Controlled wrinkling as a novel method for the fabrication of patterned surfaces

This article reviews recent applications of controlled wrinkling for creating structured and/or patterned interfaces, and its use in metrology. We discuss how wrinkles develop as a result of in-plane compression of thin sheets. As the wavelength of wrinkles is only dependent on elastic properties and thickness of the sheets, the phenomenon can be used in metrology for determination of elastic properties. The second aspect is its use for patterning and topographical structuring of surfaces. If mechanical properties and thickness are well controlled, wrinkle orientation and geometry can be tailored. Wavelengths between fractions of a micron and many micrometers are feasible. This process is based on a macroscopic deformation and upscaling to larger areas is possible which provides an attractive alternative to bottom-up or top-down approaches for surface patterning. We describe the formation of stable surface wrinkles in thin sheets of different materials having different surface chemistries, report on applications, and discuss the usefulness of wrinkles for building hierarchical structures.     

Controlling wrinkles and assembly patterns in dried graphene oxide films using lyotropic graphene oxide liquid crystals

We fabricated graphene oxide (GO) films on glass substrates by blade coating a lyotropic GO liquid crystal dispersion. Substrate temperature and blading speed were precisely controlled to manipulate the surface morphologies of GO films. The temperature and blade speed influenced the drying rate of film and the amount GO dispersion supplied. By controlling these parameters, film-thickness modulation and three types of surface wrinkle patterns were selectively achieved. We also plotted the wrinkle patterns diagram as functions of the film fabrication conditions. The films exhibited different optical anisotropies depending on wrinkle patterns. GO films with controlled wrinkles can be used as electrodes for supercapacitor applications owing to the large surface areas.     

Tuning and Erasing Surface Wrinkles by Reversible Visible‐Light‐Induced Photoisomerization

Periodic wrinkling across different scales has received considerable attention because it not only represents structure failure but also finds wide applications. How to prevent wrinkling or create desired wrinkling patterns is non‐trivial because the dynamic evolution of wrinkles is a highly nonlinear problem. Herein, we report a simple yet powerful method to dynamically tune and/or erase wrinkling patterns with visible light. The light‐induced photoisomerization of azobenzene units in azopolymer films leads to stress release and consequently to the erasure of the wrinkles. The wrinkles in unexposed regions are also affected and oriented perpendicular to the exposed boundary during the stress reorganization. Theoretical models were developed to understand the dynamics of the reversible photoisomerization‐induced wrinkle evolution. This method can be applied for designing functional materials/devices, for example, for the reversible optical writing/erasure of information as demonstrated here.     

Wrinkle engineering: a new approach to massive graphene nanoribbon arrays

Wrinkles are often formed on CVD-graphene in an uncontrollable way. By designing the surface morphology of growth substrate together with a suitable transfer technique, we are able to engineer the dimension, density, and orientation of wrinkles on transferred CVD-graphene. Such kind of wrinkle engineering is employed to fabricate highly aligned graphene nanoribbon (GNR) arrays by self-masked plasma-etching. Strictly consistent with the designed wrinkles, the density of GNR arrays varied from ～0.5 to 5 GNRs/μm, and over 88% GNRs are less than 10 nm in width. Electrical transport measurements of these GNR-based FETs exhibit an on/off ratio of ～30, suggesting an opened bandgap. Our wrinkle engineering approach allows very easily for a massive production of GNR arrays with bandgap-required widths, which opens a practical pathway for large-scale integrated graphene devices.     

Applications of multidirectional reflective light-control films on reflective polymer-dispersed liquid crystal displays for enhancement in image quality at lower viewing angles

In this paper, multidirectional light-control reflective (LCR) films are developed in order to create an active reflective structure that will enhance the image brightness and contrast ratio of reflective dye-doped polymer-dispersed liquid crystal (D-PDLC) displays at lower viewing angles. Advantages of LCR films are that their production is low cost and they require a simple photolithographic fabrication method. The optimum design prism-type light-control reflective film succeeded in minimising the surface scattering effect; thus, the contrast ratio is much enhanced. The symmetric and asymmetric LCR films produced multidirectional scattering that enhances the reflectance at lower viewing angles, which has importance in future display applications. In particular, the prism LCR film has been found to be more influential on the reflectance of D-PDLC films due to multidirectional scattering of light by non-symmetric arrays. The improvement in contrast ratio has been confirmed by the enhancement of optical properties for reflective D-PDLC displays at lower viewing angles below 30°.     

