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     

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.     

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.     

Facile synthesis and responsive behavior of PDMS‐b‐PEG diblock copolymer brushes via photoinitiated “thiol‐ene” click reaction

We present herein a mild and rapid method to create diblock copolymer brushes on a silicon surface via photoinitiated “thiol‐ene” click reaction. The silicon surface was modified with 3‐mercaptopropyltrimethoxysilane (MPTMS) self‐assembled monolayer. Then, a mixture of divinyl‐terminated polydimethylsiloxane (PDMS) and photoinitiator was spin‐coated on the MPTMS surface and exposed to UV‐light. Thereafter, a mixture of thiol‐terminated polyethylene glycol (PEG) and photoinitiator were spin‐coated on the vinyl‐terminated PDMS‐treated surface, and the sequent photopolymerization was carried out under UV‐irradiation. The MPTMS, PDMS, and PEG layers were carefully identified by X‐ray photoelectron spectroscopy, atomic force microscopy, ellipsometry, and water contact angle measurements. The thickness of the polydimethylsiloxane‐block‐poly(ethylene glycol) (PDMS‐b‐PEG) diblock copolymer brush could be controlled by the irradiation time. The responsive behavior of diblock copolymer brushes treated in different solvents was also discussed. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

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     

Modification of a hydrophilic linear polyurethane by crosslinking with a polydimethylsiloxane. Influence of the crosslink density and of the hydrophobic/hydrophilic balance on the water transport properties

A hydrophilic thermoplastic polyurethane (TPU) was modified by reactive extrusion to obtain in a first step a grafted and soluble material and to finally form by a hydrolysis condensation process a weakly crosslinked network. Different isocyanates were used as grafting agents and a α,ω‐dihydroxypoly(dimethylsiloxane) (PDMS) was used to modify the hydrophilic/hydrophobic balance of the material and the chain length between the crosslinks. The influence of the isocyanate functionality and of the PDMS content were studied on the network formation and on the thermomechanical and water sorption properties. The networks properties were also compared with those of a TPU/PDMS blend. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 48–61, 2006     

Photochemical modification of polymer surface by the pendant photobase generator containing oxime‐urethane groups and its application to an image‐recording material

Copolymers containing oxime‐urethane groups were prepared by the copolymerization of methyl methacrylate and benzophenoneoximinocarbonylaminoethyl methacrylate (BCM), and their photochemical properties were examined from the UV and IR absorption spectral changes. The decomposed fraction of oxime‐urethane groups in the copolymer increased with irradiation time, but it decreased with the content of BCM units in the copolymer. Changes of the surface properties of the copolymer film on irradiation were studied by measurements of the contact angle and dyeing with an acid dye. The surface of the copolymer film changed to become more hydrophilic upon irradiation with 254 nm of UV light. After the irradiated copolymer films were treated with HCl or methanol, changes of the contact angle of water on irradiation were compared. The copolymer film was dyed by acid dyes after treatment of the irradiated film with HCl. The degree of dyeing increased with irradiation time and BCM units in the copolymer, but it was unaffected by the film thickness. Various colors were developed on the irradiated area depending on the acid dye as the developer. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1200–1207, 2002     

A facile approach to fabricate hierarchically structured poly(3‐hexylthiophene‐2,5‐diyl) films

Microstructured surfaces have great potentials to improve the performances and efficiency of optoelectronic devices. In this work, a simple robust approach based on surface instabilities was presented to fabricate poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) films with ridge‐like/wrinkled composite microstructures. Namely, the hierarchically patterned films were prepared by spin coating the P3HT/tetrahydrofuran (THF) solution on a polydimethylsiloxane (PDMS) substrate to form stable ridge‐like structures, followed by solvent vapor swelling to create surface wrinkles with the orientation guided by the ridge‐like structures. During spin coating of the P3HT/THF solution, the ridge‐like structures were generated by the in‐situ template of the THF swelling‐induced creasing structures on the PDMS substrate. To our knowledge, it is the first report that the creasing structures are used as a recoverable template for patterning films. The crease‐templated ridge‐like structures were well modulated by the THF swelling time, the modulus of the PDMS substrate, the P3HT/THF solution concentration and the selective/blanket exposure of the PDMS substrate to O₂ plasma. UV–vis and fluorescence spectrometry measurements indicated that the light absorption and fluorescent emission were improved on the hierarchically patterned P3HT films, which can be utilized to enhance the efficiencies of organic solar cells. Furthermore, this simple versatile method based on the solvent swelling‐induced crease as the in‐situ recoverable template has been extended to pattern other spin‐coated films with different compositions. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 928–939     

