Complex pattern formation by adhesion-controlled anisotropic wrinkling

We report complex pattern formation and shape control in the confinement-induced wrinkling that occurs when a poly(dimethylsiloxane) (PDMS) mold is placed on a bilayer of metal and polymer and then heated. Various complex structures that are different from the mold pattern form through the self-organization of wrinkles. These complex structures could be inverted in shape by manipulating the work of adhesion at the interface between the mold and the metal surface. Convex wrinkles result when the work of adhesion is relatively large. However, inverted concave wrinkles emerge when it is relatively small. The ratio of the mold period to the intrinsic wrinkling wavelength is another factor that determines the shape. The ability to tailor the shape of a surface is expected to have a broad range of applications in electro-optics and microfluidics.     

基于表面起皱法的梯度图案的可控制备

报道了基于非刻蚀法的表面起皱机制来实现高分子薄膜表面的周期性梯度图案的简单可控制备.即对于处于机械拉伸状态的聚二甲基硅氧烷(PDMS)弹性基底,在其底部垫入"积木",而后对其进行紫外-臭氧(UVO)和氧等离子体(OP)的联合表面处理."积木"的加入引起了表面处理后表面硅氧层(SiOx)梯度厚度的形成,进而当释放拉伸应变后,诱导产生了梯度皱纹图案.结果表明:当UVO与OP联用处理时,不仅实现了较小拉伸应变下梯度皱纹形貌的制备,而且扩大了UVO单独使用时梯度皱纹周期的变化范围.通过OP与UVO的处理顺序和处理时间等因素的简单调节,进一步实现了不同梯度皱纹微结构的精细构筑.     

Surface initiated polymerization from integrated poly(dimethylsiloxane) enables crack‐free large area wrinkle formation

We conducted surface initiated polymerization from stretched poly(dimethylsiloxane) to construct a polymer/PDMS bilayer structure, which formed crack‐free wrinkles over large area (>6 cm²) upon recovery from the stretched state. This system further allowed us to reveal the dynamics and memory effect of wrinkling. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

Benchtop micromolding of polystyrene by soft lithography

Polystyrene (PS), a standard material for cell culture consumable labware, was molded into microstructures with high fidelity of replication by an elastomeric polydimethylsiloxane (PDMS) mold. The process was a simple, benchtop method based on soft lithography using readily available materials. The key to successful replica molding by this simple procedure relies on the use of a solvent, for example, gamma-butyrolactone, which dissolves PS without swelling the PDMS mold. PS solution was added to the PDMS mold, and evaporation of the solvent was accomplished by baking the mold on a hotplate. Microstructures with feature sizes as small as 3 μm and aspect ratios as large as 7 were readily molded. Prototypes of microfluidic chips made from PS were prepared by thermal bonding of a microchannel molded in PS with a flat PS substrate. The PS microfluidic chip displayed much lower adsorption and absorption of hydrophobic molecules (e.g. rhodamine B) compared to a comparable chip created from PDMS. The molded PS surface exhibited stable surface properties after plasma oxidation as assessed by contact angle measurement. The molded, oxidized PS surface remained an excellent surface for cell culture based on cell adhesion and proliferation. To demonstrate the application of this process for cell biology research, PS was micromolded into two different microarray formats, microwells and microposts, for segregation and tracking of non-adherent and adherent cells, respectively. The micromolded PS possessed properties that were ideal for biological and bioanalytical needs, thus making it an alternative material to PDMS and suitable for building lab-on-a-chip devices by soft lithography methods.     

Area‐selective microwrinkle formation on poly(dimethylsiloxane) by treatment with strong acid

Microwrinkles were formed by treating poly(dimethylsiloxane) (PDMS) surfaces with a mixture of concentrated sulfuric acid and nitric acid at 80 °C followed by a water rinsing step. The acid treatment oxidized the treated area, which also swelled owing to absorption of water during the rinsing step. This absorption of water induced a compressive stress on the PDMS surface that caused the formation of wrinkles. The appearance of wrinkles was controlled by the time interval between the end of the acid treatment and the beginning of the water rinsing step. Wrinkles were formed when the time interval was shorter than 5 s, as water was absorbed by the hygroscopic acid molecules within the treated surface. For longer time intervals of 300 s, this phenomenon was not observed. Based on these findings, a procedure for forming wrinkles in specific areas was developed by hydrating the chosen areas with wet filter paper cut into specific shapes after the acid treatment. The formation of wrinkles was confined to the area under the piece of paper. The orientation and alignment of the wrinkles at the boundaries were also characterized. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 167–174     

Substrate‐Induced Controllable Wrinkling for Facile Nanofabrication

In this work, we report a facile and effective strategy to generate patterned wrinkles. This strategy includes first adhering a thin poly(dimethylsiloxane) (PDMS) film (<82 µm) on porous conductive adhesive tape (CAT), followed by sputter coating of Au onto PDMS under vacuum condition, which results in formation of patterned wrinkles on the Au‐PDMS bilayer. CAT was found to induce local stretching of PDMS thin film, which was the key for controlled wrinkle formation. Compared with previous wrinkling methods, our strategy is simpler and gives smaller feature sizes (down to 300 nm).

