No abstract

We examined the dynamics of dewetting of a thin symmetric diblock copolymer film on a substrate above the bulk order−disorder transition temperature, T ODT, of the copolymer using atomic force microscopy. The dewetting mechanism proceeded with the formation of discrete holes without their characteristic peripheral rims. During this early stage, the hole radii R increased exponentially with time. This stage was followed by a narrow intermediate regime where the rim develops and R ∼ t. When the rim was fully developed, R increased as t 2/3. The shape of the rim was highly asymmetrical and its width L increased as t 1/2. At the final stage of the process, droplets of the copolymer, a few microns in diameter and with heights on the order of tens of nanometers, existed on a dense copolymer “brush” of uniform thickness 7 nm anchored to the substrate. This clearly indicates that the process is autophobic, a phenomenon first documented in small molecule liquids.

Thin symmetric poly(styrene-b-methyl methacrylate) (PS-b-PMMA) diblock copolymer films at temperatures higher than the bulk order−disorder transition temperature T ODT are shown to dewet silicon substrates, forming topographical features that depend on the initial film thickness h. Films of thickness h < 3.5 nm dewet the substrate, forming bicontinuous spinodal-like patterns. When 3.5 nm < h < h L = 7 nm, discrete holes are observed randomly throughout the film. For films of thickness in the range h L < h < h H = 35 nm, the copolymer exhibited autophobic behavior, whereby the top layer of thickness (h − h L) dewets a dense “brush” of ordered copolymer of height h L anchored to the silicon substrate. The morphologies, which include a bicontinuous spinodal pattern for films of thickness in the range h L < h < 19 nm, and discrete holes, for films of thickness in the range 19 nm < h < h H, eventually evolve into droplets. Films of h > 35 nm remained stable, with smooth surfaces. The time-dependent evolution of the “spinodal” structures that evolve in the autophobic regime is discussed. In addition, the existence of surface-induced ordering of the copolymer at temperatures above the bulk T ODT is also discussed.

The substrate is shown to induce substantial ordering in diblock copolymer thin films above the bulk order-disorder transition (ODT) where, thermodynamically, a phase mixed state is favored. Initially, uniform films reorganize to form a hierarchy of transient surface patterns and stable film thicknesses that depend on the initial film thickness and on the substrate. Self-consistent field calculations of the free energy of the system for different situations, depending on the relative tendency for the different block components to be attracted to the substrate and/or free surface, provide an explanation of the formation of the stable film thicknesses. A continuum picture proposed earlier by Brochard et al. provides an explanation of the wetting characteristics of this system. In some cases the ordering destabilizes the film so that dewetting occurs (wetting autophobicity), whereas in other cases the surface ordering results in a kinetic stabilization of a film that would otherwise dewet.

We examined the late-stage structural evolution of droplets of a polystyrene-b-poly(methyl methacrylate) PS-b-PMMA diblock copolymer on a PS-b-PMMA “substrate”. The initial droplet size distributions and shape distributions varied appreciably from one sample to another. Coarsening of the droplets occurred, wherein the average droplet size, 〈S〉, increased with time, 〈S〉 ∝ t β , accompanied by a decrease in the number of droplets per unit area with time, N(t) ∝ t -β. The value of the power law exponent β was found to increase as the size distribution and shape distribution of droplets decreased, from 1/10 to approximately 2/5. The droplet probability distributions, F(S/〈S〉) vs S/〈S〉, determined from our data were compared with theoretical probability distributions and the results strongly indicate that the mechanism of coarsening occurs via a self-similar, dynamic coalescence process, not Ostwald ripening. The differing exponents appear to be associated with quantitative differences between the details of the coarsening process as the droplets become smaller with a narrow shape distribution. We suggest evidence of a possible dimensional crossover from 3D to 2D dynamics.