Perfluoropolyether Boundary Lubricants Based on the Star Architecture

Abstract

Star PFPE (perfluoropolyether) polymers based on an aromatic benzene core and a non-aromatic cyclotriphosphazene (CTP) core are investigated as boundary lubricant films on rigid magnetic media. The effect of the benzene and CTP cores on head-disk spacing are measured by the changes in the acoustic emission signal as a function of head-disk spacing at similar lubricant film thicknesses. The earlier head-disk contact observed with the CTP core is attributed in part to its larger size perpendicular to the plane of the disk surface. The adhesion of the benzene and CTP cores to the underlying carbon film is investigated by ab initio quantum chemistry. The non-aromatic nature of the CTP ring and the lack of steric accessibility to the CTP core nitrogen atoms prevent good adhesive interactions with the underlying carbon film. Conversely, the π-electrons of aromatic benzene create a quadrupole moment in the direction perpendicular to the plane of the benzene that can interact with the underlying carbon surface and hence provide weak adhesion. Electron-withdrawing and -donating substituents on the benzene ring can be used to exert a “push-pull” effect on the π-electrons to alter the strength of the intermolecular interactions to specific functional groups of the underlying carbon surface. These effects are visualized by mapping the electrostatic potential of the benzene core as a function of electron-withdrawing and -donating substituents and computing model dimer optimized geometries and intermolecular interaction energies. Finally, the thermal stability of the star PFPE polymers are quantified by measuring the evaporation rate of thin films (11 Å) as a function of time at the HDD temperature (~ 60℃). Thermogravimetric analyses (TGA) of the neat polymers also provide direction for improvement of the synthetic lubricants.

Keywords

• star polymer
• perfluoropolyether
• disk lubricants
• electronic structure
• tribology