Fabrication of biomimetic superhydrophobic surfaces by a simple flame treatment method

A simple flame treatment method was explored to construct micro/nanostructures on a surface and then fabricate a biomimetic superhydrophobic surface at a relatively low cost. SiO₂‐containing polydimethylsiloxane (PDMS) was used as a substrate. The PDMS replicas with various micropatterned surfaces were fabricated using grass leaf, sand paper, and PET sheet with parallel groove geometry as templates via PDMS replica molding. The PDMS replica surfaces with micron structures and the surface of a flat PDMS sheet as a control sample were further treated by flame. The fabricated surfaces were characterized by scanning electron microscopy and water contact angle measurements. The effect of surface microstructures on the transparency of PDMS was also investigated. The studies indicate that the fine nanoscale structures can be produced on the surfaces of PDMS replicas and a flat PDMS sheet by a flame treatment method, and that the hierarchical surface roughness can be adjusted and controlled by varying the flame treatment time. The flame‐treated surfaces of PDMS replicas and a flat PDMS sheet possess superhydrophobicity and an ultra‐low sliding angle reaching a limiting value of 1°, and the anisotropic wettability of the PDMS replica surface with oriented microgroove structures can be greatly suppressed via flame treatment. The visible light transmittance of the flame‐treated flat PDMS surface decreases with prolonged flame treatment times. Copyright © 2016 John Wiley & Sons, Ltd.     

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.     

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.     

Influences of acid and heat treatments on the structure and water vapor adsorption property of natural zeolite

Natural zeolite was first acid treated and then heat treated. The influences of acid and heat treatments on the structure and water vapor adsorption property of natural zeolite were investigated. X‐ray photoelectron spectroscopy (XPS), X‐ray diffraction (XRD), thermogravimetric analysis (TG), and Brunauer‐Emmett‐Teller (BET) surface areas were employed to investigate the structure changes of the zeolite samples. Water vapor adsorption property of the samples was investigated in order to determine wettability of the samples. The results showed that nitric acid treatment could remove Al from the structure, decrease relative crystallinity, and significantly increase specific surface area of the zeolite samples. Heat treatment could change or even destroy the structure, effectively remove water molecules from the surface/structure, and decrease the specific surface area of the zeolite samples. Water vapor adsorptions on the acid‐treated zeolite samples were significantly lower than that for the zeolite sample, and the adsorptions further reduced for the heat‐treated zeolite samples. The increase of Si/Al ratio, decreases of silanol groups, and specific surface area of the samples were major reasons related to the reductions of water vapor adsorptions on the zeolite samples.     

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).     

Properties of poly(dimethylsiloxane)/hydrogel multicomponent systems

The properties of surface‐ and bulk‐modified poly(dimethylsiloxane) (PDMS) were examined. Laser‐induced surface grafting of poly(2‐hydroxyethyl methacrylate) (PHEMA) on PDMS and a sequential method for preparation of interpenetrating polymer networks of PDMS/PHEMA were, respectively, used for surface and bulk modifications. The hydrogel content and water‐uptake capability of the modified samples were also investigated. The modified PDMS samples were examined by performing attenuated total reflection/Fourier transform infrared spectroscopy, dynamic mechanical thermal analysis, scanning electron microscopy, and water contact‐angle measurements. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2145–2156, 2003     

Photochemically patterned poly(methyl methacrylate) surfaces used in the fabrication of microanalytical devices

