Polymer Surfaces in Motion

Structured surfaces are present in Nature for many different purposes. Self-cleaning leaves with microscopic wax patterns, antireflective properties of moth eyes, superior adhesion of gecko feet via the enhanced adhesive interaction of nano-hairs, reduced friction and wear of the skin of sandfish via nanostructured scales, or turbulence reduction near shark’ scales by micro-grooves are few examples of enhanced surface functionality by microstructuring or nanostructuring. Commercial materials with enhanced properties strongly inspired in some of these examples of Nature are already in the market. Dirt-repellent coatings and fabrics, nanostructured finish to reduce the need for car-waxing, self-cleaning vehicle windshields and mirrors, or antireflection and anti-UV coatings are some examples of surface properties enhancement by microstructuring and nanostructuring. A vast number of other applications may be anticipated for structured surfaces. Magnetic, electric, or chemical properties can be exploited to build information storage devices, displays, lighting and energy systems, actuators, and sensors. The impact of nanostructured surfaces will extend all the way from materials science and microelectronics to bioengineering. However, there is a great deal of research that needs to be conducted in this field, in order to achieve technical control of surface structure and functionality at the nanometric scale. This includes studying the structuring possibilities of new materials, achieving new structures and functionalities for old materials, as well as developing new patterning techniques.

Dewetting of thin films is a simple and thus highly convenient process for creating regularly ordered topographical patterns on various lengthscales. Here, we present a general pathway, based on dewetting of a thin polymer film, which allows to convert a chemical surface pattern of hexagonally arranged non-wettable circular patches into a sequence of ordered three-dimensional topographies. With increasing thickness of the dewetting film, cylindrical holes followed by droplets with the shape of a spherical cap and finally toroids were generated. We identified the width w of the rim, where the dewetted fluid was collected, as the crucial parameter which determined the final three-dimensional morphology. When the rim was spanning regions of varying wettability, redistribution of fluid within the rim was at the origin of morphological changes occurring in the course of dewetting. Such induced flow led to a decrease of material on the more wettable parts of the substrate and to an increase of material on the non-wettable patches, eventually causing the detachment of droplets from the rim. Such detachment of droplets occurred for films below a maximum thickness h _max determined by the maximum volume of the resulting droplet. For films thicker than h _max, flow of material between regions of different wettability was not able to create sections of the rim which were thin enough to allow for the detachment of droplets. Thus, for films thicker than h _max, the dewetted area was surrounded by a wide rim with a circular shape. Our experiments demonstrate that for a given surface pattern various three-dimensional morphologies can be obtained by simply varying the initial thickness of the thin film.

Investigation into the evaporation of a droplet consisting of nonvolatile solutes is not only for a better understanding of the mechanism that underpins the evaporation process but also for utilizing such a simple strategy it presents to craft intriguing self-assembled structures. In this chapter, we first review the recent progress on the theory of the droplet evaporation on substrate, in particular, focusing on the evaporative flux at the three-phase contact line, the radial and circular flows inside the evaporating droplet, and the interactions between solute and substrate. Subsequently, we discuss the recent advances in controlling evaporative self-assembly of solutes by restricting the evaporation process in confined geometries, including controlled evaporative self-assembly (CESA) in curve-on-flat geometries and flow-enabled self-assembly (FESA). The ability to yield well-defined dissipative structures from a variety of nanomaterials at low cost holds promise for potential applications in electronics, nanodevices, and advanced functional systems.

Block copolymers have fascinating capacities to spontaneously produce organized nanostructures, and, when shaped as thin films, chemically and topographically patterned surfaces. Once morphology is defined by the thermodynamics of the chosen block copolymer macromolecules, the type, extent, and orientation of the nanopattern obtained at the free surface of a thin film will depend on many parameters, which are now very well described in the abundant literature of the field: nanostructure period, film thickness, surface energies, type of substrate, deposition and annealing processes, to name a few. Thin films with hexagonally packed cylinders in perpendicular orientation presenting high degree of order and uniaxial symmetry are now produced with many systems, down to characteristic sizes below 20 nm, and are promising for several applications. Recent progress in copolymer synthesis, as well as in new processes, like gradient and zone processes, or addressable substrates, open many new opportunities for the design of ordered, complex and tunable patterns.

Surface modification both in terms of topography and surface functionality appears to be crucial for the development of polymeric materials for particular applications. Variations of the surface topography have been typically accomplished using different strategies based on recent technological advances such as lithography or nanoimprinting among others. Equally, changes on the chemical composition requires surface treatments either using physical approaches or chemical reactions. In general, either parameters functional groups at the surface and topography have been considered separately or the reports described combining both aspects require the use of multistep procedures. In this chapter we revise an alternative to obtain nanostructured and microstructured interfaces having in addition a particular chemical composition. The strategy reported herein takes advantage of the surface thermodynamics to induce the migration of a particular additive towards the interface. Surface segregation is the result of the preferential migration of one blend component to the interface thereby inducing selective enrichment at the near-surface level.As reported here, the use of block copolymers as additives may result in nanostructured domains at the interface. The strategies, alternatives, and methodologies reported to prepare such interfaces are thoroughly described in this chapter.

