Article pubs.acs.org/Langmuir
Mechanochemistry of Physisorbed Molecules at Tribological Interfaces: Molecular Structure Dependence of Tribochemical Polymerization Xin He and Seong H. Kim* Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *
ABSTRACT: Physisorbed molecules at a sliding solid interface could be activated by mechanical shear and react with each other to form polymeric products that are often called tribopolymers. The dependence of the tribopolymerization yield on the applied load and adsorbate molecular structure was studied to obtain mechanistic insights into mechanochemical reactions at a tribological interface of stainless steel. Three hydrocarbon precursors containing 10 carbon atomsα-pinene (C10H16), pinane (C10H18), and n-decane (C10H22)were chosen for this study. α-Pinene and pinane are bicyclic compounds with different ring strains. N-Decane was chosen as a reference molecule without any internal strain. By comparing the adsorption isotherm of these molecules and the total volume of tribopolymer products, the reaction yield was found to be proportional to the number of adsorbed molecules. An Arrhenius-type analysis of the applied load dependence of the tribopolymerization yield revealed how the critical activation volume (ΔV*) varies with the structure of adsorbed molecules. The experimentally determined ΔV* values of α-pinene, pinane, and n-decane were 3, 8, and 10% of their molar volumes, respectively. The molecule with the largest ring strain (α-pinene) showed the smallest ΔV*, which implies the critical role of internal molecular strain in the mechanochemical initiation of polymerization reaction. The tribopolymer film synthesized in situ at the sliding interface exhibited an excellent boundary lubrication effect in the absence of any external supply of lubricant molecules.
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INTRODUCTION
products as well as the in situ monitoring of reactant consumption and product formation.9,10 In tribology, the influence of mechanically applied forces on interfacial phenomena such as frictional dissipation, wear, and tribochemical reactions has been modeled with the concept that an externally applied force facilitates the thermal transition of atoms or molecules across an energy barrier, thereby promoting slip or bond dissociation. Such mechanically assisted thermal activation was first proposed by Prandtl and Eyring11,12 and further developed and applied to a wide range of interfacial phenomena.3,13−18 One common feature of all proposed models is that they can be modified and expressed as an Arrhenius-type equation19
Mechanochemical reactions initiated by a mechanical force or action are ubiquitous in nature and engineering systems. The most obvious example would be chemical reactions induced by frictional heat at sliding interfaces.1,2 There are numerous chemical reactions occurring at mechanical interfaces where frictional heat generation is negligible.3,4 In such cases, chemical reactions must be initiated or driven by directly channeling mechanical force or energy into chemical reaction coordinates, which results in the destabilization of reactants or a reduction of the activation barrier for chemical reactions.5,6 Thus, chemical reactions that usually do not occur under thermal, photochemical, or electrochemical conditions can take place at mechanical interfaces. However, a fundamental understanding of how mechanical actions alter or control reaction dynamics or mechanisms is not well established.7,8 This is in part due to experimental difficulties in isolating and purifying reaction © XXXX American Chemical Society
Received: November 9, 2016 Revised: February 12, 2017 Published: March 2, 2017 A
DOI: 10.1021/acs.langmuir.6b04028 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir ⎛ E − σ ΔV * ⎞ ⎛ E − F Δx* ⎞ ⎟ = A exp⎜− a ⎟ reaction rate or yield = A exp⎜− a ⎝ ⎠ ⎠ ⎝ kT kT
(1)
where A is the preexponential factor, Ea is the activation energy for typical thermal reactions, k is the Boltzmann constant (1.38 × 10−23 J/K), and T is the system temperature. On the basis of the dimensional analysis, the applied mechanical energy contribution can be written in two forms: force (F) times the critical bond length (Δx*) or stress (σ) times the critical activation volume (ΔV*). Equation 1 implies that, as a firstorder approximation, the applied mechanical energy effectively lowers the overall activation barrier of interfacial slip or chemical reactions. Here, Δx* can be considered to be the minimum amount of bond length change needed to reach the transition state. For example, when the Morse potential of a chemical bond is perturbed by the applied force producing a local maximum in the bond dissociation channel,20 then Δx* would be the distance between the equilibrium bond distance (at which the potential energy is the lowest) and the length where the potential energy is the highest. In laboratory experiments, it would be difficult to control the force applied to individual atoms or molecules, and thus the contact pressure or mechanical shear (σ) would be a more appropriate term to consider. Then, ΔV* must be the minimum amount of change in molecular volume to initiate chemical reactions of that specific molecule. Note that ΔV* may depend on an intrinsic Δx* term as well as an extrinsic term such as the area over which the force acts on the molecule, the dynamic coverage of molecules at the shearing interface, and the chemical interactions and orientations of molecules with the solid surface.19 This article reports how the experimentally determined ΔV* varies depending on the structure of a precursor molecule sheared at the sliding interface. This study was carried out under a vapor-phase lubrication (VPL) condition that provides several advantages for the experimental study of mechanochemical reaction mechanisms. In VPL, the supply of reactants to the sliding solid interface can be easily controlled through the vapor adsorption isotherm.21,22 In typical liquid-based lubrication, reaction intermediates at the interface are readily dissolved into the lubrication oil, which make the detection of reaction products extremely difficult. In VPL, the reaction product with lower vapor pressure will remain on the substrate,23,24 allowing the detection and quantification of the mechanochemical reaction products after VPL tests. Also, the mechanical wear of the substrate can be fully suppressed under VPL conditions,25,26 eliminating the need to consider the chemical reactions induced or caused by dangling bonds of the worn surface.20 In this work, we investigated the polymerization reaction of α-pinene (C10H16), pinane (C10H18), and n-decane (C10H22) at the sliding interface of AISI 440C stainless steel. Their molecular structures are shown in Figure 1. These 10-carbon molecules have different internal strains. Both α-pinene and pinane have strained four-membered rings.28−30 The presence of the CC double bond in the six-membered ring significantly increases the internal ring strain of α-pinene, compared to pinane. N-Decane is a linear alkane without such strains, so it is the most inert C10-hydrocarbon and can be a good reference for comparison. The critical activation volume (ΔV*) of these molecules for polymerization reactions under VPL conditions was determined from the load dependence of
