Pretransitional thinning of a polymer wetting layer

– Mixtures of two immiscible polymers A and B phase-separate into A-rich (α) and B-rich (β) phases. Thin films of such mixtures exhibit a transition from an equilibrium 3-layer structure (substrate/α/β/α/air) to a 2-layer structure (substrate/β/α/air) as the surface energy of the substrate is increased. In this letter we show that as an (equilibrium) precursor to this transition, the thickness of the α layer in the 3-layer structure at the substrate/polymer interface decreases, while that of the α layer against air increases until the α layer against the substrate vanishes leaving the 2-layer structure. This pretransitional behavior originates from the long-range nature of the van der Waals interaction between layers and is predicted by a simple model that considers the dependence of the free energies of the two α layers on their thicknesses. The behavior of polymers in confined geometries differs from that in the bulk due to additional energetic and entropic constraints imposed on the chains. In particular, the morphology of macromolecular thin films of phase-separated mixtures of polymers A and B is governed by the interplay between phase separation and wetting [1]. Phase separation is controlled by χN , where χ is a measure of the unfavorable polymer-polymer interactions and N is the number of segments of the polymers, and wetting is governed by the interactions of the polymers with the new boundary phases. It is of great interest to tailor the resultant morphology in thin phase-separated polymer films. This goal can be accomplished by varying different system parameters, e.g. the processing temperature, the overall film composition, and the air/polymer (polymer/substrate) interactions [1, 2]. Previous studies investigated the effect of annealing temperature and time on the phase separation in polymer films [1]. While such studies revealed that multilayer (> 3) films are possible [3], these are metastable and the final equilibrium morphology always consists of either an air/A-rich/B-rich/substrate (2-layer) structure or an air/A-rich/B-rich/A-rich/substrate (3-layer) structure, where A is the lower surface energy component. Which of these structures exists depends sensitively on the energies () E-mail: [email protected] () E-mail: [email protected] () Also at the Department of Chemical Engineering, University of California at Santa Barbara, Santa Barbara, California 93106-5080, USA.