Functionalized imidazolium wear‐resistant ionic liquid ultrathin films for MEMS/NEMS applications

As a kind of new material, ionic liquids (ILs) are considered a new type of lubricant for micro/nanoelectromechanical system (M/NEMS) due to their excellent thermal and electrical conductivity. However, so far, only a few reports have investigated the friction and wear of thin films of these materials at the micro scale. Evaluating the nanoscale tribological performance of ILs when applied as films of a few nanometers thickness on a substrate is a critical step for their application in M/NEMS devices. To achieve this purpose, IL thin films with four kinds of anions were synthesized and prepared on single‐crystal silicon wafers by the dip‐coating method. Film thickness was determined by the ellipsometric method. Their surface morphologies were observed by means of atomic force microscopy (AFM). The nano and micro tribological properties of the IL films were investigated by a friction force microscope (FFM)with a spherical probe and a UMT‐2MT tribotester, respectively. The corresponding morphologies of the wear tracks of the IL films were examined using a three‐dimensional non‐contact interferometric microscope. The impact of temperature on the adhesion behavior was studied, as well as the effect of sliding frequency and load on the friction coefficient, load bearing capacity and anti‐wear durability. It was found that friction, adhesion and durability of IL films were strongly dependent on their anionic molecular structures, wettability and ambient environment. Copyright © 2010 John Wiley & Sons, Ltd.     

硅表面复合自组装膜的制备及其摩擦性能

采用自组装技术在单晶硅表面制备了3-氨基丙基三乙氧基硅烷(APTES)-SiO2-APTES复合膜,并对其表面的组成、结构及摩擦性能进行了表征.结果表明:复合膜表面对水的接触角约为63°,且表面平整、致密,其平均粗糙度(Ra)约为0.963nm.通过原子力显微镜(AFM)和透射电子显微镜(TEM)观察到夹层中SiO2颗粒的粒径约为20-50nm,较均匀地分布在第一层APTES膜的表面.与APTES自组装单层膜(SAMs)相比,APTES-SiO2-APTES复合膜由于纳米SiO2颗粒的引入而表现出更低的摩擦系数和更长的耐磨寿命.     

Self-organization of polymer particles on hydrophobic solid substrates in aqueous media. II. Self-organization of cationic polymer particles on polymer films and wettability control with particle monolayers

Self-organization of cationic polymer particles through hydrophobic interaction on polymer films in aqueous system and characteristic properties of the resulting particle monolayers were investigated. Cationic polymer particles bearing quaternary ammonium groups on their surfaces effectively self-organized on polymer films. With an increase of the particle surface charge density, the surface coverage and average aggregate size (N _a) decreased. The surface coverage control was accomplished by tuning the ionic strength of the media. The wettability of polymer films for water was imparted by the formation of particle monolayers on them. Annealing of the particle monolayers resulted in the increase of the adhesive strength, while the wettability for water was lost. Further improvements of both wettability and adhesive strength of particle monolayers were achieved by the immobilization of silica colloids on the particle monolayers. This method would be effective for the hydrophilization of polymer films.     

Tribological properties of alkylsilane self-assembled monolayers

In this study, we perform molecular dynamics simulations of adhesive contact and friction between alkylsilane Si(OH)(3)(CX(2))(10)CX(3) and alkoxylsilane Si(OH)(2)(CX(2))(10)CX(3) (where X = H or F) self-assembled monolayers (SAMs) on an amorphous silica substrate. The alkylsilane SAMs are primarily hydrogen-bonded or physisorbed to the surface. The alkoxylsilane SAMs are covalently bonded or chemisorbed to the surface. Previously, we studied the chemisorbed systems. In this work, we study the physisorbed systems and compare the tribological properties with the chemisorbed systems. Furthermore, we examine how water at the interface of the SAMs and substrate affects the tribological properties of the physisorbed systems. When less than a third of a monolayer is present, very little difference in the microscopic friction coefficient mu or shear stresses is observed. For increasing amounts of water, the values of mu and the shear stresses decrease; this effect is somewhat more pronounced for fluorocarbon alkylsilane SAMs than for the hydrocarbon SAMs. The observed decrease in friction is a consequence of a slip plane that occurs in the water as the amount of water is increased. We studied the frictional behavior using relative shear velocities ranging from v = 2 cm/s to 2 m/s. Similar to previously reported results for alkoxylsilane SAMs, the values of the measured stress and mu for the alkylsilane SAM systems decrease monotonically with v.     

