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.     

Kinetics of ultraviolet and plasma surface modification of poly(dimethylsiloxane) probed by sum frequency vibrational spectroscopy

In numerous applications in microfluidics, cell growth, soft lithography, and molecular imprinting, the surface of poly(dimethylsiloxane) (PDMS) is modified from a hydrophobic methyl-terminated surface to a hydrophilic hydroxyl-terminated surface. In this study, we investigated molecular structural and orientational changes at the PDMS-air interface in response to three commonly used surface modification processes: exposure to long-wavelength ultraviolet light (UV), exposure to short-wavelength UV that generates ozone (UVO), and exposure to oxygen plasma (OP). The surfaces of two PDMS compositions (10:1 and 4:1 of base polymer/curing agent) were probed during modification, using monolayer-sensitive IR + visible sum frequency generation (SFG) vibrational spectroscopy, with two different polarization combinations. During PDMS surface modification, the peak intensities of CH3 side groups and CH2 cross-link groups decreased, while peak intensities of Si-OH groups increased. There was no significant change in the average orientation of the CH3 groups on the PDMS surface during modification. The concentration of CH3 groups on the surface decreased exponentially with time, for all three UV, UVO, and OP modification processes, with first order kinetics and time constants of approximately 160, 66, and 0.3 min, respectively. At steady state, residual CH3 groups were detected at the PDMS surface for UV and UVO treatments; however, there were negligible CH3 groups detected after OP modification.     

Rotating-disk sorptive extraction of nonylphenol from water samples

In this study the sorption of nonylphenol was implemented on a rotating Teflon disk coated with a PDMS film on one of its surfaces. In this way, the disk, which has a high surface area, contacts only the liquid sample, which can be stirred at higher velocity than with the stir bar used in stir-bar sorptive extraction (SBSE), without damaging the phase while at the same time facilitating analyte mass transfer to the PDMS surface. We refer to the procedure as rotating-disk sorptive extraction (RDSE). Extraction variables such as disk rotational velocity, extraction time, and surface area of PDMS film were studied to establish the best conditions for extraction. With increasing rotational velocity, the amount of extracted analyte significantly increases because the stagnant layer concomitantly decreases. On the other hand, the extracted amount concomitantly increases with extraction time, reaching equilibrium at approximately 20 min, which can be reduced to 10 min when the surface area of PDMS increases from 1.74 to 6.97 cm². Precision of the method was determined by using the same disk (n = 6) and different disks (n = 3), showing relative standard deviations for the analyte of 3.7% and 10%, respectively. The detection limit of the method was 0.09 μg/L NP, defined at a signal to noise ratio of 3. The method was applied to a real sample, achieving quantitative recovery. The PDMS phase on the disk could be used for at least 50 experiments. In any case, replacement of the PDMS film on the disk is very easy and inexpensive, as compared to the commercial alternative SBSE.     

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.     

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).     

Microscale patterning of organic films on carbon surfaces using electrochemistry and soft lithography

We have demonstrated three simple strategies employing poly(dimethylsiloxane) (PDMS) molds for patterning carbon surfaces with two different modifiers in an 18 microm line pattern. The PDMS molds are patterned with microfluidic channels (approximately 22 microm wide and 49 microm deep) and form a reversible, conformal seal to the pyrolyzed photoresist film (PPF) and modified PPF surfaces. Modifiers are electrochemically grafted to the PPF surface by the reduction of aryl diazonium salts and the oxidation of primary amines. For the fill-in patterning approach, the first modifier is electrografted to the PPF surface exposed within the microchannels, and in a second grafting step after removal of the PDMS mold, the second modifier fills in the remaining surface. The selective conversion strategy involves electrografting a continuous film of the modifier to the PPF surface, sealing the PDMS mold to the modified surface and carrying out an irreversible electrochemical reaction of the modifier exposed within the microchannels. In the build-up patterning approach, the PDMS mold is sealed to the modified PPF surface, and a chemical coupling reaction is effected in the microchannels to build up the pattern. The patterns are characterized using SEM, optical microscopy, the formation of condensation figures, and SEM imaging after the assembly of Au nanoparticles.     

