Professor James Economy's Group
A New Family of Lubricants Based on Sterically Hindered Aliphatic Polyester
The trend toward increasing areal density and faster data transfer rates has led to lower flying heights and higher rotational speed. The areal density of current hard disk drives is approaching 100Gb/in2, and the flying height is expected to decrease below 10 nm, while spindle speed will increase to beyond 15,000 rpm. Under these conditions the performance of the lubricant layer (<3 nm) is very critical for the durability of the head/disk interface.
Monolayer perfluoropolyether (PFPE) films are universally employed by the magnetic recording industry as disk lubricants. PFPEs offer many advantages to hard disk manufacturers, but also suffer from some inherent drawbacks. For example, they are not chemically stable in the presence of Lewis acids. The bulk structure of the slider comprises a composite mixture of aluminum oxide and titanium carbide, and the surface defects of Lewis acidity (electron deficient sites) ubiquitously present on aluminum oxide or titanium carbide catalyze the intramolecular disproportionation reaction of PFPE molecular chains. Although polar functionality has been introduced as end groups to improve adhesion and help enhance adsorption, severe migration and spin-off were observed when these PFPE lubricants were employed in high rotation systems. One trade-off of introducing polar endgroups is that coefficient of static friction rises sharply, especially when the film thickness is above 1 nm. Besides, there are increasing environmental concerns about PFPEs, since their relative insolubility requires the use of ozone-damaging solvents such as Freon during processing.
We have developed a new family of aliphatic polyester lubricants for hard disk drive applications that might address some of the above problems. The polar ester groups in the polymer main chain should provide a strong interaction between the lubricant and the carbon overcoat, hence, slow removal or loss, as well as fast recovery may be expected. In addition, polar lubricants may act like solid when the film thickness is below 5 nm, hence, stiction problem, which has been a problem for the amorphous PFPE lubricants, might be greatly reduced.
There are two major concerns in the molecular design of these aliphatic polyesters. They should have low Tgs, and they should be chemically stable. To overcome the drawbacks of common aliphatic polyester, i.e. lack of hydrolytic and thermal stability, sterically hindered polyesters were synthesized accordingly. Diacid monomers with a,a-H completely substituted, such as 2,2,5,5-tetramethylhexanedioic acid, 2,2,6,6-tetramethylheptanedioic acid (I), 2,2-diethyl-2,2-dimethylheptanedioic acid (II), and 2,2-diethyl-2,2-dimethylnonanedioic acid (III), and diol monomers with ß-H completely substituted by alkyl groups, such as 2,2-diethyl-1,3-dipropanediol and 2-ethyl-2-methyl-1,3-propanediol, were chosen or synthesized. These short alkyl branches are also desirable to lower the glass transition temperatures, as well as prevent the formation of crystalline structures. To prepare polyesters with extremely low Tg, copolymerization of a mixture of diacid and diol monomers were performed.