     

Controlled fabrication of hierarchically microstructured surfaces via surface wrinkling combined with template replication

In this paper,we present a simple method by combining surface wrinkling and template replication to create a series of hierarchical structures on polydimethylsiloxane(PDMS) elastomer.The primary stable lined patterns are formed by duplicating commercialized compact disk and digital versatile disk with PDMS.The secondary microscale patterns are from surface wrinkling,which is elicited by oxygen plasma(OP) treatment of the prestrained PDMS stamp followed with the prestrain release.By systematically varying the OP exposure duration,the prestrain,and the angle(θ) between the primary pattern orientation and the prestrain direction,we obtain highly ordered well-organized composite patterns from different patterning techniques and with different length scales and mechanical stabilities.     

Facile non‐lithographic route to highly aligned silica nanopatterns using unidirectionally aligned polystyrene‐block‐polydimethylsiloxane films

Thin films (monolayer and bilayer) of cylinder forming polystyrene‐block‐polydimethylsiloxane (PS‐b‐PDMS) were shear aligned by the swelling and deswelling of a crosslinked PDMS pad that was physically adhered to the film during solvent vapor annealing. The nanostructures formed by self‐assembly were exposed to ultraviolet‐ozone to partially oxidize the PDMS, followed by calcination in air at 500 °C. In this process, the PS segments were fully decomposed, while the PDMS yielded silica nanostructures. The highly aligned PDMS cylinders were thus deposited as silica nanolines on the silicon substrate. Using a bilayer film, the center‐to‐center distance of these features were effectively halved from 38 to 19 nm. Similarly, by sequential shear‐alignment of two distinct layers, a rhombic array of silica nanolines was fabricated. This methodology provides a facile route to fabricating complex topographically patterned nanostructures. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1058–1064     

Honeycomb pattern formation via polystyrene/poly(2-vinylpyridine) phase separation

The surface morphologies and properties of polystyrene (PS)/poly(2-vinylpyridine) (PVP) blend films cast on the mica substrate from ethylbenzene solution were investigated upon controlling different weight ratios and solvent evaporation rates. A near-honeycomblike surface morphology of the PS/PVP blend film formed under controlling the solvent evaporation rate due to the effect of Marangoni-Benard convection. The results of static water contact angles, X-ray photoelectron spectroscopy, solvent selective etching, and treatment by water illustrated that the near-honeycomblike structures on the surface of PS/PVP blend films were different for different weight ratios of PS and PVP. After treatment with water for several minutes, PVP islands-like structure emerged in the holes of the film for a PS/PVP weight ratio of 4/1, and a quasihexagonal arrangement of alternate big and small PVP droplets emerged on the top layer of the film for a PS/PVP weight ratio of 7/1. The formation mechanisms of different surface structures and their response behaviors to water were discussed.     

Topcoat‐Assisted Perpendicular and Straightly Parallel Coexisting Orientations of Block Copolymer Films

Highly ordered perpendicular orientation and straightly parallel orientation coexisting polystyrene‐block‐polydimethylsiloxane (PS‐b‐PDMS) cylindrical microdomains with 10 nm width can be realized by using polyvinyl acetate as a partially dewetted topcoat and solvent annealing with acetone vapor. During solvent annealing, the swelled topcoat begins to dewet and the dewetting rim sweeps the surface of the block copolymer films to align the cylindrical microdomains with the direction of dewetting propagation. However, the wetted region of the topcoat/PS‐b‐PDMS film forms with a perpendicular orientation due to reduced surface tension and sufficient concentration gradient in the solvent evaporation step. The orientational changes (perpendicular/straightly parallel orientation) in the dewetted/wetted area are also investigated according to the vapor pressure of solvent annealing. The degree of directionality of the swept PS‐b‐PDMS films according to the distance from the dewetting front, which is equivalent with time after sweeping, is examined. To control the direction of dewetting and complex structures within a specific area, an imprinting process is introduced to form topographical line–space patterns in the topcoat and perpendicular/parallel orientation of BCP patterns in the line–space patterns, respectively.