Figure 1. Coefficient of friction of 440C stainless steel measured in dry N2 and nitrogen containing α-pinene, pinane, and n-decane vapor at marked contact loads. The partial pressure for the organic vapor was 30% of its saturation value (p/psat). Error bars represent the standard deviation calculated from more than three measurements. The inset shows the line profile across the wear track (left), optical profilometry image (center), and optical microscope image (right) of the wear track made in dry N2 tribotesting.
the polymer production yield, which provides critical insight into the role of molecular structure on mechanical activation for chemical reactions. The tribochemically synthesized organic products, often called tribopolymers, exhibited excellent boundary lubrication with limited self-healing capability. The in situ synthesis of lubricating polymer films demonstrated a potential for the lubrication of micromechanical devices.
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EXPERIMENTAL DETAILS
All tribotests were performed using a custom-made reciprocating ballon-flat tribometer with environment control capability.26,31 The substrate was a flat plate of AISI 440C stainless steel polished with sandpaper and then a micropolish solution with a 1 μm colloidal alumina slurry (Buehler). The root-mean-square (rms) roughness of the polished stainless steel surfaces was around 20−25 nm. Commercially available 3-mm-diameter 440C stainless steel balls (McMaster-Carr, Elmhurst, IL, USA) were used as a counter surface to rub against the flat stainless steel surface. The rms roughness of the ball surface was below 10 nm.26 The elastic deformation depth calculated from the Hertzian contact mechanics (41 nm at 0.4 GPa) is larger than the surface roughness of the substrate and countersurface. Thus, the deformed solid surfaces are in intimate contact.32 The subsurface local load might be slightly higher at the asperity contact points and slightly lower outside the asperity contacts. The sliding amplitude and speed of the ball were 2.5 mm and 4 mm/s, respectively. Before tribotests, both substrate and ball surfaces were cleaned with ethanol and then UV/ozone treatment for 30 min to remove all chemical residues.33 The applied load was adjusted so that the Hertzian contact pressure varied over the range from ∼0.4 to ∼1.5 GPa. The tribotest was carried out in a continuous-flow cell with the desired relative partial pressure (p/psat) of vapor in dry nitrogen that was produced by mixing a dry nitrogen stream (N2) and the saturated organic vapor stream of interest.31 In this study, (+)-α-pinene (2,6,6trimethylbicyclo[3.1.1]hept-2-ene, CAS no. 7785-70-8), pinane (2,6,6trimethylbicyclo[3.1.1]heptane, CAS no. 473-55-2), and n-decane (CAS no. 124-18-5) were chosen to investigate the effects of molecular structure (especially internal ring strain) on mechanochemical reactions. All tribotests were conducted at room temperature and ambient pressure. Under our tribotest conditions, the maximum flash temperature was estimated to be less than 5 °C because of the slow sliding speed and low friction.34,35 Because the thermal decomposition B
DOI: 10.1021/acs.langmuir.6b04028 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 2. Cross-section line profile (left), optical profilometry image (middle), and optical microscope image (right) of sliding tracks after friction tests at the lowest and highest Hertzian contact pressures in 30% p/psat α-pinene, pinane, and n-decane. The substrates were cleaned with ethanol after the friction test to evaluate substrate damage without interference from triboreaction products. temperatures of α-pinene and pinane are ∼350 and 450 °C, respectively, thermally activated reactions of the adsorbed molecules due to frictional heat are unlikely.30 The adsorption isotherms of three precursor molecules were measured with polarization−modulation reflection−absorption infrared spectroscopy (PM-RAIRS) using a Thermo-Nicolet Nexus 670 spectrometer equipped with a mercury cadmium telluride (MCT) detector. Because the 440C stainless steel has poor IR reflectivity, a polished copper substrate was used as a model surface for conducting PM-RAIRS experiments. The higher IR reflectivity of copper made it easier to detect the adsorbed species.36 In previous studies, it was already shown that the adsorption isotherm on stainless steel under ambient conditions is similar to that on copper.31,36 The details of the PM-RAIRS system setup and experimental procedure for the adsorption isotherm measurement were discussed in a previous paper.23 The tribopolymers created during the sliding process were spread over the slide track and piled up at the periphery of the slide track. An atomic force microscope (AFM; Digital Instrument MultiMode scanning probe microscope) was used to measure the yield of tribopolymer products. Three-dimensional topographic images of tribopolymers at different segments of the slide track were collected through tapping-mode scanning in ambient air; the volumes of each segment were added to calculate the total amount of products. AFM tips (PPP-FM) were purchased from NANOSENSORS and cleaned with UV/ozone to remove organic contaminants before imaging. The nominal tip radius was about 7−10 nm, and the spring constant of the cantilever was calibrated to be 2.81 N/m using the Sader method.37 The effect of the vapor environment on the viscoelastic property of the tribopolymer was tested by performing force−distance indentation measurements in a controlled vapor environment. The maximum indentation force was set to 60 nN for all measurements. The slide track on the substrate after the removal of tribopolymers via ethanol rinsing was analyzed with optical profilometry using a Zygo NewView 7300 system.