Tunable Surface Wrinkling by a Bio-Inspired Polyamine Anion Coacervation Process that Mediates the Assembly of Polyoxometalate Nanoclusters

A bio-inspired method is used to render controlled wrinkling surface patterns on supramolecular architectures assembled from polyoxometalate (POM) clusters. It involves a polyamine-multivalent anion interaction generating positively charged coacervates, which while dictating the assembly of POM into spherical structures further facilitate an interesting surface morphogenesis with wrinkling patterns. This spontaneous surface wrinkling depends on the type of multivalent anion and the pH. As the polyamine-anion interaction becomes stronger, the wrinkles turn denser with lesser depth, which eventually undergoes post-buckling to engender a complex surface pattern. Interestingly, the order of influence exerted by different anions on the morphology follows the Hofmeister series. Moreover, the mild synthesis conditions keep the functional POM units dispersed in the sphere with a structural transformability to their lacunary form.     

Tunable Surface Wrinkling by a Bio‐Inspired Polyamine Anion Coacervation Process that Mediates the Assembly of Polyoxometalate Nanoclusters

A bio‐inspired method is used to render controlled wrinkling surface patterns on supramolecular architectures assembled from polyoxometalate (POM) clusters. It involves a polyamine‐multivalent anion interaction generating positively charged coacervates, which while dictating the assembly of POM into spherical structures further facilitate an interesting surface morphogenesis with wrinkling patterns. This spontaneous surface wrinkling depends on the type of multivalent anion and the pH. As the polyamine‐anion interaction becomes stronger, the wrinkles turn denser with lesser depth, which eventually undergoes post‐buckling to engender a complex surface pattern. Interestingly, the order of influence exerted by different anions on the morphology follows the Hofmeister series. Moreover, the mild synthesis conditions keep the functional POM units dispersed in the sphere with a structural transformability to their lacunary form.     

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.     

A novel electromagnetism‐assisted imprinting technology to replicate microstructures onto a large‐area curved surface using a flexible magnetic mold

Replication of microstructures from a mold onto a curved surface is difficult. The conformal contact between the mold and the substrate has to be ensured. The present study proposes an innovative mechanism, which employs an electromagnetic disk to provide magnetic force and a PDMS flexible mold with a layer compounded magnetic powder. This mechanism provides not only the gradual contact from center to edge to avoid air entrapment but also conformal contact between the mold and the substrate during the imprinting operation. A system based on this electromagnetic soft imprinting technology has been implemented, and imprinting to replicate microstructures from the mold onto a curved surface has been carried out. The results reveal that the PDMS magnetic mold and the electromagnetic disk‐controlled magnetic force can successfully perform the imprinting and accurately replicate the microstructures onto the large‐area, curved surface glass. The PDMS flexible magnetic mold incorporated with the magnetic disk can be employed to achieve the conformal contact between the mold and the substrate. In addition, due to the low surface free energy of the PDMS, the de‐molding without sticking can be easily accomplished. Copyright © 2008 John Wiley & Sons, Ltd.     

Self-organized anisotropic wrinkling of molecularly aligned liquid crystalline polymer

Anisotropic wrinkling which utilizes the anisotropic nature of liquid crystalline polymer (LCP) is demonstrated as a means of physical self-assembly to produce periodic microstructures. Through the plasma treatment on the molecularly aligned LCP film surface, one-dimensionally ordered wrinkle pattern was spontaneously formed on glass substrates without employing external thin-film deposition or prestrain control of the system. Experimental results indicate that the directionality of the wrinkle pattern can be tailored by the structural ordering of LCP molecules in the bilayer system of a hard skin layer on a soft substrate. Studies on process variables, such as the plasma treatment time and the film thickness, were conducted to figure out the effect on the wrinkling morphology. Due to its spatial periodicity over a large area and undemanding requirement of the process, this approach can be a candidate for the microfabrication in various applications.     

Poly(oxyethylene) based surface coatings for poly(dimethylsiloxane) microchannels

Control of surface properties in microfluidic systems is an indispensable prerequisite for successful bioanalytical applications. Poly(dimethylsiloxane) (PDMS) microfluidic devices are hampered from unwanted adsorption of biomolecules and lack of methods to control electroosmotic flow (EOF). In this paper, we propose different strategies to coat PDMS surfaces with poly(oxyethylene) (POE) molecules of varying chain lengths. The native PDMS surface is pretreated by exposure to UV irradiation or to an oxygen plasma, and the covalent linkage of POE-silanes as well as physical adsorption of a triblock-copolymer (F108) are studied. Contact angle measurements and atomic force microscopy (AFM) imaging revealed homogeneous attachment of POE-silanes and F108 to the PDMS surfaces. In the case of F108, different adsorption mechanisms to hydrophilic and hydrophobic PDMS are discussed. Determination of the electroosmotic mobilities of these coatings in PDMS microchannels prove their use for electrokinetic applications in which EOF reduction is inevitable and protein adsorption has to be suppressed.     

