Nanostructured Ultrathin Films of Silicate Clay and Polyelectrolytes: Deposition Parameters and Mechanical Properties by
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Nanostructured Ultrathin Films of Silicate Clay and Polyelectrolytes: Deposition Parameters and Mechanical Properties by Nanoindentation Xiaowu Fan and Rigoberto C. Advincula*, Department of Chemistry, University of Alabama at Birmingham 901 14th Street South, Birmingham, AL 35294, U.S.A. ABSTRACT Recently, we have investigated the electrostatic layer-by-layer (ELBL) deposition of polycation and clay platelet ultrathin films. We have investigated the properties of these films using techniques such as ellipsometry, X-ray diffraction and atomic force microscopy (AFM). In this work, we report our results regarding the formation of this type of hybrid ultrathin films focusing on their mechanical properties as probed by nanoindentation experiments. Structural information such as film thickness, platelet coverage, surface morphology, roughness, etc., is important parameters for their potential use as coatings. We have investigated the relationship of several of these parameters with their mechanical hardness and modulus properties as a function of indentor probe depth in nanoindentation experiments. The ultrathin films have remarkable mechanical properties very different from most polymer ultrathin films.
INTRODUCTION The preparation and ordering of polyelectrolyte/clay self-assembled ultrathin films have been extensively investigated in the past. [1] They are ideal systems for the study of the various theories in interface science and intercalation thermodynamics of organic species in confined and tethered environment. [2] They are also of interest as nanostructured and well-ordered multilayer thin films with tremendous potential applications as optical and electronic materials. [3] The protocol for incorporating clay species, which are negatively charged upon hydration, typically involves the electrostatic layer-by-layer (ELBL) approach. Detailed structural investigations of film thickness and interlayer spacing of clay particles was reported by Kotov and co-workers through a series of surface-sensitive techniques such as Surface Plasmon Spectroscopy (SPS), XRay Reflectivity, and X-Ray Diffraction (XRD), etc. [4] The surface morphology has been well studied by Atomic Force Microscopy (AFM). Very recently, the organic species intercalated to this system has been extended to charged dye molecules. [5] For real commercial applications, the mechanical properties of these films are essential. Very few studies have been reported regarding the mechanical properties of these films. [4] In this paper, we report the preparation and mechanical properties of relatively “thick” films of polycation/clay ELBL multilayers with a well-ordered structure and controlled thickness with optimum surface smoothness. Based on previous work, we empirically chose the optimum parameters for film preparation up to 100 layers. We then characterized the multilayer thin film by XRD, ellipsometry and AFM. Using nanoindentation measurements, we quantitatively characterized the mechanical properties of these films, which involved hardness testing and mo
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