Film conformation and dynamic properties of atomistically architectured perfluoropolyethers on the carbon overcoated surface

Abstract

Increasing magnetic recording density requires reduction in head-media spacing (HMS) down to 7 nm and introduction of new recording techniques (e.g., heat-assisted magnetic recording). Future head-disk interface (HDI) configuration demands stable protection layers with ultra thin conformation as well as enhanced tribological performance. The conventional linear perfluoropolyethers (PFPEs) lubricants with linear chain-like structure need to be atomistically modified, resulting in PFPEs such as n arms (n=3 or 4) star-like molecules (i.e., TA-30 and QA-40), which contains additional branches with functional endgroups, to improve chemical and thermal stability under the heavy load condition as well as to provide better wear protection [1]. The molecular structures of TA-30 and QA-40 especially prevent lubricant decomposition phenomena at high temperature by eliminating the acid labile unit (−OCF 2 O-), which is normally obtained from the linear PFPE manufacturing processes. We are in the progress of developing multi-scale modeling methodology covering atomistic to meso-scale levels, which will enable the massive scale simulation based on first principles by using coarse-graining procedures [2]. In this paper, we investigated the molecular conformations and dynamic responses of star-like PFPEs including TA-30 and QA-40 and compared these results to the conventional linear PFPEs (i.e., Zdol, Ztetraol, and ZTMD). We use the coarse-grained molecular level description developed for star-like PFPE films (Fig. 1) based on the force field estimation from the atomistic level model [3]. The monolayer film conformation was examined by analyzing the parallel and perpendicular components of the radius of gyration. Due to the additional adhesion of the arm to the surface, perpendicular component of radius of gyration for both TA-30 and QA-40 are smaller than Ztetraol, while larger than ZTMD since the existence of functional groups at the center of the ZTMD enable the backbone anchoring on the surface (Table 1). The film conformation was characterized by examining the functional bead density profiles, which show the distribution of endgroups and backbone exhibiting different characteristic film layering structures from linear PFPEs. The dynamic responses were also compared by calculating the self-diffusion coefficient of a tagged molecule. Beyond the TA-30 and QA-40, we investigated the physical properties of various PFPEs as a function of the molecular structure by tuning the PFPE functionality (position and strength), the number of arms, and molecular weight. From the results, we found star-like polymer structure and additional functional groups cause entanglement decreasing mobility drastically while increasing the film stability on the carbon overcoat. Novel blends combining star-like PFPEs and linear PFPEs will also be investigated to obtain an optimal lubricant combination. Our simulation results on the effect of PFPE molecular structures on the static and dynamic responses will provide the lubricant molecular design criteria for future HDI.