Transient Target Patterns in Phase Separating Filled Polymer Blends Alamgir Karim,* Jack F. Douglas,* Giovanni Nisato, Da-Wei Liu, and Eric J. Amis Polymers Division, NIST, Gaithersburg, Maryland 20899 Received March 23, 1999; Revised Manuscript Received June 16, 1999 ABSTRACT: Recent simulations suggest that the presence of filler particles in a phase separating blend can induce the development of composition waves having the symmetry of the filler particles. We investigate these predictions through atomic force microscopy (AFM) measurements on ultrathin (L ≈ 100 nm) polystyrene and poly(vinyl methyl ether) blend films containing a low concentration of model filler particles (silica particles having a nominal diameter ≈100 nm). The filled blend films were spun- cast on acid-cleaned silica wafers, and phase separation was induced by a temperature jump into the two-phase region (T ≈145 °C) of the bulk polymer blend. By rinsing off the polymer film with solvent, we show that the silica particles are associated with the substrate so that the filler particles represent a quenched disorder perturbation of the film phase separation. The presence of the filler particles leads to the development of circular composition waves (“target patterns”) about the filler particles during the intermediate stage of phase separation. These target patterns disintegrate as the background spinodal phase separation pattern becomes much larger than the filler particles. Our observations are consistent with idealized two-dimensional Cahn-Hilliard-Cook simulations of the phase separation of polymer blends having a small concentration of filler particles. Introduction The properties of fluid mixtures are characteristically insensitive to their microscopic structure near the critical point for phase separation and are instead governed by large-scale equilibrium fluctuations in the local composition. Apart from shifts in critical param- eters describing the average properties of the fluid (e.g., critical temperature and composition, apparent critical exponents), the influence of microscopic heterogeneities on the properties of near critical mixtures tends to become “washed out” in the large-scale fluid properties. This situation changes when the fluid mixture enters the two-phase region. The fluid is initially far from equilibrium and can become sensitive to small pertur- bations that can grow to have a significant influence on the large-scale phase separation morphology. In- evitably, the theoretical description of this kind of self- organization process is complicated by various non- universal phenomena associated with the details of the simulation model or the experimental system. It be- comes important, for example, whether the fluids are nearly Newtonian or viscoelastic (glassy, entangled, exhibiting hydrogen-bonding interactions or other tran- sient associations, etc.) or whether a density mismatch or some other differential response of the mixture to its environment exists between the fluid components. The beneficial aspect of this sensitivity of phase separation to perturbations is that it offers substantial opportuni- ties to control the structure of the phase separating mixture. Many previous studies have considered the applica- tion of external fields (electric and flow fields, temper- ature gradients, gravity, etc.) to perturb phase separa- tion, but there are fewer investigations of geometrical perturbations. Recent investigations of the perturbation of phase separation arising from the presence of a plane wall are perhaps the simplest and best understood example of a geometrical perturbation of phase separa- tion. 1-8 Measurements 1-5 and simulations 6-8 both show the development of “surface-directed” composition waves away from the boundary under the normal condition where one component has a preferential interaction with the interface. The coarsening dynamics of these com- position wave patterns is similar to bulk phase separa- tion. Apart from the scientific interest in studying the perturbation of phase separation by idealized surfaces, there are practical reasons for studying perturbation by more complex geometrical constraints such as in filled polymer blends. Polymer materials are hardly ever used in their pure form in applications. They are often filled with additives that improve their processability (lubri- cants and stabilizers) and filler particles that modify modulus and strength (carbon black, silica, glass beads and fibers, chalk, clay, mica), appearance (pigments and surfactants), conductivity (metal flakes, carbon black, carbon nanotubes), and flammability (flame retar- dants). 9 Moreover, a large number of applications neces- sitate the use of polymer blend materials (impact modified blends, barrier polymers for packaging, filled elastomers) so that the situation where the filler par- ticles interact with the phase separation process is widely encountered. 8 An understanding of polymer- filler interactions and the ramifications for the proper- ties of filled polymer blends is clearly a matter of practical interest that requires further investigation. Recent measurements have also shown that the surface-directed composition waves can be suppressed in blend films that are thin relative to the “spinodal wavelength” governing the initial scale of phase separa- tion. 10 In these “ultrathin” films (L < 200 nm) we observe that surface tension variations associated with phase separation within the plane of the film cause the height of the film to undulate. 10-12 These height fluc- tuations provide a source of contrast for imaging by AFM and optical microscopy (OM). Kinetic studies show that the in-plane phase separation in ultrathin blend films follows a pattern similar to light scattering and OM studies of phase separation from bulk fluid mix- tures. 10,11,13 The phase separation origin of these pat- terns is further confirmed by returning the films into the one-phase region through a temperature jump, 5917 Macromolecules 1999, 32, 5917-5924 10.1021/ma990439f CCC: $18.00 © 1999 American Chemical Society Published on Web 08/14/1999