(709c) The Molecular Rheology of Perfluoro Oligomer Nano Composite Films

During the past decade, the development of nanotechnology has enabled nanoscale engineering in diverse areas with durable and high endurance mechanical systems. Consequently, tribological characteristics of contact surfaces, which have been reduced to a few nanometer scale, are governed by the molecular deformation (e.g., shear/elongation) of the lubricant on the solid surface interfaces. Lubricant deformation affects performance and reliability of the mechanical systems since the operation failure as well as the energy loss occurs during the operational contact. Therefore, the molecular responses of lubricants become critically important in designing effective lubricants that control the friction and wear of nano mechanical systems. Performance of the lubricant is strongly related to both viscous and elastic deformations (i.e., rheology). The stringent confinements of current mechanical systems induced by the nanoscale dimensions have complicated the real-time observation of the nature of lubricant nanofilms due to difficulty in experimentally measuring film properties using conventional techniques. Therefore, it is essential to develop a computational approach enabling molecular simulation for the nano-rheological properties of lubricants by controlling molecular architectures and external operation conditions. Recently, it was found that the application of nano blended perfluoropolyether film has become one of the most promising alternatives to enhance the lubricant performance and reliability [1]. In this presentation, we examine the rheological responses of nano composite films of perfluoro oligomers including dynamic moduli (storage moduli (elastic), G¢, loss moduli (viscous), G², and complex viscosity, by monitoring the time-dependent strain-stress plots via nonequilibrium molecular dynamics (NEMD) simulations [2]. Since the oscillatory shear responses of oligomer mixture are more sensitive to the material morphology causing noise at large amplitudes, the simulated rheological data was analyzed via fast Fourier transform, which examines the frequency spectrum of nonlinear response obtained from oscillatory shear [3]. We observed that pure nonfunctional perfluoro oligomers exhibit liquid-like behavior, while “pseudoreptation-like” or “agglomeration” behavior was observed for the pure functional oligomers. The strong functionality and the agglomeration result in high stiction and shear viscosity. Endgroup couplings due to functional endgroups weaken as the temperature increases. For the binary blended oligomers, we observed similar agglomeration phenomena which depends strongly on the blend ratio and imposed condition (e.g., temperature and oscillatory frequency). By constructing a modified Cole-Cole plot (i.e., G¢-G²) [4], which provides a finger print analysis, we found that the nanostructural conformation dependence decreases with increasing frequency. Via the relaxation time spectrum analysis, we found that the existence of nonfunctional oligomers in binary mixtures enhances the relaxation of the lubricant system since the relaxation process of a binary mixture is primarily controlled by the nonfunctional oligomer molecules.

References

1. P.S. Chung, H. Park, and M.S. Jhon, “The static and dynamic responses of binary mixture perfluoropolyether lubricant films – Molecular structural effects,” IEEE Trans. Magn., 45, 4644 (2009).
2. Q. Guo, P.S. Chung, H.G. Chen, and M.S. Jhon, “Molecular rheology of perfluoropolyether lubricant via nonequilibrium molecular dynamics simulation,” J. Appl. Phys., 99,  08N105 (2006).
3. M. Wilhelm, D. Maring, and H.-W. Spiess, “Fourier-transform rheology,” Rheol. Acta, 37, 399 (1998).
4. M.S. Jhon, “Physicochemical properties of nanostructured perfluoropolyether films,” Adv. Chem. Phys., 129,  1 (2004).