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The molecular height corresponding to monolayer thickness of several types of PFPE lubricants (Z-dol2000, Z-tetraol, A20H2000, ARJ-DS, ARJ-DD, and OHJ-DS) with different molecular structures was estimated from their spreading profiles. The molecular height of Z-dol2000 was smaller than that of Z-tetraol, even though these lubricants had almost equal molecular weights. This result shows that the molecular height of the lubricant with high-polarity end groups was higher than that of the lubricant with low-polarity end groups. ARJ-DD showed higher molecular height than ARJ-DS, because the polarity of the two OH end groups in ARJ-DS was lower than that of the four OH end groups in ARJ-DD. On the other hand, OHJ-DS showed a low molecular height even though it had the highest molecular weight and four OH functional groups. Thus, a lubricant molecule with a low polarity per segment of the main chain has a low molecular height and shows a low degree of conformation on a carbon surface. These experimental results agreed with molecular dynamics (MD) simulation results. Therefore, it can be hypothesized that lubricants with multidentate functional groups having strong polarities do not exhibit a flat conformation on a disk surface because the lubricant molecules form random coil shapes on account of the strong polarity. On the basis of these results, we propose a design rule for lubricant molecules to achieve flat conformation on magnetic disks, i.e., a flat molecular conformation can be achieved on a magnetic disk surface by using molecules that have multisegments with a weak polar function like as OHJ-DS.
Date of Publication: June 2010