Phase Behavior and Lubricity of Aqueous PEO-PPO-PEO and PPO-PEO-PPO Triblock Copolymer Solutions

Aqueous triblock copolymer solutions are potential low-cost, eco-friendly lubricants. However, as a solution, their phase changes with copolymer concentration and solution temperature, raising the question, “Does the phase change affect the formation of adsorbed layer and the lubrication performance?” This article studies the copolymer solution phase behavior and lubricity in response to the copolymer structure, concentration, and solution temperature. Four different triblock copolymers, two normal PEO-PPO-PEO and two reverse PPO-PEO-PPO composed of PEO poly(ethylene oxide) and PPO poly(propylene oxide), have been investigated. From cloud point and surface tension measurements, phase change and micellization are shown to depend on copolymer type, number of hydrophilic PEO blocks, and temperature. Furthermore it is found that the phase and the presence of micelles lead to significant variation in adsorbed copolymer mass and lubricity. Based on the observed phase behaviors, the lubricity of copolymer solutions is discussed with regard to aggregation and adsorption on the solid–liquid interface.     

Automated focusing in bright-field microscopy for tuberculosis detection

Pulsed‐laser atom‐probe tomography is used to compare the field‐evaporation mass spectrum and spatial distribution of molecular fragments from various poly(3‐alkylthiophene) films deposited on sharpened aluminium specimen carriers using two different deposition methods. Films deposited via a modified solution‐cast methodology yield small fragments with a uniform structural morphology whereas films deposited via an electrospray ionization methodology yield a wide range of fragments with a very non‐uniform structural morphology. The main field‐evaporated chemical species identified for both deposition types were, in order of typical relative abundance, C₂H₅⁺, CH₃⁺, C₂H₄⁺, followed by C₃H_7,8⁺/SC⁺ and SCH⁺. Thick electrospray depositions allowed investigation of the influence of laser‐pulse energy on the analysis. Evidence is presented supporting the presence of a critical laser‐pulse energy whereby changes in film morphology are signalled by the appearance of a new mass fragment at 190 Da.     

Poly(l‐lysine)‐graft‐poly(ethylene glycol): a versatile aqueous lubricant additive for tribosystems involving thermoplastics

The adsorption and aqueous lubricating behaviour of poly(l ‐lysine)‐graft‐poly(ethylene glycol) (PLL‐g‐PEG) have been investigated for tribopairs involving thermoplastic materials, including polypropylene, polyamide‐6,6 and polyethylene. A major finding is that PLL‐g‐PEG adsorbs onto both hydrophobic, non‐polar surfaces and hydrophilic, polar (negatively charged) surfaces from aqueous solution, and thus plays as a very unique and effective aqueous boundary lubricant additive for the sliding contact of thermoplastics against themselves as well as against many hydrophilic, polar materials, including metals (e.g. stainless steel) or ceramics (e.g. zirconia, ZrO₂). Copyright © 2007 John Wiley & Sons, Ltd.     

Real‐time monitoring of the penetration of amphiphilic acrylate copolymer in leather using a fluorescent copolymer as tracer

A fluorescent tracer, poly (acrylic‐co‐stearyl acrylate‐co?3‐acryloyl fluorescein) [poly (AA‐co‐SA‐co‐Ac‐Flu)], used for real‐time monitoring the penetration of amphiphilic acrylate copolymer, poly (acrylic‐co‐stearyl acrylate) [poly (AA‐co‐SA)], in leather was synthesized by radical polymerization of acrylic, stearyl acrylate and fluorescent monomer, 3‐acryloyl fluorescein (Ac‐Flu). The structure, molecular weight, introduced fluorescent group content and fluorescent characteristics of the fluorescent tracer and target copolymer, amphiphilic acrylate copolymer, were also characterized. The results show that the tracer presents the similar structural characteristics to the target and enough fluorescence intensity with 1.68 wt % of the fluorescent monomer introduced amount. The vertical section of the leather treated with the target copolymer mixing with 7% of the tracer exhibits evident fluorescence, and the change of fluorescence intensity along with the vertical section with treating time increasing can reflect the penetration depth of the target copolymer. The introduction of the fluorescent group in polymer structure through copolymerization with a limited amount of fluorescent monomer, Ac‐Flu, is an effective way to make a tracer to monitor the penetration of the target in leather, which provides a new thought for the penetration research of syntans such as vinyl copolymer materials in leather manufacture. Microsc. Res. Tech. 78:1146–1153, 2015. © 2015 Wiley Periodicals, Inc.     

