Computing chain conformations

Drs. Wilfred T. Tysoe & Nicholas D. Spencer | TLT Cutting Edge October 2011

Quantum mechanics reveals how perfluoropolyether molecules can lower flying height.
 

Quantum calculations were recently used to explore the bonding and dynamics of a novel class of lubricants for hard-disk drives.
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With the advent of faster computers and the ready availability of “parallel” computer codes that allow various parts of a calculation to be carried out simultaneously, it has become feasible to perform quantum-mechanical calculations for physically realistic systems.

This ability has been exploited in many areas of physics and chemistry. For example, quantum mechanics has been used to calculate the elastic properties of materials from the change in energy caused by applying a strain. It is used extensively by chemists to calculate the thermodynamics of reactions and even reaction activation energies.

It is surprising that such methods have not been used to any great extent to understand tribological phenomena until now. Quantum calculations were recently used by Robert Waltman of Hitachi Global Storage Technologies in San Jose, Calif., to explore the bonding and dynamics of a novel class of lubricants for hard-disk drives, consisting of hydroxyl-terminated perfluoropolyethers (PFPE), known as TA-30 and invented by Daisuke Shirakawa and colleagues of the Asahi Glass Co., in Japan.

The work was motivated by the quest for ever larger hard-disk-drive storage densities, which requires smaller spacings between the slider element and the disk surface. These spacings have become so small that they are now limited by the thickness and rigidity of the lubricant film, which must also be sufficiently strongly bonded to the disk surface to prevent transfer to the slider. TA-30 achieves this by having hydroxyl groups terminating the PFPE backbone and another hydroxyl-terminated PFPE fragment (a so-called tether) attached to the center of the main chain, forming a Y-shaped molecule.

Waltman carried out quantum calculations to determine the optimum bonding structure for both a branched molecule analogous to TA-30 and a linear PFPE with hydroxyl groups at either end. Both molecules were found to bond to the surface via their hydroxyl groups with the linear molecule forming a loop, while the branched TA-30 analog also bonded via the tether. It was found that the tethered molecule produced a thinner film than the linear one, and the resulting structures were in agreement with the experimentally measured monolayer film thickness and polar surface energy.

The molecular rigidity was then explored by calculating the energy change caused upon expanding the molecules by moving the highest part of the loop away from the surface. The energy needed to expand the branched molecule was found to be almost an order of magnitude higher than that for the straight chain, so adding the tether substantially improved the molecular rigidity.

By scrutinizing the conformational changes caused by expanding the chains, it was found that the movement in the linear chain was accommodated almost exclusively by rotating around the bonds in the PFPE backbone or by slightly distorting the O-C-O or C-O-C angles. Both of these distortions required much less energy than stretching the bonds themselves, thereby allowing the linear molecule to expand quite easily. In contrast, the additional rigidity imposed by the tethering group on the branched molecule (the TA-30 analog) inhibited such motion, requiring some bonds to stretch. This correspondingly required more energy.

Such quantum calculations on realistic systems can provide insights into the relationship between molecular structure and tribological behavior that will undoubtedly guide our thinking on how to develop new lubricants.

FOR FURTHER READING:
1. Waltman, R.J. (2011), “Single-Chain Conformational Analysis on the Dynamic Main Chain Expansion of a Tethered Perfluoropolyether Boundary Lubricant Film,” Tribology Letters, 43 (2), pp. 175-184.
2. Shirakawa, D. and Ohnishi, K. (2008) “A Study on Design and Analysis of New Lubricant for Contact Recording,” IEEE Trans. Magn. 44 (2), pp. 3710-3714.
3. Tagawa, N. and Tani, H. (2010), “Conformation and Fundamental Properties of Novel Lubricant TA-30 for Near Contact Magnetic Recording,” Tribology Letters, 40 (1), pp. 131-137.
 

Eddy Tysoe is a Distinguished Professor of Physical Chemistry at the University of Wisconsin-Milwaukee. You can reach him at wtt@uwm.edu.


Nic Spencer is professor of surface science and technology at the ETH Zurich, Switzerland. Both serve as editors-in-chief of STLE-affiliated Tribology Letters journal. You can reach him at nspencer@ethz.ch.