Über die Autorin bzw. den Autor

Stuart Rice is a Frank P. Hixon Distinguished Service Professor in the James Franck Institute and the Department of Chemistry at the University of Chicago.

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Advances in Chemical Physics, Volume 129

John Wiley & Sons

Chapter One

MYUNG S. JHON

Department of Chemical Engineering and Data Storage Systems Center, Carnegie Mellon University, Pittsburgh, Pennsylvania, U.S.A.

CONTENTS

I. Introduction II. Experimentation and Qualitative Analysis A. Scanning Microellipsometry B. Interpretation of L-t plot and D from Phenomenological Transport Model C. Rheological Measurement D. Thermodynamic and Qualitative Description III. Simulation A. SRS Model B. Monte Carlo Simulation with Bead-Spring Model C. Molecular Dynamics Simulation IV. Conclusion Appendix A.1. Background for Hard-Disk Drive (HDD) A.2. Calculation of D(h) from Hydrodynamic Model Acknowledgments References

This chapter presents fundamental scientific tools as well as potential applications relevant to the emerging field of nanotechnology. In particular, understanding the behavior of molecularly thin lubricant films is essential for achieving durability and reliability in nanoscale devices, and the experimentation and theory for the physicochemical properties of ultrathin perfluoropolyether (PFPE) films are reviewed. A method for extracting spreading properties from the scanning microellipsometry (SME) for various PFPE/solid surface pairs and the rheological characterization of PFPEs are examined at length. The interrelationships among SME spreading profiles, surface energy, rheology, and tribology, are discussed as well. Phenomenological theories, including stability analysis and microscale mass transfer, are introduced to interpret ultrathin PFPE film nanostructures qualitatively. In addition, rigorous simulation tools, including a lattice-based simple reactive sphere model, the off-lattice bead-spring Monte Carlo method, and molecular dynamics method, are examined. These tools may accurately describe the static and dynamic behaviors of PFPE films consistent with experimental findings and thus will be suitable for describing the fundamental mechanisms of film dewetting and rupture due to instability arising from nanoscale temperature and pressure inhomogeneities. Nanotribological applications, such as finding an optimal disk lubricant based on a molecule-level interaction of the lubricant with solid surfaces, will be explored.

I. INTRODUCTION

Nanoscale confined polymers are important for their potential industrial applications. The functionalities of polymer chain and solid surfaces are key control factors in determining the material designs for these applications. A fluid confined in a nanoscale system will dramatically alter its structural and dynamic properties. Because of broad technological interest, numerous studies on nanoscale confined fluids have been investigated, both theoretically and experimentally by scientists and engineers from a variety of backgrounds, including data storage, synthetic catalysis, polymer synthesis and physics, tribology, robotics, and medicine. The behavior of materials having constituents with dimensions on the nanometer scale is remarkably different from the behavior in bulk state, which has led to a new paradigm that we now refer to as nanotechnology.

Molecularly thin lubricant film is an important application of nanoscale confined polymeric fluids, and is the focus of this chapter. Ultrathin lubricant films are necessary in high-density data storage to increase the reliability and performance of hard-disk drive (HDD) systems. Spinoff and intermittent contact between the slider (or head) and the lubricated disk [ultrathin perfluoropolyether (PFPE) films are applied to the disk's carbon-overcoated surface, as shown in Fig. 1.1] cause loss and reflow of the lubricant film. The relevant HDD technology is summarized briefly in the end-of-chapter Appendix Section A.I, which provides an overview of how certain information technology devices are controlled by nanoscale chemistry.

The lubricant dynamics can alter the nanoscale aerodynamics of the slider. Conversely, the lubricant morphology and dynamics may be altered because of the presence of the slider. For these types of applications, a molecule-level understanding of the lubricant interaction with nanoscale airbearing and solid surfaces is critical. The HDD industry must cope with problems of lubricant film uniformity, roughness, durability, and stability in order to achieve its goal of increasing areal density.

The commercially available PFPE Z and Zdol (Montedison Co. products) are random copolymers with the linear backbone chain structure

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where X (endgroup) is C[F.sub.3] in PFPE Z and C[F.sub.2]C[H.sub.2]OH in PFPE Zdol. Note that Zdol has hydroxyl groups at both chain ends, which exhibit moderate interactions with solid surfaces, e.g., silica and carbon.

In addition, we examined PFPE ZdolTX with "bulky" endgroups as a potential lubricant, which is shown below:

X = C[F.sub.2]C[H.sub.2]-[(OC[H.sub.2]C[H.sub.2]).sub.1.5]-OH

The structures of PFPE Z, Zdol, and ZdolTX are shown in Figure 1.2. Other PFPEs that have been investigated include Ztetraol and AM2001; Xs are as follows:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

The use of additives, such as X1-P, may enhance the reliability of an HDD.

Scientists and engineers working in the information storage industries (e.g., Seagate, Hitachi, and Maxtor) have conducted numerous studies on PFPEs and their thin-film properties. Several academic institutions in the United States [University of California (Berkeley, Computer Mechanics Laboratory; San Diego, Center for Magnetic Recording Research), Ohio State University (Computer Microtribology and Contamination Laboratory), and Carnegie Mellon University (Data Storage Systems Center)] have been actively investigating the role of lubricants and their applications to data storage systems. The Data Storage Institute in Singapore, the largest such institute outside the United States, is involved in nanotribology research relevant to data storage as well. However, despite the plethora of research topics, we will focus only on fundamental scientific issues and our own findings regarding to data storage applications in this review. We will discuss the details of the PFPE experiments, qualitative analysis, and full-scale simulation. Although we will concentrate on selected highlights from our research, other topics will be touched on briefly.

Although not discussed in detail in this chapter, the scientific tools we explored and developed may be applicable to other areas of nanotechnology, such as nanocomposites. The nanocomposites have emerged as a new class of materials during the 1990s. For example, confined polymers solidify at temperatures well above the glass transition temperature as the intercalation rate slows down due to the increased affinity between the polymers and inorganic plate surface. It has been suggested that the timescale of the intercalation process is relatively insensitive to the molecular weight of the polymer when compared to the diffusion coefficient in the confined slit. Various types of nanocomposites have been synthesized and characterized by our research group. In addition to the well-known conventional composite...