Self‐Lubricating Nano‐Ball‐Bearings

Abstract

Cubic symmetry in octasilsesquioxanes ([RSiO 1.5 ] 8 , diameter 1 nm) places functionality in each octant in Cartesian space, enabling nanometer-scale construction of materials with subtle tailoring of global properties. [1][2][3] Diverse studies of OPS (octasilsesquioxanes) range from their ordering in solids (nanocomposites) to catalysts, NMR standards, and spacecraft coatings. [4][5][6][7][8][9][10][11][12][13] The prototypical OPS [10] (R = phenyl) is air-stable to temperatures above 500°C; however, high symmetry and crystallinity limit its maximum solubility to 100 mg per 100 mL CH 2 Cl 2 , rendering attempts at functionalization very difficult. [10,12,13] In an effort to couple early work on liquid-crystalline (LC) silsesquioxanes with recent efforts to functionalize OPS, we explored Friedel-Crafts alkylation [14] by AlCl 3 , despite the potential for Si-C bond cleavage. [15] However, its easy nitration in fuming HNO 3 offered encouragement.[10] The impetus was to add sufficient alkyl groups to disrupt registry between OPS molecules, thereby reducing crystallinity to produce LC and/or high-temperature lubricants.[16]Here, we report that Friedel-Crafts alkylation of OPS leads to alkyl x OPS materials with rigid 1 nm cores, which are surrounded by a hydrocarbon layer that acts as a lubricant to these hard spheres or nano-ball-bearings. Surprisingly, the butyl and hexyl derivatives are air-stable and liquid over a 70-400°C range, whereas the octyl and decyl derivatives are stable only over a 30-300°C range. Molecular modeling studies suggest that the unusual 100°C difference in thermal stability arises because the shorter chains interdigitate even in the liquid state. This behavior is similar to that predicted and seen recently for alkylthiol-modified superlattices, [17,18] suggesting that alkyl x OPS materials may serve as models of superlattices. These materials offer potential as high-temperature lubricants. Furthermore, interdigitation suggests that alkyl x OPS-based systems might serve as nanogears or as nanoscale Velcro when affixed to surfaces. Friedel-Crafts alkylation (nRBr, see Supporting Information) of OPS produces alkyl x OPS, with typical [14] 7:3 ratios of branched/normal alkyl groups. Cleavage of Si-C bonds (by using nBu 4 NF) coupled with analysis by 13 C NMR and matrixassisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS, see Fig. S1 in the Supporting Information) distinguish individual alkylation products and the average numbers of alkyl groups per phenyl for each nRBr. Table 1 summarizes the results of gel-permeation chromatography (GPC) and viscosity measurements.

Surface energetics, dispersion, and nanotribomechanical behavior of POSS/PP hybrid nanocomposites

“…Increase in the surface roughness reduces the real area of contact between the AFM probe and the surface, thereby reducing the observed friction. Some authors have suggested a self-lubricating ''nano ball bearing'' mechanism for low friction nanomaterials, including POSS, 42 diamond nanoparticles, 43 and fullerene-like inorganic nanoparticles. 44 Nanoscale friction characteristics are also significantly influenced by the magnitude of the adhesive force between the AFM probe and the surface.…”

Synthesis, characterization, and properties of a novel polyhedral oligomeric octamethyldiphenylsulfonylsilsesquioxane

“…13 C NMR spectra were collected at 150.9 MHz with a 36,057.7-Hz spectral width. 29 Si NMR spectra were collected at 119.2 MHz with a 9566.3-Hz spectra width.…”

“…The meta-substituted sulfonylation was confirmed by FT-IR, 1 H NMR, and 13 C NMR spectroscopy. 29 Si NMR spectroscopy and MALDI-TOF mass analysis were used to determine the degree of substitution in the reaction. The 29 Si NMR spectra of the compounds confirmed their complete conversion from the parent OPS.…”

“…1 H NMR, 13 C NMR, and 29 Si NMR spectra were recorded on Bruker Avance 600 NMR spectrometer operating in Fourier transform mode. CDCl 3 was used as the solvent, and the solution was measured with tetramethylsilane as an internal reference.…”

“…However, functionalized (PhSiO 3/2 ) 8 products are robust and highly soluble; they offer easy purification and processing [24][25][26][27]. The synthesis of molecular (RSiO 3/2 ) 8 and functionalized (PhSiO 3/2 ) 8 via electrophilic substitution has been reviewed [28][29][30][31].…”