Self‐Lubricating Nano‐Ball‐Bearings

Abstract

Cubic symmetry in octasilsesquioxanes ([RSiO1.5]8, diameter 1 nm) places functionality in each octant in Cartesian space, enabling nanometer-scale construction of materials with subtle tailoring of global properties. Diverse studies of OPS (octasilsesquioxanes) range from their ordering in solids (nanocomposites) to catalysts, NMR standards, and spacecraft coatings. The prototypical OPS (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 CH2Cl2, rendering attempts at functionalization very difficult. In an effort to couple early work on liquid-crystalline (LC) silsesquioxanes with recent efforts to functionalize OPS, we explored Friedel–Crafts alkylation by AlCl3, despite the potential for Si–C bond cleavage. However, its easy nitration in fuming HNO3 offered encouragement. 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. Here, we report that Friedel–Crafts alkylation of OPS leads to alkylxOPS 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, suggesting that alkylxOPS materials may serve as models of superlattices. These materials offer potential as high-temperature lubricants. Furthermore, interdigitation suggests that alkylxOPS-based systems might serve as nanogears or as nanoscale Velcro when affixed to surfaces. Friedel–Crafts alkylation (nRBr, see Supporting Information) of OPS produces alkylxOPS, with typical 7:3 ratios of branched/normal alkyl groups. Cleavage of Si–C bonds (by using nBu4NF) coupled with analysis by 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. Table 2 C M M U N IC A TI O N