Bacterially induced degradation of aqueous solutions of poly(l-lysine)-graft-poly(ethylene glycol) and poly(l-lysine)-graft-dextran: consequences for their lubrication properties

Experimental data are presented comparing the degradation in the lubrication behaviour of aqueous solutions of the brush copolymers poly(l -lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) and poly(l -lysine)-graft-dextran (PLL-g-dex) when aged after exposure to bacterial contamination. While PLL-g-PEG solutions appear to be relatively unaffected by bacteria after a storage period of as much as 8 weeks, PLL-g-dex solutions exposed to bacteria during preparation lead to as much as a twofold increase in friction coefficient, when used as a lubricant in tribological tests, over an 8 week period when compared to reference data from freshly prepared and tested samples. Solutions prepared under sterile conditions and aged for up to 8 weeks do not appear to be degraded. Further experiments implementing the anti-bacterial agent sodium azide effectively prevented bacterial colonization and degradation in lubrication behaviour. Copyright © 2009 John Wiley & Sons, Ltd.     

Microstructure and interfacial chemistry of pure and La‐doped BiFeO3 thin films

We report on the microstructure and interfacial chemistry of thin films of pure and La‐doped multiferroic bismuth ferrite (Bi_1‐xLa_xFeO₃ or BLFO), synthesized on Indium Tin Oxide‐coated glass substrates by solution‐deposition technique and studied using scanning transmission electron microscopy. Our results show that undoped and La‐doped thin films are polycrystalline with distorted rhombohedral structure without any presence of any line or planar defect in the films. In addition, the films with La doping did not show any structural change and maintain the equilibrium structure. Cross section compositional analysis using X‐ray energy dispersive spectrometry did not reveal either any interdiffusion of chemical species or formation of reaction product at the film‐substrate interface. However, a closer examination of the microstructure of the films shows tiny pores along with the presence of ～2–3 nm thin amorphous layers, which may have significant influence on the functional properties of such films. Microsc. Res. Tech. 76:1304–1309, 2013. © 2013 Wiley Periodicals, Inc.     

Nanoscratch Behavior of Dendrimer-Mediated Ti Thin Films

Ramped load nanoscratch behaviors of titanium thin films with and without dendrimer mediation were extensively studied. By combining in situ investigation of scratch surface profiles and coefficients of friction curves, with ex situ microscopic (OM, AFM) and spectroscopic (AES) examinations of scratch tracks, different scratch failure modes were identified. An adhesive failure was found in the dendrimer-mediated films, whereas a cohesive failure was in the dendrimer-free samples.     

A tribological study of kinetically influenced ultrathin Au and Cu metal overlayers grown on dendrimer mediated Si

Evidence is presented here for deposition kinetic energy influences on the wear properties of Au and Cu films deposited by evaporation and sputtering on clean and poly(amidoamine) (PAMAM) dendrimer modified SiO _x substrates. Ramped load nanoscratch tests show increased resistance to wear in the presence of the dendrimer monolayer. Nanoscratch profiles indicate that the critical load to failure (scratch bearing capacity) is increased in the presence of a dendrimer interlayer. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis of the wear tracks show that following film failure plowing is the predominant mechanism of wear for sputtered or evaporatively deposited Au. No obvious changes in the wear properties (a pure cutting mechanism) of Cu thin films are observed upon changing the kinetic energy of the incoming metal.     

Friction of Poly(n-Alkyl Methacrylates)

The friction coefficient of solid poly(n-alkyl methacrylates) decreases progressively with increasing length of the alkyl group. This behavior parallels the behavior of adsorbed films of these polymers from solution on solid surfaces. Durability of thin films of these polymers (approximately 3,500 Å thick) increases with increasing length of alkyl group. For example, when the alkyl is C₂₂ the durability is four times greater than when the alkyl is C₁₂ (polymer film on a 1020 steel substrate). The film durability is strongly substrate dependent, e.g., durability of poly(n-dodecyl methacrylale) is three times greater on glass than on 1020 steel when rubbed with a 440 C rider. The friction and durability are attributed to side chain crystallinity of the polymer. Side chain crystallinity has been observed by others using x-ray and other techniques when the alkyl group is at least 10 or 12 carbons long. Friction measurements may be a more sensitive way of detecting side chain crystallinity. The friction behavior of adsorbed films from solution of the poly(n-alkyl methacrylates) suggests that side chain crystallinity also pertains to suck adsorbed films.     