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.     

Development and research for applying innovative magnetic soft mold imprinting techniques in microstructure component manufacturing

The present study employs an innovative technique, which uses PDMS soft mold, blended with magnetic powder as the transmission and imprinting methods, and integrates features from soft micromolding PMMA, an electro‐magnetically controlled, well‐proportioned, pressing technique in order to study how to create microlens arrays through a magnetic soft mold imprinting resist technique. Thus, it renders nanometer imprinting applications, and its technology, more developed and mature. The research findings revealed that, PDMS, blended with magnetic powder, can accurately recast and duplicate nanometer microstructures. Under well‐proportioned magnetic pressing, controlled by an electro‐magnetic disk, it can effectively fill and shape resist microstructures. The composite material of PDMS, with added magnetic iron powder, can effectively improve mechanical strength properties of pure PDMS soft mold, which is easily transformed for imprinting. Meanwhile, owing to the unique features of PDMS soft mold, conformal contact with the base material is possible; therefore, the effective imprinting area and the duplicated representation are significantly improved. In addition, as magnetic PDMS soft mold is easily produced and fast in recasting, the costs can be effectively reduced. In addition, due to features such as low surface free energy and a tendency not to stick to resist in imprinting, the soft mold is evenly controlled by the electro‐magnetic disk for imprinting duplication, highlighting the advantages of microstructure imprinting procedures. Copyright © 2008 John Wiley & Sons, Ltd.     

Hydrophobization of glass surface by adsorption of poly(dimethylsiloxane)

Silica or glass particles are introduced in a poly(dimethylsiloxane) (PDMS) matrix for various applications. A particular feature of these systems is that PDMS adsorbs on the surface of the dispersed particles, thus rendering them more hydrophobic with time. The mechanism of this process of in situ hydrophobization is still poorly understood. The major aims of the present study are (1) to quantify the rate of surface hydrophobization by PDMS and, on this basis, to discuss the mechanism of the process; (2) to compare the contact angles of surfaces that are hydrophobized by different procedures and are placed in contact with different fluid interfaces-PDMS-water, hexadecane-water, and air-water; and (3) to check how the type of surfactant affects the contact angles, viz., the effective hydrophobicity of the surface. We present experimental results for the kinetics of hydrophobization of glass surfaces, which are characterized by measuring the three-phase contact angle of glass-surfactant solution-PDMS. The data reveal two consecutive stages in the hydrophobization process: The first stage is relatively fast and the contact angle increases from 0 degrees to about 90 degrees within several minutes. This stage is explained with the physical adsorption of the PDMS chains, as a result of hydrogen-bond formation with the surface silanol groups. The second stage is much slower and hours or days are required at room temperature to reach the final contact angle (typically, 150-160 degrees). This stage is explained as grafting of the PDMS molecules on the surface by chemical reaction with the surface silanol groups. If the glass surface had been pretreated by hexamethyldisilazane (HMDS), so that CH(3) groups had blocked most of the surface silanol groups, the first stage in the hydrophobization process is almost missing-the contact angle slowly changes at room temperature from about 90 degrees up to 120 degrees. The experiments aimed to compare several hydrophobization procedures showed that PDMS ensures larger contact angle (more hydrophobic surface) than grafted alkyl chains. The contact angles at the PDMS-water and hexadecane-water interfaces were found to be very similar to each other, and much larger than that at the air-water interface. Interestingly, we found that the ionic surfactants practically do not affect the contact angle of PDMS-hydrophobized surface, whereas the nonionic surfactants reduce this angle. Similar trends are expected with silica surfaces, as well.     