Microtribological study of internal surfaces of fluorinated mesoporous silica films

Fluorinated mesoporous silica films were synthesized via sol-gel co-condensation and coated on glass substrates. Surfactant template concentrations were varied to examine the effect of encapsulated organic functionality on the microtribological properties of films using atomic force microscopy. Films were tested as synthesized and also after being abraded to expose interior mesostructured surfaces. Results indicate that templating allows fluorinated moieties to become encapsulated within the film, which affect the tribological properties of the exposed internal surfaces. Depending on the amount of template added, the interior surfaces were able to achieve a friction level comparable to that of conventional monolayers. The dependence of friction on sliding speed revealed that fluorinated templated films have tribological properties intermediate to those of a nonfunctional surface and a conventional fluorinated monolayer.     

Micro/macrotribological properties of several nano‐scale ionic liquid films on modified silicon wafers

Three kinds of alkylimidazolium base room temperature ionic liquids (RTILs) were synthesized and their nano‐scale lubricant films were prepared on modified silicon wafers by dip‐coating method. The thicknesses of these films were measured and their relationship between thickness and solution concentration was obtained. Their surface morphologies were observed and contact angles of water on these films were determined. The adhesions and friction coefficients of these films were detected by contact mode AFM. As comparison, their macrotribological properties were evaluated on a UMT‐2MT tribo‐tester. It was found that, in microscale, 1‐hexyl‐3‐methyl‐imidazolium hexafluorophosphate performed the best tribological properties, while in macroscale, its tribological properties were similar with that of 1‐hexyl‐3‐methyl‐imidazolium tetrafluoroborate and better than that of 1‐hexyl‐3‐methyl‐imidazolium adipate. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis and self-assembly of galactose-terminated alkanethiols and their ability to resist proteins

The synthesis of two galactose-terminated alkanethiols with the structural formula X-OC2H5NHCO(CH2)15SH (X = 2,3,4,6-tetra-O-methyl-beta-D-Gal or beta-D-Gal) is described. Single-component and mixed self-assembled monolayers (SAMs) of the methylated and nonmethylated compounds were prepared on gold and subsequently characterized with ellipsometry, contact angle goniometry, and infrared reflection-absorption spectroscopy. Studies of the irreversible protein adsorption onto the SAMs using ex-situ ellipsometry revealed very low levels of fibrinogen and lysozyme adsorption onto mixed SAMs displaying advancing water contact angles between 24 degrees and 45 degrees and below 45 degrees , respectively. A monomethylated compound (X = 6-O-methyl-beta-D-Gal) was also synthesized and assembled on gold. This particular compound was found to possess wettability properties corresponding to the low adsorption regime of the mixed SAMs, and the results from the same set of fibrinogen and lysozyme adsorption experiments showed very low levels of protein adsorption. Our findings suggest that the protein rejecting properties rely on a fine balance between the surface energy and/or hydrogen bond donating/accepting properties of the SAM surface.     

Effect of humidity and temperature on nano/microtribolgical properties of perfluorinated carboxylic acid and hydrogenated carboxylic acid self‐assembled monolayers deposited on aluminum‐coated silicon substrate

Two series of perfluorinated carboxylic acid (FC) and hydrogenated carboxylic acid (HC) self‐assembled monolayer (SAM) films were prepared on aluminum surfaces separately by chemical vapor deposition. The formation, structure and morphology of these films were characterized by measuring contact angle with ellipsometric method, x‐ray photoelectron spectrometry, and atomic force microscopy, respectively. FC and HC SAMs with long chains formed more densely packed films than those with short chains did. The comparative micro/nanoscale friction and adhesive properties of FC and HC SAMs, with various chain lengths on aluminum‐coated silicon substrate, were investigated. The influence of environmental conditions, such as relative humidity (RH) and temperature, on the friction and adhesion behavior was studied. Micro/nanotribological properties of the films were greatly influenced by their backbones and terminal groups. FC SAMs with long chain exhibited adhesion‐resistance, friction reduction, and environmental independence. Copyright © 2009 John Wiley & Sons, Ltd.     

