermolecular forces can be forced to form uniform films on the substrate by techniques such as spin coating. However, in this case the films are generally not stable. If allowed to relax (e.g., in case of a polymer film if heated to above the glass-transition temperature Tg), the films will transform, via a symmetry-breaking process named dewetting, into their equilibrium state (i.e., a series of individual isolated droplets with a finite contact angle given by Young’s equation). Because of the importance of this phenomenon, in the past 20 years a full understanding of the dewetting of unstable and metastable liquid polymer films from flat solid surfaces has evolved.1-5 The rupture mechanisms in thin films (thickness