We report here the photochemical surface modification of poly(methyl methacrylate), PMMA, microfluidic devices by UV light to yield pendant carboxylic acid surface moieties. Patterns of carboxylic acid sites can be formed from the micrometer to millimeter scale by exposure of PMMA through a contact mask, and the chemical patterns allow for further functionalization of PMMA microdevice surfaces to yield arrays or other structured architectures. Demonstrated here is the relationship between UV exposure time and PMMA surface wettability, topography, surface functional group density, and electroosmotic flow (EOF) of aqueous buffer solutions in microchannels made of PMMA. It is found that the water contact angle on PMMA surfaces decreases from 70 degrees to 24 degrees after exposure to UV light as the result of the formation of carboxylic acid sites. However, upon rinsing with 2-propanol, the water contact angle increases to approximately 80 degrees , and this increase is attributed to changes in surface roughness resulting from removal of low molecular weight PMMA formed from scission events. In addition, the surface roughness and surface coverage of carboxylic acid groups exhibit a characteristic trend with UV exposure time. Electroosmotic flow (EOF) in PMMA microchannels increases upon UV modification and is pH dependent. The possible photolysis mechanism for formation of carboxylic acid groups on PMMA surfaces under the conditions outlined in this work is discussed.     

Nanoscale hydrophobic recovery: A chemical force microscopy study of UV/ozone-treated cross-linked poly(dimethylsiloxane)

Chemical force microscopy (CFM) in water was used to map the surface hydrophobicity of UV/ozone-treated poly(dimethylsiloxane) (PDMS; Sylgard 184) as a function of the storage/recovery time. In addition to CFM pull-off force mapping, we applied indentation mapping to probe the changes in the normalized modulus. These experiments were complemented by results on surface properties assessed on the micrometer scale by X-ray photoelectron spectroscopy and water contact-angle measurements. Exposure times of < or = 30 min resulted in laterally homogeneously oxidized surfaces, which are characterized by an increased modulus and a high segmental mobility of PDMS. As detected on a sub-50-nm level, the subsequent "hydrophobic recovery" was characterized by a gradual increase in the pull-off forces and a decrease in the normalized modulus, approaching the values of unexposed PDMS after 8-50 days. Lateral imaging on briefly exposed PDMS showed the appearance of liquid PDMS in the form of droplets with an increasing recovery time. Longer exposure times (60 min) led to the formation of a hydrophilic silica-like surface layer. Under these conditions, a gradual surface reconstruction within the silica-like layer occurred with time after exposure, where a hydrophilic SiOx-enriched phase formed < 100-nm-sized domains, surrounded by a more hydrophobic matrix with lower normalized modulus. These results provide new insights into the lateral homogeneity of oxidized PDMS with a resolution in the sub-50-nm range.     

One‐step synthesis of polymer micro‐tubes tethered by polymer nanowire networks via RAFT polymerization of N,N′‐methylene bisacrylamide xerogel fibers in toluene and ethanol mixed solution

In the mixed solution of toluene and ethanol, polymer micro‐tubes (PMTs) tethered by polymer nanowire networks (PTPWNs) were fabricated facilely via one‐step reversible addition fragmentation chain transfer (RAFT) polymerization by taking N,N′‐methylene bisacrylamide (MBA) xerogel fibers as both template and monomer source. The products were analyzed by FTIR, SEM, TEM, surface area and porosity analyzer, and contact angle tester. The results indicated that PTPWNs were obtained as the sole product at ethanol content of 1.0 wt %. As the content of ethanol increases from 0 to 1.0 wt %, the specific surface area of the products became higher, indicating more polymer nanowire networks (PWNs) on the tubes. At ethanol contents of 1.5 wt % and 2.0 wt %, some particles were also obtained besides PTPWNs. The formation process of PTPWNs was studied by analyzing the products obtained at different reaction time. The results revealed that PTPWNs were formed by two steps, PMTs were formed quickly and then PWNs formed in the solution tethered to the tubes. Moreover, the effect of RAFT agent on the morphologies of the products revealed that PTPWNs could be obtained via RAFT polymerization at suitable dosage of RAFT agent, while polymer particles were generated via conventional free radical polymerization. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1862–1868     