Wrinkle morphologies with two distinct wavelengths

Wrinkles with two distinct wavelengths formed sequentially on the same surface are investigated. A series of aligned wrinkles are formed through local strain application on a partially crosslinked elastomer. After the formation of these primary wrinkles, the elastomer is fully crosslinked, and a mechanical compressive strain is applied to the sample orthogonal to the primary wrinkles. This mechanical strain results in smaller secondary wrinkles superimposed on the larger primary aligned wrinkles. Resulting biaxial morphologies suggest that the primary pattern directs the formation of the smaller wrinkles. The modulus mismatch of the substrate on primary and secondary wrinkle formation dictates the ratio between the two resulting wavelengths, as well as the specific biaxial morphologies, ranging from zigzag ridges to ellipsoidal bumps or corn‐on‐the‐cob structures to the classic herringbone. The sequential strain wrinkling process has the potential to be used on an industrial scale for the facile formation of surface topography with two discrete, tunable lateral dimensions over large surface areas. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

The effect of voltage controlled orientation order of liquid crystals in non-acrylic polymer dispersed liquid crystals films

The nematic liquid crystals (LCs) are randomly dispersed material with random orientation order in polymer dispersed liquid crystal (PDLC) films. The LCs change their orientation from random to vertical as electric field is applied. This transformation of orientation order of nematic liquid crystals in the PDLC films is controlled by many factors operating simultaneously. For instance, some factors like the internal forces of attractions among the neighboring LC molecules, anchoring with polymeric matrix, ITO glass boundaries, and chemical structures of the materials are less studied. The learning of extent of vertical orientation of liquid crystal droplets in an electric field is essential to attain optimum electro optical properties of PDLCs. In this finding, bipolar and radial LCs droplets with random orientation have been observed in non-acrylic polymeric media. It is learned that with small increase of contents of external material, the extent of vertical orientation has been varied intensely. The extent of vertical orientation of LCs molecules increases as the contents of external non-acrylic polymeric material decreased. For this study, the orientations of LCs with respect to material type/contents, external applied force, and restoration of electric filed as hysteresis have been studied in details.     

Solution-processed anodes from layer-structure materials for high-efficiency polymer light-emitting diodes

The development of low-cost, large-area electronic applications requires the deposition of active materials in simple and inexpensive techniques at room temperature, properties usually associated with polymer films. In this study, we demonstrate the integration of solution-processed inorganic films in light-emitting diodes. The layered transition metal dichalcogenide (LTMDC) films are deposited through Li intercalation and exfoliation in aqueous solution and partially oxidized in an oxygen plasma generator. The chemical composition and thickness of the LTMDC and corresponding transition metal oxide (TMO) films are investigated by X-ray photoelectron spectroscopy. The morphology and topography of the films are studied by atomic force microscopy. X-ray powder diffraction is used to determine the orientation of the LTMDC film. Finally, the LTMDC and their corresponding oxides are utilized as hole-injecting and electron-blocking materials in polymer light-emitting diodes with the general structure ITO/LTMDC/TMO/polyfluorene/Ca/Al. Efficient hole injection and electron blocking by the inorganic layers result in outstanding device performance and high efficiency.     

Crack-Free,Soft Wrinkles Enable Switchable Anisotropic Wetting

Soft skin layers on elastomeric substrates are demonstrated to support mechano-responsive wrinkle patterns that do not exhibit cracking under applied strain. Soft fluoropolymer skin layers on pre-strained poly(dimethylsiloxane) slabs achieved crack-free surface wrinkling at high strain regimes not possible by using conventional stiff skin layers. A side-by-side comparison between the soft and hard skin layers after multiple cycles of stretching and releasing revealed that the soft skin layer enabled dynamic control over wrinkle topography without cracks or delamination. We systematically characterized the evolution of wrinkle wavelength, amplitude, and orientation as a function of tensile strain to resolve the crack-free structural transformation. We demonstrated that wrinkled surfaces can guide water spreading along wrinkle orientation, and hence switchable, anisotropic wetting was realized.     

Crack-Free,Soft Wrinkles Enable Switchable Anisotropic Wetting

Soft skin layers on elastomeric substrates are demonstrated to support mechano-responsive wrinkle patterns that do not exhibit cracking under applied strain. Soft fluoropolymer skin layers on pre-strained poly(dimethylsiloxane) slabs achieved crack-free surface wrinkling at high strain regimes not possible by using conventional stiff skin layers. A side-by-side comparison between the soft and hard skin layers after multiple cycles of stretching and releasing revealed that the soft skin layer enabled dynamic control over wrinkle topography without cracks or delamination. We systematically characterized the evolution of wrinkle wavelength, amplitude, and orientation as a function of tensile strain to resolve the crack-free structural transformation. We demonstrated that wrinkled surfaces can guide water spreading along wrinkle orientation, and hence switchable, anisotropic wetting was realized.     