     

Surface restructuring behavior of various types of poly(dimethylsiloxane) in water detected by SFG

Surface structures of several different poly(dimethylsiloxane) (PDMS) materials, tetraethoxysilane-cured hydroxy-terminated PDMS (TEOS-PDMS), platinum-cured vinyl-terminated PDMS (Pt-PDMS), platinum-cured vinyl-terminated poly(diphenylsiloxane)-co-poly(dimethylsiloxane) (PDPS-co-PDMS), and PDMS-co-polystyrene (PDMS-co-PS) copolymer in air and water have been investigated by sum frequency generation (SFG) vibrational spectroscopy. The SFG spectra collected from all PDMS surfaces in both air and water are dominated by methyl group stretches, indicating that all the surfaces are mainly covered by methyl groups. Other than surface-dominating methyl groups, some -Si-CH2-CH2- moieties on the Pt-PDMS surface have also been detected in air, which are present at cross-linking points. Information about the average orientation angle and angle distribution of the methyl groups on the PDMS surface has been evaluated. Surface restructuring of the methyl groups has been observed for all PDMS surfaces in water. Upon contacting water, the methyl groups on all PDMS surfaces tilt more toward the surface. The detailed restructuring behaviors of several PDMS surfaces in water and the effects of molecular weight on restructuring behaviors have been investigated. For comparison, in addition to air and water, surface structures of PDMS materials mentioned above in a nonpolar solvent, FC-75, have also been studied. By comparing the different response of phenyl groups to water on both PDPS-co-PDMS and PS-co-PDMS surfaces, we have demonstrated how the restructuring behaviors of surface phenyl groups are affected by the structural flexibility of the molecular chains where they are attached.     

Multidirectional Wrinkle Patterns Programmed by Sequential Uniaxial Strain with Conformal yet Nontraceable Masks

Surface wrinkling is a promising route to control the mechanical, electrical, and optical properties of materials in a wide range of applications. However, previous artificial wrinkles are restricted to single or random orientation and lacks selectivity. To address this challenge, this study presents multidirectional wrinkle patterns with high selectivity and orientation through sequential uniaxial strain with conformal polymeric shadow masks. The conformal but nontraceable polymeric stencil with microapertures are adhered to a flat substrate prior to oxidation, which forms discrete and parallel wrinkles in confined domains without any contamination. By fully investigating the process, this study displays compound topography of wrinkles consisting of wrinkle islands and surrounding secondary wrinkles on the same surface. With this topography, various diffusion properties are presented: from semi‐transparent yet diffusive films to multidirectional diffusive films, which will be available for new types of optical diffuser applications.     

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.     

Simple room temperature bonding of thermoplastics and poly(dimethylsiloxane)

We describe a simple and versatile method for bonding thermoplastics to elastomeric polydimethylsiloxane (PDMS) at room temperature. The bonding of various thermoplastics including polycarbonate (PC), cyclic olefin copolymer (COC), polymethylmethacrylate (PMMA), and polystyrene (PS), to PDMS has been demonstrated at room temperature. An irreversible bonding was formed instantaneously when the thermoplastics, activated by oxygen plasma followed by aminopropyltriethoxysilane modification, were brought into contact with the plasma treated PDMS. The surface modified thermoplastics were characterized by water contact angle measurements and X-ray photoelectron spectroscopy. The tensile strength of the bonded hybrid devices fabricated with PC, COC, PMMA, and PS was found to be 430, 432, 385, and 388 kPa, respectively. The assembled devices showed high burst resistance at a maximum channel pressure achievable by an in-house built syringe pump, 528 kPa. Furthermore, they displayed very high hydrolytic stability; no significant change was observed even after the storage in water at 37 °C over a period of three weeks. In addition, this thermoplastic-to-PDMS bonding technique has been successfully employed to fabricate a relatively large sized device. For example, a lab-on-a-disc with a diameter of 12 cm showed no leakage when it spins for centrifugal fluidic pumping at a very high rotating speed of 6000 rpm.     

表面微图案化聚乳酸膜的制备及其细胞亲和性评价

利用溶剂-非溶剂法(SNS)制备表面具有微孔图案的聚乳酸(PLA)膜和聚苯乙烯(PS)膜,并以微孔PS膜为模板,构建表面具有微岛图案的PLA膜.以此为基础,对所制备的微图案表面对PLA膜亲/疏水性及成骨细胞粘附与增殖性能的影响进行研究.结果显示微图案的存在显著增强了PLA膜的表面疏水性(水接触角90°);成骨细胞在微图案表面具有良好的铺展性,其黏附数量明显高于光滑PLA膜,但细胞的生长曲线相对较平缓,显示微图案表面虽有利于细胞在PLA膜表面的粘附与铺展,但对促进细胞的增殖无贡献.     