Changes in silicon elastomeric surface properties under stretching induced by three surface treatments

Poly(dimethylsiloxane) (PDMS) substrates are used in many applications where the substrates need to be elongated and various treatments are used to regulate their surface properties. In this article, we compare the effect of three of such treatments, namely, UV irradiation, water plasma, and plasma polymerization, both from a molecular and from a macroscopic point of view. We focus our attention in particular on the behavior of the treated surfaces under mechanical stretching. UV irradiation induces the substitution of methyl groups by hydroxyl and acid groups, water plasma leads to a silicate-like layer, and plasma polymerization causes the formation of an organic thin film with a major content of anhydride and acid groups. Stretching induces cracks on the surface both for silicate-like layers and for plasma polymer thin coatings. This is not the case for the UV irradiated PDMS substrates. We then analyzed the chemical composition of these cracks. In the case of water plasma, the cracks reveal native PDMS. In the case of plasma polymerization, the cracks reveal modified PDMS. The contact angles of plasma polymer and UV treated surfaces vary only very slightly under stretching, whereas large variations are observed for water plasma treatments. The small variation in the contact angle values observed on the plasma polymer thin film under stretching even when cracks appear on the surface are explained by the specific chemistry of the PDMS in the cracks. We find that it is very different from native PDMS and that its structure is somewhere between Si(O2) and Si(O3). This is, to our knowledge, the first study where different surface treatments of PDMS are compared for films under stretching.     

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.     

Plasma stencilling methods for cell patterning

In this paper we describe plasma stencilling techniques for patterning 10 mammalian cell lines on hydrophobic and cell repellent poly(dimethylsiloxane) (PDMS), methylated glass and bacterial grade polystyrene surfaces. An air plasma produced with a Tesla generator operating at atmospheric pressure was used with microengineered stencils for patterned surface oxidation, selectively transforming the surface to a hydrophilic state to enable cell adhesion and growth. Plasma stencilling obviates the need for directly patterning cell adhesion molecules. Instead, during cell culture, adhesion proteins from the media assemble in a bioactive form on the hydrophilic regions. Critically, the removal of protein patterning prior to cell culture provides the option to also use PDMS–PDMS plasma bonding to incorporate cell patterns within microfluidic systems. Linear patterns were generated using PDMS microchannel stencils, and polyimide stencils with through holes were used for the production of cellular arrays. For the production of smaller cellular arrays, a novel microcapillary-based dielectric barrier discharge system was developed. A numerical method to characterise the cell patterns is also introduced and was used to demonstrate that plasma stencilling is highly effective, with complete patterns confined during long term cell culture (>10 days). In summary, plasma stencilling is simple, rapid, inexpensive, reproducible and a potentially universal cell line patterning capability.

Modification of micro-channel filling flow by poly(dimethylsiloxane) surface functionalization with fluorine—Substituted aminonaphthols

Microfluidics based on the capillarity-induced filling of elastomeric channels by a suitable liquid or solution represents a useful route for realizing portable diagnostic devices designed without additional mechanical or electrical micropumps. In this study, an elastomeric mold made of poly(dimethylsiloxane) (PDMS), containing relief patterns placed in intimate contact with a silicon substrate, is utilized to create a continuous network of rectangular micro-channels for the motion of water fluid. The immobilization on activated PDMS surface of suitable functional molecules such as hydrophilic and hydrophobic fluorine-containing aminonaphthols, obtained through a straightforward and versatile synthetic procedure, allowed us to modulate PDMS surface properties depending on the structural characteristics of the employed derivative. In this context, the incorporation of fluorine groups is important for improving biocompatibility of the resulting device, providing surfaces that could be chemically and biologically inert as well as resistant to surface adhesion phenomena. The functionalization from liquid phase of PDMS replicas, involving a covalent derivatization via silanization reaction of the above mentioned compounds to an oxidized PDMS surface, resulted in a successful modification of microfluidic motion of water in rectangular capillaries, moreover contact angle values evidence also how wettability of PDMS films could be modulated, with the fluorinated aminonaphthols fuctionalized PDMS exhibiting higher contact angles.     

Chains anisotropy in PDMS networks due to friction on gold surfaces

In this article, we describe an experimental friction study of poly(dimethyl siloxane) (PDMS) networks on metallic substrates such as gold-coated slides, and under different conditions. The friction generates a transfer of a thin layer of PDMS and a preferential orientation of the polymer chains at the interface. However, the characterization of this layer is complicated, given the small amount of matter and the contact with a metallic surface. The polarization-modulation infrared reflexion-absorption spectroscopy (PM-IRRAS), which is an excellent tool for anisotropy and orientation studies, was used to characterize the PDMS transferred layer. Our results showed an induced anisotropy due to the friction, and in which PDMS chains are lying parallel to the gold substrate surface. Our spectroscopic analyzes allowed us to imagine a scheme of PDMS transfer on the gold surface. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2348–2353, 2004