A “self‐pinning” adhesive based on responsive surface wrinkles

Surface wrinkles are interesting since they form spontaneously into well‐defined patterns. The mechanism of formation is well‐studied and is associated with the development of a critical compressive stress that induces the elastic instability. In this work, we demonstrate surface wrinkles that dynamically change in response to a stimulus can improve interfacial adhesion with a hydrogel surface through the dynamic evolution of the wrinkle morphology. We observe that this control is related to the local pinning of the crack separation pathway facilitated by the surface wrinkles during debonding, which is dependent on the contact time with the hydrogel. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Highly oriented tunable wrinkling in polymer bilayer films confined with a soft mold induced by water vapor

The authors report the formation of highly oriented wrinkling on the surface of the bilayer [polystyrene (PS)/poly(vinyl pyrrolidone) (PVP)] confined by a polydimethylsiloxane (PDMS) mold in a water vapor environment. When PVP is subjected to water vapor, the polymer loses its mechanical rigidity and changes to a viscous state, which leads to a dramatic change in Young's modulus. This change generates the amount of strain in the bilayer to induce the wrinkling. With a shape-controlled mold, they can get the ordered wrinkles perfectly perpendicular or leaned 45 degrees to the channel orientation of the mold because the orientation of the resultant force changes with the process of water diffusion which drives the surface to form the wrinkling. Additionally, they can get much smaller wrinkles than the stripe spacing of PDMS mold about one order. The wrinkle period changes with the power index of about 0.5 for various values of the multiplication product of the film thicknesses of the two layers, namely, lambda approximately (h(PS)h(PVP))(1/2).     

Adhesion of nonplanar wrinkled surfaces

Topological patterns on polymer surfaces can significantly alter and control adhesion. In this study, the effect of surface wrinkles on a spherical surface on adhesion has been studied. Surface wrinkling induced by swelling of a crosslinked polydimethylsiloxane elastomer constrained by a stiff, thin surface layer (silicate) is used to produce topographic features of various length scales over a large curved area. By controlling the properties of the stiff layer and the applied strain conditions, surface wrinkles of varying amplitude and wavelength are obtained. The effect of wrinkle morphology on adhesion is quantified, and the results display a transition from enhancement of adhesion to decrease depending upon wrinkle dimensions. A simple phenomenological model is proposed that describes the change of adhesion behavior as a function of wrinkle morphology. Our results provide a critical understanding toward tuning the adhesion behavior of nonplanar surfaces consisting of periodic topographic structures. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Slip-stick wetting and large contact angle hysteresis on wrinkled surfaces

Wetting on a corrugated surface that is formed via wrinkling of a hard skin layer formed by UV oxidation (UVO) of a poly(dimethylsiloxane) (PDMS) slab is studied using advancing and receding water contact angle measurements. The amplitude of the wrinkled pattern can be tuned through the pre-strain of the PDMS prior to surface oxidation. These valleys and peaks in the surface topography lead to anisotropic wetting by water droplets. As the droplet advances, the fluid is free to move along the direction parallel to the wrinkles, but the droplet moving orthogonal to the wrinkles encounters energy barriers due to the topography and slip-stick behavior is observed. As the wrinkle amplitude increases, anisotropy in the sessile droplet increases between parallel and perpendicular directions. For the drops receding perpendicular to the wrinkles formed at high strains, the contact angle tends to decrease steadily towards zero as the drop volume decreases, which can result in apparent hysteresis in the contact angle of over 100°. The wrinkled surfaces can exhibit high sessile and advancing contact angles (>115°), but the receding angle in these cases is generally vanishing as the drop is removed. This effect results in micrometer sized drops remaining in the grooves for these highly wrinkled surfaces, while the flat analogous UVO-treated PDMS shows complete removal of all macroscopic water drops under similar conditions. These wetting characteristics should be considered if these wrinkled surfaces are to be utilized in or as microfluidic devices.     

Finely Formed,Kinetically Modulated Wrinkle Structures in UV‐Crosslinkable Liquid Prepolymers

Special characteristics of wrinkles such as a scattering source and a high surface area are finding use in high‐tech applications. UV‐crosslinkable prepolymers are occasionally used for fabricating wrinkled films. Wavelength of the wrinkles formed from the prepolymers is several tens and hundreds of micrometers. Here, a UV‐crosslinkable liquid prepolymer is synthesized to spontaneously form wrinkle structures in the order of several micrometers. Double layers with a very thin hard skin and a soft and contractible foundation are formed at the same time, by ensuring that all the absorbance wavelengths of the photoinitiator are shorter than the minimum wavelength at which the prepolymer is transparent. The rate of photo‐crosslinking reaction, R_p, is also found to affect the thickness of the skin and foundation layers at the early UV‐curing stage. The first‐order apparent rate constant, k_app, is between ≈0.20 and ≈0.69 s⁻¹ for the wrinkle formation. This wrinkle structures can be simply modulated by changing R_p.