Surface Damage Characteristics of Self-Assembled Monolayers of Alkanethiols on Metal Surfaces

The effect of metal oxide layer on the nano-tribological characteristics of single chain alkanethiol (CH₃(CH₂) _n SH) self-assembled monolayers (SAMs) on various metal surfaces (gold, silver, copper) was investigated. In order to correlate the surface structures with the tribological characteristics, various surface analysis techniques such as Scanning Probe Microscopy, X-ray Photoelectron Spectroscopy, Secondary Ion Mass Spectrometry and Spectroscopic Ellipsometry were used. Results of surface analyses showed that thiols on a metal surface were susceptible to forming multilayers if the metal surface was oxidized before the thiol assembly process. From the friction and wear tests conducted using an Atomic Force Microscope and a Lateral Force Microscope, it was found that thiols on copper oxide surface could be easily removed even under a few nano-Newton normal load. On the other hand, thiols on gold and copper fresh surfaces (the surface which was made by minimizing oxide formation) could endure up to micro-Newton level loads. Based on these findings, it could be concluded that the nano-tribological characteristics of alkanethiol SAMs on various metal surfaces were largely dependent on the oxide layer that already formed on the metal surface before the thiol adsorption process.     

Self-healing behavior of a polyelectrolyte-based lubricant additive for aqueous lubrication of oxide materials

We report on the self-healing behavior of a polyelectrolyte-based aqueous lubricant additive, poly(l-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG), during aqueous lubrication of an oxide-based tribosystem. Combined pin-on-disk tribometry and fluorescence microscopy experiments have shown that stable lubricating performance was enabled by means of rapid healing of the worn tribopair surface by polymers dissolved in the adjoining bulk lubricant. This rapid ‘self-healing’ of PLL-g-PEG is attributed to electrostatic interactions between the polycationic poly(l-lysine) (PLL) backbone of the polymer and negatively charged oxide surface. In contrast, a similar healing effect was not readily achievable in the case of methoxy-poly(ethylene glycol)-trimethylsilylether (Sil-PEG), a lubricant additive that is covalently bonded to the surface prior to tribological stress.     

Boundary Lubrication of Oxide Surfaces by Poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) in Aqueous Media

In this work, we have explored the application of poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) as an additive to improve the lubricating properties of water for metal-oxide-based tribo-systems. The adsorption behavior of the polymer onto both silicon oxide and iron oxide has been characterized by optical waveguide lightmode spectroscopy (OWLS). Several tribological approaches, including ultra-thin-film interferometry, the mini traction machine (MTM), and pin-on-disk tribometry, have been employed to characterize the frictional properties of the oxide tribo-systems in various contact regimes. The polymer appears to form a protective layer on the tribological interface in aqueous buffer solution and improves both the load-carrying and boundary-layer-lubrication properties of water.     

Polyolefin and poly(alkyl methacrylate) mixed additives as mineral lubricating oil rheology modifiers

Some rheological properties of mineral lubricating oils containing polyolefin (OCP) (ethylene/propylene copolymer) and poly(alkyl methacrylate) (PAMA) mixed additives over a wide composition range have been examined. Increasing the OCP content significantly increases the viscosity and shear stability of solutions, whereas the viscosity index is directly proportional to the PAMA content. The pour point values of the additive solutions investigated are lower than −30°C even for the smallest concentration of PAMA in the polymer mixture of 15 wt.% (at a total polymer mixture concentration in oil of 2 wt.%). The viscosities of dilute mixed additive solutions obey the typical Krigbaum‐Wall equation and, due to the negative viscometric interaction parameter values (Δb₁₂ < 0), the OCP/PAMA mixture is found to be immiscible. The immiscibility becomes more pronounced on increasing the overall polymer concentrations. The viscosities of concentrated mixed additive solutions decrease significantly with an increase of OCP content, showing a minimum of half the initial value at a polymer ratio of about 50/50. On decreasing the polymer concentration to practical levels as well as increasing the temperature, the viscosity values approach the ideal additivity law. Based on the ratio of specific viscosities at 40 and 100°C (Q), the OCP additive solution shows a better thickening effect at lower temperatures (Q < 1). On the other hand, the PAMA additive solution shows a better effect at higher temperatures (Q > 1) with more coherent thickening with temperature change. The mixed polymer additive, containing ∼10 wt.% OCP and ∼90 wt.% PAMA, is a nearly optimal viscosity‐index improver showing the same thickening effect at different temperatures. The results obtained are promising as regards the design of lubricant formulations with mixed polymeric additives.     