The ability to control polymer film structure has wide-ranging applications in the fields of optical coatings, electronics, organic photovoltaics, and biofilms. Structured polymer films can be fabricated using three broad strategies: direct deposition (such as with ink-jet printing), selective etching (such as with photolithography), and template directed structuring. Each strategy currently possesses advantages, disadvantages, and opportunities for improvement. As the methods to fabricate structured polymer films have improved, so has the associated scientific understanding of the rules governing these methods. The focus of this chapter, template guided structuring, has arisen in the past two decades to encompass several exciting techniques. In this chapter the methods and principles behind these discoveries are introduced.

The control of patterns on sub-micrometre length scales is of considerable technological interest, and is especially relevant for tailoring the properties of novel functional materials. A variety of bottom-up and top-down techniques for patterning polymers have emerged in the past decade. The growing demand for better performance, reduced energy consumption, high-throughput and higher levels of complexity and integration raise the need for alternative technologies capable of generation or reproduction of patterns below the sub-100 nm. Successful cost-effective patterning processes are needed to enable straightforward high-fidelity patterning in a controlled manner on multiple-scales which will also be suitable for a broad range of materials. Development of such methods is essential for the successful future large-scale fabrication of advanced functional technologically appealing down-scaled devices. Here advances in the use of electric fields on a range of soft materials to generate complex patterns from thin films by exploiting surface instabilities are reviewed, that demonstrate a complementary and an elegant lithographic technique, limits of which are yet to be reached.

Hydrodynamics instabilities, such as Bénard–Marangoni convection, Rayleigh–Plateau, Rayleigh–Taylor, and many others, are well known to produce beautiful patterns. Indeed, capillary, viscous, and/or inertial forces could conspire to generate very regular morphologies. In this chapter, we will see that elasticity can also produce complex structures, either regular or fully random, provided that slender objects are used. These objects are usually classified as rods, shells, or sheets. In the classical picture, thin sheets constrained by external forces minimize elastic energy through focalization of deformation in singularities. We will see that, due to geometric constraints, these origami structures cannot always be obtained. In addition, very regular wrinkles with a very large range of wavelength can be observed for deformed sheets, if the sheet is tightly attached on a soft foundation. At the onset, these wrinkles reflect the competition between bending and elastic deformation forces. For large deformations, however, the morphology is surprisingly determined by the nature of the foundation. Fold localization or period-doubling bifurcations are indeed observed for liquid or solid substrate, respectively. Finally, these various studies also highlight the effort of this growing interdisciplinary researcher community to the emergence of a global picture of elastic instability. Interestingly, these wrinkled surfaces appear to be particularly useful for various applications including the design of superhydrophobic surfaces, new non-permanent adhesive, flexible electronic devices, optical devices such as anti-reflective coatings.

Surface instabilities arise on polymeric systems either during fabrication or post-fabrication in response to an external stimulus. The occurrence, mode, and size of these instabilities are determined by the heterogeneity of the polymer system, which induces dynamic changes in reaction and diffusion of species within the polymer system. This chapter mainly focuses on transient to persistent creasing patterns, creasing to wrinkling transition, and self-oscillating patterns, particularly in gel/solvent systems.

Micro/nanoporous films have found use in many applications ranging from electronics or photonic to biotechnology. The preparation of such porous structures usually requires of the templates that need to be removed upon film preparation. As an alternative, the breath figures approach has been employed for the preparation of porous surfaces and films. This approach, the topic of this chapter, has several advantages over other techniques including the use of water as template (easily removed) and possibility of controlling the chemical composition of the pore and the external surface. Herein, the strategies reported to prepare honeycomb structured porous films by the breath figures approach are summarized. In addition, the methodologies reported to control the chemical composition of the porous films are depicted. The last part of this chapter focuses on the applications in which these films have been employed.

It has been described in previous chapters how spontaneous instabilities related to interfacial phenomena can be used to produce controlled patterns on polymer surfaces. Strategies of polymer patterning assisted by dewetting or water drop condensation were described. In this chapter we present a waterborne process based on the interaction between ions in water and hydrophobic polymer surfaces, modulated by the gases dissolved in the aqueous phase. We show how by controlling this interaction the polymer surface can be conveniently modified. In the first section of the chapter we describe some aspects of the interface between water and a hydrophobic surface. We then describe how the composition of the aqueous phase can have important consequences on the morphology of the hydrophobic surface, and then illustrate how this process can be conveniently used to modify the morphology of a hydrophobic polymer in a controlled manner.

In the previous chapter it was described how degassing aqueous solutions in contact with hydrophobic polymers open pathways for polymer surface patterning. In the absence of degassing, nanobubbles can nucleate on hydrophobic surfaces. In this chapter the structuring effect of nanobubbles on hydrophobic surfaces is discussed.

The outstanding progress in synthetic polymer science of last century stimulated the development of a large number of low-cost methods of polymer production, making a growing variety of complex materials available. During the last few decades important efforts have been devoted to developing strategies of polymer patterning, aiming to achieve new functionalities for these widespread materials, largely inspired by examples found in nature.