Photo-Oxidation-Controlled Surface Pattern with Responsive Wrinkled Topography and Fluorescence

Wrinkles and photo-oxidation reactions are widely found in soft materials, which are intimately associated with the failure of materials and structures. It is expected that the photo-oxidation process could also have a positive effect on the material and its surface. Here, we report the photo-oxidation of 2-(4-dietheylaminophenyl)-4,5-bis(4-methoxyphenyl) imidazole (DEA-TAI) into a wrinkled bilayer system to control surface wrinkle and fluorescent patterns, in which a supramolecular polymer network composed of carboxylic acid-containing copolymer (PS-BA-AA; PS=poly(styrene), BA=butyl acrylate; AA=acrylic acid) and DEA-TAI were used as the skin layer. Ultraviolet (UV) irradiation can induce photo-oxidation of the imidazole ring of DEA-TAI to weaken the intermolecular hydrogen bonding between PS-BA-AA and DEA-TAI, resulting in the release of stress in the bilayer system. The wrinkled morphology and fluorescence of the surface can be simultaneously regulated by photo-oxidation of DEA-TAI under UV light, and the resulting wrinkles are extremely sensitive to the pH value, which can be quickly and reversibly erased by NH₃ gas. Smart surfaces with specific hierarchical wrinkles and fluorescence can be achieved by selective irradiation with photomasks, which may find potential applications in smart displays and multi-code information storage.     

软模板印刷法制备超疏水性聚苯乙烯膜

首次利用软模板印刷的方法,以微米-亚微米-纳米复合结构的PDMS为软模板,在平滑聚苯乙烯表面上成功制备了同样具有微米-亚微米-纳米复合结构的超疏水表面,该表面与水的接触角高达161.2º。软模板印刷方法可以用在其它热塑性聚合物如聚丙烯、聚甲基丙烯酸甲酯和聚碳酸酯等材料上,是一种简单有效地制备超疏水性表面的方法。     

Interfacial Behavior of Mixed Systems of Glycerylether-Modified Silicone and Polyoxyethylene-Modified Silicone

Undecylglycerylether-modified silicone (GES; the glycerylether-type surfactant with a silicone segment and alkyl chains (carbon number, 11) as the hydrophobic portion) forms a molecular aggregate (M.A.) with a small amount of water. This M.A. is similar to the reversed hexagonal liquid crystal formed by alpha-mono long-chain alkylglycerylether (3-isooctadecyloxy-1,2-propanediol; GE). From the investigation of the phase behavior in the water/GES/polydimethylsiloxane (PDMS) ternary system, a wide three-phase region of water (W)+M.A.+oil (O) was observed. As this M.A. is insoluble in PDMS and easily orients in the interface between water and PDMS, the high water content silicone W/O emulsion using GES as a surfactant is well stabilized. However, as the PDMS content increased this W/O emulsion became less stable. In order to improve this stability, mixtures of GES and polyoxyethylene-modified silicone (PS) were applied to the silicone emulsion as co surfactant. By application of a PS with a methyl group at the end cap of the polyoxyethylene chain (PSM), the emulsion became most stable at a GES/PSM ratio of 1 : 2, and at the same time, the interfacial tension between the oil phase and the water phase became minimal. The reason for this was studied by the measurement of spin-lattice relaxation times (T(1)) of the alkyl chains of GES in the GES/PS/water system by (13)C NMR. We assumed that the W/O silicone emulsions were stabilized by the efficient orientation of the aggregates in the interface between the silicone phase and the water phase by using PSM as a cosurfactant. Copyright 2001 Academic Press.     

Contribution toward comprehension of contact angle values on single polydimethylsiloxane and poly(ethylene oxide) polymer networks

The large application ranges of polydimethylsiloxane (PDMS) and poly(ethylene oxide) (PEO) based materials justify the importance of controlling polymer surface properties including morphology and wettability behavior. However, it appears that the reported contact angle values of PDMS surfaces show significant scattering which cannot always be interpreted in terms of sole chemical data. In addition, few values are reported concerning pure PEO surfaces, since the polymer generally swells in the presence of water. Thus, in order to correlate surface properties with sample preparation, several single PDMS and PEO polymer networks were synthesized with varying cross-linkers and different cross-linking densities. First, the sample surface topography was systematically analyzed by atomic force microscopy (AFM). It was proven that the removal process of the polymer film from the mold plays a significant role in surface topography according to the vitreous or rubbery state of the given polymer network at room temperature irrespective of mold surface treatment. AFM-scale smooth surfaces can be obtained for all the samples by removing them systematically from the mold at a temperature below the α-relaxation temperature. Dynamic water contact angles were then measured and the values analyzed as a function of cross-linker nature and cross-linking density.