Epitaxial Growth of Surface Perforations on Parallel Cylinders in Terraced Films of Block Copolymer/Homopolymer Blends.

Due to incommensurability between initial thickness and interdomain distance, thermal annealing inevitably produces relief surface terraces (islands and holes) of various morphologies in thin films of block copolymers. We have demonstrated three kinds of surface terraces in blend films: polygrain terraces with diffuse edges, polygrain terraces with step edges, and pseudo-monograin terraces with island coarsening. The three morphologies were obtained by three different thermal histories, respectively. The thermal histories were imposed on blend films, which were prepared by mixing a homopolystyrene (hPS, 6.1 kg/mol) with a weakly segregated, symmetry polystyrene-block poly(methyl methacrylate) (PS-b-PMMA, 42 kg/mol) followed by spin coating. At a given weight-fraction ratio of PS-b-PMMA/hPS = 75/25, the interior of the blend films forms parallel cylinders. Nevertheless, the surface of the blend films is always dominated by a skin layer of perforations, which epitaxially grow on top of parallel cylinders. By oxygen plasma etching at various time intervals to probe interior nanodomains, the epitaxial relationship between surface perforations and parallel cylinders has been identified by a scanning electron microscope.

Hierarchical Surface Instability in Polymer Films.

This study demonstrates hierarchical instabilities in thin films. The hierarchical instabilities display three morphological characteristics: (1) windmill-like patterns at the macroscale, (2) Bénard cells and striations at the microscale, and (3) holes at the mesoscale. Such hierarchical instabilities occurred when spin coating was performed on high-volatile solutions under a high relative humidity (RH) but were suppressed when spin coating was performed on low-volatile solutions regardless of the RH. The high-volatile solutions comprise poly(4-vinylpyridine) (P4VP) in methanol or ethanol. The low-volatility solutions comprise P4VP in propanol or butanol. P4VP molecular weights, P4VP concentrations, spin rates, and film thicknesses are not vital factors in forming hierarchical instability in spin-coated P4VP films. Instead, the formation of hierarchical instabilities depends on the RH and solvent types. Namely, the hierarchical instabilities are driven by Bénard-Marangoni convection, water vapor condensation, and disturbance of spin-up and spin-off stages during spin coating of highly volatile solutions under high RH. Mechanisms of hierarchical instabilities are interpreted in detail.

Film Instability Induced by Swelling and Drying.

Poly(2-vinyl pyridine), P2VP, films display a surface pattern of craters in a dried state after being immersed in aqueous solutions containing HAuCl4 and its mixtures with low contents of K2CO3. The morphologies of craters indicate that the formation of craters involves three stages through film blistering and drying: (i) the permeability of water and solutes to swell P2VP films, (ii) partial wetting of liquid droplets near the substrate interface in the presence of the P2VP film, and (iii) evaporation-driven flows. The three stages produce the swelling pressure, Laplace pressure, and interplays among capillary flows, Marangoni flows, and pinning effects, respectively, by which craters of different dimensions and morphologies are obtained. The first stage softens the P2VP films and produces swelling pressure. This stage relies on interactions between AuCl4- ions, water, and protonated P2VP chains. The second stage produces liquid droplets inside the film and near the substrate interface. The surface tensions of those liquid droplets at contact lines deform swollen P2VP films. Changing film thicknesses or substrate types alters craters' lateral dimension and depth. The results indicate that film thicknesses and substrate interface energies influence the shape and dimension of liquid droplets on the substrate interface. The third stage determines morphologies of craters through interplays among capillary flows, Marangoni flows, and pinning/depinning events.

Surface Wrinkling on Polymer Films.

A series of gold precursor solutions are prepared by dissolving HAuCl4 and its mixtures with K2CO3 of different contents in deionized (DI) water. Neat HAuCl4 predominately forms AuCl4- ions in an aqueous solution. In the presence of K2CO3, AuCl4- ions hydrolyze to form [AuCl4-x(OH)x]- complex ions. Increasing the content of K2CO3 in a gold precursor solution increases the content of [AuCl4-x(OH)x]- complex ions and decreases the content of AuCl4- ions. Poly(4-vinyl pyridine) (P4VP) films of two different molecular weights are deposited on SiOx/Si by spin coating, by which the thicknesses are controlled by polymer weight fractions in butanol. Those P4VP films form periodic wrinkles when immersed in aqueous solutions, followed by drying. The surface wrinkling is induced by swelling pressure that overwhelms the mechanical property of the P4VP film. The periodicity and amplitude of wrinkles grown on the P4VP films strongly correlate with initial thickness, AuCl4- ion content, and residual stress.

Influence of Osmotic Pressure on Nanostructures in Thin Films of a Weakly-Segregated Block Copolymer and Its Blends with a Homopolymer.

We studied the influence of osmotic pressure on nanostructures in thin films of a symmetric weakly-segregated polystyrene-block-poly (methyl methacrylate), P(S-b-MMA), block copolymer and its mixtures with a polystyrene (PS) homopolymer of various compositions. Thin films were deposited on substrates through surface neutralization. The surface neutralization results from the PS mats, which were oxidized and cross-linked by UV-light exposure. Thus, thermal annealing produced perpendicularly oriented lamellae and perforated layers, depending on the content of added PS chains. Nevertheless, a mixed orientation was obtained from cylinders in thin films, where a high content of PS was blended with the P(S-b-MMA). A combination of UV-light exposure and acetic acid rinsing was used to remove the PMMA block. Interestingly, the treatment of PMMA removal inevitably produced osmotic pressure and consequently resulted in surface wrinkling of perpendicular lamellae. As a result, a hierarchical structure with two periodicities was obtained for wrinkled films with perpendicular lamellae. The formation of surface wrinkling is due to the interplay between UV-light exposure and acetic acid rinsing. UV-light exposure resulted in different mechanical properties between the skin and the inner region of a film. Acetic acid rinsing produced osmotic pressure. It was found that surface wrinkling could be suppressed by reducing film thickness, increasing PS content and using high-molecular-weight P(S-b-MMA) BCPs.