Fabrication of flexible gold films with periodic sub-micrometer roughness and their wettability control by modification of SAM

Flexible honeycomb gold films supported by polymer sheets are fabricated by using polystyrene particle monolayers. The surfaces of the flexible gold films are covered with self-assembled monolayers (SAMs) of hydrophobic or hydrophilic thiol compounds, and the wettability of the modified surface is evaluated by measurements of the contact angles of water droplets. The contact angle of the film covered with hydrophobic SAM is ca. 150 degrees, which is greater than the value of 112 degrees for a flat gold surface, while the values for hydrophilic SAM are below 10 degrees.     

以TDI和IPDI为单体合成聚氨酯微胶囊及其摩擦学性能

分别以甲苯-2,4-二异氰酸酯(TDI)和异佛尔酮二异氰酸酯(IPDI)为单体,通过原位聚合法制备了离子液体@聚脲(PU)微胶囊,并与环氧树脂共混制得环氧树脂复合材料.利用扫描电子显微镜分析了微胶囊及复合材料的表面形貌,通过电子万能试验机和摩擦磨损试验机探究了微胶囊改性复合材料在不同情况下的力学性能和摩擦学性能,用傅里叶变换红外光谱对微胶囊进行表征.分析结果表明,以IPDI为单体合成的微胶囊摩擦学性能更加优异,并且随着微胶囊用量的增加,复合材料的摩擦学性能有明显提高,当微胶囊添加质量分数为20%时,含有微胶囊的复合材料具有较低的滑动摩擦系数并且摩擦面较光滑,这是由于在实验过程中,随着微胶囊壁材的破损,芯材离子液体被释放,形成了一层致密的润滑膜.     

Local packing environment strongly influences the frictional properties of mixed CH3- and CF3-terminated alkanethiol SAMs on Au(111)

Compositionally mixed, self-assembled monolayers (SAMs) derived from 16,16,16-trifluorohexadecanethiol and a normal alkanethiol, either hexadecanethiol or pentadecanethiol, were formed on Au(111) substrates. The relative composition of the films was determined using X-ray photoelectron spectroscopy and was found to approximately equal the equimolar composition of the isooctane solution from which they were formed. The frictional properties of the mixed films were measured on the nanometer scale using atomic force microscopy and were observed to decrease when the chain length of the CH(3)-terminated component was shortened by one methylene unit (i.e., when hexadecanethiol was replaced by pentadecanethiol). For comparison, the frictional properties of a mixed-chain-length CH(3)-terminated SAM derived from hexadecanethiol and pentadecanethiol in a 1:1 ratio was also examined. In contrast to the mixed CF(3)/CH(3) system, the latter mixed-chain-length system exhibited relatively higher friction when compared to single-component SAMs derived solely from either hexadecanethiol or pentadecanethiol. For both types of mixed films, the change in frictional properties that occurs as a result of modifying the position of neighboring terminal groups with respect to the surface plane is discussed in terms of the influence of local packing environments on interfacial energy dissipation (friction).     

阻抗滴定法测定巯基乙酸及其混合自组装膜表面酸度

采用循环伏安和交流阻抗法对巯基乙酸及其混合自组装膜的表面酸性基团的离解特性进行了研究,并利用阻抗滴定曲线得出了巯基乙酸及其混合自组装膜的表面酸度。研究了混合组装电极中其他组分含量的变化、支持电解质的离子强度等因素对表面酸度移动的影响,用氢键作用、疏水相互作用及分子间相互作用对实验结果进行了解释。     

Mixed alkanethiol self-assembled monolayers as substrates for microarraying applications

In current microarraying experiments, data quality is in large part determined by the quality of the spots that compose the microarray. Since many microarrays are made with contact printing techniques, microarray spot quality is fundamentally linked to the surface characteristics of the microarray substrate. In this work, surface coatings, consisting of self-assembled monolayers (SAMs) of mixed alkanethiol molecules, were used to control the surface properties of the microarray substrate. X-ray photoelectron spectroscopy and equilibrium contact angle measurements were performed in order to confirm the chemical content and wettability of these surface coatings. To test their performance in microarraying applications, sample microarrays were printed on these mixed alkanethiol films and then characterized with a noncontact visual metrology system and a fluorescence scanner. This work demonstrates that utilizing mixed alkanethiol SAMs as a surface coating provides spatially homogeneous surface characteristics that are reproducible across multiple microarray substrates as well as within a substrate. In addition, this paper demonstrates that these films are stable and robust as they can maintain their surface characteristics over time. Overall, it is demonstrated that SAMs of mixed alkanethiols serve as a useful surface coating, which enhances spot and therefore data quality in microarraying applications.     