form a surface coverage of close to a monolayer on the stainless steel surface (as shown in Figure 4).23,26 The friction coefficient value is slightly higher than the typical coefficients observed under VPL conditions,24,26 which might be due to the presence of tribopolymers produced inside the slide track.23 It is noted that even at a contact pressure lower than the yield strength of 440C stainless steel (0.42−0.45 GPa),38 the substrate wears badly in dry nitrogen, producing 3−4-μm-deep trenches after 600 cycles of reciprocating motion of the ball at a contact pressure of 0.39 GPa (Figure 1 inset). The substrate in front of the sliding ball is under compression, that in the tailing edge is under tension, and that within the contact region is under shear. The tensile yield strength is much lower than the shear modulus (∼80 GPa) for 440C stainless steel. Thus, if a cold welding between two solid surface occurs under the unlubricated, dry N2 condition, then adhesive wear can take place. The large roughness of the bottom of the slide track (∼1.1 μm) is consistent with the adhesive wear process.31 When organic vapors adsorb at the sliding interface from the gas phase, then the direct contact between solids is prevented and the shearing plane is within the adsorbed molecule layer.32 Thus, adhesive wear processes can be prevented and the adsorbed molecules can provide lubrication effects. When the contact load is lower than the yield stress of the substrate, the deformation mark in the substrate is almost invisible (Figure 2; 0.39 GPa cases). When the contact pressure is increased above the substrate yield strength, then plastic deformation takes place and a smooth compression mark is left on the substrate (Figure 2). The maximum load used in this study (1.5 GPa) is lower than the hardness (5.5 GPa) of the 440 C stainless steel determined from nanoindentation.39 Within the resolution of the optical profilometry, no evidence of surface fracture was found inside the surface track. This suggested that the native oxide layer on the stainless steel surface can conform to the elastic deformation of the substrate, which was estimated to be 510 nm at 1.5 GPa. In the case of n-decane, the substrate wear was observed when the contact pressure was increased to >1 GPa (probably as a result of the poor lubrication capability of ndecane); thus, the maximum load for the n-decane test was limited to 0.83 GPa. After tribotests for 600 reciprocating cycles, iridescent organic deposits could be seen with the naked eye. In a previous study, time-of-flight secondary ion mass spectrometry (ToF-SIMS) of the organic deposits revealed typical fragmentation patterns of polymeric materials.23 Fourier transform infrared (FT-IR) spectroscopic analysis of the organic deposits formed in the α-pinene tribotest showed the
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RESULTS AND DISCUSSION The effects of α-pinene, pinane, and n-decane vapor adsorption on the friction and wear behaviors of 440C stainless steel were tested at a Hertzian contact pressure ranging from 0.39 to 1.54 GPa. Figure 1 displays the friction coefficients measured under α-pinene, pinane, and n-decane vapor conditions at the lowest and highest normal contact pressures tested in this study. The results of tribotesting in dry N2 are also shown for comparison. In the absence of any organic vapor in the surrounding gas phase, the friction coefficient is high and very unstable. In contrast, the friction coefficients measured in the presence of 30% p/psat α-pinene, pinane, and n-decane are around 0.17 and 0.2, regardless of the adsorbate molecular structure and the contact pressure. This relative partial pressure is high enough to C
DOI: 10.1021/acs.langmuir.6b04028 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 3. Tapping-mode AFM images of tribopolymers accumulated at the ends and sides of the slide track after 600 cycles of the reciprocating slide at the lowest and highest contact loads tested in (a) α-pinene, (b) pinane, and (c) n-decane vapors (30% p/psat). The area and height scales of the image are shown above each panel of images.
Figure 4. Comparison of adsorption isotherm (black) from PM-RAIRS measurements and total yield of tribopolymer (red) after VPL tests at various vapor pressures of (a) α-pinene, (b) pinane, and (c) n-decane. The applied contact load was 0.5 GPa, and the total number of sliding cycles was 600.
isotherm shape, the monolayer coverage appears to be formed at