     

Selective deposition in multilayer assembly: SAMs as molecular templates

Self-assembled monolayers (SAMs) on surfaces may be used as molecular templates for the selective deposition of polymer multilayer films. SAMs of ω-functionalized alkane thiolates are patterned onto gold surfaces with micron scale features using the microcontact printing method; glass substrates can also be patterned with trichloroalkylsilane SAMs. Patterned polymeric monolayer and multilayer films are adsorbed atop the SAM from dilute polymer solutions using ionic macromolecular self-assembly techniques which have been developed recently. The effects of polymer molecular weight and ionic content, as well as the use of a second SAM in the unpatterned regions to promote selectivity are discussed. Surface roughness, selectivity and other film properties are presented. It is demonstrated that this technique can be used successfully in the patterning of micron scale features with multilayers of low molecular weight upon adsorption from dilute solution.     

Engineering of superhydrophobic silica microparticles and thin coatings on polymeric films by ultrasound irradiation

Superhydrophobic coatings are one of the recent hot topics in industrial applications as well as academic studies. The mimicking lotus leaves' superhydrophobic properties have been successfully transferred to real-life applications. However, the current preparation methods used to obtain superhydrophobic coatings are still complex, commonly are not transparent and/or not durable.In the present study, a new relatively simple way to prepare superhydrophobic coatings on polymeric films is described. First, superhydrophobic silica microparticles (MPs) were synthesized by fluorination of SiO₂ MPs produced by a modified Stöber method. Briefly, tetraethyl orthosilicate was polymerized in an ethanol/water continuous phase under basic conditions, and the resultant SiO₂ MPs were dispersed in heptane as a continuous phase and reacted with 1H,1H,2H,2H-perfluorododecyltrichlorosilane (FTS) to yield FTS-SiO₂ MPs, which were dried and dispersed in decane. Superhydrophobic thin coatings were then produced by a ‘throwing stones’ sonication technique and deposited onto polycarbonate, polypropylene, polyethylene, and polyurethane films. The coatings are durable, may be transparent, and exhibit self-cleaning properties for the specific practical applications. The MPs and coated polymeric films were characterized by dynamic light scattering, high-resolution scanning electron microscopy, water contact and sliding angle measurements, and infrared and x-ray photoelectron spectroscopy. This ultrasound-assisted coating process may be upscaled and applied to many polymeric films, for instance polymethyl methacrylate, polystyrene, and polyvinyl chloride. Various applications are envisaged, including but not limited to self-cleaning windows, anti-sticking of snow to antennas and windows, solar panels, roof tiles, agricultural applications, corrosion resistance, and anti-biofouling.     

聚合物Langmuir—Blodgett膜研究进展

聚合物LB膜可用两种方法制备,一种是两亲单体成膜再进行聚合反应,另一种为直接从两亲聚合物在亚相表面铺展成膜并转移。本文综合聚合物LB膜的研究状况,包括两亲聚合物和非两亲聚合物,对聚合物LB膜的成膜特点,结构和性能作了描述,并简要介绍了聚合物LB膜的应用前景。     

Interfacial rheology and structure of tiled graphene oxide sheets

The hydrophilic nature of graphene oxide sheets can be tailored by varying the carbon to oxygen ratio. Depending on this ratio, the particles can be deposited at either a water-air or a water-oil interface. Upon compression of thus-created Langmuir monolayers, the sheets cover the entire interface, assembling into a strong, compact layer of tiled graphene oxide sheets. With further compression, the particle layer forms wrinkles that are reversible upon expansion, resembling the behavior of an elastic membrane. In the present work, we investigate under which conditions the structure and properties of the interfacial layer are such that free-standing films can be obtained. The interfacial rheological properties of these films are investigated using both compressional experiments and shear rheometry. The role of surface rheology in potential applications of such tiled films is explored. The rheological properties are shown to be responsible for the efficiency of such layers in stabilizing water-oil emulsions. Moreover, because of the mechanical integrity, large-area monolayers can be deposited by, for example, Langmuir-Blodgett techniques using aqueous subphases. These films can be turned into transparent conductive films upon subsequent chemical reduction.     

Reaction-diffusion phenomena in a PDMS matrix can modify its topography

Various reagents and solvents can be absorbed into polydimethylsiloxane (PDMS), which may be a concern for many applications. We hypothesize that these absorbed reagents can also react with each other within the elastomer matrix. Here we demonstrate this phenomenon and use it as a means to physically modify the surface topography of the PDMS by generating wrinkles or pores.