Formation and tribological properties of two‐component ultrathin ionic liquid films on Si

Several single‐component and two‐component imidazolium ionic liquids (ILs) ultrathin films were formed on Si substrates by a dip‐coating and heat treatment process. The formation and surface properties of the films were analyzed by means of ellipsometric thickness measurement, X‐ray photoelectron spectra and atomic force microscope. The adhesive and nanotribological behaviors of the films were evaluated by a homemade colloidal probe. A ball‐on‐plate tribometer was used to test the microtribological performances of these films. As a result, the two‐component ILs ultrathin film containing 80% solid‐like ILs phase shows more homogenous surface morphologies and optimal micro/nano‐tribological properties as compared to single‐component ILs films, which is ascribed to a synergic effect between the steady solid‐like ILs phase as the backbone and the proper amount of flowable liquid‐like ILs phase. By studying the influence of various solid/liquid ILs ratios on tribological properties of the two‐component ILs films, we might find the way to design ILs films with excellent comprehensive tribological properties. Copyright © 2010 John Wiley & Sons, Ltd.     

Characterization and tribological investigation of self-assembled trimethoxysilyl-functionalized poly(phthalazinone ether ketone)thin films on glass substrates

<正>Trimethoxysilyl-functionalized PPEK(PKGS) films had been designed to serve as wear resistant coatings for silicon surfaces. These surface films were formed by a dip-coating technique applied to self-assembled monolayers(SAMs).The formation and wetting behavior of PKGS films were characterized by means of contact angle measurement.The friction coefficient of the film prepared is very low(about 0.1),and the anti-wear behavior is good,with a lack of failure after sliding over 1800 s.     

Tunable resistive m-dPEG acid patterns on polyelectrolyte multilayers at physiological conditions: template for directed deposition of biomacromolecules

This paper describes a new class of salt-responsive poly(ethylene glycol) (PEG) self-assembled monolayers (SAMs) on top of polyelectrolyte multilayer (PEMs) films. PEM surfaces with poly(diallyldimethylammonium chloride) as the topmost layer are chemically patterned by microcontact printing (muCP) oligomeric PEG molecules with an activated carboxylic acid terminal group (m-dPEG acid). The resistive m-d-poly(ethylene glycol) (m-dPEG) acid molecules on the PEMs films were subsequently removed from the PEM surface with salt treatment, thus converting the nonadhesive surfaces into adhesive surfaces. The resistive PEG patterns facilitate the directed deposition of various macromolecules such as polymers, dyes, colloidal particles, proteins, liposomes, and nucleic acids. Further, these PEG patterns act as a universal resist for different types of cells (e.g., primary cells, cell lines), thus permitting more flexibility in attaching a wide variety of cells to material surfaces. The patterned films were characterized by optical microscopy and atomic force microscopy (AFM). The PEG patterns were removed from the PEM surface at certain salt conditions without affecting the PEM films underneath the SAMs. Removal of the PEG SAMs and the stability of the PEM films underneath it were characterized with ellipsometry and optical microscopy. Such salt- and pH-responsive surfaces could lead to significant advances in the fields of tissue engineering, targeted drug delivery, materials science, and biology.     

