Interference of spinodal waves in thin polymer films

Abstract

We have studied the thickness dependence of the spinodal decomposition in thin films of a binary polymeric mixture. Nuclear reaction analysis (NRA) and time-of-flight forward recoil spectrometry (TOF-FRES) have been used to monitor the phase separation in thin films of poly(ethylenepropy1ene) (PEP) and perdeuterated poly(ethylenepropy1ene) (dPEP) after a quench into the two-phase region. The composition profiles are modeled by two distinct spinodal waves originating from the two surfaces of the films, and interference effects are observed as the film thickness is varied. Below a critical film thickness the coarsening of the phase structure is found to be severely altered with respect to the thick film behavior. Cell-dynamical simulations have been performed which closely resemble the experimental findings.The spinodal decomposition of binary mixtures has been a matter of considerable interest in the past, both experimentally and the~retically.'-~ Following a rapid quench from the one-phase region into their miscibility gap, bulk fluids phase separate through an exponential growth of concentration fluctuations with a characteristic, time-independent wavenumber qm. At later times, the wavenumber shifts toward zero, leading to an increasingly coarsened phase structure. This process is characterized by an isotropic structure factor, reflecting the translational and rotational symmetry of the bulk fluid. However, a quite different behavior can be expected when the symmetry is broken, e.g., in the vicinity of a surface.69 This has first been suggested by Ball and Essery on theoretical grounds6 and was confirmed shortly after by Jones et al.,l0 who observed a strongly anisotropic structure factor in the spinodal decomposition of thin films of an isotopic polymer blend. The film surface was found to be enriched in one of the components, and an oscillatory composition profile was observed perpendicular to the surface. The characteristic wavelength of this layered structure was found to grow considerably slower with respect to the bulk behavior.'l In a similar kind of experiment, Bruder and Brenn concentrated on the equilibrium phase structure at late times;12 they could show that a stable layered structure of binodal compositions can only be established when both interfaces are completely wet by one of the components. Otherwise, the competing growth of bulk domains was found to eventually destroy the layered structure.In the present study, we shall investigate the influence of the film thickness on the spinodal decomposition of a binary polymer mixture. Assuming an attraction of either of the two components to both the polymerlvacuum and the polymer/substrate interfaces, respectively, one expects two separate spinodal waves to extend into the film which eventually will interact with each other as the film thickness becomes comparable to the decay length of the oscillations. We present the first experimental evidence for both constructive and destructive interference of these waves by varying the film thickness. In additio...

Evolution of interfacial curvatures of a bicontinuous structure generated via nonconserved dynamics

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Surface-directed phase separation in nanometer polymer films: self-stratification and pattern replication

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