Professor James Economy's Group
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[Bactericides Materials] [HDD Lubricants] [New materials for Fuel Cell] [Gas Hydrates]
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. |
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