Chemical Force Microscopy Study of Adhesion and Friction between Surfaces Functionalized with Self-Assembled Monolayers and Immersed in Solvents

Adhesive and frictional forces between surfaces modified with self-assembled monolayers (SAMs) and immersed in solvents were measured with chemical force microscopy as functions of surface functionality and solvent. Si/SiO2 substrates were modified with SAMs of alkylsiloxanes (SiCl3(CH2)n-X), and gold-coated AFM tips were modified with SAMs of alkylthiolates (HS-(CH2)n-X). SAMs of alkylsiloxanes terminated in a methyl or oxidized vinyl group; SAMs of alkanethiolates terminated in a methyl or carboxyl group. Adhesive and frictional forces were measured in hexadecane, ethanol, 1,2-propanediol, 1,3-propanediol, and water. The work of adhesion (W) was calculated with the Johnson-Kendall-Roberts theory of adhesive contact. The JKR values agreed well with values derived from the Fowkes-van Oss-Chaudhury-Good surface tension model and from contact angle results. Calculated values of W for all combinations of contacting surfaces and solvents spanned two orders of magnitude. W correlated with the surface tension of the solvent for hydrophobic/hydrophobic interactions; hydrophilic/hydrophilic and hydrophobic/hydrophilic interactions were more complex. Friction forces were fit to a modified form of Amonton's law. For any solvent, friction coefficients were largest for the hydrophilic/hydrophilic contacting surfaces. The friction coefficient for any contacting pair was largest in hexadecane. In polar solvents, friction coefficients scaled with solvent polarity only for hydrophobic/hydrophobic contacting pairs. Copyright 1999 Academic Press.     

Spectroscopic analysis of the tribological behavior of a model boundary layer lubricant

Highly ordered alkanethiol self-assembled monolayers (SAMs) on gold substrates are suitable models of boundary layer lubricants and may be used in actual nanoscale device applications. Here, such monolayers were studied by spectroscopic methods as a function of tribological wear (rubbing) using a pin-on-disk microtribometer. The coefficient of friction (COF) (ratio of the frictional force to the load) was measured with the tribometer, and reflectance infrared spectra and X-ray photoelectron spectra were obtained as the monolayer film failed and the COF changed. The results show that it is possible to correlate disorder in the monolayer film with tribological failure of the film, and that continued rubbing produces a chemical change in the monolayer film. Disorder in the monolayer is distinct from the influence of wear in the underlying gold substrate. Aged SAMs, having sulfonate rather than thiol headgroups and initially less well ordered, behave differently to the well-ordered freshly prepared SAMs. Interestingly, they show a lower COF over many more cycles of exposure to the rubbing pin. The impact of the mechanism of film failure in boundary layer lubrication is discussed.     

氟硅改性聚氨酯自组装膜的合成及性能

以甲苯二异氰酸酯、聚酯二元醇、2,2-二羟甲基丙酸和十三氟-1-辛醇等为原料合成了氟醇封端的聚氨酯预聚体(FPU)。 通过FPU侧链含有的羧基官能团与异氰酸酯基硅烷偶联剂(Si-NCO)反应制备了含硅氧烷官能团功能性树脂(FPUSi)。 采用红外光谱(FT-IR) 对产物结构进行了表征,用TGA和水接触角等测试了自组装薄膜的表面性能。 结果表明,在N2气氛围下,FPU和FPUSi的热失重温度（T5％）均为178 ℃;硅基表面经FPUSi自组装膜修饰后,其表面水接触角达到81°。 微摩擦测试结果表明,当载荷为400 mN时,FPUSi自组装薄膜的稳定摩擦系数达到0.09。     

The antioxygen radiation properties of the nanocomposite film consisted of hydrogenated amorphous carbon (a-C:H) and MoS2

MoS₂/a-C:H multilayer film and MoS₂/a-C:H composite film exhibit excellent tribological properties in vacuum, which can be used as the potential space lubricant. The radiation-protective properties of these two films in atomic oxygen (AO) are evaluated. The influences of AO radiation on structure, morphology, and tribological properties of the films were investigated. The results show that AO radiation mainly causes oxidation and increases sp² C content in both of the films. Furthermore, the MoS₂ sublayer on the surface of the multilayer film is oxidized heavily, whereas both the MoS₂ and the a-C:H matrix on the surface were oxidized in the composite film. As a result of this, the multilayer film exhibits high friction coefficient and short sliding lifetime in vacuum after AO radiation. Compared with that, the composite film exhibits lower friction and longer sliding time more than 3600 seconds in vacuum, which illustrates it has a good AO radiation protection. This indicates that MoS₂/a-C:H composite film is more likely to be used as a potential space lubricant.