PDDA/PSS分子沉积膜的摩擦学行为研究

以聚对苯乙烯磺酸钠为聚阴离子,聚二烯丙基二甲基胺盐酸盐为聚阳离子在基底上交替沉积制备分子沉积膜。用紫外-可见吸收光谱仪、接触角测量仪、原子力显微镜对所制备的有序薄膜进行了表征。用UMT-2摩擦仪考察了超薄膜的摩擦学行为,结果表明,所制备的超薄膜具有良好的减摩抗磨性能,薄膜的表面电荷及亲水、疏水性对其摩擦学行为有较大影响,负电荷表面、亲水性强的表面在较高湿度下,耐磨寿命较长。     

Biobased Poly-phosphonate Additives from Methyl Linoleates‡

Poly-dialkyl phosphonates were synthesized by reacting methyl linoleate with dimethyl, diethyl, and di-n-butyl phosphites in the presence of free radical initiator and positively identified and characterized using gas chromatography–mass spectrometry (GC-MS), nuclear magnetic resonance (NMR; ¹H, ¹³C, ³¹P), and Fourier transform infrared (FTIR). Neat poly-dialkyl phosphonates and their blends in high-oleic sunflower oil (HOSuO) and polyalphaolefin (PAO-6) base oils were investigated for their physical, chemical, and tribological properties. At room temperature, the poly-dialkyl phosphonates displayed much better solubility in HOSuO than in PAO-6. Solubility in the base oils increased in the order dimethyl?<?diethyl?<?di-n-butyl. Relative to methyl linoleate, the neat poly-dialkyl phosphonates displayed higher density, higher kinematic viscosity, higher oxidation stability, and better cold flow (lower pour point and cloud point) properties. As an additive (0–10% w/w) in HOSuO, increasing concentration of poly-di-n-butyl phosphonate resulted in increasing onset and peak oxidation temperatures and decreasing cloud point. Poly-di-n-butyl phosphonate blends in HOSuO also showed lower four-ball antiwear (AW) coefficient of friction (COF) and wear scar diameter (WSD) than corresponding blends with zinc dialkyl dithiophosphate (ZDDP). As an additive (0–10% w/w) in PAO-6 base oil, poly-di-n-butyl phosphonate displayed lower four-ball antiwear COF and comparable WSD relative to similar blends of ZDDP in PAO-6. The results indicate that poly-dialkyl phosphonates are promising biobased AW additives with comparable or better performance than current petroleum-based commercial AW additives such as ZDDP.     

SEM and AFM studies of dip‐coated CuO nanofilms

Cupric oxide (CuO) semiconducting thin films were prepared at various copper sulfate concentrations by dip coating. The copper sulfate concentration was varied to yield films of thicknesses in the range of 445–685 nm by surface profilometer. X‐ray diffraction patterns revealed that the deposited films were polycrystalline in nature with monoclinic structure of (?111) plane. The surface morphology and topography of monoclinic‐phase CuO thin films were examined using scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively. Surface roughness profile was plotted using WSxM software and the estimated surface roughness was about ～19.4 nm at 30 mM molar concentration. The nanosheets shaped grains were observed by SEM and AFM studies. The stoichiometric compound formation was observed at 30 mM copper sulfate concentration prepared film by EDX. The indirect band gap energy of CuO films was increased from 1.08 to 1.20 eV with the increase of copper sulfate concentrations. Microsc. Res. Tech., 2013. © 2012 Wiley Periodicals, Inc.     