Effect of annealing on the microphase separated behavior and properties of PDMS–MDI–PEG multiblock copolymer films

A model multiblock copolymer based on (Poly dimethylsiloxane) (PDMS),–4, 4′‐diphenylmethanediisocyanate (MDI)–(poly ethylene glycol) (PEG) was synthesized by employing two step growth polymerization technique. The effect of annealing on microphase separation of the copolymer surface and bulk, surface composition, hydrogen‐bonding and some properties was investigated by AFM, SAXS, XPS, FTIR, contact angle measurement, and protein adsorption experiment, respectively. It was found that increasing the annealing temperature availed formation of microphase separation and surface enrichment of PDMS, which was accompanied by increase in average interdomain spacing, long period, and the crystallizing degree in the hard domains. But the best microphase separated structure seemed to occur at the annealing temperature of 140 °C; exorbitant annealing temperature might demolish the ordered structure. The annealing temperature dependence of microphase separation was further confirmed by the changes in urea hydrogen‐bonding and melting points characterized by FTIR and DSC, respectively. Protein adsorption experiments revealed that all annealed copolymer films possessed the low protein adsorption. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 208–217, 2007     

Reverse hydrophobic PDMS surface to hydrophilic by 1‐step hydrolysis reaction

Hydrophilic modification on the surface of polymer polydimethylsiloxane (PDMS) material is a key step for its application in biomaterial, bioengineering, and so on. In this article, a novel and effective method was proposed to reverse hydrophobic surface to hydrophilic by 1‐step hydrolysis of Si―O bond to produce hydrophilic hydroxyl group. The hydrophilizing reagent 2‐(trimethylsiloxy) ethyl methacrylate (TMSEMA) was used during the copolymerization of polydimethylsiloxane prepolymer (DMS U21). The prepared PDMS film was subjected to 1‐step surface hydrophilic reversal treatment using KOH solution to produce hydroxyl groups on the surface. The contact angle, attenuated total reflection Fourier transform infrared spectra, and equilibrium water content (EWC) measurements were conducted on PDMS films. The results showed that TMSEMA content had no obvious impact on the contact angle and EWC value of untreated PDMS. After reversal treatment, the contact angle decreased from 94° to 15°, and the EWC value increases to 10% when the TMSEMA content was 15 wt%. The spectrum proved that the reverse reaction produced hydroxyl and carboxylate on the surface. The hydrophilic stability, surface morphology, and protein adsorption properties of PDMS film were also investigated. This study can provide new ideas and further reference for improving the hydrophilicity of PDMS surface.     

The stability of radio-frequency plasma-treated polydimethylsiloxane surfaces

Polydimethylsiloxane (PDMS) is a widely used material for manufacturing lab-on-chip devices. However, the hydrophobic nature of PDMS is a disadvantage in microfluidic systems. To transform the hydrophobic PDMS surface to hydrophilic, it was treated with radio-frequency (RF) air plasma at 150, 300, and 500 mTorr pressures for up to 30 min. Following the surface treatment, the PDMS specimens were stored in air, deionized water, or 0.14 M NaCl solution at 4 degrees C, 20 degrees C, and 70 degrees C. The change in the hydrophilicity (wettability) of the PDMS surfaces was followed by contact angle measurements and Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy as a function of time. As an effect of the RF plasma treatment, the contact angles measured on PDMS surfaces dropped from 113 +/- 4 degrees to 9 +/- 3 degrees . The chamber pressure and the treatment time had no or negligible effect on the results. However, the PDMS surface gradually lost its hydrophilic properties in time. The rate of this process is influenced by the difference in the dielectric constants of the PDMS and its ambient environment. It was the smallest at low temperatures in deionized water and largest at high temperatures in air. Apparently, the OH groups generated on the PDMS surface during the plasma treatment tended toward a more hydrophilic/less hydrophobic environment during the relaxation processes. The correlation between the FTIR-ATR spectral information and the contact angle data supports this interpretation.