Tribological performance of MoS2---B2O3 compacts

A recent investigation suggests that selected oxides perform well as additives in molybdenum disulphide (MoS₂) because of their ability to soften at asperity contacts with the result that the solid lubricant can attain and retain a preferred tribological orientation.This research determined the effectiveness of boric oxide (B₂O₃), when used as an additive in MoS₂, for substrate temperatures ranging from 21°C to 316°C. This range was used to allow the asperity contact temperature to vary below and above the softening point of B₂O₃. It was found that a moderate friction coefficient and high wear, which is attributed to the additive acting abrasively, occurred when the asperity contact temperature was well below the softening point of the oxide. When the asperity contact temperature neared the softening point of the oxide, the friction coefficient increased dramatically and wear volume was reduced. It is postulated that B₂O₃ acted adhesively at the interface resulting in a higher coefficient of friction, and wear decreased due to an attainment of a preferred orientation by the MoS₂. For asperity contact temperatures significantly above the softening point of B₂O₃, the friction coefficient returned to about the same value as for temperatures below the softening point. It is speculated that wear continued to increase moderately because of localized melting of the B₂O₃, permitting the MoS₂ to be removed from the interface. These observations support a hypothesis that an additive, such as boric oxide, can soften as the asperity contact temperature approaches the softening point temperature of the additive so that the overall tribological conditions may be improved resulting in reduced interfacial wear. Significant changes in temperature, load or sliding velocity would, of course, dramatically alter the wear characteristics observed at the interface.     

Effects of supply of fine oxide particles onto rubbing steel surfaces on severe–mild wear transition and oxide film formation

This study is the first to show a quantitative condition required for the establishment of severe–mild wear transition with sliding distance, by studying the effects of supply of Fe₂O₃ particles onto rubbing steel surfaces on the transition and oxide film formation process. The supply of fine Fe₂O₃ particles was found to accelerate the wear transition, and the sliding distance at which the transition occurs was found to increase with particle diameter and applied load. Oxide films are produced on the rubbing surfaces by sintering of the supplied Fe₂O₃ oxide particles. At the severe–mild wear transition, the relative area of oxide films is the same for all diameters of supplied Fe₂O₃ particles. This finding suggests that the transition occurs when the relative area of oxide films reaches a specific value, which is proportional to the area of real contact.     

Nanotribological Behavior of Ultra-thin Al2O3 Films Prepared by Atomic Layer Deposition

Si-based nano/micro-electromechanical system (NEMS/MEMS) devices with contacting and rubbing structures cannot run reliably due to their poor tribological performance. A thin alumina (Al₂O₃) film is a promising candidate for the protective coating in the applications of NEMS/MEMS devices. In this study, nanotribological behavior of ultra-thin Al₂O₃ films prepared by atomic layer deposition on a Si (100) substrate was investigated in comparison with that of Si (100). X-ray photoelectron spectroscopy was used to determine the composition of Al₂O₃ films. Atomic force microscopy with different tips was employed to measure the scratch resistance, adhesion and friction forces of various samples. The results show that Al₂O₃ films have larger scratch resistance than that of Si (100). In addition, the adhesion and friction forces of Al₂O₃ films are smaller than that of Si (100). Thus, the Al₂O₃ films are capable of a wide application in Si-based NEMS/MEMS devices. The improved tribological performance of Al₂O₃ films is attributed to their hydrophobic properties.     

Characterization of micellar film morphologies of semifluorinated block copolymers by AFM

The micellar morphologies of well-defined amphiphilic block copolymers composed of 1H,1H-dihydroperfluorooctyl methacrylate (FOMA) and ethylene oxide (EO) blocks with different chain lengths were effectively investigated by using tapping mode-atomic force microscopy (TM-AFM). By spin-casting chloroform solutions, well-ordered spherical micellar films could be obtained for poly(FOMA(10k)-b-EO(10k)) and poly(FOMA(20k)-b-EO(20k)) copolymers. The atomic force microscopy (AFM) height and phase image analysis indicated that dark regions of the micelles corresponded to PEO blocks and the light regions were for PFOMA blocks. The spherical micelles with PEO corona and PFOMA core were also identified by transmission electron microscopy (TEM) and X-ray photoelectron spectrometer (XPS) analysis. The core diameters of the block-copolymer aggregates were 20nm for poly(FOMA(10k)-b-EO(10k)) and 30nm for poly(FOMA(20k)-b-EO(20k)) by TM-AFM, whereas slightly lower values of 17 and 26nm were obtained from TEM. A detailed study on the inverted morphological change observed in the micelles films after annealing above glass transition temperature (T(g)) was also presented.